GB2386569A

GB2386569A - A dehumidifier.

Info

Publication number: GB2386569A
Application number: GB0205337A
Authority: GB
Inventors: Alistair Allan Macfarlane
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-03-07
Filing date: 2002-03-07
Publication date: 2003-09-24
Anticipated expiration: 2022-03-07
Also published as: GB2386569B; GB0205337D0

Abstract

A dehumidifier comprises thermionic heat-pumping devices such as thermonic heat pumps or Peltier junctions 5 mounted between at least one hot sink 8 and at least one cold sink 4 so as to transfer heat from the cold sink or sinks to the hot sink or sinks, air to be dehumidified being passed over the cold sink so as to lower its temperature below its dew point allowing its extracted water to be removed. Heat pumped from the at least one cold sink to the at least one heat sink may be removed from the heat sink by passing a flow of air over the at least one heat sink, the heated air being returned to the area being dehumidified, the movement of the airflow being assisted by a fan 1. The or each cold sink may comprise a tapered rod connected to a platform 14 in thermal contact with cold poles on the thermionic heat-pumping devices water condensed from the air being dehumidified may run down the or each tapering rod until it reaches the tip where a droplet forms. When it falls, this droplet may be conveyed to a water collection tank 12. An alarm may sound, and the operation of the dehumidifier be suspended, if the water collection tank is overfull, or if the pipes connecting it to the dehumidifier become blocked. The dehumidifier may further comprise an ioniser 3 to improve the formation of water droplets within the dehumidifier.

Description

1 2386569

DEHUMIDIFIER

In the present state of the art, conventional domestic and similar small dehumidifiers use a mechanical refrigerator system to create a cold surface to cool humid air to below its dew point, enabling the deposition of the water vapour in the form of droplets. The cold surface is formed by a heat exchanger usually in the form of a coil in which the cold refrigerant circulates. The air to be dehumidified is passed over the coil causing its temperature to fall. The refrigerator system which consists of a compressor, condenser, evaporator and motor drive is both bulky and heavy and can be noisy in operation. As a result, the apparatus is often made in a portable form using wheels or castors and moved from room to room as required. While such devices work reasonably well7 they have these obvious disadvantages, and they also use refrigerant gases, which are an undesirable feature due to atmospheric pollution and disposal difficulties. Future legislation may ban the use of such refrigerants Also unless special precautions are taken at low ambient temperatures, the water extracted from the air may freeze on the cold surface, inhibiting the action of the unit.

There is a need for a small static device, permanently in position, and controlled to maintain, as far as possible, a level of humidity in an area at, or below, a fixed level, even at low ambient temperatures. Suitable rooms would be bathrooms, kitchens, cupboards and other damp areas where it is required to keep the humidity low to prevent mould growth and other damp related problems, and living areas where high humidity causes discomfort. Music rooms are another important area due to the sensitivity of musical instruments to varying humidity. Such high humidity may be due to climatic conditions, leaks and rising damp poor architectural design or caused by activities such as cooking, or in bathrooms or enclosed over heated rooms, lacking in ventilation, often prevalent due to modern living conditions. Extra ventilation to reduce humidity can cause unwanted loss of heat from a living environment with consequent high energy costs.

In our invention the cooling apparatus is formed from electronic elements. In one embodiment these devices use the well known thermoelectric effect of a Peltier Junction.

When an electric current is passed through a Peltier Junction heat is pumped from one electric pole to the other. By connecting a number of single junctions together and mounting them, suitably polarised, between a heat sink and a cold sink on either side, heat is pumped from one sink to the other. Cooling the hot sink, usually by a flow of air over it, enhances the effectiveness of this action, thus ensuring a steady flow of heat from the cold sink. When humid air from the area to be dehumidified is passed over the cold sink it is chilled and the effect may be used to lower the temperature of the air to below the dew point, enabling water vapour to condense out in the form of droplets. This runs off the cooled surfaces and is either collected in a tank, or if convenient, is piped or pumped to an outlet outside the space being controlled. Conveniently, after the vapour is extracted from it, the cold dehumidified air is passed over the hot sinks to cool them. Normally, this air is driven by a fan; the air is in a closed path from inlet to outlet. If required, further ambient air may be bled into the system to enhance the cooling of the hot sink. A simple dehumidifier using these principles is illustrated in Figurel.

