Low Load vs. Low Ambient.... I posted this on social media, but on here we go deeper! This image is from a clean room condensing unit swap we did in the summer. We needed to cut into the discharge line in order to tee in a hot gas bypass line down to the evaporator. In order to pull the refrigerant charge, these stubs were installed and evacuated so we could release the charge and recover it from the unit. Okay, low ambient controls like an ICM board or fan cycling switch is used to maintain head pressure in low ambient conditions. Low load is something else as in this case, we talk about low evaporator load or low room temperature. For instance, generally speaking a room maintained at 68⁰F will have a low evaporator pressure and low SST (saturated suction temperature) that is in jeopardy of freezing up. This particular room had a set point of 68⁰F. To prevent freeze ups or short cycling we provide hot gas directly into the evaporator, this raises the evaporator pressure and temperature at a predetermined set point, this is known as a false load. A hot gas bypass valve either mechanical or electronic is used to meter the flow of hot gas. So in short... We deal with low ambient by controlling head pressure/temperature. We deal with low load by controlling evaporator pressure/temperature. Clear as mud? Happy HVACing! #teachme

Posted by HVAC Know It All at 2021-02-25 00:58:22 UTC