1. Psychrometry
It is the study of the properties of Moist Air or a mixture
of dry air and water vapor.
Fundamental Parameters:
1. TOTAL PRESSURE
P = Pa + Pv KPa
2. VAPOR PRESSURE
Pv = Pw − PA(t d − t w ) KPa
where
A = 6.66 x 10 −4 → For tw equal or greater than 0°C
A = 5.94 x 10 −4 → For tw equal or less than 0°C
2. 3. SPECIFIC HUMIDITY OR HUMIDITY RATIO
0.622Pv kg moisture
W=
P − Pv kgdryair
4. RELATIVE HUMIDITY
Pv
φ=
x 100%
Pd
5. ENTHALPY
KJ
h = 1.0045t d + W(2501 .3 + 1.86t d )
kg dryair
3. 6. SPECIFIC VOLUME
0.287(t d + 273) m3
υ=
(P − Pv )
kgda
7. DEGREE OF SATURATION
P − Pd
µ = φ
P − P
v
Where
P – total pressure in KPa
Pw – saturation pressure corresponding tw, KPa
Pv – vapor pressure or saturation pressure corresponding
the dry bulb temperature of the water vapor in the
mixture.
Pa – partial pressure of dry air, KPa
4. Pd – saturation pressure corresponding td in KPa
td – dry bulb temperature, °C
tw – wet bulb temperature,°C
φ - relative humidity
W – humidity ratio or specific humidity, kgm/kgda
h – enthalpy, KJ/kgda
υ- specific volume, m3/kgda
µ - degree of saturation
7. Psychrometric Processes or Air conditioning
Processes
1. Sensible Heating – it is addition of heat to moist air
without the addition of moisture. It follows a constant
humidity ratio line on the psychrometric chart.
Q
m
1
t1
h1
m
2
t2
h2
9. 2. Sensible Cooling: It is the removal of heat from moist air
without the removal of moisture. It also follows a constant
W on the psychrometric chart.
Q
m
1
t1
h1
m
2
t2
h2
11. 3. Cooling and Dehumidifying: It is the removal of heat and
moisture from moist air, it involves sensible and latent
heat transfer.
Q
m
1
t1
h1
m
2
t2
h2
mw
hw
14. 4. Heating and Humidifying: It is the addition of heat and
moisture to moist air, it also involves sensible and latent
heat transfer.
Q
m
1
t1
h1
m
mw
hw
2
t2
h2
17. 5. HUMIDIFYING – It is the addition of moisture to moist
air without the addition of heat.
m
1
t1
h1
m
mw
hw
2
t2
h2
18. hw = h g @ t 1
hw < h g @ t 1
hw = hf @ tw1 or tw2
2
2
2
2
1
m(h2 – h2) = mwhw → 1
mw = m(W2 – W1)→ 2
hw > h g @ t 1
19. h2 − h1
hw =
→3
W2 − W1
. ADIABATIC MIXING – It is the mixing of two streams of
moist air without the addition of heat.
h2
W2
m2
2
m1
1
h1
W1
3
m3
h3
W3
20.
21. m1 + m2 = m3 → 1
m1h1 + m2h2 = m3h3→ 2
m1W1 +m2W2 = m3W3→ 3
m1 h2 − h3 W2 − W3 t 2 − t 3
=
=
=
→4
m2 h3 − h1 W3 − W1 t 3 − t 1
m1 h2 − h3 W2 − W3 t 2 − t 3
=
=
=
→ 5
m 3 h2 − h 1 W 2 − W1 t 2 − t 1
m2 h3 − h1 W3 − W1 t 3 − t 1
=
=
=
→ 6
m 3 h2 − h1 W 2 − W1 t 2 − t 1
22. SENSIBLE HEAT FACTOR
Sensible Heat factor is the ratio of the sensible heat to
the total heat transfer for a process.
QS
SHF =
Q
QS
SHF =
QS +Ql
24. LOAD RATIO LINE
Room air condition
Supply air condition
2
1
For air conditioning design, supply air temperature is within 3°C to 11°C lower
than the room condition.