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# Air Compressor Capacity

02/03/2015 6:10 AM

Q: What Capacity of Air compressor should i use with the requirement of the following;

1. Air Compressor pipe line has a 15 branches - Each Branches requires 30psi - Pipes size per branches is 12mm diameter each
2. Plus additional 5 branches with 85psi requirement and 10mm diameter branch pipe.

Do i simply add all this Air pressures and come up with the total pressure of 875psi plus allowance say 125psi and the total capacity of Air Compressor is 1000psi?

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#1

### Re: Air Compressor Capacity

02/03/2015 6:19 AM

In other words, no.

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#4

### Re: Air Compressor Capacity

02/03/2015 8:55 AM

You don't say! I think we've heard it all now. Jeezus!

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#2

### Re: Air Compressor Capacity

02/03/2015 6:24 AM

Find another line of work. You are not well suited to air compressor sizing.

It isn't the pressure that you should add, but it is the volume of air used that is important.

Air tools are rated in SCFM at a given pressure. Your 125 PSI number is about right for pressure. Now you need to determine the volume of air you will need to power all the anticipated tools in use.

That is your first assignment. Define "SCFM"

Then search for "sizing air compressors" using that same search engine.

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#12

### Re: Air Compressor Capacity

02/04/2015 12:33 AM

Hi Lyn,

You are always feeling great answering my questions - maybe aside from being a Guru here, you must be a Professional Leader in real life, i hope you help a lot of people specially in your vicinity with your well being and very knowledgeable witty person.

For my first assignment - SCFM is Standard Cubic Feet per Minute.

Thanks.

ken

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#22

### Re: Air Compressor Capacity

02/04/2015 1:30 PM

Wrong. Surface Cubic Feet per Minute

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#23

### Re: Air Compressor Capacity

02/04/2015 2:12 PM

Wrong SCFM. Check the Wikipedia link.

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#26

### Re: Air Compressor Capacity

02/04/2015 4:45 PM

I meant to say Square Cubic Feet per Minute. Damn speech to text function.

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#28

### Re: Air Compressor Capacity

02/04/2015 4:49 PM

We now enter the sixth dimension. A place where time is an affliction.

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#29

### Re: Air Compressor Capacity

02/04/2015 6:02 PM

Tesseracts Per Minute?

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#30

### Re: Air Compressor Capacity

02/04/2015 6:14 PM

No, it would be a Hexeract per minute.

Yes, I had to look that up.

Although it still might be a tesseract per minute. It depends if dimensions sum or multiply in this case.

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#24

### Re: Air Compressor Capacity

02/04/2015 2:31 PM

If it were right, I can't imagine what Surface Cubic Feet per Minute would mean

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#25

### Re: Air Compressor Capacity

02/04/2015 3:56 PM

Cosmological constant that controls the expansion or contraction of the universe, maybe?

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#3

### Re: Air Compressor Capacity

02/03/2015 6:26 AM

You don't size a compressor like that, you size it by the CFM required.

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#11

### Re: Air Compressor Capacity

02/03/2015 11:56 PM

Also the required pressure, psi (pounds per square inch).

Good Luck, Old Salt

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#5

### Re: Air Compressor Capacity

02/03/2015 10:01 AM

Use P=(m/V)RT; R air; V=Q/A; A=(¶D2 /4)--> P= mART/Q or Q=mART/P; Q- volumetric flow (m3/sec), mass "m" is also a rate (kg/sec), P is in (Pa), T is in Kelvin or Celcius depending on R of air units(KJ/kg K or Celcius)--look through tables.

Sum all Q's on all the branches using above and you got your maximum capacity plus your additional margin for future expansion.

That should do.

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#6

### Re: Air Compressor Capacity

02/03/2015 11:44 AM

Welcome to the forum. Now, before you post any more questions, do your homework/research first. You need to learn the basics of the question(s) you ask before you ask them. If you get stumped, ask away.

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#7

### Re: Air Compressor Capacity

02/03/2015 12:18 PM

How many and what kind of tools will be used simultaneously. Then there is the questions of pressure regulations, air line oiling, air line drying, reserve capacity tank size, recharge time, cooling, logistics of room and power for this equipment. Your initial premise of how to size this compressor tells me that you should not attempt doing this without proper supervision.

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#35

### Re: Air Compressor Capacity

02/05/2015 5:54 AM

Air line, seem like the guy got plane tickets already

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#8

### Re: Air Compressor Capacity

02/03/2015 11:02 PM

I am surprised at the answers. kendelvoxx is obviously taking the micky. If not a career change is required.

