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Title:
AIR COOLER WITH FINNED TUBE HEAT EXCHANGER
Document Type and Number:
WIPO Patent Application WO/1998/037372
Kind Code:
A1
Abstract:
A first aspect of the invention provides an evaporative cooling pad with a honeycomb-like structure which provides multiple gas flow channels extending between a pair of opposite upright side faces; the pad having two spaced vertical sections (20, 21) enclosing an air-mixing chamber (22) between them so that air cooled by passage through one section is mixed in the chamber (22) before it enters the other section of the pad. A second aspect of the invention provides an apparatus for providing an enclosed space with a continuous supply of cooled air, including: a casing (1) having a first air inlet (2) through which fresh air is drawn into the casing; a first air outlet (5) for discharging cooled air from the casing into the enclosed space; a second air inlet (4) through which stale air enters the casing from the enclosed space; a second air outlet (3) through which stale air is expelled from the casing; a heat exchanger (7) having substantially counterflow primary and secondary gas flow circuits which have relatively good heat transfer properties existing between them and so connected that the primary circuit receives fresh air (6) to be cooled from the first air inlet (2) and cools the fresh air by transferring some of its heat to the stale air (8) flowing through the secondary circuit of the heat exchanger towards the stale air outlet (3); fan means (10, 26) for producing the required air flows through the casing; an evaporative, air-cooling pad (20) through which stale air from the second air inlet (4) flows and which is cooled by the evaporation of water from the pad (20) into the stale air; a reservoir (12) for holding water cooled by passing through the pad; a finned tube heat exchanger (16) positioned in an air flow path leading to the first air outlet (5); a water circuit which includes the reservoir (12), the pad (20) and the finned tube heat exchanger (16), and a pump (13) operating to circulate water from the reservoir (12) around the water circuit and back to the reservoir.

Inventors:
Urch, John Francis (5 Marlo Road, Cronulla, NSW 2230, AU)
Application Number:
PCT/AU1998/000108
Publication Date:
August 27, 1998
Filing Date:
February 20, 1998
Export Citation:
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Assignee:
ECO AIR LIMITED (56 Bay Road, Taren Point, NSW 2229, AU)
Urch, John Francis (5 Marlo Road, Cronulla, NSW 2230, AU)
International Classes:
F24F5/00; F28C3/08; (IPC1-7): F28C3/08; F24F3/06
Domestic Patent References:
WO1995033960A1
WO1993018360A1
WO1995015203A1
Foreign References:
US4380910A
AU1757088A
AU1868167A
Attorney, Agent or Firm:
Baldwin, Shelston Waters (60 Margaret Street, Sydney, NSW 2000, AU)
Download PDF:
Claims:
CLAIMS:-
1. An evaporative cooling pad having a honeycomb like structure which provides multiple gas flow channels extending between a pair of opposite side faces; wherein the pad includes two spaced apart sections enclosing an intermediate mixing chamber so that air cooled by passage through one section is mixed in the chamber before entering the other section of the pad.
2. An apparatus for providing an enclosed space with a continuous supply of cooled air, said apparatus including: a casing; a first air inlet through which fresh air is drawn into the casing; a first air outlet for discharging cooled air from the casing into the enclosed space; a second air inlet through which stale air enters the casing from the enclosed space; a second air outlet through which stale air is expelled from the casing; a gasflow heat exchanger having substantially counterflow primary and secondary gas flow circuits disposed to facilitate heat transfer therebetween and connected such that the primary circuit receives fresh air to be cooled from the first air inlet and cools the fresh air by transferring heat to the stale air flowing through the secondary circuit towards the stale air outlet; fan means to induce air flow through the casing; an evaporative cooling pad through which stale air from the second air inlet flows and which is cooled by evaporation into the stale air of water passing through the pad; a reservoir for holding water cooled by passing through the pad; a finned tube heat exchanger positioned in an air flow path leading to the first air outlet; a water circuit which includes the reservoir, the pad and the finned tube heat exchanger; and, a pump operating to circulate water from the reservoir around the water circuit and back to the reservoir.
3. An apparatus for providing an enclosed space with a continuous supply of cooled air as claimed in claim 2, wherein said evaporative cooling pad has a honeycomb like structure which provides multiple gas flow channels extending between a pair of opposite side faces, the pad including two spaced apart sections enclosing an intermediate mixing chamber so that air cooled by passage through one section is mixed in the chamber before entering the other section of the pad.
4. An apparatus for providing an enclosed space with a continuous supply of cooled air, as claimed in claim 2 or 3, wherein the stale air entering the casing is split into two streams, one of which is supplied to the secondary circuit of the heat exchanger to cool the fresh air flowing through the primary circuit, and the other of which is mixed with the fresh air leaving the primary circuit of the heat exchanger before the air mixture is supplied to the outlet for providing air to the enclosed space.
5. An apparatus for providing an enclosed space with a continuous supply of cooled air as claimed in any one of claims 2 to 4, wherein the casing is provided internally with airguiding means for diverting a part of the stale air leaving the enclosed space before it enters the secondary circuit of the heat exchanger and passing the diverted air through the finned tube heat exchanger, the diverted air thereby mixing with cooled air from the primary circuit of the heat exchanger to provide a cooled mixed air stream for discharge through the first air outlet to the enclosed space.
6. An apparatus for providing an enclosed space with a continuous supply of cooled air as claimed in any one of claims 2 to 5, wherein the water circuit includes the pad and the tube of the finned tube heat exchanger serially arranged so that the pump circulates the cooled water from the reservoir through the finned tube heat exchanger before it enters the upper end of the pad.
7. An apparatus for providing an enclosed space with a continuous supply of cooled air as claimed in any one of claims 2 to 6, wherein the said evaporative pad is oriented generally vertically such that the cooling water discharged at the top of the pad passes down the pad and into the reservoir under gravity.
8. An apparatus for providing an enclosed space with a continuous supply of cooled air substantially as herein described with reference to any one of the accompanying Figures 1 to 6.
Description:
TITLE: AIR COOLER WITH FINNED TUBE HEAT EXCHANGER FIELD OF THE INVENTION This invention relates to air-cooling apparatus and multi-channel evaporative cooling pads through which air is passed in order to be cooled by a current of water flowing through the pads. More specifically the invention is concerned with improving the operation of such apparatus and pads.

