Cooling Tower Calculations

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Hot Water Temperature (°C) Cold Water Temperature (°C) Wet Bulb Temperature (°C) Circulating Water Flow (m³/h) Silica in Makeup (mg/L) Silica in Circulating (mg/L) Ca Hardness in Makeup (mg/L as CaCO₃) Ca Hardness in Circulating (mg/L as CaCO₃) Drift (% of circulation) Typical: 0.002% for good eliminators

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Performance

Range (°C)–

Approach (°C)–

Efficiency (%)–

Cycles of Concentration

CoC (Silica)–

CoC (Calcium)–

Selected CoC (used)–

Water Balance (m³/h)

Evaporation Loss, E–

Drift Loss, D–

Blowdown, B–

Makeup, M–

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Fills:

Cooling Tower Fill is the main heat transfer area available for Heat transfer from Hot water to Cold Air. There are two types of fills available namely Splash fills & Film Fills.  Splash fills disintegrates the hot water from the vertical direction and it splits the water to pass through next level of splash bars. Film fills forms the thin vertical film of water to make the air to contact in with to aid heat transfer. Fills are normally made of PVC, Polypropylene or Wood.

Drift Eliminators:

The purpose of Drift eliminator is to reduce the drift loss in the cooling tower. Drift eliminators normally kept next to fills in the air flow path thereby reducing the drift loss. Drift loss is the loss of entrained water through the hot air to atmosphere. Drift eliminators normally made up of PVC. More number of passes through drift eliminator decreases the drift loss but increases the pressure drop thereby increasing fan power consumption.

Cold Water Basin:

Cold Water Basin is normally made up of Reinforced Cement Concrete (RCC). It has got two functions. One is to collect the cold water from the tower and acts as storage. The other is being strong it acts as a foundation for the main structure of the cooling tower. The cold basin is either lies on the top of the soil or lies below the ground level. The height of the cooling tower is determined from the distance between the top of the cold basin to fan assembly.

Cooling Tower Structure:

Most of the Towers are made up of Chemically Treated Wood, while RCC & Fiber Reinforced Plastic (FRP) cooling towers are also available depending on the requirement of the user.

Cooling Tower Fans:

The main part of the cooling tower components. Cooling Tower fans are normally made from Aluminum, Fiber Reinforced Plastic (FRP), Glass fiber and hot-dipped galvanized steel are commonly used as fan materials. FRP being light in weight, impellers made up of FRP reduces the power requirements of the fan. The cooling tower fan blades pitch angle is varied depending on seasonal requirements.Cooling tower Fan blade Pitch angle is the angle made by the fan with the plane. Normally during the summer season, the air density is low. So the fan blade pitch angle is increased to increase the capacity of the fan.

Water Distribution Pipings:

Cooling Water Distributing pipes to hot basin must be buried underground or supported in ground to avoid thrust loading of the tower due to self-weight and water pressure inside the pipe. Individual cell inlet piping is to be independently supported.

Fan Deck & Fan cylinder:

Fan deck provides a platform for the support of the fan cylinders and acts as an access way to the fan and water distribution system.

Fan cylinder is venture shaped for enhancing the proper flow of air through the tower.  The clearance between fan blade tip and fan cylinder should be very less in order to have maximum efficiency.

Cooling Tower components – Louvers:

Louvers are made up of asbestos sheets. It serves two purposes. One is to retain circulating water within the tower, and other is to equally distribute the air flow into the fill.

Gearbox, Driveshafts & Mechanical Equipment Support:

The drive shaft transmits power from the output shaft of the motor to the input shaft of gear reduction units. Gearbox reduces the speed of the depending on the fan requirement. Torque tube unitized supports gives permanent alignment of the motor, driveshaft & gear reducer.

Distribution Valves:

Distribution Valves are used to regulate the hot water flow to distribute evenly in cells. The outlet is open to atmosphere. The valve body is designed to withstand the adverse corrosive environment. Valve should pose minimum pressure drop.

Nozzles:

Plastics are widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon. Nozzles are used to provide uniform distribution of hot water inside a cell of a cooling tower. Recent advancement of the design involves a nonclogging type nozzle.

Cooling Tower Fan Motor:

Explosion proof motors are preferred for use in Petrochemical & Refinery cooling tower applications. As the Hot Cooling water from exchangers may have explosive gas if a heat exchanger is leaky. The motor is to be provided with Protection systems like Earth fault relay and overload relay etc.

Cooling Tower Instrumentation:

Vibration Switches, Low Oil Level Switches, Level Switches for Hot & Cold Water Basin, Thermocouples for Hot & Cold Water Temperature measurement and Flow meters for Cooling water makeup & Blowdown rate are normal instrumentation system available in any cooling tower.

