GB2064099A

GB2064099A - Dehumidifier; Air Conditioner; Drier

Info

Publication number: GB2064099A
Application number: GB7941312A
Authority: GB
Original assignee: PRESTCOLD Ltd
Current assignee: PRESTCOLD Ltd
Priority date: 1979-11-29
Filing date: 1979-11-29
Publication date: 1981-06-10

Abstract

A conventional refrigeration-type dehumidifier has an air cooler 4 such as an evaporator for cooling the air in order to condense out the required amount of moisture, followed by a heater 5 to heat the airstream to the required temperature. According to the invention, an air-to-air heat exchange system 7 is provided which transfers heat from the airstream before the air cooler to the airstream after the air cooler. In so doing, the air-to-air heat exchange system reduces the load both on the air cooler and on the heater. The dehumidifier may be part of an air conditioning system or part of a drier (e.g. for drying wood or pottery). In the case of a drier, hot air passes over the material to be dried and is then recirculated via the dehumidifier. <IMAGE>

Description

SPECIFICATION Dehumidifier This invention relates to dehumidifiers.

The invention provides a dehumidifier which comprises an air cooler, the surface of which is below the dew point of air in an airstream to condense moisture out of the airstream, and a heat exchange system for transferring heat from the airstream upstream of the air cooler to the airstream downstream of the air cooler.

The heat exchange system reduces the cooling load on the air cooler and at the same time reduces or eliminates the energy input required to heat the dehumidified airstream to the desired temperature.

A heater may be provided downstream of the air cooler if the air leaving the heat exchange system needs to be further heated.

The air cooler may be part of a refrigeration system, either direct where the air cooler is the evaporator of the refrigeration system, or indirect where the air cooler is cooled by a fluid which is in turn coded by the evaporator of a refrigeration system.

The heat exchange system can be a system which uses a single or two phase fluid as the heat transfer medium, a heat wheel or simply a conducting material dividing the airstreams. For example, tubes containing water or brine may be led through a duct carrying the airstream before it impinges on the air cooler and then through a duct carrying the airstream after it has left the air cooler. The water may be pumped around the tubes if desired or a thermosyphon may be employed.

The dehumidifier may be part of an airconditioning system, but could also be part of a drier (for example, for drying wood or pottery): in the case of a drier hot air (for example, at a temperature within the range 400C to 1 200C) passes over the material to be dried, and is then recirculated into the dehumidifier to be dried before being heated and passed again over the material to be dried.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a conventional dehumidifier; Figure 2 is a schematic view of a dehumidifier in accordance with the present invention; Figure 3 is a schematic view of a modified dehumidifier in accordance with the invention; and Figure 4 is a schematic view of a modified arrangement in accordance with the invention.

Referring to Figure 1, a conventional dehumidifier comprises an air cooler which is the evaporator of a refrigeration system. A heater 2 is located downstream of the evaporator in a duct 3.

In a typical arrangement, it will be desired to cool and to reduce the moisture content of incoming air. The evaporator 1 cools the air to a temperature such that the desired amount of moisture condenses during passage through the evaporator. This temperature will generally be below that finally desired, and the heater 2 consequently heats the airstream to the desired temperature.

Referring to Figure 2, the dehumidifier according to the invention comprises an evaporator 4 of a refrigeration system (similar to the evaporator 2 of Figure 1) and a heater 5 in a duct 6 (similar to the heater 2 in duct 3 in Figure 1). In addition, the dehumidifier includes a heat exchange system 7 for transferring heat from the incoming airstream to the air downstream of the evaporator 4. The heat exchange system, whch may comprise water-filled coils, has one element 7a in the duct upstream of the evaporator 4 and another element 7b in the duct downstream of the evaporator 4, the two elements being joined by pipes 8 in which a water pump 9 is located.

The heat exchange system 7 performs the dual function of reducing the load on the evaporator by cooling the incoming air and of reducing the load on the heater 5 by heating the air leaving the evaporator. In consequence the energy required to be absorbed (for example that required to drive a compressor) is reduced and the energy supplied to the heater is reduced, resulting in lower running costs.

