Question

An air conditioning system is to take in outdoor air at 283K and 30percent relative humidity at a steady rate of 45 m3/min and to condition it to 298K and 60% relative humidity. The outdoor air is first heated to 295K in the heating section and then humidified by the injection of hot steam in the humidifying section. Assuming the entire process takes place at a pressure of 100kPa, determine (i) the rate of heat supply in the heating section and (ii) the mass flow rate of the steam required in the humidifying section. Note: Gas constant is 0.287 kPa.m3/kg.K

Answer 1

Inlet air at point 1

Dry bulb temperature of air = 283 K = 10°C

RH = 30%

Outlet air at point 3

Dry bulb temperature of air = 298 K = 25°C

RH = 60%

At point 2, outdoor air is first heated = 295K = 22°C

Enthalpy at point 1

H1 = 16.5 kJ/kg

H2 = 28 kJ/kg

Mass of air = PV/RT = (100*45/60)/(0.287*283) = 0.923 kg/s

Heat added = m (H2 - H1) = 0.923 x (28 - 16.5)

= 10.615 kJ

Specific humidity,

At point 1
w1 = 0.003 kg/kg of dry air

At point 2
w2 = 0.012 kg/kg of dry air

Moisture added = w2 - w1
= 0.012 – 0.003

= 0.009 kg/kg of dry air

Mass of steam = moisture added x mass of air

= 0.009 x 0.923

= 0.0083 kg/s