WO2019145024A1 - Air conditioning system and method based on evaporative heat transfer with air supersaturation - Google Patents

Air conditioning system and method based on evaporative heat transfer with air supersaturation Download PDF

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Publication number
WO2019145024A1
WO2019145024A1 PCT/EP2018/051633 EP2018051633W WO2019145024A1 WO 2019145024 A1 WO2019145024 A1 WO 2019145024A1 EP 2018051633 W EP2018051633 W EP 2018051633W WO 2019145024 A1 WO2019145024 A1 WO 2019145024A1
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WO
WIPO (PCT)
Prior art keywords
auxiliary
heat exchanger
air
main
passageways
Prior art date
Application number
PCT/EP2018/051633
Other languages
French (fr)
Inventor
Manuel REIS VILACA
André REMSCHAK
Original Assignee
Luwa Air Engineering Ag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Luwa Air Engineering Ag filed Critical Luwa Air Engineering Ag
Priority to PCT/EP2018/051633 priority Critical patent/WO2019145024A1/en
Publication of WO2019145024A1 publication Critical patent/WO2019145024A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
    • F24F5/00—Air-conditioning systems or apparatus not covered by F24F1/00 or F24F3/00, e.g. using solar heat or combined with household units such as an oven or water heater
    • F24F5/0007—Air-conditioning systems or apparatus not covered by F24F1/00 or F24F3/00, e.g. using solar heat or combined with household units such as an oven or water heater cooling apparatus specially adapted for use in air-conditioning
    • F24F5/0035—Air-conditioning systems or apparatus not covered by F24F1/00 or F24F3/00, e.g. using solar heat or combined with household units such as an oven or water heater cooling apparatus specially adapted for use in air-conditioning using evaporation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
    • F24F12/00—Use of energy recovery systems in air conditioning, ventilation or screening
    • F24F12/001—Use of energy recovery systems in air conditioning, ventilation or screening with heat-exchange between supplied and exhausted air
    • F24F12/006—Use of energy recovery systems in air conditioning, ventilation or screening with heat-exchange between supplied and exhausted air using an air-to-air heat exchanger
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
    • F24F3/00—Air-conditioning systems in which conditioned primary air is supplied from one or more central stations to distributing units in the rooms or spaces where it may receive secondary treatment; Apparatus specially designed for such systems
    • F24F3/12—Air-conditioning systems in which conditioned primary air is supplied from one or more central stations to distributing units in the rooms or spaces where it may receive secondary treatment; Apparatus specially designed for such systems characterised by the treatment of the air otherwise than by heating and cooling
    • F24F3/14—Air-conditioning systems in which conditioned primary air is supplied from one or more central stations to distributing units in the rooms or spaces where it may receive secondary treatment; Apparatus specially designed for such systems characterised by the treatment of the air otherwise than by heating and cooling by humidification; by dehumidification
    • F24F3/147—Air-conditioning systems in which conditioned primary air is supplied from one or more central stations to distributing units in the rooms or spaces where it may receive secondary treatment; Apparatus specially designed for such systems characterised by the treatment of the air otherwise than by heating and cooling by humidification; by dehumidification with both heat and humidity transfer between supplied and exhausted air
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
    • F24F6/00—Air-humidification, e.g. cooling by humidification
    • F24F6/12—Air-humidification, e.g. cooling by humidification by forming water dispersions in the air
    • F24F6/14—Air-humidification, e.g. cooling by humidification by forming water dispersions in the air using nozzles
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24F—AIR-CONDITIONING; AIR-HUMIDIFICATION; VENTILATION; USE OF AIR CURRENTS FOR SCREENING
    • F24F2110/00—Control inputs relating to air properties
    • F24F2110/20—Humidity
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00—Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/54—Free-cooling systems
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00—Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/56—Heat recovery units

Definitions

  • the present invention relates to an evaporative heat transfer device and a method for cooling air according to the preamble of claim 1 and 13, respectively.
  • Evaporative heat transfer can be used for cooling and is also known as evaporative cooling.
  • Evaporative cooling is the addition of water vapor into air, which causes a lowering of the temperature of air. Energy needed to evaporate water is taken from the air in the form of sensible heat, which lowers the temperature of air, and is converted into latent heat, which is present in the water vapor component of the air. The air remains at a constant enthalpy value during this conversion.
  • Such evaporative cooling causes a drop in the temperature of air proportional to the sensible heat drop and an increase in humidity proportional to the latent heat gain.
  • Direct evaporative cooling is used in cooling systems, where water is evaporated into an air stream to be cooled, by blowing this air stream over evaporative cooling pads for example.
  • Indirect evaporative cooling is used in cooling systems having a heat exchanger, in which a moist air stream and a separated air stream intended to be conditioned flow.
  • the moist air stream cools by water evaporation the separated air stream and never comes into direct contact with the conditioned air, thus maintaining constant humidity in the conditioned air.
  • WO 2015/069284 discloses an indirect evaporative air conditioning system comprising a heat exchanger having an inlet area and an outlet area and a plurality of passageways between the inlet area and the outlet area, a pump and nozzle assembly for introducing pressurized air and water to the inlet area of the heat exchanger to create a mist of water droplets suspended in air, a blower for creating a partial vacuum near the outlet area of the heat exchanger to draw the mist through the passageways and to the outlet area to remove heat from the heat exchanger through evaporative cooling, and a transfer mechanism for moving air past the heat exchanger to remove heat from the air without permitting the air to mix with the mist drawn through the heat exchanger.
  • the combination of pressurized air and water to create a mist of water droplets in the air conditioning system introduces complexity in the construction of the air conditioning system and in the control of the mist parameters, especially of the size of water droplets, which influence the efficiency of the heat exchanger. Indeed, water droplets attach to the walls of the heat exchanger and consequently increase the area for the heat exchange, which influence water evaporation and consequently the cooling performance of the air conditioning system.
  • parameters like humidity and temperature of the air fed to the heat exchanger are not considered in the generation of mist in the heat exchanger, although they also influence the cooling performance of the air conditioning system. Consequently, pressurized air and water is wasted, which leads to further increases of operating costs of such an air conditioning system.
  • the object of the present invention thus lies in providing an evaporative heat transfer device for cooling air and a method for cooling air with an evaporative heat transfer device, which allows high cooling performance of the device and the method.
  • evaporative heat transfer device for cooling air and method are meant to be used for industrial applications where an important quantity of heat must be removed from a space, for example data centres, production halls, among others for the textile industry, drying installations. However, it can also be used for houses, apartments or offices with the adequate scaling of the device .
  • the evaporative heat transfer device for cooling air comprises a heat exchanger, with a heat exchanger auxiliary inlet, a heat exchanger auxiliary outlet, a heat exchanger main inlet and a heat exchanger main outlet, having a plurality of auxiliary passageways between the heat exchanger auxiliary inlet and the heat exchanger auxiliary outlet and a plurality of main passageways between the heat exchanger main inlet and the heat exchanger main outlet, wherein the auxiliary passageways and the main passageways are separated from each other by at least one heat exchange wall in common to exchange heat .
  • the heat exchanger auxiliary inlet is connected to an auxiliary air inlet and the heat exchanger auxiliary outlet is connected to an auxiliary air outlet
  • the heat exchanger main inlet is connected to a main air inlet
  • the heat exchanger main outlet is connected to a main air outlet.
  • a mist chamber is arranged between the auxiliary air inlet and the heat exchanger auxiliary inlet.
  • the device also comprises an auxiliary ventilator for moving auxiliary air from the auxiliary air inlet into the mist chamber, and from the mist chamber through the auxiliary passageways to the auxiliary air outlet, as well as a main ventilator for moving main air from the main air inlet through the main passageways to the main air outlet.
  • the auxiliary ventilator is arranged in the region of the auxiliary air outlet to pull the auxiliary air and the main ventilator is arranged in the region of the main air outlet to pull the main air.
  • the auxiliary ventilator is arranged in the region of the auxiliary air inlet to push the auxiliary air and the main ventilator is arranged in the region of the main air inlet to push the main air.
  • the arrangement of ventilators can be chosen in such a way that an excess pressure is generated in the main passageways with the main ventilator arranged in the region of the main air inlet and a lower pressure is generated in the auxiliary passageways with the auxiliary ventilator arranged in the region of the auxiliary air outlet.
  • This arrangement avoids the introduction of auxiliary air in main air in case of leakage.
  • the main air moves at least almost completely, preferably completely, from the main air inlet to the main air outlet and the auxiliary air moves at least almost completely, preferably completely, from the auxiliary air inlet to the auxiliary air outlet.
  • the main air and the auxiliary air are at least almost completely, preferably completely, separated from each other in the device. Consequently, the main air and the auxiliary air are substantially not mixed in the device excepted in case of leakage in the device.
  • the device comprises a mist generator connected to the mist chamber and designed to form a mist in the mist chamber.
  • the mist generator is arranged, as seen in direction of flow of the auxiliary air, upstream of the mist chamber.
  • this arrangement allows a better control of mist parameters before mist flows into the heat exchanger auxiliary inlet and therefore allows an improved heat exchange in the heat exchanger.
  • An example of such parameters is the size of water droplets in the mist, which influence water evaporation and consequently humidity in the air conditioning system.
  • the mist generator adjoins the mist chamber and is arranged, as seen in direction of flow of the auxiliary air, upstream of the mist chamber.
  • the mist generator injects microscopic water droplets in the auxiliary air moving to the mist chamber in order to form in operation a mist with an absolute humidity equal or higher than the dew point absolute humidity, and the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets.
  • the total length of the auxiliary passageways and the main passageways depends on the application field and ranges from 0.5 m to 3.0 m, more preferably 0.5 m to 1.5 m.
  • the mist generator injects pressurized water at a pressure ranging from 20 to 120 bar, preferably 60 to 80 bar, in the form of microscopic water droplets.
  • pressurized water instead of pressurized air and water like in the prior art, allows a simple control of the size of the microscopic water droplets injected in the mist chamber and of the water/ air weight ratio.
