Frost Control for Space Conditioning

1. Field of the Invention

This invention relates to an apparatus and process for frost control for space conditioning wherein ambient air is passed across a heat exchanger functioning as an evaporator such as in heat pump heating systems, freezers and refrigerator-freezers. More specifically, this invention is particularly applicable to heat pump systems for residential and commercial buildings comprising a compressor, a heat exchanger mounted within the interior of the building being conditioned, and an outdoor heat exchanger subjected to ambient air flow. The heat pump system normally includes a four-way valve for reversing the flow of refrigerant. During the cooling mode of the heat pump system, the indoor heat exchanger is the evaporator for the system, and the outdoor heat exchanger serves as the condenser. During the heating mode, these two heat exchangers trade functions; the indoor heat exchanger becomes the condenser rejecting heat to the interior of the building, while the outdoor heat exchanger becomes the evaporator picking up heat from the ambient air passing through the outdoor coils. More specifically, this invention relates to an improved ambient air heat exchanger wherein the coils of the evaporator heat exchanger are contained in a fluidized bed to enhance heat transfer and to diminish or totally eliminate frost formation on the evaporator coils during absorption of heat. The ambient air heat exchanger of this invention may be an outdoor heat exchanger functioning as an evaporator in the heating mode of a heat pump system, or a heat exchanger functioning as an evaporator in a freezer or a refrigerator-freezer system.

When the ambient air heat exchanger functions as an evaporator, particularly at ambient temperatures near freezing, there is a tendency for the moisture within the ambient air stream to condense and freeze on the evaporator surface which is at or below freezing temperature. Prior art solutions to this problem have focused on various methods to periodically defrost the evaporator coils. However, such systems are quite energy inefficient. This invention provides an energy efficient fluidized bed heat exchanger apparatus and system which transfers heat more efficiently and operates frost-free at near freezing ambient temperatures.

2. DESCRIPTION OF THE PRIOR ART

There have been many prior attempts to control frost accumulation particularly on the outdoor heat exchanger of a heat pump operating in the heating mode. One method common in small residential size heat pumps comprises a momentary mode reversal of the heat pump itself, wherein the flow of refrigerant is reversed changing the outdoor heat exchanger from its evaporator function to a condenser function. Defrost of the outdoor heat exchanger is accomplished by the condensation of hot vapor refrigerant in the outdoor heat exchanger. This method is applied by means of several embodiments differing mainly by the defrost control employed and the components utilized. For example, U.S. Pat. No. 4,007,603 teaches the use of a differential pressure switch across the outdoor evaporator to initiate and terminate the defrost cycle; U.S. Pat. No. 4,024,722 teaches defrost control by monitoring the surface temperatures of selected refrigeration components as well as the ambient atmospheric temperature; and U.S. Pat. No. 4,104,888 teaches defrost control by monitoring an operational parameter of the compressor sensitive to frost accumulation, such as compressor current. U.S. Pat. No. 3,024,620 teaches an outdoor heat exchanger configuration that results in decreased defrost time, while U.S. Pat. No. 3,240,028 teaches defrost time reduction by use of an auxiliary coil immersed in a hot oil bath which superheats the hot vapor refrigerant during defrost. U.S. Pat. No. 3,529,659 teaches the use of radiant heat from hot liquid refrigerant returning from the indoor heat exchanger to warm the air flow upstream to the main outdoor heat exchanger; U.S. Pat. No. 4,171,622 teaches the use of a tandem auxiliary outdoor heat exchanger which acts as a defroster during heating operations and a subcooler during cooling operations; and U.S. Pat. No. 4,178,767 teaches automatic fan motor reversal to blow air downward over the evaporator fins to assist gravity in removing water during the defrost cycle to prevent refreezing of condensate following defrost.

Other embodiments comprise use of bypass valves to reduce the defrost cycle time. For example, U.S. Pat. Nos. 3,274,793 and 3,041,845 teach the use of bypass valves to partly bypass the refrigerant metering device to permit a more rapid loading and heating of the outdoor heat exchanger during the first part of the defrost cycle. U.S. Pat. No. 3,068,661 teaches an increase in the operating temperature of the outdoor heat exchanger during defrost by partly bypassing the hot vapor refrigerant around the outdoor coil, thereby increasing the operating pressure of the indoor heat exchanger; and U.S. Pat. No. 4,158,950 teaches the use of bypass valves upon compressor shutdown to allow a free flow of hot vapor refrigerant into the outdoor heat exchanger until the system temperature equalizes.

Evaporator defrosting by refrigerant flow reversal is both energy inefficient and damaging to equipment. For marginally designed heat pump units, the energy consumption for frost control can amount to as much as 10 percent of the seasonable energy consumption.

Another method comprises the use of direct heat to the evaporator coil. For example, U.S. Pat. No. 3,918,268 teaches the use of direct heating means comprising an electrical resistance heater in thermal contact with the fins of the outdoor heat exchanger such that heat is transferred by conduction. Although simpler and less harmful to the compressor, electrical resistance heat defrosting is characterized by slow response, increased energy inefficiency, and is maintenance prone.

Embodiments comprising heat removal from fluidized beds have focused on the high temperature heat transfer from chemical reaction systems. British Pat. No. 587,774 teaches a method for controlling the reaction temperature in a system wherein the reaction zone is in indirect contact with the fluidized bed. U.S. Pat. No. 4,158,036 teaches the use of a secondary fluidized bed to remove heat from the effluent of an upstream high-temperature fluidized reaction bed. U.S. Pat. No. 4,096,909 teaches a fluidized bed process heater structure wherein the coils are mounted horizontally and are supported by the vessel walls.

This invention provides a frost control apparatus and method applicable to space conditioning systems wherein low temperature ambient air is passed across the evaporator. This invention is particularly well suited for residential and commercial heat pumps, in the heating mode, freezers and refrigerator-freezers. The frost control apparatus of this invention comprises a fluidized bed heat exchanger wherein a fin-tube heat exchanger is contained within a fluidized bed. Frost formation on the evaporator is reduced by closer temperature control of the thermal exchange system and by the continuous abrasive actions of the fluidized bed. Furthermore, with film coefficients of about 35 Btu/hr-.degree.F.-ft.sup.2 attained in a fluidized bed heat exchanger, the overall heat transfer coefficient for the evaporator is increased from about 8 Btu/hr-.degree.F.-ft.sup.2 in conventional fin-tube evaporators, to about 30 Btu/hr-.degree.F.-ft.sup.2 for fin-tube evaporators contained within a fluidized bed. The enhancement of the heat transfer coefficient is not limited to evaporator function. This increase in heat transfer capacity also permits the use of smaller heat exchange area for a given heat load during the condenser function.

It is an object of this invention to provide frost control for the evaporator of a space conditioning apparatus which tends to form frost in its ambient atmosphere which overcomes many of the disadvantages of prior art systems.

It is another object of this invention to enable air to air heat pumps to operate with a higher seasonal coefficient of performance by eliminating the energy inefficient defrost cycles of conventional heat pump systems.

It is yet another object of this invention to enable air to air heat pumps to operate with a higher seasonal coefficient of performance by increasing the heat transfer efficiency of the outdoor heat exchanger.

It is still a further object of this invention to provide a heat pump process comprising the frost control apparatus of this invention.

It is yet another object of this invention to provide improved frost control and enhanced heat transfer capacity for freezer and refrigerator-freezer systems.

These and other objects, advantages and features of this invention will become apparent from the description together with the drawings.

FIG. 1 shows a partial perspective sectional view of an apparatus according to one embodiment of this invention;

FIG. 2 is a simplified schematic flow diagram of one embodiment of the system of this invention; and

FIG. 3 is a simplified schematic flow diagram of a freezer according to this invention.

FIG. 1 illustrates one preferred embodiment of the fluidized bed heat exchange apparatus functioning as an evaporator enclosed in housing 27. The finned-evaporator tubes 19 are contained in fluidized particulate bed 21. Fluidized bed 21 is separated from a non-fluidized support bed 17 by a fine mesh screen 29. The non-fluidized support bed 17 is separated from the distributor support plate 11 by a coarse mesh screen 13. Ambient air is supplied above the threshold fluidization velocity for fluidized particulate bed 21 by a blower means (not shown) directing air through duct 25 to distributor support plate 11. Cooled air leaving the fluidized bed enters the entrainment disengagement zone defined by duct 23 wherein exhaust air is freed of particulates before release to the atmosphere.

