Cooling induced draft

Air cooled induced draft preferred to forced draft to minimize hot gas recirculation. Include a water-cooled trim cooler . Ensure the exit tubes are flooded so that the vapor does not bypass condenser. If extreme cold conditions are expected, allow for fan to operate in reverse to counteract the overcooling by the natural circulation of cold air. 

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

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

Fig. 8. (a) Heat pipe showing the use of finned tubing for both heating and cooling and (b), heat pipe exchanger ain heater system. ID = induced draft and... 

Air recirculation. Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation. 

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 .)]... 

Air-cooled Heat Exchangers Forced vs Induced Draft... 

Advantages and disadvantages of forced and induced draft air-cooled heat exchangers ai e shown here to aid in selection. 

The plenum chamber design may be a simple box shape, formed by flat sides and bottom, or curved transition sections may be used to obtain a tapered smooth transition from the rectangular bundle to the circular fan. Either design may be used for forced-draft or induced-draft air cooled heat exchangers. 

One design that is used for installation convenience, but can also allow higher efficiency, is an induced draft fan that pulls in extra air to cool the flue products to temperatures that can be safely vented through plastic pipe. Because of the fan, long horizontal venting runs arc possible with this system. 

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... 

Figure 10-176. Typical induced draft air-cooled exchanger showing two exchanger sections and two fans. (Used by permission Griscom-Russell/Ecolaire Corporation, Easton, PA.)...

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... 

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

Air-cooled exchangers consist of banks of finned tubes over which air is blown or drawn by fans mounted below or above the tubes (forced or induced draft). Typical units are shown in Figure 12.68. Air-cooled exchangers are packaged units, and would normally be selected and specified in consultation with the manufacturers. Some typical overall coefficients are given in Table 12.1. These can be used to make an approximate estimate of the area required for a given duty. The equation for finned tubes given in Section 12.14 can also be used. 

The vent gas from the ERH flows through an electrically heat-traced duct (to prevent condensation) into an electric heat exchanger that raises its temperature to 260°C (500°F) and then into a CATOX unit to destroy residual organic contaminants. The gas is then cooled to between 12°C and 15°C (60°F) in a scrubber/con-denser that removes additional contaminants. The gas then flows through an induced draft fan, discharging into the plant s HVAC system. 

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). 

In a forced-draft air cooler, cool air is blown through the underside of the fin tube bundle. In an induced-draft air cooler, cool air is drawn through the underside of the fin tubes. Either way, road dust, dead moths, catalyst fines, and greasy dirt accumulate along the lower row of tubes. As the tubes foul, they offer more resistance to the airflow. However, note that... 

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. 

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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