Natural Draft Cooling Tower & Mechanical Draft Cooling Tower

In many industrial facilities, various pieces of equipment, as well as many fluids used in process systems need to be cooled. This cooling is mostly done with water. However, as cooling water is used, it absorbs heat, and loses its cooling effectiveness. The water needs to be kept cool.

Disposing of hot water in to ponds or basins can be detrimental to the environment. It’s also costly to replace the discharged water. The more efficient means is to cool the hot water and reuse it.

The equipment most commonly used to do this is the cooling tower. Cooling towers are part of a cooling water system in a commercial or industrial facility.

In the heat exchanger, heat from a process fluid is transferred to the cooling water. After absorbing heat from the process fluid, water flows from the heat exchanger to the top of the cooling tower. The water then falls through the tower and is exposed to air, which in-turn, cools the water. The cooled water collects at the bottom of the tower and is pumped back through the system for reuse. Cooling towers cool water by exposing it to air, which causes some of the water to evaporate. Evaporation is a process in which the heat contained in the water causes part of a water to turn to vapor. As the water turns to vapor, the heat is removed, resulting in cooled water. In a cooling tower, the heat that was contained in the water is removed from the tower in the vapor as the water evaporates.

Conduction and convection play an important role in a cooling tower. In the tower, water comes into direct contact with the air. When this occurs, some of the heat in the water is transferred to the air directly by conduction and convection.

What types of cooling tower valves are there?

All cooling towers basically operate on the same principles, however, their designs can be divided into two types- natural draft towers and mechanical draft towers(Induced Draft Cooling Tower).

What is a Natural Draft Cooling Tower?

Natural Draft Cooling Tower is an open direct contact type heat exchanger where hot water from system or condenser gets cooled by direct contact with fresh air. Cooling towers use the principle of evaporation of water against the air flow. Hot water is sprayed from the nozzles for increasing the heat transfer surface area. The temperature and humidity of the air get increased after direct contact heat transfer between hot water and fresh air. The warm and moist air being less dense goes to the top of the tower, and cold water gets collected at the bottom of the tower. Fresh air is supplied from the bottom of the cooling tower due to the density difference between hot air inside the stack and atmospheric air outside the cooling tower.

Principle of Natural Draft Cooling Tower

Air flow is obtained in natural draft cooling tower systems by way of the chimney effect of the cooling tower’s actual structure, which uses the natural pressure difference. Warm and moist air is less dense, which causes it to rise out of the cooling tower into the atmosphere and draw in denser fresh air. The difference between the warm air inside the tower and the cooler air outside creates the perfect air flow. For sufficient air flow to occur, a specific mathematical formula is used to calculate the height of the cooling tower to ensure it is almost as large as the density difference. This means cooling towers using this system tend to be large: around 200 meters tall and 150 meters in width. There is also a significant amount of water flowing in the towers. The shell itself is typically made from concrete in a hyperbolic shape. The natural draft cooling tower is the preferred choice for cool and humid climates and for heavy winter loads.

Hot water that needs cooling in the natural draft cooling tower is pumped in via the hot water inlet. The inlet is connected to nozzles that spray the water over the fill material, which provides a large surface area for heat transfer. At the bottom of the tower, the structure is open to draw in fresh air, which then flows upward and allows for direct-contact heat transfer between the warm water and the air. The hot water releases heat after coming into direct contact with the fresh air, and some of the hot water is evaporated. Cold water is collected at the bottom of the tower.

The warm and moist air is discharged from the top of the tower into the atmosphere.

Advantages of Natural Draft Cooling Towers

No electrical fan is installed so power saving
No corrosion problems
Maintenance is low
No recirculation of air occurs due to high stack outlet

Disadvantages Natural Draft Cooling Towers

Huge water flow required.
These require a large area.
Its performance depends on wind velocity and direction.

What is a Mechanical Draft Cooling Tower?

Induced draft cooling towers are a type of mechanical draft tower that features one or more fans. These fans will be located atop the tower, drawing air upwards against the downward flow of water. The water is passed around the wooden packing or decking. In these models, the coolest water is found at the bottom, coming in contact with the driest air since the airflow is counter to the water flow.

With the warmest water found near the top of the tower in contact with moist air, this results in an incredibly efficient transfer of heat. Induced towers will either come in round or square shapes, distributing water via a sprinkler or static branch system. Incoming water is injected throughout the tower with a spray distribution header. The spray is directed downwards in order to maximize the contact of water and air. This process is maximized by the cooling water fill, which is made up of wood, metal, PVC or other materials. In general, induced draft models offer the most effective solution to the cooling needs of any industry.

Principle of Mechanical Draft Cooling Tower

The flowing conditioning water is sprayed all through the cooling station with a spray dispersion head in an induced draft cooling tower. The water flows vertically through baffles to maximize the amount of time the water spends in contact with the air.

The cooling water fill, consisting of Upvc, timber, metal, or another element, maximizes the heat transfer contact area.

Through correctly constructed ‘body panels,’ air enters the inside of the cooling tower. Huge fans pull air through the baffled region, causing the water to cool. Special devices known as drift eliminators are employed to prevent water droplets from becoming entrapped in the departing airflow.

Advantages of Mechanical Draft Cooling Tower

Low-absorbed capacity.
Higher noise level than forced draft.
Humid air stream drive.
Requires little space for installation.

In a mechanical draft tower, air flow is created by using one or more fans. An induced draft cooling tower uses the fans on top of the tower create an area of low pressure. This causes the air to flow in from the sides of the tower, up to the top. In this situation, water enters through inlet pipes and is spread out in troughs. Nozzles direct the water from the troughs on to packing inside the tower. The packing slows down the water as it falls through the tower it also breaks the water up into small droplets. Both of these actions allow for better heat transfer. As the water cascades down through the tower across the layers of packing, a fan draws the air in through a set of louvers, around the packing, and then through a drift eliminator. The drift eliminator traps water droplets that could be carried along with the air leaving the tower. The water is cool by the time the water reaches the catch basin. The cold water is then drawn out of the tower through the outlet line and returned to plant equipment for reuse.

Another type of mechanical draft tower is the forced draft cooling tower. The major difference between an induced draft and a forced draft tower is that a forced draft our does not create an area of low pressure that draws air up through the tower. Instead, fans force or push the air up through the tower. Forced draft towers have no louvers on the sides, but rather fans with screens that direct the flow of air into the tower. As the cooling water cascades down from the top of the tower, the fans force air up through the tower and the heat transfer process takes place.

There are some components that are common to most towers. For example many towers are divided into cells. Each cell contains all the components of a single tower, except they share a common catch basin. To better control cooling, each cell of a tower can be taken in or out of service independently. Other common components are blow down and make up lines. As the water in the tower evaporates the concentration of the impurities increases and can cause problems. To control the concentration of impurities, water is periodically discharged from the catch basin through the blow down line. This water is then replaced with clean water through the makeup line. The flow through the makeup line is controlled by a control valve which usually has a float that senses the level in the catch basin. As the level changes the float changes position and sends a signal to the control valve which regulates the flow of water in the system.

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