Author Topic: Compressor pressure switch problems.  (Read 1674 times)

0 Members and 1 Guest are viewing this topic.

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Compressor pressure switch problems.
« on: April 18, 2021, 07:10:26 pm »
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.
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 14072
  • Country: de
Re: Compressor pressure switch problems.
« Reply #1 on: April 18, 2021, 09:35:45 pm »
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.
 
The following users thanked this post: davelectronic

Offline jmelson

  • Super Contributor
  • ***
  • Posts: 2758
  • Country: us
Re: Compressor pressure switch problems.
« Reply #2 on: April 19, 2021, 02:26:46 am »
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
 
The following users thanked this post: davelectronic

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Re: Compressor pressure switch problems.
« Reply #3 on: April 19, 2021, 07:28:19 am »
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.
 

Offline amyk

  • Super Contributor
  • ***
  • Posts: 8240
Re: Compressor pressure switch problems.
« Reply #4 on: April 20, 2021, 12:06:00 am »
Use a hand pump to put some pressure in the tank and see if it holds.
 
The following users thanked this post: davelectronic

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Re: Compressor pressure switch problems.
« Reply #5 on: April 20, 2021, 12:56:45 am »
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.   
 

Offline floobydust

  • Super Contributor
  • ***
  • Posts: 6923
  • Country: ca
Re: Compressor pressure switch problems.
« Reply #6 on: April 20, 2021, 07:39:49 am »
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.
 
The following users thanked this post: davelectronic

Offline tautech

  • Super Contributor
  • ***
  • Posts: 28136
  • Country: nz
  • Taupaki Technologies Ltd. Siglent Distributor NZ.
    • Taupaki Technologies Ltd.
Re: Compressor pressure switch problems.
« Reply #7 on: April 20, 2021, 08:05:49 am »
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.
Avid Rabid Hobbyist
Siglent Youtube channel: https://www.youtube.com/@SiglentVideo/videos
 
The following users thanked this post: davelectronic

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Re: Compressor pressure switch problems.
« Reply #8 on: April 20, 2021, 01:55:25 pm »
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.
 

Offline tautech

  • Super Contributor
  • ***
  • Posts: 28136
  • Country: nz
  • Taupaki Technologies Ltd. Siglent Distributor NZ.
    • Taupaki Technologies Ltd.
Re: Compressor pressure switch problems.
« Reply #9 on: April 20, 2021, 03:23:51 pm »
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.
Avid Rabid Hobbyist
Siglent Youtube channel: https://www.youtube.com/@SiglentVideo/videos
 
The following users thanked this post: davelectronic

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Re: Compressor pressure switch problems.
« Reply #10 on: April 21, 2021, 09:03:15 am »
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.
 

Online Ed.Kloonk

  • Super Contributor
  • ***
  • Posts: 4000
  • Country: au
  • Cat video aficionado
Re: Compressor pressure switch problems.
« Reply #11 on: April 21, 2021, 09:12:38 am »
Hey, props for putting the fridge motor on it. That is one nice hack.

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

iratus parum formica
 
The following users thanked this post: davelectronic

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Re: Compressor pressure switch problems.
« Reply #12 on: April 21, 2021, 01:47:30 pm »
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.
 
The following users thanked this post: Ed.Kloonk

Offline davelectronicTopic starter

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: gb
  • Life's too short.
Re: Compressor pressure switch problems.
« Reply #13 on: April 23, 2021, 07:36:27 am »
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
 

Online Ed.Kloonk

  • Super Contributor
  • ***
  • Posts: 4000
  • Country: au
  • Cat video aficionado
Re: Compressor pressure switch problems.
« Reply #14 on: April 23, 2021, 09:54:37 am »
I hope that's it.
iratus parum formica
 
The following users thanked this post: davelectronic


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf