Field of the Invention

This invention relates generally to a closed loop vapor cycle refrigeration system, and more particularly to apparatus and methods for defrosting the evaporator coils of the refrigeration system.

Description of the Related Art

Refrigeration systems, such as used in supermarkets for cooling food storage fixtures, contain a compressor system having one or more compressors for compressing a refrigerant fluid, a condenser for condensing the compressed refrigerant to a liquid, one or more evaporator systems, each such evaporator system often having a plurality of parallel evaporator coils with associated expansion valves, each evaporator coil being used to cool a different fixture. The different fixtures are typically used to store different products, such as the dairy products, meat products, frozen foods, etc. The refrigeration demand on different fixtures is generally different and such fixtures are often kept at different temperatures.

During normal operation of the refrigeration system, the evaporators operate at temperatures low enough to cause water vapor to crystallize on the evaporator coils, producing "frost" which reduces the efficiency of the refrigeration system. The rate at which the ice builds up on a particular fixture depends upon the type of the fixture, the load on the fixture, the temperatures of the fixture and refrigerant, and the humidity of the air within the fixture being cooled.

As a result, the surfaces of the evaporator coils must periodically be defrosted. The frequency with which a particular evaporator must be defrosted depends on the rate at which ice builds up, the cooling load on the evaporator, and the rate at which it can be defrosted. In general, the length of the defrost period is determined by the degree of ice accumulation on the evaporator and by the rate at which heat can be applied to melt off the ice. Ice accumulation will therefore vary with the type of installation, the conditions inside the fixture, and the frequency of defrosting.

Defrosting may be accomplished in a number of different ways, each of which can be classified as either "natural defrosting" or "supplementary-heat defrosting" according to the source of heat used to melt the ice from the evaporator coils. Natural defrosting utilizes the heat of the air in the refrigerated fixture to melt the frost from the evaporator, whereas supplementary-heat defrosting is accomplished with heat supplied from sources other than the fixture air. Common sources of supplementary heat include electric heating elements and hot gas from the discharge of the compressor. All methods of natural defrosting require that the evaporator system be shut down

for a period of time sufficient for the temperature of refrigerant in the evaporator to rise to a level well above the melting point of the ice.

Another common method is reverse cycle defrosting. The hot gas refrigerant from the exhaust of the compressor or the cooler gas from the receiver flows into the outlet of the evaporator such that the gas heats the cold evaporator by condensing to the liquid state.

Various apparatus and methods have been used for reverse cycle defrosting of the evaporator coils. One common method for reverse cycle defrosting includes a one-way check-valve placed in parallel with an expansion valve at the inlet end of each evaporator coil. Such a check-valve contains a compression spring that determines the pressure differential for the check-valve. When the pressure drop across the check-valve is greater than the set pressure differential for tiiat check-valve, it opens and remains open as long as the pressure drop remains above the pressure differential for that valve. The pressure differential for the check-valves varies due to the variation in the compression force of the springs. As an example and not by way of limitation, the pressure differential range for a set of one way check-valves used in a refrigeration system may be between 0.2 to 0.8 psi. One of the problems widi check valves in reverse-cycle defrost, is that shortly after initiating defrost, the frost melts and the refrigerant then ceases to condense, causing the refrigerant to remain as a gas as it flows through the check valve.

To effect reverse cycle defrost of the evaporator coils, the flow of liquid refrigerant is stopped and compressed gas refrigerant is discharged into the outlet ends of the evaporator coils (reverse flow). Because each evaporator coil is at a relatively low temperature, the compressed gas condenses in each of the evaporator coils as it gives up heat to the cold evaporator coil. The pressure drop across the check-valves causes the check-valves to open allowing condensed refrigerant to discharge from what are normally the inlets to the evaporator coils.

