Re-circulation is a condition, particularly in forced draft towers, in which a portion of tower’s discharge re-enters the air inlets along with the fresh air. Its effect is an elevation of the average entering wet bulb temperature compared to the ambient. This leads to a loss of efficiency of a cooling tower and in severe cases, causes malfunction of the equipment. Re- circulation can also occur due to presence of local heat sources upwind of the cooling tower which can elevate the wet-bulb temperature of the air entering the tower.
In this case-study we discuss the a cooling tower installation which is located in Qatar. The installation is of 2 rows of cooling towers consisting of 4 cells each which are operating at full speed. The primary aim of the cooling tower is to cool the hot water by rejecting the hot humid air in to atmosphere. The CFD study of hot humid air dissipated by these cooling tower cells are very important to assess the re- circulation & its effect on the cooling tower performance and also to assess the effect of hot humid air on the atmosphere. It also gives the designer ideas of possible design improvements to avoid re-circulation, particularly on those sites where there are many installation constraints.
The objective of the present CFD analysis is:
To study whether the “hot humid air rejected from each cooling tower cell fan outlet” is entering in to the “inlet of the cooling tower cell” or not for different outside weather conditions.
To study the “hot humid air plume” dispersion at different heights above the fan stack level for different outside whether conditions.
As per the details provided by the client, CFD simulations were carried out for 3 different weather conditions such as nominal condition, worse condition & peak summer condition.
In all three weather conditions the pressure drops for cooling tower components such as louvers, inlet, rain zone, fill, spray zone & drift eliminators were modeled with porous media boundary conditions. The CFD analysis was done for the 3 specified weather conditions.
The temperature, humidity & velocity contours of the analysis clearly indicate the occurrence of re- circulation phenomena. We use discrete particle model for determination of percentage re- circulation. 3520 particles were injected from the exhaust fans once the steady state simulation was achieved. Of these, 914 particles were found to be re- entering the cooling tower through the inlet louvers indicating a high percentage re-circulation of 25%.
As per the CFD steady state results for nominal condition, worst case condition & summer condition we can draw below conclusions
The results indicate that the 10m gap between the cooling towers is sufficient for air movement as the air velocities was varying from 0.5m/s to 6.1 m/s thus devoid of low velocity regions.
The discrete particle model revealed a substantial re- circulation of 25.5% in one of the cooling towers and 37% in the tower located downstream of ambient.
The results indicated a high wet bulb temperature at the inlets of both the cooling towers, as compared to the ambient air conditions. This will definitely lead to a lowering of performance of the cooling tower in its current proposed installation at the site.
We recommend to increase the cooling tower inlet area so that the velocity at the inlet will reduce & it will suck the air with low velocity – which should minimize the re-circulation effects.
