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12/05/2001 Split System Refrigerant Satellite BroadcastQuestions/Answers

Application Installation

Auxiliary Components Line Sizing

Circuiting/Condensing units Miscellaneous

Compressors Rules/Guidelines

Controls Sizing Components

Heat Pressure Controls Suction Accumulator

Hot Gas Bypass Traps

Application

When should I use a split system, and when should I use a chilled water system?

Chilled Water should be used when tight temperature or humidity is required. Chilledwater should be used when the loads vary greatly or when low temperature is beingmaintained. Chilled water should be used when line length or rise exceeds normalpractice. Chilled water should be used when high efficiency, or when critical reliability isnecessary.

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Auxiliary Components

If liquid riser is close-coupled to a condensing unit, is a receiver of value?

A receiver is only of value when the system has extra components that cause theoperating charge to change through the envelope. These extra components are specialand should be avoided if possible.

Are there new rules for ball valve pressure drop?

The pressure drop through a manual ball valve (typically used for isolating a filter drier forservice) is very small and is considered to be negligible.

Explain the “key point” for isolation valves on humidity sensitive applications.

[Our slide said:“Provide >5°F subcooling at expansion valves”

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and the script said:

“The key point here is that we need to maintain 5F of subcooling at the TXV.This is at all conditions within the operating envelope. We arbitrarily chose 5degrees as our factor of safety. This factor of safety is for variances such asrefrigerant charge, load, liquid filter cleanliness, and pipe construction.”

In a constant-volume system, a “trim” or unloading solenoid valve can be used to shut offa section of the evaporator coil as compressors are unloaded. This more closely matchesthe available coil capacity with the capacity of the operating compressors. The result isthat the surface of the “active” section is colder than if the entire face of the coil wereactive. A colder coil surface results in cooler air and more moisture removed from the airpassing through it. Of course, the “inactive” section of the coil does no cooling. However,the mixture of the dehumidified air leaving the “active” section of the coil and theunconditioned air leaving the “inactive” section is generally at a slightly lower dew pointthat if the entire section of the coil were to remain active.

Refer to the evaporator piping diagrams contained in the RAUC application guide (Traneliterature number SS-APG001-EN) for specific examples.

On large systems with multiple evaporator circuits, how necessary is it to havefilter-driers and sight glasses at each individual evaporator circuit versus acentral filter/drier?

An evaporator coil with multiple circuits (distributors), each distributor must be served bya dedicated TXV. In an application where a single liquid line feeds multiple TXVs, a singleliquid line filter drier and moisture-indicating sight glass can be used on the commonliquid line, prior to splitting off to feed the TXVs.

Are ball valves required on the liquid line on small split systems?

No, ball valves are not required on any system. Rather, that is a decision made forreasons of serviceability, typically for isolating and removing a filter. You can close toleave off the ball valve and instead evacuate, reclaim, and recharge a small section of linewhen service is required. In some cases, this will be a simpler approach.

Ball valves are highly preferred over globe valves because of a lower pressure drop.

Does Trane recommend liquid and suction line filter-driers, as shown in theRAUC application guide, for all split systems?

For systems about 7.5 tons and larger, both a suction filter and a liquid line filter-drier arerecommended. These protect the relatively expensive expansion valve and the even morevaluable compressor. They are generally not necessary on smaller systems

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Circuiting/Condensing Units

On an evaporator coil with 4 TXVs and 2 condensing units, how should they bestaged? (Sketch shows 2 condensers, each with 2 circuits.)

Coil connections depend on whether the system is VAV or constant-volume, and how thecoil is circuited. Please refer to the evaporator piping diagrams contained in the RAUCapplication guide (Trane literature number SS-APG001-EN) for specific examples.

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Which Trane condensing units use pump down cycles? When was the switch-over made?

As of January 4, 1999, Trane model RAUC (20- to 120-ton air-cooled scroll) condensingunits have shipped with controls to initiate a refrigerant pumpdown cycle on eachrefrigerant, when the last compressor on that circuit shuts off. This requires at least onefield-installed solenoid valve in the liquid line near the evaporator for each circuit.

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Compressors

I thought scroll compressors were not to be pumped down?

A scroll compressor can be used for pumpdown (approx 35psi). The Scroll should not beused for low pressure pumpdown, or pulling a system into vacuum.

Is Trane looking at magnetic bearings for compressors? How does this affectline sizing and system efficiencies?

Magnetic bearings are cost prohibitive on a scroll compressor at this time.

Are the guidelines for reciprocating and scroll compressors the same?