  • generating pressurized water is a less energy-intensive and expensive process than generating compressed air. The operating costs are advantageously low.
  • pressurized water is injected in the auxiliary air in the direction of flow of the auxiliary air. This improves mixing of pressurized water with auxiliary air.
  • pressurized water in the form of microscopic water droplets is mixed at least almost completely with the auxiliary air in the mist generator before it moves into the mist chamber.
  • ist refers to the two-phase mixture comprising finely dispersed water droplets in the auxiliary air.
  • Other substances can also be present in the mist, preferably to avoid the development of bacteria or mold in the device, for example ozone.
  • substances supporting the state as water droplets in the mist until the end of the passageways can also be present.
  • the mist is moved by the auxiliary ventilator from the mist chamber through the auxiliary passageways where mist droplets progressively evaporate, possibly after attaching to the heat exchange wall, by taking the energy from the heat exchange wall in the form of sensible heat.
  • This energy transfer lowers the temperature of the heat exchange wall progressively along the path of the mist and consequently reduces the temperature of the main passageways and of the main air moved into the main passageways.
  • heat is removed from the main air moved into the main passageways.
  • This energy is converted into latent heat which is present in the water vapor component of the mist moved through the auxiliary passageways. It is possible to feed the auxiliary air to a further installation to recycle energy and water contained in this air flow.
  • the cooling performance of the device is higher when the auxiliary air fed to the device has a low relative humidity. In this case the amount of water droplets that can be injected in the auxiliary air is bigger and more evaporation takes place, resulting in a greater cooling effect .
  • dew point refers to the temperature to which air must be humidified and/or cooled to become saturated with water vapor. When further humidified, the airborne water vapor will condense to form liquid water (dew) . Further, the dew point absolute humidity corresponds to the maximum mass of water expressed in g/ kg dry air that can be borne by the air at the temperature and barometric pressure at which the device is operated.
  • the absolute humidity is equal or higher than the dew point absolute humidity over 50%, preferably over 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways. Since the cooling is caused by water evaporation contained in the auxiliary air flowing through the auxiliary passageways, the cooling effect can be increased when the absolute humidity remains equal or higher than the dew point absolute humidity over the longest distance possible in the auxiliary passageways.
  • the heat exchanger efficiency is close to 100% when the absolute humidity remains equal or higher than the dew point absolute humidity over 100% of the total length of the auxiliary passageways, the gap to 100% efficiency being only related to the heat loss caused by the exterior walls of the heat exchanger. Such operation of the device allows low operational costs.
  • the absolute humidity in the mist chamber is more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity.
  • Such an absolute humidity corresponds to a relative humidity above 100% and this state is referred to as supersaturation.
  • an absolute humidity equal or higher than the dew point absolute humidity over 50%, preferably over 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways, can be reached with an absolute humidity of more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity of the auxiliary air in the mist chamber.
  • An absolute humidity of at least approximately 1.0 g water/ kg dry air above the dew point absolute humidity in the mist chamber has experimentally shown good results for most of the heat exchangers, to keep an absolute humidity equal or higher than the dew point absolute humidity over 50%, preferably over 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways. It also resulted in a minimal condensation of water. For especially large heat exchangers an absolute humidity of more than 1.0 g water/ kg dry air above the dew point absolute humidity in the mist chamber can be necessary.
  • An absolute humidity higher than 1.5 g water/ kg dry air above the dew point absolute humidity in the mist chamber can result in a separation of the microscopic water droplets from the mist in the region before the heat exchanger auxiliary inlet, leading to a reduction of the cooling performance and wasted energy. It further leads to an excessive condensation of water which is wasted and must be collected and evacuated through a water drain.
  • the microscopic water droplets have a size between 5 to 25 micrometers, preferably 8 to 15 micrometers, more preferably at least approximately 10 micrometers. Since water droplets attach to the wall of the auxiliary passageways and consequently increase the area for the heat exchange, droplets size influences the operation of the present device. Having the same water/ air weight ratio of 10%, simulations by Kumari et al . in
  • droplets size refers here to the average size of the droplets in the mist chamber.
  • an auxiliary outlet relative humidity measurement device is located near or at the heat exchanger auxiliary outlet.
  • temperature and humidity sensors are also located near or at the auxiliary air inlet, near or at the heat exchanger auxiliary inlet, near or at the main air inlet and near or at the main air outlet to monitor the temperature and humidity of auxiliary air and main air in the device.
  • the device is preferably controlled by a control system regulating the volume of auxiliary and main air, the auxiliary and main ventilators and the mist generator in function of the temperature and humidity detected by sensors arranged in the device and in function of data collected by measuring devices evaluating the external conditions in which the evaporative heat transfer device is operated.
  • Other sensors for example to detect ozone concentration, can additionally be used to control the operation of the device.
  • the mist generator increases the quantity of injected microscopic water droplets when the relative humidity measured by the auxiliary outlet relative humidity measurement device is below a predetermined level. This predetermined level is set to optimize the cooling performance of the device depending on temperature and relative humidity of the auxiliary air with which the device is operated. Injecting only the required quantity of microscopic water droplets to reach the optimized cooling performance results in a reduced energy and water consumption.
  • One possible regulation loop of the device uses the auxiliary outlet relative humidity measurement device to detect a relative humidity lower than 100% at the sensor position, which is a sign that the absolute humidity in the auxiliary passageways is below the dew point absolute humidity over at least part of the total length of the auxiliary passageways in the region of auxiliary passageways outlets.
  • the mist generator is triggered to increase the quantity of injected microscopic water droplets in the mist chamber.
  • auxiliary ventilator speed it is also possible for example to influence cooling by the heat exchanger by adjusting the auxiliary ventilator speed.
  • a reduction of the auxiliary ventilator speed will result in a longer transfer time of the mist in the auxiliary passageways and consequently in an increased evaporation, i.e. an increased cooling effect.
  • Other regulation loops are possible depending on the conditions of operation of the device and not limited to the examples given .
  • the heat exchanger is arranged in a cabinet in the form of a rectangular cuboid having a front wall and a back wall spaced in a longitudinal direction (D) , as well as a bottom wall, a top wall and lateral walls, wherein the auxiliary air inlet is connected to the heat exchanger auxiliary inlet by an auxiliary duct.
  • the auxiliary duct extends in the longitudinal direction (D) , preferably along and limited by the bottom wall and comprises the mist chamber, the mist generator and intermediate walls extending parallel to the longitudinal direction (D) to create an at least approximately laminar flow before the heat exchanger auxiliary inlet.
  • the creation of a laminar flow before the heat exchanger auxiliary inlet ensures stable operation conditions in the device.
  • This laminar flow also reduces pressure loss in the region of the auxiliary passageways inlets by reducing the interference of the flow with the heat exchanger auxiliary inlets.
  • These intermediate walls can optionally be part of the structure supporting the heat exchanger in the cabinet and contribute to stiffen the whole structure.
  • the auxiliary duct has a U-shaped cross section formed by lateral walls curved along the longitudinal direction (D) to reduce pressure loss .
  • Suitable materials for the construction of the device are metal, preferably aluminium or stainless steel, which do not promote the development of bacteria or mold and do not corrode, or plastic material, for example polypropylene. It is also possible to mix both materials: for example, the heat exchanger is made of metal and the ducts are made of plastic material or conversely. Materials are chosen depending on the environment in which the device is operated as well as cost considerations.
  • the inlets and the outlets of the auxiliary passageways are oriented towards the back wall and the front wall, respectively.
  • the inlets and the outlets of the main passageways are oriented towards the front wall and the back wall, respectively.
  • the auxiliary duct extends from the front wall towards the back wall and is deflected near the back wall towards the auxiliary passageways inlets.
  • the auxiliary duct is deflected to form a curved region near the back wall to guide the mist into the auxiliary passageways in a manner avoiding a separation of the microscopic water droplets from the air flow in the region before the heat exchanger auxiliary inlet.
  • the mist must be controlled to avoid an absolute humidity above 1.5 g water/ kg dry air above the dew point absolute humidity. Higher absolute humidity causes microscopic water droplets to flow in the auxiliary duct against the back wall and to condensate on the back wall instead of remaining in the mist and flowing into the auxiliary passageways, which leads to a reduction of the cooling performance as well as wasted energy and water.
  • the heat exchanger has a plurality of spaced heat exchange walls, which are arranged to form a repeating sequence of one auxiliary passageway and one main passageway and which are preferably planar and parallel to each other.
  • the distance between two heat exchange walls is chosen so that a construction with a compact arrangement of the heat exchanger is possible and the surface of the heat exchange walls is maximized, leading to a minimal size of the device and a minimum pressure loss, while improving its efficiency .
  • Auxiliary passageways can also be separated in a plurality of subsections of auxiliary passageways when an intermediate wall, acting for example as a spacer, is arranged between the walls of the auxiliary passageways and extends over the total length or a portion of the auxiliary passageways.
  • main passageways can also be formed in such a way that they are separated in a plurality of subsections of main passageways. It is further possible to arrange the auxiliary air inlet and the main air inlet in the front wall, the main air outlet in the back wall and the auxiliary air outlet in the top wall. Such a configuration is preferred to keep a compact arrangement in the device. However, it can be adapted to the configuration of the pipes to which the device must be connected.
  • mist generator as disclosed in EP 0 670 986 B1 and using only pressurized water to generate the mist is used because of its low energy consumption and its simplicity in controlling water droplets size and the water/ air weight ratio of the mist.
  • the mist generator comprises a duct portion of the auxiliary duct, which establishes a direction of flow, a guiding apparatus disposed to extend at right-angle to the direction of flow and atomising jets for injecting microscopic water droplets to form the mist.
  • the guiding apparatus comprises a plurality of adjacently disposed and superposed ports with fixed guide blades, the guide blades being so disposed that a twist is imposed upon the auxiliary air flowing through them to form a bundle of turbulent partial flows in the duct portion.
  • the creation of a turbulent flow maximizes the mixture of water droplets with auxiliary air flowing through the guide blades.