FIG. 2 illustrates an embodiment of the process of this invention wherein a reversible heating system of otherwise conventional design includes the fluidized bed outdoor heat exchanger apparatus of this invention in place of the conventional outdoor heat exchanger. During the heating operation, the compressor 42 passes high pressure vapor through the four-way reversible valve 45 and via conduit 47 to the indoor heat exchanger (condenser) 43 where the vapor condenses and rejects heat to the interior. The liquid refrigerant flows via conduit 46 through metering valve 49 to the low pressure fluidized bed outdoor heat exchanger (evaporator) 50 wherein it accepts heat from the ambient air. Low pressure refrigerant vapor returns via conduit 48 through the four-way reversible valve 45 to the low pressure side of the compressor 42.

FIG. 3 illustrates an embodiment of the process of this invention wherein a freezer system of otherwise conventional design includes the fluidized bed evaporator heat exchanger apparatus of this invention in place of the conventional evaporator heat exchanger inside the freezer enclosed space. During the freezer operation, compressor 52 passes high pressure vapor through the four-way reversible valve 55 and via conduit 57 to the condenser heat exchanger 53 exterior to the freezer where the vapor condenses and rejects heat to the exterior of the freezer. The liquid refrigerant flows via conduit 56 through metering valve 59 to the low pressure fluidized bed evaporator heat exchanger 60 wherein it accepts heat from the ambient air within the freezer. Low pressure vapor returns via conduit 58 through the four-way reversible valve 55 to the low pressure side of the compressor 52.

Conventionally designed evaporator heat exchangers consist of a number of turns of tubing carrying the refrigerant and are usually mounted in a horizontal plane. The tubing carries a plurality of closely spaced metal heat exchange fins which extend perpendicular to the tubing and parallel to one another. In order to accomplish the needed heat exchange, electric motor driven fans are conveniently positioned with regard to the outdoor coil. The fans operate to force air through the fins of the outdoor coil, thereby increasing the heat transfer between the ambient air and the refrigerant within the heat exchanger tubing.

The improved ambient air heat exchanger apparatus of this invention comprises an extended surface heat exchanger of conventional design mounted within a fluidized bed. Ambient air is blown or drawn upward through the distributor plate, non-fluidized support bed, and the fluidized bed.

Any distributor plate providing low pressure drop while supporting the non-fluidized support bed may be used. Metallic or ceramic distributor plates are suitable. For example, a sintered 316 stainless steel wire mesh laminate distributor plate may be used.

While not necessary, it is preferred to provide a coarse mesh screen above the distributor support plate to prevent particles in the non-fluidized support bed from passing through or becoming wedged in openings in the distributor support plate. Use of the coarse mesh screen above the distributor support plate permits larger openings in the support plate resulting in lower pressure drop across the support plate. The coarse mesh screen is sized to prevent passage of the particles of the support bed. Suitable mesh for the coarse screen for use in the apparatus of this invention is about 30 to about 40.

The non-fluidized support bed provides better distribution of incident air flow than that provided by use of a distributor plate alone. To ensure the support bed remains non-fluidized during operation, the solid support particles used in the support bed have a density-diameter relationship which prevents fluidization. Preferably the support bed particles have higher density than the fluidized bed particles and have mean diameters of about 0.5 mm to about 1.5 mm and preferably about 0.70 mm to about 0.85 mm. Preferably the particles are spherical. To reduce the pressure drop through the non-fluidized support bed, the ratio of solid support particle mean diameter to solid fluidization particle mean diameter is about 1 to about 10, and preferably about 2.8 to about 4.8. To further maintain a low pressure drop, the non-fluidized support bed used in this invention has a depth of about 0.1 to about 0.5 in. and preferably 0.25 in. to about 0.40 in. Solid support beads of glass or ceramic material are preferred for use in this invention.

While not necessary, it is preferred to have a fine mesh screen between the fluidized bed and the non-fluidized support bed to prevent the smaller particles of the fluidized bed from falling into the non-fluidized support bed. The fine mesh screen is sized to prevent passage of particles of the fluidized bed. Suitable mesh for the fine screen for use in the apparatus of this invention is about 100 to about 120. A second fine mesh screen may be used above the fluidized bed to prevent loss of particulate material. However, it is preferred, to reduce pressure drop across the entire system, to provide an entrainment disengagement zone by increasing the cross-sectional area of the air stream to reduce its velocity to below the threshold velocity so that the particles will fall back into the fluidization zone.

