Application of VSD in cooling tower

 

This article describes the application of VSD ( variable speed drive) in cooling tower.

A cooling tower is used to remove heat from the Chiller by cooling its condenser water. These towers remove heat from the condenser water by evaporation. When water evaporates it gives up/loses heat. Generally the Chiller’s condenser water is sprayed into the tower and the forced draft created by the cooling tower fan causes an increased evaporation rate resulting in the water in the tower’s basin being at a lower temperature than the incoming water. The tower is initially designed and selected based on worst case conditions (i.e. the highest condenser water temperature entering the tower (as a result of the highest cooling/Chiller load in the building) and the highest ambient air wet bulb temperature). Under these worst case conditions the fan needs to operate at full flow to ensure the return condenser water temperature is at the Chiller’s design point (typically 30°C). However, most of the situation these worst case conditions do not exist so without controlling the air flow through the cooling tower the Chiller will receive condenser water at lower than design point. 

 

For that reason, it is waste of energy to run the fan at a full speed every time. Therefore, the most efficient method to control a cooling tower fan is by adjusting the speed of the fan. To achieve the optimum control, a temperature sensor should be installed either in the cooling tower’s basin or the condenser water return line and connected directly into the VSD. Using its internal PID controller the VSD operates in closed loop control and will run the fan at whatever speed/flow is required to maintain the Chiller’s condenser water temperature at design point.

 

To optimise the performance of the Cooling Tower and minimise the installation costs, VSDs should provide the following features/functions.

• Sleep Mode – If operating conditions are such that the ambient air wet bulb temperature is very low and/or the load on the Chiller is low (e.g. at night), so the condenser water temperature entering the cooling tower is close to the design point, the cooling tower fan may not need to actually provide a forced draft. Natural evaporation may provide the necessary cooling. VSDs incorporating a sleep mode function are able to automatically detect such a situation and instead of running the fan unnecessarily at a low speed they will stop the fan (providing the best energy savings!) but continue to monitor the condenser water temperature and automatically start the fan again when necessary. This reduces the operating hours of the fan which in addition to providing the lowest operating costs minimises mechanical wear and tear.

• Bypass frequencies – When running Cooling Tower fans at varying speeds there may be some speeds which cause undesirable resonant frequencies resulting in mechanical vibration in the tower. This can possibly damage mechanical components. It should be possible to program bypass frequency ranges in the VSD to avoid operating the fan at these speeds resulting in vibration free operation.

 

 

 

Other benefits of using a VSD include smooth acceleration and deceleration of the fan, minimising mechanical stress in the drive system and therefore reducing maintenance requirements and reduced noise from the cooling tower fan when operating at lower than full speed.