Cooling towers
Cooling towers:
A cooling tower is a heat removal device that uses water to transfer process waste heat into the atmosphere. an industrial cooling tower operates on the principle of removing heat from water by evaporating a small portion of water that is recirculated through the unit.
A cooling tower is developed to remove heat from a building or facility by spraying water down through the tower to exchange heat with the inside of the building. Air comes in from the sides of the tower and passes through the falling water. As the air passes through the water, heat is exchanged and some of the water evaporates. This heat and evaporated water flow out the top of the tower in the form of a fine cloud-like mist. The cooled water is collected at the bottom of the tower and pumped back into the plant or building for reuse. Cooling towers provide large-scale air-conditioning where land and water are expensive, or regulations prohibit the return of once-through cooling waters.
Terminology Used for Cooling Tower
Cooling Range
This is the difference between the water temperature at the entry and exit from the cooling tower.
Cooling Load
The rate at which heat is removed from the water.
Make-up
The quantity of fresh water must be supplied to the water system to replace the same amount of water that evaporated.
Drift
The air droplets become entrained or carried by the air stream coming into contact with water.
Plume
The effluent mixture of heated air and water vapor discharged from a cooling tower.
Wet bulb temperature
The ambient air temperature is at 100% relative humidity. This is measured by a psychrometer where a thin film of moisture covers the bulb of the thermometer which is then exposed to flowing air. The resulting measurement is usually cooler as compared to the measurement from an ordinary thermometer depending on the relative humidity.
Blowdown
Water is deliberately removed from the system in order to remove parts of the accumulated solids due to the continuous water evaporation, and sludge due to impurities and bacterial growth.
Recirculation
Occurs when the discharge air re-enters the system by mixing with fresh air. This lowers the efficiency of the cooling tower.
Component/Parts of Cooling Tower
Fill
wet deck or surface increases the surface area of the cooling tower to allow optimum contact surface and contact time between the air and the water.
Distribution System
It depends upon the type of cooling tower according to air-to-water flow.
Cross-flow cooling towers- gravity-flow distribution system in which water is distributed at an elevation above the fill.
Counter-flow cooling towers – and pressurized water spray systems are used.
Drift Eliminators
To prevent the escape of water droplets by generating sudden changes in the route of the air stream. the air stream passes, large water droplets affect the walls of the drift eliminator, generating the water to fall back into the cooling tower.
Louvers
Air intake louvers are a part of the cooling tower as a block for water splash-out, noise, and debris located above the cold water basin, at the bottom of the panel where the fill is packed, and are placed completely around the tower.
Casing
To provide housing and to transmit loads to the tower frame.
Fan, Motor, Driveshaft, and Speed Reducer
The fans force air in or out of the cooling tower
The main driver is the motor. Torque from the motor is transferred by the driveshaft to the fan or to a gearbox.
Large cooling towers with big and heavy fans require gearboxes to increase the torque, which eliminates the need for heavy motors, and in turn, eliminates the need for a stronger structure. sometimes belt and pulley assemblies are used instead of gearboxes.
Fan Stack
The fan stack as the fan cylinder or fan cover, enhances the efficiency of the fan. This is done by a well-shaped air inlet to create smooth airflow into the fan. It also helps the discharge of air at higher elevations minimizing recirculation and interference.
Fan Deck
Support to the fan cylinders and as a structural member to transmit loads throughout the tower frame.
Valves
To control the water level inside the cooling tower and isolation valves, flow-control valves, and make-up regulator valves.
Collection Basin
It is located at the bottom of the cooling tower and collects falling water that has not evaporated or lost due to drift. Usually, the basin also serves as the foundation of the cooling tower. This is where chemical treatments for the circulating water are added.
Distribution Basin
This is a deep pan or small basin with holes or nozzles located above the fill. This is a part of the distribution system of a cross-flow cooling tower where gravity acts to distribute hot water evenly across the fill material.
Tower Structural Frame
Frame to support the whole cooling tower and transmits all loads to the foundation. Common materials used for the frame are concrete and wood.
Types of Cooling Towers
Cooling towers are classified as follows:
Air Flow Generation. Cooling towers vary on how airflow is introduced into the system. Mechanical draft cooling towers are divided into the forced draft and induced draft.
Natural Draft. Natural draft cooling towers utilize no mechanical drivers or fans to create airflow through the cooling tower. This cooling tower takes advantage of the difference in ambient air densities below and above the tower. Airflow is created as the denser air at the bottom of the tower travels to a lower-pressure area above the tower. These towers are inexpensive but can only be installed outdoors. Also, these towers have lower reliability as they are more affected by ambient wind and temperature changes.
