I want to build a compressor

[Dieselman 63]

Hi,

I was given some pieces of compressor and a few electric motors, and want to make them work. But how? How do they turn off when they reach operating pressure? And what is all this stuff I have, and is it any good? It seems to hold compression. Help gladly received!

 

[Seadog]

You need a pressure switch fed from the receiver which will disconnect power. Usually they serve a second function which is to bleed pressure from the pump head when it shuts off (the unloader valve). This means that you also need a non-return valve between the pump and the receiver after the unloader. It's possible that the unloader is mounted on the pump head in your case. I'm not familiar with the Devilfish. The Aerograph unit is a pressure regulator with drain off valve at the bottom. A nice piece from, probably, the 1930s or 40s.

 

[Dieselman 63]

I found the tank drain and undid it - only clear water came out, which seemed positive! Underneath the hammerite green is what to my mind looks like US Army paint.

 

[waddycall]

I worked at a boat yard that had a really old compressor. Everything on it had been bypassed and it just ran and ran relying on leaks to keep the pressure in check and then remembering to go and turn it off. Scared the hell out of me. I’d be a bit wary of using any antique control gear on your compressor.

 

[Morris]

I would be wary of old receivers as well.

 

[Seadog]

Old receivers are generally made from thick plate and not prone to spitting. Hydrostatically test it before use, but I'd expect it to be OK. typically you'll test to 50% more than working pressure, which seems to be 100psi for your one.
Usually working and test pressures will be stamped into the receiver or on a plate attached to it.

 

[Morris]

An old one I was given failed at 80 psi. Fortunately at that time one of my duties was hydro- static testing pressure vessels.

 

[Dieselman 63]

Old receivers are generally made from thick plate and not prone to spitting. Hydrostatically test it before use, but I'd expect it to be OK. typically you'll test to 50% more than working pressure, which seems to be 100psi for your one.
Usually working and test pressures will be stamped into the receiver or on a plate attached to it.

How do I test it? Absolute newbie here!

 

[Morris]

How do I test it? Absolute newbie here!

The only safe way is hydrostatic testing.
Fill with water and use a hydraulic pump to pressurise to test pressure ( usually +50%).
If you monitor the pressure as it rises it is easy to see the point at which failure starts should it fail.

 

[Seadog]

This is what I used to test mine

 

[Morris]

Pretty much exactly what I used to use.

 

[MoreWellie]

How do they turn off when they reach operating pressure?

Not sure all of them do, I have an old one that seemed to have been designed to open a valve on the compressor head and just let it free run once it was up to the pressure set on the valve. It would need a continuously rated motor though

I preferred to run it like a normal modern compressor so I added a standard switch with an unloader and let it stop start when needed

 

[brightspark]

the one above runs as you say motor runs all the time and when up to pressure just decompresses the valves and cuts back in on drop of pressure i ran a 90cfm v twin with a 20hp lister diesel engine and it operated the same way easy enough to convert though through an electrical pressure switch and a bit of piping

 

[minimutly]

So to recap - you need to fill the tank full with water, then pressurise it to 1.5 times the working pressure.
The working pressure is the pressure your relief valve, fitted to the tank, will lift (so you need to get one of those).
You will need an air pressure switch with adjustable differential between start and stop.
You will need a solenoid valve that gets fed from the normally open contact in the presdure switch, so that it dumps the air in the line between the compressor and a non return before the tank (unless there is a mechanical one built into the compressor).

I'm sure someone will tell me I'm wrong, but those are the bits I'd expect to see.
Edit to add, I've used a fridge compressor to pressure test before now, so long as the tank is full of water the energy release factor will be low (should it fail).

 

[brightspark]

you can use as is if you dont mind the motor running as long as it is turned on

 

[Seadog]

So to recap - you need to fill the tank full with water, then pressurise it to 1.5 times the working pressure.
The working pressure is the pressure your relief valve, fitted to the tank, will lift (so you need to get one of those).
You will need an air pressure switch with adjustable differential between start and stop.
You will need a solenoid valve that gets fed from the normally open contact in the presdure switch, so that it dumps the air in the line between the compressor and a non return before the tank (unless there is a mechanical one built into the compressor).

I'm sure someone will tell me I'm wrong, but those are the bits I'd expect to see.
Edit to add, I've used a fridge compressor to pressure test before now, so long as the tank is full of water the energy release factor will be low (should it fail).

You missed a non-return valve, but I'm pretty sure i can see one in the pictures. I can't see a mechanical unloader, neither can I see a pressure switch.

 

[brightspark]

You missed a non-return valve, but I'm pretty sure i can see one in the pictures. I can't see a mechanical unloader, neither can I see a pressure switch.

the valve decompressor unloader is in the1st picture above the t piece from the tank bottom right on second picture

 

[Seadog]

Do you mean the piece with the pressure gauge attached?

 

[brightspark]

its the one that has the wound copper pipe going in to it from the valves ,

 

[Kram]

I found the tank drain and undid it - only clear water came out, which seemed positive!

If you dont have one, get a cheap bore camera and have a look inside reciever. At that age it may be badly rotted.