GB2085143A

GB2085143A - Air-liquid heat exchanger

Info

Publication number: GB2085143A
Application number: GB8126853A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-09-04
Filing date: 1981-09-04
Publication date: 1982-04-21

Abstract

An air-liquid heat exchanger for use as an evaporator in, for example, a solar/ambient heat collecting heat pump has a cylindrical casing (1) housing a coaxial coil or coils (12, 13) of helically wound finned or corrugated tubing in which liquid flows. A centrifugal impeller (6) directs air axially into one end of the casing, and a baffle plate (4) at the other end of the casing forces the air to flow radially outwardly through the coils (12, 13) before flowing axially through an annular outlet (5) around the baffle plate, so that air flows over all the exposed surfaces of the coils. The casing (1) may be transparent to enable the coils (12, 13) to absorb direct solar radiation and solar radiation reflected by a reflective element positioned externally of the casing. <IMAGE>

Description

SPECIFICATION Air-liquid heat exchanger This invention relates to air-liquid heat exchangers, with particular, but not exclusive, application to evaporator units for use in refrigeration and heat pump systems.

A known type of air-liquid heat exchanger commonly employed in heat pumps and refrigeration systems has an array of pipes, usually arranged in parallel rows, over which air is drawn by one or more axial flow impellers. In the case of a heat pump used, for example, in air conditioning, the array of pipes constitutes the evaporator of a refrigerant circuit, and the air drawn overthese pipes is cooled as it gives up heat to the refrigerant flowing through the pipes. In the case of a refrigeration system, on the other hand, such an air-liquid heat exchanger may be used as a condenser unit. Regardless of the practical application, the effective surface area of the pipes presented to the air flow should be maximised in order to achieve the greatest possible efficiency.

With most conventional arrangements of the kind referred to, a significant proportion of the surface area of each pipe is inevitably masked or shielded from the air flow drawn through the heat exchanger, to the detriment of the overall efficiency of the unit.

The present invention seeks to provide an improved air-liquid heat exchanger with a high surface area of contact between air and liquid flows.

According to the invention there is provided an air-liquid heat exchanger comprising a generally cylindrical casing, an impeller unit for directing air axially into the casing through an inlet, at least one helically wound coil of finned or corrugated tubing located coaxially within the casing radially outwardly of the inlet, and a flow-restricting air outlet spaced axially from the inlet and radially outwardly of the coil or coils, such that air directed into the casing through the inlet flows radially outwardly under pressure between the turns of the coil or coils and is then deflected by the casing to flow axially through the outlet, whereby in use of the heat exchanger the air flows over substantially all exposed surfaces of the tubing to effect heat exchange with liquid flowing through the coil or coils.

The air directed into the casing is forced to flow radially outwardly under pressure between the turns of the or each coil and then flows axially over the coil or coils, pressure being maintained by the flowrestricting air outlet. In this way, it is ensured that the air comes into intimate contact with substantially the entire surface of the coiled tubing before being expeiled through the outlet. Preferably the fins or corrugations of the or each coiled tube are closely adjacent or touching so as to define substantially radial air flow passages in which the air is entrained to flow over the entire surface of the coiled tube or tubes.

In a preferred construction the or each coil is fabricated from corrugated tubing, for example that sold under the Trade Mark INTEGRON. ideally the heat exchanger has at least two coaxially nested coils, the corrugations or fins of the tubing from which the coils are made ensuring that the air flows over sub stantiallythe entire surface of the coils in passing from the interior of the casing to the air outlet.

Preferably the impeller unit comprises a centrifugal impeller communicating through a divergent duct with the inlet. The air outlet is preferably an annular outlet coaxial with the inlet and defined by a baffle plate at one end of the casing, closing one end of the axial airflow passage through the coil or coils, the inlet being on the axis of the casing at the opposite end from the baffle plate.