As the losses created by passing the current through the junction add to the heat pumped to the hot side it is necessary to extract more heat from the hot side than is pumped from the cold side. The Peltier devices produced at the present time in this embodiment are not very efficient. Current devices have a thermal efficiency of around 6% to 8% of the theoretical maximum, the Carnot cycle. The latest developments in thin film nanostructures using bismuth titanide and antimony telluride promise to deliver much improved performance, but these are some way off from production. New devices called thermionic heat pumps, with a trade name of 'Coolchips', are much more efficient than existing Peltier modules, with performance figures around 80% of the Carnot cycle.

These devices may preferably be used in place of the presently available Peltier junctions to enhance the efficiency of the dehumidifier by improving the rate of extraction of water vapour from the air or by substantially reducing the number of devices and the size and power consumption of the unit. Alternatively, the thermionic heat pumps may be mounted in heat series as shown in figure 2.

In a further embodiment of our invention, the air cooling the hot sinks is drawn over them by an extractor fan, and the air from the cold side which has been passed over the cold sinks in succession is drawn into the hot side air inlet. In this way the passage of the cooling air over the hot sinks is enhanced and more efficient cooling is obtained. There is a net gain of heat energy from this process due to the recovery of the latent heat of condensation of the water vapour.

Deposition of water on the cold sinks is enhanced by the shape of the cold sinks, both to increase turbulence of the air being drawn over them and to assist the formation of droplets that can run from the cooled surface. Figure 6 shows one form of cold sink in which the cold surface is formed of vertical tapering or conical rods of gradually increasing length projecting down from the body of the sink on which the heat pumps are mounted. The lower points of the rods project through holes in a screen, allowing droplets to form at the ends sheltered from the current of the air being treated and thus preventing re-evaporation of the collected water. The run-off action may be further assisted by lightly coating the surface of the heatsink with a non-wetting agent, e.g. a suitable material such as PTFE.

The hot sink may be of conventional design as is used in the cooling of electronic and similar devices. The simplest sinks are formed from extrusions, usually of aluminium alloy, which are finned to give a greater surface area to exchange heat or cold, as the case may be, between the sink and the air being passed over it. An improvement in performance over these simple sinks is obtained by the use of fabricated, bonded fin or micro-forged 'pin grid' heatsinks which achieve a greater surface area in the same volume and these are a preferred option for both the hot and the cold sides of the dehumidifier. In figure 1 the cold sink is a fabrication or die-casting of round tapered rods mounted on a flat body which carries the heat pumping devices. Another implementation might use similarly tapered pins but with a rectangular cross section to further increase turbulence.

The hot sink is fabricated, die-cast or forged with multiple fins or pins so as to obtain the greatest possible surface area in the least volume. As the hot side has to lose more heat than is extracted from the cold side the surface area of the hot side sinks is made considerably larger than that of the cold side. Adding fixed turbulence generating blades just before the inlet to both the cold and hot sinks in order to cause vortices, which increase the contact of the airflow with the hot and cold sinks, may make an improvement in the effectiveness of the heatsink.

A further preferred option is to manufacture the hot sinks from graphite or graphite foam. These are new light weight materials with a specific conductivity four times that of

aluminium thus greatly improving the heat transfer properties of the unit and reducing the overall weight of the device. The improvement in performance obtained is such that it may be possible to dispense with the fan and rely on natural convection to cool the hot sink eliminating a source of sound, with advantage in a domestic situation (See figure 5).

Varying the current supplied to the heat pumps or junctions electronically, so that the device is only activated above a measured set humidity level, controls the dehumidifying action of the device. Figure 3 shows one suitable circuit supplied from normal AC mains voltage using pulse-width modulation to control the current in the cooling devices, the details of this type of control will be well known to those skilled in the art.

If the water extracted is collected in a tank, the control circuit will be warned by means of a float switch or other water level detecting device when the tank is full, and will turn the dehumidifying action off. If the extracted water is piped to a suitable drain or to an outlet outside the area being controlled, a suitable water trap is fitted to maintain the seal on the air being drawn from the cold sinks. A small pump may be used to remove the collected discharge water if run-off due to gravity alone is insufficient or impractical. In addition, if the discharge pipe becomes blocked for any reason (for example, by ice if the water is piped outside the building in frosty conditions), this is sensed by the level detector and the dehumidifying action is turned off.

