Mechanical draft

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. 

Mechanics-draft cooling towers normally are designed for L/Q ratios ranging from 0.75 to 1.50 accordingly, the vSues of KaV/L vaiy from 0.50 to 2.50. With these ranges in mind, an example of the use of the nomograph will readily explain the effecd of changing variables. 

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... 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Lichtenstein, J. "Performance and Selection of Mechanical Draft Cooling Towers, " ASME Trans. (1943). 

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-... 

Natural draft - hyperbolic concrete shells Mechanical draft - induced draft Forced draft... 

Mechanical draft - induced, single and double sided and... 

Mechanical-draft cooling towers are normally supplied with either central baffles or inlet louvers. This depends on the tower dimensions. On these towers the wind or spray blowout is generally confined to relatively small singlecell units where an inlet may be provided on all four faces. In this case the major remedy is to provide internal diagonal baffles to prevent crossflow of air through the air inlets. 

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). 

Mechanical draft produced by a fan supplying forced air to the furnace. 

Mechanical draft developed by a fan located between the boiler and the chimney. 

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. 

Acceptance test procedure for water cooling towers of mechanical draft industrial type... 

---