THE DEVELOPMENT OF GAS PRESSURE RECIRCULATION SYSTEMS — AN AMERICAN PHENOMENA
Introduction
The concept of using high pressure refrigerant gas to transfer and recirculate liquid refrigerant in a refrigeration system was pioneered and developed here in America by two inventive genius friends; Harry A. Phillips and Jack Watkins. Watkins. Subsequent work work by Bill Richards, Herb Rosen, Bob Ross, and and Rowe Bansch has served to develop and further refine these concepts into practical and efficient refrigeration system designs that are applied today in the industry around the world. A review of these developments and their evolution in time provide great insight into how the systems work and the practical practical applications they may serve. It also gives honor to the the creativity of these men in our industry, some of whom are with us here today. Originally - Accumulator With Boilout Coil
Up until the 1940’s the traditional way to protect a compressor from liquid overfeed was to install an accumulator vessel in the the suction line with a “boilout” “boilout” coil, as shown in Figure 1. Liquid overfeed from hand expansion valves, automatic expansion valves or thermostatic expansion valves would be separated from the suction gas stream due to a change in direction and reduced gas velocity in the
accumulator. The liquid would then be boiled out of the accumulator accumulator. accumulator by heat from the warm liquid refrigerant in an internal pipe coil as it passed from the receiver to the evaporators. evaporators. In fact, many accumulators were also fabricated with steam jackets as an external source of heat to boil off accumulated liquid refrigerant more rapidly. rapidly. The vaporized refrigerant would then be recompressed recompressed by the compressor. Adjusting the hand and automatic expansion valves during normal system load fluctuations to prevent excessive liquid overfeed and carryover to the compressors was often a 24-hour hands-on maintenance job. And then there was always the intermittent periodic liquid surge, such as as a defrost cycle, that could bring back much more liquid to the accumulator than it could separate and evaporate fast enough to prevent liquid carryover carryover to the compressor. compressor. Separating out the liquid was never the challenge - it was what to do with the liquid once it was trapped in the accumulator. Suction Line Liquid Return Trap - Gravity
It was the advent of high speed reciprocating compressors in the late 1940’s that provided the need for a more effective method method of liquid separation separation and accumulation accumulation in compressor suction lines. The higher speed recips were not nearly as tolerant of refrigerant liquid carry-over as their slow speed (up to 400 rpm) predecessors. predecessors. The higher incidence of of catastrophic compressor compressor scrambles led to Harry Phillips’ 1948 patent submittal of the invention he referred to as the “Suction Line Liquid Return Trap.” Trap.” The object was to, “. . . provide improved apparatus for accumulating liquid refrigerant . . . returning in the suction line of a refrigerating system and in utilizing the high pressure refrigerant gas to deliver the accumulated liquid refrigerant to the high pressure side of the system, thereby by-passing and protecting the compressor of the system.”
This system concept, shown in Figure 2, provided for suction line liquid slop-over protection in an accumulator, which would allow the separated liquid to drain by gravity from the accumulator to a “liquid return trap” vessel vessel below it. When the “liquid return trap” vessel was filled with liquid, the level would be sensed by an electric level sensor, and a three-way valve would automatically switch this vessel from venting to the accumulator above it, to a source of high pressure compressor discharge gas. Two low pressure drop check valves would then allow the “liquid “liquid return trap” to drain by gravity to the high high pressure receiver below it. The three-way valve, the the intermediate vessel referred to as a “liquid return trap,” the inlet and outlet check valves and the electric level sensing device were all part of this system. This first patent focused on draining returned liquid to the high pressure receiver by force of gravity from the liquid trap mounted above it. Also proposed in this patent, however, however, was the concept of using hot gas to transfer liquid out of an accumulator. accumulator. Liquid from an accumulator mounted at at a lower elevation was shown being transferred to a “liquid return trap” mounted at a higher elevation by use of hot gas through an ejector. ejector. The ejector provided the vertical lift from the accumulator accumulator up to the liquid trap where it could then drain by gravity to the receiver receiver.. The added hot gas load to the refrigeration system, however, probably made this concept rather impractical in terms of operating economics.
