What is Mechanical Draft? | How does a Mechanical Draft Work?

Wet cooling towers are low-cost, thermally efficient industrial cooling equipment for the industrial, HVAC systems, and power plant. The cooling tower works by cooling the hot process water by blowing cold air through a cooling fill and distribution system. The airflow created by the exhaust gases in the stack, chimney, or mechanically process plays a vital function in the cooling tower. In fact, mechanical draft cooling towers are a versatile type of cooling tower. This article evaluates the working and many other aspects of the mechanical draft counterflow water cooling tower.

What is a Mechanical Draft?

A mechanical draft is a fan used in the cooling tower and gas-burning equipment that removes combustion gases or exhaust gases by mechanical means (e.g., fans). This can include a forced draft under positive static pressure or an induced draft section under non-positive static pressure.

A cooling tower is an industrial device that cools water through heat and mass transfer processes. The cooling towers are commonly employed in steel mills, chemical plants, and heavy industrial power plants.

A mechanical draft system contains a motor, fan, and some other drive devices. This draft uses an electric motor to drive the fans, but the natural draft system uses the only shell to generate airflow. In both draft systems, the relative humidity of the inlet air and the principle of direct evaporative cooling causes a small part of the process stream to evaporate.

Working Principle of Mechanical Draft

A cooling tower works on the evaporative cooling principle. A mechanical draft tower has a fan at the outlet (top) that forces air to flow in upward through the tower. The fan blows hot, humid air out of the exhaust. This process lowers the intake air velocity while increases the exhaust air velocity and reduces the chance of exhaust air being recirculated into the intake air. This arrangement of the fin/fan is referred to as draw-through.

When the environment cold air makes contact with hot water, it absorbs the water’s latent heat and cools it down. When the air absorbs latent heat from the hot water, the temperature of the air increases due to that some of the water evaporates, and it further lowers the water temperature.

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Types of Mechanical Draft Cooling Towers

These types of cooling towers are most commonly employed in houses that depend on electric fans to pull and push air into the tower. In the case of a mechanical draft cooling tower, a propeller fan or mechanical fan uses to blow air in the tower. These fans most commonly install outside the building.

These drafts are more efficient, flexible, and compact than natural drafts. They are best suitable for all operating and climatic conditions. In addition, they are best suitable for industrial systems such as HVAC units, wood and paper mills, petrochemical and chemical plants, power plants, air separation plants, automotive industry, steel mills, air conditioning/refrigeration plants, etc.

The mechanical drafts have the following two major types:

  • Forced Draft Cooling Tower: In this cooling tower, the fan of the forced draft installs on the side and near the bottom. It pushes air from below to an upward direction. This tower also has an eliminator which uses to stop the loss of water droplets through forced ventilation. These designs have problems with air distribution, recirculation of hot and humid air back into the tower, and leakage issues.
  • Induced Draft Cooling Tower: In this cooling tower, the fan installs on the top of the tower. This fan sucks air from the cooling tower and expels it into the environment. Air slowly enters through a large opening on the sides of the tower and flows through the fill. On the other hand, high-temperature moist air is discharged into the atmosphere by a fan. This keeps the tower under negative pressure and decreases the leaks.

Advantages of Mechanical Draft Cooling Tower

  1. The mechanical draft cooling tower has a small size than the natural draft cooling tower.
  2. They require less installation space than the natural draft cooling tower.
  3. The capacity of the mechanical draft is controllable. You can control the amount of air by handling the fan speed. In this way, you can control the capacity of these drafts.
  4. Mechanical draft towers can be installed anywhere, while natural draft towers can only install in an open area.

Disadvantages of Mechanical Draft Cooling Tower

  • These drafts require an energy source to drive the system.
  • They increase energy and utility bills.
  • This type of tower has higher operating costs due to the increased maintenance of motors, fans, and associated controls.
  • They require high maintenance costs.

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