THERMODYNAMIC ANALYSIS OF YEAR ROUND AIR CONDITIONING SYSTEM FOR VARIABLE WET BULB TEMPERATURE OF OUTLET AIR OF PRE-HEATING COIL (COLD AND DRY WEATHER)

This paper presents a study on different kinds of air conditioning systems in comparison to existing one to use through of the year. Mainly the system imparts all three regular weather conditions. Like hot and dry, hot and wet and cool and dry. For this the room condition will be fixed 25℃ dry bulb temperature (DBT) and 50% relative humidity.

1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 4, April (2015), pp. 109-116© IAEME

109












THERMODYNAMIC ANALYSIS OF YEAR ROUND AIR
CONDITIONING SYSTEM FOR VARIABLE W ET BULB
TEMPERATURE OF OUTLET AIR OF PRE-HEATING
COIL (COLD AND DRY WEATHER)


Shankar Kumar
1
, S.P.S. Rajput
2
, Arvind Kumar
3

1
Research scholar, Department of Mechanical Engineering,
Maulana Azad National Institute of Technology, Bhopal, 462051 – India

2
Professor, Department of Mechanical Engineering,
Maulana Azad National Institute of Technology, Bhopal, 462051 – India

3
Assistant Professor, Department of Mechanical Engineering,
Maulana Azad National Institute of Technology, Bhopal, 462051 – India



ABSTRACT

This paper presents a study on different kinds of air conditioning systems in comparison to
existing one to use through of the year. Mainly the system imparts all three regular weather
conditions. Like hot and dry, hot and wet and cool and dry. For this the room condition will be fixed
25
℃ dry bulb temperature (DBT) and 50% relative humidity. In the present paper, for maintaining
room condition thermodynamic simulation is being done. If wet bulb temperature(WBT) of outlet air
of pre-heating coil cooling is increased(14.5 to 17°C) for cold and dry weather condition then the
year round air conditioning system equipments change own parameters like pre-heating coil
temperature, saturation efficiency of evaporative cooler and re-heating coil temperature to maintain
the room condition. The paper also shows the variation in supply condition of air and volume of
cellulose cooling pad.

Keywords: Pre-Heating Coil, Re-Heating Coil, Evaporative Cooler, Supply Condition.




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1. INTRODUCTION

Air conditioning system plays an important role in industry, infrastructure and domestic use.
For different weather condition different air conditioning systems are used. There is no such single
air conditioning system which gives constant output (comfort) throughout the whole year .The
existing air conditioning system works better only in the specified weather condition. While human
body requires comfort throughout the year. The human body can be considered as thermal machine
with 20% thermal efficiency. The remaining 80% must be disposed off from body to the
surrounding which depends upon temperature and humidity of surrounding otherwise the
accumulation of heat result and causes discomforts in the form of body pain, heat cramp or shocks,
heat stroke, heat exhaustion.
There are following psychometric process for air conditioning. (a) Heating and
humidification (b) Cooling and dehumidification (c) Cooling and humidification
Heating and humidification: In this process for humidification, evaporative cooler is used and
heating purpose heating coil is used.
Cooling and humidification: This process is used generally in summer air conditioning to
cool and humidify the air. In which air is first partial cool and humidify by evaporating cooling and
after that is cool by sensible cooling from cooling coil.
Cooling and dehumidification: This process is used in summer (rainy) season air conditioning
to cool and dehumidify the air. The humidification is done by two methods. (1)By desiccant wheel
and (2) By cool the cooling coil up to dew point temperature. The above problem of human body due
to exiting air conditioning system and psychometric process, year round air conditioning system is
required. It have following components.(1) Fan(2)Desiccant wheel (3)Cooling pad(4)Cooling
coil(5)Heating coil. In this paper mainly analysis has done according to the variation in wet bulb
temperature of outlet air of pre-heating coil or inlet air of evaporative cooler for cold and dry weather
condition. Shankar Kumar et al [1] worked on the actual position of equipment used in year round air
conditioning system. They also studied about the parameters on which the system depends.
F.Moukalled et al [2] reported, about the use of CFD for predicting and improving the performance
of air conditioning equipment .They reported a full numerical model for the concurrent forecast of
velocity, temperature and humidity of air flowing in an air conditioning unit.Zainab Hasson et al [3]
presented an efficient methodology to design modified evaporative air –cooler for winter air-
conditioning in Baghdad city. The performance is reported in terms of effectiveness of DEC,
saturation efficiency of DEC, outlet temperature of air and cooling capacity. E.velasco et al [4] study
about actual evaporating cooling method .They explain that when in an isolated system water and air
supposed to be in contact, if air gains enthalpy then water loose it, being cooled, while if air looses
enthalpy ,water would be heated .Thus in a process where air and water are in contact, water will
always tend to adiabatic saturation temperature. Kulkarni and Rajput [5] studied about the theoretical
performance analysis of cooling pads of different materials for evaporative cooler. The material
have been considered, rigid cellulose, corrugated paper, corrugated high density polythene and aspen
fiber .It has been observed that the saturation efficiency decreases with increasing mass flow rate of
air .It also seen that material with higher wetted surface area gives higher saturation efficiency.
Dowdy, J.A., Karbash,N.S.[7]obtained the heat and mass transfer coefficient by
experimentally for the evaporative cooling process through various thicknesses of rigid impregnated
cellulose evaporative medium. E.V.Gomez et al [8] studied about the comparison of high grade
energy required in air-conditioning system and evaporating cooling system. The weather data for
analysis has taken from IMD Bhopal [11] for the year 2013.

