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Service Application ManualSAM Chapter 630-33Section 1MBy: Cecil R. Visger, Past International PipingREFRIGERANT PIPINGINTRODUCTIONIn recent years, manufacturers of condensing units have more and more gone to motor-compressor units and direct-drivecompressors, which means that the compressors are turning at motor speeds. These relatively high-speed compressorscannot operate without proper lubrication at all times. Even more than formerly, adequate oil return must be maintained.Proper piping is conducive to long life and minimum repairs to the equipment, such as reduction of damage to compressorsby broken valves, burned out bearings and scored cylinders by the lack of proper oil return; and damage to expansion valvesdue to wire drawing as a result of flash gases, to name only a few of the possible damages. After all, it is to the best interestof all to make the customer’s equipment operate as efficiently and economically as possible.The chief purpose of this discussion on refrigerant piping is to point out some of the precautions to be taken which affectthe life and efficiency of refrigerating equipment, and to offer some helpful suggestions in piping design, selection anderection.The author knows of nothing the user appreciates more, and which causes him to feel that he has received his money’sworth, than NEATNESS. That is something he can see and understand. The only way fittings will fit properly is to install thepiping runs plumb and level, except when otherwise necessary to pitch for proper drainage. The supporting of pipes is partof the job of neatness, and should be done as part of the permanent results as intended in the original engineered layout.The intervals or spacing of the supports will be governed by the type and size of pipe, but should be close enough to avoidany sagging between the supports.In order to be able to properly select the valves, fittings and pipe for an installation, and the connection of the equipment fora given job, the location of each piece of the equipment with relation to each other component must be determined. Theselocations decide the design of the piping as well as the size and length, and they also determine the necessary valves andfittings. The first thing to do then is to make a layout. For small installations, the layout may even be mental; but there shouldbe a thought-out plan before erection is started.To simplify our discussion we shall divide it into four sub-topics, namely:DISCHARGE OR HOT GAS LINESLIQUID LINESSUCTION LINES FOR R-12SIZING REFRIGERANT PIPINGMISCELLANEOUS.Copyright 1964, 2009, By Refrigeration Service Engineers Society.-1-

DISCHARGE OR HOT GAS LINESThe discharge or hot gas piping is as important as the other two lines. In many installations remote condensers are used,such as remote air-cooled condensers and evaporative condensers. Installations of these types present their problems,and should be piped to avoid excessive vibration, provide against the entrainment of oil, and for traps where necessary toprotect the equipment.The usual recommended maximum pressure drop for calculations of R-12 hot gas lines is 2 psig, and a minimum velocityof 1,500 fpm (feet per minute) in vertical, and 750 fpm in horizontal piping to maintain oil movement. These results can beobtained by referring to the manufacturers’ charts on “pipe sizing”.Care should be taken when connecting condensing units in parallel, and each with its own receiver. The hot gas linesshould be arranged so that the equalizer pipe between them is as short and as near the receivers as possible. (See Figure66F01.) This equalizer should be at least the next size smaller pipe than the discharge piping. When connecting two ormore compressors to one condenser, it is best to run a discharge line for each compressor directly into the condenser. If thismethod is not practical, each compressor discharge line should be connected to the main hot gas line in a “Y” or offset “T”,as in Figure 66F02A and Figure 66F02B, but never bull-headed, as in Figure 66F02C.Figure 66F01 Paralleling Condensing UnitsFigure 66F02A Y connection for paralleling the hot gas lines from two compressors to a condenser at a lower level (right).Figure 66F02B T or offset connection for paralleling the hot gas lines from two compressors to a condenser at a lower level (right).Copyright 1964, 2009, By Refrigeration Service Engineers Society.-2-

