referring now to fig1 the change of phase refrigeration system includes a refrigerant compressor 10 of the type normally used for compressing a common change of phase refrigerant such as freon , co 2 , etc . the compressor 10 may be hermetic or non - hermetic and also may be of single or multi - cylinder construction . the system also includes a refrigerant condenser 12 capable of condensing the refrigerant hot gas into a refrigerant liquid by cooling the hot gas below the temperature at which it becomes a liquid . in fig1 a thermostatic controlled blower or fan 14 is provided for cooling its associated condenser 12 . however , any cooling medium capable of achieving the cooling temperatures for the present system may be employed . the condenser 12 and the fan 14 are so adapted that outside air may pass into the entering side of the condenser to cool the refrigerant contained therein . 16 is a liquid receiver of the common type which collects the liquid refrigerant from its associated condenser 12 . a supply line 11 is provided between the compressor 10 and the condenser 12 to carry refrigerant gases from the compressor 10 to the condenser 12 . a two - position valve 13 is provided in the supply line 11 for a purpose to be described later . at the discharge side of the condenser 12 is provided a first liquid refrigerant supply line 15 running from the condenser 12 to the receiver 16 . the receiver 16 is connected to an evaporator 20 by a second liquid refrigerant supply line 17 running from the receiver 16 to the evaporator 20 . also provided in the liquid supply line 17 , between the receiver 16 and the evaporator 20 , is a thermostatic expansion valve 18 which monitors the amount of liquid refrigerant entering the evaporator 20 . the evaporator 20 may take various forms and typically may comprise a coil through which the liquid refrigerant passes , with the trapped air of the refrigeration system being drawn across the coil in any known manner to cool the trapped refrigerated air of the system , the cooling of the air in the refrigerated system resulting in the gasification of the refrigerant . a sensor 22 , responsive to the pressure of gases exiting the evaporator , is provided at an outlet of the evaporator 20 to regulate the amount of liquid refrigerant entering the evaporator 20 . the sensor 22 is mounted at the front portion of a suction line 24 which carries the cool evaporant discharge gases to the compressor 10 . the two - position valve 13 is responsive to external conditions to direct the flow of compressor discharge gases to the condenser 12 or to a heat recovery section 26 associated with the refrigeration system of the present invention . supply line 28 carries the compressor discharge gases from the valve 13 to a heat recovery coil 30 . a heat recovery blower 32 is associated with the coil 30 . a restricting means 34 , such as a spring - loaded check valve , is provided between the exit port of the heat recovery coil 30 and the condenser 12 . referring now to fig2 a second embodiment of the present invention is disclosed wherein the evaporator is modified to include components permitting a gas defrost cycle in the operating cycle of the refrigeration system of the present invention . in fig2 the refrigeration system includes a compressor 10 , a supply line 11 , a condenser 12 , a two - position valve 13 , a condenser fan 14 , a liquid supply line 15 , a receiver 16 , and a liquid supply line 17 . however , the system of fig2 employs a plurality of evaporator coils 35 , each coil 35 having a thermostatic expansion valve 36 , its associated sensor 37 , a check valve 38 , and a two - position valve 39 associated therewith . the coils 35 and their respective components associated therewith are further identified in fig2 by the appropriate suffix a , b , c or d . one port of the valve 39 is connected to the discharge side of the evaporator coil 35 . a compressor discharge gas supply line 40 extending from the discharge side of the compressor 10 is connected to a second port of the valve 39 . a suction line 41 having branches 42 extending thereto from respective third ports of the valves 39 connect the evaporators 35 to the suction side of the compressor 10 . the heat recovery portion 26 of the system includes the supply line 28 , the heat recovery coil 30 and the heat recovery fan 32 . however , the restricting means 34 of fig1 are replaced in fig2 by restricting means 43 and 44 , the means 43 comprising a check valve or similar device and the means 44 comprising a two - position valve or similar device . referring now to the operation of the change of phase refrigeration system shown in fig1 the compressor 10 compresses the low pressure cool refrigerant gas from the suction line 24 into a high pressure hot refrigerant gas and exhausts this gas through the valve 13 into the line 11 through which it is carried to the condenser 14 . here the hot gas is cooled to a temperature below that at which it becomes a liquid while still remaining at the high pressure imposed at the output of the compressor . it should be noted that in the improved system provided herein the discharge gas from the compressor is discharged at a pressure higher than that on the intake side but substantially lower than the discharge pressure required for the normal operation for a typical change of phase refrigerant system . with a lower discharge pressure for the refrigerant gas , the gas condenses at a relatively lower temperature . typically , the system would be operated under conditions at which the refrigerant gas would condense at temperatures 20 ° f above the ambient temperature . the rate at which the heat is removed from the refrigerant gas to cause the gas to change to a liquid varies with the temperature of the medium surrounding the coils and with the rate at which the medium is circulated across the coils . in the particular embodiment of the invention disclosed herein , the condenser coil 12 is located in the outside atmosphere whereby ambient outside air may be drawn across the coils to cool the refrigerant gas . because the ambient temperature of the air can vary considerably from day to night and extremely between the seasons , a thermostatically controlled fan 14 is utilized in order to control the heat transfer throughout these varying conditions . this fan 14 may be thermostatically controlled by ambient air temperature or by the temperature of the liquid refrigerant leaving the condenser through the line 15 . line 15 carries the liquid refrigerant to the receiver 