#### The Ideal Gas Law

The relationship between mass and volumetric flow rate, pressure and temperature for a gas can be determined from the Ideal Gas Law:

where *p *is the absolute pressure of gas (kN/m2), is the actual volumetric flow rate of the gas at the pressure *p *(m3/s), *m** *is the mass flow rate of gas (kg/s), *R *is the character- istic gas constant (kJ/kg . K) and *T *is the absolute temperature of gas (K) *t*°C + 273.

where *R*0 is the universal gas constant kJ/kg (mol . K) 8.3143 kJ/kg-mol . K and *M *is the molecular weight (mol).

Values for air and some commonly employed gases are presented in Table 9.2. Whichever gas is used, the appropriate value of *R *for that gas is simply substituted into Equation (9.4) and the design process is exactly the same.

**The use of nitrogen**

It will be noticed that there is little more than 3 per cent difference between the values of *R *for air and nitrogen, since about 78 per cent of air, by volume, is nitrogen. As a consequence little error would result if a system was inadvertently designed on the basis of air, and nitrogen was used for conveying the material instead. If carbon diox- ide or superheated steam was to be used to convey the material, however, there would be a very significant error.