gas pressure recirculation systems -- an american phenomena

8/9/2019 Gas Pressure Recirculation Systems -- An American Phenomena

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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. Subsequent work by Bill Richards, Herb Rosen, Bob Ross, 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 sys-

tems work and the practical applications they may serve. It also gives honor to the creativity of

these men in our industry, some of whom are with us here today.

Originally - Accumulator With Boilout Coil

Up until the 1940s the traditional way to protect a compressor from liquid overfeed was to install

an accumulator vessel in the suction line with a 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

THE DEVELOPMENT OF GAS PRESSURE RECIRCULATION SYSTEMS

AN AMERICAN PHENOMENA


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accumulator. The liquid would then be boiled out of the accumulator by heat from the warm liquid

refrigerant in an internal pipe coil as it passed from the receiver to the evaporators. In fact, many

accumulators were also fabricated with steam jackets as an external source of heat to boil off accu-

mulated liquid refrigerant more rapidly. The vaporized refrigerant would then be recompressed by

the compressor.

Adjusting the hand and automatic expansion valves during normal system load fluctuations to pre-

vent 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 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 to the 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 1940s that provided the need

for a more effective method of liquid separation and 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. The higher incidence of catastrophic compressor scrambles led to

Harry Phillips 1948 patent submittal of the invention he referred to as the Suction Line Liquid

Return 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 sys-

tem, 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 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 dis-

charge gas. Two low pressure drop check valves would then allow the liquid return trap to drain

by gravity to the high pressure receiver below it. The three-way valve, 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, was the concept of

using hot gas to transfer liquid out of an accumulator. Liquid from an accumulator mounted 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. The ejector provided the vertical lift from the accumulator up

to the liquid trap where it could then drain by gravity to the receiver. The added hot gas load to the

refrigeration system, however, probably made this concept rather impractical in terms of operating

economics.


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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 liquid is sepa-

rated 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 a concept that will be referred to as liquid line transfer. Suction line 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, however, the

concept was to pressurize this vessel with high pressure gas and feed the cold liquid directly toother evaporator loads in the system through the liquid line, as shown in 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 sole-

noid valve to the liquid trap. The cold liquid would then be fed directly into the liquid line and out

to the evaporator loads.


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This system certainly eliminated the problem of having to locate the liquid trap above the receiver

level. However, the problem was that when there was little demand for liquid in the 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 load on the system. If there was very low load

and very few evaporators calling for liquid, the transfer time would be very long. During this time,

however, liquid overfeed could continue to build up in the accumulator. Also, the longer the dump

cycle, the more warming of the cold liquid by hot transfer gas pressure, resulting in a system

inefficiency. In addition, the liquid lines running throughout the plant would be alternately frostedwith 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.


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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. Realizing that the major limitation of the gravity 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. The pump would then only have to provide the

additional liquid head required to transfer the cold liquid into the high pressure receiver.

The problems of this type transfer system are purely practical ones. The refrigerant pump 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 prema-

ture 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

consistently every 6 to 12 months.


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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 the compressors and the condensers. As shown in Figure 5, 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. The result would be an increase in the pressure on the liq-

uid return vessel - usually adjusted for 10 to 15 psig above the receiver pressure. This pressure dif-ference 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 pres-

sure of all the compressors to accomplish this simple refrigerant transfer. For the period of time

that the hot gas pressure rises, all the compressors in the system operate at that slightly higher dis-

charge pressure, and the operating cost is increased during the transfer cycle - which should be only

about 30 seconds.


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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 1950s, 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. The liquid refrigerant, flash

cooled to suction temperature, would drain into two pumper drums mounted below the accumu-

lator. Alternately, by means of a level sensor or timer, each would dump, being pressurized by regu-

lated high pressure gas, feeding the cold liquid out to evaporators. The liquid line pressure would

be regulated by controlling the gas pressure to the pumpers. Hand expansion valves at the evapo-

rators would control liquid flow rate. Overfed liquid and evaporated gas would be returned to the


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accumulator. The Cycle Center concept thus creates a liquid recirculation system utilizing liquid at

suction temperature.

In 1972, Watkins 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 flash gas to form. The flash gas then displaces liq-

uid in the pumper. The flash-cooled liquid also serves as makeup feed for the evaporators.

This system represents energy savings compared to the original concept, as the flash gas, reintro-

duced to the compressorsuction after the pumper drum cycles back to the fill cycle, would repre-

sent no penalty to the operating cost of the system. This concept has been applied 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 1950s, Bill Richards and others at the company founded by Harry Phillips developed the

concept of a 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.


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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 suc-

tion accumulator - thus, the constant, or Controlled Pressure Receiver (CPR). Creating a CPR pres-

sure 10 to 15 psig lower than the high pressure receiver would allow liquid transfer to the CPR from

any liquid return trap, simply by use of high 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 of liquid to a receiver using high pressure gas with-

out 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 sub-

cooled liquid in the CPR. Saturated liquid at the reduced liquid pressure at times could create prob-

lems with flash gas in the liquid line distribution to evaporators. Often a liquid sub-cooler is

installed to compensate if this condition exists.