Mechanical Draft. These towers use either single or multiple fans to generate airflow through the tower. Mechanical draft cooling towers are more reliable and stable than natural draft towers since airflow can be manipulated according to the cooling load required. Mechanical draft cooling towers can be further classified as forced or induced drafts.
Forced Draft. As the name suggests, this type of cooling tower uses fans or blowers to force air into the cooling tower. Airflow has a high entrance velocity as it is being forced by the blower. As it passes through the tower, air flow slows down. Thus, performance is less stable compared to induced draft towers due to recirculation. Forced draft cooling towers are used in indoor applications where high static pressure is a concern.
Induced Draft. These have their fans located at the top that draws air from the air intake louvers at the bottom or sides of the tower. Contrary to forced draft cooling towers, this arrangement has low entrance and high exit velocity, which results in reduced recirculation. These types of cooling towers are widely used in industrial plants requiring stable performance.
Hybrid Draft. Its operation is the same as natural draft towers but equipped with fans to augment airflow. Hence, they are also referred to as fan-assisted natural draft cooling towers. The fans in this setup have lower horsepower compared to forced and induced draft fans. Because of the additional draft, there is no need to construct a tall tower which may be economically impractical for a given application.
Air-to-Water Flow. This classification divides cooling towers into cross-flow and counter-flow cooling towers. Their difference is the method of how water comes into contact with the air stream.
- Cross-flow. In cross-flow configuration, air flows horizontally through the fills across the downward fall of water. A distribution basin distributes the hot water to evenly fall into the fill by gravity through nozzles or orifices. The action of gravity eliminates the need for a pressurized-spray system. Maintenance is easier for cross-flow cooling towers since the distribution system can be sectionalized and serviced separately, eliminating downtime.
- Counter-flow. In this type, air flows parallel but opposite to the fall of water. Counter-flow cooling towers have a pressurized-spray distribution system. The need for this pressurized system puts counter-flow cooling towers at a disadvantage. Nevertheless, counter-flow types take less space than cross-flow for the same cooling load.
Heat Transfer. it was mentioned that there are two methods of how heat is removed from water: loss of latent heat through evaporative cooling, and cooling through sensible heat transfer. Sensible heat transfer in a cooling tower is either by conduction, convection, or a combination of both. Classification by heat transfer methods differentiates cooling towers on how these principles are applied.
- Wet or Open Circuit Cooling Towers. All cooling towers described above, from natural or mechanical draft towers to cross-flow or counter-flow, are wet cooling towers. Wet cooling towers operate mainly by the principle of evaporative cooling. Some heat is also removed through sensible heat transfer. The main advantage of wet cooling towers is their efficient cooling. In a wet cooling tower, it is possible to cool water below the ambient temperature or the dry-bulb temperature.
- Fluid or Closed Circuit Cooling Towers. In this system, the returning hot water from consumers passes through tubes or coils where sensible heat transfer happens. Outside of these tubes, water is sprayed, similar to wet cooling towers. Both latent and sensible heat is removed from the sprayed water by coming into contact with the air stream. The main advantage of this system is that water used by consumers is free from contamination.
- Dry Cooling Towers. This cooling system has the same principle of operation as automobile radiators. Hot air passes through finned tubes or coils exposed to ambient air. Heat is mainly removed by sensible heat transfer.
Build or Construction. This classification divides cooling towers according to how the structure is erected or manufactured.
- Field-erected. This cooling tower is constructed or assembled at the site where it will be used. Towers with parts prefabricated or manufactured and shipped and assembled on-site are considered field-erected cooling towers. Field-erected cooling towers are usually found in industrial plants.
- Factory-assembled. These are also known as packaged or unitary cooling towers. Factory-assembled cooling towers are mechanically complete from the point of manufacture. These are shipped on-site with almost no required assembly process. These are used for commercial, residential, and some manufacturing plants.
Use of cooling towers:
Cooling towers are used for heating, ventilation, air conditioning (HVAC), and industrial purposes. Cooling towers gives a cost-effective and energy-efficient operation of systems in need of cooling. further than 1,500 industrial installations use large quantities of water to cool their plants HVAC systems are used generally in large office buildings, schools, and hospitals. Industrial cooling towers are larger than HVAC systems and are used to remove the heat absorbed in the circulating cooling water systems used in power plant facilities, petroleum refineries, petrochemical plant facilities, natural gas processing plants facilities, food processing plants facilities, and other industrial facilities.