The evaporator coils tend to defrost at different rates due to the varying nature of the fixtures and the variable amount of the ice that builds up. One of the difficulties of the prior art defrosting systems is that upon turning off the flow of liquid refrigerant from the receiver to the evaporators, a significant pressure drop develops in reverse flow through one or more evaporators. When an evaporator coil has become sufficiently warm, the compressed refrigerant ceases to liquefy in that coil and the gas passes in reverse flow through the inlet end of that coil to other evaporator coils or to other elements of the refrigeration system, which is highly undesirable. If the pressure drop in reverse flow across the check valves at the inlet of the evaporator coils becomes significant, different pressure drops may be created between the inlets of the various evaporator coils. Thus, if the pressure drop across one check valve is greater than the pressure drop across another check valve, the refrigerant will tend to flow through the smaller pressure drop and die evaporator coil with the larger pressure drop will no longer defrost. Additionally, the

check-valve having the least pressure differential remains open as long as the gas is being discharged into its associated evaporator coil while the remaining check-valves may remain open for shorter periods of time or may not open at all, thereby causing only some of the evaporator coils to defrost adequately. The refrigeration system may therefore need to be shut down for much longer periods of time to allow the remaining coils to defrost, which also is not desirable.

It is, therefore, desirable to have a refrigeration system which eliminates or reduces the above-identified problems and provides a more efficient means for defrosting the evaporator coils. The present invention overcomes the above-identified problems and provides apparatus and methods for efficiently defrosting the evaporator coils of such refrigeration systems.

Summary of the Invention

The present invention provides a closed loop vapor cycle refrigeration system that includes a compressor for compressing a refrigerant fluid, a condenser for condensing the compressed gas refrigerant into a liquid refrigerant, a receiver for storing the liquid and compressed gas refrigerants, at least two parallel evaporator coils for evaporating the liquid refrigerant to the low pressure gas refrigerant, control valves for discharging either the compressed gas refrigerant or liquid refrigerant into the evaporator coils for defrosting die parallel evaporator coils and flow controls for controlling the flow through the evaporator coils during die defrost cycle.

In one embodiment of the present invention, die flow controls include an electronic flow control valve for controlling the flow of the refrigerant during normal operation and also during the defrost cycle. In such an embodiment, an electronic flow control valve is placed at what is normally the inlet end of each of the parallel evaporator coils. Temperature and/or pressure sensors are placed at the inlet end and at the outlet end of each of d e parallel evaporator coils. During the reverse flow defrost cycle, the compressed gas refrigerant is discharged into what is normally the discharge of each of die parallel evaporator coils. The compressed gas refrigerant condenses in Λe evaporator coils and discharges dirough the inlet end of die evaporator coils.

A control circuit controls die flow of die refrigerant dirough each of die flow control valves to minimize die flow of gas passing dirough any of die control valves during die defrost cycle. This may be accomplished by ensuring diat die refrigerant liquid is subcooled. The control circuit and control valves, in conjunction wid temperature and/or pressure sensors are used to maintain sub-cooled liquid as it leaves die coil. Thus, die gas refrigerant is apportioned between the evaporator coils so that die tiiermal energy transferred to d e evaporator coils by die compressed gas refrigerant during die reverse flow defrost cycle is distributed appropriately. Thus an evaporator coil diat is more heavily frosted will receive more diermal energy during die defrost cycle d an a coil diat is only lightly frosted. Defrost is terminated by closing die control valve

when die temperature at die inlet end of its associated evaporator coil reaches a predetermined value.

In anotiier embodiment of die present invention, reverse flow of liquid refrigerant is used to defrost me evaporator coils. In this embodiment liquid refrigerant, rather dien refrigerant gas, from a refrigerant receiver or liquid line passes dirough the evaporator coils in reverse flow. The defrosting refrigerant liquid is subcooled by losing heat in reverse flow, melting accumulated frost on the evaporator coils.

In yet anodier embodiment of die present invention, die flow controls include an expansion valve coupled to die inlet end of each of die parallel evaporator coils. A one way check-valve is serially coupled to a velocity pressure drop means and placed in parallel with each of die expansion valves. During die defrost cycle, die compressed gas refrigerant is discharged into die outlet ends of the parallel evaporator coils. The velocity pressure drop means causes die pressure drop across die combination of die velocity pressure drop means and its associated one-way check valve to be greater than the largest pressure drop across any one of the check-valves used in an evaporator system, tiiereby ensuring diat all check-valves remain open during die entire defrost cycle regardless of die difference in die differential pressure of die check-valves.

An additional advantage of die present invention is that either embodiment described above may be implemented in existing refrigeration systems to increase defrost effectiveness.

