Evaporator assembly

Abstract: An evaporator assembly for incorporation into an air conditioning apparatus, particularly for automobiles, comprises a main air duct (2) defining an air inlet (6) and outlet (10), means (22) to force air through the main air duct (2), a heat exchanger (12) disposed on the main air duct (2), and an air feedback duct (18) which links the main air duct (2) downstream of the heat exchanger (12) with the duct upstream thereof, whereby a proportion of the air which has passed through the heat exchanger can be recirculated, via the feedback duct (24), to pass through the heat exchanger (12).

Description: The present invention relates to an evaporator assembly, and in particular, although not exclusively, to an evaporator assembly for incorporation into an air conditioning apparatus for use in automobiles. The invention also relates to method of cooling air using such an assembly. In automobiles, air conditioning systems are utilised to provide space cooling within a passenger compartment, by introduction of cooled air. The introduced air may be either fresh air from outside the vehicle, or air from within the passenger compartment which has been withdrawn therefrom and cooled by the air conditioning apparatus, or a mixture thereof. A conventional air conditioning apparatus employs an evaporator assembly comprising evaporative heat exchanger consisting of a plurality of tubes through which coolant is passed, and fins mounted in an air duct, and a fan to provide forced passage of air through the duct, which air is cooled on contact with the tubes and fins of the heat exchanger. An example of an air conditioning apparatus which is for use in motor trucks is disclosed in US Patent No 4382463. Although such a system can operate effectively under conditions of high load such as will exist on initial cooling, the effectiveness of the cooling operation decreases rapidly as the temperatures of the heat exchanger and the inlet air converge. This decline of cooling effectiveness is particularly dramatic when the inlet air is drawn from the space to be conditioned. This manifests itself as a rapid decline in the refrigerant flow rate after initial switching on of the refrigerating circuit during a peak demand condition. Typically, for a conventional evaporator in an automobile, the flow rate decline may be of the order of 28% after about two and a half minutes, and a further 18% after a further fifteen minutes. Whilst the condensing and pumping components of the heat exchanger circuit can maintain the throughput of refrigerant, the flow rate is limited by the rate of heat transfer of the heat exchanger. There is therefore a need to increase the efficiency of the heat exchanger. Whilst a heat exchanger which is deeper in the direction of air flow will allow a greater rate of heat transfer, the increased dimensions are undesirable; compactness of the heat exchanger, weight and cost thereof are of great importance, particularly when used in automobiles. Moreover, it can be shown that the quantity of refrigerant charge required in the heat exchanger circuit is that which will satisfy the evaporative heat exchanger under all encountered load and operating conditions, plus an equal volume in the remainder of the circuit, plus a small amount which is required to counteract anticipated leakage from the system. Therefore, if the volume of the evaporator heat exchanger can be reduced, the total quantity of refrigerant required for the system can be reduced by about twice the evaporator heat exchanger volume reduction, further reducing the cost. It is an object of the invention to provide an evaporator assembly which overcomes the problems referred to above, and which is of particularly overall compact design. DE-A-3641568 discloses a cool box arrangement for a vehicle having air conditioning, in which a portion of the cool air output of the air conditioning system is passed through the cool box to effect cooling of the contents thereof, and is returned to the input of the air conditioning system. This document does not disclose feeding the cooled air from the heat exchanger output immediately back for recirculation through the heat exchanger whereby the effect of the heat exchanger is increased. According to a first aspect of the present invention there is provided an evaporator assembly comprising a main air duct defining therein an air inlet, and an air outlet; an air feedback duct having its input at a first location in the main air duct and its output at a second location spaced along the main air duct from the first location; an evaporative heat exchanger; means for forcing air to pass through the heat exchanger; said forcing means and said heat exchanger being disposed in the main air duct between said first and second locations; wherein, in use, the input of the air feedback duct receives air cooled by the heat exchanger and the air feedback duct provides said air cooled by the heat exchanger directly to the air feedback duct output whereby the feedback duct and the forcing means recirculate said air cooled by the heat exchanger through the heat exchanger again so that the effect of the heat exchanger is increased. By recirculating a proportion of cooled air back into the main duct, the effectiveness of heat extraction is increased as compared to a conventional arrangement. Thus, for a given cooling effect, a heat exchanger which is thinner in the air flow direction than that of a conventional design can be utilised, thereby reducing the size, weight and refrigerant requirement and cost of the assembly. It is also the case that recirculation allows increased extraction of moisture (which will condense on the tubes and fins). To compensate for the reduction in volume of the air which is available for final delivery to the automobile interior, it is preferable to increase the power of the fan over the conventional design; however, this increase need only be small since the reduced thickness of the heat exchanger