Power Draw Comparison, Rotary Screw Versus Two-Stage Air Compressors

      Comparing the power needs and other aspects of rotary and piston air compressors. January 7, 2007

I am thinking of getting a rotary screw compressor. The compressor size I would like to get would be 25 Hp max. My question is, compared to a 3 phase 2 stage 80 gallon 7.5 Hp, how much more electricity will I be using?

Someone told me that I would actually draw less electricity with a rotary air compressor. It doesn't make sense to me that a 25 Hp motor will draw less electricity. I am not interested in specifics, just what would draw more power under the same usage circumstances. Can anyone please explain this, as well as the pros and cons?

Forum Responses
(Cabinetmaking Forum)
From contributor B:
I have worked on some 200 cfm 2-stage and rotary compressors. Current draw is dependent upon your air demand, compressor efficiency, compressor size, etc. If electrical demand is your concern then the manufacturer should be able to provide the information based on a continuous load. You can also operate them in start/stop modes where they just shut off instead of running unloaded, depending upon your air demand.

In your question you don't include the capacity of each compressor that you are trying to compare, but you list the Hp rating. Hp is another way of measuring work and there is a correlation to amperage of the motor (3-4-5 rule of motors - watts, hp, amps - shows the rough relationship) watt/3 = hp/4 = amps/5.

Your 25 Hp compressor will use a lot more electricity when running, but maybe it won’t have to run nearly as much because it produces more compressed air/minute/HP when operating?

From contributor P:
If you are currently using a 7.5 Hp/80 gal, going to a 25 Hp rotary is a huge jump. There are several things to consider when buying a compressor and so many ways to look at compressed air needs. If you have high demand, do you depend on 1 unit even if it is new? Two smaller units may be a better option. If your compressed air demands are not so high, then a reciprocating (piston) does a great job, maintenance is easy and cost is less.

To get these extended warranties offered by screw manufactures, you must send in your oil samples every time you change oil so they will know you are doing what you should when you should.

It is a common belief that a screw compressor last much longer than a piston but that is not necessary the case. A rotary screw that is correctly maintained should operate about 60,000 - 70,000 hours before any major rebuild work is needed. A quality cast iron / 2-stage pump compressor (Champion/Quincy/Ingersoll etc) with a 1725 rpm motor that operates the pump at around 700 to 900 RPM's will run about 60,000+ hours if properly maintained. 10 hours per day x 5 days a week x 50 weeks per year = 2500 hours per year. 60,000 hours divided by 2500 hours per year = 24 years of service. That compressor has paid for itself many times over.

The compressors you see in Lowes and Home Depot have motors that turn at 3650 RPM and the pumps turn at around 1800 to produce the CFM that is shown. They do not last long.

Other things to consider when purchasing a piston compressor is an air-cooled after-cooler (drops the air temperature down to 100 degrees, helps when using an air dryer), low level oil switch (shuts off compressor if oil level in pump drops too low) and automatic tank drain (it's hard to remember to drain a tank).

Remember to change the air filter regular if the unit is inside the shop. You may be better served to have the compressor outside to eliminate the noise and the unit will not breathe the dust of the shop. The auto tank drain is a good option once again. Personally I prefer the largest tank possible. More capacity and the compressor cycles less thus it lasts longer.

From contributor L:
We used two 10 Hp Curtis reciprocating compressors for years but were running short on air periodically so I bought a 25 Hp Curtis screw (RS25) and put one of the recips in as a backup. The RS25 has been a disappointment –it vibrates, leaks oil, and has kicked out for no apparent reason several times. I just had the Curtis dealer service it and change to all synthetic oil - it is supposed to make it run cooler. I had them replace the separators and filters, and fix the leaks at a cost of nearly $1000. They couldn't find the source of the vibrations, seems to be internal to the air end. The compressor only has 4500 hours on it. It is equipped with the auto shut down if air consumption is low. I would buy a higher quality compressor the next time, which may be much sooner than I had hoped given the vibrations.

From contributor J:
7.5 HP 3 Phase Compressor
7.5(746)*.95/240*1.73=13 Amps
5.6 KW
25 HP 3 Phase
25(746)*.95/240*1.73=42.7 Amps
18.65 KW

With my electric rates the 7.5 Hp is $56 a month for demand and 25.2 cents per hour to operate at .045 cents per kwh. The 25 Hp would add $187 a month in demand charges and cost about .84 an hour to run.

From contributor L:
Screw compressors are good where you have a fairly constant demand. Large tanks are good where you have short-term peaks of usage. Unless you are going to dramatically increase your air usage I would go to a second reciprocating compressor and put in an alternator switch. When demand was lower only one compressor would be running. Each time they have to restart the switch would alternate giving each compressor a rest to cool.

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