Flow Control Strategies

Control of tower air flow can be done by varying methods: starting and stopping (On-off) of fans, use of two- or three-speed fan motors, use of automatically adjustable pitch fans, use of variable speed fans.

On-off fan operation of single speed fans provides the least effective control. Two-speed fans provide better control with further improvement shown with three speed fans. Automatic adjustable pitch fans and variable-speed fans can provide even closer control of tower cold-water temperature. In multi-cell towers, fans in adjacent cells may be running at different speeds or some may be on and others off depending upon the tower load and required water temperature. Depending upon the method of air volume control selected, control strategies can be determined to minimise fan energy while achieving the desired control of the Cold water temperature.

Energy Saving Opportunities in Cooling Towers

– Follow manufacturer's recommended clearances around cooling towers and relocate or modify structures that interfere with the air intake or exhaust.

– Optimise cooling tower fan blade angle on a seasonal and/or load basis.

– Correct excessive and/or uneven fan blade tip clearance and poor fan balance.

– On old counter-flow cooling towers, replace old spray type nozzles with new square spray ABS practically non-clogging nozzles.

– Replace splash bars with self-extinguishing PVC cellular film fill.

– Install new nozzles to obtain a more uniform water pattern

– Periodically clean plugged cooling tower distribution nozzles.

– Balance flow to cooling tower hot water basins.

– Cover hot water basins to minimise algae growth that contributes to fouling.

– Optimise blow down flow rate, as per COC limit.

– Replace slat type drift eliminators with low pressure drop, self extinguishing, PVC cellular units.

– Restrict flows through large loads to design values.

– Segregate high heat loads like furnaces, air compressors, DG sets, and isolate cooling towers for sensitive applications like A/C plants, condensers of captive power plant etc. A 1°C cooling water temperature increase may increase A/C compressor kW by 2.7%. A 1°C drop in cooling water temperature can give a heat rate saving of 5 kCal/kWh in a thermal power plant.

– Monitor L/G ratio, CW flow rates w.r.t. design as well as seasonal variations. It would help to increase water load during summer and times when approach is high and increase air flow during monsoon times and when approach is narrow.

– Monitor approach, effectiveness and cooling capacity for continuous optimisation efforts, as per seasonal variations as well as load side variations.

– Consider COC improvement measures for water savings.

– Consider energy efficient FRP blade adoption for fan energy savings.

– Consider possible improvements on CW pumps w.r.t. efficiency improvement.

– Control cooling tower fans based on leaving water temperatures especially in case of small units.

– Optimise process CW flow requirements, to save on pumping energy, cooling load, evaporation losses (directly proportional to circulation rate) and blow down losses.