Energy efficiency rating for air compressors?

[Leveleer]

I can not find any reference to such a thing.
When I ask at various stores to see the most efficient air compressor I usually get a funny look and a "Huh?"
Then I sometimes respond with "O.K. how about the least inefficient?"
Is it possible that virtually no one really cares?

Frank

 

[A-R-K]

My guess is no one cares. Home owners duty cycle is incredibly low (typically) so efficiency isn't particularly important, as it's just not on for that much time.

For industrial applications it's a bit different. With high duty cycles you're looking at compressors with a high MTTR and paying attention to the current draw. Last time I spec'd one for industry, I made a table comparing total cost of ownership, which includes electricity consumption.

Compare the current draw draw vs cfm, that should give you some insight into efficiency, assuming you can get accurate current info.

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[Showkey]

Add the cheap air tool on the other end...........just a guess ........but bet the overall efficiency ***** ( blows).

 

[md21722]

Consider that a 1HP air tool normally used 30-35CFM. It takes about 10HP of air compressor to make this much air. Air tools are inefficient from an energy consumption standpoint. However, air tools have their place in the world. They are smaller and lighter than their electric counterparts. Less operator fatigue and can get into tighter spaces. They also don't spark and can be used in more hazardous environments. Plus if somebody cuts an electric cord... Some compressors may have more efficient motors, but typically these are ODP with lower service factors than TEFC or explosion proof motors. If you work in a dirty or dusty environment the TEFC motors are better since they direct air over the motor rather than through it. Larger motors tend to be more efficient than smaller motors. Dual stage pumps tend to be more efficient particularly at higher pressures & sometimes that higher pressure is needed. For example, a 1/2" impact that uses 37SCFM will cause a 25 psi drop across many filters and regulators and a normal 3/8" air hose. That means to get 90 at the tool you need 115 psi in the tank. And if you set the cut in that high, most pressure switches require a 25 psi differential requiring shutoff at 140. Many single stage compressors drop off efficiency past 110-115 psi.

If you're buying for home use, to run an impact you are unlikely to use enough electricity for efficiency to be a concern. In my opinion, air is more about operator productivity and overall safety rather than electrical efficiency.

 

[rockwithjason]

I can not find any reference to such a thing.
When I ask at various stores to see the most efficient air compressor I usually get a funny look and a "Huh?"
Then I sometimes respond with "O.K. how about the least inefficient?"
Is it possible that virtually no one really cares?

Frank

don't care.

 

[md21722]

The least efficient compressor is the one that doesn't meet you needs ;-)

 

[larry_g]

I know that major household appliances have energy ratings but can't think of any shop or industrial equipment that has a rating. What other shop equipment has energy ratings? not your drill press, mill, lathe, soldering iron, drill motor, or anything else. Some motors publish an efficiency rating that might sway a purchaser if the motor is in use 24/7 .

So what else are you putting in the shop where Energy efficiency ratings are published?

lg
no neat sig line

 

[Leveleer]

Since compressing air is so inefficient by itself I feel like it is irresponsible to produce equipment that makes an already inefficient process even more inefficient. I am particularly offended when I see multi thousand dollar compressors designed for 100 percent duty cycle with sharp right angle connectors coming out of the head. I can't imagine what they are thinking when they do something like that.
When I'm looking for a compressor I am forced to closely examine it for things like this while trying to make a decision. After all I am the one who has to pay the electrical bill.

Frank

 

[Farmall450]

I know that major household appliances have energy ratings but can't think of any shop or industrial equipment that has a rating. What other shop equipment has energy ratings? not your drill press, mill, lathe, soldering iron, drill motor, or anything else. Some motors publish an efficiency rating that might sway a purchaser if the motor is in use 24/7 .

So what else are you putting in the shop where Energy efficiency ratings are published?

lg
no neat sig line

A fridge

 

[A_Pmech]

Compression of air by a reciprocating two-stage compressor is about 80% efficient when compared to an is isentropic process.