The guidelines presented in this broadcast are for commercial unitary equipment (greaterthan 7.5 tons) with scroll compressors, operating with R-22. The rules are basically thesame for equipment with reciprocating compressors. Equipment with screw compressorshave their own rules and line sizes. Remember to involve the manufacturer in all cases.

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Controls

With one solenoid feeding multiple TXVs, when the system is operating at partload (less compressor capacity than the TXVs), will the TXVs be operating belowtheir rated range of control?

It depends on the type of TXV. DX split systems that have refrigerant circuit that unloadshould use balanced-port, externally equalized TXV that have a good throttling range.Refer to Table 1 in the RAUC application guide (Trane literature number SS-APG001-EN)for specific TXV suggestions.

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Heat Pressure Controls

Can the subcooling lost due to oversized lines in low ambient conditions beovercome or minimized with the use of head pressure control, such as a fanspeed controller designed for low ambient control? Penn or Hoffman variablespeed condenser fan controllers?

Head pressure control does improve subcooling at low ambient conditions. However, it isdifficult to say whether it will help enough without analyzing the specific applicationdeeper. Contact the manufacturer for assistance.

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What is your opinion on the use of a reciever and a head master set-up to controlhead pressure during low ambient conditions?

If air conditioning is necessary below 0F, then a flooded system is a viable option.However, the system should be designed so that when it is that cold, free cooling can beachieved.

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Hot Gas Bypass

If it is not possible to avoid HGBP, and it is not possible to inject into theevaporator, can a RAWAL kit work?

Certain applications can benefit from the Rawal device. Each application must be reviewedfor applicability.

It was stated not to use hot gas bypass. What do you suggest to use in itsplace?

There are many attractive alternatives to hot gas bypass in comfort-cooling applications.For VAV systems, use an intertwined evaporator coil. Better controls that sense suctiontemperature to measure coil frosting, such as Trane’s Frostat control, can be used to cyclethe compressor and prevent frosting.

For some process applications, hot gas bypass may be viable and in fact, is often crucial.

At the piping connection of hot gas bypass line into the liquid line (downstreamof TXV), is use of a standard piping tee acceptable, or is a special “hot gasbypass tee” fitting required?

The “special tee” is recommended because it allows for better distribution of gas andliquid when compared to a standard bullhead tee. The special tee has a tube inside thetee, which liquid passes through, and mixes well with the hot gas. By using this type ofspecial tee, you do not have to depend upon the distributor to do this mixing. The SporlanAuxiliary Side Connector (ASC) is one example.

I thought HGBP should be use on all VAV applications…is this wrong?

In the past, hot gas bypass was used on VAV systems to avoid coil frosting at part-loadconditions. Today, we recommend using intertwined evaporator coils and controls tomeasure suction temperature and cycle the compressor (e.g. Trane’s Frostat) to preventfrosting. This is a much more reliable and energy-efficient than hot gas bypass.

Limit HGBP piping to 35 feet?!? What is the real limit…75 ft? 100 ft?

The “real” limit is your threshold for “pain”! While you can indeed run HGBP lines longerthan 35 ft, the longer the line, the more difficult control becomes. The longer line containsmore refrigerant and refrigerant management becomes more of an issue. Recall, whenidle or in the “OFF” cycle, this long pipe can act like a sub-cooler, a condenser, an oil-trap,etc. depending upon ambient conditions. All of these necessitate more controls.

What precautions and safeties can be used if hot gas bypass to the suction line isneeded, instead of to the evaporator inlet.

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Bypassing hot gas to the suction line can only be used if the evaporator coil is locatedabove the compressor. When hot gas is bypassed from the compressor discharge to thesuction line, the mass flow of refrigerant through the condenser, liquid line, evaporator,and suction line is variable. Thus, refrigerant velocity is variable and cannot be assured tobe above minimum limits for any suction line size. This does not impose a problem in thecondenser or the liquid line since the oil is carried along with the liquid refrigerant,regardless of its velocity. However, in the evaporator and the suction line, the velocitymust be high enough to entrain the oil droplets in the refrigerant vapor. This cannot beassured since the HGBP valve modulates. This method can only be used if the suction linedrains freely to the compressor.

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Installation

Please comment on any special safeguards that should be considered whenunderground refrigerant piping is unavoidable.

Manufacture the line outside of the trench. Pressure test, evacuate, insulate, waterproofand hard case the pipe before it is placed in a trench. Avoid putting refrigerant linesunderground.

Should the liquid line be run with the suction line? I believe it can prevent vaporlock at the evaporator if the lines are routed through a hot attic. Also, it canreduce the chances of surging. I understood that the manufacturer does notrecommend this because it effects unit rating and refrigerant charge. What iscorrect?

