GB1586308A

GB1586308A - Apparatus for de-frosting low temperature heat exchanger

Info

Publication number: GB1586308A
Application number: GB4176077A
Authority: GB
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 1977-01-03
Filing date: 1977-10-07
Publication date: 1981-03-18
Also published as: JPS5385546A; FR2376379A1; FI773850A; SE7710529L

Description

(54) APPARATUS FOR DE-FROSTING LOW TEMPERATURE HEAT EXCHANGER (71) We, ELECTRIC POWER RESEARCH INSTITUTE, Inc., a Corporation organised under the Laws of the district of Columbia, United States of America, P.O. Box 10412 Palo Alto, State of California 94303, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement The invention concerns apparatus for defrosting low temperature heat exchangers.

A heat pump system generally comprises a compressor unit and a heat exchange system including a relatively low temperature evaporator and a relatively high temperature condenser. Frequently, the compressor and the evaporator heat exchanger are combined in a unit, the compressor being generally disposed interiorly of the cooling coils of the evaporator so that air cooled by the evaporator can be used to cool the compressor.

Recent studies have shown that for a variety of reasons such an arrangement is wasteful of energy. As a result, it has been suggested to separate the compressor from the evaporator, installing the former indoors and the latter outdoors to minimise heat losses during cold weather.

During operation ofthe heat pump, there is frequently a buildup of frost or ice on the heat exchanger coils of the evaporator. The amount of ice that forms is a function of the ambient temperature and the relative humidity, the buildup being greatest at temperatures around 0 C and at a saturated humidity. Since the frost buildup impedes the airflow past the heat exchanger, the unit-must be defrosted periodically. In the past, the defrosting cycle was normally initiated at predetermined time intervals. Of course, the time intervals had to be chosen to effect adequate defrosting when the frost buildup is at a maximum. Consequently, during other temperature and humidity cdnditions, when the frost buildup is less the system defrosts too frequently. This resulted in a significant waste of energy.

Attempts have also been made in the past to initiate defrost cycles by sensing the change in the pressure drop across the heat exchanger. With a buildup of frost the pressure on the downstream side of the heat exchanger decreases. This decrease is sensed with a differential barometric pressure switch. The difficulty with such an arrangement is that the differential pressure switch must close contacts reliably on pressure changes in the order of as little as 1/50th inch W.G. Up to the present it has been difficult or impossible to make such sensitive pressure switches, at least on an economically feasible basis.

Aside from problems of determining the initiation (and termination) of defrost cycles, prior art heat pump systems were less than fully satisfactory as far as the performance of the actual defrost cycle is concerned.

Normally, prior art heat exchangers are constructed so that air flows through them along an essentially horizontal path. Unless previously heated air is uniformly heated and well mixed, there are significant temperature differentials between the upper and the lower portion of the horizontal path - leading to unequal defrost times, local overheating, and the like, all of which is undesirable. As a consequence, defrosting systems have commonly been chosen 'which heat the frosted coil internally by reversing the refrigeration cycle. Such operation, with a warm evaporator and cold condensor, generates abrupt pressure reductions in the system low side, a low refrigerant flow and a high probability of inefficient cooling and lubrication of the hermetically sealed motor-compressor and, therefore, an increased compressor failure rate.

The present invention provides a heat exchanger defrosting system comprising in combination a heat exchanger having a plurality of heat exchange coils for carrying a refrigerant, a multiplicity of generally vertically oriented heat exchange fins in good thermal connection with the coils a defrost heater disposed vertically below the coils, airflow inducing means, airflow directing means surrounding the coils and the heater for directing an induced airflow vertically upwardly past the coils, the heater inducing an airflow by convection when the system is operated in its defrosting mode, and means for initiating a defrost cycle comprising means for directing an auxiliary air current to bypass the heat exchanger, means for sensing a flow rate change of the auxilary current caused by the formation of frost on the heat exchanger, and means for activating the heater in response to sensing a predetermined said change in the auxiliary current.