In many installations the location of the accumulator with respect to the receiver would not allow liquid transfer by gravity drain. So in those cases the issue still persisted: Once the the liquid is separated from the suction line, what do you do with it? Liquid Line Transfer
The next major patent development was in 1949 by Harry Phillips’ cross-town friend, Jack Watkins. Watkins patented patented a concept concept that will be referred to as “liquid line transfer.” transfer.” Suction line liquid liquid carry-over drained from an accumulator vessel into a separate liquid trap just as in Phillips “suction line liquid return trap.” Instead of returning it by gravity to the high pressure receiver, receiver, however, however, the concept was to pressurize this vessel with high pressure gas and feed the cold liquid directly to other evaporator loads in the system through the the liquid line, as shown in Figure Figure 3. This principle depends on the non-overfeeding evaporators in the system to boil off the excess liquid from the evaporators that are overfeeding. When the liquid trap was full, simultaneous electrical signals would close the king solenoid valve at the liquid feed from the receiver, close the vent solenoid valve, and open the high pressure gas solenoid valve to the liquid trap. The cold liquid would then be fed directly into the the liquid line and out to the evaporator loads.
This system certainly eliminated the problem of having to locate the liquid trap above the receiver level. However, However, the problem was that when there was little demand for liquid in the system, system, there would be little potential to move it out of the liquid trap. The time required to empty the vessel was a pure function of the amount of the evaporator evaporator load on the system. If there was very low load and very few evaporators evaporators calling for liquid, the transfer transfer time would be very long. During this time, however, liquid overfeed could continue to build up in the accumulator. accumulator. Also, the longer the “dump” cycle, the more warming of the cold liquid by hot transfer gas pressure, resulting in a system inefficiency. inefficiency. In addition, the liquid lines running throughout the plant plant would be alternately frosted with cold liquid coming from the liquid trap, and warmed by warm liquid from the receiver during normal operation. Besides the inconvenience of either insulating the liquid line or putting up with alternately frosting or condensing moisture on the liquid line and condensate dripping, there are some evaporator loads that would have difficulty in handling the sub-cooled evaporator temperature liquid being fed to them, such as some ice cream freezers which require an amount of flash gas to circulate refrigerant in the evaporator.
Pump-Assisted Transfer
An additional development was then made by both Vilter and Phillips in a pump-assisted transfer system, as shown in Figure 4. 4. Realizing that the major major limitation of the gravity drain drain liquid trap was the geometry of the accumulator, the receiver, and the liquid trap, the simple addition of a transfer pump in the outlet line of the liquid trap overcame this problem. When hot gas pressure would be applied to the transfer vessel, the pump would be initiated after the vessel pressure was equalized with the receiver. receiver. The pump would then only have to to provide the additional liquid head required to transfer the cold liquid into the high pressure receiver. The problems of this type type transfer system are purely practical ones. The refrigerant pump casing casing is required to be rated at 250 psig pressure because of its exposure to the high pressure side of the system; the initiation of pump operation must be time delayed until pressures are equalized between the transfer drum and the receiver, otherwise the pump will cavitate, resulting in premature wear; and the use of open drive pumps in this application seems to be very hard on pump seals. Some transfer systems of this type have been observed to require pump seal replacements replacements consistently every 6 to 12 months.
Interrupting Valve Transfer
The next stage of development in transfer systems was the concept of an “interrupting valve” in the hot gas discharge line between between the compressors and and the condensers. condensers. As shown in Figure 5, closing closing a differential pressure regulator in the hot gas discharge main whenever the liquid return trap calls for a transfer cycle will provide a source of hot gas pressure above receiver condensing pressure. When the liquid return vessel was filled and hot gas pressure was applied to the transfer vessel, either a mechanical pilot would actuate this interrupting valve or an electrical pilot solenoid would actuate a differential pressure regulator. regulator. The result would be an increase in the pressure on the liquid return vessel - usually adjusted for 10 to 15 psig above the the receiver pressure. This pressure difference would allow liquid to flow from the transfer unit to the high pressure receiver and overcome any additional pressure losses due to the piping, valves or the vertical location of the receiver above the transfer vessel. While this system concept works well, one slight drawback is that it does raise the discharge pressure of all the compressors to accomplish this simple refrigerant transfer. transfer. For the period of time that the hot gas pressure rises, all the compressors in the system operate at that slightly higher discharge pressure, and the operating cost is increased during the transfer cycle - which should be only about 30 seconds.