DISCLOSURE OF THE INVENTION In the applicant's accepted Australian Patent No. 683,758 is described and claimed apparatus for cooling an air stream by passing it through an evaporative cooling pad with a honeycomb like structure. Cool water flows down the interior walls of the pad, with a portion of it being evaporated into the air stream. The latent heat of evaporation of the water acts to cool the pad so that the air leaving the pad is at a lower temperature than when it enters the pad.

The above system of cooling air can be incorporated into apparatus for cooling domestic or industrial premises. Advantageously, the use of a supplementary separate compressed gas refrigeration circuit can be avoided in some applications. In others, the refrigeration circuit does not have to be used to the same degree with the result that the apparatus is less expensive to operate and is also quieter. As such apparatus can be used to handle large rates of flow of air with a relatively high efficiency, it can be used to remove gaseous pollutants, such as cigarette smoke, from an enclosed space such as a house, public premises, or a food storage facility.

A first aspect of the present invention provides an evaporative cooling pad with a honeycomb like structure which provides multiple gas flow channels extending between a pair of opposite side faces; wherein the pad includes two spaced apart sections enclosing an intermediate mixing chamber so that air cooled by passage through one section is mixed in the chamber before entering the other section of the pad.

In use the cooling pad is provided with means for supplying water at a controlled rate to an upper end of the pad so that, as it descends, a portion of the water evaporates into a current of air flowing through the channels of the pad.

