Cooling Towers

Question 1

Describe how a basic cooling water system works

Answer 1

Cooling water is pumped from a basin at the bottom of the cooling tower to the heat exchanger. Here heat is transferred from the process fluid to the cooling water. The cooling water, which has picked up heat from the process fluid, flows to the top of the cooling tower and falls down through the tower from the top. As it falls the water is exposed to air, which causes the water to cool and it collects at the bottom of the tower and reused.

Question 2

How does cooling in the cooling tower take place

Answer 2

It is a result of evaporation - heat escapes with the vapour, causing the remaining water to cool

Question 3

Give the two classes cooling towers can be classified into

Answer 3

• Natural circulation towers

- Mechanical draft towers

Question 4

Describe a natural circulation tower

Answer 4

Consists of an empty shell constructed either of timber or ferroconcrete - the upper portion is empty and serves to increase the draught. The lower portion is fitted with grids on to which the water is fed.

Question 5

Draw the two types of natural circulation towers

Answer 5

Pg 36, 37 of study guide

Question 6

List the two types of mechanical draught cooling tower

Answer 6

• Forced draught

- Induced draught

Question 7

List the two types of natural circulation cooling tower

Answer 7

• Atmospheric

- Natural draught

Question 8

Describe how a induced draft cooling tower works

Answer 8

The air that flows through an induced draught cooling tower is maintained by one or more fans situated at the top of the cooling tower. The fans lower the pressure which causes the air to flow up the tower. The lower pressure area in the tower also increases the rate of evaporation, which speeds up cooling

Question 9

Draw the two types of mechanical draught cooling towers

Answer 9

Pg 40, 43

Question 10

Describe how a forced draft cooling tower works

Answer 10

Similar to induced draught, but the fans are located at the side and push or force the air through the tower.

Question 11

List the four classes if contaminants in cooling water

Answer 11

• Suspended solids
• Dissolved solids
• Dissolved gases
• Micro-organisms

Question 12

Why is the pH value important in cooling water

Answer 12

It controls the chemical properties - too acidic and it’s corrosive, too alkaline and it forms scale

Question 13

Describe suspended solids

Answer 13

Dust and other solid particles can concentrate and settle in the cooling tower catch basin and in the pipes to form sludge. This interferes with heat transfer and flow.

Question 14

Describe dissolved solids

Answer 14

The main dissolved solids are calcium and magnesium and cause scale if they are above a certain concentration. Scale lowers efficiency and restricts flow

Question 15

Describe dissolved gases

Answer 15

Dissolved gases such as CO2, hydrogen, sulphide and oxygen react, for example with iron, speeding up corrosion. Dissolved gases tend to lower the pH

Question 16

Describe micro-organisms

Answer 16

Cooling water is exposed to sunlight and air, making it a good breeding ground for algae and bacteria. This reduces the heat transfer. Chemicals such as chlorine, copper sulphate, or potassium permanganate can be added to control micro-organisms

Question 17

Discuss the maintenance of cooling towers

Answer 17

• Keep equipment clean
• Regular inspections
• Clamps and bolts tightened
• Motors and housing inspected for vibrations
• Debris and scale must be removed
• Storage basin must be drained and washed
• Nozzles inspected for clogging
• Keep an eye on water temperatures - they may indicate a fault

Question 18

List the components the make up a basic cooling water system

Answer 18

• Cooling tower
• Circulation pump
• Heat exchanger
• Pipelines