The temperature of the cold sink is sensed electronically so as to detect the conditions which would cause ice to form on its surface. Ice is a thermal insulator and would reduce or prevent the dehumidifying action taking place if allowed to build up on the cold surfaces. If temperatures less than about +0.5 to +1 degrees Celsius are detected, the current through the heat pumping device is reduced by the pulse-width modulating control circuitry to prevent this and held at a level which enables the devices to operate to optimum capacity even at low ambient temperatures without freezing up. (See Figure 3) Ionising the air being passed over the cold sinks may enhance nucleation of the chilled water vapour. Figure 4 shows a suitable circuit to generate and discharge negative ions into the air stream via a conductive brush whose many discharge points produce a flow of ions into the air stream. The ions provide nuclei that act to cause water droplets to form, enhancing the extraction of water from the air. An additional advantage arising from the use of an ioniser for this purpose is that the dehumidified air returned to the room may carry negatively charged ions known both to improve the quality of the air and reduce the content of dust, mould spores, live viruses and bacteria carried in the air. This principle is used in air ionising devices intended for this purpose. The details of the circuitry to generate a high voltage supply of this nature will be well known to those skilled in the art.

While, in the majority of cases, the use of a low noise level fan to propel the air through the dehumidifier will not be a disadvantage, there may be situations where it is desirable to reduce any noise disturbance to a minimum. In dehumidifiers with fan assisted airflow, if the device would normally be switching quickly between full fan speed and off due to low humidity, this condition may be controlled electronically and the speed changed progressively with a corresponding reduction of perceived noise.

By mounting several modules in air series utilising the new high efficiency thermionic heat pumps as in figure 5 (if necessary in heat series), and with an ioniser if needed, a form of the device is possible which does not require fan assistance. The device works by convected air rising from the heat sinks and cooled air drawn into the cold sinks acting together to cause a continuous convection airflow through the device. Water is removed from the air by condensation on the cold sinks in a similar manner as described previously.

This may be further improved by the use of graphite or high conductivity graphite foam to

form the heat sinks. This form of the dehumidifier will be less effective but is useful when a silent working device is necessary.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a side cross-sectional view of the whole device.

Figure 2 shows a side view of the whole device using the cooling elements in heat series.

Figure 3 shows one embodiment of the control circuitry.

Figure 4 shows one embodiment of the ioniser circuitry.

Figure 5 shows a side view of the silent version without fan.

Figure 6 shows a more detailed drawing of the cold sink in side and plan view.

As shown in figure 1, the moist air (17) from the surrounding environment is drawn through the device by a low-noise computer type modular fan (1), first impinging upon a set of turbulence causing vortex blades (2) and then passing the ioniser brush (3). The vortex blades are simply flat structures fitted across and at an angle to the air stream so that the air starts to spin in small vortices at the trailing edges of the blades. The ioniser brush is connected to the output of the ioniser circuitry (Figure 4) which is mounted on the electronic control circuitry (9). The moist, ionised and turbulent air then passes across the tapered pins (22) of the cold sink (4), which is cooled by the action of the Peltier or thermionic cooling device (5) mounted on the flat surface (21) of the cold sink. These devices are connected to and supplied with the required amount of current from the electronic control circuitry (9) shown in more detail in Figure 3. Figure 6 shows one arrangement of the tapered pins (22) on the cold sink. Figure 2 shows a version of the device where the cooling devices are connected in heat series to cope more effectively with higher ambient temperatures. Operation of this device is otherwise the same as for Figure 1, except for a flat aluminium or copper heat spreading plate (15) interposed between the series connected electronic cooling devices.

The air is cooled below its dew point by contact with the cold sink, and excess water vapour condenses on to the tapered cold sink pins (22) and runs down due to gravity to sharp points (23) projecting through a perforated sheet (10). This perforated sheet has holes that are larger than the pins by a small amount leaving a space so that the water can run down the pin below the sheet, and this reduces the re-evaporation of the condensed water due to the airflow passing over the cold sink. The sharp points (23) on each cold sink pin concentrates the condensed water into droplets too large to remain on the pin.

