Cooling towers induced-draft tower

Countercurrent induced draft towers are the most common in process industries. They are able to cool water within 2 F of the wet bulb. 

Counterflow-induced draft towers, Figure 9.17(d), are the most commonly used in the process industries. Mechanical draft towers are capable of greater control than natural draft and in some cases can cool water to below a 5°F approach. The flow of air is quite uniform at a high velocity and the discharge is positive so that there is a minimum of backflow of humid air into the tower. The elevated fan location creates some noise and structural problems. It has been reported that mechanical draft towers at low water rates (19,800 gpm) perform better than natural draft towers. 

Fig. 8. Transverse cross-sectional view of double-flow induced-draft cooling tower. Courtesy of The Madey Co.

FIG. 12-14 Sizing chart for a coiinterflow induced-draft cooling tower, for induced-draft towers with (1) an iipspray distributing system with 24 ft of fill or (2) a flume-type distributing system and 32 ft of fill. The chart will give approximations for towers of any height. (Ecodyne Carp.)... 

FIG. 12-15 Horsepower chart for a coiinterflow induced-draft cooling tower. [Fluoi Coij). (now Kcodyne Cotj .)]... 

Figure 9-103A. Counterflow induced draft cooling tower. Used by permission of The Pritchard Corp. (now, Black and Veatch Pritchard).

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

Example 9-16 Wood Packed Cooling Tower with Recirculation, Induced Draft... 

Gears should be specified as American Gear Manufacturer s Association (AGMA) requirements for cooling tower service in order to ensure an adequate minimum service factor rating of 2.0. The spiral bevel type is probably used a litde more often than the worm gear. It is also cheaper. When gears are used with induced draft applications, the... 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

In July of 1997, a cooling tower at an ammonia and urea plant, originally constructed in 1968, caught fire and was destroyed. The plant produced 1,450 tons/day (1,315 tonnes/day) of ammonia and 240 tons/day (218 tonnes/day) of urea. The coolingtowerwasa 5-cell, induced draft, cross flow unit. It was constructed of redwood with steel supports and fiberglass fill. The capacity of the cooling tower was 50,000 gallons (190,000 liters). 

Figure 9.18. Main types of cooling towers, (a) Atmospheric, dependent on wind velocity, (b) Hyperbolic stack natural draft, (c) Hyperbolic assisted with forced draft fans, (d) Counterflow-induced draft, (e) Crossflow-induced draft, (f) Forced draft, (g) Induced draft with surface precooler for very hot water also called wet/dry tower, [(fc)-(e) from Cheremisinoff and Cheremisinoff, 1981).

In the smaller cooling tower installations employed for refrigeration and air conditioning service, induced-draft propeller-type air movers are used. A favored design utilizes the rotary centrifugal squirrel cage blower, which is normally driven by electric motors connected to the rotors by V-belts. Because of lower fan efficiency, these often use more electric power to achieve the same results as propeller fans. 

In an atmospheric spray tower the air movement - is dependent on atmospheric conditions and the aspirating effect of the spray nozzles. Natural-draft cooling tower operation depends on a chimney or stack to induce air movement. Mechanical-draft cboling towers utilize fans to move ambient air through the tower. Deck-filled towers contain tiers of splash bars or decks to assist in the breakup of water drops to increase the total water surface and subsequently the evaporation rate. Spray-filled towers depend only on spray nozzles for water breakup. Coil shed towers are comprised of a combination structure of a cooling tower installed on top of a substructure that contains atmospheric section coils. Hyperbolic natural-draft cooling towers are typically large-capacity systems. 

Under some wind conditions, a portion of the warm moist air leaving the tower may recirculate back through tire tower inlet and thus degrade performance. Forced-draft towers have recirculation rates that are about double those of induced-draft towers. Both water loading and tower height play the dominant role in- recirculation. Correlations exist in the literature for defining the effects of these parameters, and corrections can be applied to the wet-bulb temperature [2,3], Cooling tower fabricators can supply data to estimate the severity of the problem. 

Provide automatic sprinkler protection for induced-draft water-cooling towers where property damage and business interruption could result. In estimating the extent of possible damage, it should be remembered that the exterior and certain interior portions of the towers are normally dry and that whole towers as well as single cells of multicell towers may be taken out of service for repairs. 

Design, supply all materials for delivery to site and supervise erection on prepared foundations, testing and commissioning one mechanical-induced draft cooling tower in accordance with the requirements of this requisition. 

---