The main purpose for adding Hot Gas Bypass (HGBP) to any system is to prevent condensate from freezing on the surface of the evaporator coil when the system is operating at extremely low load conditions. The HGBP valve is controlled by suction pressure, and its set point should be adjusted as low as possible to limit the amount of gas being bypassed and to minimize the negative effect of HGBP on the operating cost of the system. A set point between 50 and 55 PSIG for R-22 and 88 and 98 PSIG (26 to 30 F) will prevent the finned surface of the evaporator coil from dropping below 32 F. Once the HGBP valve starts to open, the cooling output of the unit will remain constant, but its compressor KW will continue to increase.
Although HGBP will never hurt the reliability of the system, it shouldn’t be added to every unit. There is always a chance of contaminating a system during the installation, and HGBP may be ineffective on systems where:
- The supply air CFM is constant and above 320 CFM per nominal ton.
- Most of the air is being re-circulated.
- The indoor design temperature is 70 F or higher.
On these systems, the evaporator temperature will never approach 32 F. The evaporator temperature will drop as the internal load and outside temperature drop, but the supply air will also drop and this lower temperature air will satisfy the room thermostat before the HGBP valve starts to open.
Adding HGBP for capacity reduction on these systems will not be practical because the set point of the HGBP valve may have to increase to 68 PSIG for R-22 and 118 PSIG for R-410A (40 F) or higher to get it to open, and this will increase the operating cost of the system.
Hot Gas Bypass (HGBP) is more commonly used on systems with:
- 100% Outside Air.
- A variable supply air CFM (Frequency drives or VAV systems)
- An indoor design temperature below 70 F.
- A supply air below 300 CFM per nominal ton.
On the first two of these systems, the cooling load will vary over such a wide range that the evaporator temperature could easily drop below 32 F.
On the last two of these systems, the evaporator temperature will be low at full load and could easily drop below 32 F with a small change in operating conditions. This only applies to units with 1 stage of cooling or face split coils. Units with interlaced DX coils will seldom require any HGBP.
Please note that a system should never be required to bypass more than 30% of its compressor’s lowest stage of capacity. If too much gas is bypassed, too little refrigerant will go into the condenser coil and the unit may experience low head pressure problems.
Also note, HGBP is always piped into the first stage of cooling. It will require a TXV metering device and head pressure controls, and the HGBP line must be routed to the OUTLET of the TXV at the evaporator coil. The HGBP tubing should be insulated with minimum ½ wall thickness insulation for its entire length. The “Rule of Thumb” for sizing is to use the same size as the liquid line.
Attached at the end of this posting is a “typical” HGBP piping layout.
(Most of the information provided for this post was taken from an application bulletin written by George Simonson York UPG Technical Service retired who taught me a lot.)