The more practical issue is that in larger systems, the size of this valve in the main hot gas header can become large, which raises the cost of the valve significantly and adds to the capital cost of the system for tranferring liquid. In a large multiple compressor system, however, one or more of the compressors could be utilized with a smaller differential pressure regulator installed in its discharge line as the source of higher pressure gas to operate the liquid return vessel. Cycle Center - Dual Pumper Drum Recirculation
Meanwhile, in the late 1950’s, Jack Watkins developed his liquid line transfer concept further into what he called a “Cycle Center,” using high pressure gas to recirculate the liquid refrigerant. Shown in Figure 6, this involved controlling a liquid level in an accumulator. accumulator. The liquid refrigerant, flash cooled to suction temperature, would drain into two “pumper drums” mounted below the accumulator.. Alternately lator Alternately,, by means of a level sensor or timer, timer, each would dump, being pressurized by regulated high pressure gas, feeding feeding the cold liquid out out to evaporators. The liquid line pressure would would be regulated by controlling the gas pressure to the “pumpers.” “pumpers.” Hand expansion valves at the evapoevaporators would control liquid flow rate. Overfed liquid and evaporated evaporated gas would be returned to the
accumulator. The Cycle Center concept thus creates accumulator. creates a liquid recirculation system utilizing liquid at suction temperature. In 1972, Watkins patented patented the concept of recirculating the liquid by use of flash gas. Instead of pressurizing the “pumper” with high pressure gas, high pressure warm liquid would be introduced into the trap. The drop in liquid pressure causes causes flash gas to form. The flash gas then displaces liquid in the pumper. pumper. The flash-cooled liquid also serves as makeup feed for for the evaporators. This system represents energy savings compared to the original concept, as the flash gas, reintroduced to the compressorsuction after the pumper drum cycles back to the “fill” cycle, would represent no penalty to the operating cost of the system. system. This concept has been applied extensively extensively by Holmsten Ice Rinks in their “Direct Liquid Refrigeration” systems, which utilize that other refrigerant, circulating R-22 directly in evaporator tubes in the ice rink floor. Controlled Pressure Receiver CPR Transfer
In the late 1950’s, Bill Richards and others at the company founded by Harry Phillips developed the concept of a controlled controlled pressure receiver (CPR) transfer system as illustrated in Figure 7. The genius of this concept is that instead of struggling to generate a higher pressure gas to transfer the overfed liquid from evaporators up to condensing pressure in the receiver, a liquid receiver pressure lower than condensing pressure was created.
