Trane Engineers Newsletter, volume 32-2

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most often involve refrigerant-to-airevaporators. With countlesscombinations of equipment and layoutspossible, how can you best and mostresponsibly implement hot gas bypass?Figure 1. Hot gas bypassed to evaporator inlet (preferred arrangement for HGBP)Apply HGBP as a last resort. Onlyuse hot gas bypass if all other designoptions fail to meet the demands of theapplication. To make this determination,it is critical to …Understand the year-round loadsand requirements of the system.“Most comfort cooling applicationscan operate between steps of loadingwithout loss of temperature or humiditycontrol.” Not only does this statementhold true between the third and fourthsteps of cooling, when the outdoorlatent load is high, but it also holds truefor the step between off and first-stagecooling when the outdoor latent loadis comparatively low.Choose the right arrangement. Hotgas bypass is arranged in one of twoways (see the comparison in Table 1,p. 4); both require special care duringdesign and installation. 1 HGBP to theevaporator inlet delivers hot refrigerantvapor between the expansion valve andthe distributor (Figure 1). During HGBPoperation, the expansion valve metersenough liquid refrigerant to bothdesuperheat the bypassed vapor andsatisfy the evaporator load. Theresulting refrigerant flow rate issufficient to carry oil through the coiland suction line.HGBP to the suction line bypassesboth the condenser and the evaporator,diverting hot vapor from thecompressor discharge directly to thesuction line (Figure 2). A liquid-injectionvalve meters liquid refrigerant into thestream of bypassed vapor, cooling it1 For more information, see Hot Gas BypassControl (Trane publication AM-CON10) and the1998 ASHRAE Handbook–Refrigeration.Keys to successful implementation:1 Position the HGBP valve above the dischargeline, near the compressor. If the systemincludes pump-down, provide a means to shutoff refrigerant flow.2 Pitch the line upstream of the HGBP valve todrain oil back into the discharge line.3 Pitch the line downstream of the HGBP valvetoward the evaporator, away from the valve.4 If the HGBP line includes a riser, regardless ofheight, provide a drain leg of the sameenough to prevent the compressormotor from overheating.Do not oversize the system. Anoversized system quickly meets thesensible load without satisfying thelatent load. Adding HGBP may mitigatethis error … but at the expense ofunnecessarily high power bills.Once again, understanding the yearroundloads and properly sizing thesystem to match them is paramount toproviding thermal comfort (controllingtemperature and humidity) andeliminating the need for HGBP.Select equipment with multiple-steprefrigerant circuits. The logic foravoiding system oversizing alsosupports the choice of multiple-steprefrigerant circuit(s) whenever possible.The capacity of an unloading refrigerantcircuit will attempt to match a reducedload at better balance points.diameter as the riser. Add an oil-return line1 in. (25 mm) from the bottom of the trap; usetubing that is 1/8 in. (6 mm) and at least 5 ft(300 mm) long. Precharge the trap with oil.5 Divert hot gas to each active distributor at theexpected operating points for hot gas bypass.6 If the HGBP line feeds multiple distributors,provide a check valve for each distributor.7 Insulate the entire length of the HGBP line.Select an evaporator coil thatcan maintain a high suctiontemperature to permit the system toaggressively stage down before a frostcondition develops. In all cases, thedesign should maximize the suctiontemperature while maintaining thedesired conditions. For comfort coolingapplications, the minimum saturatedsuction temperature at design shouldbe 43°F to 45°F for variable-volume airdistribution (VAV), or 40°F to 43°F forconstant-volume air distribution (CV).Coils with intertwined circuits tend toreduce the risk of coil frosting becausethey use more of the available finsurface (than face-split coils) at partloadconditions. This allows the systemto operate at higher suctions and morereliable balance points.■ 2 Trane Engineers Newsletter — Vol. 32, No. 2
