Pressure switch unloader valve won't close on its own

[larryq]

Any experts on air compressors and pressure switches here?

I'm building a homemade airbrush compressor using a refrigerator motor and a 1 gal. pancake compressor tank, and it's all set with the exception of the unloader valve.

It never closes / seals, always leaks air and thereby prevents tank pressure from building. I suspect it's because the initial airflow from the fridge motor isn't strong enough to push the spring closed but I'm scratching my head on how to modify it / and or find a replacement unloader that will seal under low cfm.

It's a furnas style pressure switch, similar to this:

I've looked around and don't see different pressure cut-in variations on the unloader valves. They all appear to be the same. I've got several furnas switches and they all behave the same so I don't believe it's a defective unloader. I'm open to suggestions if you have them.

 

[Milton Shaw]

You might want to rethink this. A refig compressor will pump some oil out as it works and that will mess up your air brushing . It also will run out of oil and lock up unless oil is circulating. Get a small compressor from HD , Lowes, Northern, or HF you will end up with a lot less problems and it will end up being a lot better paint experience.

 

[rlitman]

I'm missing something here. Every unloader I've had, has a snap action switch that flips based on tank pressure. The closed electrical position powers the pump, and closes the unloader valve simultaneously, while the open electrical position closes the unloader valve.

I don't get where CFM comes into the equation.

 

[Jeff May]

The unloader valve on the pressure cut-off switch will leak without a "check valve" to keep the air in the tank from coming back to the compressor.

 

[Motorman55]

The unloader only 'unloads' the pressure that remains in between the tank check valve and the exhaust valve on the compressor itself.

You must install a check valve in the tank.

 

[larryq]

You might want to rethink this. A refig compressor will pump some oil out as it works and that will mess up your air brushing . It also will run out of oil and lock up unless oil is circulating. Get a small compressor from HD , Lowes, Northern, or HF you will end up with a lot less problems and it will end up being a lot better paint experience.

The advantage of the refrigerator motor is it's whisper quiet. In fact, many of the commercial 'silent' airbrush compressors, from Silentaire and the like, use refrigerator motors. An airbrush filter will take the oil out before it hits the line, so that should keep the paint etc clean.

 

[larryq]

The unloader valve on the pressure cut-off switch will leak without a "check valve" to keep the air in the tank from coming back to the compressor.

Yes, correct, without a check valve none of this works. I do have a check valve in the system however, in fact that's what the unloader line is connected to. Here's a picture of the one I'm using:

The unloader port is the one on the bottom, shown capped.

 

[KenC]

Double check the flow direction on the check valve. If it's backward you will be relieving tank pressure instead of line pressure.

 

[larry4406]

Interesting build!

Slightly off topic but I bought my daughter the HF air brush kit and the compressor noise was not obscene. I think I paid maybe ~$40?

After the oil all gets soaked up in the discharge air filter I assume you have to refill to avoid meltdown? How do you monitor and add oil?

 

[Todd.Brock]

If the pressure switch won’t stop leaking, means your check valve isn’t working correctly. No air should get past it. Check the disc for dirt or something. May not take much to force it open. If you had it backwards, in theory you wouldn’t be able to put air in the tank

 

[The Cobbler]

a lot the Chinese switches are ****.
If I understand, the unloader is always leaking air, even before the tank pressure is up and compressor shuts off.
in the closed ( or pump on position) the unloader should be closed , no air escaping. if it is leaking while in the run position, there's a defect with the unloader valve

 

[Cruzan80]

Going with first stupid guess...

You made sure all the lines are air-tight, taped, etc, right? It isn't leaking around a fitting but from the unloader?