This was accomplished by the simple addition of a high side float control draining liquid into the main receiver and controlling the pressure in this vessel by regulating the flash gas back to the suction accumulator - thus, the constant, or “Controlled “Controlled Pressure Receiver” Receiver” (CPR). Creating a CPR pressure 10 to 15 psig lower than the high pressure receiver would allow liquid transfer to the CPR from any liquid return trap, simply simply by use of high pressure pressure discharge gas in the system. Liquid could then be fed from the CPR to the system at liquid supply line pressure only slightly lower than condensing pressure. This really simplified the transferring transferring of liquid to a receiver receiver using high pressure gas without having to artificially raise the pressure in the transfer vessel. In this system, however, if there is no liquid carryover from the evaporators, there would be no subcooled liquid in the CPR. Saturated liquid at the reduced liquid pressure pressure at times could create problems with flash gas in the liquid line distribution to evaporators. Often a liquid sub-cooler sub-cooler is installed to compensate if this condition exists. CPR Recirculation System
The CPR concept was further extended extended in 1959 by Bob Ross to a recirculating system. This development recognized that the control pressure receiver could be operated not only at 10 to 15 psig lower,, but perhaps as much as 100 psig lower than condensing pressure. In fact, if the CPR vessel is lower controlled by the flash gas pressure regulator to about 25 psig above evaporator pressure, then the liquid supply pressure to the evaporators would become very much like that of a pump recirculating system as shown for comparison in Figure 8. The control pressure receiver (CPR) in the recirculation system provides a constant pressure source of liquid for the evaporators. evaporators. The liquid supply is throttled throttled at the evaporators evaporators by a hand expansion expansion valve, with overfed liquid being returned to the accumulator accumulator.. Cold liquid transferred to the CPR from the accumulator by the Liquid Transfer Unit (LTU) blends with the warmer liquid makeup to the CPR, providing sub-cooled liquid to the evaporators at the regulated supply pressure. Most of the ammonia gas pressure recirculation systems being applied and in use today are of the CPR type design. Some of the beneficial features of this system are: • Sim Simple ple prin princip ciple le of ope operat ration ion • Lo Low w ma main inte tena nanc nce e • Con Conven ventio tional nal com compon ponent entss • Sea Sealed led syst system em - no shaf shaftt seal seal leaks leaks • Kee Keeps ps accumulato accumulatorr free of liquid - even even in abnormal abnormal flood flood back conditio conditions ns • CPR pressure pressure allows allows direct transfe transferr of liquid using using hot gas as transfer transfer medium medium • Cons Constant tant pressu pressure re liquid liquid feed to the evapo evaporator ratorss • Oil drains drains from CPR - additiona additionall separation separation before before liquid liquid is fed to the evaporat evaporators ors • High pressur pressure e liquid availab available le for evaporat evaporators ors where where required required • Sub-cooled liquid supplied to evaporators evaporators - not not saturated at evaporator temperature
Further Developments
The companies that specialize in the manufacture of the CPR recirculation systems have further developed some of the operating features to provide the following system benefits: • Allows Allows suction suction press pressure ure regulat regulation ion of of overfed overfed coils coils - The fact that liquid can be supplied to the evaporators at a higher pressure and temperature than that of saturated suction allows evaporator pressure regulation over a range of evaporator temperatures not possible with pump recirculated systems. • Multiple Multiple evapor evaporator ator temper temperature aturess at a common common suction suction pressure pressure - Sub-cooled liquid supplied to the evaporators at a higher pressure and temperature than suction allows for individual evaporators or zones of evaporators to be controlled at different evaporator temperatures from a common suction pressure while still gaining the system benefit of liquid overfed coils. • Reduce Reduced d produ product ct mois moistur ture e loss loss - In product cooling where weight loss is a consideration, such as meat and produce, the ability to feed the evaporators at higher liquid temperatures allows the evaporators to be automatically controlled individually, or by zone, to saturated liquid temperature temperature in the the evaporator closer to the desired room temperature. temperature. The lower ∆ T (air-to-refrigerant) can result in drastically reduced moisture loss in the product when compared to pump recirculated coils which have all liquid entering the coil at saturated suction temperature. • Hot gas def defros rostt relie relieff to CP CPR R - If a CPR in the system is designed to operate at approximately 75 psig, then all hot gas defrost coils in the system could be piped to this one vessel without requiring individual defrost defrost relief pressure regulators. In a two-stage system the defrost load for low temperature temperature coils could be taken taken entirely out of the low stage load. In addition, defrost relief piping is simplified by requiring only a check valve at each defrost zone and eliminating the need for individual defrost pressure regulators. • Seri Series es Li Liqu quid id Fe Feed ed - Illustrated in Figure 9, this system utilizes overfed liquid from high temperature coils, say 30 psig (17F) suction, to feed directly to the low temperature coils, say 0 psig (-28F). The 17F accumulator accumulator thus serves also as the CPR CPR for the low temperature temperature evaporators. If the evaporator loads loads are in correct proportion, proportion, all or part of the liquid overfeed from the high temperature evaporators may be fed directly to the low temperature evaporators. There is then no operating operating energy cost associated with liquid transfer from the the high temperature accumulator. accumulator. The high temperature compressor is flash cooling the liquid makeup to the low temperature evaporators. • Flash Flash gas to transfe transferr liquid liquid rathe ratherr than than hot hot gas - In a two-stage or a single stage “economizer” refrigeration system as shown in Figure 10, flash gas from a control pressure receiver (CPR) can be used as the source of transfer gas for a liquid transfer unit (LTU). (LTU). In the two stage system there is no net energy requirement for the booster compressor recompression of the transfer gas. In the single stage system, system, the flash gas generator vessel can also be designed utilizing the “economizer” capability of screw compressors.