The heat exchanger according to the invention finds useful application in an evaporator unit for extracting heat from air, for example stagnant air in a roof space, to provide a low-grade heat input for a heat pump. For this purpose the heat exchanger would be installed in an evaporator unit having a capillary tube or expansion valve connected to an inlet end of the coil or coils and a gas outlet for connection to the outlet end of the coil or coils. Two heat exchangers may be mounted in tandem, having their respective coils connected in parallel with a common capillary tube or expansion valve.With this application in view, it is envisaged that in a preferred embodiment of the invention the casing would be made of transparent material or would be provided with a window, the tubing forming the coil or coils having light-absorbent surfaces which are exposed in use ofthe heat exchanger to solar radiation through the casing or the window. To maximise the absorption of solar radiation a reflective element extending parallel to the axis ofthe casing may be disposed on the opposite side of the casing from that exposed to solar radiation to direct such radiation onto the absorbent surfaces of the coil or coils.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partly cutaway perspective view of an air-liquid exchanger according to one embodiment of the invention; Figure 2 is an axial sectional view of the heat exchanger shown in Figure 1; Figure 3 is a partly cutaway end view of the heat exchanger, at the air outlet end thereof; Figure 4 is a perspective view of a heat exchanger unit according to a second embodiment of the invention; Figure 5 is a diagrammatic end view of the heat exchanger shown in Figure 4, equipped for use as an evaporator unit, and Figure 6 is a diagrammatic partly cutaway perspective view of a heat exchanger according to another embodiment of the invention, installed in a loft or roof.

In the drawings the same reference numerals are used throughout to designate the same or corresponding component parts.

Referring first to Figure 1 to 3, the illustrated air liquid heat exchanger according to the invention comprises a cylindrical sheet-metal casing 1 having at one end a radially inwardly projecting flange 2 defining a circular air inlet 3. The other end of the casing 1 is open and defines, with a concentrically supported circular baffle plate 4, an annular air outlet 5.

An electrically driven centrifugal impeller 6 is supported by the end flange 2 of the casing 1, the outlet of the impeller 6 communicating with the air inlet 3 of the casing through a divergent inlet duct 7. The impeller 6 is preferably a self-contained unit incorporating an electric motor with vibration-damping mountings. The impeller unit incorporates a divergent air outlet throat 8 which has a flanged coupling 9 to the inlet end of the divergent inlet duct 7, so that the entire impeller unit can be removed for replacement or repair as necessary.

The casing 1 is reinforced internally by four longitudinally extending channel section support members 10 to which the casing 1 is riveted. The baffle plate 4 is supported from the members 10 by means of radially inwardly projecting brackets 11. The four support members 10 together form a support "cage" for two coaxially nested helically wound coils 12, 13 which extend coaxially throughout the entire length of the casing 1. As shown in Figure 2, the coils 12,13 surround the air inlet 3, and the radially outer coil 13 has a smaller diameter than the baffle plate 4.Each coil 12, 13 is wound from corrugated tubing, in this example corrugated tube available commercially under the Trade Mark INTEGRON. The corrugations of adjacent turns of each coil are closely spaced or touching, as shown diagrammatically in Figure 2, and similarly the corrugations of adjacent turns of the radially inner and outer coils 12,13 are closely spaced or touching, as shown diagrammatically in Figure 3.

In use of the heat exchanger the impeller 6 draws air from the surrounding atmosphere, for example a roof or loft space in which the unit is mounted, and directs the air axially th rough the inlet 3, as shown diagrammatically by arrows in Figure 2. The presence of the outlet baffle plate 4forces the air to flow radially outwardly between the turns of the coils 12, 13. The corrugations of the tubing from which the coils 12, 13 are fabricated form in effect a number of flow passages which direct the radially outwardly flowing air over substantially the entire surfaces of the coils 12, 13, as illustrated by the arrows in Figure 2.

The heat exchanger illustrated in Figures 1 to 3 is designed for use as an evaporatorunitin a heat pump system for utilising ambient heat, for example in loft spaces or other enclosures heated directly or indirectly by solar radiation. For this purpose a refrigerant liquid, after passing through an expansion valve, capillary tube, or other pressure-reducing device, is admitted to one end ofthe nested coils 12, 13 through an inlet connector 14. After flowing through the two coils 12,13 in series the refrigerant leaves through an outlet connector 15, both connec tors 14, 15 projecting through holes in the baffle plate 4.In passing through the expansion device the refrigerant cools to a sub-ambient temperature, and as the cold refrigerant vapour flows through the coils 12,13 it absorbs heat from the coils 12,13 and is evaporated, the refrigerant gas leaving through the outlet connector 15. In this way the refrigerant absorbs heat from the air flowing over the coils 12, 13, the efficiency of the heat transfer between the air and the refrigerant liquid being enhanced by the large effective surface area of contact between the air and the surfaces of the corrugated tubing forming the coils 12,13.

An auxilliary electric resistance heater 16, shown in broken outline in Figure 2, is housed in the air outlet throat 8 of the impeller 6. The heater 16 is energised for short periods as necessary in order to prevent ice accretion on the coils 12, 13.

In place of the corrugated tubing illustrated in the drawings, finned tubing may be employed. Where finned or corrugated tubing of small bore is used, the helical coils 12, 13, or a single such coil, may be formed from coiled finned or corrugated tube, so that the or each coil 12,13 is in effect a "coiled coil".

