Efficiency of Air Compressors

Variable speed rotary screw is what you want. 2 years ago I replaced our old 75hp and 50hp fixed speed compressors with a 150hp variable. We received a sizeable rebate from the electric company and seen about a 60% decrease in energy used. Added bonus is there is 25hp more capacity when needed.

Thanks. I forgot to mention that I had already decided that if we get a rotary, it MUST be variable speed. The additional initial cost clearly has a very short ROI payoff time.

Thanks for the post! My involvement does not necessarily concern air compressors, but it could in the future.

Small correction here - "payoff time." at the end of your last sentence is redundant; "payoff time" is inherent in ROI. I'm sure you know this, but for our readers...

It is for our readers that I added the "payoff time". Speakers whose first language is not English may well not know what ROI means. I have stated many times that I hate it when people use acronyms without definitions. I probably should just have added (Return On Investment).

Our company is saving about $3000 a month by swapping from the rotary to the reciprocating compressor, and that is the savings from swapping just one single compressor! At this rate, the reciprocating compressor would still be cheaper, even if we had to buy a new one every six months!

I believe rotary screw compressors can produce more CFM's, while reciprocating compressors can produce a higher PSI.

You don't give a lot of information on your air requirements (7 day/24 hr operation, demand variability, total demand, oil-free requirements (apparently none since you're currently using rotary lubricated), quality (breathing air?), etc?), so a detailed recommendation is not possible.

However, your findings correlate with my experience. In a nutshell (I'm assuming we're talking nominal 100PSI requirement):

1. On-off control with a properly sized receiver is the most efficient, regardless of compressor type and size.

2. However, the larger motors, required as air demand increases, are limited in the allowed starts/hour (I've seen 4/hour and some allow fewer). The additional start-up stresses also become more apparent on larger equipment. To accomodate on-off control, the required receiver volume gets out of hand.

3. Therefore, the larger compressors need some form of capacity control, preferably with corresponding energy usage. These include suction throttling, cylinder unloaders, inlet guide vanes, lift valves, variable speed, etc. If your requirements are in the rotary screw range, the variable speed option is the way to go, assuming you can convince the purse holders to loosen the purse strings. I couldn't on a project a few years ago, and we ended up buying a 100 HP rotary screw with 4 lift valves down to 50% capacity instead of variable speed.

Thanks for your input!

You're right, i didn't give any details about our requirements, partly because I don't know! We had a compressor company log our use for a week in one plant, but they recorded pressure and current, so any flow information is inferred/calculated. They claim we use 200 SCFM in that plant, but I'm virtually certain that is a peak value. I believe our average use is about a quarter of that, but I can't prove it... I've tried to get the company to buy a flow meter, but so far have not convinced them...

It is pretty standard industrial air. In that plant, we have about a 500 gallon receiver/accumulator, and do have a drier running, so the air going into the receiver is pretty close to ambient temperature, at roughly 100 psig. We currently have two shifts, with a much smaller crew in the evening shift. The air is used for a set of about 15 CNC mills, lathes, and VTLs, plus a number of manual machines, of course including chip blow-off, plus some occasionally used air-powered hand grinders, and a number of other machines that have air cylinders, but require very little flow. We have no need for oil-free air or breathable air.

We did originally have some starting problems with that 15hp reciprocating compressor, in the other main plant, but that was because it was designed for 220V 3Ø, and was connected with about 150 feet of cable to a 208V transformer, so during startup, it only got about 200 Volts. I rewired it for 440V, and connected it to 480V (our incoming voltage). Since then no problems. Under the most common circumstances, it has about 5 starts per hour with roughly a 30% duty cycle. During peak use, I have seen as many as 8 starts per hour, with a duty cycle approaching, but never exceeding 50% This unit currently has about a 250 gallon receiver. Another is available, but is not yet installed.

Unless something drastic changes, I believe our next purchase will be a reciprocating compressor. If it turns out to be a rotary unit, I think I may have enough influence to go the extra mile for a variable speed unit.

After a lot of twisting of the accountants arm we replaced five 60Hp Vee’s with two 75Hp variable speed screws. One would usually be stopped in standby mode for normal plant operation. A second plant that ran intermittently was connected in on a ring main system and the new compressors took the demand easily. That meant two more 40Hp Vee’s could be shut down.

Of the Vee’s usually one would be in bits while we tried to patch it up with an ever dwindling supply of spares.

Lower running costs, lower maintenance costs, quieter running and a quieter accountant.

Wow, you do use a lot of air!

I assume that by "Vee", you refer to a two or more stage/cylinder reciprocating compressor. So in the end you replaced 7 of them with two variable speed screws, and one of those is commonly shut down. Excellent! Do you have any info on the energy used before and after?

How long has it been since you made these changes?

I had never heard of a ring main system for air, or any other fluid; only for electricity. In fact, the only place I recall ever having seen the phrase is right here on CR4.

I remember once, probably 20 years ago, visiting a a small factory in Denmark, and being amazed that their air compressor sounded like a refrigerator. Our rotary compressors are definitely NOT quiet. I won't work around any of them for more than a minute without using ear protection.

Can you tell me the brand you selected?

Thanks! Dick.

It's very easy to see how your rotary air compressors are drawing so much power. From what you said, your reciprocating compressors are on a demand switch, powering off when the pressure set point is satisfied. Your screw compressors are on all the time. Why? Almost all units have a control available (or should) that allows the unit to shut off if pressure is satisfied. Or, they can be set to stay on, and run in stand-by mode.

Perhaps your screw compressors are leaking badly internally, and are unable to satisfy the pressure set point?

A few years back, when I was the maintenance engineer for a foundry, we had 8 (50 hp) Quincy screw compressors. These units provided air for 3 foundries, 3 processing plants. The entire site was fed by a 6" id steel pipe loop (about a mile in circumference). Our units cycled on and off as demand called for air based on pressure readings. All units were put on sequential timers, all units were on soft starters. Only 1 unit was on at any time (normally), although demand might call for 2 units in parallel occasionally. The compressors were logically programmed in such a way that unit 1 would power up, unit 2 would come on if it was determined that air usage was too high for #1 alone. When the pressure switch was satisfied, the next unit to come on line was #3. Then, when the pressure was satisfied, and #3 shut down, the next unit to power up was unit #4, and so forth. This allowed for system redundancy for maintenance, etc. None of our units stayed on continuously, even at no load.

I think you have a setting problem on your controls, or have worn screw compressors.

Interesting! Thanks for your input. One of the current main compressors is indeed a 75hp Quincy.

I have read, and have been told in person, that screw compressors should never be operated on pressure switches. As I recall, this was due to something related to water condensation inside the compressor during the off times.

All of our compressors were acquired as part of the process of purchasing existing companies to add to our capabilities, so we have no record of maintenance or anything else. They have been serviced regularly by "professional" air compressor people since they were acquired and moved to our location about 2.5 years ago, but to my knowledge, those people have never said anything about checking or changing settings.

As I have stated previously, these are all old, so undoubtedly do have reduced efficiency, but there has never been a problem supplying the demand. I still don't understand why they continue to use essentially full power regardless of demand!

I would get in touch with the Quincy folks, and ask them. There is a modulating valve at the compressor input, as I recall. I do know that we shut ours down; they did not stay running in standby mode whether or not there was a load. We had no problems shutting them down, or restarting them.

Our main problem was air leaks....but that's a whole 'nother matter.

I signed up with Quincy on Feb 6, and sent an email on the 7th. I never received a reply...

I have to admit that I did not try the phone yet...