  • this turbulent flow is transformed further in the auxiliary duct in a laminar flow by means of intermediate walls.
  • the ports comprise in their respective centres a continuous aperture extending in the direction of flow and in which there is an atomising jet projecting beyond the guide apparatus in the direction of flow and wherein the atomising jet is surrounded preferably by no additional guide means projecting beyond the atomising jet in the direction of flow.
  • ultrasonic humidifying is preferred to generate mist in case the evaporative heat transfer device is used for air conditioning of smaller spaces, typically houses, and requires little maintenance only.
  • An ultrasonic humidifier uses a ceramic diaphragm vibrating at an ultrasonic frequency to create water droplets that exit the humidifier in the form of mist.
  • the auxiliary passageways and the main passageways are arranged to form a cross-counter air flow in the heat exchanger.
  • the cross-counter air flow is preferred because of its higher efficiency.
  • a water drain is arranged in the region near the auxiliary passageways inlets to collect condensed water in the device.
  • This water can be reused by recirculating it to the mist generator to reduce the water requirements of the device, so that the device has a lower environmental impact and lower operational costs.
  • air filters are used to ensure that particles and dust do not clog the evaporative heat transfer device. Moreover, such particles and dust result in increased water condensation in a humidity supersaturated atmosphere and have a negative impact on the cooling performance.
  • the method to cool air by evaporative heat transfer uses a heat exchanger, with a heat exchanger auxiliary inlet, a heat exchanger auxiliary outlet, a heat exchanger main inlet and a heat exchanger main outlet, having a plurality of auxiliary passageways between the heat exchanger auxiliary inlet and the heat exchanger auxiliary outlet and a plurality of main passageways between the heat exchanger main inlet and the heat exchanger main outlet, wherein the auxiliary passageways and the main passageways have at least a heat exchange wall in common to exchange heat.
  • the heat exchanger auxiliary inlet is connected via a mist chamber to an auxiliary air inlet and the heat exchanger auxiliary outlet is connected to an auxiliary air outlet
  • the heat exchanger main inlet is connected to a main air inlet
  • the heat exchanger main outlet is connected to a main air outlet.
  • Auxiliary air is moved into the mist chamber by an auxiliary ventilator, and from the mist chamber into the auxiliary passageways to the auxiliary air outlet, and main air is moved by a main ventilator from the main air inlet into the main passageways to the main air outlet.
  • a mist is formed in the mist chamber by a mist generator connected to the mist chamber.
  • the mist generator injects microscopic water droplets in the auxiliary air moving to the mist chamber in order to form a mist with an absolute humidity equal or higher than the dew point absolute humidity; and the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways.
  • the mist generator injects microscopic water droplets until the absolute humidity in the mist chamber is more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity.
  • the absolute humidity is equal or higher than the dew point absolute humidity over at least 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways .
  • the mist generator injects microscopic water droplets having a size between 5 to 25 micrometers, preferably 8 to 15 micrometers, more preferably at least approximately 10 micrometers.
  • the mist generator increases the quantity of microscopic water droplets injected when the relative humidity measured by an auxiliary outlet relative humidity measurement device located near or at the heat exchanger auxiliary outlet is below a predetermined level.
  • the device may be operated such that air is extracted from a building and fed to the device as auxiliary air which is rejected through the auxiliary air outlet to the outside, while outside air fed as main air is cooled by the device and fed through the main air outlet to the building.
  • the device may be operated such that outside air is fed as main air and as auxiliary air to the device. Auxiliary air is rejected through the auxiliary air outlet to the outside, while main air is cooled by the device and fed through the main air outlet to the building.
  • the device may be operated such that outside air is fed to the device as auxiliary air which is rejected through the auxiliary air outlet to the outside, while air extracted from a building fed as main air to the device is cooled by the device and fed through the main air outlet to the building.
  • the cooling performance of the device and of the method is higher and bigger temperature differences between main air fed to the device and cooled main air produced by the device can be achieved, when the relative humidity of the auxiliary air fed to the device is low.
  • the lower this relative humidity of the auxiliary air the more amount of water droplets can be injected in the auxiliary air and therefore the bigger the cooling effect is .
  • the main air dew point must be higher than the auxiliary air dew point.
  • the device may be operated in several stages, including the use of further air treatment devices.
  • the main air can be cooled in a first stage using a device according to the present invention and the main air resulting from this first stage can be used as auxiliary air for the next cooling stage involving a further device according to the present invention .
  • the device according to the present invention can also be used as a heat exchanger when it is not operated as a cooling device, so that it works as a heat recovery system.
  • Fig. 1 shows a perspective view of the device equipped with a mist generator as disclosed in
  • FIG. 2 shows a vertical cross section of the device represented in Fig. 1 along the section line II- II extending through an auxiliary passageway
  • Fig . 3 shows a schematic representation of the mist generator in a horizontal section along the section line III-III of Fig. 1,
  • Fig. 4 shows a schematic representation of the portion of the front wall closing the auxiliary duct, viewed from the inside of the device of fig. 1 in the direction opposed to the direction D
  • Fig. 5 shows a perspective view of the device equipped with an ultrasonic humidifier
  • Fig. 6 shows a psychrometric chart corresponding to the process taking place in the device of Fig. 1 with exemplary parameters for temperature and humidity .
  • Fig. 4 shows a schematic representation of the portion of the front wall closing the auxiliary duct, viewed from the inside of the device of fig. 1 in the direction opposed to the direction D
  • Fig. 5 shows a perspective view of the device equipped with an ultrasonic humidifier
  • Fig. 6 shows a psychrometric chart corresponding to the process taking place in the device of Fig. 1 with exemplary parameters for temperature and humidity .
  • Fig. 4 shows a schematic representation of the portion of the front wall closing the auxiliary duct, viewed from the inside of the device of fig. 1 in the direction opposed
  • FIG. 1 is an exemplary illustration of an embodiment for an evaporative heat transfer device 10 according to the present invention, comprising a heat exchanger 20, with a heat exchanger auxiliary inlet 22, a heat exchanger auxiliary outlet 24, a heat exchanger main inlet 26 and a heat exchanger main outlet 28, having a plurality of auxiliary passageways 30 between the heat exchanger auxiliary inlet 22 and the heat exchanger auxiliary outlet 24 and a plurality of main passageways 32 between the heat exchanger main inlet 26 and the heat exchanger main outlet 28.
  • the heat exchanger 20 is arranged in a cabinet 34 in the form of a rectangular cuboid having a front wall 36 and a back wall 38 spaced in a longitudinal direction D, as well as a bottom wall 40, a top wall 42 and lateral walls 44.
  • a cabinet 34 in the form of a rectangular cuboid having a front wall 36 and a back wall 38 spaced in a longitudinal direction D, as well as a bottom wall 40, a top wall 42 and lateral walls 44.
  • Fig. 1 only one lateral wall is shown.
  • the front wall 36 has an opening forming an auxiliary air inlet 46 and an opening forming a main air inlet 48
  • the back wall 38 has an opening forming the main air outlet 50
  • the top wall 42 has an opening forming an auxiliary air outlet 52.
  • separation plates 106 are arranged between the heat exchanger 20 and the top wall 42 to avoid mixing of auxiliary air 12 and main air 16.
  • the auxiliary air inlet 46 is connected to the heat exchanger auxiliary inlet 22 by an auxiliary duct 54 extending in the longitudinal direction (D) , preferably along and limited by the bottom wall 40 and an intermediate wall 41.
  • the auxiliary duct 54 comprises a mist generator 74, a mist chamber 56 and vertical intermediate walls 58 extending parallel to the longitudinal direction (D) to create an at least approximately laminar flow before the heat exchanger auxiliary inlet 22.
  • the heat exchanger auxiliary outlet 24 is connected to the auxiliary air outlet 52, the heat exchanger main inlet 26 is connected to the main air inlet 48 and the heat exchanger main outlet 28 is connected to the main air outlet 50.
  • the auxiliary passageways 30 and the main passageways 32 are completely separated from each other by at least one heat exchange wall, in the present case by a plurality of heat exchange walls 60, in common to exchange heat.
  • the heat exchanger 20 has a plurality of spaced heat exchange walls 60, which are arranged to form a repeating sequence of one auxiliary passageway 30 and one main passageway 32 and which are preferably planar and parallel to each other.
  • the space between two consecutive heat exchange walls 60 is partially closed by a wall 61 to form a passageway with an inlet and an outlet.
  • the auxiliary passageways inlets 62 and outlets 64 are flow-connected and the main passageways inlets 66 and outlets 68 are flow-connected.
  • auxiliary passageways 30 and main passageways 32 have a plurality of heat exchange walls 60 in common to exchange heat.
  • the inlets 62 of the auxiliary passageways are oriented towards the back wall 38 and the inlets 66 of the main passageways are oriented towards the front wall 36.
  • the outlets 64 of the auxiliary passageways are oriented towards the front wall 36 and the outlets 68 of the main passageways are oriented towards the back wall 38.
  • the auxiliary duct 54 extends from the front wall 36 towards the back wall 38 and is deflected near the back wall 38 towards the auxiliary passageways inlets 62.
  • the device also comprises a auxiliary ventilator 70 (represented in Fig. 2), arranged at the auxiliary air outlet 52, for moving auxiliary air 12 from the auxiliary air inlet 46 into the mist chamber 56, and from the mist chamber 56 through the auxiliary passageways 30 to the auxiliary air outlet 52, as well as a main ventilator 72 (represented in Fig. 2), arranged at the main air outlet 50, for moving main air 16 from the main air inlet 48 through the main passageways 32 to the main air outlet 50.
  • a auxiliary ventilator 70 represented in Fig. 2
  • main ventilator 72 represented in Fig. 2
  • Fig. 1 shows the mist generator 74, comprising a duct portion 76 of the auxiliary duct 54 and atomising jets 86, designed to inject pressurized water into the mist chamber 56 to form a mist.
  • the mist generator 74 in Fig. 1 is represented schematically without water feeding system for the sake of figure clarity. An embodiment of the mist generator is described in more details in Fig. 3 and Fig . 4.