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#9

### Re: Air Compressor Capacity

02/03/2015 11:27 PM

Don't listen to all these guys. Just take the easy way out- if you've been looking into the price of high pressure compressors to meet your needs, then I can see why you're posting this question.

Here's a little trick that will drop the cost big time. Just take your figure- 1,000 psi with a buffer margin and go looking at compressors. You'll find that a typical 2 stage industrial piston compressor can run up to 175 or 200 psi- just get 5 of them!!! 5 times 200 psi gives you your 1,000 psi, right? And if you're careful and look for the type that have 1 motor on a tank driving 2 separate compressor heads, you might be able to get by with only 3 units!!!! WAY cheaper than a big high pressure unit, and way safer too as nothing develops over 200 psi!

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#10

### Re: Air Compressor Capacity

02/03/2015 11:29 PM

.

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#13

### Re: Air Compressor Capacity

02/04/2015 1:11 AM

What work are you asking the compressed air to do? If you follow the end user of the air's requirements, you will be able to add the total air that can be used at one time, in scfm. That is the minimum size of air that must be produced at the pressure the end user recommends. Now how often do you want the compressor to run. If you need 50 scfm@ 125psi, and the compressor can produce 100scfm @125psi, your compressor will run about every 15 seconds. So the capacity of your compressor needs to be many times more than the air being used to allow the air compressor to not run at 100% duty cycle. Find out what is going to be using the compressed air, and then come back for more help.

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#14

### Re: Air Compressor Capacity

02/04/2015 5:04 AM

Hi Bob C,

thank you for your comment here's how it works;

1.0 In my 15 ports - we need 30psi with 30CFM each

2.0 the 5 ports with 35psi with 120CFM each

3.0 the 9 ports with 85psi with 40CFM each

Assuming each port will run/operate alternately with the time interval of 60sec each of the port.

at this data how can i compute or how can i select for the Compressor Capacity?

Thanks & Best Regards,

Ken.

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#15

### Re: Air Compressor Capacity

02/04/2015 5:40 AM

Here's a start:

120CFM at 35psi is about 400SCFM

40CFM at 85psi is about 270SCFM

(If the back of my envelope serves me well).

So total requirement is 15x90 + 5x400 + 9x270 ≈ 6000SCFM.

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#16

### Re: Air Compressor Capacity

02/04/2015 6:24 AM

That's if his quoted CFM figures are at the pressure. This is possible, but unlikely. Air consumption figures are usually given in SCFM (or some other standard units).

OP confirm?

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#17

### Re: Air Compressor Capacity

02/04/2015 6:57 AM

Agreed. If, however, they were calculated by e.g. by adding the volume of cylinders etc. the figures would be at pressure.

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#21

### Re: Air Compressor Capacity

02/04/2015 8:35 AM

Correct. I hope OP confirms one way or the other, but I won't hold my breath!

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#27

### Re: Air Compressor Capacity

02/04/2015 4:48 PM

There are a few other things that should be considered after the initial calculations for the pressure and volume are determined. The initial investment in the equipment to generate compressed air is very expensive. The operation of that equipment is also very expensive. Compressed air is about the most costly utility for any operation. So the higher the compressed air needs are the higher the cost of the equipment to do so. Compressed air is very expensive since the energy path is mined coal or drilled natural gas---> transportation--->change from material to usually heat---> heat to mechanical --->mechanical to electrical---> transportation of the electricity--->electrical to mechanical and mechanical to compressed air. Even with hydroelectric power or nuclear generation it is expensive.

The OP doesn't state how many or which drop pipes will be operating simultaneously, sequentially or on demand only. If all are operating at the same time the compressor must be sized to the sum of all the individual usages. If some are used sequentially, not operating at the same time, the compressor can be sized to the largest load that could be needed at that intermittent usage. That's the most you will need at any time. If the intermittent peak load is very short in length of time and not frequently, a compressor with a slightly higher volume capacity and connected to a storage tank can be used. The compressor is then sized to replenish the storage tank at the same time it is operating the normal loads or normal intermittent loads.

Why all this? Compressed Air is expensive to buy and to feed the equipment. A large amount of electricity (\$\$\$\$\$) is required to start the compressor motor. The less the motor is stopped and restarted the cheaper it is to operate. All this is similar to the operation of a water reservoir. You have to have the peak capacity based on highest short term usage and not the highest theoretical possible needs.