Excessive Operating Cost
Because gas pressure recirculation systems receive their motive power from compressed refrigerant gas, they are subject to abuse by improper operation or inadequate maintenance. Excessive operating cost for a CPR recirculation system can usually be attributed to one of the following causes: • Unregula Unregulated ted hot hot gas pressur pressure e to the the liquid liquid transfer transfer unit - The pressure required to transfer the liquid to the CPR should be regulated regulated to within 10 to 15 psig above CPR pressure. The use of unregulated high pressure gas burdens the refrigeration system with excessive gas flow released from the transfer transfer vessel each time liquid is transferred. transferred. If the system is sized correctcorrectly, the time to transfer should not exceed approximately 30 seconds. • Exces Excessiv sive e recir recircul culati ation on rate rate - Overfeed recirculation rate of 3:1 will take twice as much energy to transfer the liquid liquid to the CPR than than a recirculation rate of 2:1. 2:1. Coil performance changes changes are small once overfeed conditions are met (in excess excess of 1:1). Coil manufacturers can use refrigerant distributors in the circuit design to insure equal distribution to each circuit. Consider design at recirculation recirculation rate of 2:1 for maximum maximum evaporator load. Use hand expansion valves whose manufacturer will provide calibration curves or tables - tons of refrigeration as a function of turns turns open and pressure pressure drop across the valve. Several manufacturers provide this information. • Defective Defective inle inlett or or outlet outlet check valv valves es - This can allow either transfer gas blowback into the accumulator or frequent frequent cycling of the transfer transfer unit. This condition is easily detected. detected. • CPR CPR Hot Hot Gas Reg Regula ulator tor set too too high high - Often a hot gas line with an outlet pressure regulator is connected to the CPR vessel. Its purpose is to maintain maintain a minimum CPR pressure pressure when the system load is small and a large surge of cold liquid has been transferred into the CPR - when flash gas will not be adequate adequate to maintain CPR pressure. If this pressure is adjusted adjusted too high (above the setpoint of the flash gas relief pressure regulator) it will be a constant source of hot gas leaking over to compressor suction. • Tran ransfe sferr time time set too lon long g - Causes transfer gas to be blown directly through the liquid transfer unit into the CPR creating a false load on the system. • Tran ransfe sferr time time set too low - Does not completely clear the LTU of liquid, causing the equivalent of “short cycling.” Practical Recommend Recommendations ations
Some practical recommendations for efficient operation of a CPR recirculation system are: 1. Adjust Adjust hand hand expans expansion ion valve valves - To set the system to minimum recirculation rate requires setting individual hand expansion valves at each evaporator evaporator.. Use hand expansion valves whose manufacturer publishes pressure drop and capacity ratings - typically tons of refrigeration based on number number of turns open and pressure drop across the the valve. Flow characteristics of
different manufacturer’s manufacturer’s valve vary widely depending depending on the valve design. Set the valve for recirculation rate of 1:1 to to 2:1 for maximum anticipated anticipated coil load. Adjust from there. 2. Use thermos thermostat tat control control of evapor evaporators ators - Always shut off liquid supply to evaporators when temperature conditions conditions are satisfied. There will be a direct reduction reduction in the cost of operating the CPR transfer system. 3. Adjust Adjust LTU LTU contro controls ls - Set the transfer gas pressure as low as practical, usually 10 to 15 psig above CPR pressure. With transfer gas pressure set properly, properly, adjust the transfer timer so the vessel clears almost completely of liquid before terminating the cycle. 