These eventually build up to a sufficient weight where surface tension cannot support it, drop onto a downward sloping surface and (20) run offinto the water reservoir (12), from where the water may be piped or pumped to a suitable drain. A float combined with a vertical actuator rod (11) mounted in the mouth of the reservoir actuates a mechanical or optoelectronic switch or other electronic detector' connected to or part of the electronic control circuitry (9) to switch off the device should the reservoir become full.

The cooled air ( 18) leaving the left-hand side of the cold sink is then mixed if necessary with additional cooling air from outside the device via a vent (6) and passed through a second set of vortex blades (7) to increase turbulence. This combined turbulent air is then drawn through the hot sink (8) to remove the heat from the hot side of the Peltier or thermionic modules (5). It then passes over the electronic control circuitry (9) to enhance

the cooling of that circuitry, and leaves the device through the extraction fan (1) as warm dry air (19). A humidity sensor (13) connected to the electronic control circuitry is fitted with access to the surrounding or incoming moist air, to measure the level of humidity at which the device controls. An adjustment of the humidity setting may be provided by a potentiometer (37) in the electronic control circuitry (9) as shown in Figure 3. This may be fitted as a physical part of the electronic control circuitry.

A thermistor (14) connected to the control circuitry is mounted on the cold sink and thermally connected to it, to measure its temperature and prevent freezing of condensed water on the cold surface. This is shown in more detail in Figure 3.

Figure 5 shows a silent version of the dehumidifier, where a fan is not used to cause the airflow through the device. In this case the cold sinks (16) are made up of one or more finned extrusions, mounted with the fins vertical. Instead of the fan, the well known chimney effect' caused by cooling the air column engenders a considerably slower flow of air (17) to pass over down the cooling fins (16), becoming cold and more dense in the process. This action is assisted by the same effect in reverse on the hot side of the device, which warms and draws the drier air up over the hot sinks, as it becomes less dense. Such a circulating airflow around the device acts in a similar way to the fan assisted version, in that the cold sinks cause the water vapour in the air to condense on to their surfaces and run down to the bottom and (20) into a reservoir or outlet pipe at the base. The other features of this implementation are identical to that used in Figure 1, namely the electronic control circuit (9), ioniser (3), float switch (11) etc. Figure 3 shows one possible implementation of a suitable control system for the dehumidifier; other possibilities exist and the details of such systems will be well known to those skilled in the art. In our implementation, the AC mains voltage from a suitable mains outlet (24) is full-wave rectified by rectifier diodes (25) and smoothed by storage capacitance (26), giving a DC voltage supply of around 300V across this capacitor. A switching device (27) which may be a bipolar transistor, field effect transistor, IGBT or

other suitable device, is controlled to be on or off by a pulse width modulator (32), such that the voltage applied to the cooling devices (5) and the fan (1) is controlled by the feedback signals (31), (33a) and (33b). When the switching device is on, current builds up through inductor 29 and charges smoothing capacitor 30 until it reaches the maximum required voltage to supply the cooling devices and fan. This voltage is sensed by a feedback circuit, which provides signal (31), and this acts to switch off the switching device (27). When this happens, the current already flowing in the inductor (29) continues to flow through flywheel diode (28). The switching action takes place at a high frequency relative to the mains frequency and the averaging effect of the inductor (29) and capacitor (30) results in a smooth DC voltage being applied to both fan (1) and cooling devices (5).

If the humidity measured by the humidity sensor (13) is higher than desired, the voltage from it will be higher than the voltage set on the humidity adjustment potentiometer (37), and the feedback signal (33a) from the operational amplifier (35) does not cause the voltage applied to the cooling devices to be reduced via the pulse width modulator (32) and switching device (27). However, should the humidity fall due to the drying action of the device or other external cause, the voltage from the humidity detector (13) may become equal to, or less than the voltage set on (37) and then signal (33a) acts to reduce the voltage on the cooling devices (5) and fan (1) by reducing the 'on' time of the switching device. In this way, at lower than the set value of humidity, the cooling of the cold sink and the fan speed are reduced with the result that the dehumidifying action is reduced or prevented.