The air cooler 4 need not be an evaporator: instead, it may carry cooling coils which are cooled by an evaporator elsewhere in the system; the cooling coils may carry water, brine or other liquid. The heater 5 may be electric, steam or hot water, or it may form part of the condenser of the refrigeration system (the condenser being where the refrigerant condenses and gives up heat). In the latter case, however, the condenser may have too much heat available and may need to be split so that one part only formed the heater; this in turn could make the refrigeration system difficult to control. The air-to-air heat exchange system 7 should be of a type requiring minimum external energy for its effective operation, that is, of a type where the energy supplied to move the heat energy is much less than the amount of heat energy moved.As a further possibility, the temperatures of the heat exchange system, of the air cooler, and of the airstream may be such that moisture may condense out on the first element of the heat exchange system, that is, the first element of the heat exchange system may be below the dew point of the air in the airstream.

Similarly, in Figure 4, the duct 12 is convoluted to allow a single unit 13 air-to-air heat exchange system to be used. The other parts are the same as in Figure 2 and have been accorded like reference numerals.

The dehumidifiers described above may be air conditioning systems or driers, for example, for pottery or wood. In the case of a drier, hot air, for example, at 1 000C is blown across wood and picks up moisture which needs to be removed since the air is recirculated continuously, in order to control its moisture content accurately.

Example As an example of the energy savings to be achieved with the dehumidifier according to the invention compared with a conventional dehumidifier, assume that 5000 cubic feet per minute of air at 750F and relative humidity 50% is to be cooled to 700F and relative humidity 48%.

In order to achieve the latter relative humidity, the air must be cooled to 500F and 95% relative humidity.

Conventional System Evaporator cools air from 750F 50% r.h. to 500F 95% r.h. Calculated evaporator duty~187 000 Btu/hr.

In order to obtain correct air final temperature, the heater must heat the air from 500F to 700F.

Calculated heater duty~109 000 Btu/hr.

System According to Figure 1 Assume the air-to-air heat exchange system 7 is 40% efficient. To achieve the same final temperature and relative humidity as before, that is, 700F r.h. 48%, the evaporator still has to cool the airstream to the same temperature as before to achieve the required precipitation, that is, 500F r.h. 95%. Thus, the element 7a receives air at 750F and the element 7b receives air at 500 F.

The air temperature change through each element will thus be 40%x (75-50)=1 00F. (This assumes that no moisture is removed as the air passes through the element 7a).

The evaporator now only need cool air from 650F r.h. 70% to 500F r.h. 95%.

Calculated evaporator duty~132 000 Btu/hr.

Calculated heat pick up of element 7a in cooling air at 750F r.h. 50% to 650F r.h.

70%#55 000 Btu/hr.

This heat must be rejected in element 7b, and the heating duty of the heater 5 is reduced by this amount.

Thus it will be seen that the evaporator duty has been reduced by about 55 000 Btu/hr and the heater duty by the same amount, giving a substantial reduction in running costs. In addition, in this example, since the evaporator duty has been reduced a smaller air cooler and a smaller compressor can be used, effecting a reduction in equipment costs.

Claims

1. A dehumidifier which comprises an air cooler, the surface of which is below the dew point of air in an airstream to condense moisture out of the airstream, and a heat exchange system for transferring heat from the airstream upstream of the air cooler to the airstream downstream of the air cooler.

2. A dehumidifier as claimed in claim 1, wherein there is provided a heater downstream of the heat exchange system.

3. A dehumidifier as claimed in claim 1 or claim 2, wherein the air cooler is part of a refrigeration system.

4. A dehumidifier as claimed in claim 1 or claim 2, wherein the air cooler is the evaporator of a refrigeration system.

5. A dehumidifier as claimed in any one of claims 1 to 4, wherein the heat exchange system is arranged to transfer the heat via a single or two phase fluid.

6. A dehumidifier substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

7. An air-conditioning system incorpornring a dehumidifier as claimed in any one of claims 1 to 6.

8. A drier incorporating a dehumidifier as claimed in any one of claims 1 to 6.