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#31

### Re: Air Compressor Capacity

02/04/2015 11:06 PM

Hi JohnDG,

for this 6000SCFM - what is the required air pressure? does the 125psi can be suitable if i have a receiving tank with that volume?

Ken.

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#32

### Re: Air Compressor Capacity

02/05/2015 3:06 AM

I'm not a pneumatics specialist - and those where just guide figures (which should be checked). I suggest you get on the 'phone to a couple of pneumatics suppliers; they know all the sums (and how much safety margin to allow). If the answers from two suppliers vary greatly, consult a third supplier.

They probably won't underspecify, because they'll get the blame if it is unsuitable when in service, and probably won't overspecify (much), because they want to make a sale.

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#33

### Re: Air Compressor Capacity

02/05/2015 4:36 AM

Are your consumption figures in #14 in standard or actual CFM? (See earlier posts) It makes a lot of difference!

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#43

### Re: Air Compressor Capacity

02/06/2015 5:09 AM

Hi Codemaster,

All in Standard CFM.

Ken.

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#46

### Re: Air Compressor Capacity

02/06/2015 9:56 AM

OK thanks.

Not sure what you mean by the alternate operation and time intervals in #14, so just added the flows as JohnDG did in #15. Can revise if total flow is different.

You don't give a lot of detail, but based on where I used to work an approach is as follows.

Total air required 190SCFM from #14. Compressor output 1.7 x that, = 325SCFM, so it's running ~ 60% of the time. Output in SCFM or FAD, it doesn't make a lot of difference.

Output pressure 20psi above required max pressure, 85 + 20 = say 110psig including a margin for losses and regulator differential. This allows 20psi ΔP between compressor start and stop, start at 90psig, stop at 110psig. Air receiver sized to give max n starts/h, given by the compressor supplier.

Receiver size, ft3 = 60/n*14.5 psi (atm pressure)*325cfm (compressor output)/20psi (start/stop ΔP)/4. The 4 in the bottom line comes from a bit of calculus - max start frequency occurs when air demand = ½ compressor output, so above is worst case.

Assuming a fairly generous 15 starts/h, that comes to a hefty receiver, ~ 240 ft3, but then the scheme is pretty big, I make compressor motor 75kW. It would have been a lot bigger if your flows were in actual CFM!

That was all from a few years back and maybe can be improved on - modern controls load/unload the compressor without stopping, the motor stops after a set time without reloading. That can be at much higher frequency so would reduce receiver size. Worth talking to manufacturers. You might also get away with a lower compressor output, but at bare minimum 200SCFM it's still 45kW.

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#47

### Re: Air Compressor Capacity

02/07/2015 12:26 AM

First off I lack the expertise to be confident enough to post a compressor size, but the 190 figure is what I thought was being consumed. My question to you is why store at 110psi, as opposed to storing at a higher figure? In my work experience, we have always stored at the high end of the compressor's range. My belief was that it would allow the compressor longer off cycles.

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#48

### Re: Air Compressor Capacity

02/07/2015 5:04 AM

Basis of compressor size was to give a reasonable duty cycle. If the quoted SCFM only happens for a short time each day (say) something closer to the peak could be appropriate.

You're right about the pressure, I thought about mentioning it but I'd been long-winded enough already! Higher start/stop DP gives smaller receiver, and optimum can be found. No doubt it's worth going to max pressure of the particular machine being considered. But higher pressure means more energy used as it has to be regulated down to what's needed at point of use, this might be an issue.

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#34

### Re: Air Compressor Capacity

02/05/2015 5:06 AM

I'm still talking guidelines (and note Codemaster's comment), but 125psi from the compressor is OK. You'll need pressure regulators (and Safety Relief Valves) for the lower pressure branches. Again, a supplier is the person to ask - he'll also know all about sizing the receiver so that the compressor doesn't cycle on and off too frequently.

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#36

### Re: Air Compressor Capacity

02/05/2015 6:04 AM

Sorry, thought your highest pressure was 125psi. If you only need 85psi - then size the compressor for that (otherwise you'd need yet another regulator & Safety Relief Valve).

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#40

### Re: Air Compressor Capacity

02/05/2015 3:38 PM

The regulator, and relief valve should be there. Storing air at higher pressures is a way to cut down on compressor run time. As compressors get larger in volume, the normal storage pressure usually goes from 125 to 175 psi.

