Standard cubic feet per minute, or SCFM and pounds per hour, or PPH, are units used to describe gas flow. The volume that a gas occupies depends upon its temperature and pressure. With a higher pressure, the gas will expand in volume if it is not constrained, by say a container. The pressure depends upon the density and temperature of the gas. Standard cubic feet represents the amount of air in a volume that is a cubic foot in size under standard conditions, where the standards specify the gas pressure and temperature to be used. These standards vary globally. Pounds per hour measures the mass flow rate of liquids and gases, which again is pressure and temperature dependent.
- Skill level:
Find the density of the gas, such as air at standard conditions. In the USA the standard conditions are defined using a gas temperature of 15.6 degrees C and atmospheric pressure. Note that one atmospheric pressure is the density of air at sea level, and it decreases in value as the elevation increases. The density of air under these conditions in metric units is 1.2754kg/m^3.
Convert the density obtained in step one to a value in the appropriate units, which are pounds over standard cubic feet. For example, to change the units of the density 1.2754kg/m^3 multiply by (m^3/sft^3) (lbs/kg).
To make the conversion you need to use the relationships 1m^3 = 37.2397 scf and 1kg = 2.2046lbs. The value for the density becomes:
1.2754 * 2.2046 / 16.9kg/sft = 0.034kg/scf, and so the density is now density_air = 0.034kg/scf.
Multiply the quantity that is in units of standard cubic feet per minute by the product of the density found in Step two and 60 1/hr.
For example, if you are given a value of 2 scfm for air, then multiply this by density_air* 27.2kg / (scf * hr) to get:
2 * 0.0755 * 27.2kg/hr = 4.11kg/hr.
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