Question

a) What is the physical interpretation of the wet-bulb temperature? b) During winter time a room is maintained at a steady temperature and humidity by an air-conditioning system shown. 200 m/min of room return air at 30°C, 50% relative humidity is first mixed with 40 m/min of outdoor at 10°C, 20% relative humidity. The mixed air is then introduced to a heater where the temperature of the air is raised to 45°C and supplied to the room. [Note that air and the steam in the heater do not mix. Steam simply supplies the heat.) Do the following using the ASHRAE Psychrometric chart only: i) Plot the processes on the ASHRAE Psychrometric chart and submit. Show all the states clearly on the chart. Determine: ii) the required capacity of the steam heater, kW ill) the relative humidity, 6, of the air supplied to the room, % iv) is the calculated relative humidity of the supply air acceptable in common engineering practices? Explain why. v) the required mass flow rate of steam through the heater if saturated vapor is supplied at 150 kPa and exits the heater as saturated liquid at the same pressure, kg/min at vap 12 MPa 200 30C, -50% Moned air 45°C Room 40 min 100, 20% Steam heater Sa. 1501

Answer 1

(Ь) return room air - 200 m z = 30°c 200 0.88 seun - Soy from psy chrometric chart; Ja 0.88 mily ing - t 227.27 Kg/min h = 64.2 KJ/kg : my = 3.78 kg/f mixing ail (2) it = 40m /min; tz=2006; 8=20%. = 0.35 w/ty nie = 42 +819 Kg ſenin 0.8032 Kg/4 प h = 27.4 kJ/kg 40 0.83 oftel mixing from (conservation of mass) inaing til - 4.583 kgle from (energy conservation) mighing = night why hy = (0.8032)( 27.4) + 6.48)(64.27 (4.583) ha 3- 57.75 KJ/kg also; in W = m₂ W₂ + mawa (0.002 88X, 8032+(0.01331) (3.78) w = 0.011482 Kg/kg of dry out - 4.583

i from psychrometric chart corresponding to be; and wz = 28.3°C RH = P = 47.8% (a) @ 150kPa saturated steam 269.4 kJ/kg lelsokra 3 4 @ 150kPa 7 (6) a 150 kla; saturated liquid ha leff = 467.2 Kulky given; te=45°C W₃ = Wp = 0.0 11482 kg/kg of dry air corresponding to (ty and WA) from psychosometry chart, = 74.9 kJ/kg RH = 0 = 19.1% ha (ii) capacity of steam heater in a nie ni (h4-43) = 9 (4.583)(749–57.75) = ♡ 195 = 7.8.59 Khi FANS)

(T=45°C) so lower RH Chellative humidity) should ih from psychromatic chart 194 = 19.1% a ~194. balance across HX, energy = 78.59 ng (2693.4 - 467.2) ing = 0.03525 kg/€ 2.115 kg / min msteam 4 ly As the temperature supplied in the rom is high also be supplied to achieve thermal comfort. hence; Q4 is justified,