WO2018073344A1

WO2018073344A1 - Plant and process for production of hot water from humid air

Info

Publication number: WO2018073344A1
Application number: PCT/EP2017/076713
Authority: WO
Inventors: Niels Hansen
Original assignee: Hsl Energy Holding Aps
Priority date: 2016-10-20
Filing date: 2017-10-19
Publication date: 2018-04-26

Abstract

Plant for production of hot water from humid air, comprising a first conduit (10) configured for transporting a first stream of humid air, a heat exchanger (11) provided in the first conduct (10) and configured to heat exchange the first stream of humid air with water to be heated, a second conduit (20) configured for transporting a second stream of humid air having a temperature lower than the temperature of the first stream of humid air, a second conduit evaporator (22) provided in the second conduit (20) and configured so that the second stream of humid air is cooled by evaporation of a liquid refrigerant to a gaseous refrigerant, a first conduit evaporator (12) provided in the first conduit (10) and configured so that the stream of humid air from the heat exchanger (11) is cooled further by evaporation of a liquid refrigerant to a gaseous refrigerant, a compressor (24) configured for compressing the gaseous refrigerant, and a condenser (25) for heating the water from the heat exchanger (11) further and condensing the gaseous refrigerant to a liquid.

Description

Title

Plant and process for production of hot water from humid air Introduction

The present invention relates to a plant and a process for production of hot water from humid air. The humid air typically emanates from a drying plant, in which two or more drying steps are used . The produced hot water may be used for different purposes, such as heated water in a district heating system.

Background art

Hot humid air from drying plants is traditionally discharged to the ambient air, optionally after particles have been removed. However, the hot humid air contains energy, which may be used for various purposes.

A number of prior art documents suggest using the energy in the humid air for reheating the drying air used for drying the wet product. Thus, FR2347087 discloses a method for drying humid air using a number of heat pumps. The humid air from a drying plant is first conveyed to two or more evaporators in which the humid air progressively is cooled and a refrigerant is evaporated . The evaporated refrigerant from each evaporator is compressed and delivered to two or more condensers. Condensed water is discharged from the cooled humid air and the air is subsequently heated in the two or more condensers that receive the evaporated refrigerant. The heated air is returned to the drying plant for drying the wet product.

FR2341342 discloses a plant for drying humid air. Initially, the humid air is cooled in a heat exchanger and subsequently further cooled in an evaporator. The condensate is collected and the cooled air is conveyed to the heat exchanger for heating before it is further heated in a condenser that receives the refrigerant from the evaporator after it has been compressed. The heated air is returned to the drying plant.

WO 2004/046629 discloses a method for drying of a wet product in two or more steps. In a first step the wet product is dried by a hot drying gas. The resulting humid gas is subsequently condensed and water in a closed loop is heated . The heated water is used to heat drying air to be used in a subsequent drying step. Other prior art documents have suggested in addition to reheating the drying air also to heat water in a district heating system. Thus, EP 129631 relates to a plant for production of hot water for district heating systems and cooling or condensing of steam. The steam is used in a fluid bed that dries biofuels, such as peats, and two streams of exhaust steams streams are produced . A top stream from the fluid bed is cleaned in a cyclone and subsequently conveyed to a condenser that produces a condensate and heated water in a district heating system. The water of the district heating system is further heated by a second steam stream from the fluid bed in a heat exchanger. Further documents defining the general state of the art are the following : US 4653198 A, US 201154684 Al, US 2007251115 Al, DE 2630853 Al, and DE 3319348 Al .

WO 82/02939 shows a method for reducing the heat loss in a drying plant for paper production. The paper is heated by a drum, which uses steam for the heating. The humid air from the drying plant is treated by several heat pumps. In a first heating pump, the humid air is initially cooled by an evaporator, which evaporates a refrigerant. The refrigerant is subsequently compressed and condensed in a condenser, which evaporates condensate from the drum. The evaporated condensate or steam is reintroduced in to the drum for heating purposes. The cooled humid air from the first heating pump is subsequently further cooled by a second heat pump, which heats process water. In a final heat pump the humid air from the second heat pump is further cooled and water for room heating is heated.

The present invention provides an improved plant and process for production of hot water by recovering energy from humid air from a production facility having two or more outlets for humid air.

