Process plant cooling towers are available in a range of designs. The most common are cross flow and counter flow with air drawn in by induced draft fans. Figures 8.19 and 8.20 below illustrate the main differences. For some reason, cross flow towers are common in North America and counter flow towers predominate in Europe and Asia.
In a cross flow tower, air is drawn through the falling water from the side, over the full height of the packing. In a counter flow tower, air enters underneath the packing and then rises vertically through the packing in the opposite direction to the falling water.
Cross flow towers are usually wooden construction. Counter flow are mostly concrete. Note that wooden towers are a serious fire risk when they dry out during turnarounds. A well designed and maintained tower should be able to cool the water to within 3 °C of ambient wet bulb temperature.
Water distribution deck and nozzles
Figure 8.20 Down-flow cooling tower.
One attraction of the counter flow type is that it requires less plot space than a cross flow tower of the same rating. Cross flow towers also generally require a higher elevation of the water return lines-increasing pump power required.
There are many potential problems that can reduce cooling tower performance. For all types it is important that the water distribution is evenly spread through the packing. If there are areas with reduced water flow, the air sucks through the packing preferentially in these locations, reducing the cooling where water is flowing more densely.
Cross flow towers have open decks so that operators can check whether distribution nozzles are plugged with debris. However, the current environmental regulations mean that certain chemicals for preventing build of algae are now banned, and with sunlight playing directly on the open decks this is difficult and expensive to control. As a result, it is necessary to add sunshades over the decks. Sun also shines down the fan shafts promoting algal growth on the drift eliminators.
Depending on local conditions, it can be attractive to use a hybrid counter flow cooling tower. These towers route return water through finned tubes above the drift eliminators to achieve sensible cooling before the water is passed through the packing for evaporative cooling. In the past, the main reason for adopting the hybrid design was to eliminate the visible plume above the tower. However, the hybrid design also reduces water make iup rate; in some cases up to 30% water saving has been reported.
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