offers a reduced resistance to air flow. Preferably, the assembly further comprises means for selectively closing said air feedback duct. According to a second aspect of the present invention there is provided a method of operating the evaporator assembly of the first aspect, comprising forcing air through said evaporative heat exchanger whereby said air is cooled; and causing recirculation of said cooled air via the feedback duct and again through the heat exchanger whereby the effect of the heat exchanger is increased. The air feedback duct may be closed by means for selectively closing said air feedback duct, on initial operation of the evaporator assembly, and only opened to allow recirculation of air via the feedback duct subsequent to this initial operation. The means for selectively closing the air feedback duct may comprise valve means and control means therefor, adapted to open the air feedback duct after a predetermined period of time from initial operation of the evaporator assembly, or alternatively, to open the air feedback duct once a detected temperature of cooled air has reached a predetermined value. An embodiment of the invention is described, by way of example only, with reference to the following drawings, in which: Figure 1 is a schematic diagram of an evaporator assembly in accordance with the invention; Figure 2 shows in simplified form a prior art evaporator assembly; and, Figure 3 shows in simplified form the evaporator assembly in accordance with the present invention. A conventional evaporator assembly for incorporation into an air conditioning system is illustrated, in simplified form, in Figure 2 of the drawings. Figure 2 shows a duct 2 comprising duct portions defining an inlet duct 4 having an inlet 6, and an outlet duct 8 having an outlet 10. Inlet duct 4 is arranged for connection to further ducting leading from a position externally of the passenger compartment of the automobile, through which fresh air can be introduced, and/or to ducting connected to the passenger compartment of the vehicle, through which air can be withdrawn from the passenger compartment. A heat exchanger 12 through which a coolant is circulated is arranged in the duct 2 at a position between the inlet 6 and outlet 10, and includes a plurality of heat exchanger coils, ends 14 of a number of which are shown in Figure 2. In the example illustrated, five rows of coils in the air flow direction are employed. A fan 16 is arranged immediately upstream of the heat exchanger 12 to force air through the duct 2 to the outlet 10, and into the passenger compartment. This conventional apparatus operates by drawing air externally of the passenger compartment of the vehicle (fresh air) and/or from the passenger compartment itself, which air is cooled on contact with the heat exchanger 12, and then introduced into the passenger compartment via outlet 10. Figures 1 and 3 show the evaporator assembly in accordance with the invention, those parts in common with the conventional assembly illustrated in Figure 2 being denoted by the same reference numerals. Thus, this system includes a duct 2, comprising an inlet duct 4 having inlet 6, and an outlet duct 8 having an outlet 10. A heat exchanger 12 is disposed between the inlet duct 4 and outlet duct 8. The heat exchanger 12 is an evaporative heat exchanger in which a refrigerant is circulated, which refrigerant is changed from a liquid to gaseous state within the heat exchanger. In use, the heat exchanger 12 is connected to a refrigerant circuit further including, as is conventional, a compressor, condenser, an expansion device and a plurality of connecting plumbing (not shown). A fan 16 is mounted for rotation in the inlet duct 4 in order to draw air into and through the duct 2 and heat exchanger 12. The fan 16 is driven by an electrically powered motor. A feedback duct 18 links the outlet duct 8 downstream of the heat exchanger 12 with the inlet duct 4, upstream of the heat exchanger 12. The duct is arranged so that a proportion of air which has passed through the heat exchanger 12 is recirculated via the feedback duct 18 to the inlet duct 4. In Figure 3 it can be seen that the junction of the feedback duct 18 with the outlet duct 8 is arranged at a sloping face of the outlet duct 8 in order to assist in the division of the air flow. Valve means 20 in the form of, for example, a pivotably mounted vane are provided in the feedback duct 18 at the junction with the outlet duct 8 to allow selective opening and closing of the feedback duct 18, although the valve means 20 may be located elsewhere in the duct, such as at the junction with the inlet duct 4. Control means 22 are provided to effect opening and closing of the valve means 20. The evaporator assembly in accordance with the invention is operated as follows. On initial start up of the cooling operation, the valve means 20 is arranged to be in the closed position, closing the feedback duct 18. Air is drawn by the fan 16 from outside the automobile or from the passenger compartment into and through the duct 2 and through the heat exchanger 12, which on initial start up, can operate effeciently to give rapid and effective air cooling, the entire throughput of the duct 2 being fed directly to the outlet 10, and to the interior of the passenger compartment. Also where the air inside the passenger compartment has become much warmer than ambient air, which occurs where a vehicle has been left in the sun, introducing cooler outside air into the passenger compartment will purge the hot air from the passenger compartment, and the maximum air flow into the passenger compartment is required initially to effect maximum cooling, to a large degree merely by air replacement. A further point is that on start-up there is little or no condensed moisture on the heat exchanger coils, so that supplying the entire throughout to the passenger compartment presents little danger of moisture carry-over. However, as cooling continues moisture condenses on the heat