Summary of the invention

The present invention relates to a plant for production of hot water from humid air, comprising

a. a first conduit configured for transporting a first stream of humid air,

b. a heat exchanger provided in the first conduct and configured to heat exchange the first stream of humid air with water to be heated, c. a second conduit configured for transporting a second stream of humid air having a temperature lower than the temperature of the first stream of humid air of the first conduit,

d. a second conduit evaporator provided in the second conduit and configured so that the second stream of humid air is cooled by evaporation of a liquid refrigerant to a gaseous refrigerant, e. a first conduit evaporator provided in the first conduit and configured so that the stream of humid air from the heat exchanger is cooled further by evaporation of a liquid refrigerant to a gas- eous refrigerant,

f. a compressor configured for compressing the gaseous refrigerant, and

g. a condenser for heating the water from the heat exchanger further and condensing the gaseous refrigerant to a liquid.

Humid air that results from an industrial process, such as a drying plant, contains energy which may be recovered . However, often the humid air is delivered to the environment without extracting this energy. The present invention suggests a plant and a process for recovering the energy by heating water, which for instance may be used in a district heating system. Thus, the humid air from the production facility is cooled prior to the discharge to the environment.

Some industrial processes produce several humid air streams as a consequence of drying being performed in two or more stages. A product to be dried may, as an example, initially be dried at a high temperature and in a subsequent step be dried at a lower temperature to prevent heat damage. Such drying process will produce two or more streams of humid air having different temperatures, which are applied in the present invention.

According to the invention, a first stream of humid air having the highest temperature is transported in a conduit to the heat exchanger, which transfers heat to water to be heated and cools the humid air stream. The water that has been heated in the heat exchanger is then transferred to further heating in a condenser, which recovers the energy from a second stream of humid air having a lower temperature and from the first stream of humid that has been cooled in the heat exchanger.

The two-step heating process for heating of the water increases the overall efficiency compared to heating in a single step. The extraction of heat from the humid air in the first conduit is also more efficient when performed in two or more steps, i.e. in a first step the humid air is cooled in a heat exchanger and in a second step the humid air is further cooled in an evapora- tor.

The evaporators provided in the first and second conduit may apply the same compressor and condenser, i.e. they are working in tandem or in parallel. In this way the complexity of the plant is reduced and the cost for erection is also lower. In a certain embodiment of the invention it is, howev- er, suitable that separate compressors and condensers are configured for the second conduit evaporator and the first conduit evaporator, respectively. Separate compressors and condensers for each of the evaporators allow for heating of the water to be heated in three (or more) steps. Thus, after the water in a first step initially has been heated in the first conduit heat ex- changer it is conveyed to the condenser configured for the second conduit evaporator for heating in a second step. The water leaving the second conduit condenser is in a third step heated in the first conduit condenser. Due to Car- not considerations, a three step heating of the water is more energy efficient than heating in single step or in two steps.

It is generally suitable to reuse the refrigerant and liquid and gaseous connections are therefore usually present, which allows the refrigerant to circulate in a closed system.

The hot water that has been heated by the plant according to the invention may be used for any suitable application, including chemical process- es, household water, and heating purposes, etc. In a preferred aspect of the invention the water to be heated in step b. is return water from a district heating system and the heated water from g. is supplied to the district heating system. The heating needed for the return water of a district heating system suits well with the temperature of the streams of the humid air for opti- mizing the transfer of energy. The heating range for the water in the district heating system is suitable for the recovery of the energy in the humid air. Typically, the return water in the district heating system is heated from 25 - 50°C to 55 - 80°C. The water in the district heating system is subsequently circulated in a closed loop to recipients in need of heating, e.g . households and industries having rooms or enclosures to be heated. A number of refrigerant types are possible, such as butane, isobu- tane, pentane, isopentane, diethyl ether, methyl formate, methylamine, ammonia, carbon dioxide, chlorinated and/or fluorinated alkanes or alkenes. Ammonia is generally preferred due to its relatively low toxicity and eco- friendliness.

