Calculating Compressed Air Consumption in an Existing Plant

This needs a study of the nature of the machines, the power needed and the run time of each. The sum is what your compressor produces. You can measure the power and the time if electrical. Minus the losses brings you close to the total of the consumption of your machines. When all the same and same runtime = your compressor production divided by the number of machines. If you use all different tools and machines you will need to monitor each individual consumer.

I have done this before.

Basically, you have to go to the manufacture's catalog to get the rated usage at rated pressure. Then, adjust the usage upward by the ratio of actual absolute pressure vs. the rated absolute pressure.

If you cannot find this information, put a plastic bag with a known volume (say a large kitchen trash bag with a 13 gallon content around the exhaust port (bag fully empty and flat at the start and the "open" end sealed around the exhaust with very little excess). Measure how many seconds it takes to fill the bag and correct for minutes and multiply times 13 / 7.5 (the number of gallons in a cubic foot) to get the CFM of that product- do this at least twice and take the average.

If there are machines with air cylinders, you will need a bag for each if the outputs are just dampened and free to air.

do this at least twice and take the average. Wear a face-guard when you do that.

Energywood does not claim any damage caused due bursting of plastic bags during testing. in "twice" the bursting does not count!

Hi there. Can you please explain further on how you do this with the garbage bag? Please excuse my ignorance but I just want to learn. YOu have a machine and the incoming pressure supply connected to the pressure regulator and the machine is big with numerous actuators and valves in it at various spots. I want to calculate the consumption of this entire machine in CFM. How and where will I use this bag to getthat? Please help

For a machine with multiple devices, depending on the size of each you may need to use several bags (one at a time) using the "known" volume for each and the time needed to "fill" the bag.

Because you will be taking several readings, do this as a team with one measuring the time to fill the bag and another to HOLD the bag over its outlet. Hold it tightly to prevent by-pass airflow but not so tight that you cannot easily release it when "full" to avoid bursting.

Just pick bag sizes that are approximate to the expected airflow and that will "fit" in the area around the outlet.

Do following. On holiday , when no machine is running. Run compressor , fill up to say 7 bar. Move along the line and machies and look for hissing sound of leakages. Take care of leakages. Some valves may need to be changed , which normally discharges air when machine is not running.

Stop the compressor and see how long it takes to come to 4 bar. That is an indication of leakages. Take action and review periodically.

Add up catalogue/manual consumption levels specified and see you are ok.Another way to take care of short supply is to reduce peak pressure to 6bar. And run equipments and see if you are ok. If something does not run/has problem, either change the design or usea booster. Cu.m per minute at 6 bar will be higher at 6 than at 7. Set the optimum pressure at which you can operate. chakraborty02@gmail.com

All very good suggestions-

Goes beyond the original question but goes straight toward how to control costs and minimize the likelihood of buying more capacity sooner than you might need to.

Add to this a training "manual" for your employees telling them the actual COST of several things they typically do- like using a nozzle to clean up loose junk on the floor rather than get a broom.

You can calculate this cost fairly easily- just look at the rated capacity of your compressor at whatever your final operating pressure ends up, then check out the operating kW of the compressor and measure how often it runs per hour (a cheap recording meter can do this for you). Look at your utility bill and find out the cost of a kW of power (demand charge) and the cost of the cheapest kWh (usage) including any fuel costs. Then, look back in to maintenance records and determine how much has been spent, on average, per year for the last 5 years.

Calculate the TOTAL average hourly operating cost- electricity plus maintenance- then break that down to a cost per minute. Divide that cost by the average CFM produced in an hour. The answer will shock you. Compressed air is the most misunderstood utility in any plant. It is by far the highest unit cost of any utility and the most wasted.

Calculate the typical usage of, say a nozzle (based on rated capacity) and then calculate the cost of usage of the nozzle. By telling your employees how much air costs per Cubic Foot (you might even put a sign near nozzle connections telling them the cost for just 10 seconds of usage) you will encourage them to minimize waste AND to report leaks for repair. Most employees think that air is "free". It's always available and no one has told them (before) how much it costs.

http://www.engineeringtoolbox.com/orifice-air-volume-leakage-d_1191.html

http://www1.eere.energy.gov/industry/bestpractices/pdfs/compressed_air_sourcebook.pdf

Compressed air can be a very energy wasteful medium and the above links will serve you well.

How totally true-

Just consider that the most efficient compressors generate about 4.3-4.7 CFM per HP, using the average, that is 6 CFM per kW or 360 CuFt per kWh. Using average costs of $15.00 per kW and $0.08 per kWh, that is $0.000248 (demand) + $0.000222 (usage) per CuFt, or $0.00047 per CuFt. just for the electricity and without maintenance costs.

Doesn't sound bad until you realize that a nozzle shooting 100 CFM will cost about $0.01 for every 10 seconds it is used, or that each machine using 75 CFM is costing $4,266 a year to operate, before you add in maintenance costs on the compressor.

Use the same costs to determine the actual costs of leaks found based on the link above and you will see what a problem this can be.