Experiments with a cooling pad vertically split into two spaced sections have shown that the cooling of an air stream is greater when the two sections of the pad are spaced apart than the cooling which occurs in a single cooling pad of equivalent thickness to the two sections. The reason why the cooling is enhanced by introducing a mixing chamber between the pad section is not clearly understood, however it is believed that the gas flow through each channel tends to be streamlined rather than turbid. Thus the air flowing next to the wall of a channel vaporises some of the water flowing down the channel wall and becomes saturated so that it cannot absorb any more water vapour. The air flowing through the centre of the channel on the other hand, never contacts the channel wall and it leaves the pad drier than the air in contact with the channel wall. It is believed that the effect of separating the two sections of the pad by, for example, 1.5 centimetres, is to introduce a chamber in which the air leaving the first pad section is mixed before it enters the second pad section. Thus the air flowing through the second pad section no longer has its portion flowing along the walls of the channels in a saturated condition and is therefore able to absorb more water vapour. As the cooling effect of the pad is a function of the latent heat of evaporation absorbed by the evaporation of the water into the gas stream, there results a greater cooling effect with the vertically split pad than is obtained with an unsplit pad providing the same length of flow channel.

A second aspect of the present invention provides an apparatus for providing an enclosed space with a continuous supply of cooled air, said apparatus including: a casing; a first air inlet through which fresh air is drawn into the casing; a first air outlet for discharging cooled air from the casing into the enclosed space; a second air inlet through which stale air enters the casing from the enclosed space; a second air outlet through which stale air is expelled from the casing; a heat exchanger having substantially counterflow primary and secondary gas flow circuits disposed to facilitate heat transfer therebetween and connected such that the primary circuit receives fresh air to be cooled from the first air inlet and cools the fresh air by transferring heat to the stale air flowing through the secondary circuit towards the stale air outlet; fan means to induce air flow through the casing; an evaporative, air-cooling pad through which stale air from the second air inlet flows and which is cooled by evaporation of water from the pad into

the stale air; a reservoir for holding water cooled by passing through the pad; a finned tube heat exchanger positioned in an air flow path leading to the first air outlet; a water circuit which includes the reservoir, the pad and the finned tube heat exchanger; and, a pump operating to circulate water from the reservoir around the water circuit and back to the reservoir.

Preferably the cooling pad has a honeycomb like structure which provides multiple gas flow channels extending between a pair of opposite upright side faces; the pad having two spaced vertical sections enclosing an air-mixing chamber between them so that air cooled by passage through one section is mixed in the chamber before it enters the other section of the pad.

Preferably the casing is provided internally with air-guiding means, which are preferably adjustable, for diverting a part of the stale air leaving the enclosed space before it enters the secondary circuit of the heat exchanger and passing the diverted air through the finned tube heat exchanger, the diverted air mixing with cooled air from the primary circuit of the heat exchanger to provide a cooled mixed air stream which is discharged through the first air outlet to the enclosed space. Such an arrangement has a number of advantages. Firstly, the air flow through the heat exchanger can be reduced, which in turn improves its thermal efficiency. Secondly, the energy consumed by the fans driving the air through the casing is reduced as less fresh air is drawn in from outside, which in turn reduces running costs. Thirdly, as lower air-flow rates are involved than with a total air replacement system, the capital cost of the apparatus can be reduced.

Conveniently the diverted air is passed through the finned tube heat exchanger before being mixed with the cooled fresh air. However it is equally practical to mix the diverted air with the fresh air before passing the mixture through the finned tube heat exchanger.

In a preferred arrangement, the water circuit has the pad and the tube of the finned tube heat exchanger serially arranged so that the pump circulates the cooled water from the reservoir through the finned tube heat exchanger before it enters the upper end of the pad. After trickling down through the pad, the cooled water is returned to the reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of examples, with reference to the accompanying schematic drawings, in which: FIGURE 1 is a vertical section through a first embodiment; FIGURE 2 is a modification of the apparatus shown in Figure 1; FIGURE 3 is a vertical section of a third embodiment; FIGURE 4 shows the apparatus of Figure 3 in end elevation, as viewed from the left of the figure, and has some interior parts shown in broken outline; FIGURE 5 is a vertical section through a fourth form of apparatus; and FIGURE 6 is a vertical section through a fifth embodiment of the invention.

PREFERRED EMBODIMENT OF THE INVENTION The apparatus shown in Figure 1 includes a casing 1 having on one side a fresh air inlet 2 and a stale air outlet 3. On the opposing side of the casing is provided a stale air inlet 4 and fresh air outlet 5 through which cooled air is supplied to an enclosed space, such as the interior of a room or a building.

The path followed by fresh air entering the casing 1 is denoted by full arrows 6.