The present invention provides an efficient and reliable system for defrosting heat exchangers of heat pumps. The heat exchanger is mounted within an upright air duct fitted at its downstream end with a draw-through or suction fan to draw air vertically upward through the heat exchanger. The heat exchanger itself is mounted so that the air flows vertically past it. A defrost heater is positioned in the duct beneath an upstream end of the heat exchanger. The heater can be an electric resistance heater such as are commercially available under the trade designation CALROD in which electric resistance wires are insulated with magnesium oxide and disposed within flat metal tubes. This heater substantially evenly heats the air beneath the heat exchanger so that the air rises by convection upwardly through the exchanger during a defrost cycle.The heretofore common, relatively complicated and expensive reverse cycle defrost systems can be replaced with rugged, low cost, simple to install electric resistance heaters.

The defrost cycle is initiated by sensing the pressure drop across the heat exchanger which, as is above summarized, increases as the frost builds up. The pressure drop, however, is not directly sensed to avoid the above-discussed difficulties encountered with conventional barometric differential pressure sesnors.

Instead, the present invention provides a relatively small diameter bypass pipe which extends through the air duct and terminates downstream of the heat exchanger. As a result, during normal operation of the fan an auxiliary air current is drawn through the pipe into the section between the coil and the fan duct. As long as there is no frost buildup on the heat exchanger and, therefore, no change in the pressure drop across it, the volume of air flowing through the pipe remains constant.

A constant energy output heater is placed inside the pipe to heat the auxiliary air current.

As long as the amount of air flowing through the pipe remains constant, its temperature rise remains also constant. If there is a buildup of frost on the heat exchanger and a resulting increased pressure drop across it, the volume of air flowing through the pipe will increase correspondingly. As a result, the temperature of the heated air will de crease. This decrease in the heated air temperature rise is reliably and inexpensively sensed and used to initiate a defrost cycle.

For this purpose, a temperature sensor is mounted in the pipe downstream of the heater and coupled with a differential temperature switch which gets a second, relatively constant reading from another temperature sensor disposed outside the conduit or inside the conduit where the temperature of air flowing past the heat exchanger remains substantially constant. If a predetermined temperature drop in the auxiliary air current has been determined, the switch is closed which energizes the resistance heater while it deactivates the compressor, the suction fan and the constant energy output heater inside the pipe. The defrost cycle continues until completed as determined, for example, by measuring the temperature of the airflow through the heat exchanger or by measuring the temperature of the heat exchanger itself.

The defrost cycle is simply terminated by again opening the differential temperature switch and operating the system as abovedescribed until the next defrost cycle is required.

It is apparent that the defrost cycle initiator of the present invention eliminates the heretofore common energy wasting fixed time interval, defrost cycle. Also, it eliminates the unreliable and sometimes dangerous low, pressure differential switches. Instead, the present invention indirectly senses the pressure drop across the heat exchanger in a simple, inexpensive, highly sensitive and completely reliable manner. Thus, it assures both better performance of the heat pump and a reduced overall energy consumption.

The invention will now be described further by way of example only, with reference to the single figure of the accompanying drawing which shows an upright, schematic illustration of a heat pump constructed in accordance with the present invention and in particular it illustrates the construction of the heat exchanger, its relative positioning in a surrounding air duct, and the construction of the defrost initiator.

Referring to the drawing, heat pump 2 generally comprises a compressor unit 4 shown mounted separately of and spaced apart from an evaporator heat exchanger 10 and a defrost system 6 therefor. Other conventional heat pump components such as a condenser or an expansion valve are also provided. For simplicity, they are neither separately shown nor described herein. Generally speaking, the defrost system comprises an upright air duct 8 surrounding heat exchanger 10 and having an air intake 12 adjacent a base 14 and an air exhaust 16 at a top 18 of the duct. Normally, the heat exchanger has a rectangular (which for the purposes of this application includes a "square") plan configuration and the air duct has a complementary cross-section.

A suction fan 20 driven by a motor 22 is coaxially mounted within the duct adjacent the top thereof and is supported by a suitable mounting structure 24 which in turn is carried by upright side walls 26 of the duct. During normal operation, the fan draws air through duct air intake 12 and the air rises upwardly in a vertical direction past heat exchanger 10 and is then exhausted.

As was mentioned above, frost frequently forms on the heat exchanger. The amount of frost buildup is a function of the ambient temperature and humidity, and it is largest at temperatures in the vicinity of about 0 C and at a saturated humidity. As the frost builds up on the heat exchanger, it impedes the passage of air and the heat transfer between the air and the heat exchanger.

Consequently, the heat exchanger must be periodically defrosted, and it is the purpose of the present invention to facilitate the defrosting cycle and to render it less wasteful of valuable energy.

