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Active Contributor

Join Date: Jul 2007
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How to Calculate Air Consumption?

09/25/2007 4:14 PM

Is there any thumb rule or actual calculations to determine the air consumption of the plant so that the person can easily select the desired capacity of the Air compressors and Air receivers.

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Guru
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#1

Re: Air Consumption .

09/25/2007 11:22 PM

I believe Festo has software that can do this although I would suspect that it would only be accurate if you use Festo products.

The next alternative is to get the air consumption figures for each of your equipment (if available) and add them all up.

The third alternative is to install flowmeters that would tell you how much each equipment or area is consuming. If you install one flowmeter at the main line, you'll get the total. This way, you'll get the actual figure instead of just calculating it.

The downside of alternative #3 is that if your system has leaks, your measurement will be higher than what it normally would be.

At least, however, once you know how much you're consuming and enter into a compressed air audit and correction program, you can measure your improvement directly.

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#9
In reply to #1

Re: Air Consumption .

09/26/2007 9:28 AM

We are not using festo products , beside your recommendation for the air cunsumption for each equipment and cycle of operation for the same can be calculated . Thanks for your reply.

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Anonymous Poster
#25
In reply to #1

Re: Air Consumption .

11/22/2007 3:31 AM

dear vulcan.

your method of calculating the air compressor is not accurate , as if some want to know the already istalled cmpressor,s flow rate than it will work out but if some one has to isntall a new compresor , how should be the procedure of calculating tht sytem ,

thanxs

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#28
In reply to #25

Re: Air Consumption .

11/22/2007 10:15 PM

Hi,

I'm not a design engineer or even a pneumatics expert. However, I do know that a good designer or engineer can determine the air consumption of a plant even when it hasn't been built yet.

If you know the equipment that will be installed (and no one adds to it without you knowing), the pipe sizes and layouts, you can calculate the size of your compressor with reasonable accuracy.

I am not the correct person to ask for formulas or methods. I only know that engineers can calculate the required size by having all the information that I mentioned because I've seen them do it.

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Anonymous Poster
#30
In reply to #25

Re: Air Consumption .

05/13/2008 10:30 AM

this is the problem i am having new skid mounted plant ineed a formula

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#31
In reply to #30

Re: Air Consumption .

05/13/2008 3:10 PM

Suggest you read my post #3 in this discussion.

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Guru

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#2

Re: Air Consumption .

09/25/2007 11:59 PM

I do not know any rule of thumb that I would trust.

Simplest way I know is build an Excel spreadsheet with each piece of equipment. Multiple by the air flow consumption, and multiply again by a usage or diversity factor. If you anticipate future growth or increased production add that in. That will provide a target for a base line system.

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Anonymous Poster
#26
In reply to #2

Re: Air Consumption .

11/22/2007 3:35 AM

dear Ried,

Can u please elabrate your explanation with example , it will be more benificaial and easy to understand

regard

engr

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#3

Re: Air Consumption .

09/26/2007 2:45 AM

I assume you are trying to size the compressed air system for a facility that has not been constructed. As mentioned by previous respondents a good start would be to add the consumption rate of each piece of equipment or system then double or triple it for the stuff that is not readily apparent and growth. If those who would supply your equipment do not give you a number for compressed air consumption I would insist or change supplier.

If you know of a company or facility that does approximately the same thing that you are planning you might find out the size, square feet, of their production area as well as the size of their compressors. Then use your proposed size divided by their size times their compressor capacity. Since most new facilities add equipment as they grow you would be wise to oversize your plumbing by a factor of four or more.

If you under estimate the compressed air need but left room for growth in your compressor room it is not hard to add a compressor and another receiver or two.

It is very hard approaching impossible to go back and increase the size of your compressed air supply lines in the plant. Over sized supply lines act as additional reservoir and reduce the pressure drop in the pipes themselves. There is no draw back (except a minimal one time cost for larger pipe) to greatly over sizing your distribution plumbing and multiple benefits that keep on giving. The labor to install big pipes is similar to installing small pipes. If your pipes are enormous you will save the cost the pressure lost in distribution. Your compressed air will travel in laminar flow, under 1200 fpm, well below the speed that causes turbulent flow and the inherent inefficiency. In summary use plumbing that is audaciously over size and if your best guess for the compressor is too small add another. You can even rent a portable compressor to maintain production while another compressor and accessories are procured. Trust me.

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Guru

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#10
In reply to #3

Re: Air Consumption .

09/26/2007 10:27 AM

Definitely agree with your "rule of thumb" pipe sizing guide, except now I try to state it differently.

Our electrical distribution friends try to limit voltage drop to a fixed percentage when sizing cable sizes for plant supply. Borrowing a note from electrical criteria, I try to specify or design compressed air piping with a 5% pressure drop from the end of the compressor (not the compressor room) to the farthest point in the shop. I have noticed that the compressor accessories manufacturers are notorious for undersizing there equipment resulting in higher pressure drops in air driers, filters, etc.