Upon passing through the air inlet 2, the air enters a counterflow heat exchanger 7 having isolated primary and secondary air-flow circuits designed to have good heat- transfer characteristics between them. One form of such a heat exchanger is described in detail in the applicant's Australian Patent No. 660,781 and another form is shown in the applicant's International Patent Application PCT/AU96/0073 1, the disclosures of both being hereby incorporated into this specification by way of cross-reference. The warm fresh air flows through the primary circuit of the heat exchanger and is cooled by cool stale air which flows through the secondary circuit of the heat exchanger in the direction of the chain line arrows 8. The stale air is drawn through the heat exchanger by a suction fan 10 which discharges the stale air 8 through the outlet 3.

The fresh air cooled by its passage through the heat exchanger 7 flows into a turning space or chamber 11 containing a water reservoir 12 at its lower end. The reservoir 12 is provided with a dump valve device, a water supply device for ensuring

the water level in the reservoir is maintained at a sufficient level, and an overflow device for draining away excess water if a predetermined water level in the reservoir is exceeded. None of these devices are illustrated as they form no part of the working of the invention. A pump 13, driven by a motor 14, draws cool water upwardly from the reservoir through a pipe 15 and feeds the water into the lower end of a finned tube heat exchanger 16. The design of the heat exchanger 16 can take many forms. However, a preferred form is that of the type used for motor vehicle radiators. Such heat exchangers have a sinuously wound tube provided with parallel thin metal fins which are aligned with an air path through the heat exchanger. The fins provide a low resistance to air- flow through the heat exchanger, whilst providing an extended heat-transfer surface for transferring heat between the air passing between the fins and the water flowing through the tube.

The water flows upwardly through the finned tube heat exchanger 16 to a pipe 17 which leads upwardly to a pair of perforated tubes 18 respectively located above water distributors 19 located above a pair of upright cooling pads 20,21 which are spaced from one another by a mixing chamber 22 approximately one centimetre wide. The cooling pads 20,21 provide sets of parallel air-flow channels extending between their opposite upright faces and through which the stale air is passed. The water released from the tubes 18 trickles down the walls of the channels of the pads 20,21 and results in part of the water vaporising into the stale air stream flowing through the channels. This vaporisation extracts latent heat from the water and consequently cools the water. The applicant's International Patent Application No. PCT/AU95/003 15 describes the construction and use of an air cooling pad in detail, and the disclosure in this International Patent Application is hereby incorporated into this specification by way of cross-reference. A water collector 23 located beneath the two cooling pads 20,21 collects the water leaving their lower ends and directs it downwardly to the upper end of a third cooling pad 24, the lower end of which is located in the water of the reservoir 12.

The pad 24 is arranged alongside and downstream of the finned tube heat exchanger so that the fresh air which passes between the fins of the heat exchanger then passes through the pad 24 before entering a first compartment 25 containing a suction fan 26.

The fan 26 draws the air from the first compartment 25 through to a second compartment

27 containing a highly efficient, low toxicity, burner shown at 28. These burners produce virtually total combustion of gas passing through them so that the hot combusted gas stream leaving the burner contains a negligible amount of noxious fumes.

Such a burner is described in more detail in the applicant's International Patent Application PCT/AU96/00176 the disclosure of which is hereby incorporated into this specification by way of cross-reference. The burner is not used when the apparatus illustrated is supplying a stream of cool air to the enclosed space. The burner is only used when the temperature of the fresh air stream entering the casing is lower than is required in the enclosed space. In this mode the pump 13 is not operated and the apparatus behaves as an air heater rather than an air cooler.

The operation of a first embodiment of the invention will now be described with reference to Figure 1. It is first assumed that the ambient fresh air is at a higher temperature than that required in the enclosed space. The pump 13 and the fans 10 and 26 are then operated. The stale air from the inlet 4 passes through the two evaporative pads 20 and 21. Some of the water trickling down through these pads evaporates into the stale air. The latent heat of evaporation is extracted from the water in the pads 20,21 and is thus cooled. The flow of stale air may be as high as 750 to 1200 litres per second.

