compressor not knocking off

[justin]

My air compressor has been barely used for years. when i turned it on this week it didn't shut down when it reached pressure, Is it possible to clean out these switches or do they just have to be replaced. my thinking is that it is just sticking through lack of use.

 

[dobbslc]

I had the same on my one and no matter what I did to adjust it it wouldn't work reliably.
I just replaced it.

 

[rtbcomp]

Does it stop if you operate the knob on the switch?

 

[justin]

yes the switch knob is fine, the compressor has probably only been turned on once or twice in the last ten years.

 

[rikrobson]

the swithces are about £10-20 so its wont break the bank to replace it. Its probably just stuck though, they are just spring loaded. Take the power of and give it a clean

 

[rtbcomp]

If the knob works OK the contacts &c will be OK - it sounds like the diaphragm may be punctured, probably perished.

Or the torsion spring on the toggle mechanism may have come dislodged.

 

[justin]

I didn't know that there was a diaphragm in them, The compressor is 30 years old so i think i should replace it.
thanks guys

 

[the snooper]

try adjusting it first it could be just stuck

 

[rikrobson]

Does it stop if you operate the knob on the switch?

That reminds me of the goons joke

Bluebottle: Yes, why do you not open the door?
Eccles: Okay, I'll- How do you open a door?
Bluebottle: You turn the knob on your side.
Eccles: I haven't got a knob on my side!
Bluebottle: On the door!
Eccles: Oh! I'll soon get the hang of that.

 

[rikrobson]

if the diaphram is gone I suspect it would be leaking air

 

[justin]

It is leaking air when it didn't knock off I turned it off with the knob and a few hours later it was about half empty. I had just replaced the drain valve so i had assumed that it was that leaking and the pcl coupling was loose.

 

[rtbcomp]

if the diaphram is gone I suspect it would be leaking air

My Next question "Is it leaking air"

It is leaking air when it didn't knock off I turned it off with the knob and a few hours later it was about half empty. I had just replaced the drain valve so i had assumed that it was that leaking and the pcl coupling was loose.

Answered.

New switch then, you could replace the diaphragm, but you risk springs flying all over the place!

 

[rikrobson]

there are plenty of switches in ebay for £10-£20

 

[justin]

New switch ordered. Can anybody recommend a suitable cut off pressure to set it too
it is an old 50l 3hp airmaster compressor. the plate on the tank says a mwp of 150 psi.

 

[TurtleDude]

150 psi x 0.0689 = 10.33 bar

I would suggest you set it for the lowest pressure you can get away with, which will be the highest pressure required for whatever you use the air for.

If it's air tools, normally around 6.5 bar is sufficient, so set it at 7.5 and work down until you have problems. The. Go back up a step.

Running at lower pressure saves you money and will prolong the life of your compressor as it will be 'on load' less.

 

[rtbcomp]

The problem with setting it to cut out just above the working pressure is that the motor will switch off and on more frequently, this could cause it to heat up more and any savings in electricity could be lost due to more frequent starting surges.

If you increase the cut-off pressure the stop/start frequency will be reduced (obviously you'll need to regulate the output pressure). The compressed air will get hotter as the pressure increases, a higher temperature will cause more heat to be lost which means the air will be cooled more as the pressure drops across the regulator. The greater the pressure drop the greater the cooling and consequently more moisture will condense resulting in drier air.

 

[TurtleDude]

The problem with setting it to cut out just above the working pressure is that the motor will switch off and on more frequently, this could cause it to heat up more and any savings in electricity could be lost due to more frequent starting surges.

If you increase the cut-off pressure the stop/start frequency will be reduced (obviously you'll need to regulate the output pressure). The compressed air will get hotter as the pressure increases, a higher temperature will cause more heat to be lost which means the air will be cooled more as the pressure drops across the regulator. The greater the pressure drop the greater the cooling and consequently more moisture will condense resulting in drier air.

Without knowing the size (and output) of the compressor, the size of the receiver, and the application of the compressed air, we can't generalise and say that a lower pressure will cause lots of stop/starts. For example if he's got a 15kW compressor feeding a 350 litre air receiver and he's blowing swarf off the bench, it'll make little or no difference what pressure he's running in the grand scheme of things.

I doubt he'll see much of a difference in môisture content just by upping the pressure, unless he's using a refrigerant dryer or routing the pipework outside or through a colder room. I suspect he's using flexible hose which won't condense much at all.

Maybe the OP can fill in some of the blanks for us ?

 

[rtbcomp]

I agree the frequency of stop/starts will be less the bigger the tank, but the pressure will still affect this figure.

The condensation will occur mostly in the regulator, which is where the pressure drop occurs. A good reason for using a combined regulator/trap.

 

[justin]

3hp 50 litre kw? I was looking to buy a die grinder and when I saw the difference in price between electric and air powered I thought that I would try to use the compressor that I had been given.

 

[the snooper]

3hp 50 litre kw? I was looking to buy a die grinder and when I saw the difference in price between electric and air powered I thought that I would try to use the compressor that I had been given.

kw is usually around 2.2 for a 3hp