Important features of die present invention have been broadly summarized above in order mat the following detailed description diereof may be better understood, and in order diat die contribution to die art may be better appreciated. There are, of course, many additional features of die present invention diat will be described in detail hereinafter and which will form die subject of the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of die drawings of die present invention wherein like elements have been identified by like numerals.

FIG. 1 shows a closed loop vapor cycle refrigeration system according to die present invention. FIG. 2 is schematic diagram of die refrigerant flow control system utilizing a velocity pressure drop means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to Figure 1, there is shown an embodiment of a closed loop vapor cycle refrigeration system 10 according to die present invention. Refrigeration system 10 contains a plurality of parallel compressors 12, 13 for compressing a low pressure gas refrigerant 14 to a high pressure, high temperature gas refrigerant 16, a condenser 18 for condensing die compressed gas refrigerant 16 into a liquid refrigerant 20, a reservoir or receiver 22 for storing the liquid and compressed gas refrigerants 14, 20, a plurality of evaporator systems 30, 40 each having at least two parallel evaporator coils 32, 34 and 42, 44 respectively, means, such as control valves 120, 124, for selectively discharging die compressed gas refrigerant 14 into die evaporator coils 32, 34 and 42, 44 of die evaporator systems 30, 40 during die defrost cycle, flow controls 36, 38 and 46, 48 coupled to die inlet end of each evaporator coil 32, 34 and 42, 44, respectively, for controlling die refrigerant flow through dieir associated evaporator coils during die defrost cycle and during die normal operation of die refrigeration system 10. The operation of die refrigeration system is controlled by a control circuit 50.

Compressors 12, 13 d en are coupled at dieir inlet end 52, 53 to a suction manifold 54 and at dieir outlet end 56, 57 to the condenser coil 58 of condenser 18 via a line 60. Compressors 12,

13 receive die low pressure gas refrigerant 14 from die suction manifold 54, compress it to die high pressure, high temperature gas refrigerant 16 and discharge the compressed gas refrigerant 16 into die condenser coil 58. A temperature sensor 62 is placed in die line 60 to provide signals (information) representative of the temperature of die compressed gas refrigerant 16 in die line 60. Also, temperature sensors 64, 65 are coupled to the compressors 12, 13 for providing signals representative of die temperature of the compressor crank cases.

The compressed gas refrigerant 16 condenses in die condenser 18 as air 66 is passed across die condenser coil 58 by a fan 68. The liquid refrigerant 20 from die condenser coil 58 discharges dirough a liquid return line 72 and into the receiver 22. A pressure sensor 74 and a liquid level sensor 76 are coupled to die receiver 22 for respectively providing signals representative of die pressure and die level of die liquid refrigerant 78 in die receiver 22. The liquid refrigerant 78 from die receiver 22 discharges dirough a solenoid operated valve 80 and into a manifold 82 containing a plurality of liquid lines, such as lines 84 and 86, to evaporator systems 30, 40. Solenoid operated valve 80, placed between die receiver 22 and die manifold 82, permits die liquid refrigerant 78 from die receiver 22 to flow into d e manifold 82. A pressure differential valve 81 is coupled in parallel widi solenoid operated valve 80. Pressure differential valve 81 may have a differential pressure setting, such diat when solenoid valve 80 is closed, liquid refrigerant 78 flows from receiver 22 to manifold 82 if die receiver pressure is greater dian die manifold pressure by a predetermined amount.

