Compressor pressure switch problems.

[davelectronic]

Recently I put together an air compressor for airbrush use, it had a silent compressor motor on it from new. It was to loud for use in the evening, so I out two refrigeration compressors on it instead, the compressors work fine. But the pressure switch just leaks air from the unloader valve, so I bought another pressure switch in the hope this would cure it. And it hasn't, it's still leaking air from the unloader valve.

Also I have removed and had a good look at the receiver check valve, the spring and valve seat look fine. I am at a complete loss as to why the unloader valve is leaking air, and not filling the tank. All I can come up with is the two refrigeration compressors don't put out nearly as much air volume as the original compressor twin piston unit does. If there are any air engineers out there that might know what's going on with the unloader valve leaking air ? I'm starting to think I'm going to have to think up another type of air control system. But I've watched all the YouTube videos on this similar projects, and there using the same pressure switch, and have no problems. Thoughts appreciated, thanks for reading. The pressure switch is the common one with the pull push on off knob on top of the pressure switch housing.

[Kleinstein]

The usual problem is from the check valve at the pressure vessel. Than the unloader valve releases air when the motor is off.

If the air is leaking out, when the motor is running it is usually a problem with the unloader valve.  There are 2 types: one is controled be the switch. The other, rarer version closes when the air flow is high enough - this version may not work with the small silighent compressor.

[jmelson]

The built-in unloader on the switch depends on there being a check valve on the tank where the compressor feeds air to the tank.  The idea is to release air from the compressor delivery pipe, so that the compressor starts with no load.

You have likely removed the check valve in your retrofit.  If not, then maybe a small bit of debris was left in the line and it has blown into the check valve so it no longer seals.

Jon

[davelectronic]

I have looked at the receiver check valve, the valve seat and spring move freely, and looks fine. This is a second pressure switch, doing exactly the same as the first switch which was also new. The receiver is getting some air, but it's not enough to move the gauges. I had noticed Bambi compressors use a pressure switch with an on of rotation knob, and the commonly found other switch has a push pull knob. I haven't adjusted either switch in anyway at all, and the check valve moves freely, so looks like it would hold air in a non return configuration. I'm completely out of ideas what's wrong with this red knob push pull pressure switch.

[amyk]

Use a hand pump to put some pressure in the tank and see if it holds.

[davelectronic]

I did think of adding about 20 psi then turn on, see if it holds like you said. the original compressor put out a huge volume of air compaired to the two refrigeration compressors. i've purchased a pressure switch 70 psi on 120 psi off, no idea if its any good yet. The refrigeration compressors have a high head pressure in the specs sheet. I might still need to purge or unload the compressor when it cuts out, no idea how to achieve that. It might start up at 70 psi, although that's more strain on the motors. They could run continuous and just bleed off the unwanted air, but again its wasted power and excessive strain on the motors. I don't think the current generic pressure switch will work with a much lower volume of air. There is another similar pressure switch, but that uses a rotary red knob, and not the push pull start control of current switch. I don't know if there different at all.   

[floobydust]

Unloader valves need a good flow rate to close, when the compressor starts up. Airflow has to over come the return spring and push the piston out of the way. You might not have enough flow with the two fridge compressors, I think they are designed for low flow but high pressure.
You can hit the unloader valve with something like a small hammer, or use try a magnet to try move the pintle so it closes but it's likely brass.
I had an unloader valve that had some debris in it, so it couldn't close. Make sure you go metal chips in it.

[tautech]

Unloader valves need a good flow rate to close, when the compressor starts up. Airflow has to over come the return spring and push the piston out of the way. You might not have enough flow with the two fridge compressors, I think they are designed for low flow but high pressure.
You can hit the unloader valve with something like a small hammer, or use try a magnet to try move the pintle so it closes but it's likely brass.
I had an unloader valve that had some debris in it, so it couldn't close. Make sure you go metal chips in it.

This ^^
The receiver may need 20+ PSI in it before some unloader valves close.

[davelectronic]

I'm sure your both right, is flow volume that just not enough to operate the check valve properly. After I installed the second new pressure switch, I removed the check valve first to inspect it. The spring and valve seat where rather stiff to move with a blunt object ( rounded end of a pencil ) I removed half a turn of the spring, but not to much so it wouldn't close the valve seat. I tried that twice more and started to get some air in the tank. But not enough to register on the gauge. Only know this as loosening the check valve the air rushed out. But the unloader valve still kept leaking the all the time. I've given up on this generic red button push pull pressure switch and ordered a couple of small preset pressure switches. So on at 70 psi off at 100 psi, it doesn't deal with the unloader issue, but I have been looking at solianoid valves to come up with some ideas.