Figure 66F02C Bullhead (Wrong) and two permissible connections for paralleling two hot gas lines.The opposing flow of refrigerant gases, as shown in the left diagram in Figure 66F02C, can set up a turbulence that maycause pulsation in the piping. If the rise to the main hot gas line is over 4 or 5 ft, the branch lines should connect to the mainline above the center of the main line piping. Another suggested method for hot gas lines, when paralleling two compressors,is shown in Figure 66F03A.Another method of equalizing hot gas and receiver pressures and crankcase oil.If the condenser is as much as 8 or 10 ft above the compressor, provisions should be made to trap the oil and refrigerantthat might otherwise condense in the compressor heads when the compressors are stopped. (See Figure 66F03B andFigure 66F03C.) Using this method, oil and refrigerant will soon return to the system when the machine starts again, withoutdamage to valve plates or heads. For each additional 30 ft rise, another trap should be provided, dividing the amount of pipeproportionately to each trap. These traps should be made as short as fittings will permit.Copyright 1964, 2009, By Refrigeration Service Engineers Society.-3-

Figure 66F03C Separate hot gas lines from compressors to one condenser at a higher level, showing traps to prevent oil, and refrigerantfrom running back down into the compressor heads when the compressors stop.Figure 66F03C Similar connections to those in Figure 66F03B, except one hot gas line is used for both compressors. Note use of Yconnections and trap.If it is anticipated that the compressor temperature may be lower than the receiver temperature, and that the receiver islocated above the compressor, a check valve should be installed in the discharge piping near the receiver (Figure 66F03D).This will prevent refrigerant evaporated out of the receiver from condensing in the compressor head, and causing seriousdamage when the compressor starts.Figure 66F03DUse of check valve in hot gas line with condenser above the compressor if compressor is in cooler location thancondenser.EVAPORATIVE CONDENSER DISCHARGE LINESToday we are becoming concerned with water-saving devices, and one of these is the evaporative condenser. As has beenpreviously pointed out, the inlet and outlet piping of a multi-coil evaporative condenser should not be bull-head connected.The liquid outlet of an evaporative condenser should be piped downward from the condenser coil outlet and full size, beforea reduction in pipe size is made. This will avoid trapping any refrigerant in the coils of the evaporative condenser (Figure66F04A and Figure 66F04B).Copyright 1964, 2009, By Refrigeration Service Engineers Society.-4-

Figure 66F04A Bullhead tee connection (wrong) of hot gas line to two sections of evaporative condenser. T connection (right) of liquidlines. Note check valve in main liquid line to receiver.Figure 66F04B Correct hot gas and liquid lines for two section evaporative condenser.Again, we have the temperature difference affecting the refrigerant in the discharge circuit. To avoid any difficulty, it isrecommended that a check valve be installed at the inlet of the receiver, to prevent the refrigerant from recondensing out ofthe receiver back into the condenser coils, should the refrigerant temperature in the receiver exceed that in the evaporativecondenser coils.In cases of two or more evaporative condensers connected in parallel, the outlets should have a drop of at least 24” toprovide a static head to insure the condensed refrigerant draining into the receiver.Under certain weather conditions when the evaporative condenser is installed outside, and sometimes under what appearsto be normal operating conditions, liquid refrigerant will have a tendency to hang up in the condenser coil, caused by gasbinding in the receiver. This makes the system appear to be short of refrigerant, but after adding refrigerant and makinganother inspection, the system may be found to be over-charged. To help this condition, a control can be used to operatethe fan motor or spray pump to maintain a pressure in the condenser sufficient to overcome the pressure to the receiver.Copyright 1964, 2009, By Refrigeration Service Engineers Society.-5-