Bottom line is that you pay a lot of operating cost to pressurize air for plant use. Don't waste it with large pressure drops in piping and accessories. Insist on properly sized equipment with low pressure drops.

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#14
In reply to #10

Re: Air Consumption .

09/26/2007 2:55 PM

With compressed air even 5% is rather costly. It is not difficult to get a theoretical 2 or even 1% without being ridiculous. Doubling the pipe size squares the flow area and kicks pressure drop to the curb.

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#16
In reply to #14

Re: Air Consumption .

09/26/2007 3:39 PM

Typically, I see "value engineered", rule of thumb, nominal 100 psi, compressed air systems with 10 to 20 psi pressure drop from the compressor outlet to the far point of the shop. So people solve the problem by paying the electric company $$$$ to run a compressor at 120 psi for 100 psi air.

NEC requires electrical conductors to be sized for 2% drop from meter to distribution panel, than a 3% drop from the panel circuit breaker to the farthest circuit. Total is a 5% drop.

Most "value engineers" have difficulty arguing with this logic, where as if you go in with "bigger is better" they can bring out the canaries ("cheep, cheep, cheap").

A cost saving concept I would like to kick out for feedback is something you touched on. Most air recievers are sized by size of compressor, ie; a 100 cfm compressor need a 100 gallon reciever, 200 cfm needs 200 gal, etc. My opinion is this is garbage without some other justification.

Recip compressors do need recievers to dampen air pulsations. Rotary-screw and centrifugals do not. Air storage is only useful if the air is stored at high pressure to cover surges in air usage, which in my experience does not occur often in most standard manufacturing (exception may be forging operations).

Instead of investing money and floor space for an air tank, oversize the shop header to develop the reciever volume that people think they need. One foot of 4" pipe is about 0.66 gallons volume. If you "need" 200 gallon reciever, install a 300' 4" header for compressed air main to store the air and reduce your pressure drop.

Does this make sense from a first cost aspect?

(Now I'm guilty of being long-winded!)

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#17
In reply to #16

Re: Air Consumption .

09/26/2007 3:47 PM

Sounds like you are right on top of it.

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#4

Re: Air Consumption .

09/26/2007 7:10 AM

<there any thumb rule or actuall calculations to determine the air consumtion >

NONE CAN BE TRUSTED unless:

  • You have a fixed production ,manpower,air-consuming machines deployment
  • With fixed air-consuming machines
  • With same working hours.

Best

MM

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Power-User

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#5

Re: Air Consumption .

09/26/2007 8:16 AM

There is no rule of thumb to determine the air consumption. You have to calculate how many users e.g. air actuated valves, control panel purge etc. the facility is going to have. The air usage for each device can be obtained from equipment catalogs or you can call the vendor of each device to get the information. Once you have the total usage add 15 % extra for the losses/leaks.

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Commentator

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#6

Re: How to Calculate Air Consumption?

09/26/2007 9:06 AM

Are you designing a new plant or trying to figure out an existing plant?

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#8
In reply to #6

Re: How to Calculate Air Consumption?

09/26/2007 9:24 AM

Existing Plant .

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#18
In reply to #8

Re: How to Calculate Air Consumption?

09/26/2007 5:54 PM

OK! You have an existing plant and you did ask about a formula, close enough.

You need a Reservoir with a shut off valve between it and the compressor. A butterfly or ball valve will work much better than a multiple turn gate valve. We need to block the pressure smartly if possible.

You do not need expensive equipment but your accuracy will depend upon having a reliable gauge on the reservoir and a decent timer, stop watch or similar device. An assistant is a big help.

With the plant in normal operation:

1. Observe and remember the pressure in the reservoir. This will be P1

2. Close the inlet valve to the reservoir and start the timer simultaneously.

3. When the reservoir pressure decays approximately 10 psig (This will be P2) stop the timer and open the reservoir supply valve to keep from screwing up production.

Calculate the size of the reservoir in cubic feet. For example 100 gallons divided by 7.48 = 13.37 cubic feet.

Multiply the cubic feet by the compression factor (at psig not psia). For example the pressure, P1 might have been 93 psig. Then 93 divided by 14.7 = 6.33 = Cf1

Next calculate the compression factor at P2. Example 83 divided by 14.7 = 5.65 (Cf2)

Subtract Cf1 - Cf2 = SCF consumed. 6.33 - 5.65 = .68 Delta Cf (Cfd)

Multiply Cfd x Cubic Feet 13.37 x .68 = 9.14 SCF consumed

Multiply SCF x 60 sec/min. and divide by Time (sec) = SCFM

Use 13 seconds for the elapsed time for this example. Then:

(9.14 x 60) / 13 = 42 SCFM for a close approximation

[R gallons x (P1-P2) x 60 ] / [7.48 x 14.7 x Tsec] = 60R(P1-P2) / 110T = SCFM

Many of the devices using this compressed air will have regulators so the small change in supply pressure would not affect their consumption. Leaks and unregulated consumption will be slightly affected. By using 5 psid your answer would be a little more accurate. This is better than the SWAG (Scientific Wild Ass Guess) system.