I've studied carefully retail airbrush compressors, I can see the same pressure switch that I purchased, and a solianoid in the compressors pipe work. And a slightly different looking air pressure switch on Bambi compressors. I think I can get there eventually, it's just a matter of persisting with different configurations.

[tautech]

Find the unloader valve vent and hold your finger on it until you get some pressure up and momentarily release finger pressure until you find where it shuts off venting. A solution may be just simply bushing down the vent hole to allow pressure to build but still unload when cutout pressure is reached.

[davelectronic]

I had tried blocking the unloader valve port, it was hard to get to, I wrapped some PTFE tape round the end of a rounded pencil. The air just squeeled past my attempt to block it. The compressor motor that was originally on it is in the picture below. I'm trying to work out a relay circuit that could work with a stand alone pressure switch device, and incorporate an unloader solenoid valve. Most small solenoid valves of the smaller variety are 12 Volt or 24 Volt. That would mean adding a small PSU in the wiring junction box I put on the unit. It should house a small SMPS brick, say from an old power adapter. Adding other voltages just seems to complicate things. Although I think I could use pressure switch, solenoid, controlled with a suitable relay. The original compressor was 750 watt monster, not sure what to do with that lol.

[Ed.Kloonk]

Hey, props for putting the fridge motor on it. That is one nice hack.

Beats my portable cold-room-panel box by a mile.

[davelectronic]

Hopefully it will be worth it once it's finished, of only been waiting 23 years to try airbrushing. I could have bought a quite cheap retail compressor, but most only have a e litre storage receiver. The better silent Bambi compressors cost a lot, cheapest about £400 or so. Many years ago I started to put an airbrush compressor together, but got side tracked by something back then and never finished it. I've purchased a couple of NC pressure switches, and a NC compressed air solenoid, that's 12 Volts. So I figure I need two small 12 Volt wall warts and a couple of relays to achieve a pressure control system. The compressed air switch isn't super accurate, but on at 70 psi and off at 100 psi. And relay control of the solenoid to unload the compressor input hose should work. I didn't want to use two PSU modules for the relay and solenoid, but I can't figure out to do it with one 12 Volt PSU. There quite small though, so shouldn't be a problem.

[davelectronic]

I think I've finally found the answer, I've haven't tried this yet. But it's at 4:30 into the video in the link below, I happened on it just generally watching home brew compressor builds. It's the spring in the unloader valve, and seems to be the lower air volume from hematically sealed compressors. Soon I will try this, just wish I had found this video before buying alternative pressure switch and solenoid valve.

https://youtu.be/5hc4lMEFurg

[Ed.Kloonk]

I hope that's it.

[Dannyx]

Hi. I know this is a really old thread, but I'm having the same issue......sort-of. What instantly caught my eye is that I have the same compressor......sort-of Mine is its bigger, beefier brother, with TWO motors on it, but aside from that, it's the same brand and I'm positive uses the same parts. I was curious about how the unloader valve on the pressure switch works, since you seem to have fiddled with it. The part I'm mostly interested in is in post #11.

While I haven't actually modified mine in any way, it too began slowly losing pressure overnight, which causes it to start up on its own, wasting power and wearing out the motors unnecessarily. I started by replacing one of the leaky quick connect fittings on the pressure switch outlets (mine has two) and that did improve things slightly, but it's still not perfect, so further digging was required.

One thing I find interesting is the way they implemented the dual motor situation when it comes to the discharge procedure: each motor obviously has its own check valve which connects the heads to the tank itself. The check valves are the kind that have a second, much smaller port Teeing off on one side, and that's connected via the aluminum tube to the pressure switch's unloader valve. In this case only one of them is connected to the unloader valve in this fashion (see the picture - front). The other one uses a solenoid valve on that discharge port. I haven't taken it off to inspect it, but logic would indicate it's got to be a N/O solenoid valve, so it closes when the motor is running and opens when the pressure switch cuts the power to the whole rig, so the valve goes PSSSSSHHHHT, releasing the air in the compressor head via that discharge port of the check valve. Meanwhile, the check valve on the first motor discharges through the thin tube and the unloader valve itself.