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
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111

Nomenclature
T Dry bulb temperature of air
h Enthalpy of air(kj/kg)
T
w Wet bulb temperature
W Specific humidity (g/kg)
R Ratio of ambient air mass to Re-circulated air mass
RHL Room heating load (kw)
RSHF Room sensible heat factor
RSHO Room sensible heat out
C
pa Specific heat of air at constant pressure (j/kg-k)
C
pv Specific heat of vapour at constant pressure (j/kg-k)
V Volume of cooling pad
B Bi-pass factor
T
c Cooling coil temperature
T
rh
Temperature of re-heat coil
T
ph Temperature of pre-heat coil
P % mixing of re-circulated air to atmospheric air
M
m Mass flow rate of air in duct after mixing (kg/s) of ambient air and re-circulating air

Greek letters
φ Relative humidity of air(%)

ρ Density of air(kg/m³)
ɳ Saturation efficiency of evaporative cooler (%)

Subscripts
Ph pre-heating coil
rh re-heating coil
c cooling coil
phi pre- heating coil inlet
pho pre –heating coil outlet
ei evaporative cooler inlet
eo evaporative cooler outlet
ci cooling coil inlet
co cooling coil outlet
rhi re-heat coil inlet
rho re-heat coil outlet
a ambient
s supply
r room
m mixed air

2. METHODOLOGY

Following assumptions and equations have been taken for analysis.

2.1. Assumption
(1)The wet bulb temperature before and after the evaporative cooler will be constant.
(2)The cooling coil and heating coil be only used for sensible cooling and sensible heating.
(3)The mass flow rate (or air velocity) will be same throughout the duct.

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
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112

(4) Axial heat conduction and water vapour diffusion in the air is negligible.
(5)The channels are assumed adiabatic and impermeable.
(6)The heat and mass transfer coefficient are constant.

2.2 Equations

2.2.1 Equation of specific humidity for mixed air.
111110
5 =
346 86
m86
(1)

2.2.2 Equation of enthalpy for mixed air [9].
ℎ
5=
34h 8h
m83
(2)


2.2.3 Equation of DBT for mixed air [9].
1111111o
5=
ehrjftrmcr)
(..×
r)
(3)

2.2.4 Equation of pre-heat coil temperature [9].
111111o
=
(
×r
)
(
)
(4)

2.2.5 Equation of enthalpy for supply air [9].
ℎ
=
(!r4h"#)
!
r
(5)

2.2.6 Equation of DBT for supply air [9].
1111111o
2 o
$+
(&"#)
(.'(')*×!
r)
(6)

2.2.7 Equation for saturation efficiency of evaporative cooler [4, 10].
+ =
(
,-,)
(
,-
.,-)
(7)
Where

o
/02 o
12341o
5/02 o
5/12 o
51

2.2.8 Equation for volume of cooling pad.[4]
6
=
(7 86
,-×78)×!r×9:(;)
esh
<×(*)
(8)

Where

111111110
12 0
52 0
/0

2.2.9 Equation of re-heating coil temperature.
o
$h=
(
×,=)
(
)
(9)

3.2.10 Equation for percentage mixing of re-circulated air with atmospheric air.
> =

em85
?@5A)
=

()
(10)