  • Fig. 2 is an illustration of a vertical cross section along the line II-II of the embodiment described in
  • auxiliary outlet relative humidity measurement device 78 is located at the heat exchanger auxiliary outlet 24 and the mist generator 74 increases the quantity of injected microscopic water droplets when the relative humidity measured by the auxiliary outlet relative humidity measurement device 78 is below a predetermined level. This predetermined level is set to optimize the cooling performance of the device 10 depending on the external temperature and relative humidity conditions in which the device is operated.
  • temperature and humidity sensors 80 are also located at the auxiliary air inlet 46, at the heat exchanger auxiliary inlet 22, at the main air inlet 48 and at the main air outlet 50 to monitor the temperature and humidity of auxiliary air 12 and main air 16 in the device 10.
  • An ozone sensor to detect the ozone concentration is also located at the heat exchanger auxiliary outlet 24.
  • a water drain 69 is arranged in the region close to the auxiliary passageways inlets 62 to collect water condensation taking place in the device 10. This water can be reused to reduce the water requirements of the device.
  • the mist generator 74 as disclosed in EP 0 670 986 B1 is represented in Fig. 3.
  • the mist generator 74 uses only pressurized water to generate the mist and is preferred because of its low energy consumption and its simplicity in controlling water droplets size injected in the mist chamber 56.
  • the mist generator 74 comprises a duct portion
  • the guiding apparatus 84 extends over the clear cross- section of the duct portion 76 and has webs 104, which delimit a plurality of adjacently disposed ports 108.
  • the ports comprise in their respective centres a continuous aperture 88 extending in the direction of flow and in which there is an atomising jet 86 projecting beyond the guiding apparatus 84 in the direction of flow.
  • the atomising jets 86 are fed via a pipe 102 with pressurized water.
  • the atomising jets 86 are preferably embodied in the form of nozzles, like in the embodiment represented in the zoomed excerpt A of Fig. 3, wherein the pipe 102 feeds pressurized water to the nozzle 86.
  • the pressurized water is projected against a conical piece 110 arranged in front of the pressurized water flow and scattered in the form of microscopic water droplets .
  • the embodiment represented in Fig. 1 to Fig. 4 comprises six adjacently disposed ports 108, each having one nozzle 86.
  • Auxiliary air 12 moving into the mist chamber 56 through the apertures 88 of the ports 108 is split into a bundle of at least approximately parallel partial flows 100 in the duct portion 76.
  • Guide blades 90 are fixed to the ports 108, the guide blades 90 being so disposed that a twist is imposed upon the partial flows 100 flowing through them to form turbulent partial flows 100.
  • the creation of a turbulent flow is important to maximize the mixture of water droplets with auxiliary air 12.
  • this turbulent flow is transformed further in the auxiliary duct 54 in a laminar flow by means of the vertical intermediate walls 58.
  • the portion of the front wall 36 represented in Fig. 4 closes the duct portion 76 of the auxiliary duct 54 and is viewed from the inside of the device 10 in the direction opposed to the direction D.
  • the guide blades 90 are fixed to the ports 108 and the nozzles 86 are disposed behind the guide blades 90.
  • the auxiliary duct lateral walls 57 are curved in the longitudinal direction (D) to reduce pressure loss.
  • Fig. 5 shows a perspective view of the device according to Fig. 1 equipped with another type of mist generator 74.
  • the mist generator 74 comprises a duct portion 76 of the auxiliary duct 54 and an ultrasonic humidifier 112 designed to inject water droplets into the mist chamber 56 to form a mist.
  • the mist generator 74 is represented schematically without water feeding system for the sake of figure clarity.
  • Auxiliary air 12 flows through the auxiliary air inlet 46 above the ultrasonic humidifier 112 and collects water droplets that exit the humidifier in the form of mist.
  • the mist generator 74 is connected to the heat exchanger auxiliary inlet 22 by an auxiliary duct 54 extending in the longitudinal direction (D) to create an at least approximately laminar flow before the heat exchanger auxiliary inlet 22 by means of the vertical intermediate walls 58.
  • Fig. 6 shows a psychrometric chart with a schematic representation of the process taking place in the device, wherein main air and auxiliary air state changes are represented during operation of the device. Each state is represented with a point in the chart.
  • auxiliary air 12 according to state A for example outside air, is fed to the device through the auxiliary air inlet 46.
  • Auxiliary air 12 flows through the auxiliary duct 54, which comprises the mist generator 74 and the mist chamber 56, to the heat exchanger auxiliary inlet 22.
  • Microscopic water droplets are injected by the mist generator 74 in the auxiliary air 12 moving to the mist chamber 56 to form a mist with an increasing absolute humidity and correspondingly a lower temperature.
  • the auxiliary air state changes with the absolute humidity increase and the associated temperature decrease along the line from A to B in the psychrometric chart. In this process, the auxiliary air absolute humidity reaches the dew point absolute humidity represented by the state A1.
  • Microscopic water droplets are further injected so that the auxiliary air absolute humidity becomes higher than the dew point absolute humidity, i.e.
  • auxiliary air becomes supersaturated, until the absolute humidity auxiliary air reaches the target absolute humidity, which is represented by state B in the psychrometric chart.
  • the quantity of water in g water/ kg dry air above the dew point absolute humidity in the auxiliary air is represented by the quantity S in the psychrometric chart.
  • the auxiliary air 12 is moved through the heat exchanger auxiliary inlet 22 to the heat exchanger 20, namely to the plurality of auxiliary passageways 30 between the heat exchanger auxiliary inlet 22 and the heat exchanger auxiliary outlet 24.
  • main air 16 according to state E in the psychrometric chart for example air extracted from a building, is fed to the device through the main air inlet 48, which is connected to the heat exchanger main inlet 26, and moved through the heat exchanger main inlet 26 to the heat exchanger 20, namely to the plurality of main passageways between the heat exchanger main inlet 26 and the heat exchanger main outlet 28.
  • the auxiliary passageways 30 and the main passageways 32 have at least one heat exchange wall 60 in common to exchange heat.
  • Mist droplets in the auxiliary air 12 moved into the auxiliary passageways 30 progressively evaporate, possibly after attaching to the heat exchange wall 60, by taking the energy from the heat exchange wall 60 in the form of sensible heat.
  • Supersaturation in the auxiliary passageways 30 becomes lower because of the heat transfer between the main passageways 32 and the auxiliary passageways 30, and the auxiliary air temperature increases.
  • Auxiliary air state changes along the line from B to C in the psychrometric chart.
  • auxiliary air 12 is rejected through the auxiliary air outlet 52, for example to the outside, while main air 16 cooled by the device is fed through the main air outlet 50, for example to the building.

Abstract

Evaporative heat transfer device (10) for cooling air, comprising a heat exchanger (20), having a plurality of auxiliary passageways (30) and a plurality of main passageways (32), wherein the auxiliary passageways (30) and the main passageways (32) are separated from each other by at least one heat exchange wall (60) in common to exchange heat, a mist chamber (56) and a mist generator (74) connected to the mist chamber (56) designed to inject microscopic water droplets in the auxiliary air (12) in the mist chamber (56) to form in operation a mist with an absolute humidity equal or higher than the dew point absolute humidity; and the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways (30). Preferably, the mist generator (74) injects pressurized water in the form of microscopic water droplets.

Description

Air conditioning system and method based on evaporative heat transfer with air supersaturation
The present invention relates to an evaporative heat transfer device and a method for cooling air according to the preamble of claim 1 and 13, respectively.
Evaporative heat transfer can be used for cooling and is also known as evaporative cooling. Evaporative cooling is the addition of water vapor into air, which causes a lowering of the temperature of air. Energy needed to evaporate water is taken from the air in the form of sensible heat, which lowers the temperature of air, and is converted into latent heat, which is present in the water vapor component of the air. The air remains at a constant enthalpy value during this conversion. Such evaporative cooling causes a drop in the temperature of air proportional to the sensible heat drop and an increase in humidity proportional to the latent heat gain.
Direct evaporative cooling is used in cooling systems, where water is evaporated into an air stream to be cooled, by blowing this air stream over evaporative cooling pads for example.
Indirect evaporative cooling is used in cooling systems having a heat exchanger, in which a moist air stream and a separated air stream intended to be conditioned flow. The moist air stream cools by water evaporation the separated air stream and never comes into direct contact with the conditioned air, thus maintaining constant humidity in the conditioned air.
WO 2015/069284 discloses an indirect evaporative air conditioning system comprising a heat exchanger having an inlet area and an outlet area and a plurality of passageways between the inlet area and the outlet area, a pump and nozzle assembly for introducing pressurized air and water to the inlet area of the heat exchanger to create a mist of water droplets suspended in air, a blower for creating a partial vacuum near the outlet area of the heat exchanger to draw the mist through the passageways and to the outlet area to remove heat from the heat exchanger through evaporative cooling, and a transfer mechanism for moving air past the heat exchanger to remove heat from the air without permitting the air to mix with the mist drawn through the heat exchanger.
Generating compressed air is a very energy-intensive and expensive process. In addition to its capital cost, there are important operating costs related to a compressor. Up to 90% of the energy needed to generate pressurized air is dissipated as heat into the environment of the compressor. Said heat must then be also evacuated, in addition to the heat that is initially intended to be evacuated by the air conditioning system. As a result, such air conditioning system using compressed air involve an important energy consumption .
On top of an energy-intensive operation, the combination of pressurized air and water to create a mist of water droplets in the air conditioning system introduces complexity in the construction of the air conditioning system and in the control of the mist parameters, especially of the size of water droplets, which influence the efficiency of the heat exchanger. Indeed, water droplets attach to the walls of the heat exchanger and consequently increase the area for the heat exchange, which influence water evaporation and consequently the cooling performance of the air conditioning system.
Further, in the air conditioning system disclosed above, parameters like humidity and temperature of the air fed to the heat exchanger are not considered in the generation of mist in the heat exchanger, although they also influence the cooling performance of the air conditioning system. Consequently, pressurized air and water is wasted, which leads to further increases of operating costs of such an air conditioning system.