exchanger surfaces and the carry-over of moisture may increase. The control means 22 for the valve means 20 are arranged to open the valve means 20 subsequent to the initiation of cooling. A proportion of the cooled air is thereby allowed to recirculate through the feedback duct 18 to mix with the warmer air in the inlet duct 4, thereby improving the efficiency of the heat exchanger. This recirculation also increases the extraction of moisture from the air as compared to a conventional arrangement, the condensed moisture being collected and led away from the assembly. The control means 22 may be arranged to operate on a timed sequence, i.e. to open the valve means 20 after a predetermined interval of time from initial operation of cooling, or alternatively may be temperature controlled, for example in response to the temperature of air immediately downstream of the evaporator 12 or alternatively may be arranged to operate on the basis of detected humidity. The embodiment of the invention employing the feedback duct 18 is able to utilise a heat exchanger 12 of reduced dimensions in the direction of air flow, for a given cooling effect, in comparison with the conventional arrangement of Figure 2. This is illustrated schematically in Figure 3, where it should be noted that a row of heat exchanger coils three deep can be utilised, in contrast to the conventional arrangement which requires an array of coils five deep. Thus, the size, weight and cost of the evaporator assembly can be reduced over the conventional arrangement for a given cooling requirement. The control means 22 may also be arranged to close the flap 20 on detection of a low fan speed operation and intake of air from outside of the automobile. In such a situation the pressure of air entering the duct, which may be augmented by the movement of the automobile, may exceed the pressure reduction generated by the fan which draws the air through the duct 2; if the feedback duct 18 was not closed in this state there would be a risk that air would enter the feedback duct 18 upstream of the evaporator, circulating in the opposite direction to that required. In a computer model of the assembly, a heat exchanger employing three rows of coils, and having a face area of 0.63 square feet (0.058 m 2 ) was presented with 390 cubic feet per minute (11.0 m 3 min -1 ) of air at 90 o F (32 o C), dry bulb, 75 o F (24 o C), wet bulb. The conditions of the air being passed once through the heat exchanger were determined and 95 cubic feet per minute (2.69m 3 min -1 ) of this air blended with 295 cubic feet per minute (8.35m 3 min -1 ) of air in the original state, this being fed into the heat exchanger. The total calculated heat transfer was within 2% of that for an air-conditioning apparatus employing a heat exchanger having five rows of heat exchanger coils operated at 295 cubic feet per minute (8.35m 3 min -1 ), this being the original air flow rate available for delivery to the air conditioned space.

Claims: An evaporator assembly comprising: a main air duct (2) defining therein an air inlet (6), and an air outlet (10); an air feedback duct (18) having its input at a first location in the main air duct and its output at a second location spaced along the main air duct from the first location; an evaporative heat exchanger (12); means (16) for forcing air to pass through the heat exchanger (12); said forcing means (16) and said heat exchanger being disposed in the main air duct between said first and second locations; wherein, in use, the input of the air feedback duct receives air cooled by the heat exchanger and the air feedback duct provides said air cooled by the heat exchanger directly to the air feedback duct output whereby the feedback duct and the forcing means recirculate said air cooled by the heat exchanger through the heat exchanger again so that the effect of the heat exchanger is increased. An evaporator assembly according to claim 1, further comprising means (20,22) for selectively closing the air feedback duct (18). An evaporator assembly according to claim 2, wherein said means for selectively closing the air feedback duct comprise valve means (20) and control means (22) therefor. An evaporator assembly according to claim 3, wherein the control means (22) are adapted to open said valve means (20) after a predetermined time interval from initial operation of the evaporator assembly. An evaporator assembly according to claim 3, wherein the control means (22) includes a sensor for detecting the temperature of cooled air, and is adapted to open said valve means when the temperature of cooled air reaches a predetermined value. An evaporator assembly according to any preceding claim wherein the means (16) for forcing air comprises a fan (14) disposed in the main air duct. An automobile air conditioning apparatus comprising an evaporator assembly according to any preceding claim. A method of operating the evaporator assembly of any of claims 1-6 comprising: forcing air through said evaporative heat exchanger whereby said air is cooled; and causing recirculation of said cooled air via the feedback duct and again through the heat exchanger whereby the effect of the heat exchanger is increased. A method according to claim 8 comprising closing said air feedback duct (18) on initial operation of the heat exchanger, and subsequently opening the duct to allow recirculation of said cooled air via the feedback duct. A method according to claim 9, in which said opening step is performed after a predetermined time interval from said initial operation of the heat exchanger. A method according to any one of claims 8,9 or 10 further comprising detecting the temperature of said cooled air; and causing recirculation through said feedback duct when said temperature reaches a predetermined value.

Inventors: CLEVELAND DAVID V, CLEVELAND, DAVID V.
Applicants: VALEO CLIMATE CONTROL CORP, VALEO CLIMATE CONTROL CORPORATION
Application year: 1993
Main classification: B60H1
IPC codes: F24F11, B60H1