does this old compressor need an unloader valve?

  1. MoreWellie Member

    Messages:
    983
    Location:
    Bedfordshire, UK
    I have an air industrial developments compressor and wanted to understand a bit more about how it works

    There is an obvious pressure relief valve that has a narrow windy copper tube that runs up to the inlet side of the compressor pump and I have assumed that this is meant to help it restart under pressure by somehow balancing the pressure across the pump

    It didn't have any control gear when I got it so I have fitted a standard automatic pressure cut off switch but I haven't hooked it up to an unloader valve

    It starts and initially restarts when the pressure drops but when it is worked hard it sometimes goes to restart and stalls humming loudly. Switching off and on again several times gets it running again so I think the motor and capacitor are ok

    Do I just need to be kinder to an old compressor or should I fit an additional unloader valve and plumb it to the pressure switch?


    IMG_20190505_110046147.jpg
     
  2. frank horton

    frank horton V twins are great but 4"s rule.........

    Messages:
    1,316
    Location:
    Soon 2 B Crete
    Personally I'd go the unloaded route, cheaper than a bigger motor or diff gearing...
    They r really just a few squids from the bay...
     
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  3. Lazurus

    Lazurus Member

    Messages:
    517
    Location:
    Norfolk uk
    Wot he said. :clapping:

    I had a similar issue with a small 24litre but only when it was cold, guess it just got very tight and the small motor couldn't overcome the tighter tolerances when the castings contracted in the cold.
     
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  4. Bill Edwards Member

    Messages:
    4,798
    Location:
    Scarborough, North Yorkshire
    It looks like the copper pipe that goes from the pump to the receiver has no connection with the valve that has the curly pipe to the inlet. Not certain that that’s a safety relief valve or just a manual one.

    You need a non return valve at the end of the copper pipe where it enters the tank, and then a small pipe on the pump side of that valve that’s connected to the unloader valve on the pressure switch.
    This dumps the pressure in the pump outlet pipe when the motor is shut off (with a very audible hiss) and allows the compressor to start without load.

    Unless it’s plumbed like this starting up with air in the receiver will place a huge strain on the motor and electric supply, and is causing it to sometimes fail to start at all.
     
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  5. MoreWellie Member

    Messages:
    983
    Location:
    Bedfordshire, UK
    Thanks All, bits now on order

    I was just confused about the purpose of the wiggly pipe up to the inlet side of the pump and wondering if that was to assist in restarts

    Edit: and terminology failure I have ordered a non return valve as I already have the unloader in the pressure control switch
     
  6. gordon stephenson

    gordon stephenson Forum Supporter

    Messages:
    6,361
    Location:
    Skelton in Cleveland U.K.
    The "wiggly" pipe is pressurised when the unloader operates and usually lifts a valve so that the compressor just free-wheels until the tank pressure falls.
     
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  7. MoreWellie Member

    Messages:
    983
    Location:
    Bedfordshire, UK
    then I still don't understand as that wiggly pipe is plumbed off of a separate tank outlet and nothing to do with the pump to tank input line where I was planning to put the non return valve

    IMG_20190507_173635719.jpg

    edit: and it is an adjustable pressure relief valve as I had to adjust it to be able to up the pressure on the tank to 90psi which is where I wanted it

    This tank now has three pressure relief valves, this adjustable, another original on a different outlet and a new one on the new pressure switch
     
  8. Bill Edwards Member

    Messages:
    4,798
    Location:
    Scarborough, North Yorkshire
    Normally the case, except this seems to be plumbed into the air intake rather than a valve.
     
  9. gordon stephenson

    gordon stephenson Forum Supporter

    Messages:
    6,361
    Location:
    Skelton in Cleveland U.K.
    Our big dry air compressors that were used for all the instrument lines had unloaders, They worked very well when set up correctly, Set the pressure max pressure when you want the unloader to operate and adjust how much pressure drop before it started compressing again, What is the round knob on the cylinder head? Looks like the system operates through some sort of lever mechanism on the inlet valve.
     
  10. MoreWellie Member

    Messages:
    983
    Location:
    Bedfordshire, UK
    So the electric motor is supposed to run continuously and the unit standing up from the tank controls the on off pressures by switching on and off the pressure through the pipe which makes the pump freewheel when it isn't needed

    I assumed the knob on the head would control something, It turns freely but doesn't seem to adjust anything and doesn't pull or push either.

    These questions are why I was seeking assistance understanding the correct operation.

    The bits I have ordered will allow me to ignore the pipe and knob and make it work like a modern compressor but I would prefer to understand what the original bits are there for
     
  11. gordon stephenson

    gordon stephenson Forum Supporter

    Messages:
    6,361
    Location:
    Skelton in Cleveland U.K.
    You have got it, Motor running continuously and unloader only lets the pump work when the tank pressure drops. Is the round knob on the head threaded or spring loaded?
     
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  12. MoreWellie Member

    Messages:
    983
    Location:
    Bedfordshire, UK
    the round knob is threaded and winds in and out with no perceivable effect other than it gets taller or shorter :laughing:, possibly because my newly added pressure control is set to cut off at 90psi whereas the original design pressure for the compressor was 120psi

    the motor running continuously doesn't sound a particularly nice way of managing the compressor for home use so I will go ahead and install the non return valve and pipe to the unloader in the pressure control which will be kinder to the motor and let it survive a bit longer

    Edit: the stand up bit for the wiggly pipe must server two purposes. I suspect the bolt with the locknut sets the pressure that the pump was supposed to switch to freewheel and the locknuts and the thread at the top set the pressure at which it vents to atmosphere
     
  13. gordon stephenson

    gordon stephenson Forum Supporter

    Messages:
    6,361
    Location:
    Skelton in Cleveland U.K.
    You always have the option of taking off the head and valve plate to see what is going on inside,
     
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  14. MoreWellie Member

    Messages:
    983
    Location:
    Bedfordshire, UK
    I could but its working and I have plenty of other things higher up the "List" or that aren't working

    I'll put it on the round tuit list
     
  15. Turbo Member

    Messages:
    3,050
    Location:
    Fermanagh, Northern Ireland
    If the compressor is designed to run all the time the motor start windings might not be happy having to start & stop with a modern setup. Could I suggest fitting a new start capacitor, possibly of a slightly higher rating to help things along.
     
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