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Guru

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#21
In reply to #18

Re: How to Calculate Air Consumption?

09/26/2007 8:20 PM

Simple and brilliant for an existing plant! Makes me wish I did a bit more reading in my IR Air Data handbook!

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#22
In reply to #21

Re: How to Calculate Air Consumption?

09/26/2007 9:30 PM

It pleases me to say that you would not find this in the IR handbook. The guy who thunk it up didn't read it anywhere and you are one of the first to see it in print.

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Guru

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#23
In reply to #22

Re: How to Calculate Air Consumption?

09/26/2007 10:24 PM

If I wus smart enough to understand sizing and how receivers worked (and I'm not!), the receiver sizing guide on pages 5-8 to 5-10 in the second edition 1969 gives enough hints to develop your method. I really don't do enough work with existing systems now, and I always need to refer to the book for sizing new systems when it does come up.

I salute your wisdom in this area and thank you for sharing your insights.

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Anonymous Poster
#32
In reply to #18

Re: How to Calculate Air Consumption?

07/29/2009 8:30 AM

Hi,

I read this solution of yours on net and found it very interesting. It seems that you have a lot of hands On the engineering systems.

I have a small problem related to Air consumption .. Can I trouble you for the solution ?? Please let know .... My email ID is vishwas.joglekar@iaplindia.com

Regards

Vishwas Joglekar

India

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Anonymous Poster
#33
In reply to #18

Re: How to Calculate Air Consumption?

11/03/2010 1:01 AM

Hello there.

I got some info; total volume of air (based on cylinder total size), and the time needed to advance & return d cylinders. So, i calculated the total litre/ sec. Another parameter that is not so clear to me, the pressure (i need at 5 bar). So, how can i calculated required flowrate from the compressors (7 bar)...?

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Anonymous Poster
#27
In reply to #6

Re: How to Calculate Air Consumption?

11/22/2007 3:45 AM

Dear pebble,

i am trying to design a new plant

regard

textile engr

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Anonymous Poster
#7

Re: How to Calculate Air Consumption?

09/26/2007 9:21 AM

Listen to Tom K, he KNOWS air.

Even an egotist, know-it-all like myself will defer to Tom's knowledge when it comes to pneumatics.

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#13
In reply to #7

Re: How to Calculate Air Consumption?

09/26/2007 2:49 PM

One of the two Bernoulli cousins, replied to an anonymous answer by Newton to a posted problem with "I recognize the Lion by his paw" Is that you Bigfoot? Thanks for the kind words.

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Commentator

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#11

Re: How to Calculate Air Consumption?

09/26/2007 11:11 AM

If you have the budget, it is best to install a flowmeter. The one I used when I was analyzing my plant is from CDI Meters. The 2" meter was about $500 usd and I also purchased a logger for a couple hundred more. With the flowmeter installed, it takes alot of guesswork out of equation.

Not only will the flowmeter tell you your consumption, it will help you determine leaks. If all of a sudden you start losing pressure and the compressor seems to be running fine, look at your flowmeter and see what CFM you are consuming. If you have a 240 CFM compressor and you are consuming 80 CFM, the problem is on the compressor side of your meter. If you are consuming 240 to 260 CFM the problem is on the plant side. There has been a few times where someone forgot to turm off a bleeder valve and pressure was dropping and my men or I know where to go to immediately.

I'm not trying to sell this company's product, McMaster and many other companies sell flowmeters. But with the logger you can actually see the average consumption. While you are looking at you gauge, it will be fluctuating between say 30CFM to 320CFM while everything is working normally, due to the pulses caused by cylinders, machines starting and stopping and other factors. Your actual consumption could be anywhere between those two numbers and the logger software will show you the average consumption. This also gives you the proof you need to show the "powers that be" your calculations are correct.

Now that you have your numbers you can decide to go with a variable speed compressor, 3 or 4 smaller compressors controlled by a PLC for varying loads. These compressor companies want to do a pnuematic audit for 1 to 5 thousand usd depending on your plant size and only monitor you for a month. For less money you can monitor yourself all the time.

Oversizing your piping for future expansion, I definately agree with, within reason. Oversizing your compressor, unless it is a variable speed, I don't agree with. Your air supply is one of your largest utilities. It needs to be matched to your usage. If you are oversized by 25 HP in my area, you are wasting over $14,000 usd a year. That is running the 25 HP compressor unloaded.

