Question

Air is heated from 25c to 150c . The flow rate of air at the exit of the heater is 1.25m^3/min and the air pressure is 122 Kpa absolut .

calculate the heat requirement in KW , assuming ideal gas behavior and that the kinetic & potential energy are negligible.

Answer 1

I am not sure what is the use of the pressure related data in this question, because making use of that would require some other data as well, such as specific enthalpy change for this process, because then I can incorporate the pressure term into the solution.

But since you have not provided any other data, I am giving a simple solution without using the pressure term. Idea is simple as illustrated below:

Heat requirement per unit time = mass flow rate * Cp * dT

Mass flow rate can be calculated from density, assuming that the air density doesn't change much in the given temperature range.

SInce both Cp and density change appreciably with temperature, I am making use of average values for them over the specified temperature range.

d = 1.029 kg/m^3

Cp = 1.008 kJ/kg.K

Mass flow rate = volumetric flow rate * d = 1.25 * 1.029 /60 = 0.0214 kg/s

Putting all these values in above eqn :

Heat per unit time = 0.0214*1.008*(150-25) = 2.69 kJ/s = 2.69 kW

Hope this helps !