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 mº/min of room return air at 30°C, 50% relative humidity is first mixed with 40 mº/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: D) 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, , 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 SR 150 200 Mbod air 450 SON Room Shamheter low

Answer 1

A )

In simple terms Wet bulb temperature tells you how easy it is for water to evaporate

If you consider air which can be cooled by evaporation
Upto - 100% relative humidity , (that is you can't add any more water in it )
then "The colling effect that occurs by adiabatic evaporation of water is called
WB temp"

WB thermometer measure the latent heat of a moisture loaded air

Humans sweat by means of evaporation normally, when you want to know how well water will evaporate, you need to know the difference between the wet bulb and dry bulb temperatures , if the difference is 0 there will be no evaporation and no heat transfer could occur between skin and ambient.

B ) Psychrometric Chart of Process :

3 30 ASHRAE ASHRAE PSYCHROMETRIC CHART NO. 1 NORMAL TEMPERATURE SEA LEVEL BAROMETRIC PRESSURE 101.325 kPa COPYRIGHT 1902 ASHRAE 100 281 30 120 26 1.0 10 90 287 -10 040 96 24 05 3.0 00 SENSIBLE HEATH TOTAL HEAT - % 22 110 254 mone end 20 A 95 ENTHALPY HUMIDITY RATIO an aw VA 100 2.28 4A 25 WET BULB FEMPERATURE 18 % 050 ºc 16 ES ENTHALPY KILOJOULES PER KILOGRAM DRY AIR 204 A1 90 - 14 OS 40 SATURATION TEMPERATURE -0.80 12 A3 ONS 15 80 A4 0.90 10 30 20 70 10% RELATIVE HUMIDITY 60 A2 0 20 30 50 Pred CENTER FOR ATLID THRONAMIC STUDIES. Uity of DRY BULB TEMPERATURE C

RJ from given information & Ma = 200 m² m/min Dry bulb temp = 30°c. (taj relative humidity = 50% (6) from chart Heemidity (w.) = 0.0135 leg of dry air enthalpych, ) = 65 kolleg Mag: 40m/min Dery bulb temp (de) = loc relative humidity &q= from chart Humidity (W2) = 0.0018 IN u/ng of day wir enthalpy (hq) = 24 kolleg M = M, & M. 200t Goa 240 m3/mon Mh+ Mahaa Mg hz 200(65) + 40/240 - hz 20%. 240 hg = 58.16 les/reg

Jw, t Me Web M₂ W3 90 200 (0.035) + 40 (0.0018) = 240 x w z Wg = 0.01157 kJ /tag or dig wir So have 58.16 after mixing of air ha kJ lkg 0.0 1157 kJ /kg of day kJ /kg of dry ens. 3 = 48"). (from chast 2 st tag = 28°C (from chant) وفى (ii) Cupacity of heater 240 m/min 240 x 1.204 60 (density of cin- 1.204) 4.816 kals d = m Cod 4.816 (19/3) x 1.05 kgxc Q = 81.872 KW y (45-28).

(iii) after heating air Supply to room Tay - 45°c Øu Wha0.01157 kJ ling of dry cis 19 82 hu 78

iv ) Standart Relative relative humidty range is 30% - 50%

as in this process after heating it is less than 20%.

so it is not acceptable to supply in room.