 

[md21722]

Since compressing air is so inefficient by itself I feel like it is irresponsible to produce equipment that makes an already inefficient process even more inefficient. I am particularly offended when I see multi thousand dollar compressors designed for 100 percent duty cycle with sharp right angle connectors coming out of the head. I can't imagine what they are thinking when they do something like that.
When I'm looking for a compressor I am forced to closely examine it for things like this while trying to make a decision. After all I am the one who has to pay the electrical bill.

Frank

When 1/2" pipe flows 100 SCFM and the compressor head flows 15, a 90 degree bend is not decreasing the efficiency of the unit.

Compressing air makes hotter air. Hotter air holds more water. What's the most efficient way to deal with the water? And how much water do you need to get rid of? It would be inefficient to pay for a high temp air dryer if you don't need it.

I would take a step back and evaluate from a different point of view.

 

[Leveleer]

Here is an IR compressor rated just under 20 CFM.
Note the brass connector coming out of the 2nd stage head.

Since the air flow coming out of the head is highly intermittent, and only flows while the exhaust valves are open, the actual flow hitting the fitting is more like 60 to 80 CFM or possibly more. Not very efficient to say the least.
When the valves open the air has to suddenly accelerate and the first thing it does is hit a wall of brass.

Frank

 

[md21722]

The CFM air rating of an air compressor is determined by how much air it draws on its (first stage) intake stroke.

There are less expensive dual stage pumps that make 15.2-15.8 CFM and more expensive ones that make in the 17's. Not a large difference in CFM but a large difference in price.

 

[CNGsaves]

OP . . . Is this a pedagogical goosechase ????

What is BUDGET for compressor ??

What are your CFM requirements ?? (ie all the air tools that you plan to use)

If that IR 2-stage you show above is "Not" good enough . . . what do you want ??

 

[Leveleer]

OP . . . Is this a pedagogical goosechase ????

What is BUDGET for compressor ??

What are your CFM requirements ?? (ie all the air tools that you plan to use)

If that IR 2-stage you show above is "Not" good enough . . . what do you want ??

My interest in this started about a year ago while I was designing air hose couplings for my associates.
They asked me if I could develop a portable air compressor that could power their contractor tools and run off a 120V extension cord.
During this process I looked at a lot of compressors and was quite surprised at how inefficient they were for the most part.

I am trying to figure if there is any interest in efficiency so I have an idea whether or not to continue this endeavor.

Frank

 

[dnschmidt]

No, there really isn't.

 

[md21722]

No, there really isn't.

I'd agree. Restricting air flow through quick couplers is far different than making compressing air in the first place.

 

[Boilerhouse]

Compressed air one of the most expensive forms of energy. At home, our little compressors run occasionally on the weekends, so it really isn't a big deal. Industry, which will often have hundreds of HP producing thousands of cfms costing hundreds of thousands of dollars per year, compressor system efficiency should be a big deal but even then, it often isn't.
For those industries that do care;
>Plant wide air leak detection and repair
>2 stage compressors with inter-cooling between stages
>Screw compressors with variable speed drives vs full rpm and load/unload
>lower plant air pressure (each 2 psi drop equals 1 % energy savings)

 

[MacMcMacmac]

Compressed air is more expensive than natural gas. Especially if it is dried.

4 cfm per horsepower is about the limit of what you are going to get out of the most efficient designs, which is the double acting reciprocating compressor.

You are only going to get 1 hp or so out of a 120v outlet. Max 15A. That's not enough power to make a whole lot of air. They are also starting off on the wrong foot by planning to use extension cords.

There is no home owner/contractor sized compressor that is going to flow enough air that those fittings will be a source of restriction. When flow testing compressors, we use a specific sized orifice and put a pressure gauge upstream from it. The pressure it can maintain at that point, flowing through an orifice of an accurate size tells us the CFM flow. We had a 750CFM CompAir portable with a either a Cummins L10 or Big Cam running it and a 3/4" orifice was enough to flow what that machine could put out for testing.