The object of the present invention thus lies in providing an evaporative heat transfer device for cooling air and a method for cooling air with an evaporative heat transfer device, which allows high cooling performance of the device and the method.
The invention is achieved by the evaporative heat transfer device and the method as claimed in claim 1 and 13, respectively. Advantageous embodiments are represented in the dependent claims. Such an evaporative heat transfer device for cooling air and method are meant to be used for industrial applications where an important quantity of heat must be removed from a space, for example data centres, production halls, among others for the textile industry, drying installations. However, it can also be used for houses, apartments or offices with the adequate scaling of the device .
The evaporative heat transfer device for cooling air comprises a heat exchanger, with a heat exchanger auxiliary inlet, a heat exchanger auxiliary outlet, a heat exchanger main inlet and a heat exchanger main outlet, having a plurality of auxiliary passageways between the heat exchanger auxiliary inlet and the heat exchanger auxiliary outlet and a plurality of main passageways between the heat exchanger main inlet and the heat exchanger main outlet, wherein the auxiliary passageways and the main passageways are separated from each other by at least one heat exchange wall in common to exchange heat . The heat exchanger auxiliary inlet is connected to an auxiliary air inlet and the heat exchanger auxiliary outlet is connected to an auxiliary air outlet, the heat exchanger main inlet is connected to a main air inlet and the heat exchanger main outlet is connected to a main air outlet.
A mist chamber is arranged between the auxiliary air inlet and the heat exchanger auxiliary inlet.
The device also comprises an auxiliary ventilator for moving auxiliary air from the auxiliary air inlet into the mist chamber, and from the mist chamber through the auxiliary passageways to the auxiliary air outlet, as well as a main ventilator for moving main air from the main air inlet through the main passageways to the main air outlet.
Preferably, the auxiliary ventilator is arranged in the region of the auxiliary air outlet to pull the auxiliary air and the main ventilator is arranged in the region of the main air outlet to pull the main air. Alternatively, the auxiliary ventilator is arranged in the region of the auxiliary air inlet to push the auxiliary air and the main ventilator is arranged in the region of the main air inlet to push the main air. A mixing of both systems is also possible. The arrangement of ventilators can be chosen in such a way that an excess pressure is generated in the main passageways with the main ventilator arranged in the region of the main air inlet and a lower pressure is generated in the auxiliary passageways with the auxiliary ventilator arranged in the region of the auxiliary air outlet. This arrangement avoids the introduction of auxiliary air in main air in case of leakage. The main air moves at least almost completely, preferably completely, from the main air inlet to the main air outlet and the auxiliary air moves at least almost completely, preferably completely, from the auxiliary air inlet to the auxiliary air outlet. The main air and the auxiliary air are at least almost completely, preferably completely, separated from each other in the device. Consequently, the main air and the auxiliary air are substantially not mixed in the device excepted in case of leakage in the device.
Further, the device comprises a mist generator connected to the mist chamber and designed to form a mist in the mist chamber.
Preferably, the mist generator is arranged, as seen in direction of flow of the auxiliary air, upstream of the mist chamber. Compared to the device disclosed in WO 2015/069284, this arrangement allows a better control of mist parameters before mist flows into the heat exchanger auxiliary inlet and therefore allows an improved heat exchange in the heat exchanger. An example of such parameters is the size of water droplets in the mist, which influence water evaporation and consequently humidity in the air conditioning system. Preferably, the mist generator adjoins the mist chamber and is arranged, as seen in direction of flow of the auxiliary air, upstream of the mist chamber.
According to the invention, the mist generator injects microscopic water droplets in the auxiliary air moving to the mist chamber in order to form in operation a mist with an absolute humidity equal or higher than the dew point absolute humidity, and the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets.
The total length of the auxiliary passageways and the main passageways depends on the application field and ranges from 0.5 m to 3.0 m, more preferably 0.5 m to 1.5 m. Preferably, the mist generator injects pressurized water at a pressure ranging from 20 to 120 bar, preferably 60 to 80 bar, in the form of microscopic water droplets. The use of pressurized water only, instead of pressurized air and water like in the prior art, allows a simple control of the size of the microscopic water droplets injected in the mist chamber and of the water/ air weight ratio. Furthermore, generating pressurized water is a less energy-intensive and expensive process than generating compressed air. The operating costs are advantageously low.
Preferably, pressurized water is injected in the auxiliary air in the direction of flow of the auxiliary air. This improves mixing of pressurized water with auxiliary air.
Preferably, pressurized water in the form of microscopic water droplets is mixed at least almost completely with the auxiliary air in the mist generator before it moves into the mist chamber.
In the following, "mist" refers to the two-phase mixture comprising finely dispersed water droplets in the auxiliary air.
Other substances can also be present in the mist, preferably to avoid the development of bacteria or mold in the device, for example ozone. Preferably substances supporting the state as water droplets in the mist until the end of the passageways can also be present.
The mist is moved by the auxiliary ventilator from the mist chamber through the auxiliary passageways where mist droplets progressively evaporate, possibly after attaching to the heat exchange wall, by taking the energy from the heat exchange wall in the form of sensible heat. This energy transfer lowers the temperature of the heat exchange wall progressively along the path of the mist and consequently reduces the temperature of the main passageways and of the main air moved into the main passageways. As a result, heat is removed from the main air moved into the main passageways. This energy is converted into latent heat which is present in the water vapor component of the mist moved through the auxiliary passageways. It is possible to feed the auxiliary air to a further installation to recycle energy and water contained in this air flow.
The cooling performance of the device is higher when the auxiliary air fed to the device has a low relative humidity. In this case the amount of water droplets that can be injected in the auxiliary air is bigger and more evaporation takes place, resulting in a greater cooling effect .
In the following, "dew point" refers to the temperature to which air must be humidified and/or cooled to become saturated with water vapor. When further humidified, the airborne water vapor will condense to form liquid water (dew) . Further, the dew point absolute humidity corresponds to the maximum mass of water expressed in g/ kg dry air that can be borne by the air at the temperature and barometric pressure at which the device is operated.
In an embodiment, the absolute humidity is equal or higher than the dew point absolute humidity over 50%, preferably over 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways. Since the cooling is caused by water evaporation contained in the auxiliary air flowing through the auxiliary passageways, the cooling effect can be increased when the absolute humidity remains equal or higher than the dew point absolute humidity over the longest distance possible in the auxiliary passageways. The heat exchanger efficiency is close to 100% when the absolute humidity remains equal or higher than the dew point absolute humidity over 100% of the total length of the auxiliary passageways, the gap to 100% efficiency being only related to the heat loss caused by the exterior walls of the heat exchanger. Such operation of the device allows low operational costs.
In a preferred embodiment, the absolute humidity in the mist chamber is more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity. Such an absolute humidity corresponds to a relative humidity above 100% and this state is referred to as supersaturation.
Depending on the characteristics of the heat exchanger, an absolute humidity equal or higher than the dew point absolute humidity over 50%, preferably over 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways, can be reached with an absolute humidity of more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity of the auxiliary air in the mist chamber.
An absolute humidity of at least approximately 1.0 g water/ kg dry air above the dew point absolute humidity in the mist chamber has experimentally shown good results for most of the heat exchangers, to keep an absolute humidity equal or higher than the dew point absolute humidity over 50%, preferably over 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways. It also resulted in a minimal condensation of water. For especially large heat exchangers an absolute humidity of more than 1.0 g water/ kg dry air above the dew point absolute humidity in the mist chamber can be necessary. An absolute humidity higher than 1.5 g water/ kg dry air above the dew point absolute humidity in the mist chamber can result in a separation of the microscopic water droplets from the mist in the region before the heat exchanger auxiliary inlet, leading to a reduction of the cooling performance and wasted energy. It further leads to an excessive condensation of water which is wasted and must be collected and evacuated through a water drain.
According to a preferred embodiment of the invention, the microscopic water droplets have a size between 5 to 25 micrometers, preferably 8 to 15 micrometers, more preferably at least approximately 10 micrometers. Since water droplets attach to the wall of the auxiliary passageways and consequently increase the area for the heat exchange, droplets size influences the operation of the present device. Having the same water/ air weight ratio of 10%, simulations by Kumari et al . in
International Journal of Heat and Mass Transfer 53 (2010) pp . 3346-3356 have shown that a mist with droplets size of 10 micrometers compared to a mist with droplets size of 50 micrometers increases the heat transfer by 50% in the heat exchanger .
Given the fact that the size of a droplet evolves during the transfer of the mist through the auxiliary passageways, droplets size refers here to the average size of the droplets in the mist chamber.
According to a preferred embodiment of the invention, an auxiliary outlet relative humidity measurement device is located near or at the heat exchanger auxiliary outlet.
Preferably temperature and humidity sensors are also located near or at the auxiliary air inlet, near or at the heat exchanger auxiliary inlet, near or at the main air inlet and near or at the main air outlet to monitor the temperature and humidity of auxiliary air and main air in the device. These parameters can be used to control the device operation. The device is preferably controlled by a control system regulating the volume of auxiliary and main air, the auxiliary and main ventilators and the mist generator in function of the temperature and humidity detected by sensors arranged in the device and in function of data collected by measuring devices evaluating the external conditions in which the evaporative heat transfer device is operated. Other sensors, for example to detect ozone concentration, can additionally be used to control the operation of the device.
In operation, supersaturation in the auxiliary passageways becomes lower because of the heat transfer between the main passageways and the auxiliary passageways. Consequently, the humidity measured by the auxiliary outlet relative humidity measurement device also becomes lower. In one regulation loop the mist generator increases the quantity of injected microscopic water droplets when the relative humidity measured by the auxiliary outlet relative humidity measurement device is below a predetermined level. This predetermined level is set to optimize the cooling performance of the device depending on temperature and relative humidity of the auxiliary air with which the device is operated. Injecting only the required quantity of microscopic water droplets to reach the optimized cooling performance results in a reduced energy and water consumption. One possible regulation loop of the device uses the auxiliary outlet relative humidity measurement device to detect a relative humidity lower than 100% at the sensor position, which is a sign that the absolute humidity in the auxiliary passageways is below the dew point absolute humidity over at least part of the total length of the auxiliary passageways in the region of auxiliary passageways outlets. Thus, the mist generator is triggered to increase the quantity of injected microscopic water droplets in the mist chamber.
