Compressor @ Netto

I presume that '2hp' is rather like the '2hp' on a little compressor that I have?

Tim

I quite expect they have stretch every possible interpretation to come up with the 2hp figure. I have a similar one from the Aldi stable. It runs an air nailer Ok and is a general purpose for blow gun duties. It is a noisy little b'stard though but pretty good value for the price. I would not rely on it for professional use though.

Bob

Hamster Power?

Nick

I'll be like my '15 cu ft/min' 3HP Clarke Air Compressor bought from Machine Mart, when coupled up to Machine Mart air fed mask consuming 9 cu ft/min. Starting with a full tank, I connected the mask. The air was rapidly consumed. The compressor ran continuously and never cycled until I disconnected the mask. Then I found out that 15 cu ft /min was air displacement and that the 9 cu ft/min demanded of the mask was free air delivery.

Conclusions?

Compressors are always hyped by the marketing departments and until you start spending many thousands of pounds the real world performance doesn't match *anything* quoted in the specs.

What is the difference please? Henry

The first is input, the second output, at a guess. Or before and after regulator.

The swept volume or displacement is the total volume that the piston moves through in a minute ie piston area x stroke length x number of strokes per minute.

This is not however the amount of air that actually goes through the compressor because of the effect of needing to have a small clearance volume at top dead centre. This contains high preesure air which must expand as the piston descends. It must expand until its pressure is low enough to allow the inlet valve to be opened by the pressure difference, ie the pressure in the cylinder must be below atmospheric pressure to open the weakly spring loaded valve.

The end result is that a different means of describing the output of a compressor is used and this is the 'free air delivered' or FAD. This is the air delivered per minute expressed as a volume of air measured at ambient conditions.

The FAD will always be less than the swept or displacement volume per minute. The FAD will vary with the pressure you require the compressor to deliver because of the expansion of the trapped air in the clearance space at TDC. A higher delivery pressure will reduce the FAD until eventually at a really high pressure there will be no delivery.

This can be cured by using multiple stage compressors and things can also be improved by adding intercooling between stages. Reducing the clearance space at TC also helps significantly, However all of these fixes add cost to the original purchase.

H> Drag =EF=BF=BDThen I found out that 15 cu ft /min

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Plans and free books - *Now with forum*

I am familiar with FAD and the fact that it varies with delivery pressure. Once got involved in an argument a compressor supplier who quoted FAD and delivery pressure which matched our needs. However the FAD was quoted with an open outlet (i.e. low delivery pressure) but that was not made clear. Soon became clear when we measured the flow rate as part of our use!

Had not met the "air displacement" term before so didn't recognise it as swept volume.

Henry

er.http://www.workshop-projects.com-> > Plans and free books - *Now with forum*

Like the duty cycle on welders. Often mentioned but never explained.

The duty cycle is based on a ten minute cycle, now take your average Seeley stick welder rated at 140 amps. Now if they were to put the correct duty cycle label on them and explain then they wouldn't sell any. It will probably read

50 amps 100% 60 amps 90% 70 amps 80% 80 amps 70% 90 amps 60% 100 amps 50% 110 amps 40% 120 amps 30% 130 amps 20% 140 amps 10%

So basically what this means is if you want to continually weld you are limited to 50 amps and if you want to weld flat out you weld for 1 minute and drink tea for 9

A decent welder of the same rating should read something like

140 amps at 60% 100 amps at 100%

But you won't buy that for =A329.99

John S.

It's a measure of the flow of air available at the outlet of the compressor rather than the amount displaced by the compressor reciprocating motion.

In the case of the compressor I had, a 3HP 150psi 15 cu ft/ minute compressor became something around an 6 cu ft / minute one despite being regulated down to 60psi.

If you want to measure and do the maths