estimating compressed air flow

[Herman2]

I need to install flow meters in our plant air system (90 psig). Total capacity of all compressors is about 7500 cfm. The only other information I have is pipe sizes (4, 6 and 8 inch steel, and 2-inch copper)

Please suggest a typical design velocity or other method to estimate flow meter requirements. A quick note on cfm vs. scfm would also help this EE. Thanks.

 

[TD2K]

Roughly, ignoring compressibility effects which won't be significant for air at these pressures:

scfm = cfm * (14.7/P) *((520/(T + 460)

P is the system pressure, psi absolute (gauge + 14.7)
T is the system pressure, deg F

For your question on flow meter, are you talking about sizing the meter? You are likely looking at an orifice meter. That is sized based on the pipe it is installed in, the flow rate and the system pressure and temperature. There's no quick rule of thumb for sizing an orifice plate, there are rules of thumb for the beta ratio (orifice diameter/pipe ID) that you want to stay within but I don't think that is what you are talking about.

 

[Herman2]

Clarification - I need to estimate the volumetric flow rate in my compressed air system (90 psig) based on pipe size. What velocity might the original pipe designer have designed for?

 

[ccfowler]

Rather than investing in test instrumentation or trying to guess at the intentions of the system's designer(s) based on pipe size, you would probably do better to monitor the compressors' duty cycles. This can provide a reasonable measure of current actual compressed air usage. Most compressors operate close enough to their ratings to provide a credible estimate of actual compressed air usage. This monitoring can be done with the investment of some time, a stop watch, and some paper. Depending on the system's characteristics, the monitoring periods can be quite short while still providing the needed information. Be sure to interview operating personnel to gain valuable insight into actual system operating characteristics.

A by-product of such a survey is likely to be knowledge of which compressors are actually carrying the load and which ones are serving primarily as reserve capacity.

If the compressors are distributed around the system, it may be practical to isolate portions of the system to gain a measure of the distribution of usage in the different sections of the system.

This method of estimating usage does include all air consumption including parasitic losses. If your monitoring is done during periods of high ambient temperature and humidity, you may want to run some calculations to see if your estimates should be adjusted to recognize moisture condensed in intercoolers, aftercoolers, and the piping system.

If your monitoring is done with a reasonable understanding of the timing of the monitoring periods with respect to your plant's operating characteristics, the anecdotal data that this survey will produce can provide an amazingly accurate estimate of actual usage and usage characteristics. It may be necessary to monitor the compressors during several different time periods to provide the needed data to provide the needed insights into the plant's compressed air needs. Don't be surprised if you find that you actually have significantly more compressor capacity than you actually need if this is an older plant.

Unless you have really valid information to indicate that a particular compressor is not performing near its rated capacity, you should presume that it is operating near its ratings. This is particularly true for reciprocating air compressors. I've run performance test on old compressors that had significant known problems (such as worn valves) only to find that the delivery was still greater than the nameplate ratings!

If the metering project is intended to provide a means of segregating usage (and apportioning operting costs) to different departments or areas, you should seriously consider installing positive displacement meters despite their significant initial costs. These meters can provide the very large turndown ratios needed while maintaining excellent accuracy.

Continuing with the assumption that your metering project is for the purpose of apportioning costs, you can expect to find quite dramatic reductions in compressed air usage once it it no longer perceived to be "free."