Refrigeration & Air-Conditioning

Module 3. Refrigeration plant components

Lesson 13
COOLING TOWERS AND SPRAY PONDS-TYPES, CONSTRUCTION, WORKING AND MAINTENANCE

13.1 Introduction

The purpose cooling tower is to cool the water used in the water cooled condenser of the refrigeration plant so that the same water can be reused to absorb the heat at the condenser. The basic principle of cooling of water is the evaporative cooling achieved by using atmospheric air. The condenser water is sprinkled through nozzles and air on passing over the water droplets creates evaporative cooling of water. The heat lost from the water is carried by the air which increases the enthalpy of air leaving the cooling tower. The temperature drop achieved depends on the following factors.

• Surface area of water exposed to air stream.
• Dry bulb and wet bulb temperature of the air.
• Velocity of air.
• Water inlet temperature
• Direction of air flow in relation to water
• Contact time period of air with water.

Theoretically, it is possible to cool the water up to wet bulb temperature (wbt) of the air entering the cooling tower. Generally, the temperature of water coming out from the cooling tower is 3 ºC to 5ºC above the wbt of the entering air. It is very important to operate the cooling tower at optimum efficiency to get water at lowest possible temperature for the condenser.

13.2 Terminology Used in Cooling Tower

13.2.1 Range of cooling tower

The difference between the temperature of water entering the cooling tower and temperature of water leaving the cooling tower is known as range of cooling tower.

13.2.2 Cooling tower approach

The difference between the temperature of water leaving the cooling tower and wbt of entering the air is called cooling tower approach.

13.2.3 Wind ward and lee ward side of cooling tower

The side from which ambient air enters the cooling tower is called wind ward side and the opposite side of wind ward is called lee ward side of the cooling tower. It varies as per the direction of ambient air in different seasons of the year. While evaluating the performance of cooling tower, it is essential to know the wind ward and lee ward side to measure the psychrometric properties of air as per the requirement.

13.2.4 Cooling tower capacity (load)

The rate of heat rejected at cooling tower is the measure of cooling tower capacity.

Cooling tower load (kJ/h)

=

Mass flow rate of water, kg/h

×

Specific heat of water, kJ/kg K

×

(t1 – t2)

Where,

t₁ = temperature of water entering the cooling tower, K

t₂ = temperature of water leaving the cooling tower, K

13.2.5 Efficiency of cooling tower

The efficiency of cooling tower is defined as the ratio of actual drop in temperature of water to the ideal drop in the temperature of water.

η (%) = (t₁-t₂/t₁-wbt₁ ) X 100

Where, wbt of entering air (on wind ward side)

The efficiency of cooling depend on several factors such as surface area of water exposed to air stream, dbt and wbt of the air, velocity of air, flow rate of water, in coming water temperature etc.

Efficiency values obtained in commercial of cooling tower are as under.

Table 13.1

Types of cooling tower	Efficiency (%)
Spray ponds	50-60
Natural draft cooling tower	50-75
Mechanical draft cooling tower	70-90

The temperature of water leaving the cooling tower can be approximated by the following empirical formula.

t₂ = t₁+dbt₁+2wbt₁ / 4

Where, dbt₁ = dry bulb temperature on wind ward side

wbt₁ = wet bulb temperature on wind ward side

The quality of air change when it passes through the cooling tower is depicted in ) The quality of air on wind ward side and leeward side is at A and B respectively as indicated on psychrometry chart. The enthalpy of air increases when air passes through the cooling tower. It is possible to determine the quantity of air required for the cooling tower based on the change of enthalpy of air when air passes through the cooling tower.

13.3 Types of Cooling Tower

There are two main categories of cooling towers.

1. Natural draft cooling tower

2. Mechanical draft cooling tower

13.3.1 Natural draft cooling tower

The natural draft atmospheric cooling tower consists of a sump and arrangement for the spray of water from the height of about 5 m to 8 m . It is necessary to provide side louvers to minimize carry over water loss. The warm water from the condenser is nuzzled and the cooled water is collected in the sump constructed at the bottom. The performance of this cooling tower varies due to variation of air velocity and it requires more space for the installation of the cooling tower.

13.3.2 Mechanical draft cooling tower

In case of mechanical draft, fan or blower is used to supply the air through the cooling tower. In recent design of the cooling tower, warm water is passed over the supporting medium in form of thin sheet and air is blown through the cooling tower. Induced draft cooling is preferred over the forced draft cooling tower as air distribution is more uniform in induced draft cooling tower.

13.4 Heas and Mass Balance of Cooling Tower

The heat from the water is transferred to air in cooling tower. Thus,

Heat lost by water	=	Heat gained by air
mw· Cpw· (t1– t2)	=	ma· (h2 – h1)

Where, m_w = mass of water flow in the cooling tower, kg/h

m_a= mass of dry air flow in the moist air, kg/h

C_pw = Specific heat of water, kJ/kg K

h₁= Enthalpy of air entering the cooling tower, kJ/ kg dry air

h₂ = Enthalpy of air leaving the cooling tower, kJ/ kg dry air

The above equation is used to find the amount of air required for the cooling tower by considering the load of the tower and the enthalpy of air on wind ward and lee ward side of the cooling tower. Enthalpy of air can be obtained from psychrometric chart by measuring dbt and wbt of the respective air.

13.5 Maintenance Aspects of Cooling Tower

· Use the soft water as make up water to compensate the quantity of water evaporated.

· Cleaning of nozzles, filling material, sump etc. is necessary at regular interval.

· Replacement of cooling tower water at regular interval as salt concentration of water increases due to evaporation of water.

· Follow general maintenance requirement of pumps, fans etc.

· Adopt corrosion control measures depending on the material of construction of the cooling tower.

13.5.1 Example

Water is cooled in a natural draft cooling tower. The temperature of water supplied to cooling tower is 35 ºC and ambient conditions are 30 ºC dbt and 24 ºC wbt. Find the efficiency of the cooling tower.

Solution

Using the equation to find the temperature of water leaving the cooling tower, ºC

t₂ = t₁+dbt₁+2wbt₁ / 4

= (35+30+2) * 24 / 4

= 28.25 ºC

The cooling tower efficiency can be calculated using the following equation.

η (%) = (t₁-t₂/t₁-wbt₁ ) X 100

η (%) = = 59.09 %