Ok guys so I need some help. I have an aircompresser that is running my waste oil burner. The burner needs very little air pressure. I want to lower the start up pressure so that the compresser runs longer when it is on and stays off longer. Does anyone know who makes a pressure switch that is adjustable. Like maybe 35psi lower to 135 upper?
Thanks, Zach
Check with your local refrigeration tech. He has access to all kinds of adjustable pressure switches. Any voltage. Most refrig controls turn on with a high pressure and shut off with a low pressure, but he can get one that is the opposite.
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Zach
My Craftsman compressor has an adjustable pressure switch, 2 springs with adjusters if I remember right. There is significant interaction between the adjustments and maybe not the wide off/on range you are looking for.
What are you doing with the waste oil burner? I like to see pictures or drawings. I am thinking about building an oil burner for a crucible furnace.
Carl Boyd
Hey Zach,
That's a substantial amount of differential. What about just using a
2nd switch, say like one used for water pressure, and that way you can get the differential you want.
Two standard pressure switches, a little bit of relay logic - The low pressure switch pulls up a "Run" relay that locks even after the low-pressure switch has opened, and the compressor runs until the stock high-pressure switch is satisfied and turns it off.
You will need a small DPDT relay for the latching start, and a plain power contactor to start the motor. Or a motor contactor with an auxiliary SPDT contacts.
A full-on motor starter is better - first it has the aux. latching contacts built in for using "Start" and "Stop" buttons, so you could wire it all by itself, no external logic or second relay. (Follow the directions on the inside of the motor starter cover.) And you have overload heaters in case of a locked rotor no-start - make sure you know the motor full-load amps to select the right overload heaters.
And do NOT use the bleeder style unloader valve built into the stock high-pressure switch with this setup - if you lose power and try to start the motor on a high head pressure load, it might not start. Or the pressure could build up on the compressor output side if you have a leaky tank check valve - the high pressure switch's built-in bleeder valve closed at 120 PSI, and has been sitting there closed till the 35 PSI start switch finally kicked in...
Get an 1/8" NO solenoid valve (closes on power), hook the coil across the motor leads, and make it the unloader valve. Motor stops, valve unloads.
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Your logic is sound, but there is another thing to consider - an electric motor in severe duty (as on an air compressor) is usually only rated for four to five starts an hour maximum. Even if it's lightly loaded, as it would be on a 110-135 PSI single-stage compressor derated to running at 35-50 PSI duty.
When you start an electric motor it's drawing "locked rotor" current for a half second till it gets up to speed, and that builds up a lot of heat in the interior of the motor windings. The heat only gets a chance to start dissipating when the motor gets up to speed and the cooling fan gets moving. Multiple short cycles per hour will quickly burn up the motor - it's better to let it run than to have it stop for three minutes and start again.
If he was going to do this the /right/ way, that really is the proper job for a small diaphragm or "wobble piston" oilless air compressor sized to the pressure needs and air consumption of the babbington burner - say 1/4 or 1/6 HP. Have it start when the oil burner lights off, and run continuously while the burner is lit. And when it wears out they are rebuildable or disposable.
Another solution is a continuous-run compressor that leaves the electric motor running and unload the compressor head for the short lulls between calls for air. But that makes more sense on a 25-HP or bigger compressor supplying a large plant than a 2-HP compressor at a residence.
But if he doesn't want to do that, the large differential is second best. Then the compressor only has to cycle on once or twice an hour.
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Agreed, that sounds like the winning approach.
In addition, you might consider adding air storage. That way it does run longer when it runs, and runs less often.
Both of those are pretty simple, standard ways to approach the original goal.
sdb
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