Large Splits Refrigrant Piping Design - Engineers Newsletter

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© American Standard Inc. 1998 Volume 27, No. 4 December 1998 s

providing insights for today’s HVAC system designer

As Equipment Evolves, So Must Piping Practices …

Split Systems And Interconnecting Refrigerant Lines

The efficiency and reliability of an R-22,split air-conditioning system hingeslargely on the piping that connects therefrigerant-condensing and air-handlingsides of the system. Operational

difficulties are inevitable if thisinterconnecting piping is improperlydesigned or installed, regardless of howcarefully the equipment was selected andapplied.

Interconnecting piping designs thatsuccessfully avoid these difficulties shareseveral common traits:

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A simple, direct layout that reduces theamount of system refrigerant.

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A refrigerant tube size that consistentlyreturns oil to the compressors.

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A refrigerant tube size that doesn’t causeexcessive pressure drops which reducecompressor efficiency and capacity.

Some of the piping techniques discussedin this newsletter wouldn’t have beenconsidered good practice in the past, butthey work best and most reliably for

modern commercial Trane equipmentwith scroll compressors.

This Engineers Newsletter previewsthe selection and refrigerant piping

guidelines now recommended for Trane’scurrent line of commercial split-system/ scroll-compressor equipment. (Literaturedetailing these guidelines is slated forpublication in 1999.)

Equipment Selection

The success of any HVAC applicationbegins with choosing the right systemand properly sizing the equipment. If asplit air-conditioning system best suitsthe project, then observe theseguidelines:

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Base your selection on a minimum coil

suction temperature of 40 F for constant-volume systems and 43 F for variable-air-

volume (VAV) systems. These limitspromote stable performance over thesystem’s entire operating range.

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Select the system using design conditionsand check for proper operation at theexpected operating extremes.

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Avoid using hot gas bypass in comfortcooling applications. It squanders energyand is seldom necessary for properly

designed systems. If coil-frosting is aconcern, consider adding an evaporator

defrost control such as Frostat™ coil frostprotection.

Interconnecting Piping

Keep these general guidelines in mind asyou review the recommendations specific

to suction, liquid, discharge and hot gasbypass lines:

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Limit overall line length, including thevertical suction rise. Pay particularattention to liquid risers. Enoughsubcooling may be lost as refrigeranttravels up the riser to cause flashing.

Review any questionable applicationswith the manufacturer.

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Use Type L copper tubing for R-22. Forcopper-to-copper joints, use BCuP-6

without flux. For copper-to-steel orcopper-to-brass joints, use BAg-28 with anon-acid flux.

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Use pipe sizing software such as TraneEngineering Toolbox to quickly determineproper sizes for refrigerant lines based oncurrent engineering data.

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s 2 Trane Engineers Newsletter — Vol. 27, No. 4

Suction Line. Pressure drop in thesuction line adversely affects unit

capacity and efficiency. A pressure drop

of 6 psid in matched commercial TraneRAUC units reduces compressor capacityby 2.5 percent and efficiency by1.5 percent.

With that in mind, consider thesesuction-line recommendations:

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Route the suction line from theevaporator to the compressor by theshortest path.

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Use different pipe sizes for horizontal

and vertical lines to make it easier tomatch line pressure drop and refrigerantvelocity to suction-line requirements.

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To assure proper oil entrainment andavoid sound transmission, size the

suction line so that the refrigerantvelocity equals or exceeds the required

minimum velocity stated in Table 1 and remains below 4,000 fpm.

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Riser traps are unnecessary. If the riser isproperly sized to maintain velocity,adding a trap only increases the suction-line pressure drop.

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Double risers are unnecessary. The stepsof unloading for commercial Trane unitswith scroll compressors are such that asingle, carefully selected riser canmaintain oil circulation at any loadcondition.