The heat exchanger 10 comprises multiple, horizontally oriented heat exchanger coils or conduits 28 which are arranged in a plurality, e.g. three parallel coil layers 30.

A multiplicity of spaced apart, parallel heat exchange fins are thermally coupled, e.g.

brazed to the exterior of each coil 28 and arranged so that air can flow in a vertical direction past the fins and the coils as is indicated in the drawing by the vertical arrows leading through the heat exchangers.

For simplicity, the fins are not individually shown but are only schematically illustrated in the drawing by a generally rectangular box having an outline corresponding to the combined outline of all fins attached to all coils. The individual fins have a rectangular or square shape.

The coil layers 30 are horizontally arranged or slightly inclined relative to the horizontal by an angle not exceeding approximately 10 . It is preferred that the edges 33 of the fins be parallel to the coil layers 30 since such an inclination of the edges facilitates the drainage of water during a defrost cycle.

Mounted beneath, that is upstream of the heat exchanger 10 is an electric resistance heater 34, constructed as above described and secured to the duct side wall 26 in a conventional manner. During a defrost cycle, the resistance heater is energized so that heated air rises by convection upwardly through the spaces between heat exchange fins 32 to melt ice that has formed on the fins and on the coils. In instances in which the coil layers are inclined to the horizontal (as shown in the drawing) an airflow control baffle 36 is provided to assure that the air flows uniformly to all portions of the heat exchanger. The baffle comprises multiple, parallel, upright walls 38 which columnize convection airflow. A more efficient, complete and uniform defrosting of the heat exchanger is thereby obtained.

To control, that is to start and terminate the defrost cycles the present invention provides a defrost cycle initiator 40 which generally comprises an auxiliary air current pipe 42 of a relatively small, e.g. 3/4 inch dimaeter. The pipe extends through duct side walls 26 and has an intake 44 disposed outside the duct and an outlet 46 disposed inside the duct at a downstream duct portion located between the heat exchanger 10 and fan 20.

A flow resisting packing such as stainless steel wool 48 or multiple baffles are placed in the pipe to control the airflow therethrough. For example, the density and depth of the stainless steel wool can be adjusted to produce an auxiliary air current flow of 1.0 CFM during normal operation, that is with fan 20 running and little or no frost buildup on coils 28 and cooling fins 32.

A constant energy output heater 50 energized via a power source 52 is disposed within pipe 42 for heating the air current therethrough. It is apparent that as long as the volume of air (per unit time) remains constant, the heater raises the temperature of the air current by a constant value. If the volume of air flowing through the pipe changes, however, the temperature of the heated air rises or decreases depending on whether or not the volume is smaller or larger. For the above-discussed example in which the pipe packing is adjusted so as to yield a normal 1.0 CFM airflow therethrough, the provision of a heater having a constant energy output of 10 watts leads to a temperature increase of the air flowing through the pipe of approximately 25"F.

A first temperature sensor 54 is disposed within pipe 42 downstream of heater 50 and is operatively connected with a control 56 that includes a main switch. The switch is preferably a differential temperature switch (of an electrical or mechanical construction) and is coupled with a second temperature sensor 58 shown mounted exteriorly of the duct to measure the temperature of the ambient air. Alternatively, the second temperature sensor may be mounted interiorly of the duct (but upstream of the pipe 42) to measure the temperature of air flowing through the duct. For practical purposes, the temperature of this air fluctuates with ambient air temperature fluctuations.

Switch 56 in turn controls a defrost relay 60 so that resistance heater 34 is connected with a power supply 62 to thereby initiate the defrost cycle. Switch 56 at the same time operates a main control relay 63 to shut down the heat pump during the defrost cycle, e.g.

to shut down compressor 4 and fan 22.

Lastly, switch 56 also de-energizes constant energy output heater 50 during the defrost cycle.

In operation and with heat exchanger 10 completely or substantially defrosted approximately 1.0 CFM of air is drawn through pipe 42. This air is heated approximately 25 above the ambient temperature. Switch 56 is set so that resistance heater 34 is deenergized and fan motor 22 is operating at that auxiliary air temperature. As frost builds up between coils 28 and fins 32, the pressure drop across the heat exchanger increases. As a result, the pressure in the downstream portion of duct 8 between the heat exchanger and fan 20 decreases slightly which in turn increases the volume (per unit time) of air flowing through pipe 42.

