HVAC Processes

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

5. Psychrometric Chart
φ
P = 101.325 KPa
h
tW
tdP
W
υ
td

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

8. h2
h1
1
t1
2
W1 = W 2
t2
Q = m(h2 - h1) → 1
Q = mCp(t2 - t1) → 2
Cp = 1.0045 + 1.86W KJ/kg-C → 3

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

10. h1
h2
2
t2
1
W1 = W2
t1
Q = m(h1 – h2) → 1
Q = mCp(t1 – t2) → 2
Cp = 1.0045 + 1.86W KJ/kg-C → 3

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

12. h1
1
ha
W1
h2
2
t2
a
t1
Q = m(h1 – h2) - mwhw → 1
mw = m(W1 – W2)→ 2
Q = Q s + QL → 3
W2

13. Qs = mCp(t1 – t2) → 4
Cp = 1.0045 + 1.86W2 → 5
Qs = m(ha – h2) → 6
QL = m(W1 – W2)hfg → 7
QL = m(h1 – ha) → 8
Where:
Qs – sensible heat
QL – latent heat
For air conditioning design
Cp = 1.02 KJ/kg-°C
hfg = 2500 KJ/kg

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

15. h2
2
ha
W2
h1
1
t1
a
t2
Q = m(h2 – h2) - mwhw → 1
mw = m(W2 – W1)→ 2
Q = Q s + QL → 3
W1

16. Qs = mCp(t2 – t1) → 4
Cp = 1.0045 + 1.86W1 → 5
Qs = m(ha – h1) → 6
QL = m(W2 – W1)hfg → 7
QL = m(ha – h1) → 8

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

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

23. APPARATUS DEW POINT
COOLING AND DEHUMIDIFYING
PROCESS
1
2
tADP
t ADP – apparatus dew point

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.

25. By: ENGR. YURI G. MELLIZA