The compressor used in the present invention may be of the reciprocating, rotary screw or rotary centrifugal type. Generally, the compressor is a high-pressure reciprocating compressor. Reciprocating compressors are positive displacement machines in which the compressing and displacing element is a piston having a reciprocating motion within a cylinder. Arrangements may be of single- or dual-acting design. In the dual-acting design, compression occurs on both sides of the piston during both the advancing and retreating stroke. Some dual-acting cylinders in high-pressure applications will have a piston rod on both sides of the piston to provide equal surface area and balance loads. Tandem cylinder arrangements help minimize dynamic loads by locating cylinders in pairs, connected to a common crankshaft, so that the movements of the pistons oppose each other. The compressor may be driven by a conventional driver such as an electrical motor, a gas engine, a diesel engine, a turbine etc.

The evaporator of the invention may include a throttle that reduces the pressure of the liquid refrigerant at the entrance of the evaporator. A number of different types of evaporators are available including natural or forced circulation evaporators, shell-and-plate evaporators, falling film evaporators, rising film evaporators, climbing or falling-film plate evaporators, mul- ti-effect evaporators and agitated thin film evaporators. While any of these evaporators are potential candidates for the evaporator used in the present invention it is generally desired to use an evaporator of the shell-and-plate type.

As the humid air may exceed the saturation point during cooling thereof, the evaporator is suitably provided with a liquid separator for the separation of condensed humid air from the humid air. In a preferred embodiment, the evaporator of the present invention is a shell-and-plate evaporator with built-in liquid separator.

The condenser is an apparatus in which the refrigerant is trans- formed from its gaseous form to the liquid form. The heat generated by this process is transferred to the water to be heated . Examples of suitable condenser are shell-and-coil condensers, tube-in-tube condensers, and shell- and-tube condenser. Usually, the water to be heated is present in a tube or coil and the gaseous refrigerant is present on the outside. When heat is transferred from the gaseous refrigerant to the water, the condensation takes place so that a liquid refrigerant is produced.

The evaporator, compressor, and the condenser may be separate parts of the plant but is preferably parts of the same heat pump unit. Suitable examples of useful heat pump units are commercially available from Sa- broe (HeatPAC-recip and HeatPAC-screw), Vilter Manufacturing, and Neatpump. Presently, HeatPAC-recip from Sabroe is preferred .

The present plant is suitable for production facilities in which a first and the second stream of humid air are provided. The first and the second stream of humid air may be provided by a drying plant having at least two stages with separate outlets of humid air. When drying a product it is often suitable to dry at different temperatures depending on the content of water in the product to be dried. Thus, when the product contains the highest amount of water it is generally preferred to dry at a higher temperature. When the product is partially dried it is subjected to a lower drying temperature. The lower drying temperature in later drying stages may be used to prevent heat damages.

The drying of the product may occur in any suitable apparatus, such as a spray drier, a fluid bed, a belt drier, a drum drier etc. Furthermore, the fluid bed and the belt drier may be divided in several stages, each providing a separate stream of humid air for the present plant. The produced humid air is in many industrial facilities discharged to the surroundings. With the present invention the humid air is used to produce energy, e.g. for heating rooms in houses or industry. The humid air may be produced in a process where a product is dried. The dried product may be potato starch, grains, minerals, phosphates, peat, wood chips, timber, cellulose pulp, hay, veneer, foodstuff, sludge, bark, enzymes, pharmaceuticals, instant coffee, etc.

In industrial facilities exhaust gases occurs from various sources, including internal and external combustion engines, and boilers. According to a further development of the present invention a conduit configured for trans- porting exhaust gases from an internal or external combustion apparatus, is used wherein the conduit is provided with a heat exchanger configured to heat exchange the exhaust gases with water to be heated. The presence of a heat exchanger in the exhaust gas pipe further increases the recovery of energy from an industrial facility.

The water heated in the heat exchanger provided in the exhaust gas stream may be further heated in a condenser. Optionally, the water stream may be mixed with one or more other stream(s) of heated water prior to the introduction into a condenser. In a preferred embodiment, the condenser is configured for the first conduit evaporator. The recovery of heat in more than a single step increases the over-all efficiency.