It has been found that the cooling effect on the water passing down through the pads is significantly greater if the two pads are slightly separated from one another by, for example, a space of about one centimetre. It appears that this spacing provides an air mixing chamber between the pads which significantly increases the cooling effect on the water passing down through them.

The stale and humidified air from the pad 21 flows into the secondary circuit of the heat exchanger 7. The stale air stream follows the U-shaped path in the heat exchanger indicated by the chain arrows 8, and cools the stream of incoming fresh air 6 which enters the primary circuit of the heat exchanger from the fresh air inlet 2. The stale air stream 8 is drawn through the heat exchanger by the fan 10 and is discharged into the ambient air by way of the stale air outlet 3.

The fresh air cooled by its passage through the U-shaped primary circuit of the heat exchanger 7 flows down into the turning space or chamber 11. The fresh air then passes between the fins of the finned tube heat exchanger 16 where it is further cooled.

The heat exchanger 16 is of the type such as is used in water-cooled radiators of cars and is kept cold by the water from the reservoir 12 which is pumped through the heat exchanger 16 by the pump 13.

After travelling through the heat exchanger 16, the fresh air is further cooled by being passed through the evaporative pad 24 down which is passed cold water from the collector 23 at the lower ends of the pads 20, 21. Some of the water evaporates into the fresh air to slightly increase its humidity, and the water discharging into the reservoir 12 from the lower end of the pad 24 is further cooled. The fresh air from the pad 24 enters the compartment 25 and is discharged in a cooled and partially humidified condition into the enclosed space by the fan 26 as shown by the full arrows.

The water leaving the upper end of the finned tube heat exchanger 16 passes upwardly through the feed pipe 17 to the perforated tubes 18 above the respective pads 20,21.

In cool conditions the apparatus is required to circulate warm air through the enclosed space. The pump motor 14 is then switched off and the burner 28 is ignited.

This discharges hot combusted gases containing negligible quantities of toxic fumes, into the pre-warmed air leaving the fresh air outlet 5. This air has been pre-warmed by its earlier passage through the heat exchanger 7, with heat transferred to it from the stream of warm stale air 8 flowing through the secondary circuit of the heat exchanger 7.

Thus the burner 28 is required to provide little more than make-up heat as the heat exchanger 7 has a relatively high thermal efficiency. The amount of combusted gases discharged into the fresh air stream by the burner 28 is therefore small, and is insufficient to breach the safe health limits controlling the amount of noxious gas allowed in an inhabited enclosed space.

The apparatus shown in Figure 2 is similar in layout and operation to that already described with reference to Figure 1, and the same reference numerals have been used to denote identical parts which will not therefore be again described. The following description is thus limited to the differences between Figures 1 and 2, respectively.

It will be noticed from Figure 2, that the third evaporative cooling pad 24 of Figure 1 is absent. Instead, the water cooled by passage through the two evaporative

cooling pads 20 and 21 and passing into the collector 23, descends directly through a vertical pipe 30 to the reservoir 12.

Fresh air cooled by passage through the finned tube heat exchanger 16 is further cooled by passage through an evaporator coil 32 of a compressed gas refrigeration circuit which includes a motor driven compressor 33 and supply and return piping 34 and 35 which extends between the compressor 33 and the evaporator coil 32. The cooled air leaving the evaporator coil 32 is supplied to the enclosed space by the fan 26 as previously described with reference to Figure 1.

Controls (not shown but conventional in the domestic air-conditioning art) are used to control the operation of the compressor 33 in accordance with the temperature requirements of the enclosed space. Any condensation collecting on the external surfaces of the evaporator coil 32 flows down into the reservoir 12. When the apparatus is required to provide warm air, the compressor 33 and the pump 13 are de-energised, and the burner 28 is ignited and operated, as previously described with reference to the apparatus shown in Figure 1.

In both of the embodiments of Figures 1 and 2, provision is made for removing any excess water which collects in the reservoir 12, and for supplementing the water level with top-up water, should the reservoir water level fall beneath a predetermined minimum level.

The apparatus of the embodiment shown in Figures 3 and 4 includes a casing 40 having on one side a fresh air inlet 41 and a stale air outlet 42, and on the other side an air outlet 44 and a stale air inlet 45 both of which communicate via ducting, partially shown at 46 and 47, with an enclosed space such as a room.

