Four students designed and modeled a counter flow cooling tower for an air conditioning system with 578 ton cooling capacity. They developed a thermodynamic model based on Merkel's theory and implemented it in a computer program. Their model determined the optimal water to air mass flow ratio matched the cooling tower characteristic and packing function. Their results showed the cooling tower area increases with higher inlet air humidity and larger temperature range.

1. DESIGN AND COMPUTER MODELING OF
COOLING TOWER FOR AIR-CONDITIONING
SYSTEM
Project Members:
1) Jehanzeb Hassan (11-ME-057)
2) Kamran Waris (11-ME-059)
3) Khawar Mehmood (11-ME-060)
4) Muhammad Danial (11-ME-083)
Project Supervisor:
Dr. Abdul Waheed Badar
Project Co-Supervisor:
Engg. Usman Manzoor
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 1

2. MOTIVATION
 Cooling towers is an integral component of a number of applications,
such as in HVAC systems and other industrial processes
 Need for the development of more efficient cooling towers
 Lack of detailed mathematical/analytical modeling procedure in
literature for designing of cooling towers
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 2

3. OBJECTIVES
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 3
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
 To design a Counter Flow Direct Contact Cooling Tower for an existing water
cooled air-conditioning system having a cooling capacity of 578 TR(cross flow)
 To develop a detailed thermodynamic model of the Cooling Tower
 To implement the model in a computer program
 To perform a parametric analysis to study the effects of various key design
parameters

4. INTRODUCTION
 Cooling tower is the equipment used to reduce temperature of a water stream
using evaporative cooling.
 Theoretically lowers water temperature to wet bulb temperature of inlet air.
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 4
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
Cooling
tower
Natural
draft
Forced draft
Induced
draft
Mechanical
draft
Counter
flow
Cross flow

5. INDUCED DRAFT COUNTER FLOW CT
 Air enters at bottom, leaves at top.
 Components:
—Fill: slats which provides heat transfer area.
—Air inlet: present near the bottom of
cooling tower for air to enter into cooling
tower.
—Water basin: attached to bottom of
cooling tower to collect the water cooled
down by cooling tower.
—Fan: to draw in air(induced draft)
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 5
Design And Computer Modeling Of Cooling Tower For Air Conditioning System

6. DESIGN OF COOLING TOWER
Background:
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 6
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
Merkel’s theory- based on a falling water drop surrounded by a film of
saturated air having same enthalpy as water and exchanges heat and
mass through that film with upstream of unsaturated air.

7. Merkel Theory Mathematical Modeling:
Mathematical form of Merkel Theory:
(
𝑲𝒂𝑽
𝑳
) 𝒎𝒆𝒓𝒌𝒆𝒍= Cp,w * 𝑪𝑾𝑻
𝑯𝑾𝑻 𝒅𝑻
(𝒉𝒔−𝒉𝒂)
(KaV/L)merkel = Tower characteristic- also called Merkel number.
Cp,w = Specific heat capacity of water
hs= Saturation enthalpy of air water mixture
ha= Air enthalpy at exit
dT= Water temperature difference
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 7
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
DESIGN OF COOLING TOWER

8. Packing Function:
A function of:
—Fill height.
—Water to air mass flow rate (L/G).
—Type of packing used.
(
𝑲𝒂𝑽
𝑳
) 𝒑𝒂𝒄𝒌𝒊𝒏𝒈= BZ(
𝑳
𝑮
)−𝒏
(KaV/L)packing= Packing Function
Z= Fill height
B, n = Fill coefficients- determined based fill type
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 8
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
DESIGN OF COOLING TOWER

9. Optimal Point For Design:
For a cooling tower based on Merkel Theory
(
𝐾𝑎𝑉
𝐿
) 𝑚𝑒𝑟𝑘𝑒𝑙 = (
𝐾𝑎𝑉
𝐿
) 𝑃𝑎𝑐𝑘𝑖𝑛𝑔
—For optimal design conditions value of Merkel Number should
essentially be equal to packing function.
—Given the fill type, above equation can give optimal ratio of
mass flow rate of water to air.
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 9
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
DESIGN OF COOLING TOWER

10. Outlet Air Properties:
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 10
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
Determination of Outlet air density-using outlet air properties.
Determination of outlet air-enthalpy:
(ℎ𝑎) 𝑜𝑢𝑡= (ℎ𝑎)𝑖𝑛𝑙𝑒𝑡+((L/G)*𝐶 𝑝,𝑤*(𝑇 𝑤,𝑖𝑛𝑙𝑒𝑡 - 𝑇 𝑤,𝑜𝑢𝑡𝑙𝑒𝑡))
Determination of outlet air humidity-Finite Element Method
Δω =
𝐿
𝐺
x Δℎ𝑙 x
(ω 𝑠
−ω)
(ℎ 𝑠−ℎ 𝑎)
ω 𝑜𝑢𝑡 = ω𝑖𝑛𝑖𝑡𝑖𝑎𝑙 + Δω
DESIGN OF COOLING TOWER