4. Adjus Adjustt CPR regula regulator torss - Set the flash gas relief (inlet-type) pressure regulator to the desired CPR control pressure. Be sure the hot gas gas (outlet-type) pressure regulator regulator is set a few psig lower to maintain a “minimum” pressure. 5. Monitor Monitor the trans transfer fer time time - With the transfer pressure properly set, the transfer time should not exceed approximately approximately 30 seconds. seconds. Excessive transfer time causes increased losses in the system due to warming of the cold liquid and the LTU LTU vessel from the transfer gas. This indicates a need to change to a larger sized outlet check valve and/or transfer line size to the CPR. For a new system, specify 30 seconds maximum transfer time from the system manufacturer. 6. Know the trans transfer fer volum volume e - Have the system manufacturer provide the estimated gallons of liquid transferred in a “dump cycle,” or calculate it yourself using basic vessel dimensions. 7. Monitor Monitor the the number number of transfer transfer cycles cycles - Digital counters costing less than $50 can be installed on the timer control panels to count the number of times the liquid transfer unit is energized. Knowing the transfer volume volume of the vessel will allow you you to track the specific specific gallons (or pounds, or tons refrigeration) per day being transferred and relate that to overfeed ratio for the evaporators. evaporators. In an automated control system, excessive circulation circulation rate could be set as an alarm. In a manual data gathering gathering system, trends can be determined and operating problems with the LTU can be determined quickly by knowing the number of transfer cycles expected in normal normal operation. Digital counters are now offered offered as a standard option by some system manufacturers on their timer control panels.
Summary
Refrigerant liquid circulation systems powered by refrigerant gas evolved from basic concepts developed to transfer liquid refrigerant out of low temperature temperature accumulators. Transfer systems and recirculation systems have continued to be developed over the years to meet specific system requirements and will continue to be used in applications that particularly match their unique performance features. Correctly applied and maintained these these systems can operate simply and efficiently efficiently,, with very minimal maintenance cost.
REFERENCES
2,589,859. Suction Line Liquid Return Trap , Harry A. Phillips; Applied November 12, 1948, Patented March 18, 1952. 2,590,741. Liquid Return Trap in Refrigeration Systems , John E. Watkins; Applied January 24, 1949, Patented March 1, 1952. 2,871,673. Liquid Return System (Control Pressure Receiver), William V. V. Richards and Herbert Rosen; Applied October 8, 1956, Patented February 3, 1959. 2,931,191. Refrigerating System with means to Obtain High Liquid Line Pressure, John E. Watkins; Applied March 3, 1959, Patented April 5, 1960. 2,952,137. Low Pressure Refrigerating Systems (Pumper drum recirculation), John E. Watkins; Applied January 2, 1959, Patented September 13, 1960. 2,959,934. Oil Separator and Return Apparatus, Robert R. Ross; Applied March 9, 1959, Patented November 15, 1960. 2,966,043. Balanced Circulating System for Refrigeration (CPR recirculation), Robert R. Ross; Applied August 17, 1959, Patented December 27, 1960. 3,315,484. Pressurized Refrigeration Recirculating System (CPR liquid at evaporator temperature), Robert R. Ross; Applied May 17, 1965, Patented April 25, 1967. 3,848,425. Low Pressure Refrigeration System (Flash gas pressurization), John E. Watkins; Applied December 4, 1972, Patented November 19, 1974. 3,919,859. Refrigerating System (Intercooler/CPR combination), Robert R. Ross; Applied November 18, 1974, Patented November 18, 1975. 3,988,904. Refrigeration System (High pressure gas subsystem), Robert R. Ross; Applied December 5, 1974, Patented November 2, 1976. 4,059,968. Refrigeration System (Flash gas transfer, economizer compressor), Robert R. Ross; Applied April 5, 1976, Patented November 29, 1977. 4,324,106. Refrigeration System (Dedicated flash gas compressor), Robert R. Ross and Daniel Rowe Bansch; Applied October 3, 1980, Patented April 13, 1982.