The fluidized beds of solid particles for use in the ambient air heat exchangers of this invention have a depth, when in the fluidized state, of about 0.25 inch to about 2 inches and preferably of about 0.50 to about 0.75 inch. The shallower fluidized beds are desired for the conservation of power required to maintain their fluidized state. Suitable solid particles for the fluidized beds used in this invention have mean particle diameters of about 0.06 to about 0.60 millimeters and preferably about 0.20 to about 0.30 millimeters. Fluidized beds of solid particles are known to the art for thermal transfer and a variety of materials are known to be suitable. Solid particles of silica and alumina are preferred for use in this invention, but any suitable particulate material enhancing heat transfer may be used. Particle attrition can be controlled by using particles with greater hardness than that of ice. Fluidized beds of the above depths and particle sizes may be maintained in a fluidized state by passing gas streams through them at velocities sufficiently high to maintain proper fluidization. It is preferred that the height of the fluidized bed to particle diameter ratio be about 85 to about 350. Spherical particles are most preferred.

When the air flow reaches the threshold velocity for the specific particulate bed, dependent upon particle density and size and bed depth, the particulate bed "expands" and becomes fluidized. Conversely, the air velocity can reach a velocity above which the particles are carried from the bed. Suitable fluidization velocities can be readily ascertained by one skilled in the art. A blower to provide fluidization velocity may be placed below support plate 11 to push air through the bed or may preferably be placed above the particle disengagement zone to draw air through the bed.

An energy efficient method of frost control on the evaporator ambient air heat exchanger of a heat pump in the heating mode, a freezer, a refrigerator-freezer, or the like, is to enhance the heat transfer characteristics of the exchanger sufficiently to permit operation at a tempeature difference, .DELTA.T, (T.sub.evaporator -T.sub.ambient) smaller than the difference between ambient dry bulb and dewpoint temperatures necessary to bring about condensation.

With film coefficients of about 35 Btu/hr-.degree.F.-ft.sup.2 attained in a fluidized bed heat exchanger, the overall heat transfer coefficient for the evaporative exchanger is increased from about 8 Btu/hr-.degree.F.-ft.sup.2 in conventional fin-tube evaporators, to about 30 Btu/hr-.degree.F.-ft.sup.2 for fin-tube evaporators contained in a fluidized bed. The optimal heat transfer is obtained at a fluidization velocity between the minimum and maximum fluidization velocities which may be determined empirically. The enhancement of the heat transfer coefficient provided by the fluidized bed evaporator ambient heat exchanger permits a substantial reduction in .DELTA.T required for normal operation. This increase in heat transfer capacity also permits the use of smaller heat exchange area during the condenser function.

Simple enhancement of heat exchanger effectiveness by increasing the overall heat transfer coefficient may not be adequate to control frost formation in those climates where relative humidity levels can reach the 90 percent range during the heating season. However, enhancement of evaporator effectiveness by the fluidized bed approach utilizing abrasive and/or desiccant action can be relied on to cope with such occasional frosting conditions, and, hence, protect the evaporator in a heat pump system operating in the heating mode or the evaporator in freezers and refrigerator-freezers. The protection of the heat exchange surface from frost or ice formation is enhanced by the continuous abrasive action of the fluidized bed. Abrasive removal of frost is relatively easily accomplished by the mechanical action of a fluidized bed since the ice exists as fine filaments. Ice existing as a continuous film, on the other hand, is more difficult to remove and requires particle momentum considerably greater than ordinarily encountered in conventional fluidization. Increased particle momentum is attained by operating at higher fluidization velocities.

Frosting could be further diminished by drying the ambient air before contact with the evaporator surface. This can be accomplished by using solid desiccant as the fluidized bed particles or by using solid desiccant particles in the non-fluidized support bed. However, energy would be required for desiccant regeneration. Suitable synthetic zeolite desiccants are available in different forms from small pellets to various mesh sized powders.