Plants according to the present invention may be combined in a system. The advantage of combining two or more plants is that the plants can be spaced apart so that the energy in the humid air may be recovered in a larger geographical area. Each of the plant of the system is in fluid communi- cation with at least one other plant through a pipeline that transports heated water. According to a suitable embodiment, two or more plants are combined in a system according to which the heated water of a first plant exiting at the outlet is supplied to a condenser in a second plant for further heating . Thus, the water can be heated in more steps and equipment may be concentrated in certain areas.

The present invention also relates to a process for the production of hot water from a humid air, comprising the steps of:

a. transporting a first stream of humid air in a first conduit, b. heat exchanging the first stream of humid air of the first conduit with water to be heated,

c. transporting a second stream of humid air in a second conduit, said second stream of humid air having a temperature lower than the temperature of the first stream of humid air of the first conduit,

d. evaporating a liquid refrigerant to a gaseous refrigerant by cooling the second stream of humid air,

e. evaporating a liquid refrigerant to a gaseous refrigerant by cooling the first stream of humid air further,

f. compressing the gaseous refrigerant, and

g. condensing the compressed gaseous refrigerant to a liquid re- frigerant by heating the water from step b. further.

In a suitable embodiment, the gaseous refrigerant of steps are d . and e. are separately compressed according to step f. and condensed according to step g.

In a suitable embodiment, the liquid and gaseous connections are present, which allows the refrigerant to circulate in a closed system.

In a suitable embodiment, the water to be heated in b. is return water from a district heating system and the heated water from g . is supplied to the district heating system.

In a suitable embodiment, the district heating system supplies hot water for room heating.

In a suitable embodiment, the refrigerant is ammonia.

In a suitable embodiment, the relative humidity of the first and/or the second stream of humid air is 90% or more before the cooling in step d. or e.

In a suitable embodiment, the condensate produced during the cooling in steps d. or e. of the stream of first or second humid air is separated from the humid air stream and discarded.

In a suitable embodiment, the first stream of humid air results from a first stage and the second stream of humid air results from a second stage of a drying plant. In a suitable embodiment, the drying plant dries a wet product in two or more stages by conveying a hot air stream through the wet product, thereby producing at least a first and a second stream of humid air having different temperatures.

In a suitable embodiment, the stream of exhaust gas produced in a secondary process of the drying plant is heat exchanged with water to be heated .

In a suitable embodiment, the heated water heat exchanged with the exhaust gas is further heated in a condenser.

The invention also relates to the use of the plant as defined above for heating water in a district heating system.

While the present invention has been explained using water at the medium to be heated, a person skilled in the art will understand that other liquids may be used if desired or considered suitable. Examples of liquids which may be used as the medium to be heated includes mineral, marine or vegetable oils; refrigerants like chlorofluorocarbons, chlorofluoroolefin, hy- drochlorofluorocarbons, hydrochlorofluoroolefins, hydrofluorocarbons, hydro- fluoroolefin, hydrochlorocarbons, hydrochloroolefins, hydrocarbons (butane, isobutane, pentane, isopentane, cyclo-pentane, diethylether, methyl formate, propane, ethane, dimethylether), hydroolefins (propene), perfluorocarbons, perfluoroolefins, perchlorocarbons, perchloroolefins, and halon/haloalkanes; methylamine; ethylamine; and ammonia.

Summary of the drawings

The present invention will now be described in greater detail based on preferred embodiments with reference to the drawings on which :

Figure 1 discloses a depiction of an embodiment of the invention. Figure 2 shows the embodiment of figure 1 further provided with a heat exchanger in a conduit for exhaust gas.

Figure 3 discloses an embodiment using separate compressors and condensers for separate evaporators provided in the first and the second conduit.

Figure 4 shows the embodiment of figure 3 further provided with a heat exchanger in a conduit for exhaust gas.

Figure 5 depicts a system of two plants, in which the output heated water from a first plant is further heated in the second plant.

Figure 6 shows the embodiment of figure 5 further provided with a third and fourth conduit for humid air to assist the second plant. Detailed description

Figure 1 discloses a depiction of an embodiment of the invention according to which water to be heated introduced at the inlet "In" is heated in several steps before it exits as heated or hot water at "Out". In each of the heating steps, energy is extracted from at least two streams of humid air.

