Natural Draught Cooling Tower-A Natural Choice
This article critically and scientifically analyses merits and demerits of both types of cooling tower.
Salient features of Natural draught cooling towers.
This type of cooling tower consists essentially of hyperbolic shell whose conical profiles add to strength, stiffness and economy. In these towers the air is set in motion merely by the chimney effect and hence no energy is consumed to induce the air into the tower. the hyperbolic shell encloses the packing(fill) and the water distribution system.
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Diameter of shell at base and throat, shell height and air inlet height are governed by thermic design of cooling tower. Ratio of tower height to base diameter varies from 1.48 to 1.15 depending on tower height; value of ratio decreasing with increased tower height. Ratio of throat height from the top to overall height varies between 0.15 to 0.25.
The shell is supported on RCC racker column resting on pedestals which are integral with the peripheral wall on cold water basin. In order to make direct transfer of shell loads, the racker column and basin wall are inclined. The tower foundation consist of a continuous ring foundation resting either on soil or piles, if soil bearing capacity is poor.
The internal cooling fill consists of either precast prestressed concrete louvers of pear shape or PVC film flow type fill. Hot water is discharged evenly from the top of the fill by system of RCC duct and AC/PVC pipes and is allowed to flow down through fill by gravity. The supporting structure of the fill consists of a grillage of RCC column usually at 2.5m to 3m centres and RCC beams at various levels.
The accessories include desludge chamber, RCC staircase upto hot water duct, G.I.ladders upto the top, landing platforms and aviation warning light and lightening arrester systems. There are no moving components in the natural draught cooling tower.
Salient features of induced draught cooling towers.
This type of tower consists essentially of either one or more square or rectangular cells which enclose the fill and form the boundary of the air stream. Each cell is provided with a power driven impeller at the roof level to induce a controlled draught of air through the cell. The structural frame consists of RCC column and beams tied together at various level and with external RCC cladding above air inlet height on longitudinal side and upto full height on the transverse sides. The internal fill is usually either precast prestressed concrete louvers of ASCU treated timber or PVC film flow type fill.
The clod water basin is provided at bottom from where water is recycled through the plant by water distribution system. The fans are located within tapered RCC flumes in each cell above roof level and the hot water duct is usually provided at the external face of the tower on the longitudinal side and hot water is discharged evenly from the top of the fill by AC/PVC distribution pipes. The foundation are either isolate footing or raft or piles depending on the soil conditions.
The accessories for the tower include RCC staircase, G.I.ladders, desludge chambers, lifting arrangement for erection and maintenance of fan and motors and a lightning arrestor system. I.D.C.T. involves mechanical /electrical equipment like gear box, motor and fan unlike Natural draught cooling tower wherein such equipment are not needed.