The liquid refrigerant 78 from the manifold 82 passes to die evaporator systems 30, 40 respectively via liquid lines 84 and 86. The liquid refrigerant on liquid line 84 passes through flow controls 36, 38 and into parallel evaporator coils 32, 34, respectively. Likewise, the liquid refrigerant from liquid line 86 passes dirough flow controls 46, 48 and into parallel evaporator coils 42, 44, respectively. The refrigeration system 10 of die present invention however, may contain any number of evaporator systems, each such system having any number of evaporator coils. The refrigeration system 10 may contain only one evaporator system, such as die evaporator system 30, having parallel evaporator coils, such as coils 32 and 34, or it may contain a plurality of evaporator systems, each evaporator system having any number of evaporator coils. The flow controls 36, 38 and 46, 48 operated according to die present invention are placed between the inlet ends 88 Of coils 32, 34 and 42, 44 of each parallel evaporator coil and die manifold 82. In Figure 1, flow controls 36, 38 are respectively coupled between the evaporator coils 32, 34 and die liquid line 84. Similarly, flow controls 46, 48 are respectively coupled between the inlet ends 88 of die evaporator coils 42, 44 and the liquid line 86. A temperature sensor is placed at the inlet end 88 and at the outiet end 89 of each evaporator coil for providing signals representative of die temperatures at such inlet end 88 and oudet end 89. In die refrigeration system 10 of Figure 1, temperature sensors 90, 92 are coupled to die inlet end 88 and outlet end 89 respectively, of the evaporator coil 32. Similarly, temperature sensors 94, 96 are coupled to the evaporator coil 34, temperature sensors 98, 100 to die evaporator coil 42 and temperature sensors 102, 104 to the evaporator coil 44. Additionally, a temperature sensor is placed at each evaporator coil to provide signals representative of die discharge air temperature for each such evaporator coil. Temperature sensors 106, 108, 110, and 112 respectively provide signals representative of die temperature of die discharge air for dieir associated evaporator coils 32, 34, 42, and 44. Additional sensors may be used in die refrigeration system 10 to obtain information about o er system parameters, such as die compressor oil pressure, suction pressure, fan speed, etc.

The outlet ends 89 of die evaporator coils of each evaporator system are coupled to die compressors 12, 13 via a common suction line manifold 54. The outlet ends 89 of die evaporator coils 32, 34 are coupled to die suction line manifold 54 via a suction line 114 while die outlet ends 89 of the evaporator coils 42, 44 are coupled via a suction line 116. Flow control valves 24, 26 are respectively placed in die suction lines 114 and 116 to control die flow of die refrigerant from die evaporator systems 30, 40 to die suction line manifold 54 and hence die compressors 12, 13.

A line 118, coupled to die receiver 22, provides access by die evaporator systems 30, 40 to die compressed gas refrigerant in die receiver 22. Line 118 is also coupled to die suction line 114 to provide passage for die compressed gas to the outlet end 89 of the evaporator coils 32, 34

of die evaporator system 30. A control valve 120 is placed in die line 118 to control die flow of the gas refrigerant to die evaporator coils 32, 34. Similarly, a line 122 and a control valve 124 provide passage of the gas refrigerant to the coils 42, 44 of die evaporator system 40. Alternately or in addition to the line 118, a line 118A widi a control valve 120A may be provided to discharge the compressed gas refrigerant from the line 60 to die line 118 and hence the evaporator coils 32,

34.

As noted earlier, the operation of die refrigeration system 10 of the present invention is controlled by a control circuit 50. Such a control circuit preferably is a microprocessor based circuit. A microprocessor based circuit typically contains, among odier diings, a microprocessor, analog to digital converters, switching circuitry, memory elements and odier electronic circuitry.

The use of circuits containing microprocessors and circuits containing discrete electronic components to control die operation of refrigeration systems is known in die electrical engineering art and is, tiierefore, not described in greater detail here.

The control circuit 50 is operatively coupled to each of the sensors via input ports 126 for receiving electrical signals from die sensors and is coupled via output ports 128 to the refrigeration system elements, such as compressors 12, 13, fan 68, control valves 24, 26, 120 and 124, and die flow controls 36, 38, 46 and 48 for controlling die operation of die refrigeration system 10. The control circuit 50 receives signals from die various sensors in die refrigeration system 10 and in response diereto and in accordance with programmed instructions controls die operation of die various system elements.

During normal operation, die flow control valves 120,124 remain closed while die valves 24,26 remain open. Compressors 12, 13 receive die low pressure gas 14 from die evaporator systems 30,40 via the suction line manifold 54 and compress die low pressure gas 14 to a high pressure, high temperature gas refrigerant 16. The compressed gas refrigerant 16 passes via die line 60 to die condenser coil 58, wherein it condenses as die air 66 is passed over die condenser coil 58 by die fan 68. The air passing over die condenser coil 58 removes tiiermal energy from die gas refrigerant 16 in die coil 58, diereby causing die gas refrigerant to condense.

