As desired hold temperature is approached, the engineer should start checking the system more often. The compressor is a vapor pump, and if liquid refrigerant enters through the suction side, the compressor suction valves will be stressed and eventually will break. Most systems are equipped with a suction accumulator, which acts as a “slopover” tank to trap liquid refrigerant and allow it to boil off, helping prevent floodback. During refrigerant floodback conditions, such as when the chiller freezes up, the accumulator can fill completely and liquid refrigerant continues on to the compressor. A liquid level probe installed in the accumulator to signal an alarm condition is a recommended safeguard.
Floodback can be recognized by frost extending down the suction line all the way to the compressor and covering a portion of the compressor. The cause is generally freezeup, caused by low salinity in the tank, too low a thermostat setting, chillers clogged with debris, too many fish in the tank, or circulating pump failure. A pressure gauge to measure circulating pump water pressure on one side of the chiller or the other becomes useful as desired hold temperature is reached. A reading that starts to drop on the downstream side, or increase on the upstream side of the chiller, indicates the chiller is freezing up.