Page 321 - Chemical Process Equipment - Selection and Design

REFERENCES 285
was 1.2 M pounds and that of the mechanical draft was 0.75 M packed section where it is cooled further by direct contact with
pounds, but the fan power was 775kW. The opinion was air. Separate dampers for air to the dry and wet sections can
expressed that mechanical draft towers are more economical at throw greater load on the wet section in summer months.
water rates below 1.25 m3/sec (19,800 gpm).
Crossflow induced draft offer less resistance to air flow and can
operate at higher velocities, which means that less power and WATER FACTORS
smaller cell sues are needed than for counterflows. The shorter
travel path of the air makes them less efficient thermally. The
cross flow towers are made wider and less high, consequently Evaporation losses are about 1% of the circulation for every 10°F of
with some saving in water pumping cost. cooling range. Windage or drift losses are 0.3-1.0% for natural
Forced draft towers locate the fans near ground level which draft towers and 0.1-0.3% for mechanical draft. Usually the salt
requires simpler support structures and possibly Bower noise content of the circulating water is limited to 3-7 times that of the
levels. A large space must be provided at the bottom as air inlet. makeup. Blowdown of 2.5-3% of the circulation accordingly is
Air distribution is poor because it must make a 90" turn. The needed to maintain the limiting salt concentration.
humid air is discharged at low velocity from the top of the tower
and tends to Ietnrn to the tower, but ab the same time the drift TESTING AND ACCEPTANCE
loss of water is less. The pressure drop is on the discharge side of
the fan which is less power-demanding than that on the intake At the time of completion of an installation, the water and air
side of induced draft towers. conditions and the loads may not be exactly the same as those of the
Wet-dry towers employ heat transfer surface as well as direct design specification. Acceptance tests performed then must be
contact between water and air. Air coolers by themselves are analyzed to determine if the performance is equivalent to that under
used widely for removal of sensible heat from cooling water on a the design specifications. Such tests usually are performed in
comparatively small scale when cooling tower capacity is limited. accordance with recommendations of the Cooling Tower Institute.
Since dry towers cost about twice as much as wet ones, The supplier generally provides a set of performance curves
combinations of wet and dry sometimes are applied, particularly covering a modest range of variation from the design condition, of
when the water temperatures are high, of the order of 160"F, so which Figure 9.19 is a sample. Some of the data commonly required
that evaporation losses are prohibitive and the plumes are with bids of cooling tower equipment are listed in Table 9.20, which
environmentally undesirable. The warm water flows first through is excerpted from a 10-page example of a cooling tower requisition
tubes across which air is passed and then enters a conventional by Cheremisinoff and Cheremisinoff (1981).

REFERENCES 14. A. Williams-Gardner, Industrial Drying, Leonard Rill, Glasgow, 1971
Drying Cooling Towers
I. W.L. Badger and J.T. Banchero, Introduction to Chemical Engineering, 1. N.P. Cheremisinoff and P.N. Cheremisinoff, Cooling Towers: Selection,
McGraw-Hill, New York, 1955. Design and Practice, Ann Arbor Science, Ann Arbor, MI, 1981.
2. C.W. Hall, Dictionary of Drying, Dekker, New York, 1979. 2. Cooling Tower Institute, Performance Curves, CTI, Spring, TX, 1967.
3. R.B. Keey, Drying Principles and Practice, Pergamon, New York, 1972. 3. AS. Foust et al., Principles of Unit Operations, Wiley, New York, 1980.
4. R.B. Keey, Introduction So Industrial Drying Operations, Pergamon, 4. D.Q. Kern, Process Heat Transfer, McGraw-Hill, New York, 1950.
New York, 1978. 5. T.K. Sherwood, R.L. Pigford, and C.R. Wilke, Mars Transfer,
5. K. Krol1, Trockner und Trocknungsuerfahren, Springer-Verlag, Berlin, McGraw-Hill, New York, 1975.
1978. 6. J.R. Singham, Cooling towers, in Heat Exchanger Design Handbook,
6. P.U. McComick, Drying, in Encyclopedia of Chemical Technology, Hemisphere, New York, 1983, Sec. 3.12.
Wiley, New York, 1979, Vol. 8, pp. 75-113.
7. K. Masters, Spray Drying, George Godwin, London, 1976.
8. A.S. Mujumdar (Ed.), Advances in Drying, Hemisphere, New York,
1980-1984, 3 vols. Data on Performance of Cooling Tower Packing
8. 6. Nonhebel and A.A.H. Moss, Drying of Solids in the Chemical
Industry, Butte.morths, London, 1971. 1. Hayashi, Hirai, and Okubo, Heat Transfer Jpn. Res. 2(2) 1-6 (1973).
10. R.E. Peck, Drying solids, in Encyclopedia of Chemical Processing and 2. Kelly and Swenson, Chem. Eng. Prog. 52, 263 (1956), cited in Figure
Design, Dekkar, New York, 1983, Vol. 17, pp. 1-29. 9.16.
18. E.U. Schliinder, Dryers, in Heat Exchanger Design Handbook, 3. Lichtenstein, Tram. ASME 66, 779 (1943), cited in Figure 9.16.
Hemisphere, New York, 1983, Sec. 3.13. 4. London, Mason, and Boelter, Trans. ASME 62, 41 (19401, cited in
U. G.A. Schurr, Solids drying, in Chemical Engineers Handbook, Figure 9.16.
McGraw-Hill, :New York, 1984, pp. 20.4-20.8. 5. Lowe and Christie, Proceedings, International Heat Transfer Conference,
13. T.H. Wentz and J.R. Thygeson, Drying of wet solids, in Handbook of Boulder, CO, 1961, Part V, pp. 933-950.
Separation Techniques for Chemical Engineers, (Schweitzer, Ed.), 6. Simpson and Sherwood, Refrig. Eng. 52, 535 (19461, cited in Figure 9.16.
McGraw-Hdl, 'New York, 1979. 7. Tezuka, Heat Transfer Jpn. Res. 2(3), 40-52 (1973).