The liquid refrigerant 20 from die condenser coil 58 discharges via the liquid return line 72 into die receiver 22. The liquid refrigerant 78 from die receiver 22 passes to the evaporator coils 32, 34 and 42, 44. The flow controls 36, 38 and 46, 48 meter die refrigerant flow into their associated evaporator coils 32, 34 and 42, 44, respectively. The liquid refrigerant 78 evaporates in die evaporator coils 32, 34 and 42, 44 into a low pressure gas refrigerant and discharges into the associated suction lines 114, 116. For example, die low pressure gas from die evaporator coils 32, 34 discharges into die suction line 114 while die gas refrigerant from die evaporator coils 42, 44 discharges into the suction line 116. The low pressure gas refrigerant 14 from die suction lines

114, 116 and die like discharges into die common suction line manifold 54, from where it is compressed by d e compressors 12, 13, repeating die closed loop vapor cycle.

During normal operation of die refrigeration system 10 described above, die control circuit 50 receives signals from the various sensors in the refrigeration system 10 and in response diereto and in accordance widi instructions provided to die control circuit 50 by a software means controls the operation of die various system elements including refrigerant flow into the evaporator coils 32, 34 and 42, 44. For example, the control circuit 50 may be programmed to control die refrigerant flow into an evaporator coil as a function of die superheat, which may be measured as die difference between the temperature at the coil outlet 89 and die temperature at the coil inlet 88. Also, as an example, die operation of die compressors 12, 13 may be controlled as a function of die suction pressure. Similarly, the operation of other system elements may be controlled as a function of certain desired system parameters. Additionally, odier control criteria may be used to control die operation of die elements of the refrigeration system 10. The apparatus and methods used in die refrigeration system 10 of the present invention during die defrost cycle are described below.

In one embodiment of the present invention, die flow controls 36, 38 and 46, 48 include electronic control valves connected and controlled by control circuit 50. The operation of die refrigeration system 10 during the defrost cycle when electronic control valves are used is described below widi respect to die evaporator system 30, and is equally applicable to die other evaporator system 40 in die refrigeration system 10.

To effect defrost of die evaporator coils 32, 34 in die evaporator system 30, reverse flow is effected dirough evaporator coils 32, 34. The solenoid operated valve 80 is actuated to the closed position. This allows flow of liquid from receiver 22 to manifold 82, only dirough pressure differential valve 81. Pressure differential valve 81 will allow flow only when die pressure across it exceeds a direshold value, for example, 20 psi. When die pressure in the manifold 82 drops below die pressure in die receiver 22 by die direshold value of die pressure differential valve 81, die pressure differential valve 81 opens and discharges die liquid refrigerant 78 into die manifold 82. Thus pressure differential valve 81 will allow liquid to flow from receiver 22 to manifold 82 only if die pressure in receiver 22 exceeds die pressure in manifold 82 by 20 psi or more. Valve 24 is closed to prevent fluid communication between die evaporator coils 32, 34 and die compressors 12, 13. Valve 120 is opened to allow gas refrigerant 14 to discharge from receiver 22 via line 118, into die outlet ends 89 of die evaporator coils 32, 34 diereby reversing its normal flow direction. The gas refrigerant dien passes dirough the coils 32, 34 releases heat, condenses to a liquid refrigerant, and may be subcooled, as it passes dirough die evaporator coils 32, 34, which are at a relatively low temperature. The electronic control valves may also be

controlled to maintain subcooling of die refrigerant in reverse flow across the evaporator coils 32, 34. Subcooling control may be maintained by modulating die duty cycle of die pulse modulated solenoid valve 120. For example, when the amount of subcooling increases, the refrigerant flow is increased. Similarly, when die amount of subcooling decreases, the flowrate of refrigerant is decreased.

As the gas refrigerant condenses to a liquid refrigerant, it gives up thermal energy (heat) thereby heating die evaporator coils, melting die ice, and defrosting the evaporator coils. The electronic control valves used as flow controls 36, 38 are opened to allow die liquid refrigerant to pass to die manifold 82 and to the odier evaporator systems such as system 40. During the defrost cycle, flow control is performed across each evaporator coil 32, 34 and die control circuit 50 continually monitors the temperature of die refrigerant at die inlet end 88 and outlet end 89 of each of die evaporator coils 32, 34. The control circuit 50 receives signals from die various sensors in die refrigeration system 10 and in response diereto and in accordance widi instructions provided to die control circuit 50 by a software means controls the operation of die various system elements including refrigerant flow into the evaporator coils 32, 34 and 42, 44.

