Measurement in Compressed Air and Natural Gas Industries
I have said before that if you don’t have a measurement, you only have an opinion, but in our industry measurements themselves are sometimes a source of confusion.
In compressed air and the natural gas industry, there are certain ways that you have to express how much and how many. Most of who work with compressed air are familiar with Standard Cubic Feet per Minute (SCFM). Many of us who work with applications outside the U.S. also deal with metric flow rates. Metric flow rates are usually stated in terms of standard cubic meters per hour or normal cubic meters per hour, shown as m3/hour and Nm3/ hour respectively. The two forms are not that far apart, but may be different enough to cause problems in a closely sized application. This is because the standard meter (m3) is based on a temperature of 20°C/68°F while the Normal cubic meter is based on a temperature of 0°C/32°F, which is the STP that the scientists use (this is Standard Temperature and Pressure, not the motor oil additive). In more recent times, some are based on one atmosphere of pressure (14.7 PSIG) and others on one bar (14.5 PSIG) as the base pressure. The conversion is 35.31 Standard Cubic Feet / 1 Standard cubic meter, and the other is 37.32 Standard cubic feet/ Normal cubic meter. These conversions may vary if the basis is wet or dry air. Again this is important in closely sized systems, so it pays to double check to find the exact base conditions that were used to calculate volume.
Those using the English system are used to working flow per minute, as in SCFM. Metric people usually use flow per hour, an in Nm3 per hour. This has been a source of many cross cultural technical goofs.
The problem comes when a specification uses a hybrid such as normal cubic meters per minute or standard cubic feet per hour. Your brain may see these units and think that the time shown is a typo, but sometimes it is not. Of course, I never tripped up on this.
The DLR principle (Doesn’t Look Right), is a good one. Never hesitate to double check by matching the dryer size to the compressors' horse power. Confusing minutes and hours will result in an error by a factor of 60.
On the moisture front, we most commonly use dew point in compressed air industry. Remember dew point always has to be indexed to a pressure, as it will change when pressure increases or decreases. You often see dew point expressed as PDP (Pressure Dew Point) or ADP (Atmospheric Dew Point).
In the North American natural gas industry, moisture is usually measured in pounds of water per million standard cubic feet of gas, normally expressed as Lbs Water/MMSCF, which turns out not to be dew point or relative humidity. Rather this is an expression of "absolute humidity". As the scientists express it via Wikipedia:
Absolute humidity is the mass of water vapor, per unit volume of total air [or gas] and water vapor mixture, which can be expressed as:
AH = mw/Vnet
In this case, we are using gas, and the unit volume is a million standard cubic feet.
A few things about Lbs water / MMSCF:
M does not mean not million. It stands for a thousand, so you have to have two of them together to make a million. M x M = 1,000 x 1,000 = 1,000,000.
The moisture is measured by content at atmospheric pressure. This is why the S shows up- to designate Standard conditions, or 14.7 PSIG.
This is not dew point. You cannot tell the temperature at which moisture forms until you know the operating pressure of the gas stream. It tells moisture content only and leaves the dew point up to the pressure in the system.
Dew point tells you at what temperature condensation occurs, but not necessarily the moisture content if you do not know pressure. Lbs Water /MMSCF tells you the moisture content, but not when you will condensation, because you do not know pressure. As long as you are aware of this, you can go back to the charts or the technical sales department to double check.
Sometimes the specification is like the “telephone game” you play at parties, where you say something to one person who passes it on five or ten times. The end result is anywhere surprising to unbelievable. This also happens when requirements pass through many hands on their way to you. Just think of the DLR concept that was mentioned earlier.
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