Improving Efficiency of Air Compressors

There are a lot of good references/resources here that may help you.

http://www1.eere.energy.gov/industry/bestpractices/compressed_air.html

split compressors into groups use the most economical ones where they will be running almost continuously ie the screw ones, then scale down possibly using some as back up ones to supplement when system is near max load.

use the least economical ones where they are only required occasionally.

if they are being used in one large place set them up so the screw ones bleed of some surplus to the other areas which are then boosted as required by the use of compressors that come in on demand

Also by using large diameter pipe work this will add to the systems storage capacity without the need for large receivers everywhere, fit valves so if on part of the system has a major leak it is removed from the rest of the system so you don't loose the whole network,

Using check vales as used in hydraulic systems, ie so long as there is pressure in the system air can go in any direction, but if there is a catastrophic failure then the valve shuts of automatically

Pete you should be getting GA's for both answers.

What the OP is requesting should be referred to a qualified consultant from someone like Atlas Copco. This "problem" cant be resolved in a few lines on a forum..

Sir,

The short answer to your question is: Yes, this can be done and can result in savings in maintenance and operating expenses. BUT, "the devil is in the details":

• If all your compressors are in one area and there is significant distance to the equipment that consumes the air, then you have frictional losses in the pipe that result in loss of pressure and the cost of boosting the main line pressure higher to compensate for this.
• If your logic for starting/stopping compressors is based on system pressure only, then you have to include sufficient dead-band between setpoints for each compressor.
• If you include in your logic a time delay between starting units, then rapid changes in load can cause unacceptable reduction in pressure.
• If you have some units that are running much more than similar ones, you may want to consider balancing the operating hours by including in the control logic lead/lag assignment on a daily or weekly schedule.
• If you have equipment that can cause sudden changes in air consumption or large shifts in air consumption, you have to include reserve margin and may need to have inputs into your logic to signal when this equipment is running or when the air flow is above a certain value (so the compressors can be started before the pressure has been harmed.
• If some of your compressors have controls to unload a portion of their capacity, this needs to be factored in.
• If one or more of your compressors is powered by a synchronous motor, then you may want to look at how its starter sets the exciter current, so it can best correct for your power factor in the plant.
  

Beyond all this, of course, is the need to do a careful audit of the equipment to eliminate air leaks. Etc.

I'd say to go ahead, but be ready for mistakes and a lot of fine-tuning of the control scheme. Also, consider using a PLC system that has redundant processors and pressure sensors, or a well laid-out set of backup mechanical controls. There are major costs of production losses from the failure of an integrated system such as you are considering. (I once had a customer pay the costs to deliver a replacement fuse via courier to the airport and pay for a seat on the plane for it to be delivered to the airport nearest their plant, while simultaneously paying for a separate taxi trip to deliver a second fuse of the same size to their plant--their production losses were around $5,000 per hour.)

--John M.

Let's start with some basics-

All compressors are designed to run for a long time, so the concept of "equalizing" the run hours is misguided.

You indicate that you are planning to link all of the compressors with a common header. Good- make sure that it is large enough. A few dollars saved to install a pipe that is too small will mean that your entire installation cost will be wasted because the system will not be able to do what you want.

Next- After the interconnection main is in place, run your recip machines as base load- always on. You did not indicate how many of each type you have, or state how evenly spaced the machines were throughout your plant. So- I am guessing that your recip machines are relatively evenly spaced along your main and I am guessing that you have 5 of them.

If the recip machines only supply say, 50% of the minimum plant air load, then you should run enough of your highest efficiency screws base loaded as well to NEARLY cover your minimum air needs. Relocate the rest of the screws near the mid-point of the air loads and feed them into a central storage tank with an operating pressure at least 15 PSIG higher than your design main pressure- 25 to 30 PSIG or more would be great.

Install a load-responsive fast-acting feeder valve to the output of the pressure tank to the main. Then, you will start and stop the tank-mounted screws as required to maintain the pressure in the tank. This assures that all compressors are operating at full load and peak efficiency and it allows you to "dial up" the operating pressure in the plant very easily.