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Even the most careful installation allowsforeign matter into the system. That’swhy it’s critical to install suction filtersand properly maintain them. Once

cleanup is complete, a fresh core should

F gure 1Recommended Suction-Line Arrangements From Evaporator Headers

To compressor located above evaporator

Evaporator headers join at a point lower than thelowest suction header outlet

To compressor located below evaporator

Height of common riser exceedsheight of evaporator

prevent any refrigerant that condenses inthe suction line from flowing to thecompressor when the unit is off.

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Specify suction-line insulation if moisturecondensation and dripping pose aproblem, or to prevent refrigerant fromcondensing inside the lines if long runswill be exposed to low temperatures.

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Don’t install suction lines underground.The likelihood of corrosion, vibration,condensation of water outside—andrefrigerant inside—the line, combinedwith inaccessibility, make this practiceunwise. If underground installation isunavoidable, make provisions to insulate,

waterproof and encase the lines in a hardsleeve.

Liquid Line. Sufficient subcooling mustbe maintained at the expansion valve. Toprovide proper operation throughout the

range of operating conditions, the liquid-line pressure drop should not exceed theunit’s minimum subcooling value less 5 F.To achieve this objective, keep theseliquid-line considerations in mind:

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Select the smallest, practical line size forthe application. Limiting the refrigerantcharge improves compressor reliability.

be installed in the shell to prevent oiltrapping.

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Figure 1 illustrates one method oftrapping the evaporator coil. The suctionlines from the evaporator headers should

join at a point lower than the lowestsuction header outlet. This arrangementdrains the coils and prevents oil andrefrigerant from “backfeeding” from one

coil to the other(s).The common connection should then riseabove the height of the coil beforeproceeding to the compressor to preventthe refrigerant and oil in the evaporatorfrom “free-draining” into the suctionline.

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Provide a 1-inch pitch toward theevaporator for every 10 feet of run to

Table 1

Minimum Suction-Line VelocitiesFor Oil Entrainment

Nominal

Pipe Size, in.

Refrigerant Velocity, fpm

Riser Horizontal

1-1/8 700 525

1-3/8 780 585

1-5/8 840 630

2-1/8 980 735

2-5/8 1080 810

3-1/8 1180 885



“providing insights for today’s HVAC system designer” 3 s

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When designing the liquid line for a

typical air-conditioning application (i.e.

one with an operating range of 40 F to115 F), remember that every 10 feet ofvertical rise will reduce subcooling by2.8 F, while every 10 feet of vertical dropwill add 1.1 F of subcooling.

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Specify one expansion valve perdistributor. An expansion valve servingmore than one distributor will distributethe refrigerant unevenly.

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Provide a 1-inch pitch toward theevaporator for every 10 feet of run. Sincethis pitch equals that of the suction line,

the two may be run together.

If the system is designed with a liquid linerise, a column of liquid refrigerantremains atop the volume of refrigerantgas in the condenser whenever the unitstops. The liquid refrigerant willeventually drain down the line and mayfill the condenser … perhaps even

overflow to the compressor. Sloping theliquid line toward the evaporator createsa gas trap at the line’s highest point,preventing liquid refrigerant frompassing.

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Solenoid valves are required. Theyprevent liquid refrigerant from filling theevaporator when the compressor stopsand slugging when the compressorrestarts. Adding solenoid valves alsoprevents siphoning which could allow anelevated column of liquid to overcomethe gas trap and flow back into thecondenser and compressor.

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If the liquid line must be routed throughan area warmer than outdoor airtemperature, insulate the line to preventthe refrigerant from flashing.

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Be sure to include a replaceable-core

filter drier to permit proper system

cleanup and moisture removal. The coreshould be changed whenever the systemis opened for service.

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Consider adding a moisture-indicatingsight glass to permit a visual check of theliquid column for bubbles. However,never use the sight glass todetermine whether the system isproperly charged! Instead, eithercharge the system based on the requiredsubcooling or calculate the amount ofrefrigerant needed and add it based onweight.

Discharge Line. Limit the pressuredrop in the discharge line to 6 psidwhenever possible to minimize theadverse effect on unit capacity andefficiency. While a pressure drop of asmuch as 10 psid is usually permissible,note that a 6-psid pressure drop reducesunit capacity by 0.9 percent andefficiency by 3 percent.