If air shut off point was 85 psi, the compressor would have to run each and every time anything on the 85psi pipeline was operated.

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#42

### Re: Air Compressor Capacity

02/05/2015 7:36 PM

Good point.

As I've said, pneumatics isn't my field - I've just had to pick up a bit as I've gone along (being the only person in the company I contract to who has the faintest clue about contol systems). But I remember a little bit of physics (my original training), and try to apply it where applicable.

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#52

### Re: Air Compressor Capacity

02/10/2015 9:36 AM

The air pressure required is dependent on the device(s) rating being serviced by the air supply.

You may have to locally install pressure reducing stations or small regulators for each device if the pressure ratings are different from device to device.

You also must identify the allowable contaminants/particulates such as oil, water, dust, and or other particulates in the process and specify the type of compressor best suited for the application and/or filters and/or separators required for each device.

If the highest pressure required for any device being serviced by the system is 125PSI constant then your compressor system must be capable of producing the required SCFM flow while maintaining 125PSI. (The compressor required for this system would be a constant pressure type.)

Normally devices have an acceptable operating range rating in PSI such as 100-125PSI or other variation in operating pressure. (Verify the requirements by reading the Installation & Operations Manual for each device to be used in the system.)

This type of system does not require a constant pressure type compressor and can be pretty much any type of compressor as long as the allowable particulates for each device is met.

Most compressor manufacturers offer on site field application services and advice for free if you will contact them. I suggest talking to at least three different manufacturers and get equipment installation references from each then contact their customers for performance and reliability information.

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#44

### Re: Air Compressor Capacity

02/06/2015 5:17 AM

Dear All,

I would like to Thank you all in participating here in the forum.. . every witty man has a different point of view and sometimes everyone is agreed from one another meaning we are all in the trail. . . From experience based on the actual condition or being a part of their daily work- and further study.

Others like feeling too much high in their Position sometimes making some non-sense comment - don't know if trying to bully someone or want to show that he's the best- for there is no acceptable comment being delivered-- well. .. .

Again to those who spend some time here, weather its non-sense or not - Related or Off-topic . . .THANK YOU VERY MUCH!

Ken.

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#18

### Re: Air Compressor Capacity

02/04/2015 7:10 AM

Andy,

I found one manufacturer here in North America. They make Silicon and Teflon based oil formulations which are good for delicate machinery. Their website has detailed specs on each of their product. I have used their PTFE based oil and it is does not leave any deposits after months of use. Check them out at www.super-lube.com.

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#19

### Re: Air Compressor Capacity

02/04/2015 7:14 AM

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#20

### Re: Air Compressor Capacity

02/04/2015 7:43 AM

Damn right John. Don't know what I pressed to send it here.

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#37

### Re: Air Compressor Capacity

02/05/2015 9:32 AM

It's the air volume you add, not pressure. the maximum air pressure you need will suffice for all lower pressure requirements.

CFM = cubic feet per minute. But a cubic foot could be anything because air is compressible. Therefore you need to specify the pressure and temperature as well.

ACFM = Actual cubic feet per minute. This is the actual volume occupied by the air when it is compressed at whatever pressure and temperature prevails.

SCFM = Standard cubic feet per minute. This is a volume based on standard pressure and temperature conditions being specified. These vary around the world. But usually it is safe to assume the local ambient conditions, but to be exact it is usually specified.

For instance, in my line of business which is compressed air for breathing (long since retired) the ISO air quality limits are currently specified at a pressure of 1013 hPa and a temperature of 20oC. This does not mean the air must be at this pressure and temperatures in use - but it does means all samples when taken/measured at whatever pressure and temperature in use must be converted to 1013 hPa and 20oC for comparison.

Most compressor people use SCFM in technical specs. Some use FAD = Free air delivered, which is the air actually delivered by a compressor, expressed as the space the air would occupy when it expands back in the ambient atmosphere.

Some use PD = Piston displacement, which is the volume of the cylinder swept by the piston (which excludes any loss due to inefficiency) Thus usually giving an output volume well above exactly the same compressor when expressed in FAD or SCFM.

On the safe side, use SCFM as a base for sizing and selecting compressors.

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#38

### Re: Air Compressor Capacity

02/05/2015 12:46 PM

Interesting you use hPa = hectopascal. I thought only the French used that, most people say mbar (it's the same pressure of course). If ISO spec said 1 atm it would be simpler and save some users wondering where a number like 1013 comes from!