Other things you can do to prevent pressure drops is the plant system should be a loop. I actually created a triple loop in my plant due to the layout of the machines it only took about 40' of pipe to reconnect to the main run. Always take your drops from the top of your run. Oops... I got carried away!!!

Sorry for the longwinded post. I get very passionate about the waste I see in pnuematic systems in some plants. The amount of waste is unbelievable.

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#12
In reply to #11

Re: How to Calculate Air Consumption?

09/26/2007 12:26 PM

Amen brother, keep preaching!

There are ways to cut costs of operating compressed air systems, but they will not be in the first cost of the system.

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#15
In reply to #11

Re: How to Calculate Air Consumption?

09/26/2007 3:39 PM
  1. It is dangerous to assume but I did. This seemed like a question about sizing for a yet to be built facility. If the air consumption is a future event a flow meter is premature.
  2. If you over size plumbing by a factor of 8 or more it will only cost the price of the original material. In operation the very low pressure drop and reservoir affect will pay back the difference in initial plumbing material. As you know the most expensive part of a compressed air system is the high pressure which takes more energy to squeeze it. Unfortunately that is the part that is lost to pressure drop.
  3. If I had four 250 horse power compressors and found that one would do nicely I would shut three off and cut operation expenses and wear. Then I would plan to sell two of them. However if the system plumbing is undersized and my 100 horse power compressor won't 'get er done' the solution is messy.
  4. Where electricity cost $14,000 a year for an "unloaded" 25 horse power motor I would run two 15 hp and use a pressure switch to cut in the second on demand for peak loads. With a duty cycle timer on #2 you could determine excess flow as in a broken pipe in short order.

If this is an existing facility I have an original formula to calculate air consumption easily and accurately. False modesty requires that I only present it when asked.

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#19
In reply to #15

Re: How to Calculate Air Consumption?

09/26/2007 7:52 PM

If the air consumption is a future event a flow meter is premature

Back when I proposed a compressed air flowmeter, my proposal was shot down because,

"I've been putting up factories left and right and I've never put a flowmeter in a compressed air system. Furthermore, I've never seen a factory that had a flowmeter. We don't need them!"

These were the words of the project manager at the time.

Funny how, years later, the company decides that monitoring the compressed air flow would be helpful in monitoring and troubleshooting the air consumption in the plant. They ordered flowmeters and a data management system. Now, they send us reports on our consumption either congratulating us for reducing or maintaining and, on occasion, asking why, why, why are we consuming so much air.

Okay, my point is that a flowmeter is a good thing to have. It's saved my bacon (actually, the company's bacon) a few times already.

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#20
In reply to #19

Re: How to Calculate Air Consumption?

09/26/2007 8:19 PM

"Value engineering" at its finest. I did manage to talk a plant manager into it once (installed the RMS style, because they were relatively inexpensive). Glad I did and would recommend it IF plant people will be intelligent to understand what it means. One rap on the RMS is they are a high pressure drop, I would prefer to spend a little more for a true flow nozzle.

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#24
In reply to #20

Re: How to Calculate Air Consumption?

09/27/2007 4:45 AM

I attended a seminar by Siemens a few months ago. They presented an ultrasonic flowmeter that you just needed to clamp on to the pipe. On the demo unit they used, they clamped it to a three inch pipe and blew into one end. That thing registered the flow though it was only a soft and short blow. In fact, while the presenter was holding it up, the air flow from the air conditioning was registering on it!

I had never used ultrasonic flowmeters before, mainly because when I first heard about them, they were intrusive and required some debris or solids to be in the stream. Plus they could only be used in liquids. Today, they can just be clamped on and useable even for air. I've just submitted a proposal for a trial order.

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Anonymous Poster
#29

Re: How to Calculate Air Consumption?

05/08/2008 11:31 AM

Is there a list some where that give air consumption rates to the most used insruments and valves not a perfect one just guessing

karen

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#34

Re: How to Calculate Air Consumption?

09/27/2012 5:40 AM

Thanks you all for the inputs. Yet to clarify whether i have understood properly let me explain how I will calculate. please correct me if I am wrong.

Assume I have 100 valves each operating 10 times a day with the air capacity of 1L @ 1Kg pressure.

My total air requirement per valve = 1L x 10 = 10 L

for 100 valves = 10L x 100 = 1000 L or 1m3

at 4 kg = 4 x 1m3 = 4m3

My requirement is 4m3/day

Considering 50 % duty cycle my compressor requirement in m3/hr is 4/12 = 0.33m3/hr

Assuming I need a receiver for catering peak load.

Assuming at any point of time 5 valve will operate together in a span of 2 mins.

5 x 1L x 4kg = 20 L / 2 min

20L x 60 / 2 min = 60L @ 4 KG is my receiver sizing requirement.

Is my understanding correct?

Thanks

Jeyaraman

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