CPR Recirculation System

The CPR concept was further extended in 1959 by Bob Ross to a recirculating system. This develop-

ment 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

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. The liquid supply is throttled at the evaporators by a hand expansion

valve, with overfed liquid being returned to the accumulator. Cold liquid transferred to the CPR

from the accumulator by the Liquid Transfer Unit (LTU) blends with the warmer liquid makeup to theCPR, 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:

Simple principle of operation

Low maintenance

Conventional components

Sealed system - no shaft seal leaks

Keeps accumulator free of liquid - even in abnormal flood back conditions CPR pressure allows direct transfer of liquid using hot gas as transfer medium

Constant pressure liquid feed to the evaporators

Oil drains from CPR - additional separation before liquid is fed to the evaporators

High pressure liquid available for evaporators where required

Sub-cooled liquid supplied to evaporators - not saturated at evaporator temperature


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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 suction pressure regulation of overfed 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 evaporator temperatures at a common suction pressure - Sub-cooled liquid sup-

plied to the evaporators at a higher pressure and temperature than suction allows for individ-

ual 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.

Reduced product moisture 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 in the evaporator closer to the desired room temperature. The lower T

(air-to-refrigerant) can result in drastically reduced moisture loss in the product when com-

pared to pump recirculated coils which have all liquid entering the coil at saturated suction

temperature.

Hot gas defrost relief to CPR - If a CPR in the system is designed to operate at approximate-

ly 75 psig, then all hot gas defrost coils in the system could be piped to this one vessel with-

out requiring individual defrost relief pressure regulators. In a two-stage system the defrost

load for low temperature coils could be 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.

Series Liquid Feed - Illustrated in Figure 9, this system utilizes overfed liquid from high tem-

perature coils, say 30 psig (17F) suction, to feed directly to the low temperature coils, say 0

psig (-28F). The 17F accumulator thus serves also as the CPR for the low temperature evapo-

rators. If the evaporator loads are in correct proportion, all or part of the liquid overfeed

from the high temperature evaporators may be fed directly to the low temperature evapora-

tors. There is then no operating energy cost associated with liquid transfer from the high

temperature accumulator. The high temperature compressor is flash cooling the liquid make-

up to the low temperature evaporators.

Flash gas to transfer liquid rather than hot gas - In a two-stage or a single stage econo-

mizer 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). 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, the flash gas generator vessel can also be

designed utilizing the economizer capability of screw compressors.


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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 fol-

lowing causes:

Unregulated hot gas pressure to the liquid transfer unit - The pressure required to transfer

the liquid to the CPR should be 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 vessel each time liquid is transferred. If the system is sized correct-

ly, the time to transfer should not exceed approximately 30 seconds.

Excessive recirculation rate - Overfeed recirculation rate of 3:1 will take twice as much ener-

gy to transfer the liquid to the CPR than a recirculation rate of 2:1. Coil performance changes

are small once overfeed conditions are met (in 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 rate of 2:1 for maximum evaporator load. Use hand expan-

sion valves whose manufacturer will provide calibration curves or tables - tons of refrigeration

as a function of turns open and pressure drop across the valve. Several manufacturers pro-

vide this information.

Defective inlet or outlet check valves - This can allow either transfer gas blowback into the

accumulator or frequent cycling of the transfer unit. This condition is easily detected.

CPR Hot Gas Regulator set too high - Often a hot gas line with an outlet pressure regulator

is connected to the CPR vessel. Its purpose is to maintain a minimum CPR pressure when the

system load is small and a large surge of cold liquid has been transferred into the CPR - whenflash gas will not be adequate to maintain CPR pressure. If this pressure is 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.

Transfer time set too long - Causes transfer gas to be blown directly through the liquid

transfer unit into the CPR creating a false load on the system.

Transfer time set too low - Does not completely clear the LTU of liquid, causing the equiva-

lent of short cycling.

Practical Recommendations

Some practical recommendations for efficient operation of a CPR recirculation system are:

1. Adjust hand expansion valves - To set the system to minimum recirculation rate requires set-

ting individual hand expansion valves at each evaporator. Use hand expansion valves whose

manufacturer publishes pressure drop and capacity ratings - typically tons of refrigeration

based on number of turns open and pressure drop across the valve. Flow characteristics of


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different manufacturers valve vary widely depending on the valve design. Set the valve for

recirculation rate of 1:1 to 2:1 for maximum anticipated coil load. Adjust from there.

2. Use thermostat control of evaporators - Always shut off liquid supply to evaporators when

temperature conditions are satisfied. There will be a direct reduction in the cost of operating

the CPR transfer system.

3. Adjust LTU controls - Set the transfer gas pressure as low as practical, usually 10 to 15 psigabove CPR pressure. With transfer gas pressure set properly, adjust the transfer timer so the

vessel clears almost completely of liquid before terminating the cycle.

4. Adjust CPR regulators - Set the flash gas relief (inlet-type) pressure regulator to the desired

CPR control pressure. Be sure the hot gas (outlet-type) pressure regulator is set a few psig

lower to maintain a minimum pressure.

5. Monitor the transfer time - With the transfer pressure properly set, the transfer time should

not exceed approximately 30 seconds. Excessive transfer time causes increased losses in the

system due to warming of the cold liquid and the LTU vessel from the transfer gas. This indi-cates 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 transfer volume - 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 the number of transfer 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 of the vessel will allow you to track the specific gal-

lons (or pounds, or tons refrigeration) per day being transferred and relate that to overfeedratio for the evaporators. In an automated control system, excessive circulation rate could be

set as an alarm. In a manual data 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 operation. Digital counters are now 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 devel-

oped to transfer liquid refrigerant out of low temperature accumulators. Transfer systems and recir-culation systems have continued to be developed over the years to meet specific system require-

ments and will continue to be used in applications that particularly match their unique performance

features. Correctly applied and maintained these systems can operate simply and efficiently, with

very minimal maintenance cost.


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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. 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, PatentedNovember 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 tempera-

ture), 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.

gas pressure recirculation systems -- an american phenomena

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