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#39

### Re: Air Compressor Capacity

02/05/2015 3:13 PM

Yes you are right, we've used cubic feet and psi in the UK for years but metrication changed all that where in respiratory protective systems it was common practice to use litres-per-minute and and mbars for equipment worn on the person, and bars for distribution pressures and normal cubic metres per hour. The ever common use of ppm for contamination has been changed to ml/m3.

The purists in ISO insist we went over to approved SI units, and then leave each country to publish a National Foreword if they wanted to personalise the standard.

I am getting too old to learn all these new tricks and I will be retiring from my committee work in a few weeks, but the youngsters coming along will grow up with it, and it will become second nature to them.

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#41

### Re: Air Compressor Capacity

02/05/2015 4:43 PM

Yes, it would be a poor thing if ISO did not use SI units! But they could relate specs to 1000 hPa (or mbar) and alter other data to correspond.

For gas calculations I use mbar and molar volume 22.7 m3/kmol at 0°C instead of 22.4 at 1 atm and 0°C.

Also it makes you wonder why we try to help the OP when he can't or won't come back with basic information - are his air consumptions in standard or actual CFM?

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#45

### Re: Air Compressor Capacity

02/06/2015 6:48 AM

I suppose we all settle into habits and use values that we are comfortable with.

In a sort of defense of the respiratory protection standards they have been related to actual practice where traditionally all measurements and calculations have been based on ambient conditions of 1013.25 mbar which has been pinned down at 1013 hPa, and 20C. It simplifies tests because readings can be taken at face value.

If the OP is using air for respiratory protection he will need to know this, albeit the ISO standards will not be published until later this year.

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#49

### Re: Air Compressor Capacity

02/08/2015 6:14 AM

Dear Mr. kendelvoxx,

You have to add the capacity of compressed air cumulatively in terms of SCFM (Standard Cubic Feet per Minute) or SCMM (Standard Cubic Metres per Minute) for a given pressure.

Alternatively for capacity it will can be termed as FAD, (Free Air Delivery) and the pressure should not be arithmatically added and invariably the pressuure will remain constant, if the pressure drop in pipe line is ignored.

In your case you have stated 2 different pressures namely, 85 psi and 30 psi. You may have to use 2 different compressors one with pressure of 85 psi and the other at 30 psi for which the quantity or capacity for both pressures seperately.

If not you can use one compressor with 85 psi, directly use where 85 psi is in use and for other at 30 psi, reduce to the pressure from 85 psi to 30 psi for the cumulative capacity and distribute.

Instead of saying the pipe line size, you have to be specify the quantity of air requied per equipment. and based on the quantity, and pressure you can calculate in which Volumetric efficiency of compressor, compensation for air leakage is involved.

Pl. refer any standard Thermo Dynamics Book - which will help you to calculate the power.

DHAYANANDHAN.S

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#50

### Re: Air Compressor Capacity

02/08/2015 11:08 AM

I guess they do not have step down pressure regulators in India. I see a market opportunity.

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#51

### Re: Air Compressor Capacity

02/08/2015 4:37 PM

Calculation of a compressed air system's operational capacity can not be done based solely on the sum of the requirements of each station requiring air. Automatic condensation traps are needed to remove condensation. These work best if located in a drop in the piping. If necessary, a chiller must be used for low condensation levels in the supply air. For a dual pressure system such as this one, a pressure regulator should be used to drop from the higher pressure to the lower pressure. If he wants a second compressor, install it as a back-up to the primary one.

Each user supply station should have the branch piping coming out of the top of the main piping with a "T" and then drop towards the unit supplied. This leaves most of the condensation in the supply piping. For each one of these stations a filter and a pressure regulator (and possibly a lubricator) should be installed. Also the possibility of future changes should limit all the drop lines to no smaller than 1/2" NPT piping until it gets to the device using the air. Depending on the location of drop lines, a loop configuration of the main supply piping should be considered. This will reduce the hydraulic losses due to piping, probably down to near half of that for one connection.

What does all this mean? A compressed air system's capacity needs to have everything on the system and the configuration of each station considered when calculating. Use a storage tank for periods of high usage, pitch the piping down as it goes through the supply piping, use automatic condensate drains with pre-filters installed and any other factor effecting the operation. Include them and all the other operational configurations, location of loads, rest of the other items mentioned above and any other factor. Almost every component causes a drop in air pressure. Now you are ready to calculate the capacity of the compressor. A compressed air system is a whole system with it's whole requirements and not just the sum of the simple requirements of each supplied component.

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