For certain practical applications where a high throughput of air is required two heat exchangers of the type shown in Figures 1 to 3 may be mounted side by side in tandem, as shown diagrammatically in Figures 4 and 5, where the same reference numerals have been used to identify the same or corresponding component parts. In the version illustrated in Figures 4 and 5 there are two separate impeller units 6, but the coils of the two heat exchangers are connected together in parallel to a common capillary tube 17 or other expansion device, and to a common outlet connector 18.

The embodiment illustrated in Figure 6 is designed to make use of direct solar radiation as well as heat stored in the roof space of a building to impart heat to liquid, in this case refrigerant, circulating in the heat exchanger coils. In this embodiment the casing 1 is positioned directly below a roof light or window 19 and is made of transparent material, for example, clear plastics, the coils 12,13 having matt blacksurfaces for optimum absorption of thermal radiation.A reflective element 20 is suspended below the casing 1 to reflect solar radiation which, is not incident directly on the casing onto the coils 12, 13. In the illustrated embodiment the reflective element 20 has a parabolic cross section and its focal line coincides with the common axis of the casing 1 and the coils 12, 13. The reflective element 20 also acts as a drip tray, for the collection of condensation water which, during defrosting, falls from the coils 12,13 and through a drainage hole or slot (not shown) in the casing 1. A drain pipe 21 is connected to the reflec- tive element 20 to conduct away any condensation water. Under certain operating conditions, for example, in winter sunshine, the ambient air temp- erature in the roof space may be too low for useful work to be done by the heat pump connected to the evaporator. In this case, the solar radiation incident on the avaporator, which could typically be 500 watts per square metre, imparts useful heat to the heat exchanger coils 12,13.

Claims

1. An air-liquid heat exchanger comprising a generally cylindrical casing, an impeller unit for directing air axially into the casing through an inlet, at least one helically wound coil of finned or corrugated tubing located coaxially within the casing radially outwardly of the inlet, and a flow-restricting air outlet spaced axially from the inlet and radially outwardly of the coil or coils, such that air directed into the casing through the inlet flows radially outwardly under pressure between the turns of the coil or coils and is then deflected by the casing to flow axially through the outlet, whereby in use of the heat exchanger the air flows over substantially all exposed surfaces of the tubing to effect heat exchange with liquid flowing through the coil or coils.

2. A heat exchanger according to Claim 1, in which the air outlet is an annular outlet coaxial with the inlet and defined by a baffle plate at one end of the casing, closing one end of the axial airflow passage through the coil or coils, the inlet being on the axis of the casing at the opposite end from the baffle plate.

3. A heat exchanger according to Claim 1 or Claim 2, in which the impeller unit comprises a centrifugal impeller communicating through a divergent duct with the inlet.

4. A heat exchanger according to any one of.the preceding claims, in which an electrical resistance auxiliary heater is located in or adjacent the inlet.

5. A heat exchanger according to any one of the preceding claims, in which the fins or corrugations of the or each coiled tube are closely adjacent or touching to define substantially radial air flow passages.

6. A heat exchanger according to any one of Claims 1 to 5, in which the or each coil comprises a finned or corrugated tube, or a coiled tube, wound into a helical coil.

7. A heat exchanger according to any of Claims 1 to 6, in which the casing is made of transparent material or is provided with a window, the tubing forming the coil or coils having light-absorbent surfaces which are exposed in use of the heat exchanger to solar radiation through the casing or the window.

8. A heat exchanger according to Claim 7, in which a reflective element extending parallel to the axis of the casing is disposed on the opposite side of the casing from that exposed to solar radiation to direct such radiation onto the absorbent surfaces of the coil or coils.

9. An air-liquid heat exchanger according to Claim 7 or Claim 8, installed in a loft or roof to absorb heat from air forced through the heat exchanger by.

the impeller unit, and to absorb radiant heat from solar radiation impinging on the coil or coils.

10. An air-liquid heat exchanger substantially as herein described, with reference to and as shown in Figures 1 to 3, Figures 4 and 5 or Figure 6 of the accompanying drawings.

11. An air-liquid heat exchanger according to any one of the preceding claims, installed in an evaporator unit having a capillary tube or expansion valve connected to an inlet end of the coil or coils and a gas outlet connection at the outlet end ofthe coil or coils.

12. An evaporator unit comprising two air-liquid heat exchangers according to any of Claims 1 to 10 having their respective coils connected in parallel with a common capillary tube or expansion valve.