According to Figure 1, a stream of humid air is provided in first conduit 10 and a heat exchanger 11 is provided in the stream of humid air such that heat exchange with the water to be heated is ensured. By the heat exchange the humid air will be cooled and the water will be heated. Depending on the relative humidity of the stream of humid air, a condensate may or may not be produced by the cooling. In the event, a condensate is produced it is separated out from the stream of humid air and discarded or used in another part of the process.

In a second conduit 20 a stream of humid air is provided, wherein the temperature of the second stream of humid air is lower than the temperature of the first stream of humid air. Typically, the difference in temperature is above 5°C to ensure effective heating of the water. In the second conduit a second conduit evaporator 22 is provided . The evaporator contains a liquid refrigerant 23, which may evaporate to a gaseous refrigerant when subjected to the heating of the stream of humid air. The evaporation extracts energy from the stream of humid air, which in consequence is cooled. The gaseous refrigerant is subsequently compressed in compressor 24. The compressor 24 is driven by a suitable driver M, such as an electrical motor or a gas engine.

In the first conduit 10 is also provided an evaporator 12, which is connected to the same refrigerant system as the second conduit evaporator, i.e. first and second conduit evaporators are connected in parallel using a common compressor and condenser unit. Evaporator 12 contains a liquid refrigerant 13, which may evaporate to a gaseous refrigerant when subjected to the heating of the stream of humid air. The evaporation extracts energy from the stream of humid air, which in consequence is cooled . The evapo- rated refrigerant is mixed with the vapours from the second conduit evaporator and condensed in the condenser 25. The condensing process is an exothermic process, which develops energy. The energy is delivered to the water from the heat exchanger 11, thereby providing for further heating of the water. The heated water leaves the plant at "Out" and may be used for district heating, i.e. heating of rooms in households and industry.

In an exemplary embodiment of the invention depicted in Figure 1, the plant receives a stream of humid air in the first conduit having a temperature of 43°C and a relative humidity of 95-100%. The water being heated is entered into the heat exchanger 11 at a temperature of 35°C. By heat ex- changing the water is heated to around 40°C and the first stream of humid air is cooled to 39°C.

The second conduit receives a stream of humid air having a temperature of 35°C from a stage of a drying plant. The second conduit stream of humid air is directed to an evaporator 22, in which the humid air is cooled to 30°C. The evaporator 22 works in tandem with the first conduit evaporator 12, which cools the humid air from 39°C to 30°C. In the evaporators the refrigerant, typically ammonia, is evaporated to a gaseous refrigerant. The gaseous refrigerant is compressed and subsequent condensed. By the condensing the temperature of the water to be heated is increased from around 40°C to around 70°C.

Figure 2 further discloses the recovery of energy from an exhaust gas. In addition to the embodiment shown in figure 1, the embodiment shown in figure 2 has been supplemented with an exhaust gas conduit or duct 30. The exhaust gas is conveyed through a heat exchanger 31 for cool- ing thereof. Water to be heated is introduced at "In" and is subsequently mixed with the water heated in the first conduit heat exchanger 11 before introduction into the condenser 25.

In an exemplary embodiment of the invention depicted in figure 2, the plant receives an exhaust gas with a temperature of 70°C in conduit 30. The exhaust gas in the conduit 30 is cooled in the heat exchanger to about 37°C. The water to be heated is introduced at a temperature of about 35°C and is heated in the exhaust gas heat exchanger 31 to about 40°C. The stream of water from the exhaust gas heat exchanger 31 and the first conduit heat exchanger is mixed before it enters the condenser 25. In the condenser 25, the water is further heated to a temperature of 70°C.

Figure 3 shows a depiction of an embodiment of the invention according to which water to be heated is introduced at the inlet "In" is heated in several steps before it exits as heated water at "Out". In each of the heating steps, energy is extracted from at least two streams of humid air.

According to Figure 3, a stream of humid air is provided in first conduit 10 and a heat exchanger 11 is provided in the stream of humid air such that heat exchange with the water to be heated is ensured. By the heat exchange the humid air will be cooled and the water will be heated. Depending on the relative humidity of the stream of humid air, a condensate may or may not be produced by the cooling. In the event, a condensate is produced it is separated out from the stream of humid air and discarded or used in another part of the process.

