When it comes to heat rejection cooling towers are the essential equipment for almost all type of industries. Cooling towers are a special type of heat exchangers where air and water are brought in direct contact with each other to reduce water temperature. cooling tower works on the principle of evaporative cooling. Water is broken into smaller particles to facilitate evaporation for rejecting heat to the atmosphere.
Based on method of heat exchange, cooling towers can be categorized as :
1 Open loop cooling tower
In this type of cooling tower, the same water is used for cooling and generating an evaporation cooling effect. The salt concentration of cooling water increases due to the evaporation of water. In this type of cooling tower, cooling water is directly exposed to the atmosphere.
2. Closed loop cooling tower
Unlike an open-loop system where cooling water is exposed to the atmosphere, in closed-loop cooling tower cooling water is not exposed to the atmosphere. In a closed-loop cooling tower, two different cooling circuits are created, external and internal circuits. In external circuits cooling water is used to mix with atmospheric atmosphere with spray and generating a cooling effect, while in internal circuit heat transfer is performed though process heat exchangers. Both internal and external circuits are connected with a heat exchanger as shown below fig-1 and this heat exchanger is crucial for cooling operation. This intermediate heat exchanger can be installed inside or outside the cooling tower.( refer fig-2)
Both types of cooling towers are having their own advantages and disadvantages; thought must be given while selecting the right cooling tower for your process requirement.
Let us discuss which cooling tower can be selected based on different requirements :-
1 Heat transfer efficiency :
With closed-loop heat transfer coefficient between cooling, water and process can be maintained at its highest design conditions, technically heat transfer coefficient shall be maximum with the closed-loop cooling tower because of the use of clean cooling water. in some closed type cooling tower intermediate heat exchanger can also be installed separately than cooling tower for ease in cleaning or reducing capital cost. Fouling of heat exchangers shall be only with an intermediate heat exchanger which is directly taking part in the heat rejection, this can be managed easily.
2. Lower cooling tower approach:
With an open-loop cooling tower, a lower approach can be achieved easily. it must be noted that in an open-loop cooling tower we need to count two approaches one with the cooling tower and another with the heat exchanger. In the case of the lower approach, the open-loop cooling tower shall be beneficial.
3. Power Saving at high head operation :
As in closed-loop cooling water is not exposed to the atmosphere return head can be utilized effectively. The Head required for cooling water circulation shall be only resistance of heat exchanger and frictional head. The static head required for the pump can be completely eliminated.
As the cooling water used for heat transfer is not exposed to atmosphere, corrosion and scaling problem can be eliminated completely.
4. Volume of water treatment:
As in case of closed loop cooling tower because of two circuits, water volume for water treatment is low and it can be managed more efficiently.
5. Corrosion and other water related problem
With a closed-loop cooling tower corrosion problem is completed eliminated from process heat exchangers as the water used for cooling is not directly exposed to the atmosphere. Due to the Non-exposer of cooling water to the atmosphere in closed-loop cooling tower airborne particles do not contaminate water and prevent heat exchangers from corrosion and other water-related problems.
6. Maintenance requirement
While maintenance required in standalone open loop cooling tower is low, but considering the complete cooling system including pipe and heat exchangers the maintenance required for open loop cooling tower is much higher than closed-loop cooling tower.
7. Water requirement :
As both the cooling tower works on the principle of evaporation of water. For equal heat load of cooling tower, both types of cooling towers require an equal amount to makeup water but with a small change in design benefit of air cooling or dry cooling can be added in the closed-loop cooling tower to reduce water consumption.( shown in fig-3)
8. Capital investment
Open cooling towers are lower in initial cost because of unavailability of intermediate heat exchanger.
9. Operational cost
Operational costs of closed-loop cooling towers are lower because of better consistency in approach, reduction in pumping power, and overall approach.
10. Expansion Flexibility :
Expansion of open-loop cooling tower can be done easily while in case of closed-loop cooling tower capacity expansion requires higher design skills involved because of the use of an intermediate heat exchanger.