There is some benefit to running the lines together because of the heat transfer thatoccurs. However, there may be some problems like rubbing and leaks, and what affect aloss of charge may have on gas temperatures.

You said to insulate liquid lines only if routed through warm spaces. Pleasedefine “warm”.

“Warm” means anything warmer than the refrigerant inside the liquid line. For an air-cooled condenser, it is easiest to define it by saying if routed through any area warmerthan ambient conditions.

What if I have a system installed with oversized lines? Should I change them?

If no problems have occurred then just leave the system as is. If problems are evident,then the entire system as a whole needs to be reviewed for correctness. This includescontrols. Then, the appropriate corrective action can be taken.

As engineers we are invariably faced with long-line applications. Any basicrecommendations on line sizing, use of accumulators, etc.?

Layout the shortest, simplest, most direct line routing as possible. Use the smallest linesizes possible. Recommend chilled-water when the distance is too great. Finally, consultthe manufacturer for line sizes.

What is Trane’s recommended charging procedure? Use the subcooling at theTXV? At full load?

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The proper method of charging is the subcooling method. To do this the unit needs to befully loaded and all the condenser fans should be on. With this, the measurements shouldbe taken at the condensing unit.

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Line Sizing

Where should you change sizes on liquid line sizes to multiple expansion valves?Example: 7/8” liquid line serving 3 valves. Valves have 5/8” liquid connection.Should you downsize at the valve which then leads to other components beinglarger, or downsize where liquid line splits?

It depends on whether there are any refrigerant specialties between the point where theline splits and the TXVs. If not, then it does not matter where you reduce from 7/8” to5/8”. If each TXV has a dedicated solenoid valve upstream of it, then downsizing thebranch lines from 7/8” to 5/8” right after the split would allow you to use slightly smallersolenoid valves.

Can you briefly describe line sizing between a self-contained indoor cooling unitand a condenser? (i.e., liquid line, hot gas line)

This isn’t a brief topic. The discussion in the entire b-cast applies to these systems as well.

What software do you want Trane sales personnel to use for our large and smallsplit systems?

None. For RAUC use SS-APG001-EN and for TTA/TWA use the cataloge line sizes.

Does the Trane Reciprocating Refrigeration Manual reflect the revised R/d ratioequivalent length data for the pipe sizing tables. If not where is this informationavailable?

No, the R/d data used to calculate equivalent lengths for refrigerant fittings is notcontained in this manual.

The R/d equivalent length data is used by the Trane Engineering Toolbox software, andcan be found in the program’s online help file.

Since the pressure drop of elbows is less, can I use a longer suction line withmore elbows?

Since the liquid line is the same length as the suction line, increases to the length of thesuction line will also increase the length of the liquid line. This in turn increases therefrigerant charge. That is not what we want. Use the simplest and most direct line layout

What if the line lengths or risers go beyond the envelope?

You need to contact the manufacturer because there may be something specific they cando, or there may not be anything they can do! Specifics about that job are required andthe manufacturer must be involved.

You stated that an incorrectly sized suction line could cause frosting at thesuction line or compressor…why?

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If the suction line is sized too small, the resulting high pressure drop may cause thesaturated suction temperature to drop and the compressor can build frost.

Do the requirements for piping and accessories vary between tonnage ranges ofcondensing units; i.e., 1 – 5 tons, 6 – 20 tons, 20 –-120 ton s?

Good question. The focus of this broadcast was on “commercial” split system, which isdefined as 7.5 tons and larger. Thus, all the requirements discussed are applicable forthat range. Smaller equipment is a little different, and the manufacturer should beconsulted.

When sizing liquid lines and determining the subcooling lost from pressure drop,what is considered a rise? Condenser above evaporator, or evaporator abovecondenser?

A rise, or riser, is that section of the system piping where the refrigerant travels upward.If the condenser is located below the evaporator, then the liquid line must “rise” andconversely the suction line must “drop”.

Does the Trane Engineer’s Toolbox software incorporate the new rules for linesizing?

Yes, this software incorporates the R/D method described in the broadcast, which moreaccurately portrays equivalent lengths of fittings. Also, the “new” minimum line velocitiesdiscussed in the broadcast are also used in that software. However, this program does notknow what condensing unit is being used. Therefore, it cannot indicate when a suctionriser must be downsized to allow for proper oil return at part load. Also, it can be difficultto choose the proper liquid line because subcooling at the condenser is a required input.As Paul mentioned, the subcooling “produced” by the condenser varies throughout theoperating envelope; it is not a constant value. Therefore, always use the line sizesrecommended by the manufacturer, if provided.

