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jcclark 08-22-2005 04:51 AM

Compressor pressure?
Some 2 stage compressors shut off at 175 psi
while others like my Eaton shut off at 135 to 150 psi.
Is there an advantage to the higher shut off pressures?
Or is that more heat and water while just making the
compressor work harder?

baddbob 08-22-2005 06:48 AM

My old Emglo was factory set at 150 but I bumped it up to 175, didn't see any difference in compressor temps but I did notice I could sandblast for a longer amount of time before having to break for compressor catch up. The only difference I could see was on initial startup it ran just a bit longer to reach full. I've got three 80 gallon tanks now and she really works works to fill all of them on initial startup but the reserve is great. Eventually I'll need to upgrade but the Emglo just won't die. Bob

bracketeer 08-22-2005 10:45 AM

Depends what you are doing with your compressor. Higher pressure will give you a little more use time but as said takes longer to get up to speed on initial start up. Compressors are built by stages for higher pressures, single acting or double acting for faster initial start up, working pressure for various kinds of work, and flow for volume demand. Spray painting requires low pressure high volume. A jack hammer requires high pressure and high volume. Testing safety valves requires high pressure low volume. Testing ballast valves on a submarine requires extremely high pressure and low volume.

BondoKing 08-22-2005 11:30 AM

Jim and I have been discussing this over the phone and I think he needs to up the pressure as well...

Another question for the compressor guru's..... his company told him not to break in the compressor by letting it run for an hour with the pic **** valve open at the bottom of the tank to seat the rings... My compressor company said that step was very important to do for seating..... His said it would polish the cylnder walls and the rings may never seat.... What is the deal here???? THey told him to break in the compressor under pressure.... That sounds more appropiate to me after thinking about it, but of course I broke mine in the other way a long time ago......

Any thoughts on that??? Jim did I leave anything out??


bracketeer 08-22-2005 11:33 AM

Like a car, compressors can come with several types of rings. Follow the manufacturers recommendations or you may void the warranty.

jcclark 08-22-2005 12:06 PM

BondoKing, you pretty much said it all.
I don't think I'm getting the cfm I should with my Eaton.
It's a 2 stage 5hp rated pretty high for cfm but it
won't keep up with my da running for more than 25 min straight.
Does anyone out there know if this is about right?
Can I expect better performance after it gets broke in real good?
It has a continuous run feature and I'm not sure if it's disabled
completely when running

oldred 08-22-2005 01:22 PM

You hit the nail on the head with the CFM being the problem if your DA won't keep up since tank volume has very little to do with it although a lot of people will not accept this fact. A higher shut-off pressure does exactly the same thing as a bigger tank, that is store more cubic feet of air. I have been building and installing air systems as part of my welding business for over thirty years and I guess pressure and tank size has been the biggest misunderstanding I have had to deal with. It seems a lot of people think higher pressure and/or a bigger tank will help an undersized compressor keep up with an air hungry tool but this is not true since it is the CFM capabilities of the compressor pump that determines if it will keep up or not and the start/ cut-off pressure and size of the tank simply controls the on-off cycle rate. Any extra run time gained by increasing cut-off pressure/using bigger or multiple tanks is immediately lost to the extra time it takes to recharge the bigger volume so it is a trade off and you gain nothing. Cut-off pressure/tank size is chosen based on expected demand so the cycle times can be set to allow a proper cooling period for the pump within normal operating range and demand should not be exceeded for extended periods. If a compressor will not keep up with demand then it's CFM capabilities are being exceeded and about the only solution is to go with a bigger compressor since changing storage volume by upping the pressure or adding tanks will only change the cycle rate and do nothing for compressor output in spite of common belief, it is simple laws of physics that you can get no more out of a tank than you put in no matter what the pressure or size.

jcclark 08-22-2005 01:26 PM

Very true, and I understand that.
But wouldn't a lower pressure cut off allow the pump to put more out
at a lower pressure so it works less in the long run?
It puts out less cfm the higher the pressure goes so why not shut
it off at 140 instead of 175?