The compressed gaseous refrigerant is conveyed to the second conduit condenser 25, in which the refrigerant is condensed to a liquid refriger- ant. The liquid refrigerant is subsequently returned to second conduit evaporator, thereby forming a closest circuit. The condensing process is an exothermic process, which develops energy. The energy is delivered to the water from the heat exchanger 11 for further heating.

In the first conduit 10 is also provided an evaporator 12 for further heating of the water. The evaporator 12 contains a liquid refrigerant 13, which may evaporate to a gaseous refrigerant when subjected to the heating of the stream of humid air. The evaporation extracts energy from the stream of humid air, which in consequence is cooled. The gaseous refrigerant is subsequently compressed in compressor 14. The compressor 14 is driven by a suitable driver M, such as an electrical motor or a gas engine.

The compressed gaseous refrigerant is conveyed to the first conduit condenser 15, in which the refrigerant is condensed to a liquid refrigerant. The liquid refrigerant is subsequently returned to first conduit evaporator, thereby forming a closest circuit. The condensing process is an exothermic process, which develops energy. The energy is delivered to the water from the heat exchanger 11, thereby providing for further heating of the water. The heated water leaves the plant at "Out" and may be used for district heating, i.e. heating of rooms in households and industry.

In an exemplary embodiment of the invention depicted in Figure 1, the plant receives a stream of humid air in the first conduit having a temperature of 43°C and a relative humidity of 95-100%. The water being heated is entered into the heat exchanger 11 at a temperature of 35°C. By heat exchanging the water is heated to around 40°C and the first stream of humid air is cooled to 39°C.

The second conduit receives a stream of humid air having a tempera- ture of 35°C from a stage of a drying plant. The second conduit stream of humid air is directed to an evaporator 22, in which the humid air is cooled to 30°C. In the evaporator the refrigerant, typically ammonia, is evaporated to a gaseous refrigerant. The gaseous refrigerant is compressed and subsequent condensed. By the condensing the temperature of the water to be heated is increased from around 40°C to around 60°C.

In the evaporator provided in the first conduit, the water is further heated to around 70°C. The heated water may be circulated in a district heating system.

Fig . 4 discloses a further development of the embodiment of figure 3.

In addition to the elements in the embodiment shown in figure 3, the embodiment shown in figure 4 has been supplemented with an exhaust gas conduit or duct 30. The exhaust gas is conveyed through a heat exchanger 31 for cooling thereof. Water to be heated is introduced at "In" and is after the passing in the heat exchanger 31 mixed with the water heated in the second conduit condenser 25. The combined streams are subsequently introduced into the first conduit condenser 15 for heating thereof.

In an exemplary embodiment of the invention depicted in figure 4, the plant receives an exhaust gas with a temperature of 70°C in conduit 30. The exhaust gas in the conduit 30 is cooled in the heat exchanger to about 40°C. The water to be heated is introduced at a temperature of about 35°C and is heated in the exhaust gas heat exchanger 31 to about 60°C. The stream of water from the second conduit condenser 25 having a temperature of 60 °C as indicated above for the embodiment of Fig . 3 is mixed with the heated stream of water from the exhaust gas heat exchanger 31 before it enters the first conduit condenser 15. In the first conduit condenser 15 the combined streams are heated to temperature of around 70°C.

The embodiment of Fig . 5 may be regarded as a combination of the embodiments shown in Fig. 2 and Fig . 3, in which the outlet "out" from fig. 3 is mixed with the stream of heated water from the second conduit condenser before entry in to the first conduit condenser and final heating therein. The combination of the embodiments allows the elements of the plant to be distanced from each other, only connected with a pipe. In this way, waste energy productions in two or more industry plants may be used in the production of heated water for a common district heating system. In an exemplary embodiment of the invention depicted in figure 5, a first part of the plant receives a stream of humid air in the first conduit 10 having a temperature of 43°C and a relative humidity of 95-100%. The water being heated is entered into the heat exchanger 11 at a temperature of 35°C. By heat exchanging the water is heated to around 40°C and the first stream of humid air is cooled to 39°C.

