compressor line

[bowtieboy77]

On the back of many bigger air compressor pumps there is a small copper tube that comes off the back of the compressor that goes to the unloader. Some compressors have the line some do not. What is this line for?

My old 5hp compressor pump had the line on it that tee into the tank check valve and the other side of tee goes to pressure switch unloader. My new pump does not have the line from the back of the pump. I believe the operation that will occur on my pump is when pressure reaches pressure switch cut out the pressure switch will disconnect power to the mag switch coil stopping motor. Also at same time the unloader valve will be activated which will bleed off the air between the pump heads and above the check valve.

Only purpose I can think of is the pumps that have this line purge crankcase pressure during unloading where as my new pump has a vent tube at the front between the cylinders. Is this correct?

 

[Milton Shaw]

That is the line that actually unloads the compressor when the pressure switch cycles and opens the valve in it. The air bleeds off the head/valves and check valve holds the pressure in the tank. This is so that the compressor starts the next time without a load. Some compressors had an automatic check valve that does the unloading when the air flow stops so no line is required. This means that if you loose power with the compressor running. When it restarts it will have full pressure on the compressor and may not start as the motor should be overloaded. Some pressure switches have a manual lever that can be used to cycle the pressure switch to overcome this problem with restart after power outage.

 

[bowtieboy77]

Thanks Milton I understand the purpose of the unloader I was just curious why some compressors have a line coming from what looks to be the crankcase of the pump that tees into the line from the tank check valve to the unloader.

 

[Milton Shaw]

Some compressors have a valve activated by compressor rotation on the crankshaft that releases the air pressure, that's why they have the line to them. Some other as I said use the pressure switch, others use a combination check valve that releases pressure.