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4.7 Standard Conditions

We often see the rating of small compressors or other gas handling equipment given as so many standard cubic feet (SCF) at some pressure (psi) for a specified amount of time. But, what is a standard cubic foot and why is that measurement specified with a pressure?

As we saw with the Ideal Gas Law, the volume of a gas is proportional to the amount of that gas. However, most people do not think in terms of moles of a gas and it is even more difficult to think in terms of the mass of a gas. Yet we can and do think in terms of a volume. The only caveat is that we must set the conditions of the gas’s temperature and pressure to equate it to an amount of that gas. Thus, a specific temperature and a specific pressure have been adopted to define the conditions of the gas in a cubic foot and thus the number of moles of that gas in that cubic foot. Usually, when making specification, the acronym S.C. is used to indicate standard conditions or sometimes S.T.P is used for standard temperature and pressure.

Unfortunately, different scientific, engineering, governmental, and even international organizations have adopted different temperatures and pressures. Table 4.2 summarizes those temperatures and pressures per the defining organizations.

Table 4.2 Some Standard Conditions for an Ideal Gas

System

Temperature

Pressure

Volume/Unit

SI

273.15°K

101.325 kPa

22.415 m3/kg mol

Universal Scientific

0.0°C

760 mm Hg

22.415 L/g mol

American Engineering

491.76°R (32°F)

1 atm

359.05 ft3/lb mol

Natural Gas Industry

59.0°F (15.0°C)

14.696 psia (101.325 kPa)

379.4 ft3/lb mol

Source: David M. Himmelblau and James B. Riggs, Basic Principles and Calculations in Chemical Engineering, eighth ed. (Upper Saddle River, NJ: Prentice Hall, 2012): 353.

Inspection of Table 4.2 shows that the SI, the Universal Scientific, and the American Engineering standard conditions have the same conditions of temperature and pressure. However, they have different mass for their standard volumes, but when volumetric flow rates through compressors and other processing equipment are being reported relative to standard conditions, the volume per unit mole is irrelevant for the purpose of defining the condition of temperature and pressure.

By specifying standard cubic feet for our volume per amount of time, we have specified a mass or rather the number of moles for that amount of time. The pressure term is then added to indicate how much force the compressor will have to work against. As will be seen in the next chapter on thermodynamics, a volume against a pressure is equivalent to a force over a distance that tells us how much work the compressor is capable of performing.

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