compressor unloader valve

If the original installation did not haave an unloader valve you can safely ignore it on the new switch. An unloader valve simply bleeds the pressure out of the compressor (in-line check valve prevents the tank from being drained). An unlader valve makes for easier starts.

WROTE: (clip) Can I ignore it and leave it unconnected?, or did I buy the wrong switch? What does an unloader valve do? ^^^^^^^^^^^^^^^^^^ It vents the compressor output until the thing is up to speed. You probably don't need it, but it wouldn't do any harm to hook it up. It will allow your motor to come up to speed a little faster, draw a little less starting current, and maybe increase the life of the starting switch (not applicable to 3 phase.)

This may vary by the compressor, but the one in our shop vents everything before the check valve located where the pipe enters the tank from the compressor. The unloader is operated by the same plate that carries the moving contacts in the pressure operated switch, so it opens as soon as the motor stops running, and is closed as soon as the compressor starts. This means the air in the pipes and compressor heads is at ambient pressure as the motor begins it's rotations but rapidly compresses as the pump reaches operating speed.

In the winter, our compressor would benefit from a delay before the unloader closes, if time permits this summer, I may rig a solenoid valve on a delay relay.

Weld up a small reservoir that fits into the discharge line. The compressor will have to fill that and will take more time to allow the thing to come up to speed. Simpler.

Dan

You can buy the proper pressure switch with the unloader valve from McMaster Carr or Grainger. That is the first thing to do.

Some Champion compressor have centrifugal unloaders that automatically do this. There would be a air line running to a housing at the end of the crankshaft if your machine has this.

If you cannot get the right part, consider adding a NO (normally open) solenoid valve into the air circuit. When energized (compressor motor on) the valve is closed forcing all compressed air into the tank. Then when off the air is blead out of the compressor head. This is what the mechanical valve did.

This is a critical feature of larger compressors to get them rolling.

The way I read the OP's post is that his compressor did not have an unloader valve, but his new pressure switch does have a provision for an unloader valve.

Your motor will thank you for the properly functioning unloader.

wrote: Weld up a small reservoir that fits into the discharge line. (clip) ^^^^^^^^^^^^^^^^^ Welding up a pressure vessel is simpler than hooking up valve? Ennyhoo--if you do that, be sure to put a check valve between your "surge" tank and the main tank. Otherwise, the compressor will still be starting against tank pressure.

No. The unloader valve will drain the added surge tank when the motor is off. Adding the extra surge tank size allow the compressor more time to build up pressure, and have aneasier start in the winter if needed. And it doesn't need to be a "tank" just more air line T'd into the line from th ecompressor head to the check valve in the main tank. Anything that adds volume to the compressor head output, and gets drained by the unloader valve.

"Half-Nutz" wrote: No.

^^^^^^^^^^^^^^^^^^^ Well, "No" right back atcha. If you connect a surge tank to the compressor output/air tank input, with a Tee and no checkvalve, you will drain all the air pressure each time the machine shuts down. Sure, that will allow the motor an easy startup, but you will have to wait long time for the pressure to build, AND YOU WON'T BE ABLE TO RUN ANY TOOLS EXCEPT WHEN THE COMPRESSOR IS RUNNING.

Or are you talking about adding more air capacity WITHOUT an unloader valve? In which case, that is really a different question.

The unloader system always has a check valve going into the main tank. That is how they work. Otherwise you are proposing that the unloader valve drain the main tank!!

Dan suggested adding more volume to the unloader circuit. Unloader circuit. You can't have that without a check valve feeding the main tank. As I read your reply, you suggested adding a check valve to the added unloader volume. That would defeat it's purpose.

The other type of unloader system opens the compressor valves until the compressor crankshaft s up to speed somehow.

Here is your response: " Welding up a pressure vessel is simpler than hooking up valve? Ennyhoo--if you do that, be sure to put a check valve between your "surge" tank and the main tank. Otherwise, the compressor will still be starting against tank pressure. "

That was in response to someone suggesting adding a "surge" tank to an existing unloader system. IN THAT CASE, you suggestion to add a check valve to the "surge" tank did not make sense, since in the context of where you responded, the discussion was aobut modifiing an existing unloader circuit.

So.... What you said might have been correct, but not -in the context- of responding to the suggestion that you responded to.

Clear now?

^^^^^^^^^^^^^^^^ Yep. But I want to make sure we are BOTH clear. The OP said he was replacing the pressure switch on a used compressor, and inadvertently got one with an unloader valve. Then, Dan Thomas suggested the surge tank, which he said would be simpler. So he must have meant to use it instead of the unloader. That would mean he would just be adding volume to the existing tank. In that context, I think my suggestion to use a check valve was appropriate.

IOW, a surge tank, in combination with an unloader, would allow the compressor to run slightly longer before feeling back pressure.

I meant to add volume, using a small tank, to the compressor's discharge line. It would get drained by the unloader, and would provide more opportunity for the compressor to achieve speed before the pressure built against it. The usual check valve would be between the small tank and the main tank. A coiled section of 3/4" soft copper tubing would do the same thing, and would cool the air off more, getting the water condensed sooner so it doesn't end up in the air tools or paint or whatever.

Dan

wrote: I meant to add volume, using a small tank, to the compressor's discharge line. It would get drained by the unloader, and would provide more opportunity for the compressor to achieve speed before the pressure built against it. The usual check valve would be between the small tank and the main tank. ^^^^^^^^^^^^^^^^ Thanks, Dan. I think it's time to sound the all-clear signal.