1) Mechanical draft cooling towers use fans to circulate air through the tower to maximize heat transfer between water and air. There are two main types - counter-flow and induced draft. 2) Counter-flow towers have water flowing down and air flowing up, while induced draft towers have water flowing in at the top and air drawn in at the bottom and exiting at the top. 3) The operational cost of cooling towers includes electricity costs from pumps and fans circulating water and air, as well as water costs which are influenced by makeup water usage from evaporation and blowdown. Minimizing these costs can improve efficiency.

1. Cost of Running an Industrial Mechanical Counter-Flow Cooling
Tower
Chukwuchendo, E. & Mukengeshayi, L.M.
(ENSIGHT ENERGY SOLUTIONS)
Introduction
A cooling tower is simply a device that rejects waste heat to the atmosphere through a cooling fluid at a lower
temperature. The concept of cooling towers was developed many years ago through the establishment of
condensers for use with steam engines. Industrial machines and processes generate enormous amount of heat
which is required to be dissipated if the machines and processes are to continue to operate efficiently. The
application of cooling towers and their significance are essential in industrial processes and/or facilities (buildings).
The use of cooling tower together with a chiller have shown greater significance in their applications. The main
purpose of the cooling tower is to reduce the temperature of circulating water for reuse in condensers and other
heat exchange equipment.
Cooling towers fall into two main categories based on airflow namely; natural/passive draft and mechanical draft.
Not all types are suitable to applications for every heat load configuration. Since there is a high degree of variation
in cooling towers, knowledge of the different types, together with their benefits and restrictions can be significance
when in use. In this paper, only mechanical draft cooling towers are discussed.
Mechanical Draft Cooling Towers
This type of cooling towers consists of walled enclosures provided with means for spraying water through and
many various baffles over which the water may drip through the tower. This method has an improved cooling rate
as it has a prolonged contact time between the water and the air, hence, maximizing the heat transfer between the
two fluids media. Mechanical draft cooling towers uses either single or multiple fans for circulating air through the
spray and around the baffles. These fans, in-turn, either blow air into the tower or suck air through the tower
(Brainard 1942). There is greater stability in this type of towers due to their greater thermal performance and is
affected by fewer psychometric variables compared to natural draft towers. The cooling capacity is controlled by
means of regulating the air flow in order to compensate for varying atmospheric and load conditions by fan capacity
manipulation (Hensley 1985). The cooling rates of mechanical draft towers depends on the diameter of the fan
and as well as the speed of operation. Mechanical draft cooling towers can be categorised into three main types
of cooling towers based on the manner in which both water and air flows through them. These types include
counter-flow draft, cross-flow and hyperbolic. In order to achieve a desired cooling water temperature, a large
volume of air is required. The effect of cooling air is, in turn, influenced by the humidity of the air and as well as
its temperature (Brainard 1942). The following types of cooling towers are commonly used in the petrochemical,
oil refineries and other chemical plants:

2. 1 Counter-flow forced draft
This type of towers is characterized by high air entry and low exist velocities. The in-flow of air travels in a vertical
path over the splash fill as the water streams down from the reservoir above. This creates difficulties due to water
distribution and maybe susceptible to recirculation, thereby considered to have less performance stability, as it can
be affected by severe icing. Relative to cross flow cooling towers, this system setup is expensive due to the fact
that more energy is required to push air upward against down-flowing water.
Figure 1: Counter-flow forced draft
The fan in forced draft towers is located at the ambient air stream entering the tower, with air blown through the
tower. The tower is generally noisier than towers with axial flow fans and it is incorporated with fills. The tower is
also equipped with centrifugal fans with high resistance capability.
2 Counter-flow induced draft
This type of towers is categorized by high air exist and low entry velocities. It requires little or no inclination for a
reduced pressure zone to be created at the air inlets by the fan, thereby, less prone to recirculation. It has lower
drift potential and large fans with low speed & low noise. The potential for recirculation in this type of tower is not
self-initiating, and it can be easily quantified on the basis of ambient wind conditions (Hensley 1985). The hot
water is injected into the tower with a spray distribution header at the top while the air is introduced at the bottom
and at air exist at the top. The air outlet draws air upward through the tower and cools the water. Induced draft
cooling towers have an air discharge velocity of 3 to 4 times higher than their entrance air velocity. The location of
the fan within the warm air stream provides excellent protection against the formation of ice on the mechanical
components, however the presence of moisture is a source of possible corrosion. Therefore, such setup will
require weather-proofing for corrosion prevention. This type of cooling of tower can operate with or without fill.

3. (a) (b)
Figure 2: Induced draft (a) with fill, and (b) without fill
Operations of Cooling Towers and Chillers
The application of cooling towers evidently varies, and different types are applicable to different load configurations.
The mostly widely used type of cooling tower is the mechanical draft. Seemingly, the operating characteristics of
all cooling towers are governed by the laws of thermodynamics, psychometrics and physics, and these laws maybe
describe occasionally for the purposes of promoting comprehensive understanding and as well as the advantages
and limitations to the prospect user.
The heat into the cooling water comes from the condenser side of the centrifugal chiller, this heat is then cooled
and return to the chiller to keep the chiller cooled on the side of the refrigeration cycle. The cooled water from the
chiller is supplied to the facilities, the chilled water loop absorbs heat from the facilities and then returns it to the
chiller. The evaporator in the chiller absorbs the heat from the chilled water loop and rejects the heat along with
the heat of compression to the condenser. This heat is transported from the condenser to a cooling tower, where
the heat is transferred to the atmosphere.
The cooling tower efficicency is a function of the wet-bulb temperature. The higher the entering wet bulb
temperature, the higher the required nominal condenser water flowrate and the tighter the approach needs to be
in order to achieve optimum system energy usage. The performance of a cooling tower is often expressed in terms
of range and approach as these parameters both affects the performance of a cooling tower. The “approach” is
the temperature difference between the cooled water leaving the tower and ambient wet bulb temperature. The
cooling “range” is difference between the hot water into the cooling tower and the temperature of the cold water
leaving the tower. Both the range and the approach should be monitored, however, an approach is a better indcator
of cooling water performance.