There are at least two, and likely more, companies that market a system like just described. They have all of the controls as well.

Good luck- and you are moving in the right direction.

As already stated,the devil is in the detail.

Generally screw compressors are most efficient at full load. A screw compressor running in modulating mode (ie, there is a valve on the intake adjusts to regulate input to maintain output pressure) will consume nearly as much power on tick-over as it does running flat out - after all it is an oil pump as well.

Recips usually do not modulate, (they run 'on-or-off) either by off-loading valves or stop-start, or both. But beware, frequent stop-starts can overheat the motor. Also when not pumping the load on the motor is much reduced - but so is the power factor - which might push you into penalty charges with the supply company.

All this logic must be built into the PLC programme

But whatever controls you install, first make sure there are no leaks or wastage. Then make sure you have plenty of manual over-ride valves for re-routing, isolation and servicing.

I'm guessing that the newer compressors were purchased to replace older compressors that have failed. You might want to examine each compressor and replace the ones that are the oldest and therefore more likely to fail in the future. This way, you may be able to use less compressors and more efficient ones in your installation.

If amongst the new screw types you happen to have one that is having a Variable Speed Drive with the controls to modulate the speed so as to maintain the system pressure constant, Then do the following:

find out what is your maximum required load in compressed air (m3/h) (= Qm)

Find out what is the maximum capacity of the Screw compressor with the VSD (= Qv)

select the best condition compressors you have and select the required number of them that will give you a capacity =

Qo = Qm-(Qv/2)

The remaining compressors should be left as backup for any failure that can happen with the others or to use when maintenance is required.

now run The combination that gives Qo + the Qv (that is the VSD compressor). the VSD unit will keep the system pressure allways correct without the need to stop, Unload or else. This will be very economical in energy (in fact maybe the best efficiency you could get without having to worry about too much programming on the PLC).

If no VSD unit, then Choose the Screw unit that can give you enough capacity to cover the possible load variations and use it in load/Unload mode so as it does stay loaded more than Unloaded most of the time.

If you use the PLC with none of the above considerations, you will need a lot of tuning and will allways encounter situations that will make your programming to need re-visiting...!

I had a similar scenario some time back. I received assistance from my power company. The end result was this: Removed the 4 recip units, sell them, purchase another screw comp to compliment the one I had. Do your controls to cover the volume needed at any given time. Increased the header size. Our PF was determined on a 15 minute load. Should your multiple units cycle errantly in one 15 minute period, the charge for power used could increase. By doing what I did, I am regulating my power consumption and usable air. Repair all leaks!

We have a mixture of various makes and capacities of compressor and a wild demand. Our HP totals 1250 and our demand from 500 to 3000CFM. We had an air audit from Air Technologies and they determined our system efficiency was slightly under 4 kWh/MCF. After installing their control system, ManagAir, we have increased our efficiency to 3.5 kWh/MCF and are looking for better by different sequencing, reducing our pressure band, as well as pinpointing when the spikes occur and what we can do about changing our demand. We expect a payback of less than 1 year.

You can't fix it unless you know what is happening. Air Technologies has sold "over the wall" compressed air for years and have made their mark by succesfully selling air from unmanned substations using this monitoring and control system.

One of the suppliers that I referenced earlier.

They have a great product and thoroughly understand AIR.

Reciprocating compressors are more efficient as they give almost 100% swept volume but are expensive to maintain. Screw compressors are less efficent and are cheaper to maintain. Maintenance is done on running hours and therefore a recprocating compressor running on no load will require serving sooner. These machines load and unload faster. It is easier to get an oil free screw compressor that a reciprocating machine. Each machine should run through its own air preperation system then into your manifold. Reciprocating compressors produce air at a higher temperature and thus require more cooling if you are using refrigeration dryers.

I have some notes if you want.

bskinner@illovo.co.za