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Compressors

Whatís the requirements?

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David GW18/01/2020 22:36:53
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32 forum posts

I’m trying to choose a compressor.

Can anyone tell me roughly what the air requirements are to run (not drive) a 2” scale single cylinder TE please?

Paul Lousick19/01/2020 12:24:27
1501 forum posts
571 photos

You don't need a lot of pressure to run a TE. 20 - 30 psi will run it over but you need a lot of volume. Air does not have the expansion rate of steam and the reservoir is quickly emptied.

I have a larger engine with a 3.25" piston dia and have an 8 cfm compressor. It takes about 10 minutes to charge the air receiver to 80 psi and will run my engine for a minute or two before it is exhausted. The compressor cannot supply enough air to run continually. A 5 cfm compressor would probably keep a 2" engine running continually without a load.

Paul.

Paul Kemp19/01/2020 12:42:30
548 forum posts
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David,

Good advice from Paul. It's a bit of a minefield as compressor sellers often exaggerate their claims! Short answer is get one as big as you have room for and with a motor that can be supported by the power supply you have available, pretty well all hobby machines start on load so starting current is the limiting factor.

I think if you want to try to calculate it, work out the displacement of your engine cylinder (bore X stroke), double it (because it's double acting) and multiply by the maximum speed (rpm) you want to run the engine at (say 30?). That will give you a volume per minute at atmospheric pressure. Your compressor then needs to deliver more than this to maintain any pressure in the reservoir. A larger tank will give you a greater buffer.

Paul.

Bob Brown 119/01/2020 13:17:51
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1010 forum posts
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Compressor output is usually quoted as x cfm but often miss the last bit "free air delivered" or running at atmospheric pressure.

JasonB19/01/2020 13:35:31
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Also depends on what you mean by "run" if it is just ticking over very slowly out of gear you are not going to need anything as big as if you want to run it up and down the workshop or in circles on the patio.

They don't miss the last bit off they tend to quote displacement rather than FAD which can be quite abit less, Just like machine sellers quoting a motors input watts rather than output.

David GW19/01/2020 14:01:05
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32 forum posts

Thanks all, much appreciated. Yes I’m just talking about running the engine in neutral with no load, not propelling itself. I was hoping not to have to get a full on garage type compressor (I sold mine last year because I never used it!) and that a little airbrush compressor would do it as it would be smaller & quieter. I had a meander through eBay last night but the pidgin specifications of these machines are ambiguous and written as if to disguise their performance.

David GW20/01/2020 22:33:52
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32 forum posts

Thinking outside the box here but would a pump up garden sprayer be capable of ticking her over for 20/30 seconds or so?

Paul Kemp20/01/2020 22:43:43
548 forum posts
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Not sure what pressure they attain? That have a reasonable volume but pressure would drop off quickly so personally I doubt it but prepared to be proved wrong!

Paul.

Paul Lousick20/01/2020 23:47:26
1501 forum posts
571 photos

David,

An air brush compressor would probably run a little Stuart type portable engine but not a 2" TE. You will need a bigger garage type compressor. An air brush compressor has ample pressure to turn over your engine but does not have enough air volume for running continually. It may run your engine for 10 seconds before the reservoir was exhausted.

not done it yet21/01/2020 06:46:04
4889 forum posts
20 photos

Join your local model engineering club, is my advice. Someone will have a possible suitable compressor to demonstrate if it runs or not and would likely help you out in making your decision.  A new engine might need rather more pressure than when run in a bit.

Lidl recently sold a small compressor of about a kiloWatt, claimed FAD of 180 litres per minute. Very noisy and without a particularly long continuous run time, but might suffice. They cost about fifty quid.  There is a review video on you tube.

Time of run will clearly depend on tank pressure and size, for a conventional type of compressor.

I personally would not want to buy a modern “buzz box” compressor. An old cast iron unit, running slowly would be a far nicer prospect for me.

PI D^2 h/4 the volume of a cylinder, where Pi is 22/7, D is diameter and h is length of stroke in this case. Paul’s suggestion is a good starting point. Less than that volume would be no good without some form of pressure regulation, fed from a much higher pressure air receiver., for continuous running.

A profile, with location, might help you get some practical assistance. My air line would likely not reach your location!