The increased air volume flowing therethrough causes a corresponding decrease of its temperature rise. Switch 56 is set so that it operates when there is a predetermined reduction in the temperature of the heated air flowing through the pipe, e.g. when the temperature rise of the heated air is reduced by a factor of approximately 50%, or to 12 5 F above ambient in the above example.

Operation of the switch energizes the resist ance heater 34 to defrost the heat exchanger and de-energizes motor 22 as well as power supply 52 for the constant energy output heater 50. The defrost cycle is conventionally terminated as, for example, by another temperature sensor 57 mounted to sense the temperature of air flowing through the heat exchanger or by sensing the temperature of the heat exchanger itself. Upon termination of the defrost cycle the switch 56 is reset to deactivate heater 34 and energize motor 22 and constant energy output heater 50.

To avoid interference form wind, rain and to avoid radiation losses, suitable baffles and protective walls, such as outside cover 64 and inside shield 66 mounted to protect the pipe intake and outlet, respectively, are provided.

WHAT WE CLAIM IS:- 1. A heat exchanger defrosting system comprising in combination a heate exchanger having a plurality of heat exchange coils for carrying a refrigerant, a multiplicity of generally vertically oriented heat exchange fins in good thermal connection with the coils, a defrost heater disposed vertically below the coils, airflow inducing means, airflow directing means surrounding the coils and the heater for directing an induced airflow vertically upwardly past the coils, the heater inducing an airflow by convection when the system is operated in its defrosting mode, and means for initiating a defrost cycle comprising means for directing an auxiliary air current to bypass the heat exchanger, .means for sensing a flow rate change of the auxiliary current caused by the formation of frost on the heat exchanger, and means-for activating the heater in response to sensing a predetermined said change in the auxiliary current.

2. A system according to claim 1, wherein the heat exchanger comprises a multiplicity of parallel coils each fitted with a multiplicity of heat exchange fins, the heat exchanger having a substantially rectangular plan configuration, and wherein the airflow directing means includes a vertically oriented air duct surrounding a peripheral portion of the heat exchanger.

3. A system according to claim 1 or 2, wherein the heat exchanger includes a plurality of coil layers, each layer having several spaced apart, co-planar, parallel coils and being spaced from the coils in the other layer.

4. A system according to claim 3 when dependent on claim 2 wherein the fins have a generally rectangular shape, and wherein an edge of each fin is substantially parallel the layers.

5. A system according to claim 4, wherein the layers are inclined relative to the horizontal.

6. A system according to claim 5, wherein the layers have an angular inclination relative to the horizontal of not more than 10 .

7. A system according to any one of the preceding claims including a baffle assembly disposed between an underside of the heat exchanger and the heater, the baffle assembly defining a plurality of parallel, vertical, independent air passages, whereby an equal heat convection airflow through all portions of the heat exchanger is assured.

8. A system according to any one of the preceding claims wherein the heater comprises an electrical resistance heating element.

9. A system according to any one of the preceding claims wherein the means for directing the auxiliary current comprises means for directing an air current at a point downstream of the heat exchanger from the exterior of the airflow directing means to the interior thereof.

10. A system according to any one of the preceding claims wherein the means for directing the auxiliary air current comprises a relatively small cross-section conduit member having an intake end disposed outside the airflow directing means and an outlet disposed inside the airflow directing means.

11. A system according to claim 10 including a flow reducing packing in the conduit member for limiting the normal flow of the air current through the conduit member to a predetermined amount.

12. A system according to claim 11 wherein the packing is selected to limit the flow of the auxiliary air current in the conduit member to about one cubic foot per minute under normal operating conditions in which