The casing 40 contains a heat exchanger 48 having U-shaped substantially counterflow primary and secondary airflow circuits and which corresponds to the heat exchanger 7 in earlier figures. A central vertical partition 52 located beneath the heat exchanger 48, separates a lower space 53 containing a fan 54 for drawing stale air along the full-line arrow path 51, from a fresh air turning space 55 containing a water reservoir 56 and a water pump 57. The stale air stream 51 drawn through the secondary circuit of the heat exchanger 48 cools the fresh air stream 50 passing in counterflow through the primary circuit of the heat exchanger 48, as indicated by the broken arrow path 50. The

air from the turning space 55 is drawn through an evaporative cooling pad 58 standing in the reservoir 56 and enters an air-stream mixing compartment 60.

As shown in Figure 4, stale air from the enclosed space enters the casing 40 through the stale air inlet which comprises two inlets 46A and 46B leading into a triangular plenum 61 shown in Figure 3. A division plate 62 divides the plenum 61 into two triangular zones 63,64 each of which receives a portion of the stale air passing through the inlets 46A and 46B. The position of the plate 62 is adjustable to alter the proportion of the incoming stale air supplied to the two zones 63,64, preferably so that the zone 64 receives stale air at substantially the same flow rate as, or slightly less than, the rate of flow of fresh air entering the fresh air inlet 41.

The stream of stale air entering the zone 64 flows serially through two spaced evaporative air-cooling pads 66 and 67 which are separated by a narrow mixing chamber 68. The function and operation of the pads 66 and 67, and the separating chamber 68, is identical to that of the pads 20 and 21 and the mixing chamber 22 described in Figure 1, and their function will not therefore be again described in detail. Suffice to say that the pads serve to cool water passing down them and are further cooled by the evaporation of water into the stale air stream 51. The stale, humid and cool air stream 51 from the zone 64 flows through the heat exchanger 48 and serves to cool the incoming warm fresh air stream 50 flowing through its primary circuit. The water from the lower ends of the pads 66 and 67 passes into a collector 70 which directs it into a distributor 71 at the upper end of the evaporative pad 58 where it is further cooled before discharging into the reservoir 56.

The portion of the stale air stream entering the zone 63 flows down through a finned tube heat exchanger 72 which cools it further, and then passes into the compartment 60. The cooled stale air admitted to the compartment 60 mixes with the cooled fresh air entering it via the pad 58, and the air mixture is discharged by a fan 73 via the air outlet 44.

As shown in Figure 4, cold water from the reservoir 56 is forced by the pump 57 through piping leading to the tube of the finned tube heat exchanger 72 so that it acts to cool the fins between which the stale air flows from the zone 63 to the compartment 60.

The warmed water from the heat exchanger 72 passes upwardly through piping (not

shown) to perforated distributor tubes 74 respectively located at the upper ends of the pads 66,67. It then passes down through these pads and the pad 58 beneath, where it is further cooled, before re-entering the reservoir 56.

When heating of the fresh air is required, the pump 57 is de-energised and a burner 74 having the same characteristics as the burner 28 of earlier figures, is ignited to provide hot combustion products with a low toxicity to the air flowing through the air outlet 44.

In practice, about half of the stale air passing through the inlet 45 is recycled from the zone 63 through the compartment 60. The power consumption of the fan 54 is only about half of that of the fan 73 and the rate of flow of air through the primary and secondary circuits of the heat exchanger 48 is halved so that the rate of heat-transfer between them is substantially increased. The apparatus is thus cheaper to construct and operate than the apparatus of earlier embodiments. Although a small part of the stale air is re-cycled through the enclosed space, the slight increase in carbon dioxide content is not noticeable and is perfectly acceptable from a health point of view.

When air heating is required, the rate of flow of air into the enclosed space can be halved to about 750 litres per second. The burner 74 is only required to provide make up heat, as a high proportion of the heat content of the stale air passing through the heat exchanger 48 is transferred to the incoming fresh air.