11. Pressure Drop:
Total pressure drop in a mechanical draft cooling tower.
𝜟𝑷 𝒕𝒐𝒘𝒆𝒓 = 𝜟𝑷 𝒔𝒕𝒂𝒕𝒊𝒄 + 𝜟𝑷 𝒅𝒚𝒏𝒂𝒎𝒊𝒄
𝜟𝑷 𝒔𝒕𝒂𝒕𝒊𝒄 = 𝝆 𝒂,𝒊𝒏 − 𝝆 𝒂,𝒐𝒖𝒕 ∗ 𝐠 ∗ 𝐡
𝜟𝑷 𝒅𝒚𝒏𝒂𝒎𝒊𝒄 =
𝟏
𝟐𝝆 𝒂,𝒐𝒖𝒕
∗ (
𝑮
𝒏 𝒇 𝑨 𝒇
) 𝟐
ρa= density of air
g= gravitational acceleration
G= mass flow rate of air
Af= fan exit area
nf = number of fans
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 11
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
DESIGN OF COOLING TOWER

12. Tower Cross-sectional Area Calculation
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 12
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
Determination of Area Area=
𝒎 𝒘
𝑴 𝑳
Determination of water-loading- Mass flow rate of water per unit area of tower.
𝑴 𝑳 = 𝑴 𝒂 ∗
𝑳
𝑮
Determination of air-loading – Mass flow rate of air per unit area of tower.
𝑴 𝒂 =
𝟐∗𝝆 𝒂,𝒎𝒆𝒂𝒏∗𝜟𝑷 𝒕𝒐𝒘𝒆𝒓
𝑲∗𝒁
Z= fill height , ρa= density of air
K= correction factor
DESIGN OF COOLING TOWER

13. Numerical Modeling Flow Diagram
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 13
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
Determination of
outlet air
properties using
Finite Difference
Method
Pressure Drop
calculations
ΔPtotal= ΔPstatic +
ΔPdynamic
Calculation of
Area using
pressure drop
Design of various
cooling tower
components
Determination of the packing
function with different L/G
(Ip)
Ip=𝑩𝒁(
𝑳
𝑮
)−𝒏
Determination of L/G
required for design point by
plotting
Ip, Im vs L/G
Input parameters
1. Dry bulb temperature (oC)
2. Wet bulb temperature (oC)
3. Hot water temperature (oC)
4. Cold water temperature (oC)
5. Relative humidity (%age)
6. Water flow rate (Kg/s)
Calculation of tower characteristic (NTU)
also called Merkel number at different L/G
ratio
𝑲𝒂𝑽
𝑳
= 𝑪 𝒑,𝒘 ∗
𝒅𝑻
𝒉 𝒔 − 𝒉 𝒂
𝑻 𝒘,𝒉𝒐𝒕
𝑻 𝒘,𝒄𝒐𝒍𝒅

14. MODELING RESULTS AND DISCUSSION
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 14
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.3 0.6 0.9 1.2 1.5
Ip,Im
L/G
Merkel number/Packing function vs L/G
Merkel number
Packing Function
Optimal value of
water mass flow rate
to air flow rate

15. DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 15
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
0
5
10
15
20
25
30
35
40
45
15 25 35 45 55 65
Area(m2)
Relative Humidity (%age)
Relative Humidity Of Ambient Air Vs Tower Cross-
Sectional Area
MODELING RESULTS AND DISCUSSION
Parametric Study Results
Cross sectional area of cooling
tower,
—Increases as relative humidity
of inlet air increases.
—Decreases as relative humidity
of inlet air decreases.

16. DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 16
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
0
2
4
6
8
10
12
14
16
18
2 3 4 5 6 7 8 9 10
Area(m2)
Range (oC)
Tower Cross Sectional Area Vs Range
MODELING RESULTS AND DISCUSSION
Parametric Study Results
Cross sectional area
of the cooling tower
has a direct relation
with range of cooling
tower.

17. COOLING TOWER DIMENSIONS
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 17
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
MODELING RESULTS AND DISCUSSION
Single cell dimensions
Total number of cells=2

18. Discussion
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 18
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
Compared to cross flow cooling tower:
—Is compact in sizing
—Consumes more power
—Higher cost of operation
Limitations of Merkel theory
—Does not incorporate evaporative losses
—Heat transfer from water to film is proportional to heat transferred
from film to bulk air.

19. —Counter Flow type cooling tower for air-conditioning system having
578TR cooling capacity is mathematically modeled in a step-wise
manner based on Merkel Theory
—Model was implemented in a computer program for detailed
parametric analysis.
—Heat transfer volume= 19 m3 which is less than existing cooling tower
—It is found that area increases with increasing RH and range.
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 19
Design And Computer Modeling Of Cooling Tower For Air Conditioning System
CONCLUSIONS

20. THANK YOU
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 20

21. QUESTIONS?
DEPARTMENT OF MECHANICAL ENGINEERING, HITEC UNI 21