How does a hot gas bypass system work and what is its primary application?

A hot gas bypass system works by diverting a portion of the hot, high-pressure refrigerant gas directly from the compressor discharge line to the evaporator inlet or the suction line. This bypasses the condenser and expansion valve for that portion of the refrigerant. The primary application of a hot gas bypass system is to prevent evaporator freeze-up and maintain stable suction pressure during low load conditions. When the cooling load is low, the refrigerant flow rate through the evaporator decreases, which can lead to a drop in suction pressure and potential evaporator freeze-up. The hot gas bypass system compensates for this by introducing hot gas into the evaporator, increasing the refrigerant mass flow rate and maintaining a higher suction pressure. This prevents the evaporator from freezing and ensures stable compressor operation. The hot gas mixes with the refrigerant returning from the evaporator, effectively raising the evaporator pressure. For example, in a walk-in cooler or freezer, the cooling load may be significantly lower during off-peak hours. A hot gas bypass system can be used to maintain a stable evaporator temperature and prevent the coil from icing up, even when the cooling demand is minimal. This comes at the cost of efficiency, as the bypassed hot gas does not contribute to cooling, but it prevents damage to the equipment.