It will be apparent to one of ordinary skill in the art, upon reading the above disclosure, that the frost control apparatus and process of this invention are applicable to any space conditioning system having an ambient air heat exchanger. By the term "space conditioning" as used throughout this disclosure and the appended claims, we mean conditioning systems such as heat pumps in the heating mode, freezers and freezers in refrigerator-freezers. While the above description has emphasized the application to heat pumps, it is readily apparent that the freezer and refrigerator-freezer applications may be effected in the same manner. In the case of the freezer and refrigerator-freezer, the evaporator is within the space which is cooled to below 0.degree. C. By the terminology "ambient air" as used throughout this disclosure and the appended claims, we mean to include air surrounding the evaporator, the outside atmosphere in the case of heat pumps in the heating mode, and low temperature air comprising the atmosphere inside a freezer or a refrigerator-freezer which may be used for heat input to form an extended surface evaportor heat exchange means connected to such a space conditioning system. By the terminology "freezer" as used throughout this disclosure and the appended claims, we mean an apparatus and process in which the evaporator is within a confined space maintained at the ambient temperature below about 0.degree. C. surrounding the evaporator most of the operating time.

It is an important aspect of our invention in a space conditioning system having an ambient air heat exchanger that a substantially vertical duct means defining a confined passage for the ambient air and has within that duct means a support means extending substantially across the confined passage supporting a plurality of fluidizable solid particles. Blower means capable of fluidizing the fluidizable solid particle bed passes ambient air through the vertical duct means in an upward direction maintaining the fluidizable particle bed in fluidized state during operation. An extended surface heat exchange means for heat transfer connected to the space conditioning system is immersed in the fluidized bed.

The following examples are set forth for specific exemplification of preferred embodiments of the invention and are not intended to limit the invention in any fashion.

From an effective area of heat transfer surface of 300 ft.sup.2, as used with conventional heat pump installations, with a fin efficiency of 50 percent, the .DELTA.T can be expressed as follows: ##EQU1## where:

.DELTA.T=T.sub.(evaporator) -T.sub.(ambient), .degree.F.

Q=Heat load on evaporator, Btu/hr

a.sub.h =effective area of heat transfer, ft.sup.2

U=overall heat transfer coefficient, Btu/hr-.degree.F.-ft.sup.2

For evaporators of conventional design for 50,000 Btu/hr output and U of 8: ##EQU2## For fluidized bed evaporators for 50,000 Btu/hr output and U of 30: ##EQU3##

The example presented above shows that with fluidized bed heat transfer, the heat pump can either operate at significantly lower .DELTA.T's, thus reducing significantly the amount of frosting and/or permit reductions in heat transfer area requirements, thereby reducing cost. The optimum compromise between frost control and cost would have to be determined for each particular case. However, the above example shows heat transfer does offer significant reduction in the frosting of the evaporator coil under high moisture cold ambient air conditions.

Field data was obtained for a whole year on a conventional three ton residential heat pump installation located in a Minneapolis, Minn. residence. This work is described in more detail in Groff, G. C., Reedy, W. R., Investigation of Heat Pump Performance in the Northern Climate Through Field Monitoring and Computer Simulation, ASHRAE Transactions, 84, Part 1, pps. 767-785 (1978). The total energy consumption for frost control was 1517 kWhr or about 7 percent of the total seasonal heat pump consumption. Based on the reported 3249 operating hours, the equivalent installed power requirement for frost control was estimated to be 0.442 kW. Another 0.124 kW was used by the installed fan power requirement of the outdoor heat exchanger and, therefore, a total of 0.566 kW would be the maximum equivalent power requirement available for the fluidized bed heat exchanger. It is estimated one can provide effective frost control using only 0.166 kW for the fluidized bed heat exchanger. The heat transfer area requirement would be decreased 25 percent, but would require slightly larger frontal area. The net energy savings by incorporating a fluidized bed outdoor heat exchanger is estimated to be greater than 6 percent of the seasonal energy consumption.

The space conditioning apparatus and method of frost control of this invention permits smaller ambient air heat exchangers and accommodates greater transient conditions due to enhanced heat transfer and physical prevention of ice formation resulting from the fluidized bed.

This invention provides a method of frost control in an ambient air heat exchanger of a space conditioning apparatus by passing heat exchange medium of the space conditioning apparatus through an extended surface heat exchanger, the extended surface heat exchanger immersed in a fluidizable bed and passing ambient air in thermal exchange relation to the heat exchanger at sufficient velocity to fluidize the bed thereby enhancing heat exchange between the heat exchange medium and ambient air and reducing tendency of frost formation by physical vibration and abrasive action.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.