air conditioning engineering

1. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
Air-conditioning Engineering
References:
1.Jan F. Kreider, Peter S. Curtiss “Heating and cooling of Building”. Mc Graw Hill, 2000.
2.W. P .Jones “Air conditioning Application and design”. Arnold international students’
edition, 1998.
3.W. P .Jones" Air conditioning engineering", Arnold international Students edition, 1977.
4.W. E. Stoecker “Refrigeration and air conditioning",. Mc Graw Hill, 1982.
5.ASHRAE, Fundamental. 1997.
6.R.S.KHURMI and J.K.GUPTA “A textbook of refrigeration and air conditioning”.
7.R.K.Rajput“A textbook of refrigeration and air conditioning”.
1. Review of basic principles
Air conditioning : Is the science and practice of controlling the indoor climate in term
of temperature , air motion , humidity , air purity and noise.
Refrigeration :Is the process of removing the undesirable heat from a given body to
maintain it at a desired lower temperature .
.Fundamental properties of Air and Water vapour mixture:
2
There are number measures of the amount of moisture contained in atmospheric air,
before we begin the discussion of thermodynamics of moist air mixture, we shall define
their most important properties and indicate where each is used.
1) Dry Bulb Temperature DBT: is the temperature of moist-air mixture measured
by a perfectly dry sensor, such as thermocouple or glass thermometer.

2. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
2) Wet Bulb Temperature WBT: is the temperature of moist – air mixture measured
by thermometer's bulb covered by a wick that has been thoroughly wetted with
water.
3) Wet bulb depression : It is the difference between dry bulb temperature and wet
bulb temperature at any point.
4) Humidity or Moisture content (w): It is the mass of water vapour present in 1 kg
of air, or is the ratio of the mass of water vapour to the mass of dry air in a moist-
air mixture. Generally expressed in terms of gram per kg of dry air (g / kg of dry
air). It is also called specific humidity or humidity ratio.
5) Relative humidity ϕ: It is the ratio of actual mass of water vapour in a given
volume of moist air to the mass of water vapour in the same volume of saturated
air at the same temperature and pressure.
6) Partial pressure Ps: is the pressure exerted by one gas component on a mixture
of several gases. The partial pressure of water vapour is used in define of relative
humidity.
7) Dew point Temperature Tdew : It is the temperature of air recorded by a
thermometer, when the moisture (water vapour) begins to condense.
.The composition of dry air
3
Dry air is a mixture of five main component gas together with traces of number
of gases: It is reasonable to consider all these as on homogenous substance but to deal
separately with the water vapour present, because the later condensable at every day
pressure and temperature. The dry air portion of the atmosphere, then, may be thought
of gas being composed of true gases. These gases are mixed together as follows, to form
the major part of the working fluid;

3. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
Table (1.1) Composition of dry air.
The molecular mass of dry air is taken as 28.966 and the gas constant of air ( a
R ) equal
to 0.287 K
.
kg
/
kJ or 287 K
.
kg
/
J .
.Standard Adopted
4
1) Density of air is 1.293 kg/m3
at 101325 Pa and 0o
C
2) Density of water is 1000 kg/ m3
at 4o
C & 998.23 kg/m3
at 0o
C
3) Barometric pressure is 101325 Pa
4) Specific force due to gravity is 9.802 N/kg or m/sec2
5) Specific heat of air at constant pressure is 1.005kJ/kg.K
6) Gas constant of air is 0.287kJ/kg.K
7) Gas constant of water vapour is 0.461kJ/kg.K.
8) Universal gas constan is 8.31447kJ/kmol.K.
S.N Constituent By volume By mass Molecular
mass
1 Nitrogen ( )
2 Oxygen( )
3 Argon( )
4 Carbon-dioxide ( )
5 Hydrogen ( )
2
N %
03
.
78 %
47
.
75 28
2
O %
99
.
20 %
19
.
23 32
Ar %
94
.
0 %
29
.
1 40
2
CO %
03
.
0 %
05
.
0 44
2
H %
01
.
0 ‫ـــــ‬ 2

4. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
.The General Gas Law
5
It is possible, as we saw from the 1st year of thermodynamics subject" to combine
Boyle's and Charle's laws as one equation:
PV= m.R.T (1.1)
P Pressure of gas kPa
V Volume of gas m3
m Mass of gas kg
R Gas constant kJ/kg K
T Absolute temperature K
.Dalton's law of partial pressure:
6
If a mixture of gases occupies a given volume at a given temperature, then, the
total pressure by the mixture equals the sum of the pressure of the constituents, each
being considered at the same volume and temperature.
T 20o
C
ma 1kg
mv 0kg
Pa 100143Pa
Pv 0Pa
Pat 100143Pa
T 20o
C
ma 0kg
mv 0.0073kg
Pa 0Pa
Pv 1182Pa
Pat 1182Pa
T 20o
C
ma 1kg
mv 0.0073kg
Pa 100143Pa
Pv 1182Pa
Pat 101325Pa
Dry air
a
Water vapour
b
Air vapour mixture
c

5. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
As shown from the figure an air-tight container under three different conditions, from
which it can be seen that the partial pressure exerted by the water vapour in (b) and (c)
are equal, as are those exerted by dry air in(a) and (c), and in (a), (b) and (c), the total
pressure equals the sum of the partial pressures.
.Saturation Vapour Pressure
7
There are two requirements for the vapour of liquid water to occur:
1) Thermal energy must be supplied to the water.
2) The vapour pressure of the liquid must be greater than that of the steam in the
environment.
The following equation may be used for the vapour pressure of steam over water up to
100o
C.
log(p) = 28.59051- 8.2log (t+273.16) + 0.0024804 (t+273.16) -
3142.31
t+273.16
(1.2)
Where:
t= temperature in o
C
p= pressure in bar
Over ice, the equation to be used is the following:
log(p)=10.5380997-
3142.31
t+273.16
(1.3)
)
v
p
air (
-
pressure of steam in moist
.The vapour
8
The question arises; how do we determine the vapour pressure for relative
humidity less than 100%, an empirical equation exist which answer this question;

6. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
Pv = psw – pb .A. (DBT - WBT) (1.4)
Pv Partial pressure of water vapour in wet air
psw Saturation vapour pressure at wet bulb temperature (WBT)
pb Barometric pressure
A constant A=6.66×10-4 o
C-1
when WBT ≥ 0
A=5.94×10-4 o
C-1
when WBT<0
Example 1:
Calculate the vapour pressure of moist-air at 20o
C DBT, 15o
C WBT and 95kPa.
Solution:
From table at 15o
C WBT psw= 1.7044kPa
Pv = 1.7044 - 95×6.66×10-4
(20-15)= 1.3876kPa
:
ϕ
.Relative humidity
9
Relative humidity is the ratio of the partial pressure of the water vapour in the
moist-air, at a given temperature, to the partial pressure of the water vapour pressure in
saturated air, at the same temperature.
Φ =
pv
ps
=
mv
ms
(1.5)
ps= saturated water vapour pressure at dry bulb temperature kPa.

7. Air Conditioning A/C Techniques Dep.
2nd
Year Class First Term 2021-2022
Lecture one: Fundamental properties of Air and Water vapour mixture
by: Asst. lect. Karrar Al-Mansoori
Example 2:
Calculate the relative humidity of moist air at 30o
C DBT, 20o
C WBT and 100kPa
barometric pressure?
Pv = psw – pb.A.(DBT -WBT)
psw = 2.3373 kPa at 20o
C WBT
pv = 2.3373 -100×6.66×10-4
(30-20)= 2.2704kPa
ps = 4.2431kPa 30o
C DBT
Φ=
pv
ps
×100% =
2.2704
4.2431
×100% = 53.3%