It is also possible for example to influence cooling by the heat exchanger by adjusting the auxiliary ventilator speed. A reduction of the auxiliary ventilator speed will result in a longer transfer time of the mist in the auxiliary passageways and consequently in an increased evaporation, i.e. an increased cooling effect. Other regulation loops are possible depending on the conditions of operation of the device and not limited to the examples given .
In a preferred embodiment, the heat exchanger is arranged in a cabinet in the form of a rectangular cuboid having a front wall and a back wall spaced in a longitudinal direction (D) , as well as a bottom wall, a top wall and lateral walls, wherein the auxiliary air inlet is connected to the heat exchanger auxiliary inlet by an auxiliary duct. The auxiliary duct extends in the longitudinal direction (D) , preferably along and limited by the bottom wall and comprises the mist chamber, the mist generator and intermediate walls extending parallel to the longitudinal direction (D) to create an at least approximately laminar flow before the heat exchanger auxiliary inlet. The creation of a laminar flow before the heat exchanger auxiliary inlet ensures stable operation conditions in the device. This laminar flow also reduces pressure loss in the region of the auxiliary passageways inlets by reducing the interference of the flow with the heat exchanger auxiliary inlets. These intermediate walls can optionally be part of the structure supporting the heat exchanger in the cabinet and contribute to stiffen the whole structure. Preferably the auxiliary duct has a U-shaped cross section formed by lateral walls curved along the longitudinal direction (D) to reduce pressure loss .
Suitable materials for the construction of the device are metal, preferably aluminium or stainless steel, which do not promote the development of bacteria or mold and do not corrode, or plastic material, for example polypropylene. It is also possible to mix both materials: for example, the heat exchanger is made of metal and the ducts are made of plastic material or conversely. Materials are chosen depending on the environment in which the device is operated as well as cost considerations.
In a more preferred embodiment, the inlets and the outlets of the auxiliary passageways are oriented towards the back wall and the front wall, respectively. The inlets and the outlets of the main passageways are oriented towards the front wall and the back wall, respectively. The auxiliary duct extends from the front wall towards the back wall and is deflected near the back wall towards the auxiliary passageways inlets. Such a construction allows a compact arrangement of the heat exchanger and of the auxiliary duct in the cabinet, leading to a minimal size of the device while keeping the same efficiency.
The auxiliary duct is deflected to form a curved region near the back wall to guide the mist into the auxiliary passageways in a manner avoiding a separation of the microscopic water droplets from the air flow in the region before the heat exchanger auxiliary inlet. The mist must be controlled to avoid an absolute humidity above 1.5 g water/ kg dry air above the dew point absolute humidity. Higher absolute humidity causes microscopic water droplets to flow in the auxiliary duct against the back wall and to condensate on the back wall instead of remaining in the mist and flowing into the auxiliary passageways, which leads to a reduction of the cooling performance as well as wasted energy and water.
In an even more preferred embodiment, the heat exchanger has a plurality of spaced heat exchange walls, which are arranged to form a repeating sequence of one auxiliary passageway and one main passageway and which are preferably planar and parallel to each other. The distance between two heat exchange walls is chosen so that a construction with a compact arrangement of the heat exchanger is possible and the surface of the heat exchange walls is maximized, leading to a minimal size of the device and a minimum pressure loss, while improving its efficiency .
Auxiliary passageways can also be separated in a plurality of subsections of auxiliary passageways when an intermediate wall, acting for example as a spacer, is arranged between the walls of the auxiliary passageways and extends over the total length or a portion of the auxiliary passageways. Similarly, main passageways can also be formed in such a way that they are separated in a plurality of subsections of main passageways. It is further possible to arrange the auxiliary air inlet and the main air inlet in the front wall, the main air outlet in the back wall and the auxiliary air outlet in the top wall. Such a configuration is preferred to keep a compact arrangement in the device. However, it can be adapted to the configuration of the pipes to which the device must be connected. There are several techniques available on the market that can be used to generate mist in the present heat transfer device, preferably more efficiently than by using compressed air. Preferably, a mist generator as disclosed in EP 0 670 986 B1 and using only pressurized water to generate the mist is used because of its low energy consumption and its simplicity in controlling water droplets size and the water/ air weight ratio of the mist.
The mist generator comprises a duct portion of the auxiliary duct, which establishes a direction of flow, a guiding apparatus disposed to extend at right-angle to the direction of flow and atomising jets for injecting microscopic water droplets to form the mist. The guiding apparatus comprises a plurality of adjacently disposed and superposed ports with fixed guide blades, the guide blades being so disposed that a twist is imposed upon the auxiliary air flowing through them to form a bundle of turbulent partial flows in the duct portion. In the region of the mist generator the creation of a turbulent flow maximizes the mixture of water droplets with auxiliary air flowing through the guide blades. As already explained above, this turbulent flow is transformed further in the auxiliary duct in a laminar flow by means of intermediate walls. The ports comprise in their respective centres a continuous aperture extending in the direction of flow and in which there is an atomising jet projecting beyond the guide apparatus in the direction of flow and wherein the atomising jet is surrounded preferably by no additional guide means projecting beyond the atomising jet in the direction of flow.
Another technique known as ultrasonic humidifying is preferred to generate mist in case the evaporative heat transfer device is used for air conditioning of smaller spaces, typically houses, and requires little maintenance only. An ultrasonic humidifier uses a ceramic diaphragm vibrating at an ultrasonic frequency to create water droplets that exit the humidifier in the form of mist.
In a preferred embodiment, the auxiliary passageways and the main passageways are arranged to form a cross-counter air flow in the heat exchanger. The cross-counter air flow is preferred because of its higher efficiency. Depending on constructions constraints, it is also possible to use a cross-flow based heat exchanger.
It is also possible to adjoin two or more heat exchangers one after the other in a modular manner to increase the total length of the auxiliary passageways and the main passageways, increasing correspondingly the cooling performance of the device.
Preferably a water drain is arranged in the region near the auxiliary passageways inlets to collect condensed water in the device. This water can be reused by recirculating it to the mist generator to reduce the water requirements of the device, so that the device has a lower environmental impact and lower operational costs.
Preferably, air filters are used to ensure that particles and dust do not clog the evaporative heat transfer device. Moreover, such particles and dust result in increased water condensation in a humidity supersaturated atmosphere and have a negative impact on the cooling performance.
The method to cool air by evaporative heat transfer uses a heat exchanger, with a heat exchanger auxiliary inlet, a heat exchanger auxiliary outlet, a heat exchanger main inlet and a heat exchanger main outlet, having a plurality of auxiliary passageways between the heat exchanger auxiliary inlet and the heat exchanger auxiliary outlet and a plurality of main passageways between the heat exchanger main inlet and the heat exchanger main outlet, wherein the auxiliary passageways and the main passageways have at least a heat exchange wall in common to exchange heat. The heat exchanger auxiliary inlet is connected via a mist chamber to an auxiliary air inlet and the heat exchanger auxiliary outlet is connected to an auxiliary air outlet, the heat exchanger main inlet is connected to a main air inlet and the heat exchanger main outlet is connected to a main air outlet. Auxiliary air is moved into the mist chamber by an auxiliary ventilator, and from the mist chamber into the auxiliary passageways to the auxiliary air outlet, and main air is moved by a main ventilator from the main air inlet into the main passageways to the main air outlet. A mist is formed in the mist chamber by a mist generator connected to the mist chamber.
According to the invention, the mist generator injects microscopic water droplets in the auxiliary air moving to the mist chamber in order to form a mist with an absolute humidity equal or higher than the dew point absolute humidity; and the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways.
In a preferred method the mist generator injects microscopic water droplets until the absolute humidity in the mist chamber is more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity. In a more preferred method, the absolute humidity is equal or higher than the dew point absolute humidity over at least 75% of the total length of the auxiliary passageways, measured from the auxiliary passageways inlets, most preferably until the end of the auxiliary passageways .
In a more preferred method, the mist generator injects microscopic water droplets having a size between 5 to 25 micrometers, preferably 8 to 15 micrometers, more preferably at least approximately 10 micrometers.
In a more preferred method, the mist generator increases the quantity of microscopic water droplets injected when the relative humidity measured by an auxiliary outlet relative humidity measurement device located near or at the heat exchanger auxiliary outlet is below a predetermined level.
In a preferred method, the device may be operated such that air is extracted from a building and fed to the device as auxiliary air which is rejected through the auxiliary air outlet to the outside, while outside air fed as main air is cooled by the device and fed through the main air outlet to the building.
In a preferred method, the device may be operated such that outside air is fed as main air and as auxiliary air to the device. Auxiliary air is rejected through the auxiliary air outlet to the outside, while main air is cooled by the device and fed through the main air outlet to the building.
In a preferred method, the device may be operated such that outside air is fed to the device as auxiliary air which is rejected through the auxiliary air outlet to the outside, while air extracted from a building fed as main air to the device is cooled by the device and fed through the main air outlet to the building. In a preferred method, it is possible to mix air extracted from a building and outside air in a mixing device connected to the auxiliary duct, and to feed this mixture to the device as auxiliary air which is rejected through the auxiliary air outlet to the outside, while outside air fed as main air is cooled by the device and fed through the main air outlet to the building.
In any case, the cooling performance of the device and of the method is higher and bigger temperature differences between main air fed to the device and cooled main air produced by the device can be achieved, when the relative humidity of the auxiliary air fed to the device is low. The lower this relative humidity of the auxiliary air, the more amount of water droplets can be injected in the auxiliary air and therefore the bigger the cooling effect is .
For the optimal operation of the device, the main air dew point must be higher than the auxiliary air dew point.
