I thought it was time to start a thread on how to add a pressure switch to a compressor that permits re-starting with pressure on the outlet.Today I realized it wasn't getting hot it just has trouble starting when there is pressure in the output! I had a 25' hose on the end of it before that would stay at about 200psi,, no wonder it had trouble starting!
Most fridge compressors use a small tube between the low pressure side and the high pressure side to act as the expansion valve in a refrigerator. They bleed off the pressure before it re-starts. They were not built to start with a high pressure on the output.
Most shop air compressors are built to start with fairly high pressure in the tank. To make the starting easier, they remove the pressure off the compressor when it shuts off, so it starts with no load. Here is how they do it.
Two components that work together does the job. One is a pressure switch that has an "unloader" valve in it. More on the unloader in a moment.
The second component is a check valve.
The pressure switch has two plumbing fittings on it. One is normally 1/4 inch and screws directly onto the regulator manifold to monitor the tank pressure. The second port for the unloader is connected to the tube from the compressor to the tank.
The diaphragm pressure port is on the bottom of this switch. The unloader port is the smaller fitting on the side.
The pressure switch when it operates pushes a button much like a blow gun so when the compressor cycles off, the button is pushed letting air out of the unloader valve. A check valve at the tank keeps the pressure in the tank and allows the compressor output pipe to bleed off to no pressure. Once the pressure is down a spring in the unloader valve holds this valve open, unlike a blowgun which has a spring to hold it closed.
The reason for the spring is to allow the compressor to start with the valve remaining open. When the compressor is nearly up to speed, the air flow overcomes the spring holding it open and it snaps shut. Now the pipe will fill with pressure and open the check valve into the tank and add pressure to the tank.
If you are doing a fridge compressor and want reliable restarts, consider adding the check valve to the tank and an unloader style pressure switch.
This photo shows a typical in tank check valve with a port for the pipe from the compressor and the small tube that goes to the pressure switch unloader valve.
It appears to be about 4 years old so still fairly young.