I had a call out to diagnose a fault with a dairy cabinet not maintaining temperature. The case thermostat is reading 15 degrees C, it should be around 2 or 3.
There are two hermetic pots. The refrigerant is R404A, and the suction pressures are 40 psi and 80 psi on each circuit. The compressor head has a temperature of 105 degrees C on the 80psi circuit and 55 degrees C on the 40psi circuit-the first one is way too hot. The discharge temp is also very high on the first circuit. When the unit goes to defrost the standing presssure on the first circuit is 95psi (but there is alot of heat in the compressor and lines) and 75psi on the other circuit. During the defrost cycle the cabinet temperature actually falls !!. When the defrost ends the compressors start up, and the first one does not pull down by more than 5-6 psi. The condenser inlet pipes are at 55-65 degrees C, and the outlet is 27 degrees C, but the dT across the condenser(air cooled) is only 4 degrees C.
As there are no service valves on the rig I can't try and pull a vac on the compressor to test the valves. The suction line measured before the compressors are at about 20degrees C or so- way too high. I had a listen to the compressors-using a screwdriver blade against them, and the hot one that is not sucking down sounds noticeably different to the other.
My thought is that the first compressor has bad suction valves-hence the lack of pull down, meaning that the refrigerant in the evaporator is boiling at nearly room temperature, and also as a result the condenser is not condensing properly etc etc.
If anyone else can give an opinoion, I would appreciate it.