oldred 08-22-2005 02:03 PM

First off let me say you chose a really good compressor those Eatons are excellent compressors that seem to last forever. Cut-off pressure/tank size are chosen based on expected demand with a lot of consideration given to pump efficiency vs motor power over the entire charge cycle. This is engineered at the factory and with a well built unit like an Eaton you can figure it is about the optimum set-up as is. Changing pressures, like adding tanks, is usually a mistake since it can be counter productive and increase compressor wear without gaining anything but unless you make really big changes like really huge pressure changes or adding huge tank volumes then you probably will not notice much difference. Lowering pressure will have the effect of shrinking tank size and produce more frequent but shorter run times that may not allow for proper cooling periods but in the range you are talking about I don't think it would do any harm it just would not gain anything either IMO. With most 5-6 HP two stage units 150-175 psi with a 60-80 gal tank is about right with a good balance of run vs cooling time.

BondoKing 08-22-2005 03:14 PM

Im glad you saw this Red... I knew you would shed the correct light on it... I have read allot of your post concerning compressors before.

Jim crank that trick up to 175 and forget about it. Have it kick back on at 125!!


jcclark 08-23-2005 10:10 AM

Bondoking: you need to go back and read what Oldred said,
Bigger tank or higher pressure doesn't get you anything.
After what I've read and understand I'll definetly keep mine
at going off at 150 psi. like Eaton recommended
Less wear and tear on the pump and motor,
and less heat and water.
It'll recover quicker too.

Bee4Me 08-23-2005 12:05 PM

A side note.
IF you decide to "up" the PSI cut off on any compressor, be SURE to check what PSI the RELIEF VALVE open's at. Most are just a few pounds above the factory setting on the switch.

A highr tank PSI does nothing for adding capacity as oldred stated, It just provides more "power" for tool's like cut off's and impacts,ect.
Mine is at 130 and my cut off tool is almost useless on some cut's.

oldred 08-23-2005 12:20 PM

JC, Jacking up the pressure will have the same effect on run time as switching to a bigger tank(more air pumped in) which means it will run a little longer but you will have to wait longer for a recharge so nothing is gained, but I think you already know that. Over the years I have seen many, many times where someone raises pressure(believe it or not I know one guy who runs 250 PSI in his 40 gal tank on his service truck!) or adds multiple tanks in a futile effort to gain compressor performance. This may add some useful run time on tools like large impact wrenches when they are used for only a few seconds and then lay idle anyway giving the compressor time to catch up. On longer running tools such as DA sanders anything gained is lost during the recharge and over about a 5-10 min period the run time vs recharge time will be the same regardless and in fact on some really huge tank volumes such as 2 or even 3 extra 40-60 gal tanks on a 12-16 CFM compressor one may even lose performance due to heat build-up affecting pump efficiency. The bottom line is if the SCFM is insufficient to operate a tool the way you need it to then nothing you can do to your compressor, short of increasing pump CFM in some manner, will change that fact. A tool needs a given amount of volume over a given time to run properly and higher pressure or bigger tanks only store pump volume they cannot add to it so over any operating period that uses enough air to kick on the compressor more than once nothing can be gained from adding volume. Simple laws of physics.

oldred 08-23-2005 12:37 PM

Bee, Good point about higher pressure and tool power, I know that most air tools are supposed to run at about 90 PSI but on heavy equipment like large bulldozers a lot of mechanics run 175-200 PSI(I never recommend more than 175) :nono: to get more torque from their tools. It's hard on tools but something like a Caterpillar D11 dozer needs all the grunt you can get! That pressure I am talking about often runs at 150-160 PSI at the tool!, That too is not recommended!

jcclark 08-23-2005 12:40 PM

Thanks, I understand everything you said and kind of always
thought that. What bothers me right now is my new Eaton
will not keep up with my DA. The Eaton is suppose to be
in the 20 cfm range and this sander is the typical cheapie
DA that shouldn't draw more than 10 @ 90 psi.
I'm going to time the pump up times tonight and talk to
Eaton like they asked me to and see if something is not right.
It has a continuous run feature and I suspect it's not operating
correctly, I sure hope we find something because this is very disappointing.
A 5hp 2 stage should do better.

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