Research & Development

Plant Layout - an Analysis


CFD Cooling towers reject heat using evaporative cooling which is very much dependent on the moisture already present in the ambient air. The exit air from cooling tower is nearly saturated. If some portion of this air manages to get back again in the cooling tower inlet then we say that recirculation is taking place. The percentage of inlet air which is coming due to recirculation gives the magnitude of recirculation. Recirculation decreases the cooling capability of cooling tower. It is very much dependent on the layout of the cooling tower, wind velocity and direction. Wind with high velocity tends to increase recirculation. Wind direction also play crucial role. Any water body or heat source can cause the inlet air to have more moisture and higher wet bulb.

If there are more than one cooling towers then saturated exit air from one cooling tower may enter the inlet of other cooling tower. Proper design of fan stack also reduces recirculation as the role of fan stack is to discharge moisture laden air into ambient atmosphere.


          Recirculation


Cooling Tower site selection-Cooling tower operation requires that the tower area should be free of any obstruction near its inlet and the site should offer uninterrupted flow of inlet air as well as the exit air must not mix with inlet of its own or any other cooling tower. There should not be any water body and heat source nearby which may increase wet bulb temperature. Site selection criteria must have wind flow speed, its direction, contour of the place, nearby objects and heat sources, water bodies and its placement should be kept in mind. All these factors can be judged on qualitative and quantitative basis. Qualitatively some thumb rules and guidelines can be followed. If there are more than one cooling towers then proper study of the orientation with respect to each other preferably by Computational Fluid Dynamics (CFD) may give least interference for all the towers. To assess proper impact, quantitative analysis is required which is done by computational tools like Computational Fluid Dynamics (CFD).

The recommendations of Cooling Tower Institute (CTI) for recirculation and interference are as follows :

Recirculation

  • Tower less than approx. 80 meter long should be aligned with the prevailing summer wind direction.
  • Tower longer than 80 meter should be placed perpendicular to prevailing summer wind direction.


Interference

  • CTI recommendations for orientation of a new cooling tower near existing cooling tower can be seen below:

    • Where the long axis of the existing tower is perpendicular to the prevailing summer wind direction, the influence of the old tower on the new is minimised if the distance between the towers is greater than their average length (“d”). The new tower’s long axis will be in line with the old tower’s long axis.

      Plant Layout
    • Where the long axis of the existing tower is parallel to the wind direction, the influence of the old tower on the new is minimised if the distance between the towers is greater than their average length.

      Plant Layout
    • Where the long axis of the existing tower is at a 45 deg angle to the wind direction, the influence of the old tower on the new is minimised if the distance between the towers measured normal to the wind direction is greater than their average length. The ends of the old tower and the new tower should be in line so that an offset of at least one tower length exists.

      Plant Layout