In some embodiments, the device may be operated in several stages, including the use of further air treatment devices. For example, the main air can be cooled in a first stage using a device according to the present invention and the main air resulting from this first stage can be used as auxiliary air for the next cooling stage involving a further device according to the present invention . For the sake of completeness, it should also be noted that the device according to the present invention can also be used as a heat exchanger when it is not operated as a cooling device, so that it works as a heat recovery system.
The method disclosed in the present application is implemented preferably with a device having the features disclosed in claims 1 to 12.
The invention is illustrated in detail with reference to the appended figures, wherein:
Fig. 1 shows a perspective view of the device equipped with a mist generator as disclosed in
EP 0 670 986 B1 with a vertical cross section along the direction D, Fig. 2 shows a vertical cross section of the device represented in Fig. 1 along the section line II- II extending through an auxiliary passageway,
Fig . 3 shows a schematic representation of the mist generator in a horizontal section along the section line III-III of Fig. 1,
Fig. 4 shows a schematic representation of the portion of the front wall closing the auxiliary duct, viewed from the inside of the device of fig. 1 in the direction opposed to the direction D, Fig. 5 shows a perspective view of the device equipped with an ultrasonic humidifier, and Fig. 6 shows a psychrometric chart corresponding to the process taking place in the device of Fig. 1 with exemplary parameters for temperature and humidity . Fig. 1 is an exemplary illustration of an embodiment for an evaporative heat transfer device 10 according to the present invention, comprising a heat exchanger 20, with a heat exchanger auxiliary inlet 22, a heat exchanger auxiliary outlet 24, a heat exchanger main inlet 26 and a heat exchanger main outlet 28, having a plurality of auxiliary passageways 30 between the heat exchanger auxiliary inlet 22 and the heat exchanger auxiliary outlet 24 and a plurality of main passageways 32 between the heat exchanger main inlet 26 and the heat exchanger main outlet 28.
The heat exchanger 20 is arranged in a cabinet 34 in the form of a rectangular cuboid having a front wall 36 and a back wall 38 spaced in a longitudinal direction D, as well as a bottom wall 40, a top wall 42 and lateral walls 44. In Fig. 1 only one lateral wall is shown. The front wall 36 has an opening forming an auxiliary air inlet 46 and an opening forming a main air inlet 48, the back wall 38 has an opening forming the main air outlet 50, and the top wall 42 has an opening forming an auxiliary air outlet 52. In the present embodiment, separation plates 106 are arranged between the heat exchanger 20 and the top wall 42 to avoid mixing of auxiliary air 12 and main air 16. The auxiliary air inlet 46 is connected to the heat exchanger auxiliary inlet 22 by an auxiliary duct 54 extending in the longitudinal direction (D) , preferably along and limited by the bottom wall 40 and an intermediate wall 41. The auxiliary duct 54 comprises a mist generator 74, a mist chamber 56 and vertical intermediate walls 58 extending parallel to the longitudinal direction (D) to create an at least approximately laminar flow before the heat exchanger auxiliary inlet 22. The heat exchanger auxiliary outlet 24 is connected to the auxiliary air outlet 52, the heat exchanger main inlet 26 is connected to the main air inlet 48 and the heat exchanger main outlet 28 is connected to the main air outlet 50. The auxiliary passageways 30 and the main passageways 32 are completely separated from each other by at least one heat exchange wall, in the present case by a plurality of heat exchange walls 60, in common to exchange heat.
The heat exchanger 20 has a plurality of spaced heat exchange walls 60, which are arranged to form a repeating sequence of one auxiliary passageway 30 and one main passageway 32 and which are preferably planar and parallel to each other. The space between two consecutive heat exchange walls 60 is partially closed by a wall 61 to form a passageway with an inlet and an outlet. The auxiliary passageways inlets 62 and outlets 64 are flow-connected and the main passageways inlets 66 and outlets 68 are flow-connected. In this construction, auxiliary passageways 30 and main passageways 32 have a plurality of heat exchange walls 60 in common to exchange heat.
The inlets 62 of the auxiliary passageways are oriented towards the back wall 38 and the inlets 66 of the main passageways are oriented towards the front wall 36. The outlets 64 of the auxiliary passageways are oriented towards the front wall 36 and the outlets 68 of the main passageways are oriented towards the back wall 38. The auxiliary duct 54 extends from the front wall 36 towards the back wall 38 and is deflected near the back wall 38 towards the auxiliary passageways inlets 62.
The device also comprises a auxiliary ventilator 70 (represented in Fig. 2), arranged at the auxiliary air outlet 52, for moving auxiliary air 12 from the auxiliary air inlet 46 into the mist chamber 56, and from the mist chamber 56 through the auxiliary passageways 30 to the auxiliary air outlet 52, as well as a main ventilator 72 (represented in Fig. 2), arranged at the main air outlet 50, for moving main air 16 from the main air inlet 48 through the main passageways 32 to the main air outlet 50.
Further, Fig. 1 shows the mist generator 74, comprising a duct portion 76 of the auxiliary duct 54 and atomising jets 86, designed to inject pressurized water into the mist chamber 56 to form a mist. The mist generator 74 in Fig. 1 is represented schematically without water feeding system for the sake of figure clarity. An embodiment of the mist generator is described in more details in Fig. 3 and Fig . 4.
Fig. 2 is an illustration of a vertical cross section along the line II-II of the embodiment described in
Fig. 1, wherein the section is taken between two heat exchange walls 60. An auxiliary outlet relative humidity measurement device 78 is located at the heat exchanger auxiliary outlet 24 and the mist generator 74 increases the quantity of injected microscopic water droplets when the relative humidity measured by the auxiliary outlet relative humidity measurement device 78 is below a predetermined level. This predetermined level is set to optimize the cooling performance of the device 10 depending on the external temperature and relative humidity conditions in which the device is operated. In the preferred embodiment shown in Fig. 2, temperature and humidity sensors 80 are also located at the auxiliary air inlet 46, at the heat exchanger auxiliary inlet 22, at the main air inlet 48 and at the main air outlet 50 to monitor the temperature and humidity of auxiliary air 12 and main air 16 in the device 10. An ozone sensor to detect the ozone concentration is also located at the heat exchanger auxiliary outlet 24.
A water drain 69 is arranged in the region close to the auxiliary passageways inlets 62 to collect water condensation taking place in the device 10. This water can be reused to reduce the water requirements of the device. The mist generator 74 as disclosed in EP 0 670 986 B1 is represented in Fig. 3. The mist generator 74 uses only pressurized water to generate the mist and is preferred because of its low energy consumption and its simplicity in controlling water droplets size injected in the mist chamber 56. The mist generator 74 comprises a duct portion
76 of the auxiliary duct 54, which establishes a direction of flow, a guiding apparatus 84 disposed to extend at right-angle to the direction of flow and atomising jets 86 for injecting microscopic water droplets to form the mist. The guiding apparatus 84 extends over the clear cross- section of the duct portion 76 and has webs 104, which delimit a plurality of adjacently disposed ports 108. The ports comprise in their respective centres a continuous aperture 88 extending in the direction of flow and in which there is an atomising jet 86 projecting beyond the guiding apparatus 84 in the direction of flow. The atomising jets 86 are fed via a pipe 102 with pressurized water. The atomising jets 86 are preferably embodied in the form of nozzles, like in the embodiment represented in the zoomed excerpt A of Fig. 3, wherein the pipe 102 feeds pressurized water to the nozzle 86. The pressurized water is projected against a conical piece 110 arranged in front of the pressurized water flow and scattered in the form of microscopic water droplets . The embodiment represented in Fig. 1 to Fig. 4 comprises six adjacently disposed ports 108, each having one nozzle 86. Auxiliary air 12 moving into the mist chamber 56 through the apertures 88 of the ports 108 is split into a bundle of at least approximately parallel partial flows 100 in the duct portion 76. Guide blades 90 are fixed to the ports 108, the guide blades 90 being so disposed that a twist is imposed upon the partial flows 100 flowing through them to form turbulent partial flows 100. In the region of the mist generator 74 the creation of a turbulent flow is important to maximize the mixture of water droplets with auxiliary air 12. As already explained above, this turbulent flow is transformed further in the auxiliary duct 54 in a laminar flow by means of the vertical intermediate walls 58.
The portion of the front wall 36 represented in Fig. 4 closes the duct portion 76 of the auxiliary duct 54 and is viewed from the inside of the device 10 in the direction opposed to the direction D. The guide blades 90 are fixed to the ports 108 and the nozzles 86 are disposed behind the guide blades 90. The auxiliary duct lateral walls 57 are curved in the longitudinal direction (D) to reduce pressure loss.
Fig. 5 shows a perspective view of the device according to Fig. 1 equipped with another type of mist generator 74. The mist generator 74 comprises a duct portion 76 of the auxiliary duct 54 and an ultrasonic humidifier 112 designed to inject water droplets into the mist chamber 56 to form a mist. The mist generator 74 is represented schematically without water feeding system for the sake of figure clarity. Auxiliary air 12 flows through the auxiliary air inlet 46 above the ultrasonic humidifier 112 and collects water droplets that exit the humidifier in the form of mist. In a similar manner as for the device according to Fig. 1, the mist generator 74 is connected to the heat exchanger auxiliary inlet 22 by an auxiliary duct 54 extending in the longitudinal direction (D) to create an at least approximately laminar flow before the heat exchanger auxiliary inlet 22 by means of the vertical intermediate walls 58. Fig. 6 shows a psychrometric chart with a schematic representation of the process taking place in the device, wherein main air and auxiliary air state changes are represented during operation of the device. Each state is represented with a point in the chart. In operation, auxiliary air 12 according to state A, for example outside air, is fed to the device through the auxiliary air inlet 46. Auxiliary air 12 flows through the auxiliary duct 54, which comprises the mist generator 74 and the mist chamber 56, to the heat exchanger auxiliary inlet 22. Microscopic water droplets are injected by the mist generator 74 in the auxiliary air 12 moving to the mist chamber 56 to form a mist with an increasing absolute humidity and correspondingly a lower temperature. The auxiliary air state changes with the absolute humidity increase and the associated temperature decrease along the line from A to B in the psychrometric chart. In this process, the auxiliary air absolute humidity reaches the dew point absolute humidity represented by the state A1. Microscopic water droplets are further injected so that the auxiliary air absolute humidity becomes higher than the dew point absolute humidity, i.e. auxiliary air becomes supersaturated, until the absolute humidity auxiliary air reaches the target absolute humidity, which is represented by state B in the psychrometric chart. The quantity of water in g water/ kg dry air above the dew point absolute humidity in the auxiliary air is represented by the quantity S in the psychrometric chart.