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. energy output heater 50 during the defrost cycle. In operation and with heat exchanger 10 completely or substantially defrosted approximately 1.0 CFM of air is drawn through pipe 42. This air is heated approximately 25 above the ambient temperature. Switch 56 is set so that resistance heater 34 is deenergized and fan motor 22 is operating at that auxiliary air temperature. As frost builds up between coils 28 and fins 32, the pressure drop across the heat exchanger increases. As a result, the pressure in the downstream portion of duct 8 between the heat exchanger and fan 20 decreases slightly which in turn increases the volume (per unit time) of air flowing through pipe 42. The increased air volume flowing therethrough causes a corresponding decrease of its temperature rise. Switch 56 is set so that it operates when there is a predetermined reduction in the temperature of the heated air flowing through the pipe, e.g. when the temperature rise of the heated air is reduced by a factor of approximately 50%, or to 12 5 F above ambient in the above example. Operation of the switch energizes the resist ance heater 34 to defrost the heat exchanger and de-energizes motor 22 as well as power supply 52 for the constant energy output heater 50. The defrost cycle is conventionally terminated as, for example, by another temperature sensor 57 mounted to sense the temperature of air flowing through the heat exchanger or by sensing the temperature of the heat exchanger itself. Upon termination of the defrost cycle the switch 56 is reset to deactivate heater 34 and energize motor 22 and constant energy output heater 50. To avoid interference form wind, rain and to avoid radiation losses, suitable baffles and protective walls, such as outside cover 64 and inside shield 66 mounted to protect the pipe intake and outlet, respectively, are provided. WHAT WE CLAIM IS:-

1. A heat exchanger defrosting system comprising in combination a heate exchanger having a plurality of heat exchange coils for carrying a refrigerant, a multiplicity of generally vertically oriented heat exchange fins in good thermal connection with the coils, a defrost heater disposed vertically below the coils, airflow inducing means, airflow directing means surrounding the coils and the heater for directing an induced airflow vertically upwardly past the coils, the heater inducing an airflow by convection when the system is operated in its defrosting mode, and means for initiating a defrost cycle comprising means for directing an auxiliary air current to bypass the heat exchanger, .means for sensing a flow rate change of the auxiliary current caused by the formation of frost on the heat exchanger, and means-for activating the heater in response to sensing a predetermined said change in the auxiliary current.

there is substantially no frost build-up in the heat exchanger.

13. A system according to any one of the preceding claims, wherein the sensing means comprises heater means disposed in the auxiliary current and having a constant energy output, and temperature sensing means for determining a temperature change in the auxiliary current resulting from variations in the auxiliary current flow rate due to the formation of frost on the heat exchanger.

14. A system according to claim 13 wherein the temperature sensing means comprises first means for sensing the temperature of the auxiliary air current, and second means for sensing a substantially constant reference temperature.

15. A system according to claim 14, wherein the second means comprises means for sensing the temperature of the ambient air exterior of the airflow directing means.

16. A system according to claim 14, wherein the second means comprises means for sensing the temperature of air flowing past the coil assembly.

17. A system according to any one of the preceding claims wherein said airflow inducing means is disposed downstream of the heat exchanger and of the means for directing the auxiliary air current.

18. A system according to claim 17 when dependent on claim 13, wherein the activating means includes means responsive to the temperature sensing means for initiating a defrost cycle for the heat exchanger in response to a predetermined decrease in the temperature of the heated auxiliary air current, whereby a reduction in the airflow past the exchanger causes the airflow inducing means to increase the rate of flow of the auxiliary air current and thereby cause a corresponding reduction in the auxiliary air current temperature and the initiation of the defrost cycle in response to a predetermined frost build-up on the heat exchanger.

19. A system according to claim 13, when dependent on claim 10, wherein the heater means is mounted interiorly of the conduit member.

20. A system according to claim 13 when dependent on claim 10 wherein the initiating means comprises second temperature sensing means for sensing the temperature of one of the ambient air and of the air flow in the airflow means downstream of the heat exchanger and upstream of the conduit member outlet, and means for starting the defrost cycle when the temperature rise of the heated auxiliary air current has been reduced by about 50% over the temperature rise encountered when the heat exchanger is substantially frost-free.

21. A system according to claim 17, wherein the airflow directing means comprises an upright duct and the heat exchanger is disposed within the duct and constructed for flowing air vertically past it, and wherein the defrost heater comprises an electrical resistance heater disposed vertically beneath the heat exchanger for heating air during the defrost cycle so that such heated air rises past and defrosts the heat exchanger, and including means for deactivating the airflow inducing means and for energizing the resistance heater during operation of the heat pump in its defrost mode.

22. A system according to any one of the preceding claims including means for deactivating the heater means when the heat pump is in its defrost mode, and for reenergizing the heater means in response to a return of the heat pump from its defrost mode to its normal mode.

23. A heat defrosting system substantially as hereinbefore described with reference to and as illustrated in the single figure of the accompanying drawing.