The second conduit 20 receives a stream of humid air having a temperature of 35°C from a stage of a drying plant. The second conduit stream of humid air is directed to an evaporator 22, in which the humid air is cooled to 32°C. The evaporator 22 works in tandem with the first conduit evaporator 12, which cools the humid air from 39°C to 30°C. In the evaporators the refrigerant, typically ammonia, is evaporated to a gaseous refrigerant. The gaseous refrigerant is compressed and subsequent condensed. By the condensing the temperature of the water to be heated is increased from around 40°C to around 59°C.

The plant receives an exhaust gas with a temperature of 70°C in conduit 30. The exhaust gas in the conduit 30 is cooled in the heat exchanger to about 37°C. The water to be heated is introduced at a temperature of about 35°C and is heated in the exhaust gas heat exchanger 31 to about 54°C. The stream of water from the exhaust gas heat exchanger 31 and the stream exiting the condenser 25 are mixed to a common temperature of 58°C and transferred in a pipeline to a second part of the plant.

In the second part of the plant a stream of humid air having a temperature of 57°C is received in the first conduit 10'. The water being heated is entered into the heat exchanger 11' at a temperature of 35°C. By heat exchanging the water is heated to around 43°C and the first stream of humid air is cooled to 49°C.

The second conduit 20' receives a stream of humid air having a temperature of 43°C from a stage of a drying plant. The second conduit stream of humid air is directed to an evaporator 22', in which the humid air is cooled to 32°C. In the evaporator the refrigerant, typically ammonia, is evaporated to a gaseous refrigerant. The gaseous refrigerant is compressed and subsequent condensed in second conduit condenser 25'. By the condensing the temperature of the water to be heated is increased from around 43°C to around 58°C. The stream of heated water from the first part of the plant having a temperature of 58°C is mixed with the heated stream leaving the second conduit condenser 25' to obtain a common temperature of 58°C. In the first conduit condenser 15' the combined streams are heated to temperature of around 70°C and the humid air in the first conduit is cooled from 49°C to around 41°C.

Figure 6 shows a further development of the embodiment shown in figure 5. In the further development a third conduit 40 and a fourth conduit 50 for humid air is present, wherein the temperature of the humid air is high- er in the third conduit than in the fourth conduit. The third conduit is provided with a heat exchanger 41, which cools the humid air and heats water to be heated . The heated water is mixed with the water from the first conduit heat exchanger 11' before the mixed stream is introduced into the second conduit condenser 25'. An evaporator 42 is provided in the third conduit so that the humid air from the heat exchanger 41 is further cooled. The evaporator 42 works in tandem with the first conduit evaporator 12' and supplies the compressor 14' with vapour. The fourth conduit 50 receiving a humid air is provided with an evaporator 52, which works in tandem with the evaporator 22' of in the second conduit.

In an exemplary embodiment of the invention depicted in figure 6, the third conduit 40 receives a humid air having a temperature of 57°C. The water to be heated enters at "In" and is heated from 35°C to around 43°C. In consequence, the humid air is cooled from 57°C to 49°C. In a subsequent cooling process, the humid air is further cooled to 41°C in evaporator 42. The vapour from the evaporation is transferred to the compressor 24', optionally after being mixed with the vapour from the first conduit evaporator 12'. The fourth conduit receives a humid air having a temperature of 40°C. In the evaporator 52, the refrigerant is evaporated and in consequence the humid air is cooled to 32°C. Reference numbers on the drawings :

10 First conduit

1 1 Heat Exchanger

12 First conduit evaporator

13 First conduit refrigerant

14 First conduit compressor

15 First conduit condenser

20 Second conduit

22 Second conduit evaporator

23 Second conduit refrigerant

24 Second conduit compressor

25 Second conduit condenser

30 Exhaust gas conduit

31 Exhaust gas conduit heat exchanger

40 Third conduit

41 Third conduit heat exchanger

42 Third conduit evaporator

50 Fourth conduit

52 Fourth conduit evaporator

Out Outlet for heated water

In Inlet for water to be heated

M Driver

Claims

P A T E N T C L A I M S

A plant for production of hot water from humid air, comprising

a. a first conduit configured for transporting a first stream of humid air,

b. a heat exchanger provided in the first conduct and configured to heat exchange the first stream of humid air with water to be heated,

c. a second conduit configured for transporting a second stream of humid air having a temperature lower than the temperature of the first stream of humid air of the first conduit,

d. a second conduit evaporator provided in the second conduit and configured so that the second stream of humid air is cooled by evaporation of a liquid refrigerant to a gaseous refrigerant, e. a first conduit evaporator provided in the first conduit and configured so that the stream of humid air from the heat exchanger is cooled further by evaporation of a liquid refrigerant to a gaseous refrigerant,

f. a compressor configured for compressing the gaseous refrigerant, and

The plant according to claim 1, wherein separate compressors and condensers are configured for the second conduit evaporator and the first conduit evaporator, respectively.