The thermistor (14) fitted to the cold sink (4) is connected via resistor (38) to a stable voltage supply, and the voltage across the thermistor is adjusted by the choice of values to be equal to the reference voltage (34) at around 0.5 to 1 degree Celsius. Should the temperature on the cold sink drop to this point (i.e. close to freezing) the feedback signal (33b) from operational amplifier (36) acts to reduce the current to the cooling devices to a value just sufficient to maintain this minimum temperature. This action and the action of the humidity control are independent of each other. In this way the device can work effectively at low ambient temperatures and still provide dehumidifying action. Should the float switch (11) be lifted by the condensed water level reaching the neck of the water reservoir or pipe (12), it will cause switch (47) to make and switch off the device to avoid water leakage.

Figure 4 show one possible implementation of the ioniser circuit; other possibilities exist and the details of such systems will be well known to those skilled in the art. In our implementation, the AC mains supply (24) is rectified by rectifier (25) giving a DC voltage across smoothing capacitor (26) of around 300 Volts. A resistor (39) allows capacitor (41) to charge from the 300 Volts via the primary of a high voltage output pulse transformer (42). A switching device (43), which may be a transistor, thyristor, field effect

transistor or IGBT, is driven by an oscillator (40) in order to discharge capacitor (41) abruptly through the primary of transformer (42). Oscillator (40) has a short positive going pulse and a frequency chosen such that capacitor (41) can adequately recharge between discharges. The step up transformer (42) output is negatively rectified by a high voltage diode (44) and the resultant high DC voltage is stored on high voltage capacitor (45). The high negative voltage is fed to the ioniser brush (3) via a high value resistor (46) to limit current. The ioniser brush may be metal, carbon fibre or other conductive material, comprising a large number of fine points which maximises the ion flow into the . surrounumg alr.

Claims

Claims:

1 A domestic dehumidifier consisting of heat pumping devices such as thermionic heat pumps or Peltier junctions mounted between a hot sink and a cold sink or a plurality of these in which an electric current is passed through the devices to pump heat from the cold sinks to the hot sinks and where the air to be dehumidified is passed over the cold sinks so that its temperature is lowered below the dew point and water is extracted from the air thus lowering its humidity.

2 A dehumidifier as in I in which the heat pumped from the cold sinks into the hot sinks and the heat produced by the electronic devices is extracted from the hot sinks by a flow of air over the sinks and returned to the area being dehumidified.

3 A dehumidifier as in 2 in which the flow of air is assisted by a fan.

4 A dehumidifier as above in which the cold dehumidified air is used to cool or assist cooling of the hot sinks before being returned to the area being dehumidified...

5 A dehumidifier as above in which the cold sinks are formed from a number of vertical tapered rods of varying length joined to a platform which is in close heat contact with the cold poles of the electronic heat pumping devices.

6 A dehumidifier as in 5 above, in which the lower ends of the tapered rods project through a screen to shield the ends from the flow of cooling air to enable water drops to form on them without re-evaporation.

7 A dehumidifier as above in which the air to be dehumidified is drawn over the individual cold sinks in series so as to cool the air progressively and ensure that the air temperature attains the required value.

8 A dehumidifier in which either or both the cold and hot sinks are manufactured from graphite or graphite foam.

9 A dehumidifier in which the heat pumping devices are mounted in vertical columns which are cooled by natural convection.

10 A dehumidifier as in any of the above in which the current through the junctions is automatically altered to maintain a humidity in the controlled area at, or below, a set level.

11 A dehumidifier as in any of the above in which the current in the heat pumping device is controlled to prevent water Reezing on the cold sinks to maintain an optimum output.

12 A dehumidifier in which the discharge water is collected in a tank and in which the dehumidifying action is turned off if the water collection tank is full and a warning signal IS glVCn.

13 A dehumidifier in which the discharge water is piped away from the device to a suitable drain or to outside the dehumidified area and in which the dehumidifying action is turned off if the discharge pipe is, for any reason, blocked.

14 In combination with any of the above, an air ionising circuit to provide nucleating ions to improve the formation of water droplets in the dehumidifier.