Sometimes for an Operating Room (a second coil or a packaged DX for a mobile unit hospital) a commercial Split DX System with a Constant Volume Draw-Thru AHU and matching condensing unit will require a 49° F LAT off the coil to maintain space temperature and humidity, the system must operate continuously with no fluctuation. The biggest concern is to avoid coil freeze up from a low SST, dirty coil, clogged condensate drain, or coil restriction. When ice begins to form it can take several hours to thaw, and could trip the fan motor due to high static. Low suction pressure will cycle the compressor off; it will take a minimum of five minutes to restart the compressor, which is unacceptable if a surgery is in progress. To avoid coil ice up, SST must be controlled.
Saturated suction temperature is the temperature of the refrigerant when it is 100% vapor at the specified pressure. Any increase in the temperature to the vapor will result in a super heated state. Super heat ensures no liquid refrigerant will be circulated back to the compressor, avoiding damage to the compressor. The industry standard is 8° F to 12° F of super heat.
Liquid refrigerant is metered through the coil by a Thermal expansion valve (TXV) or capillary tube(s), the metering device is the dividing point between the high-pressure and the low pressure side of the system. The refrigerant leaving the metering device is about 75% liquid and 25% gas. Liquid refrigerant will boil off approximately 90% of the way through the coil; reaching saturated vapor (saturated suction temperature).
Refrigerant has a temperature pressure relationship, meaning that at a specified pressure it will correspond to a specific temperature, any temperature above that is super heated vapor, and any temperature below that is sub-cooled liquid. To calculate super heat the suction pressure must be known and the actual temperature of the suction line must be known, the closer to the coil, the more accurate, for example, R-22 at 68.51 PSIG has a saturated vapor temperature of 40° F, per the chart, but actual suction line temperature is measured at 50° F, thus yielding 10° of superheat.
In a low load condition the TXV will throttle down, reducing the suction pressure and correspondingly the SST will also decrease. Many publications indicate that with a SST of 38° F ice begins to form, this will create a low suction pressure, the compressor will cycle off or hot gas by-pass will initiate, thus a fluctuation in temperature and humidity will occur. To prevent this some manufacturers recommend a SST of 40° F – 43° F for a Constant-Volume AHU. For critical applications where cooling cannot cycle or have temperature fluctuation, having a SST less than 40° F SST creates an undesired risk, even when utilizing a defrost cycle.
