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Mechanical Draft Towers: Forced Draft Towers And Induced Draft Towers | Mass Transfer - Chemical Engineering PDF Download

(A) Mechanical Draft Towers: forced draft towers and induced draft towers 
Fans are used to move air through the tower in mechanical draft cooling towers. Two types of mechanical draft towers are there, namely, forced draft tower and induced draft tower.
Forced draft towers: 
It can be seen from Figure 6.7 that it has one or more fans located at the tower bottom to push air into tower.

Advantages:
(a) A part of the velocity head of air thrown by the blower is converted to pressure head on entering into the tower. It makes energy efficient than induced draft.
(b) Less susceptible to vibrations as fans are installed near the ground.

Disadvantages:
(a) Air flow through the packing may not be uniform
(b) Some of the warm and humid air may be recirculated back. Recirculation rate becomes low if the wind velocity is high. It is not popular except for small capacities.

Mechanical Draft Towers: Forced Draft Towers And Induced Draft Towers | Mass Transfer - Chemical Engineering
Figure 6.7: Schematic of forced draft towers.

Induced draft towers: One or more fans are installed at the top of the tower. Depending on the air inlet and flow pattern, induced draft towers are of two types, cross-flow and counter flow towers.
Major advantages of countercurrent induced draft cooling tower
(a) Relatively dry air contacts the coldest water at the bottom of the cooling tower
(b) Humid air is in contact with the warm water and hence maximum average driving force prevails for both heat and mass transfer.

Disadvantage of induced draft towers compared to forced draft towers 
It consumes more horse power. Cross-flow induced draft cooling tower requires less motor horse power than countercurrent induced draft cooling towers.

(B) Cross-current and counter-current 
Cross-flow induced draft cooling tower supplies horizontal air flow along the packed height and requires less motor horse power than the counter-flow type. Additional ‘cells’ may be added to raise the capacity. The schematic of induced draft counter-flow and cross-flow cooling towers are presented in Figure 6.8 and Figure 6.9, respectively.
Mechanical Draft Towers: Forced Draft Towers And Induced Draft Towers | Mass Transfer - Chemical Engineering
Figure 6.8: Schematic of mechanical draft counter-flow tower.

Mechanical Draft Towers: Forced Draft Towers And Induced Draft Towers | Mass Transfer - Chemical Engineering
Figure 6.9: Schematic of mechanical draft cross-flow tower.

The document Mechanical Draft Towers: Forced Draft Towers And Induced Draft Towers | Mass Transfer - Chemical Engineering is a part of the Chemical Engineering Course Mass Transfer.
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FAQs on Mechanical Draft Towers: Forced Draft Towers And Induced Draft Towers - Mass Transfer - Chemical Engineering

1. What is a mechanical draft tower?
Ans. A mechanical draft tower is a type of cooling tower used in industrial processes to remove heat from a system. It utilizes fans or blowers to create air movement and assist in the cooling process.
2. What is the difference between forced draft towers and induced draft towers?
Ans. The main difference between forced draft towers and induced draft towers lies in the location of the fan or blower. In forced draft towers, the fan is located at the air intake, while in induced draft towers, the fan is located at the air outlet. This difference affects the airflow direction and overall performance of the cooling tower.
3. What are the advantages of forced draft towers?
Ans. Forced draft towers offer several advantages, including better control over air distribution, lower power consumption due to reduced fan energy requirements, and increased resistance to freezing in colder climates. Additionally, the fan location in forced draft towers makes them easier to access and maintain.
4. What are the advantages of induced draft towers?
Ans. Induced draft towers have their own set of advantages. These include better thermal performance due to the fan being located at the air outlet, lower noise levels as the fan is positioned away from the intake, and enhanced resistance to recirculation of warm air. Induced draft towers also tend to have a smaller footprint compared to forced draft towers.
5. How do mechanical draft towers contribute to energy efficiency?
Ans. Mechanical draft towers contribute to energy efficiency by utilizing fans or blowers to assist in the air movement, resulting in enhanced heat transfer. This allows for the removal of heat from the system more efficiently, reducing the energy required for cooling. Additionally, the ability to control air distribution in mechanical draft towers helps optimize the cooling process and minimize energy waste.
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