Figure 5 illustrates a further embodiment of the present invention. The apparatus shown in Figure 5 includes a casing 100 having a first wall 101 provided with a fresh air inlet 102 and, at a level beneath the inlet 102, a stale air outlet 103. Preferably, the casing is designed to stand against the inside face of an outside wall of a building, with the inlet 102 and outlet 103 registering with corresponding openings formed through the outside wall.

The casing 100 has a second wall 105 within which is provided an upper stale air inlet 106 for connection via ducting 107 to an enclosed space from which stale air is to be extracted. An air outlet 108 located beneath the inlet 106, is connected by ducting 110 to supply air to the enclosed space.

The stale air inlet 106 opens inside the casing 100 into a plenum 111 having an upper zone and a lower zone. The upper zone is flanked by one upright face of two

spaced evaporative pads 112,113 provided with a mixing chamber 114 between them.

The evaporative pads are similar to those described with reference to earlier embodiments and hence will not be further described. An isolating heat exchanger 121 occupies the upper region of the casing 100 and is spaced from the back surface of the pad 113 to leave a narrow gap 115 therebetween. The bottom of the gap is closed and the gap allows stale humid air which has passed through the pads 112,113 to enter a secondary circuit of the heat exchanger 121. The path followed by the stale air stream is denoted by full-line arrows 120 and it follows a substantially U-shaped path through the heat exchanger 121. The stale air leaves the underside of the heat exchanger adjacent the back wall 101, and is drawn downwardly by a suction fan 126 which discharges the stale air through the outlet 103. The heat exchanger design is similar to that already described with reference to earlier embodiments and will not therefore be again described.

Fresh air stream from the inlet 102 follows a U-shaped path in the primary circuit of the heat exchanger 121, as indicated by the chain-line arrows 123. The primary and secondary circuits of the heat exchanger 121 are in good heat transfer relationship and convey air along parallel counterflow paths to maximise the transfer of heat from the primary circuit to the secondary circuit so that the fresh air is cooled in the heat exchanger 121. The cooled fresh air stream leaves the primary circuit of the heat exchanger 121 by way of an outlet at the front portion of its undersurface, and enters a mixing compartment 124 which also receives stale air from the lower zone of the plenum 111. The air mixture is drawn down through a finned tube heat exchanger 125 which is water cooled and cools the air mixture still further. The cooling water for the heat exchanger 125 is provided at its underside from a pipe 132 which is connected to a water pump 136 mounted in a reservoir 127 located on the floor of a second compartment 128 which also contains a suction fan 130. The fan operates to draw cooled air downwardly through the finned tube heat exchanger 125 and discharge it into a third compartment 129 containing a burner 137. This burner is of the type referred to in earlier embodiments of the invention and is designed to produce fully-combusted products of combustion with virtually no noxious fume content. Cooled air from the third compartment flows through the outlet 108 and into the ducting 110 which conveys it to the enclosed space.

A partition 134 in the casing 100 separates the stream of stale air 120 leaving the heat exchanger 121, from the stream of fresh air 123 leaving the heat exchanger 120.

Cooling water which has passed upwardly through the finned tube heat exchanger 125 flows upwardly through a pipe 140 to two perforated tubes 110,111 located above respective water distributors 109 disposed above respective pads 112,113.

Water from the distributors 109 percolates down the walls of the pads 112,113, with some of it being evaporated into the stale air passing through the pads. The extraction of the latent heat of evaporation from the percolating water acts to cool the water as it passes down the pads. The cooled water enters a collector 141 located beneath the two pads 112,113 and which has a drain pipe for returning the cooled water to the reservoir 127. The reservoir is provided with devices (not shown) for emptying it when cooling of the fresh air is not required, and for maintaining the water level in the reservoir between upper and lower limits. Such devices are well-known in the art and will not be described.

The apparatus of Figure 5 operates as follows. When cooled air is to be supplied to the enclosed space, the burner 137 is switched off and the fans 126, 130 and water pump 136 are operated. Fresh air 123 entering the heat exchanger 121 is pre-cooled by the cool room air flowing in counterflow to it. The pre-cooled room air is then mixed with approximately half of the cool stale air entering the inlet 106, and this air mixture is further cooled by its passage through the finned tube heat exchanger 125. The fan 130 supplies the cooled air mixture to the enclosed space via the outlet 108 and ducting 110.

