Induced-draft towers

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

Two types of mechanical-draft towers are in use today the forced-draft and the induced-draft. In the forced-draft tower the fan is mounted at the base, and air is forced in at the bottom and discharged at low velocity through the top. This arrangement has the advantage of locating the ran and drive outside the tower, where it is convenient for inspection, maintenance, and repairs. Since the equipment is out of the hot, humid top area of the tower, the fan is not subjected to corrosive conditions. However, because of the low exit-air velocity, the forced-draft tower is subjected to excessive recirculation of the humid... 

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 .)]... 

Induced Draft Air mover, usually an aerial fan, on top of the tower pulls air up through the fill and out the stack. 

This type of tower uses fans at the base to force air through the tower fill or packing (Figure 9-102). Due to the relatively low oudet air velocity, there is a tendency for discharged hot air to recirculate into the fan intake and reduce tower performance. The fan handles only atmospheric air thereby reducing its corrosion problem when compared to the fan on an induced draft tower. The tower size for the forced as well as the induced draft unit is considerably less than for an atmospheric or natural draft unit due to the higher heat transfer rates. 

Recirculation the portion of exit or outlet air from the tower that recirculates back to the inlet of the fresh air to the tower. To keep this low it is important to space towers away from each other as well as from any structures which can deflect the exit moist air back to the inlet. Due to recirculation the wet bulb temperature at the tower inlet may be different from that at a point 100 yards away. The recirculation of induced draft towers is usually less than forced draft due to the upward velocity of discharge of the air. 

Normal recirculation in average installations for forced draft may run 3-10% of total inlet air, and 1-8% for induced draft towers, all depending upon the location and wind conditions during any day or season. Some towers can be arranged to have less than 1% recirculation. If conditions are suspected of being conductive to recirculation, it should definitely be allowed for in design of the tower. Recirculation increases the wet bulb temperature of entering air, increases the total air required (and hence size of... 

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... 

The tower pressure losses are (1) tower packing or fill (70-80% of loss) (2) air inlet if induced draft (3) mist eliminators at top (4) air direction change losses and entrance to packing on forced draft units. These losses are a function of air velocity, number and spacing of packing decks, liquid rate and the relation between L and Ga. 

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

Pressure loss through louvers for induced draft tower Assume louvers are along 24-ft dimension Total louver face area... 

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... 

Mix products - placing a forced-draft tower beside an induced-draft one causes problems for both designs (Figure 34.12). 

Figure34.12 Mixing forced draft with induced. The overloaded forced-draft tower with excess plume results in elevated wet bulb at air inlets on new tower. Removing the forced draft and adding one more cell to the induced draft resolved the problem...

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

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 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

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. 

Air paths may be crossflow or counterflow. Fan placement can be ahead of the fill section (forced draft) or behind it (induced draft). Manufacturers use different criteria in sizing units. Figure 4.18 reviews one tower manufacturer s approach. Specific design details should be obtained from the tower manufacturer. 

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. 

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