The auxiliary air 12 is moved through the heat exchanger auxiliary inlet 22 to the heat exchanger 20, namely to the plurality of auxiliary passageways 30 between the heat exchanger auxiliary inlet 22 and the heat exchanger auxiliary outlet 24.
Simultaneously, main air 16 according to state E in the psychrometric chart, for example air extracted from a building, is fed to the device through the main air inlet 48, which is connected to the heat exchanger main inlet 26, and moved through the heat exchanger main inlet 26 to the heat exchanger 20, namely to the plurality of main passageways between the heat exchanger main inlet 26 and the heat exchanger main outlet 28.
The auxiliary passageways 30 and the main passageways 32 have at least one heat exchange wall 60 in common to exchange heat.
Mist droplets in the auxiliary air 12 moved into the auxiliary passageways 30 progressively evaporate, possibly after attaching to the heat exchange wall 60, by taking the energy from the heat exchange wall 60 in the form of sensible heat. Supersaturation in the auxiliary passageways 30 becomes lower because of the heat transfer between the main passageways 32 and the auxiliary passageways 30, and the auxiliary air temperature increases. Auxiliary air state changes along the line from B to C in the psychrometric chart.
This energy transfer lowers the temperature of the auxiliary passageways 30 wall progressively along the path of the mist and consequently reduces the temperature of the main passageways 32 and of the main air 16 moved into the main passageways 32. Main air state changes along the line from E to F in the psychrometric chart. As a result, heat is removed from the main air moved into the main passageways .
Finally, auxiliary air 12 is rejected through the auxiliary air outlet 52, for example to the outside, while main air 16 cooled by the device is fed through the main air outlet 50, for example to the building.

Claims

Claims
1. Evaporative heat transfer device (10) for cooling air, comprising a heat exchanger (20), with a heat exchanger auxiliary inlet (22), a heat exchanger auxiliary outlet (24), a heat exchanger main inlet (26) and a heat exchanger main outlet (28), having a plurality of auxiliary passageways (30) between the heat exchanger auxiliary inlet (22) and the heat exchanger auxiliary outlet (24) and a plurality of main passageways (32) between the heat exchanger main inlet (26) and the heat exchanger main outlet (28), wherein the auxiliary passageways (30) and the main passageways (32) are separated from each other by at least one heat exchange wall (60) in common to exchange heat, wherein the heat exchanger auxiliary inlet (22) is connected via a mist chamber (56) to an auxiliary air inlet (46) and the heat exchanger auxiliary outlet (24) is connected to an auxiliary air outlet (52), the heat exchanger main inlet (26) is connected to an main air inlet (48) and the heat exchanger main outlet (28) is connected to an main air outlet (50); an auxiliary ventilator (70) for moving auxiliary air (12) from the auxiliary air inlet (46) into the mist chamber (56), and from the mist chamber (56) through the auxiliary passageways (30) to the auxiliary air outlet (52); a main ventilator (72) for moving main air (16) from the main air inlet (48) through the main passageways (32) to the main air outlet (50); wherein the auxiliary air (12) and the main air (16) are at least almost completely separated from each other in the evaporative heat transfer device (10); a mist generator (74) connected to the mist chamber (56) and designed to form a mist in the mist chamber (56) ;
characterized in that
the mist generator (74) injects microscopic water droplets in the auxiliary air (12) moving to the mist chamber (56) to form in operation a mist with an absolute humidity equal or higher than the dew point absolute humidity; and in that the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways (30), measured from the auxiliary passageways inlets (62) .
2. Device according to claim 1, characterized in that the absolute humidity in the mist chamber is more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity.
3. Device according to claim 1 or 2, characterized in that the absolute humidity is equal or higher than the dew point absolute humidity over at least 75% of the total length of the auxiliary passageways (30), measured from the auxiliary passageways inlets (62) .
4. Device according to any one of claims 1 to 3, characterized in that microscopic water droplets are injected in the auxiliary air (12) in the direction of flow of the auxiliary air (12) .
5. Device according to any one of claims 1 to 4, characterized in that an auxiliary outlet relative humidity measurement device (78) is located near or at the heat exchanger auxiliary outlet (24) and in that the mist generator (74) increases the quantity of injected microscopic water droplets when the relative humidity measured by the auxiliary outlet relative humidity measurement device (78) is below a predetermined level. 6. Device according to any one of claims 1 to 5, characterized in that the mist generator (74) is arranged, as seen in direction of flow of the auxiliary air (12), upstream of the mist chamber (56), and preferably adjoins the mist chamber (56) . 7. Device according to any one of claims 1 to 6, characterized in that the auxiliary passageways (30) and the main passageways (32) are arranged to form a cross-counter air flow in the heat exchanger.
9. Device according to claim 1 to 8, characterized in that the mist generator (74) injects pressurized water in the form of microscopic water droplets.
10. Device according to any one of claims 1 to 9, characterized in that the heat exchanger (20) is arranged in a cabinet (34) in the form of a rectangular cuboid having a front wall (36) and a back wall (38) spaced in a longitudinal direction (D) , as well as a bottom wall (40), a top wall (42) and lateral walls (44), wherein the auxiliary air inlet (46) is connected to the heat exchanger auxiliary inlet (22) by an auxiliary duct (54) extending in the longitudinal direction (D) , preferably along and limited by the bottom wall (40), comprises the mist generator (74), the mist chamber
(56) and intermediate walls (58) extending parallel to the longitudinal direction (D) in order to create an at least approximately laminar flow before the heat exchanger auxiliary inlet (22) .
11. Device according to claim 10, characterized in that the auxiliary passageways inlets (62) and outlets (64) are oriented towards the back wall (38) and the front wall (36) , respectively, and the main passageways inlets (66) and outlets (68) are oriented towards the front wall (36) and the back wall (38), respectively, and in that the auxiliary duct (54) extends from the front wall (36) towards the back wall (38) and is deflected near the back wall (38) towards the auxiliary passageways inlets (62) .
12. Device according to any one of claims 9 to 11, characterized in that the mist generator (74) comprises a duct portion (76) of the auxiliary duct (54), which establishes a direction of flow, a guiding apparatus (84) disposed to extend at right- angle to the direction of flow and atomising jets (86) for injecting the microscopic water droplets in order to form the mist, wherein the guiding apparatus (84) comprises a plurality of adjacently disposed and superposed ports with fixed guide blades (90), the guide blades (90) being so disposed that a twist is imposed upon the auxiliary air flowing through them in order to form a bundle of turbulent partial flows (100) in the duct portion (76) and wherein in their respective centres the ports comprise a continuous aperture (88) extending in the direction of flow and in which there is an atomising jet (86) projecting beyond the guiding apparatus (84) in the direction of flow and wherein the atomising jet (86) is surrounded preferably by no additional guide means projecting beyond the atomising jet (86) in the direction of flow .
13. Method to cool air by evaporative heat transfer, using a heat exchanger (20), with a heat exchanger auxiliary inlet (22), a heat exchanger auxiliary outlet (24), a heat exchanger main inlet (26) and a heat exchanger main outlet (28), having a plurality of auxiliary passageways (30) between the heat exchanger auxiliary inlet (22) and the heat exchanger auxiliary outlet (24) and a plurality of main passageways (32) between the heat exchanger main inlet (26) and the heat exchanger main outlet (28), wherein the auxiliary passageways (30) and the main passageways (32) have at least a heat exchange wall (60) in common to exchange heat, wherein the heat exchanger auxiliary inlet (22) is connected via a mist chamber (56) to an auxiliary air inlet (46) and the heat exchanger auxiliary outlet (24) is connected to an auxiliary air outlet (52), the heat exchanger main inlet (26) is connected to an main air inlet
(48) and the heat exchanger main outlet (28) is connected to a main air outlet (50); wherein auxiliary air (12) is moved into the mist chamber (56) by an auxiliary ventilator (70), and from the mist chamber (56) into the auxiliary passageways (30) to the auxiliary air outlet (52), and main air (16) is moved by a main ventilator (72) from the main air inlet (48) into the main passageways (32) to the main air outlet (50); and wherein a mist generator (74) is connected to the mist chamber (56) and designed to form a mist in the mist chamber (56) ; characterized in that the mist generator (74) injects microscopic water droplets in the auxiliary air (12) in the mist chamber (74) in order to form in operation a mist with an absolute humidity equal or higher than the dew point absolute humidity; and in that the absolute humidity is equal or higher than the dew point absolute humidity over at least 50% of the total length of the auxiliary passageways (30), measured from the auxiliary passageways inlets (62) . 14. Method according to claim 13, characterized in that the mist generator injects microscopic water droplets until the absolute humidity in the mist chamber (56) is more than 0 and up to 1.5 g water/ kg dry air above the dew point absolute humidity. 15. Method according to any of the claims 13 to 14, characterized in that the absolute humidity is equal or higher than the dew point absolute humidity over at least 75% of the total length of the auxiliary passageways (30), measured from the auxiliary passageways inlets (62), most preferably until the end of the auxiliary passageways (30) .
16. Method according to any one of claims 13 to 15, characterized in that the mist generator (74) increases the quantity of microscopic water droplets injected when the relative humidity measured by an auxiliary outlet relative humidity measurement device (78) located near or at the heat exchanger auxiliary outlet (24) is below a predetermined level.
PCT/EP2018/051633 2018-01-24 2018-01-24 Air conditioning system and method based on evaporative heat transfer with air supersaturation WO2019145024A1 (en)

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