4. Operational Cost in Cooling Towers
To achieve savings on an existing system, a thorough economic analysis must be performed. The findings from
the techanical analysis performance of a given system will subsequently unwind opportunities to minimize energy
conumption (cost), while maintaining and/or improving efficiceny. In a cooling tower, minimizing the consumption
of energy is essential. There has been several literatures on cooling tower, and a number of them reported on
approach aimed at minimzing the operational cost. At the supply water side, measuring the flowrate of water, its
temperature and temperature of the returned water on the return water side is important. Once this measurement
are performed, the sensible heat can be determined from Eq. (1).
𝑄 = 𝑚̇ 𝐶 𝑝(𝑇 𝑤𝑖𝑛 − 𝑇 𝑤𝑜𝑢𝑡) (1)
As described in 0, the following cooling tower performance indications can be calculated as follows:
𝑅𝑎𝑛𝑔𝑒 = 𝑇 𝑤,𝑖𝑛 − 𝑇 𝑤,𝑜𝑢𝑡 (2)
𝐴𝑝𝑝𝑟𝑜𝑎𝑐ℎ = 𝑇 𝑤,𝑜𝑢𝑡 − 𝑡 𝑎,𝑤𝑏,𝑖𝑛 (3)
𝑁𝑇𝑈 =
ℎ 𝑐.𝐴
𝑐 𝑝,𝑚
(4)
𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑑𝑒𝑔𝑟𝑒𝑒 (𝜂 𝐶𝑇) =
𝑇 𝑤,𝑖𝑛−𝑇 𝑤,𝑜𝑢𝑡
𝑇 𝑤,𝑖𝑛−𝑡 𝑎,𝑤𝑏,𝑖𝑛
(5)
The model used by Cortinovis, Paiva et al. (2009) and Castro, Song et al. (2000) is as follows:
𝐶 𝑇 = 𝐶𝑒 + 𝐶𝑐𝑤 (6)
Where the operational cost CT, is composed of the electricity cost and cooling water cost. However, the elecricity
cost Ce is composed of pumping cost Cpump for the cooling water circulation and the fan operation cost. Cfan. This
information together with eq. (6) yields Eq. (7).
𝐶 𝑇 = 𝐶 𝑝𝑢𝑚𝑝 + 𝐶𝑓𝑎𝑛 + 𝐶𝑐𝑤 (7)
Moreso, the cooling water is a function of the make-up flowrate. A derivation will therefore yields Eq. (8), where
Ce is defined as the cost coefficicent of electricity and Ccw as cost coefficicent of cooling water,and with wmu, Ppump
& Pfan as water make-up flowrate, pump power and fan power respectively.
𝐶 𝑇 = 𝐶𝑒(𝑃𝑝𝑢𝑚𝑝 + 𝑃𝑓𝑎𝑛) + 𝑐 𝑐𝑤. 𝑤 𝑚𝑢 (8)
with 𝐶𝑐𝑤 = 𝑐 𝑐𝑤. 𝑤 𝑚𝑢 (9)
The importance of make-up flow rate is due to water loss by evaporation (we), entrainment (wd), blow down (wp)
and hot water intentionally removed (wr). Thus, the make-up flow rate (wmu) of the system can then be calculated
from Eq. (10). The water loss due to evaporation being the main factor in the process of water cooling can be
estimated using Eq. (11). The water flowrate through the tower is defined as w’.

5. 𝑤 𝑚𝑢 = 𝑤𝑒 + 𝑤 𝑑 + 𝑤 𝑝 + 𝑤𝑟 (10)
𝑤𝑒 = 0.00153 . 𝑤′
. (𝑇 𝑤𝑖 − 𝑇 𝑤𝑜) (11)
The power consumed by fan is the function of air flowrate and can be calculated from the expression in Eq. (12).
The pump power is related with the characteristic pump curve, and its effiicency ŋp as shown in Eq. (13).
𝑃𝑓𝑎𝑛 = 0.0548 [
𝑤 𝑎𝑖𝑟
𝜌 𝑎𝑖𝑟
] (12)
𝑃𝑝𝑢𝑚𝑝 = 1283
1
𝜂 𝑝
[
𝑤 𝑒
𝜌
]
0.476
(13)
References
Brainard, D. D. (1942). Mechanical draft cooling tower, Google Patents.
Castro, M. M., et al. (2000). "Minimization of Operational Costs in Cooling Water Systems." Chemical
Engineering Research and Design 78(2): 192-201.
Cortinovis, G. F., et al. (2009). "A systemic approach for optimal cooling tower operation." Energy Conversion
and Management 50(9): 2200-2209.
Hensley, J. C. (1985). Cooling tower fundamentals.
Schwedler, M. C., et al. (1997). Near optimization of cooling tower condenser water, Google Patents.