The plant according to claim 1 or 2, wherein liquid and gaseous connections are present, which allows the refrigerant to circulate in a closed system.

The plant according to anyone of the claims 1 to 3, wherein the water to be heated in step b. is return water from a district heating system and the heated water from step g. is supplied to the district heating system.

The plant according to anyone of the claims 1 to 4, wherein the district heating system supplies hot water for room heating.

6. The plant according to anyone of the claims 1 to 5, wherein the refrigerant is ammonia.

7. The plant according to anyone of the claims 1 to 6, wherein the compressor is a high-pressure reciprocating compressor.

8. The plant according to anyone of the claims 1 to 7, wherein the evaporator is a shell-and-plate evaporator.

9. The plant according to anyone of the claims 1 to 8, wherein the evaporator is provided with a liquid separator for the separation of condensed humid air from the humid air.

10. The plant according to anyone of the claims 1 to 9, wherein evaporator, compressor, and the condenser are parts of the same heat pump unit.

11. The plant according to anyone of the claims 1 to 10, wherein the first and the second stream of humid air are provided by a drying plant having at least two stages with separate outlets of humid air.

12. The plant according to anyone of the claims 1 to 11, further comprising a conduit configured for transporting exhaust gases from an internal or external combustion apparatus, wherein the conduit is provided with a heat exchanger configured to heat exchange the exhaust gases with water to be heated.

13. The plant according to claim 12, wherein heated water exiting the heat exchanger provided in the exhaust gas duct is further heated in a condenser.

14. The plant according to claim 13, wherein the condenser is configured for the first conduit evaporator.

15. The plant according to anyone of the claims 1 to 14, wherein two or more plants are combined in a system according to which the heated water of a first plant exiting at the outlet is supplied to a condenser in a second plant for further heating.

16. A process for the production of hot water from a humid air, comprising the steps of:

d. evaporating a liquid refrigerant to a gaseous refrigerant by cool- ing the second stream of humid air,

f. compressing the gaseous refrigerant,

17. The process according to claim 16, wherein the gaseous refrigerant of steps are d . and e. are separately compressed according to step f. and condensed according to step g .

18. The process according to claims 16 or 17, wherein liquid and gaseous connections are present, which allows the refrigerant to circulate in a closed system.

19. The process according to anyone of the claims 16 to 18, wherein the water to be heated in b. is return water from a district heating system and the heated water from g . is supplied to the district heating sys- tern.

20. The process according to anyone of the claims 16 to 19, wherein the district heating system supplies hot water for room heating .

21. The process according to anyone of the claims 16 to 20, wherein the refrigerant is ammonia.

22. The process according to anyone of the claims 16 to 21, wherein the relative humidity of the first and/or the second stream of humid air is 90% or more before the cooling in step d. or e.

23. The process according to anyone of the claims 16 to 22, wherein condensate produced during the cooling in steps d. or e. of the stream of first or second humid air is separated from the humid air stream and discarded .

24. The process according to anyone of the claims 16 to 23, wherein the first stream of humid air results from a first stage and the second stream of humid air results from a second stage of a drying plant. 25. The process according to claim 24, wherein the drying plant dries a wet product in two or more stages by conveying a hot air stream through the wet product, thereby producing at least a first and a second stream of humid air having different temperatures.

26. The process according to anyone of the claims 16 to 25, wherein the stream of exhaust gas produced in a secondary process of the drying plant is heat exchanged with water to be heated.

27. The process according to claim 26, wherein the heated water heat exchanged with the exhaust gas is further heated in a condenser.

28. Use of the plant according to anyone of the claim 1 to 15 for heating water in a district heating system.