Cooling Tower efficiency calculation | Cooling Tower Approach

Cooling Tower efficiency calculation

Cooling Tower – Cooling tower is a crucial part when it comes to running a chemical process plant as cooling tower is necessary for cooling water utility in the plant. There are different types of cooling tower which we had discussed in previous post along with working principle of cooling tower. In this article we will discuss important cooling tower terms and cooling tower glossary. Not only terms but we will see cooling tower calculations i.e. cooling tower efficiency calculation and we also learn about cooling tower approach, cooling tower approach temperature, cooling tower blowdown etc.

Important Terms of Cooling Tower

Drift: During cooling tower operation, some water droplets get entrained and carried out to atmosphere along with air which comes from the bottom. This result in water loss of cooling tower. It is independent water lost by evaporation.

Concentration  : The process of increasing solids per unit volume of solution.concentration of liquid in cooling towers according due to evaporation that cools the water.

Blow Down ; To maintain TDS/concentration of salt water discharged/removed  from the system of cooling tower is known as cooling tower blowdown water.

Evaporation Loss : During operation, due air and hot water inter Water evaporated from circulating water into atmosphere.

Drift eliminators : To Minimize the water drifted with air, drift eliminator and hence water loss in cooling water.

Cooling water : Water circulated through cooling the system to cooling system remove heat from areas.

Exhaust Air : The mixture of air and its vapor leaving the cooling tower system.

Louvers : Horizontals in a cooling tower wall to provide the opening through which enters into system falling water within the tower.

Nozzles: Hot water is sprayed through this device to increase surface area of water and to distribute water evenly in cooling tower

Makeup water:- Water quality which is added to maintain level of cooling tower which is lost due to drifting and evaporative loss while cooling tower operation is called makeup water.

Cooling tower calculation

Cooling Tower Approach
Cooling Tower Approach

1. Cooling tower makeup calculation

Water Make Up = D + E +B

D = Drift loss
E = Evaporation Loss
B = Blow loss

2. Evaporation Loss  =  0.00085 * 1.8 * C * Δt

C = Circulating water in {{m}^{{3~}}}/hr

Δt = Temperature  difference between inlet and outlet

3. Blowdown : B =  \frac{E}{{\left( {COC-1} \right)}}

E = Evaporation Loss in {{m}^{{3~}}}/hr

B = Blowdown in {{m}^{{3~}}}/hr

4. COC = Cycle of Concentration

Concentration of Cycle :This is dimensionless number and it is ration difference between parameter in solid of cooling water and solid in makeup water. This value is between 3.0 to 7.0

COC = \frac{{Hardness~in~cooling~water}}{{Hardness~in~makeup~water}}

5. Capacity of cooling Tower

Units in Tons in Refrigeration = \frac{{500*Q*\Delta T}}{{12000}} \frac{{kcal}}{{hr}}
Q =water flow rate
Δt = Temperature  difference between inlet and outlet

As you had read, there are different cooling tower term which are associated with the equation to derive cooling tower equations. Cooling tower performance depends on various parameters which are cooling tower approach, range. Let go deep and be familiar with these terminologies and derive cooling tower efficiency.

Prior to know what is cooling tower approach and cooling tower range, let us know what is wet bulb temperature and dry bulb temperature.

Wet bulb temperature: Wet-bulb temperature (WBT) is the temperature measured by a thermometer covered in water-soaked cloth/muslin over which air is passed. It is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it.

Dry bulb temperature: Dry Bulb Temperature refers to the ambient air temperature. It is called “Dry Bulb” because the air temperature is indicated by a thermometer not affected by the moisture of the air.

Cooling Tower Approach / What is approach temperature in cooling tower?

Cooling tower approach can be defined as the The difference between the Cold Water Temperature (Cooling Tower Outlet) and ambient Wet Bulb Temperature.

 Approach = Cold Water Temperature – Wet Bulb Temperature

Cooling Tower Range

Cooling tower range can be defined as the difference between the Hot Water Temperature (Cooling Tower Inlet) Temperature and Cold water (Cooling Tower Outlet) temperature .

Range = Hot Water Temperature – Cold Water Temperature

Cooling Tower efficiency calculation

Now as we had learn Cooling tower approach and range, let us dive into calculation of cooling tower efficiency. It involves the Range and approach of the cooling Tower. Cooling tower efficiency is limited by the ambient wet bulb temperature. Ideally, cold water temperature will be equal to the wet-bulb temperature which is practically impossible to achieve. Hence, cooling tower efficiency will be in between 70 to 75%.

Cooling tower efficiency = \frac{{\left( {Hot~water~temperature-Cold~water~Temperature} \right)*100}}{{\left( {Cooling~tower~inlet~temperature-wet~bulb~temperature} \right)}}

= \frac{{Range}}{{Range+Approach}}

 

Wrapping Up

Hope you like this tutorial on cooling tower equation i.e Cooling Tower efficiency calculation and cooling tower approach and range. If you have internet in cooling towers then do read our tutorial on types of cooling tower where we had cover different types of cooling tower along with diagrams, what is cooling tower and working principle of cooling tower. If you have any problem regarding this post then feel free to use comment section.

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