5. International Journal of Mechanical Engin
ISSN 0976 – 6359(Online), Volume 6, Issue 4, April (2015), pp.

2.2.11 Equation for temperature of cooling coil.
111111o
B=
(=×,)
()


2.2.13 Equation for velocity of air
C =
!r
(D ×E)


3. SYSTEM DESCRIPTION

The positions of equipments used in year round air conditioning system are shown in the
diagram of year round air conditioning system(Fig.1).The analysis is done on the concept that,
supply air condition is depends on the room conditions, mass flow rate,
We take the room conditions is constant. So the supply condition will be depends on mass flow rate,
CLC or RHL and RSHF. The main equipments like pre
heating coil of year round air conditionin
condition(shown in Fig.2). In this weather condition, the mixed air is passed through pre
coil in which air is heated at constant specific humidity ,after that air is passed through evaporative
cooler in which air is humidified at constant wet bulb temperature and lastly air is heated up to
required supply condition by re-heating coil.



Fig.1. Diagram of year round ai

Fig.2. Diagram used for cold and dry weather condition

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976
6359(Online), Volume 6, Issue 4, April (2015), pp. 109-116© IAEME
113
.2.11 Equation for temperature of cooling coil.

.2.13 Equation for velocity of air

The positions of equipments used in year round air conditioning system are shown in the
diagram of year round air conditioning system(Fig.1).The analysis is done on the concept that,
supply air condition is depends on the room conditions, mass flow rate, CLC
constant. So the supply condition will be depends on mass flow rate,
The main equipments like pre-heating coil ,evaporative cooler and re
heating coil of year round air conditioning system are used for cold and dry weather
In this weather condition, the mixed air is passed through pre
coil in which air is heated at constant specific humidity ,after that air is passed through evaporative
n which air is humidified at constant wet bulb temperature and lastly air is heated up to
heating coil.
Diagram of year round air conditioning system
Diagram used for cold and dry weather condition
eering and Technology (IJMET), ISSN 0976 – 6340(Print),
© IAEME
(11)
(12)
The positions of equipments used in year round air conditioning system are shown in the
diagram of year round air conditioning system(Fig.1).The analysis is done on the concept that,
CLC or RHL, and RSHF.
constant. So the supply condition will be depends on mass flow rate,
heating coil ,evaporative cooler and re-
g system are used for cold and dry weather
In this weather condition, the mixed air is passed through pre-heating
coil in which air is heated at constant specific humidity ,after that air is passed through evaporative
n which air is humidified at constant wet bulb temperature and lastly air is heated up to



Diagram used for cold and dry weather condition

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online), Volume 6, Issue 4, April (2015), pp. 109-116© IAEME

114

4. RESULTS AND DISCUSSION

The result are found for the variation in WBT of outlet air of pre-heating coil for the range
13.85°C to 17.72°C.The results and their discussions are given below.

Twpho 14.5 15 15.5 16 16.5 17
Tm(°C) 17.90712 17.90712 17.90712 17.90712 17.90712 1 7.90712
Tpho(°C) 21.94 23.33 24.72 26.22 27.67 29.17
Tph(°C) 22.94822 24.68572 26.42322 28.29822 30.11072 3 1.98572
Teo(°C) 14.78 16.17 17.5 19 20.5 21.94
Trh(°C) 30.29815 29.95065 29.61815 29.24315 28.86815 2 8.50815
Ts(°C) 27.19452 27.19452 27.19452 27.19452 27.19452 2 7.19452
Wm(g/kg) 7.922 7.922 7.922 7.922 7.922 7.922
Wpho(g/kg) 7.922 7.922 7.922 7.922 7.922 7.922
Weo(g/kg) 10.88304 10.88304 10.88304 10.88304 10.88304 10.88304
Ws(g/kg) 10.88304 10.88304 10.88304 10.88304 10.88304 10.88304
η (%) 96.23656 85.95438 78.30803 70.64579 64.18979 5 9.40838
V(cm³) 5523.683 3305.718 2573.74 2064.3 1729.5 1518 .429

Table 5: variation of output parameter w.r.t. T
wpho at 60% mixing of re-circulated air, 7°C
DBT and 60 % relative humidity of ambient air, 523m altitude , 25°C DBT and 50% relative
humidity of room, 1.25kg/s mass flow rate, 3.5KW RHL Value, 0.9 RSHF and 0.2 BPF.