Another condition may be due to difference in levels, and may be corrected by repositioning the inlet and outlet of thecondenser piping, as shown in Figure 66F05.Figure 66F05 Evaporative condenser should be at higher level than its receiver. Note trap in liquid line and equalizer from top of receiverto hot gas line.In some installations, it may be necessary to raise the evaporative condenser to obtain a minimum of 18” drop between theoutlet of the condenser and the inlet to the receiver. Also, there must be a trap in this line at the lowest run to prevent gasfrom rising to the condenser coil. The small equalizer line from the receiver to the inlet piping of the condenser should havea valve for shutting off the receiver. The size of this pipe can be 3/8” up to 25 tons capacity, and 1/2” up to 60 tons.When designing a system, the engineer needs to maintain a solid column of liquid refrigerant to the flow control device,such as a thermostatic expansion valve, low-side float valve or high-side float valve, and make sure that column will stayat a reasonable working temperature. Any flashing of liquid refrigerant in the liquid line affects the capacity of flow-controldevices and of the evaporator. Also, it may cause damage to these controls, or may be noisy in operation.If the evaporator is located above the receiver, there are three important precautions that should apply:a. The liquid refrigerant should be delivered to the evaporator without flashing due to heating of liquid from surroundingair, or to drop in pressure due to friction in the piping or to static head.b. The expansion valve or other flow control devices should be selected according to the manufacturers’ rating for thecapacity required after pressure drop due to friction and static head have been considered.c.The liquid refrigerant should reach the flow control device sub-cooled at least 1 F below the exit temperature at thereceiver.The limits in height to which liquid refrigerant can be piped without flash gas differ among engineers, but a maximum rise of15 ft for R-12 is usually recommended. The controlling factor is the static head of the refrigerant at a given temperature. ForR-12 at 97 F, this is .55 psi per ft of rise, and .57 psi per ft of rise at 70 F. For R-22 at 97 F, it is .50 psi per foot of rise, and.52 psi per foot of rise at 70 F, so the maximum rise may be a foot or so more than for R-12.Several steps can be taken to reduce flash gas in lifts higher than that mentioned above:1. Increase liquid pipe size one size over the calculated size, to reduce pressure drop due to pipe friction.2. Increase shut-off valves, strainers, solenoid valves and dryers to match above piping. These measures (1 and 2)will not reduce pressure drop due to static head.Copyright 1964, 2009, By Refrigeration Service Engineers Society.-6-

3. If, due to excessive static head, the above recommendations cannot be used a heat exchanger installed betweenthe suction and liquid line in (Figure 66F06A) to sub-cool the liquid will accomplish satisfactory results. It is importantthat the heat exchanger be installed in a position that will not allow oil to be trapped in the suction side of it, whichin turn reduces the oil level in the compressor.Figure 66F06A Use of heat exchanger in liquid and suction line to sub-cool the liquid in case of excessive static head.4. Another alternative is to locate the receiver on the same floor level and near the evaporator. This method will bemore applicable when a receiver is used with a shell and tube condenser-receiver or other type condenser.The following are precautions to be followed in liquid line piping that will improve the service of the equipment: Never locate a stop valve directly below a”Y” type strainer in a vertical pipe see left drawing (Figure 66F06B), asballs of solder, scale and foreign particles will drop into the valve when the flow of refrigerant ceases, causingdamage to the valve disc and seat, as the valve is turned to the closed position. Figure 66F06C and Figure 66F06Dshow suggested installations of stop valves and strainers in the liquid line. The use of an angle strainer usuallyeliminates one fitting.Figure 66F06B Stop valve directly beow strainer in vertical liquid line (Wrong).Figure 66F06C Stop valve ahead of angle strainer in horizontal liquid line (Right).Figure 66F06D Stop valve ahead of Y strainer in horizontal liquid line (Right). Dryers should be installed in a vertical position and the liquid fed upward. The large volume of the drier causes areduced velocity of the liquid. If the drier is placed in a horizontal position, oil may tend to separate out and fill thatCopyright 1964, 2009, By Refrigeration Service Engineers Society.-7-

portion below the outlet, thus reducing the capacity of the drier. According to recent research, a good cleanablestrainer should be installed in the liquid line between the receiver and drier, to save the drier from becomingcontaminated with the foreign substances that may be in the refrigerating system. This procedure will improve andincrease the effectiveness of the drier. If the liquid line is run through an area with an ambient temperature above the design refrigerant temperature, thenthe line should be insulated the required thickness to protect it from out