When air at a higher temperature than the ambient air is to be supplied to the enclosed space, the pump 136 is switched off, the burner 137 is ignited, and, the water is drained from the reservoir. The cold fresh air 123 from the inlet 102 is first pre-warmed in the heat exchanger 121 by the counter-flowing stale air. The pre-warmed fresh air is mixed with an approximately equal quantity of warm stale air and the resultant mixture is driven by the fan 130 through the third compartment 129 where it is further warmed by the hot combustion products from the burner 137, and then passed through the outlet 108 and ducting 110 to the enclosed space.

The apparatus shown in Figure 5 is quiet in operation, economical to run, cheap and simple to manufacture and is able to optionally provide cool or warm air over a wide range of climatic conditions.

In all of the embodiments shown in the drawings the use of a mixing chamber between two parallel pads is, of course, optional. In some cases the additional expense involved in such an arrangement is not warranted and a single pad can be used although it has a lesser cooling effect on the water.

Figure 6 illustrates a further embodiment of the present invention which operates as follows. Approximately half of the stale air stream is passed through the evaporative cooling pad 112 prior to entering the counterflow heat exchanger 121. This in turn cools the incoming fresh air stream 123 passing through the heat exchanger. The fresh air stream 123 is then passed through a cooling water coil 150 in the form ofa finned tube heat exchanger. The remainder of the stale air stream is passed through a finned tube water coil 151 and then mixed with the incoming fresh cooled air, and supplied to the enclosed space by means supply fan 130.

The finned tube water coils 150, 151 are cooled with cold water from the cooling pad 112. A similar finned tube water coil 152 is fitted to the lower entry area of the cooling pad 112 and this is operated in conjunction with one of the cooling coils 150 or 151 which supplies cool air to the room. This coil also acts to lower the wet bulb temperature therefore lowering the temperature of the water supply to the air cooling coils. This has significant advantages in dropping the water temperature by around 2"C below wet bulb temperature which translates to a significant increase in cooling performance and which could not be achieved using the room wet bulb temperature.

Water is circulated through the system from a reservoir (not shown) by means of apump 136.

Another preferred feature of this embodiment is a smaller "celdek" cooling pad 153 located above the finned tubed water cooling coil 152 which supplies water to this cooling pad. This has the advantage of pre-cooling the stale air leaving the room before entering the main cooling pad 112 and heat exchanger 121. The cold water flowing from this pre-cooling pad 153 flows into and down the small finned tube water cooling coil

152 and in turn evaporates on this cooling coil and lowers the wet bulb temperature. It is possible to get very close to dew point with this arrangement.

When heating is needed this same unit can include a direct or indirect gas heating system as previously described in the applicant's co-pending Australian Patent Application No. 50005/96. This system supplies approximately 50% fresh air at a very high efficiency with the gas heating apparatus. The cooling side of the system is not used when the apparatus is in a heating mode.

With the indirect gas burning system, the flue gas is exhausted to the outside air, via the flue pipe 154, to the exhaust side of the small water coil and heat exchanger.

This has the advantage of reclaiming the flue gas which would normally be wasted to the atmosphere. The gas is thus drawn through the heat exchanger to reclaim the flue gas heat to the incoming cold fresh air. The effect of this system is that approximately 20- 80% of the heat from the flue gas exiting the system is reclaimed. This would normally represent a loss of around 25% of the total input heat, whereas in this instance, the loss is reduced to around 5%.

Another improvement to this system is to have a diverter valve in the water circuit which pumps water to the small water coil to absorb the heat from the flue gas before entering the heat exchanger, which in turn pumps the now heated water into the incoming return air to the supply air water coil. This has the ability to absorb almost 95% of all the energy from the flue gas which in turn achieves an efficiency rating of up to 96% even with return fresh air input.

This whole system has been specifically designed to fit into low modern constructed truss roof cavities by using a heat exchanger which is split in the center to allow a low truss beam to fit into the heat exchanger profile without downgrading the performance. The unit has been designed to be fitted into low truss roofs in 3 sections to accommodate the very limited space of some of the modern constructions.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.