Fig3: Variation in T
m, Tph, Tpho, Teo, Trh and Ts w.r.t. Twpho


Fig4: Variation in saturation efficiency of evaporative cooler w.r.t. T
wpho
10
15
20
25
30
35
14 15 16 17 18
Temperatures(°C)
T
wpho(°C)
Tm
Tph
Tpho
Teo
Trh
Ts
0
20
40
60
80
100
120
14 15 16 17 18Saturation efficiency(%)
T
wpho(°C)
Twpho

7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
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Fig5: Variation in volume of cooling pad of evaporative cooler w.r.t. T wpho

The figure (3) shows that the variation in WBT of outlet air of pre-heating coil (T
wpho) has no
effect on the inlet condition of pre-heating coil because inlet condition of pre-heating coil only
depends on the room condition, ambient condition and mixing ratio of air and these parameters are
taken to be constant. The pre-heating coil heated the mixed air sensibly and T
wpho increases i.e.
enthalpy of the outlet air of pre-heating coil increases. In other ways we can say that T
pho increases.
The inlet air DBT of pre-heating coil and BPF of pre-heating coil are constant, where as T
pho
increased, it means increase in T
ph. The result shows that the supply condition depends on the mass
flow rate, room condition, room heating load (RHL) and room sensible heat factor (RSHF) value and
these parameters are also taken to be constant. In our simulation the outlet air specific humidity of
evaporative cooler remains constant and the operating wet bulb temperature of evaporative cooler
increases. So the DBT of outlet air of evaporative cooler increases also. The DBT of inlet air of re-
heating coil or outlet air of evaporative cooler increases and the outlet air DBT of re-heating coil or
supply air DBT is constant as well as BPF of re-heating coil also taken to be constant .Hence with
the increasing in T
wpho the re-heating coil get more heated air i.e. the required heating work of re-
heating coil will be less. It means the re-heating coil temperature decreases. The specific humidity of
mixed air and the outlet air of pre-heating coil is same (Table: 1) because only sensible heating is
done by the pre-heating coil. Similarly for same region of re-heating coil, specific humidity of supply
air and the evaporative cooler outlet air is same. With the increase in T
wpho at constant inlet and
outlet specific humidity of air for evaporative cooler, the outlet DBT of evaporative cooler also
increases. The increasing rate of T
eo is more than Twpho, so the saturation efficiency of evaporative
cooler decreases (Fig: 4). The volume of evaporative cooling pad decreases because saturation
efficiency decreases (Fig:5). When saturation efficiency decreases then less evaporation phenomena
is done by cooler. Less evaporation means the flowing air has lesser contact time with water for
same mass flow rate and which is possible when thickness or volume of cooling pad will be less.

5. CONCLUSION

After going through thermodynamic analysis of year round air conditioning system for
different value (14.5 to 17°C) of WBT of outlet air of pre-heating coil (cold and dry weather), the
following conclusions were made:

• The analysis of year round air conditioning system depends mainly on the performance of the
evaporative cooler, heating coil and cooling coil.
• DBT of supply air remains constant and their value is 27.19°C.
0
1000
2000
3000
4000
5000
6000
14 15 16 17 18Volume of cooling pad(cm³)
T
wpho(°C)
Twpho

8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
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• The specific humidity of supply air also remains constant and their value is 10.88g/kg.
• The volume of cellulose cooling pad decrease from 5523.68to 1518.42cm
3
.
• The saturation efficiency of evaporative cooler decreases from 96.23to 59.40%.
• The pre-heating coil temperature increases from 21.94 to29.17°C.
• The re-heating coil temperature decreases from 30.29 to 28.50°C.
• T
pho increases from 21.94 to 29.17°C for constant BPF 0.2 of pre-heating coil.
• Specific humidity of supply air and evaporative cooler outlet air is equal and their values are
10.88g/kg.
• Specific humidity of inlet and outlet air of pre-heating coil is same and which is equal to
7.92g/kg.

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