For example, when die temperature of the refrigerant at die inlet end 88 of a particular coil 32 reaches a predetermined temperature as compared to die temperature at die outlet end 89, die control valve 36 associated widi diat coil 32 throttles down, decreasing die flowrate of refrigerant to the remaining coil 32. Alternatively, control valve 36 may also be controlled based on only the measured temperature and pressure at the inlet end 88 of a particular coil 32. This allows calculation of die amount of subcooling by control circuit 50, and allows control of control valve 36 to provide subcooling of refrigerant flowing dirough coil 32 during defrost. In practice, die evaporator coils 32, 34 tend to defrost at different rates due to die differences in die amount of product stored in die fixtures, d e amount of ice diat has been accumulated on die coils, and die temperature of die coils. By apportioning die flow of refrigerant between evaporator coils 32, 34 and 42, 44, the thermal energy transferred to die evaporator coils during die defrost cycle is distributed only where needed. This optimizes die defrost cycle, and minimizes me energy required to defrost a refrigeration system with multiple evaporator coils.

As mentioned, die refrigerant flow is preferably controlled to maintain subcooling of die refrigerant in reverse flow across die evaporator coils 32, 34. Subcooling may be monitored by die control circuit 50 via monitoring of die temperature and pressure of die refrigerant at die inlet end 88 of each of die evaporator coils 32, 34. Alternatively, die difference in temperature between Λe oudet end 89 and die inlet end 88 of each of die evaporator coils 32, 34 may be monitored and used to control die amount of subcooling. Subcooling control may be maintained by pulse modulating die solenoid valve 120. For example, when the amount of subcooling as determined

by control circuit 50 increases, die refrigerant flow is increased. Similarly, when die amount of subcooling as determined by control circuit 50 decreases, die flowrate of refrigerant is decreased.

When the defrost cycle is complete, i.e. die temperature of die refrigerant at the inlet end 88 of each of the parallel evaporator coils 32, 34 has reached the predetermined temperature, preferably above die freezing point of water, valve 120 closes to shut off die gas refrigerant supply to die oudet ends 89 of evaporator coils 32, 34. Valve 24 is then opened and solenoid operated valve 80 is deactuated to place it in its normal open position, allowing direct flow of refrigerant from receiver 22 to manifold 82, to resume die normal operation of die refrigeration system 10. The above described apparatus and method provides an effective defrost means wherein each evaporator coil 32, 34 is controlled independent of die odier and which prevents excessive discharge of the gas refrigerant from die evaporator coils 32, 34 being defrosted to odier evaporator coils 32, 34 or odier elements of the refrigeration system 10. The energy required to defrost die evaporators is tiius minimized by distributing the thermal energy transferred during die defrost cycle only to where it is needed.

The cost of defrosting is reduced because, during defrosting and melting of die ice, die liquid refrigerant passing dirough die evaporator coils 32, 34 is subcooled and thus refrigeration is performed on die refrigerant liquid by die melting of die ice. The cooling diat was stored in die frost on die evaporator coils is recaptured by subcooling die refrigerant. Referring now to Figure 2, diere is shown anodier embodiment of die present invention which may be used as die flow controls 36, 38 during defrost instead of die electronic control valve. The flow control apparatus 130 of this embodiment includes a valve 132 coupled to the coil inlet 88. A serial arrangement of a one way check-valve 134 and a velocity pressure drop means 136 is placed in parallel widi die valve 132. The flow dirough line 88 is controlled in reverse flow by die velocity pressure drop means 136.

The velocity pressure drop means 136 includes a line 138 having a restriction 140. The length of the restriction 140 is die same for all evaporator coils.

