How to measure superheat and subcooling

[Peter_1]

When I started in refrigeration, I learned the job from an older tech (then +/-55 years old) or let me sya...an experienced tech.
When we did an install in a restaurant or a hotel for a smaller unit and like in many places, the compressors were placed in the cellar, he never installed a thermostat, only a LP switch.
He regulated the LP switch always with a ***** bottle and his manifold before connecting it to the circuit.
Once the start point was set at 2°C, he locked it with a little bit of leak lock so that they shouldn't turn on this knob.

I often still see nowadays on older installations 2 thermostats: the room thermostat shuts off the evaporator fan and a second with the bulb fitted between the fins shuts off the compressor (set at start 2°C and a DT of +/- 8K)
The second thermostat serves as a defrost thermostat.

It is a relay simple setup - often used in the past - and the evaporators never frost.

[iceman007]

Hello Ken,

You mention the piping runs being short unless they are built for remote installation, so I would imagine these have condensers inside the space. To achieve the temperatures you are talking about, you are probably needing to evaporate at at least minus 10 degrees C, maybe lower. I assume that you are using electric defrost, on a timer system with a pump down system off the thermostat, which many european manufacturers sometimes do. I used to do alot of work with dairy cabinets, usually I have not seen a HP control, due to the low evaporating temperatures used, because the pressures used in the evaporator to do the job are lower than the point at which ice build up will form. HP control is generally used for situations where you don't necessarily utilise a defrost, for example cellar coolers,and IT rooms a/c systems where you are running at much higher evaporating temperatures. If you are using a remote condenser/receiver outside then depending on where in the country you are you may need to install a check valve to stop migration of refrigerant to the condenser (especially if lower than the compressor). Normally two thermistors can be used to control- one for cabinet temp, and one in the coil for defrost (which I think Peter mentioned) Set a 5 minute delay on the evaporator fans follwing a defrost cycle to ensure that the coil comes down to temp before hot air etc gets blown all over the place. If you do that, and build on the advice from others in the thread you will be fine.

Best Wishes

[US Iceman]

He regulated the LP switch always with a ***** bottle and his manifold before connecting it to the circuit.

This is a very good idea to use. Any pressure switch should be checked for the intended operation. You can use the refrigerant pressure from a bottle (I would not recommend this in today's legal world). However, you can obtain the same result by operating the compressor down to a lower suction pressure to make sure the switch operates at the desired pressure.

Otherwise, you can plan on setting there to watch the switch operation.