Edited By not done it yet on 21/01/2020 07:01:30

David GW21/01/2020 08:00:34
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32 forum posts
Posted by not done it yet on 21/01/2020 06:46:04:

Join your local model engineering club, is my advice. Someone will have a possible suitable compressor to demonstrate if it runs or not and would likely help you out in making your decision. A new engine might need rather more pressure than when run in a bit.

Lidl recently sold a small compressor of about a kiloWatt, claimed FAD of 180 litres per minute. Very noisy and without a particularly long continuous run time, but might suffice. They cost about fifty quid. There is a review video on you tube.

Time of run will clearly depend on tank pressure and size, for a conventional type of compressor.

I personally would not want to buy a modern “buzz box” compressor. An old cast iron unit, running slowly would be a far nicer prospect for me.

PI D^2 h/4 the volume of a cylinder, where Pi is 22/7, D is diameter and h is length of stroke in this case. Paul’s suggestion is a good starting point. Less than that volume would be no good without some form of pressure regulation, fed from a much higher pressure air receiver., for continuous running.

A profile, with location, might help you get some practical assistance. My air line would likely not reach your location!

Edited By not done it yet on 21/01/2020 07:01:30

Ha funny you should mention it I tried doing my profile a few times on day one but when submitted the fields clear. This site just doesn’t work properly on Safari, I constantly get logged out also but hey ho.. I’ve tried asking about it on another thread.

Paul Lousick21/01/2020 10:04:10
1501 forum posts
571 photos

"no good without some form of pressure regulation"

Most shop compressors come with a reducing valve which can output air to a lower pressure

David GW21/01/2020 11:21:07
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32 forum posts

Well I can get a low-noise (60dB) compressor that fits the criteria for about £90 or a standard noisy one for about £60 but it’s down to whether it’s worth me getting one at all.
My TE has clearly been fired, nothing seems amiss, everything turns freely so I’m fairly confident it will run I would just like to have seen it running before having its hydraulic & steam tests, however I don’t see me wanting to run it on air for entertainment thereafter.
Will running it on air tell me much more than I already know about the engine or just wait for the steam test?

Mike Poole21/01/2020 12:26:08
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A large receiver could extend the runtime of a small compressor as long as you are prepared to charge it up, of course once you run out of air then it’s game over until you charge up again.

Mike

not done it yet21/01/2020 12:32:04
4889 forum posts
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Take it along to a garage forecourt and use their air supply?

Edited By not done it yet on 21/01/2020 12:32:23

HOWARDT21/01/2020 13:50:21
583 forum posts
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Posted by not done it yet on 21/01/2020 12:32:04:

Take it along to a garage forecourt and use their air supply?

Edited By not done it yet on 21/01/2020 12:32:23

You have to pay for air these days at most garages. May be cheaper to gat a compressor in the long run.

David GW21/01/2020 15:11:06
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32 forum posts
Posted by not done it yet on 21/01/2020 12:32:04:

Take it along to a garage forecourt and use their air supply?

Edited By not done it yet on 21/01/2020 12:32:23

Ha! Nice one!

The issue has been resolved by a kindly soul who is going to bring over his compressor so we can have a play. Thanks for all the replies, sorry I haven’t responded to all but I have to log on each time.

old mart21/01/2020 15:15:36
1913 forum posts
151 photos

There are lots of small compressors with 25 litre recievers which would not only power models during testing, but also be very useful for a home shop airline and for blowing up your car tyres.

not done it yet21/01/2020 17:36:34
4889 forum posts
20 photos
Posted by HOWARDT on 21/01/2020 13:50:21:
Posted by not done it yet on 21/01/2020 12:32:04:

Take it along to a garage forecourt and use their air supply?

Edited By not done it yet on 21/01/2020 12:32:23

You have to pay for air these days at most garages. May be cheaper to gat a compressor in the long run.

My response was to this, posted by the OP at 11:21:07 today, so actually only a very ‘short run’.🙂

My TE has clearly been fired, nothing seems amiss, everything turns freely so I’m fairly confident it will run I would just like to have seen it running before having its hydraulic & steam tests, however I don’t see me wanting to run it on air for entertainment thereafter.

As I see it, a forecourt air supply would be far cheaper with no storage issues. Could sell on a compressor immediately afterwards, I suppose. Still likely cheaper to buy some air. Certainly cheaper than buying a compressor (for one job) if you join the local club and find one that way

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