The pressure of fluid flowing dirough restriction 140 will decrease as die fluid passes through die restriction. The size of the line 138 and die size of die restriction 140 determine die pressure drop, for a given flow rate, across die velocity pressure drop means 136. The amount of refrigerant flow dirough 88 depends on the refrigerant flowing dirough restriction 140 and its physical properties, i.e., whedier it is liquid or gas. Once defrosting is complete, small amounts of gaseous refrigerant begin to pass dirough die line 88. A small restriction allows relatively more liquid refrigerant dirough die line dian it does gaseous refrigerant. Gaseous refrigerant is one-tenth

(1/10) to one-fifteenth (1/15) die volume of die liquid. Thus, die restriction is a flow limiting device at die completion of die defrost cycle.

The device 136 is designed so tiiat d e pressure drop across die device is equal to or greater tiian die largest pressure differential of any of die one-way check valves used in die parallel evaporators 32, 34. Generally, it is desirable to use velocity pressure drop devices 136 which have a pressure drop that is substantially greater than the pressure drop of any of die one-way check valves of die evaporator system 30. This ensures mat during die defrost cycle, all check-valves remain open when die gas refrigerant is being discharged into the evaporator coils 32, 34 during die defrost cycle, diereby assuring diat all evaporator coils 32, 34 will defrost. Widi the velocity restriction, die problem of die check valves not allowing gas refrigerant to flow through is solved because the liquid is able to pass through the restriction 140. However, once die frost has melted and disappeared, the flow of the mass of refrigerant in the gaseous phase is reduced dirough die restriction. Thus, in this embodiment, die restriction 140 acts as a velocity flow controller, and minimizes die transfer of diermal energy to an evaporator coil tiiat needs no further defrosting.

Inanodier embodiment of die present invention employing liquid, radier dian gas, refrigerant in reverse flow, d e apparatus required to effect defrost is essentially die same. In mis case die connection of line 119 widi valve 121 to receiver 22 is made below die level of the liquid refrigerant 78 in receiver 22. This ensures tiiat defrost is performed by refrigerant liquid. Line 119 and die evaporator coils are flushed of liquid following defrosting by die gas from line 118A by cycling valve 120A on and 120 off at die end of die defrost cycle.

The operation of die refrigeration system using me alternative embodiment of Figure 2 as the flow control means is described below widi reference to tiie evaporator system 30. This explanation equally applies to odier evaporator systems in die refrigeration system 10, such as die evaporator system 40. During die defrost cycle, die gas refrigerant is discharged into die evaporator coils 32, 34 of die evaporator system 30. The gas refrigerant condenses into a liquid refrigerant in die evaporator coils 32, 34 and die one way check-valves open because die pressure drop between the gas refrigerant and die line 84 is greater dian die direshold pressure differential of die one way check-valves, diereby allowing die refrigerant to pass in reverse flow from die evaporator coils 32, 34 to die line 84. The velocity pressure drop means 136 assures at die combined pressure drop of check valve 134 and restriction 140 remains above die direshold pressure drop value of each of d e one-way check valves in die evaporator system 30, thereby ensuring at all such check valves will remain open during die defrost cycle. When die desired amount of defrost has occurred, die control valve 120 is closed to resume the normal operation of the refrigeration system 10.

The pressure drop means 136 provides a relatively inexpensive mechanical means for ensuring diat refrigerant will continue to flow dirough each of die parallel evaporator coils 32, 34 during die entire defrost cycle, diereby ensuring that tiiermal energy is distributed to coils tiiat are frosted and therefore that each such coil 32, 34 will defrost. In die prior art refrigeration systems using one-way check valves to control die refrigerant flow, die one-way check valve having die lowest pressure drop will remain open while die remaining check valve may remain closed, diereby not effectively defrosting all die evaporator coils. Such prior art systems also allow the gas refrigerant from the evaporators to pass into the line 84 and thereby to other evaporator systems, such as system 40, which as described earlier is highly undesirable. The above-described apparatus and metiiod provides a more efficient means for effecting die defrost of the evaporator coils 32,

34 and 42, 44 in a refrigeration system 10 compared to a system utilizing check-valves alone, and also reduces die discharge of the gas refrigerant dirough the evaporator coils 32, 34 and 42, 44 during die end of die defrost cycle.

The foregoing descriptions are directed to particular embodiments of die invention for die purpose of illustration and explanation. It will be apparent, however, to one skilled in die art tiiat many modifications and changes to die embodiments set forth above are possible witi out departing from die scope and die spirit of die invention. It is intended diat die following claims be interpreted to embrace all such changes and modifications.