Measurement of other parameters of moist air
To determine state of moist air, one property in addition to the pressure and temperature must be known. It can be v, I, (f>, or W. However, none of them can be directly measured. As an alternative, we seek an indirect measuring technique. In this section, the method used to determine air humidity will be introduced.
First we look at a special process:
The equation that describes above process is:
O 1st Law
O The process is adiabatic, and only flow work is present.:
O where /??, = ma + W] ma, m2 = ma + W2tiia, and form mass balance
The equation that describe above process becomes:
Dry air + water vapor+water vapor added =dry air + water vapor (in) (in) (added) (out) (out)
So we have O
Where ifg is enthalpy difference between liquid water and saturated vapor at the temperature T2
W7 = 0.622 Pv’2 P-Pv.2
Then the state of moist are can be determined.
T2 is called wet-bulb temperature.
Find W and 0 of the above adiabatic saturation device.
Given: P=1.01325xl05Pa, Ti=30°C, T2=26°C
Find: Wi, 01
Since the state 2 is in saturation, from the Table A-lb (McQuiston & Paker, p587), we can find:
For T2 = 26°C, pv,2=ps = 0.03363x10s (0=100%), 7fg = 2439.1 kJkg, iv.= 9.01 kJkg for Ti = 30°C, 7v, i = 2555.3 kJ. kg
1) Find Wt
2) Find 01
(h = — xl00%
/A j can be found via
From Table A-lb
• The Psychrometer
Psychrometer TWB used in place of T2 for practical humidity measurement In wet-bulb, heat transfer from air —>bulb P+Tdb+Twb =>Moist air state Key issues to measure TWr
• Wet bulb unshielded
• Wet bulb well ventilated (V> 100 fpm)
For thermocouples V could be lower.
Then the accuracy is in order of 0.27°C (0.5°F).