How does a petrol compressor work?

[gasket999]

Hi folks – bit of a basic question here, but the chap at machine mart couldn’t help and the stickies hint at the answer but would like some confirmation.
If you buy a compressor powered by a Honda petrol engine, does the petrol engine run constantly (in which case, upon reaching pressure, there would be a valve that diverted the excess air to atmosphere) or does the engine turn on and off (like a normal direct drive compressor) upon reaching pressure?

Similarly, if you buy a big 4hp electric, belt driven, compressor – does the motor run contstantly or does it turn on and off as required (like a direct drive compressor)?

Many thanks.

 

[bc30se]

For the petrol engine they use unloader systems, either in the form of unloadable intake valves, or through an unloader valve usually located somewhere on the compression outlet. I think they will run constantly... however there is no power limit for the electric motor driven compressor to be switched off and on again with a pressure switch. I myself have 22kw compressor which can work on a regular on/off basis

examples here:
http://www.mig-welding.co.uk/forum/threads/piloted-unloader-valves.50797/#post-584240

 

[DanielW]

Never seen one in operation but I would imagine it still runs but disengages from the compressor when the pressure has reached its required PSI,think it might along the lines of like a cars clutch system?

 

[Cobbler]

See if you can have a word & look with one of the mobile tyre vans that fix the lorry tyres, they all have a petrol or diesel compressor in the front of the van. You should be able to see how it's done. Nothing better than the mk1 eyeball to help understand it.

 

[rtbcomp]

This is what they use

https://www.machinemart.co.uk/shop/product/details/1-2in-pressure-unloader

When the pressure reaches the set level the valve opens and diverts the pump output to atmosphere.

 

[brokenbiker]

my fathers comp used to run continually and just diverted the air to another place when the tank was full. think he had a pipe on the outlet that ran through the shed wall but may be worng about that bit

 

[gasket999]

Thanks everyone.

This is what they use

https://www.machinemart.co.uk/shop/product/details/1-2in-pressure-unloader

When the pressure reaches the set level the valve opens and diverts the pump output to atmosphere.

Perfect thank you!

I’m building up a new garage workshop on a ludicrously small budget. I’ve just been given a 200 litre 5.5hp 3-phase compressor (24 CFM - 17.7 FAD). I’ve already got a little direct drive compressor.
I’m aiming for around 20 CFM FAD and want to get there using the bits I have around rather than shelling out a grand on one of the big twin pump single phase units form Machine Mart-but was really confused about whether I'd have to worry about stop-start if I used a different power unit, hence the query.
I’ll be having plumbed airlines and tandem compressors as per the guidelines in the Spray Painting article on the website - the small cheap existing one for day-to-day and the big one in an outside shed for more demanding activities (the blasting cabinet and my small spray gun, etc).
I’m just trying to work out how best to run the big compressor:
· Don’t fancy a phase converter – too expensive for digital and don’t think a rotary would run it.
· Converting to a 240v motor is fine but I'd imagine that getting the rest of the plumbing to work properly would require a lot of electronic knowledge I don’t have, plus the limit on 240v I believe is about 4hp which might not be up to efficiently spinning the existing pump.
· Converting to twin 3hp motors to run a single flywheel – would have to rig up pulleys and work out a way of having them start in sequence without damaging the belts (clutch or similar)
· Covert to petrol engine – I already have access to a few 5.5+ HP Honda engines from plant equipment and I’m a lot more comfortable with engines than electrics. Would need the valve, plus a correctly sized pulley for the revs. The big compressor would only be run while spraying or blasting so would see occasional use.

 

[wristpin]

As well as the uploader valve the better ones have "auto idle" so that the petrol or Diesel engine is not running flat out while the unloaded is dumping air.

 

[armalites]

Mine has a pressure valve with a plunger that cuts the throttle when the pressure is high enough. It has a centrifugal clutch on the engine that only engages when the revs are up so it will tick over until the pressure drops

 

[gasket999]

Brilliant thanks folks. Looks like a petrol engine is the simplest approach - would effectively create a mini road compressor. I'll only be running it for an hour or two most weekends.

Going off similar posts off the forum it looks like I'll need:

- A small petrol or diesel motor, I've got access to these for buttons - I understand that a petrol/diesel engine is typically quoted at momentary HP and that it produces approx 2/3 the stated power consistently. That means that for a 5.5hp replacement I'll need a 8-9HP petrol engine - such as Briggs and Stratton or Honda GX240 or a Chinese clone. Obviously I'll have to make sure the engine turns the right way.

- A pulley to get the RPM to match - I've heard about a 3:1 reduction but there are numerous calculators around. Obviously a belt too. I'll pick up a hand held tacho to check th existing wheel speeds.

- An unloader valve such as the one pointed out above. On a VAT free day that's £65. My understanding is that this will support continuous running and will divert air to atmosphere when the tank reaches pressure and will return flow to the tank when the pressure drops below a certain level.

 

[Maker]

the limit on 240v I believe is about 4hp

3 hp is the most you can effectively get from a 13 amp supply but 240 volt single phase motors can be had up to 5hp with a higher current supply(FWIW, using an inverter, a three phase 240 volt motor and an even bigger supply you can get 10hp).

 

[gasket999]

EDIT: Just been having a chat with my Dad. He's got a 3hp electric motor sitting in the garage. A friend of mine also has a 3hp electric motor that I can have for a crate of beer. Both 240v.

That's 6hp single phase. Just need to know if there's a way of running both motors in tandem. Given its an odd question I'm going to start a dedicated thread. If nothing comes of it then I have the petrol option as a backup - many thanks for your help. New thread can be found here

 

[gasket999]

Maker - thanks mate. I'm actually in the process of running a rewire of the house and garage and will have 40amps to play with which might make 240v possible. As I had an odd query about two 240v motors I've put this in a new thread - see here

However, I've been told that I can't run twin motors so I'll be going down the petrol route.