Question

- What is the physical interpretation of the wet-bulb temperature During winter time a room is maintained at a steady temperature and humidity by an air-conditioning system shown. 200 min of room return air at 30°C. SON 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: Plot the processes on the ASHRAE Psychrometric chart and submit. Show all the states clearly on the chart. Determine: 1) the required capacity of the steam heater, kW 1) 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 Sat. vap150 0 - 50% Mixed air 45°C Room 2 40 ml 100.0 -20% Steam heater Sat. liq. 150 kPa

Answer 1

wet the air. thermometer when it is covered comes in a) sio Physical significance of bulb temperature The wet bulb temperature indirectly indicates the moisture content present in ais que is offected by Relative humidity of Temperature of ordinary air measweed wet cloth is wet bulb temperature. When air contact with wet cloth it absorbs some moisture and give up some heat due to which air temp. Reduces. This reduced temperature bulb temperature. Therefore het mooiectusce content of an so wery low, it will give up 'more heat to cloth and those will be comparatively And if high, it will loose lesses heat to air and weet bulb temp will be higher more is the moisture ore water vapor content of air, Bulb temperature. is heet lower moisture is So. The more is the weet

Solution. N Assume Adiabatic mixing Satrap. @150 kPa rate at mass flow L ( ma.) 30°c, & -50% m3/min Q = 200 mm V 0.8763 milig 45°C Room 3 ma, = 3.8 kg/sec 0-2016 • at 2- 40m (min, 10c V=0.8033 using ma 40 0.8033 x 60 Steam Heater sat liq = 0.83kg 1sec @ 150kla mon = h = 7445 ma, & ma, {mass Balance} Saturation eine (6=1804) - 3.8 +0.83 Plsssu 8147- ес, 4.63 kg/sec z = mo, wat mo, Wz maz 0-50% h₂ = 13.86 - 3.8 (0.01381) +0.83(0.00191) 4.63 7W, =0.01331 y W -0.0112 0 - 18.7% +207 Enthaupt line W = 0.0112 kgrap! a dryair 2226.00151 Humidity Ratio kg vapleg dry air = mah, tma, ha maz 72 = 10°c T = 30°c 45°C = T4 =3.8 (64-18) + 0.83 (13-86) 4²63 Dry Bulb 55.16 kg 1 kg dry air Temperature (²) 200 ml min Mixed -0.1 %

in Required capacity of steam heater, kW -(@s). Qs = mag (hu-hy) = 4.63 (94.19.55.16) kW = 88.11 kW S hee have. W = 4 supply. so S98 air and stream ") Relative humidity. ¢ of the aio supplied to the room, % P = ? As there is no air and stream moisture is added W4= = 0.622 Pu P-pu Rv s vapour pressure) 0.0112 = 0.622 Pv 101.325 - Pv Quot8006) fin ( on floor) (toto 325-fo) to 180064 h 78. (0.0980064) C101.325 - Pu) = Pv 1.8245 1.0 180064 Pu P = 10 792kPa] Pu Where Ps Ps saturation pressure at 45°c = 9-5944 kPa = 18.7% & Relative humidi humi clity of air supplied to Room "") Moss flow rate of steam through heater (mis) Applying energy Balance to heater- maz hat ms (hg - hy) he hig Saturation enthalpies at 150 kPa steam table- from - 467.08 kg 1kg 2693.1 kJ lig o d man hy moss flow rate of dry air remains the Same ht hg where het mau

= 4.63 (55.16) + ing (2693.1- 467-08) = 4.63 (74.19) mg = 4.63 (74-19-55-16). (2693.1-467.08) = 0.0396 kg 1sec 0.03 96 x 60 kg 1 min eglmin ng = 2.375 Agimin iu) is not acceptable in according common The calcul ated Relative humidity of supply it is 18.77 and engineering proctices because to Ashrae, the value relative hum humidity according Human comfortable zone lies between 40-50 %, generally 40 4. in winter conditions and is app. 19 % which is lesser than 40 % % to our calculated value much