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View Full Version : Why are 2-stage compressor quieter?



dvo
10-21-2011, 10:06 AM
Is it just their low rpm or is there more to it?

EddyCurr
10-21-2011, 10:52 AM
I'm not sure that all two-stage compressors run at 1,725, nor that all single
-stage compressors run at 3,450. But compressors that run at 1,750 sound
quieter and less frantic to me than those that run at 3,450, regardless
whether they are 1 or 2-stage

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Black_Moons
10-21-2011, 01:12 PM
Most compressors are heavily belt ratio changed too, you should take that into account more then the motor RPM.
I have a 3600rpm motor 5HP compressor that sounds like a warzone when it runs, the main compressor runs around 1000rpm I believe (1 stage 3 cylinder)

I have a 2 cylinder, 1800rpm motor 2HP compressor that you can TALK over. the main compressor runs around 300~400rpm iirc.

I have considered buying a 500rpm compressor head for my 5HP compressor, but I think the motor pully would end up too small with a 3600rpm motor.

You gotta change the ratio too much to get 3600rpm down to 300~ rpm to have pratical pully sizes, hence why quiter compressors often have the 1800rpm motors.

Scottike
10-21-2011, 01:28 PM
Most 2 stage compressors are more heavily built than their single stage counterparts - so the extra mass helps reduce the noise issue. But, the rpm is the main reason, most 2 stage units run at 1000 rpm or less, that's not so for a lot of single stage units.

Rosco-P
10-21-2011, 02:25 PM
Much of the noise is from the intake valves and you hearing that through the intake/air cleaner. Beside the pump running more slowly (below 1000 rpm), a two stage compressor is probably equipped with a more effective air filter/silencer like a Solberg.

danlb
10-21-2011, 05:09 PM
I thought it was because the 2 stage has it's intake via the first stage, which should be low pressure, high volume. That muffles the intake to the second stage which is lower volume and higher pressure.

I could have that all wrong.

Dan

dvo
10-21-2011, 07:06 PM
Thank you all. Seems like the rpm seem to come up a lot so I decided to experiment with my HF 60 single stage. This thing is nice for the price but it's one loud motha. One of these days I'm going to get hurt cause I'll crap my shorts every time it comes on when I'm working on the mill or lathe.

Base on the current pulley setup the pump is running at 1488 rpm. Had a 3" pulley available so I used that which brought the rpm down to 744. Oh baby....no more crappin in me shorts :D when she starts up. With such a low rpm you can hear the valve/intake noise...put....put...put...put. So I made an adapter to use oil filters as the intake filter but that did very little to reduce the noise any further. I suppose you could pipe the intake outside but seem too much hassle since it's already pretty quiet. Oh well at least it's clean air going into the head vs the swiss cheese thing they called a filter. I used the same filter as my Toyota pickup so no need to keep track of which one to buy.

SCFM drop accordingly but not a problem for me so far. I could bring the rpm up to like 900-1000 with a bigger motor pulley but I won't bother. Instead I'm going to see if I can Craiglist's a 5hp motor for cheap and snag this http://www.harborfreight.com/145-psi-5-hp-twin-cylinder-air-compressor-pump-67698.html with a 20% coupon. Target it for 800-900rpm. That should give me the noise level I can tolerate and plenty of SCFM for my need without having to go the two stage pump.

Rosco-P
10-21-2011, 07:22 PM
One of these (4" size):http://www.solbergmfg.com/ViewSeriesDetail.aspx?SeriesId=17&src=hscroll

dvo
10-21-2011, 08:47 PM
One of these (4" size):http://www.solbergmfg.com/ViewSeriesDetail.aspx?SeriesId=17&src=hscroll

The oil filter is doing fine, got two of them. The SCFM drop is from the drop in rpm not intake restriction. Can't beat $2 per filter.

duckman
10-22-2011, 12:46 AM
dvo before you buy that pump read the specs the volume is at 1000+ rpm and the rated speed is a little over 600 RPM. :confused:

Rosco-P
10-22-2011, 09:50 AM
The oil filter is doing fine, got two of them. The SCFM drop is from the drop in rpm not intake restriction. Can't beat $2 per filter.
That's not the point.

The $2 filter does nothing to silence the noice from te intake.

A.K. Boomer
10-22-2011, 09:51 AM
Much of the noise is from the intake valves and you hearing that through the intake/air cleaner. Beside the pump running more slowly (below 1000 rpm), a two stage compressor is probably equipped with a more effective air filter/silencer like a Solberg.


yes that's where my head went - like you and danlb stated - there's only one "exposed" intake - the other is linked directly to the large cylinders compression stroke - between that and the lower RPM's is the reason I would think but that's only speculation on my part also.

Mike Nash
10-22-2011, 01:11 PM
The $2 filter does nothing to silence the noice from te intake.

Using an automotive oil filter for the air intake on my 60 gallon upright single stage compressor made for a huge reduction in overall noise level.

I don't know about anyone else's, but I tried the original felt filter again to compare noise and recharge time against either filter or none at all. The oil filter extended motor run time very slightly, but was much quieter. No filter was not a significantly shorter time but was very much louder.

EddyCurr
10-22-2011, 02:15 PM
It is possible that the addition of a Helmholtz resonator to the intake
manifold would reduce the runtime back to that which the compressor
experienced with the low restriction air filter, while still retaining the benefit
of reduced dB provided with the more restrictive filter.

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