Gauge Readings

The suction pressure gauge reading gives an indication of the chiller temperature. Pressures correspond to saturated temperatures, which may be thought of as the boiling point of the refrigerant. The pressure reading should correspond to a saturated temperature reading about 10 to 15 degrees colder than the circulating water temperature, depending on the peculiarities of your particular chiller. Often a saturated temperature scale is inscribed on the gauge face parallel to the pressure scale.
Compressor discharge pressure is controlled by the amount of raw seawater being pumped through the condenser. Decreasing condenser water flow, which decreases the heat exchange capacity of the condenser, increases the compressor discharge pressure. For most efficient operation, the discharge pressure should correspond to a saturated temperature of 85-95F. Once the system has been running a few minutes, adjust the condenser water by opening or closing the water control valve to obtain the optimum pressure. Adjust gradually, as it takes a few minutes for the pressure to stabilize. If you find the pressure remains undesirably high, the cause may be high seawater temperature (late summer out on the ocean), a condenser that needs to have sludge and scale rodded out, a condenser water pump that is restricted or worn, or non-condensables (air) in the lines with the refrigerant. If high side (discharge) pressure is undesirably low on startup, even with water flow through the condenser restricted or bypassed, then you might be short on refrigerant or you have no load on the chiller. Be sure the circulating pump is turning, and be sure water valves are all in correct positions so all the water is going through the chiller. If there is no load on the chiller the compressor suction line will frost heavily all the way to the compressor.
Don’t forget the possibility a pressure gauge may be faulty, giving a inaccurate reading.
Discharge pressure will drop as suction pressure drops, as brine temperature is pulled down, so occasional adjustment of condenser water flow is desirable during temperature pulldown to keep system operating at best efficiency. If the desired temperature pulldown isn’t too great, as when starting with 45 degree ºF water, some operators will set the discharge pressure a little high on startup and allow it to “float”, and won’t readjust it until desired temperature is reached just before fishing commences. On some systems discharge pressure is controlled by an automatic water valve that controls condenser water flow.