Overview of Cooling Tower working principle, components and types.

Generally we think that cooling tower is Heat transfer equipment, but the actual thing is cooling tower is work on mass transfer and result is Heat transfer.

As we know Cooling towers are a very important part of many chemical plants.

The primary task of a cooling tower is to reject heat into the atmosphere.

They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water.

Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.

Some of the Applications are listed below..

• Oil refineries,
• Petrochemical industries
• Chemical Industries,
• Thermal Power plans,
• HVAC system for cooling buildings...

Working Principle:

Humidification Operation: This operation is concerned with the interphase transfer of mass and of energy which result when a gas is brought into contact with a pure liquid in which it is essentially insoluble.

Hear air is brought into contact with Water which is insoluble in air.

The matter transferred between phases in such cases is the substance constituting the liquid phase, which either vaporizes or condenses. 

Reference: Google Images

Mass transfer is only feasible when gas is unsaturated with vapor.

Evaporation is a natural phenomenon. Water will evaporate till air in contact with it gets saturated with moisture. Evaporation cannot be more than the saturation point of air. Thus total evaporation will depend upon the moisture holding capacity of air, which depends on humidity.

The hot water is usually caused by air conditioning condensers or other industrial processes. That water is pumped through pipes directly into the cooling tower. Cooling tower nozzles are used to spray the water onto to the “fill media”, which slows the water flow down and exposes the maximum amount of water surface area possible for the best air-water contact. The water is exposed to air as it flows throughout the cooling tower. The air is being pulled by an motor-driven electric “cooling tower fan”.

When the air and water come together, a small volume of water evaporates, creating an action of cooling. The colder water gets pumped back to the process/equipment that absorbs heat or the condenser. It repeats the loop over and over again to constantly cool down the heated equipment or condenser.

Types of Cooling Towers:

Comparison between Natural and Forced Cooling Tower

PARTICULARS	NATURAL OR FANLESS COOLING TOWER	FORCED OR FAN OPERATED COOLING TOWER
Design	To work out evaporation required and quantity of air required for this evaporation	Same
Air draft	Created with nozzles. Numbers of nozzles are worked out.	Created with fan. Size of fan is worked out.
Evaporation losses	Will depend on wet bulb temp. and delta ‘T’	Will depend on wet bulb temp. and delta ‘T’
Drift losses	Controlled by overlapping louvers	Drift eliminators used, consumes more power
Approach	Up to 3-4 0 C   Steady all the time.	Can be less than 3 oC. Increases as fills breaks and chokes
Range	Can be designed as per requirement or higher range two towers in series to be installed	Can be designed as per requirement For higher range two towers in series to be installed
Capacity (Flow rate)	Can be designed for any capacity. Higher capacity to be split in two or more Numbers of modules.	Can be designed for any capacity. For higher capacity to be split in two or more numbers of cells
Maintenance	No moving parts   No breakdown maintenance  No stoppage of cooling tower so long as circulation pump is running	Possibility of breakdown of bearings, fan, gear, motor, fills, etc. Cooling tower to be stopped for repairing Breakdown and maintenance can’t be avoided.
Spare parts	No spares required   few nozzles can be kept Nozzles can be changed without stopping cooling tower.	Require spares for drive and fills Have to depend on supplier
Stand by cooling tower	Even for continuous operation stand by cooling tower not required	For continuous operation stand by cooling tower is required to take care against breakdown
Cost and investment	Initial cost is less. No investment for stand by cooling tower. Thus two ways saving in investment.	Higher initial cost. Cost of stand by cooling tower. So added initial investment.
Power consumption	No power consumption. Equivalent power available for other critical process equipment	Power is a must at the cost of other critical process equipment.
Payback period	No power consumption. No power bill. Payback period is almost one year.	No payback period On the contrary there is running expense on power bill
Power failure	In case of power failure only circulation pump is to be run on D G.	In case of power failure D G is to be unnecessarily loaded for fan motor.