Air Compressors

J

jmellc

Senior Member
Location
Durham, NC
Occupation
Facility Maintenance Tech. Licensed Electrician
Our plant has a lot of air compressors. I ran power to lots of compressors over the years but never managed their operation until being here. Lots of opinions are floating around by guys more experienced than me. Certain units run nearly all the time and others as needed to beef up supply. Some guys think we should alternate every quarter or so and have some of the other units run more, giving the main ones a break. Others say a unit is better to run continuously as much as possible. I personally agree more with the first ones, that run time should be alternated more. A few units barely run at all, even at peak. Seems that's not good and they may not be ready if other units break down.

Opinions please......
 
I base my opinion on nothing scientific, only gut instinct. I think its better to cut down on the number of motor starts, so having the few run all the time is better. That is if the bearings and belts on all the compressors are kept in good shape and all of them get exercised on some sort of schedule, like say, monthly or quarterly. Oh, wait. That is what you are already doing. Carry on.
 
I spent a good part of my career designing compressor controls and plant air systems. One important factor you haven’t told us is what kind of compressors are they and are they indentical units?
It’s not uncommon to have one compressor whose design is such that it is most efficient when fully loaded and another that has better efficiency at part load. In that case, you run the first one flat out and trim with the second one. VFD-driven compressors were just becoming a thing when I got out of the industry, but they mitigated much of this efficiency variation. Compressors are huge electricity users and efficient operation is important.

If the compressors are identical, we would devise a system that would alternate usage in order to equalize the wear and tear.

If your plant works with a competent compressor distributor/dealer, they should have access to manufacturer’s software for system design.
 
ActionDave said:
I base my opinion on nothing scientific, only gut instinct. I think its better to cut down on the number of motor starts, so having the few run all the time is better. That is if the bearings and belts on all the compressors are kept in good shape and all of them get exercised on some sort of schedule, like say, monthly or quarterly. Oh, wait. That is what you are already doing. Carry on.
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No, the lesser used ones barely run at all. I have asked the question about running them occasionally but no answers ever came.
 
kwired said:
Are you talking piston compressors or screw compressors?

What kind of demand does your facility have for air?
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Embarrassed to say, not sure. They are Ingersoll/Rand, from 1 yr old to about 20 yrs old.
 
retirede said:
I spent a good part of my career designing compressor controls and plant air systems. One important factor you haven’t told us is what kind of compressors are they and are they indentical units?
It’s not uncommon to have one compressor whose design is such that it is most efficient when fully loaded and another that has better efficiency at part load. In that case, you run the first one flat out and trim with the second one. VFD-driven compressors were just becoming a thing when I got out of the industry, but they mitigated much of this efficiency variation. Compressors are huge electricity users and efficient operation is important.

If the compressors are identical, we would devise a system that would alternate usage in order to equalize the wear and tear.

If your plant works with a competent compressor distributor/dealer, they should have access to manufacturer’s software for system design.
Click to expand...
Various sizes but all are Ingersoll/Rand. Our main compressor room has 8 units. 2-3 of them run nearly full time, 2 almost never and 2-3 others off and on. Larger ones definitely run more than the smaller ones, with 1 exception. A smaller one that runs nearly nonstop. Not a dedicated one that I can tell.
 
jmellc said:
Various sizes but all are Ingersoll/Rand. Our main compressor room has 8 units. 2-3 of them run nearly full time, 2 almost never and 2-3 others off and on. Larger ones definitely run more than the smaller ones, with 1 exception. A smaller one that runs nearly nonstop. Not a dedicated one that I can tell.
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Your arrangement is too complex for me to come up with advice. Your IR dealer should be able to do a site evaluation for you. With as much of their equipment that you have, they’ll likely do it for no charge.
 
retirede said:
Your arrangement is too complex for me to come up with advice. Your IR dealer should be able to do a site evaluation for you. With as much of their equipment that you have, they’ll likely do it for no charge.
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They may be involved already. Some things we don't hear much about.
 
jmellc said:
Various sizes but all are Ingersoll/Rand. Our main compressor room has 8 units. 2-3 of them run nearly full time, 2 almost never and 2-3 others off and on. Larger ones definitely run more than the smaller ones, with 1 exception. A smaller one that runs nearly nonstop. Not a dedicated one that I can tell.
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How big of motors driving them?