I have heard that some very large compressors will get a false unload signal if they discharge straight into a 90 deg bend, or into a main from a tee with other machines also pumping into that main, but we are talking very large industrial compressors. At that point, it makes sense to use 45 deg or a gentle loop of pipe to bring the air into the main trunk line.

 

[Xicaque]

I don't care about that efficiency factor. I have a Quincy QT7 and it is a lot of compressor for my weekend/random use. I think it uses a lot less electricity than my clothes dryer. This is my last compressor for as long as I live on this planet. My previous CH 30 gal was a good one but I could not stand the noise plus I was doing a lot of spraying and it sucked waiting for air.

If you are going for it, go big the first time and you will be not regretting it when you go for cheaper/lesser options.


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[Oggy]

FWIW I work for a pump manufacturer and the dept of energy recently published new guidelines on motor efficiency ratings.

However, these guidelines have only affected our larger motor pumps that are 3 phase. Other motors are largely unaffected. I would imagine that along with others, the duty cycle for air compressors in a residential or even a light commercial settingwould be so low that the typical buyer wouldn't consider that as a factor for purchase, and therefore the numbers would be very difficult to find, and if you could there would likely not be a standard of measurement for said number.

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[MikeF2316]

FWIW I work for a pump manufacturer and the dept of energy recently published new guidelines on motor efficiency ratings.

However, these guidelines have only affected our larger motor pumps that are 3 phase. Other motors are largely unaffected. I would imagine that along with others, the duty cycle for air compressors in a residential or even a light commercial settingwould be so low that the typical buyer wouldn't consider that as a factor for purchase, and therefore the numbers would be very difficult to find, and if you could there would likely not be a standard of measurement for said number.

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I'm with the OP on this. Even if I use a compressor so little that it uses 60¢ of electricity, I'll still buy one that only uses 45¢ if everything else is equal. It's like buying gasoline - you're going to go to the station that 1¢ per gallon cheaper, even if the 15¢ you save will make no difference in the long term.

 

[bsaint]

Here is an IR compressor rated just under 20 CFM.
Note the brass connector coming out of the 2nd stage head.

Since the air flow coming out of the head is highly intermittent, and only flows while the exhaust valves are open, the actual flow hitting the fitting is more like 60 to 80 CFM or possibly more. Not very efficient to say the least.
When the valves open the air has to suddenly accelerate and the first thing it does is hit a wall of brass.

Frank

Hey Frank, if you think that is inefficient you should see the valve design. I wouldn't even worry about that elbow. If you do some fluid calcs, you'll notice the loss is hardly anything.

It might be 80 cfm at atmosphere but certainly not under pressure.

 

[bsaint]

The only way to increase compressor efficiency is to utilize the heat energy in another way. Our energy conscience customers would use heat exchangers on their rotary screws to heat their building in the winter. In the summer it was a loss.

 

[pcmeiners]

"I see multi thousand dollar compressors designed for 100 percent duty cycle with sharp right angle connectors coming out of the head. I can't imagine what they are thinking when they do something like that. "

Money, money, money, the industry and consumers don't care much about efficiency. Considering compressor/air supply inefficiency, you would think the government would **** in with mandates.... the lobbyists are doing a good job.

The fitting you pictured, 90 degree turn is bad enough, the size of the port should be larger, but that would cost a couple bucks more to manufacture.

Common inefficient compressor/air system components....

Pitifully under sized check valves.
Small pipe size feed off the tank (restriction).
Under sized intercoolers, both in pipe size and heat capacity.
Under sized components, filters, regulators, air supply lines.
General air supply piping design has too many short Els and Tees
Lack of true full flow valves, common valves are poorly designed (for max profit)
Aftercoolers should be standard on large shop reciprocating compressors.
Even expensive shop compressors have undersized intake filters.
Lack of high efficiency motors as a common option, at a reasonable markup.

 

[dnschmidt]

Junior Johnson once said: "There is no substitute for cubic inches." in air compressors the equivalent sage wisdom is: BIGGER IS BETTER. Get a five or 7 1/2 HP two stage and your air problems are handled for the rest of your life. That's just the way it is.