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Cooling Towers by Becher from Flipkart.com

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Cooling Tower Efficiency Calculation is described in this article. Cooling Tower plays a major role in Chemical Process Industry. They reject process heat from the cooling water to atmosphere and keep the water cool. The performance of the cooling tower depends on various parameters like Range & Approach. We shall see those terminologies in Detail.

Cooling Tower Approach

The difference between the Cold Water Temperature (Cooling Tower Outlet)

And ambient Wet Bulb Temperature is called as Cooling Tower Approach.

 Approach = Cold Water Temperature – Wet Bulb Temperature

Cooling Tower approach is the better indicator of the performance.

Cooling Tower Range

The difference between the Hot Water Temperature (Cooling Tower Inlet) Temperature and Cold water (Cooling Tower Outlet) temperature is called Cooling Tower Range.

Range = Hot Water Temperature – Cold Water Temperature

Cooling Tower Efficiency Calculation

The calculation of cooling tower efficiency involves the Range and approach of the cooling Tower. Cooling tower efficiency is limited by the ambient wet bulb temperature. In the ideal case, the cold water temperature will be equal to the wet-bulb temperature. This is practically not possible to achieve. This requires very large tower and results in huge evaporation and windage or drift loss resulting in a practically not viable solution. In practice, the cooling tower efficiency will be in between 70 to 75%.

Cooling Tower Efficiency =    

(Hot Water Temperature – Cold water Temperature) x 100/

(Hot Water Temperature – Wet bulb temperature)

Or Simply

Cooling Tower Efficiency = Range/ (Range + Approach) x 100

In summer the ambient air wet bulb temperature raises when compared to winter thus limiting the cooling tower efficiency.

Other Cooling Tower Calculations

This includes determination of cycle of concentration, Evaporation loss, Drift or Windage Loss, Blow down water requirement makeup water requirement.

Cycle of Concentration

The cycle of concentration is a dimensionless number. It is a ratio between parameter in Cooling Water to the parameter in Makeup water. It can be calculated from any the following formulae.

COC= Silica in Cooling Water / Silica in Makeup Water

COC = Ca Hardness in Cooling Water/ Ca Hardness in Makeup water

COC = Conductivity of Cooling Water / Conductivity of Makeup water

The cycle of concentration normally varies from 3.0 to 7.0 depending on the Process Design. It is advisable to keep the Cycle of concentration as high as possible to reduce the makeup water requirement of the cooling tower. At the same time, the higher cycle of concentration increases the dissolved solids concentration in circulating cooling water which results in scaling and fouling of process heat transfer equipment.

Draw off or Blowdown

As the cooling water circulates the cooling tower part of water evaporates thereby increasing the total dissolved solids in the remaining water. To control the Cycle of Concentration blow down is given. Blowdown is the function of Cycle of concentration. Blowdown can be calculated from the formula:

B = E/ (COC-1)

B = Blow Down (m³/hr)

E = Evaporation Loss (m³/hr)

COC = Cycle of Concentration. Varies from 3.0 to 7.0 depending upon Manufactures Guidelines

Evaporation Loss Calculation

Evaporation Loss in the cooling tower is calculated by the following empirical equation.

E = 0.00085 x R x 1.8 x C

E = Evaporation Loss (m³/hr)

R= Range

C = Circulating Cooling Water (m³/hr)

(Reference: Perry’s Chemical Engineers Hand Book )

Alternatively, The Evaporation loss can be calculated from the heat balance across the cooling tower. The amount of heat to be removed from Circulating water according to Q =  m C_p  DT is C x C_p x R _.  The amount of heat removed by evaporative cooling is Q = m x Hv is E x H_V

On Equating these two, we get

E = C x R x C_p / H_V

E = Evaporation Loss in m³/hr

C= Cycle of Concentration

R= Range in °C

Cp = Specific Heat = 4.184 kJ / kg / °C

H_V = Latent heat of vaporization = 2260 kJ / kg

Windage or Drift Loss Calculation

Drift loss of the cooling tower is normally provided by the cooling tower manufacturer based on the Process design. If it is not available it may be assumed as

For Natural Draft Cooling Tower D = 0.3 to 1.0 * C /100

For Induced Draft Cooling Tower D = 0.1 to 0.3 * C /100

For Cooling Tower with Drift Eliminator D = 0.01* C /100

Cooling Tower Mass Balance – makeup water

Cooling tower mass balance gives an idea about make-up water requirement. Cooling Tower Makeup has to substitute the water losses resulting from Evaporation, Windage and Blowdown.

M = E + D + B

M = Make up water Requirement in m³/hr

B = Blow Down in m³/hr

E = Evaporation Loss  in m³/hr

D = Drift Loss in m³/hr

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