Figure 2. Hot gas bypassed to suction lineKeys to successful implementation:1 Position the HGBP valve above the dischargeline, near the compressor. If the systemincludes pump-down, provide a means to shutoff refrigerant flow.2 Pitch the line upstream of the HGBP valve todrain oil back into the discharge line.3 Pitch the line downstream of the HGBP valvetoward the suction line, away from the valve.4 Assure that the evaporator and suction linefreely drain to the suction-line HGBPconnection.Target the cooler, drier part of thethermal comfort envelope.Compressor cycling usually isn’t anissue for comfort-cooling VAV or CVapplications. But this strategy providesmore latitude for the space condition to“float” during compressor stagingwithout sacrificing thermal comfort.Comply with local codes. ASHRAE/IESNA Standard 90.1, Energy Standardfor Buildings Except Low-RiseResidential Buildings, precludes theuse of hot gas bypass for HVACsystems larger than 7.5 tons unlessthey are designed with multiple stepsof unloading or continuous capacitymodulation. The maximum amount ofHGBP allowed by the standard varieswith system size: 25 percent of totalcapacity for systems larger than20 tons, 50 percent for systemsranging from 7.5 to 20 tons. 25 Site the suction-line HGBP connectionupstream of the pilot-line tap for the HGBPvalve and at least 5 ft (1.5 m) upstream from thecompressor inlet. Angle the connection into thesuction flow.6 Attach the remote bulb for the liquid-injectionvalve to the suction line, downstream of theHGBP connection.7 Provide a solenoid valve upstream of the liquidinjectionvalve. Synchronize the operation of theHGBP and liquid-line solenoid valves.8 Insulate the entire length of the HGBP line.Line Design ConsiderationsMake gravity work for you. A typicalHGBP valve can infinitely vary the rateof hot gas flow. The resultingrefrigerant velocity can become so lowthat it “traps” oil in the HGBP line.Gravity then becomes the sole meansfor returning oil to the compressor. It istherefore critical to design therefrigerant piping system so that all oil(and condensed refrigerant) drainsfreely out of the HGBP line.Suction-line lift and the design of theevaporator coil particularly limit the useof HGBP to the suction-line. Because2 Standard 90.1 (which excludes processapplications and low-rise residential buildings)grants an exception to systems smaller than7.5 tons. Be sure to consult the standard forspecific requirements.this arrangement bypasses theevaporator and part of the suction line,the refrigerant flow rate upstream ofthe HGBP point of entry may becometoo low to move oil. To overcome thislimitation, restrict the maximumamount of HGBP to a quantity thatmaintains sufficient refrigerant velocityfor oil entrainment.Note: If this design cannot beaccomplished, it is critical to assurethat oil drains freely from both theevaporator and the suction line to theinlet of the HGBP line. Failure to do sowill starve the compressor of oil andshorten the life of the compressor.Independent of which HGBParrangement is applied, the HGBP linemust connect to the top of both thedischarge line and the evaporator inlet(or suction line). Otherwise, a mixtureof oil and refrigerant will pour into theHGBP line, starving the compressor ofoil and causing a liquid slug when theHGBP valve opens.The refrigerant that collects in theHGBP line also reduces the refrigerantcharge available to the system, whichmay lower the suction pressure.Ironically, the HGBP valve may respondby opening and allowing bypass flow ofhot gas. The repetitive, unstableoperation and slugging that result makeit critical to prevent this condition.Select the appropriate pipediameter. With HGBP lines, typicallysmaller is better to promote oilmovement; however, a design thatallows gravity to purge any liquid fromthe HGBP line lessens thisrequirement. Size the line for apressure drop of 6 psid to 20 psid.Keep the HGBP line short.Refrigerant vapor will condense in, andfill any portion of, the HGBP line that iscolder than the saturated temperatureof the suction or discharge gas.Incidences (and consequences) of“providing insights for today’s HVAC system designer”3 ■
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Trane Engineers Newsletter, volume 32-2

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