To design the discharge line properly,follow the guidelines recommended

below:

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Choose the shortest route from thecompressor to the condenser.

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Use different pipe sizes for horizontaland vertical lines to make it easier to

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match line pressure drop and refrigerantvelocity to discharge-line requirements.

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To assure proper oil entrainment andavoid sound transmission, size the

discharge line so that refrigerant velocityequals or exceeds the minimum velocityin Table 2 and remains below 3,500 fpm.

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Prevent oil and condensed refrigerantfrom flowing back into the compressorduring “off” cycles by: (a) pitching thedischarge line toward the condenser and(b) routing the discharge line so that itrises to the top of the condenser, thendrops to the level of the condenser inlet,creating an inverted trap.

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Double risers are unnecessary. The scrollcompressors in current Trane commercial

units unload to the extent that a single,properly sized riser can transport oil at

any load condition.

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Riser traps are also unnecessary. Avoidusing riser traps. If the discharge riser issized to maintain the proper refrigerantvelocity, adding a trap will only increasethe pressure drop.

Hot Gas Bypass Line. “Hot gasbypass” (HGBP) describes the refrigerantsystem design historically used to providestable operation at low loads while

avoiding coil freeze-up and compressorcycling problems. When the load fallsbelow the compressor’s minimum stageof loading, the modulating HGBP valveattempts to maintain suction pressure by

Table 2

Minimum Discharge-Line VelocitiesFor Oil Entrainment

Nominal

Pipe Size, in.

Refrigerant Velocity, fpm

Riser Horizontal

7/8 375 285

1-1/8 430 325

1-3/8 480 360

1-5/8 520 390

2-1/8 600 450



s 4 ENEWS-27/4

A publication ofThe Trane CompanyWorldwide Applied Systems GroupLa Crosse, WI 54601-7599

Printed on recycled paper as part ofThe Trane Company’s recycling program.

diverting hot gas from the compressordischarge to the evaporator inlet.

This strategy reduces the system’scooling capacity since the divertedrefrigerant bypasses the evaporator. Italso increases compressor energyconsumption significantly and can causeunstable operation if improperly applied.

It’s good design practice to avoid hotgas bypass for comfort coolingapplications, particularly those withvariable-air-volume (VAV) systems.

For other types of applications, observethese guidelines:

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Size the HGBP line to carry only thedesired amount of gas at a pressure dropequalling the sum (usually 20 to 40 psid)of the pressure drops for the dischargeline plus the condenser and subcooler

plus the liquid line. Oversizing the linecan cause the HGBP and thermalexpansion valves to “hunt.”

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To assure proper oil entrainment, size theHGBP line so that refrigerant velocityequals or exceeds the minimum velocityrecommended in Table 2.

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Elevate the HGBP valve above the

discharge line and the distributor inlet to

promote free drainage away from theHGBP valve and toward the distributor ordischarge line.

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Insulate the HGBP line to prevent therefrigerant from condensing during lowloads.

Bottom Line …

Reliability determines the success of asplit air-conditioning system.

Interconnecting refrigerant lines play aninstrumental role in that success. It’s upto us to ensure that our system designpractices evolve with equipmenttechnologies.

This article revisited split-systemapplication guidelines in light of currentscroll compressor technology. They canbe summarized as five fundamental“rules”:

1 Choose the right system, i.e. don’tspecify split-system equipment whena chiller is best suited for the job.

2 Size the interconnecting lines to avoidthe use of traps and double risers.

3 Slope the suction and liquid linestoward the evaporator.

4 Minimize the length of theinterconnecting tubing.

5 Keep the system clean. s

By Paul Solberg, applications engineer,and Brenda Bradley, informationdesigner, The Trane Company. (Paul plansto publish more detailed information on

this subject in 1999.)

If you’d like to comment on this article, send a note to The Trane Company,Engineers Newsletter Editor, 3600Pammel Creek Road, La Crosse WI54601, or to www.trane.com.

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Large Splits Refrigrant Piping Design - Engineers Newsletter