16 . the liquid refrigerant line 17 carries the refrigerant from the receiver to the evaporator 20 . the thermostatically controlled valve 18 adjacent the inlet of the evaporator 20 controls the flow of liquid refrigerant into the evaporator . the valve or metering device 18 is controlled thermostatically by an element 22 secured adjacent an outlet of the evaporator 20 . if the pressure of the outlet gases of the evaporator decreases below a predetermined level , more refrigerant is introduced into the evaporator 20 . the liquid refrigerant in the evaporator coils of the evaporator 20 draws heat from air circulating across the coils to vaporize the refrigerant . this refrigerant vapor or gas is then exhausted through an exit port into line 24 at relatively low temperature and pressure . the gas then returns to the compressor 10 through the line 24 to start the refrigeration cycle over again . heat recovery refers to the use of a refrigerating compressor to build up the pressure and consequently the temperature of compressor discharge gases so that the excess heat generated by such gases may be used for heating purposes in an integrated thermal system to reduce the demand on other components ( furnaces , heaters , etc .) in the system . the heat recovery section 26 of the present invention is initiated by an external control ( not shown ) associated with the two - position valve 13 . the heat recovery cycle of the refrigeration system is initiated when the first path through the valve 13 to the condenser 12 is closed and compressor discharge gases flow through the supply line 28 to the heat recovery coil 30 . restrictive means 34 , such as a spring loaded check valve , blocks the flow of compressor discharge gases out of the heat recovery section 26 of the refrigeration system until the pressure and the temperature of the discharge gases have built to a predetermined level . means are provided with the heat recovery section of the refrigeration system to carry off excess heat generated in the heat recovery coil 32 for use in an integrated thermal system . such means may include , but are not restricted to , the heat recovery blower 32 . when the pressure in the heat recovery section 26 has built to a predetermined level the restrictive means 34 is opened to release the compressor discharge gases to the condenser 12 . in certain refrigeration systems it may be desirable to use hot compressor gases to defrost the evaporator coil . gas defrost involves the so - called &# 34 ; reverse flow &# 34 ; of hot compressor discharge gases through the evaporator coil . unless means are provided to port these hot gases back to the compressor , the efficiency of the system is severely degraded . consequently , it has become customary in the trade to employ gas defrost with no less than four evaporator coils , so that no more than 25 % of the cooling capacity of the refrigeration system is affected during periods of gas defrost . referring now to fig2 during periods of gas defrost the valve 13 would be opened to permit flow from the compressor into the supply line 11 . however , the restricting means 44 would be closed to block flow from the supply line 11 to the condenser 12 and the restricting means 43 would block &# 34 ; reverse &# 34 ; flow to the heat recovery coil 30 . after the discharge pressure of the compressor had reached the desired higher level , for example , valve 39a would be open to permit flow of the compressor discharge gases through the supply line 40 into the evaporator operator coil 35a . the natural flow of the discharge gases would be from a region of higher temperature to a region of relatively low temperature . thus , gas would flow from the discharge side of the compressor 10 through the supply line 40 through the valve 39a and into the coil 35a . the defrost gases would be exhausted out of the coil 35a through the check valve 38a to be mixed with condensate from the supply line 17 and carried into the remaining evaporator coils 35 through the thermostatic expansion valves 36 to pass through the remaining coils 35 and return to the compressor 10 through the suction line 41 . it is easily seen that the system of fig2 may be used for heat recovery purposes with the shifting of the valve 39 to permit suction from the evaporator coils 35 , the shifting of the two - position valve 13 to the position blocking flow of compressor discharge gases to the supply line 11 and permitting flow of discharge gases to the supply line 28 into the heat recovery section 26 , and the opening of means 44 to permit gas flow to the condensor 12 of the refrigeration system shown in fig2 . it should also be apparent to one skilled in the art that the heat recovery section and the gas defrost section of the refrigeration system shown in fig2 could operate independently of one another so that the heat recovery section 26 could be eliminated and the gas defrost section could be retained . because of the relatively low pressure of gases entering the condenser 12 , it becomes necessary to cool the gases in the condenser 12 to a relatively lower temperature in order to achieve liquification of the refrigerant gases . further , it becomes necessary to provide sufficient means to transfer the relatively low temperature condensate of the condenser 12 to the evaporator in the liquid stage . this is achieved by enlarging the liquid refrigerant supply line 17 between the condenser and the evaporator or appropriately insulating the liquid refrigerant supply line 17 . the present invention provides for an improved change of phase refrigeration system wherein the compressor is operated at relatively low compression ratio with resultant high efficiency . the condenser is cooled by a low temperature medium to condense the refrigerant gases at a relatively low condensate temperature and the metering device between the condenser and the evaporator is operable at a relatively low pressure differential . the present system also includes a means for temporarily increasing the discharge pressure of the compressor to provide relatively high pressure compressor gases for use during a heat recovery cycle or a gas defrost cycle of the refrigeration system . having thus described the preferred embodiment of the present invention , it will be , of course , understood that various changes can be made in form , details , arrangements and proportions of the parts without departing from the scope of the invention which consists of the matter shown and described herein and set forth in the appended claims .