Mechanical-draft towers

The thermal design of cooling towers follows the same general procedures already presented. Integration of equation 35 is usually done numerically using the appropriate software, mass-transfer coefficients, saturation enthalpies, etc. In mechanical-draft towers the air and water dows are both suppHed by machines, and hence dow rates are fixed. Under these conditions the design procedure is straightforward. 

Cooling-Tower Plumes. An important consideration in the acceptabiHty of either a mechanical-draft or a natural-draft tower cooling system is the effect on the environment. The plume emitted by a cooling tower is seen by the surrounding community and can lead to trouble if it is a source of severe ground fog under some atmospheric conditions. The natural-draft tower is much less likely to produce fogging than is the mechanical-draft tower. Nonetheless, it is desirable to devise techniques for predicting plume trajectory and attenuation. 

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... 

This tower uses fans at the top of the tower to draw air in the base of the tower through the fill and out the fan discharge (Figures 9-103-105). In this type of mechanical draft tower the hot moist air discharges vertically (usually) to the atmosphere with such a velocity as to eliminate the possibility of recirculation of this air in at the base of the tower. This moist air is corrosive to the fan parts and therefore requires protection of coated plastic or special metal blades and sealed motors and reduction gears. 

Drift Loss or Windage Loss the amount of water lost from a tower as fine droplets entrained in the leaving air. For an atmospheric type tower this is usually 0.1-0.2% of the total water circulated. For mechanical draft towers it is usually less. 

The evaluation of atmospheric and natural draft towers has not been completely presented in the detail comparable to mechanical draft towers. Some data are available in estimating form, but the evaluation of transfer rates is only adequate for estimating purposes [4]. The design of such towers by the process engineer must be made only after due consideration of this, and ample factor of safety should be included. Figure 9-130 presents general information on water loss due to wind on the tower. 

Performance, 387 Ground Area vs. Height, 391 Pressure Losses, 393 Fan Horsepower for Mechanical Draft Tower, 392 Water Rates and Distribution, 393 Blow-Down and Continuation Build-Up, 394 Example 915 Determining Approximate Blow-Down for Cooling Tower, 395 Pre-... 

On larger multi-celled mechanical-draft towers of both counterflow and crossflow variety, the air inlets are confined to the two opposing faces and windage or drift loss is unlikely to occur, except under exceptionally high wind conditions. Here again, remedial work, depending upon the location, can be applied but at additional cost (see Figure 34.10). 

Evaporation losses are 1 % of the circulation for every 100 F of cooling range. Windage or drift losses of mechanical draft towers are 0.1-0.3%. Blowdown of 2.5-3.0% of the circulation is necessary to prevent excessive salt buildup. 

Crossflow mechanical draft towers are thermally less efficient. They offer less resistance to air flow and can operate at higher velocities than counterflow towers, which means less horsepower and smaller cell sizes for comparable outputs. In practice, however, both crossflcw and counterflow designs have advantages and limitations, depending on the type of application. 

A cross-sectional view of a mechanical-draft tower is illustrated in Figure 4.16. The primary design elements are numbered in this figure and are summarized below ... 

Recent hyperbolic tower designs have included fans for intermittent operation. This type tower combines the features of both natural-draft and mechanical-draft towers. These resemble natural-draft systems in that they employ a hyperbolic shell however, they are smaller and have large... 

Applications Economics favor mechanical-draft towers over hyperbolics, except in very large installations. Considering the climatic and load conditions, natural draft has its best application in the power industry. Selected when (a) operating conditions consist of low wet-bulb temperature and high relative humidity (b) a combination of low wet-bulb and high inlet and exit water temperature exists and (c) heavy winter load is possible. 

Figure 4.21 Direct, dry-type cooling tower condensing system utilizing a mechanical-draft tower.

Equation 6.8 was obtained from mechanical-draft tower testing. The equation is shown to correlate the data in Figure 6.11. 

Windage losses or drift vary with the type of tower and local conditions. Average estimates for normal tower operations are 0.3-1% of circulation for natural-draft towers and 0.1-0.3% of circulation for mechanical-draft towers. 

The magnitude of the heat rejection of large modern power generating plants is so great that the problem of potential environmental effects due to dry cooling systems must be studied. The plumes from both natural-draft and mechanical-draft towers designed for 1000-MW plants of several representative types are examined with respect to... 

Cooling systems using evaporative cooling towers are often called open systems because the systems are open to the atmosphere. The evaporative cooling towers can be classified in a number of ways. There are natural draft and mechanical draft towers. (Incidentally, dry cooling towers are also available in natural draft or mechanical draft designs.)... 

Mechanical draft towers are always used for cooling systems requiring a low approach and typically have an L/G (liquid/gas) ratio within the range of 1 0.75 to 1 1. 

Mechanical draft tower design with fans on side or ends of tower at air inlets. 

Finally, Table 3.2.1 contains two economic relations or rules-of-thumb. Equation 3.2.20 states that the approach temperature differences for the water, which is the difference between the exit water teir jerature and the wet-bulb temperature of the inlet air, is 5.0 "C (9 °F). The wet-bulb temperature of the surrounding air is the lowest water temperature achievable by evaporation. Usually, the approach temperature difference is between 4.0 and 8.0 C. The smaller the approach temperature difference, the larger the cooling tower, and hence the more it will cost. This increased tower cost must be balanced against the economic benefits of colder water. These are a reduction in the water flow rate for process cooling and in the size of heat exchangers for the plant because of an increase in the log-mean-temperature driving force. The other mle-of-thumb. Equation 3.2.21, states that the optimum mass ratio of the water-to-air flow rates is usually between 0.75 to 1.5 for mechanical-draft towers [14]. 

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