Reciprocating vs Rotary Screw Air Compressors

[EddyCurr]

I am looking for opinions about enclosed rotary screw compressors (RSC)
in comparison to equivalently rated enclosed reciprocating piston
compressors (RPC).

For example:

1. Quincy QGS5-1-60
   • 5HP/1Ph
   • 16.5 CFM @ 150 PSI
   • 60 Gal
   • 62 dB
   • LxWxH - 56" x 22" x 50"
   
   .
2. Quincy 251CS80HCBX
   • 5HP/1Ph
   • 17.2 CFM @ 175 PSI
   • 80 Gal
   • 64 dB
   • LxWxH - 70" x 28" x 58"

Before beginning my search, I thought that rotary screw compressors
made less noise and required less room than reciprocating compressors.
Yet, as shown above, the RSC version is not much quieter than the RPC.
The RSC's smaller dimensions are due in large part to its smaller capacity
tank (60 gal vs 80 gal).

Another consideration is that the RSC appears to be more complex. The
rotary has an oil separator and some other components which the
reciprocating does not have. How will an RSC compare with an RPC as
far as routine maintenance cost & frequency, average service life and
rebuild costs?

Which design would stand up better under an intermittant operating
cycle where the compressor might stand idle for 2-3 weeks between
usage? Would either or both require preparation before use after
standing for extended periods?

.

[CCWKen]

I have a Sullair RSC but it's much larger--160 CFM. Yes, the decimal is in the right place. They are pricey to replace parts but maintenance isn't all that bad. Oil and filters is all I've used since the original repairs and adjustments. (used 1984 model) It can sit weeks/months at a time since I only use it for media blasting. It's 4-cylinder GM powered so I worry more about the battery staying charged. As long as you use it long enough to get hot and evaporate moisture out of the oil, they'll probably last forever. But that's true with a piston pump too.

I have a piston pump for the shop compressor (17.0/175). It's a Coleman Commercial that's given me great service since new (about 1995). I had to replace the head gasket once, around 1998, but other than that it's been filters and oil since then. And it gets heavy use with air tools and painting just about every day. Replacing the head gasket is when I found out the compressor pump came from Italy.

[hardtail]

We have both multiple rotary screws and recips at work, the rotaries are great for continuous duty provided they are sized properly for the load.......depending on the brand, maintenance with filters and seperators isn't cheap.......then theres the continuous duty power consumption. They're not necessarily quiet either, we had a small Quincy rotary installed in the last plant I was at and it came with a soundproof enclosure, open it up for inspecting and it was loud. We service the rotaries every 2000 hrs.......

I wouldn't consider one for homeshop duty, the recips seem to shine there.....

[Robo]

I think rotaries are the future of compressors no pump up time instant air. With that being said recips have been doing the job for a long time and in most cases can be had for cheap. Hard to beat in the home shop IMO all depending on intended use. Just my 2 cents

[EddyCurr]

Thank you for the replies.

I hadn't been aware that some/all ? rotary compressors run continuously
rather than 'on demand' as do reciprocating compressors. That alone
is a deal-breaker.

For this and other reasons, it looks like I best confine myself to looking
at quiet reciprocating units.

.

[hardtail]

They don't have to run continuously but thats where they shine, like Ken said if you have periodic air intensive work they would be ok, but they either load/unload while running or have some form of speed control to keep them loaded continously. They are more geared to constant operation but if you have some application that requires lots of air and access to a cheap one in good shape go for it........

You could just turn it on/off as required but I don't know if it would reach optimal operating temperatues at times.......

One type I have never been around is a Hydrovane but guys rave about them.........

[hardtail]

Eddy perhaps the Hydrovane is the best of both worlds?.......did a search and Sir John is a fan and has a link to a UK one for auction......rotary and intermittent operation, seems quiet compared to most screws......

http://bbs.homeshopmachinist.net/sho...ight=hydrovane

[Jaakko Fagerlund]

I would say to check on what dB means, as 62 and 64 dB has quite a difference.

[hardtail]

Actually thats quite close to one another.......

[Rustybolt]

We have two Ingersol-Rand rotary compressors at work 30 hp or something. They ran continuously for three shifts until we decided to move the airlines from under the floor and mount them overhead. Now we just use one, intermittantly, for two shifts.

[small.planes]

I've not done the sums, but the rotary will be quieter than the figures suggest.
db's are log, not linear. From my sound engineering days iirc 3 db difference is twice as loud.

Dave

[EddyCurr]

Eddy perhaps the Hydrovane is the best of both worlds?.......

Thanks for the suggestion and link.

I'm not familiar with the Hydrovane, time to find out more about them.

.

[EddyCurr]

I would say to check on what dB means, as 62 and 64 dB
has quite a difference.

Actually thats quite close to one another.......

I've not done the sums, but the rotary will be quieter than the figures
suggest. db's are log, not linear. From my sound engineering days iirc
3 db difference is twice as loud.

As decibel is a 10ⁿ logarithmic scale, the change in sound intensity
represented by an increase of 10 dB is a ten fold increase in sound
intensity. An increase of 20 dB is a one hundred fold increase in intensity.
30dB is one thousand fold.

However, testing has led to a Rule-of-Thumb that an increase of 10 dB
is 'perceived' by humans as being twice as loud as the initial sound. (A
bit of searching suggests that this value varies: apparently amongst a
cross-section of people will feel that increases of anywhere from 6 dB
to 10 dB represent a doubling in perceived loudness.)

Although I saw a lower value at least once, 3 dB is a common value held
out as the lowest increase usually perceptible by humans.

Other factors figure into perceived loudness, but for my purposes of
comparing compressors, the above will do. A quick glance shows that
enclosed RSC & RPC compressors operate in the low 60 dB's. Quality
unenclosed RPC's with motors that run at 1750 RPM are rated in the
mid-70 dB's. Other unenclosed compressors with motors that run at
3400 RPM are rated in the mid 80 dB's. With these kinds of deltas, there
is no sense in splitting hairs over the frequency of the sound or other
factors.

.

[jdunmyer]

Recips are supposed to use a lot less electricity to pump the same amount of air. Ingersol-Rand claimed that the largest expense, by far, in the lifetime of a compressor is the power used.

Regarding that, it pays to eliminate air leaks.

[EddyCurr]

Recips are supposed to use a lot less electricity to pump the same amount of air.

The comparison in the OP has both machines utilizing 5 HP/1 PH motors to
produce similar ACFM outputs. Provided that each cycled only as necessary
to keep the tank at the same regulated pressure, wouldn't the power consumption
be comparable?

Ingersol-Rand claimed that the largest expense, by far, in the lifetime of a
compressor is the power used.

Regarding that, it pays to eliminate air leaks.

This is my understanding, too. It was impressed upon me at an early stage
that air is expensive. To date, I have relied on two smaller compressors and
an third tank for additional capacity, using only as much compressor and
storage as necessary for a job rather than firing up a large compressor to
fill a large storage tank for whatever little task came up.

.