Miscellaneous Solved Sample Problems Thermodynamics

Calculation of NM3/hr and SM3/hr gas flow

Sample Problem Statement

What is the NM3/hr (normal cubic meter per hour) and SM3/hr (standard cubic meter per hour) flowrate corresponding to 100m3/hr of nitrogen flowing through a pipe? Operating temperature of nitrogen stream is 800C. Operating pressure at the point of flow measurement is 25 bar.

Normal temperature and pressure conditions to be used are –
T = 250C and P = Atmospheric pressure

Standard conditions defined by NIST to be used.

Solution

The calculation of normal and standard gas flowrates (NM3/hr and SM3/hr calculation), first requires the normal and standard conditions to be fixed. Normal temperature and pressure conditions are already defined in the problem statement and standard temperature and pressure conditions defined by NIST are to be used.

Normal Temperature TN = 250C = 298.16 K
Normal Pressure PN = 101.3 kPa (atmospheric pressure)

From a list of standard temperature and pressure conditions defined by various organizations, the standards conditions defined by NIST are noted as,

Standard Temperature TS = 00C = 273.16 K
Standard Pressure PS = 101.325 kPa

The operating conditions are,
Volumetric gas flow V = 100m3/hr
Gas temperature T = 800C = 353.16 K
Gas pressure P = 25 bar = 2500 kPa

Gas volume is directly proportional to temperature and inversely proportional to pressure.

Hence, gas flow at normal conditions is calculated as,
NM3/hr = V×(TN/T)×(P/ PN)
= 100×(298.16/353.16)×(2500/101.3)
Gas flow = 2083.57 NM3/hr

Gas flow at standard conditions is calculated as,
SM3/hr = V×(TS/T)×(P/ PS)
= 100×(273.16/353.16)×(2500/101.325)
Gas flow = 1908.4 SM3/hr

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