If they are all like 10 HP or less they are likely piston type compressors

A screw type compressor has lot less friction and wear and higher efficiency and a single 50 to 100 HP unit will easily out perform and be more efficient than 8 to 10 piston type compressors of 10HP or less. It may or may not run continuously and may or may not have variable speed drive and varies speed with air demand.

I've been to places that have several smaller piston type compressors, always seems to be at least one or two down and being rebuilt or even up for replacement. Places with screw drive compressor seldom have down time of the compressor. Yes they still need some maintenance, if anything a plant that doesn't really shut down much will have at least a back up unit or even alternate the usage of two units but seldom needs to run both at same time.

The screw compressor doesn't really need any storage tank in most cases either or at least not as much of a storage tank, particularly in applications where there is nearly always a demand for air. When there is periods of little to no demand yet you want air readily available that's where you may see a unit that fully shuts down yet has a reserve tank to supply those little demands when necessary.
 
I am going to tell you what I think.

Smaller compressors suck. Anything under about 25 HP is inefficient as heck and made to be as cheap as possible, so tend to fail more often.

Larger compressors tend to be of higher quality.

However, smaller compressors seem to take start/stop cycles better than larger ones.

So the answer might be to use larger ones for the almost continuous loads, and smaller ones to kick in when extra air is needed.

But my sample size is small, so my observations might reflect sample size bias.
 
One thing i do with extra compressors is pipe it in parallel for extra capacity (piped in after the radiator of course) and give it its own ball valve.Works great.
We also use the 120v condensate drains religiously.. they are annoying when they release but very worth riding the lines of water. Of course i have a hydraulic hose crimper, but you can buy them custom made everywhere....
Edit- to be clear the second comps motor is disconnected... just the extra tank is used.
 
rambojoe said:
One thing i do with extra compressors is pipe it in parallel for extra capacity (piped in after the radiator of course) and give it its own ball valve.Works great.
We also use the 120v condensate drains religiously.. they are annoying when they release but very worth riding the lines of water. Of course i have a hydraulic hose crimper, but you can buy them custom made everywhere....
Edit- to be clear the second comps motor is disconnected... just the extra tank is used.
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I was in a plant down south one time where an air compressor tank had developed a leak. Their response was basically to remove the tank and replace a section of pipe that was like 1 inch diameter with what looked to be 6 or 8 inch pipe. Maybe thirty feet of it.

I do not know if this is true or not but I have been told that you can dramatically increase the efficiency of your compressed air system by increasing the pipe size.
 
petersonra said:
I do not know if this is true or not but I have been told that you can dramatically increase the efficiency of your compressed air system by increasing the pipe size.
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I had a customer install a 12" air line from one building to another, maybe 1500'. They no longer needed the 'new' airtank they had installed the previous year and were able to get rid of the miscellaneous backup compressors in their facilities.
 
petersonra said:
I was in a plant down south one time where an air compressor tank had developed a leak. Their response was basically to remove the tank and replace a section of pipe that was like 1 inch diameter with what looked to be 6 or 8 inch pipe. Maybe thirty feet of it.

I do not know if this is true or not but I have been told that you can dramatically increase the efficiency of your compressed air system by increasing the pipe size.
Click to expand...
Not true... comp starts on demand from a pressure gauge. The volume you have ahead of that gauge on demand is what works. Bigger lines just costs more.
3/4" is better velocity than 1"....
 
Our facility had several compressors of various types both piston and screw type. I never checked the efficiency but the old I/R piston type required almost no maintenance whereas the screw type needed expensive filter changes relatively often, so when demand was low the piston did the work.
 
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augie47 said:
Our facility had several compressors of various types both piston and screw type. I never checked the efficiency but the old I/R piston type required almost no maintenance whereas the screw type needed expensive filter changes relatively often, so when demand was low the piston did the work.
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Same at a plant I do work at, they have a glass tempering furnace that eats a lot of air, so I talked them into a 25 hp air screw. It gives them a fair amount of trouble, especially during the winter. They have it sitting outside in the weather uncovered, so it constantly freezes, and even fried the cooling fan on top when it froze. Finally got them to build a room around it. They complained their old piston compressor didn’t give them trouble, and they sit outside. Finally got them to cover those too, because the new ones they got have nema 1 starters on them. (Put a plastic bag on them until they did)
 
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