Now this is the part that doesn't make sense to me: why would they NOT use a solenoid on the first check valve as well, but instead use the tube leading to the unloader on the pressure switch unit Having a solenoid seems a bullet-proof solution to ensure no air escapes when the compressor is running (unless the N/O valve itself becomes faulty, which should be easier to replace). I'm pretty sure I can feel a faint puff of air escaping from somewhere around the housing of the pressure switch when the compressor is running.....which may not necessarily mean the unloader is leaking. Could be the pressure switch itself, but having a solenoid on that check valve, instead of that discharge tube leading to the unloader, would eliminate the possibility of that air coming from the compressor through the check valve's discharge port...

In post #11 you mentioned blocking off the unloader port, so I was mostly curious which side you tried plugging up exactly: was it the check valve end, or the the pressure switch end ? From what I understand, the unloader end (in the pressure switch) isn't actually connected to the tank itself, so if you were to plug that side, there should be no pressure there. Only if you tried to plug the side coming from the aluminum tube from the check valve there would be pressure, as it's coming straight from the motor (when it's running).....yet you say had some pressure there. Then again, you said it was hard to get to, so maybe you are referring to the unloader side after all (since on mine, it does seem quite tough to disconnect the tube from the underside of the pressure switch without taking off the entire assembly, gauges, regulator and all), but then where was all that pressure coming from ? From what I gathered, the unloader is simply actuated by the pressure switch - it is not actually connected to any pressure, so if I were to disconnect it, nothing should happen. It would just sit there doing nothing...

The reason I'm interested is because I was planning to install a solenoid on the first check valve as well and just ditch that return tube entirely, but I'm not sure what happens to the unloader side if I just leave that floating. I might end up with air escaping from the tank if it turns out it's under pressure.

[Psi]

The pressure switch in my compressor is a push-switch but there's a small square of butyl rubber sandwiched between the switch housing and the cylinder housing.  The pressure deforms the rubber and pushes on the switch while maintaining pressure separation between each side. 

The rubber developer a hole so I had to find some butyl rubber of the same thickness at the local hardware store and cutout a small section the same size as the blown piece.  Luckily they carried that rubber in 20cm rolls by the meter. So now I have a 20cm x 100cm piece with a 30x30 square cut out of the corner  .
Was cheaper than a new pressure switch assembly.  Aliexpress sell new rubber seals , but I needed it fixed that day.

[tautech]

For compressors the pressure switch often doubles as the unloader valve so when set pressure is reached it both switches OFF the motor and dumps the line pressure.
The dump valve needs be good/sound or it will leak out any pressure the check valve lets past so there are 2 things to check, the check valve and the dump valve.

[Dannyx]

Yes, in this case the pressure switch also houses and actuates the unloader valve (dump valve, as you called it). I was curious what would happen if I leave the dump valve floating and use a solenoid on the check valve's dump tube (like Einhell did on the second motor here). Would the tank leak out the dump valve ? Unlikely, since it shouldn't be connected to the tank and under pressure....

[Kleinstein]

Using a separate solenoid actuated valve is possible, in stead of the valve inside the pressure switch is possible. However it adds costs and a little power consumption.  The extra valves can also fail, though they may be a bit more reliable.

Usually it is anyway the check valve that fails and this is needed in both cases.

[Dannyx]

I already found a fitting N/O solenoid valve on Ali for relatively cheap. It runs at 230v, just like the one already installed on the second motor. The only real question is what happens to the unloader in the pressure switch if I just disconnect its tube. Do I need to cap it off or it doesn't matter...

[Kleinstein]

The valve at the pressure switch should not matter und could be left open or capped to keep dirt and insects out.

[Dannyx]

I'll start by *checking* the *check* valves  because no extra solenoid valve would help in that case, since they're already open....

[Dannyx]

As I suspected, the unloader is indeed leaking when the motors are running: I took the plastic cover off the pressure switch and the wisp of air I thought I felt the first time around is much more noticeable now. Unfortunately, I can't really see or do much to the unloader valve with the pressure switch installed on the compressor, since it's just one solid piece of metal and plastic, with no screws or other way of accessing the unloader valve portion, so I'll just have to leave it as is....too much trouble to take off the pressure switch. It just means it takes longer for the tank to fill up...