Is the Trane SSP selection program still current for line sizing?

No, it is not current.

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Miscellaneous

Do liquid refrigerant pumps help for efficiency or odd-ball applications?

Liquid line pumps can solve some installation problems. But, these problems should beavoided on new designs. The cost/benefit analysis does not allow for an attractivepayback to many users.

They are also another part in the system which can fail.

Does Trane plan on revising its Reciprocating Refrigeration Manual toincorporate the new ideas presented in this seminar?

Yes, indeed we do. Our goal is to write several new product-specific new applicationguides in 2002 and revise the refrigeration manual in 2003.

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Rules/Guidelines

With all the changes/improvements discussed today, does Trane have availablea “master” guideline which customers can use?

Currently we have SS-APG001-EN and Engineers newsletter Vol 27 #4. We are workingon Application guides for TTA and TWA.

Can you discuss the applications of these guidelines to split system heat pumps?

In the cooling mode, the liquid line is the critical line, just as it is in cooling-only (non-heat pump) applications. In the heating mode, subcooling often increases and so is not ascritical.

Again, use the manufacturer’s line size recommendations.

What are the rules and guidelines for refrigerant blends that replace R-22?

The line sizing process will be common between R-22 and replacement refrigerants..However, the specific values of minimum velocity, acceptable operating envelope, andpressure drops will likely change.

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Sizing Components

I have seen a number of mismatched split systems in the past year. I’m not sureif this is a mistake or there is some kind of reason. For instance, a TWE120 witha TTA180 on the roof. What would be a reason or purpose of mismatching likethis? I’m confused on this subject of mismatching.

In the Trane Odyssey light commercial product line (7.5- to 20-ton model TTA condensingunits and matched TWE air handlers), the evaporator coils are fixed. In other words,unlike in larger commercial air handlers, the number of rows and fin spacing are notselectable.

In order to meet specific job conditions, particularly when the required cooling capacity isrelatively high for the airflow required (cfm/ton), it may be advantageous to use a largercondensing unit. The example you site connects a nominal 10-ton air handler with anominal 15-ton condensing unit. The resulting operating capacity (balance point) will besomewhere in between.

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Suction Accumulator

In what applications would you recommend a suction accumulator?

In all our experience, for commercial applications, we are not aware of one.

Is the application of an accumulator recommended when condensation in thesuction line is probably unavoidable?

The specific circumstances need to be investigated. There are other ways to avoid thistype of problem.

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At what suction line length does Trane recommend using a suction lineaccumulator?

We do not suggest using accumulators on any system that is within the operatingenvelope.

Please explain why we used to recommend using suction accumulators if lengthwas over 70 feet and add a solenoid valve if over 125 feet, and now we don’t.

Suction line accumulators can cause problems by logging oil. They also increase thecharge of the system (the thing we try best to avoid). Accumulators may not work in allcases as they may pass liquid directly through them.

In units with high variances of coil load, i.e., large ratios of outdoor air intake, isthere a need for a receiver or a suction line accumulator for managing therefrigerant?

Coil load can not vary enough to cause the requirement for a receiver or acccumulator.Receivers may be necessary on built-up systems that use modulating or active/inactiveheat recovery coils.

Please comment on the use of field-installed subcoolers and accumulators forboth cooling units and heat pumps.

More than likely, when a system requires subcoolers or accumulators, the validity of itbeing a split system application, is in question. Reliability is the key. The fewer the parts,the greater the reliability.

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Traps

What are all the purposes of a trap? In other words, what actually happensinside the piping to cause manufacturers to require traps at the base of vaporrisers, and at periodic intervals in long vertical risers?

Why do some manufacturers insist on use of traps and others not?

Historically, it was believed that suction riser traps caused enough turbulence in the baseof the riser that the oil droplets, which is circulated with the refrigerant and accumulatesin the trap, is broken up into a mist which can be carried up to the top of the riser. Today,we understand that oil is moved in a tube by the force of mass flux density. With thisunderstanding, it is possible to size a line, without a trap, which will carry the oil duringminimum loads. The storage capability of the trap is unnecessary if the suction line isslightly sloped away from the compressor.

Bottom line, if a suction riser is properly sized, oil will return to the compressor regardlessof whether a trap is present. If a suction riser is oversized, adding a trap will not restoreproper oil movement.

Can you trap the 1/8 “ line from the drip leg of the HGBP in the followingconfigurations: as a traditional “vertical” trap; “horizontal trap”; or a vertical orhorizontal loop?

Yes, all of the configurations you list are valid. We suggest you minimize the use andsize, however.

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