Page 263 - Chemical Process Equipment - Selection and Design

YERS AND COOLING TOWERS

he processes of the drying of solids and the industries, drying practices of necessity have outpaced drying
evaporative cooling of process water with air have theory. In the present state of the art, it is not possible to
a co1nmon foundation in that both deal with design a dryer by theory without experience, but a reasonably
interaction of water and air and involve satisfactory design is possible from experience plus a little
simultaneous heat and mass transfer. Water cooling is theory.
accomplished primarily in packed towers and also in spray Performances of dryers with simple flow patterns can be
ponds or in vacuum spray chambers, the latter for described with the aid of laboratory drying rate data. In other
exceptionally low temperatures. Although such equipment is cases, theoretical principles and correlations of rate data are
comparatively siinpb in concept, it is usually large and of value largely for appraisal of the effects of changes in some
expensive, so that efficiencies and other aspects are operating conditions when a basic operation is known The
considered proprietary by the small number of manufacturers essential required information is the residence time in the
in this field. particular kind of dryer under consideration. Along with
In contrast, a great variety of equipment is used for the application of possible available rules for vessel proportions
drying of solids. Thomas Register lists about 35 pages of U.S. and internals to assure adequate contacting of solids and air,
manufacturers of drying equipment, classified with respect to heat and material balances then complete a process design of
type or the nature of the material being dried. In a major a dryer.
respect, dryers are solids handling and transporting In order to aid in the design of dryers by analogy,
equipment, notable examples being perforated belt conveyors examples of dimensions and performances of the most
and pneumatic conweyors through which hot air is blown. common types of dryers are cited in this chapter. Theory and
Solids being dried cover a range of sizes from micron-sized correiation of heat and mass transfer are treated in detail
particles to largst slabs and may have varied and distinctive elsewhere in this book, but their use in the description of
dryhg behaviors. As in some other long-established drying behavior will be indicated here.

9.1. ~ ~ ~ ~ OF ~AIR AAND WATER l O ~ on the same saturation line, one of those sloping upwards to the left
~
l
~
on the charts. For example, all of these points are on the same
Eesides the obvious processes of humidification and dehumidifica- saturation line: (T, H) = (250, O.OOS), (170,0.026)1 and (100,0.043);
tion of air for control of environment, interaction of air and water is the saturation enthalpy is 72Btu/lb dry, but the individual
a major aspect of the drying of wet solids and the cooling of water enthalpies are less by the amounts 2.5, 1.2, and 0, respectively.
for process needs. Heat and mass transfer then occur simul- Properties such as moisture content, specific volume, and
taneously. For equilibrium under adiabatic conditions, the energy enthalpy are referred to unit mass of dry air. The units employed on
balance is Figure 9.1 are lb, cuft, OF, and Btu; those on Figure 9.2 are SI. The
data are for standard atmospheric pressure. How to correct them
(9.1) for minor deviations from standard pressure is explained for
example in Chemical Engineers' Handbook (McGraw-Hill, New
where ps is the vapor pressure at the wet bulb temperature T,. The York, 1984, 12.10). An example of reading the charts is with the
moisture ratio, Hlb waterJlb dry air, is related to the partial legend of Figure 9.1. Definitions of common humidity terms and
pressure of the water in the air by their units are given following.
Humidity is the ratio of mass of water to the mass of dry air,

H = W,/W,. (9.5)
the approximation being valid for relatively small partial pressures.
Accordingly, the equation of the adiabatic saturation line may be Relative humidity or relative saturation is the ratio of the
written prevailing humidity to the saturation humidity at the same
temperature, or the ratio of the partial pressure to the vapor
H, - If = (h/lik)( 2" -- Tw) pressure expressed as a percentage,
= (C/li)(T - &I.
%RH = 100H/H, = lOOp/p,. (9.6)
For water, numerically C = h/k, so that the wet bulb and adiabatic
saturation temperatures are identical. For other vapors this The relative absolute humidity is
conclusion is noit correct.
For practical purposes, the propeIties of humid air are
recorded on psychrometric (or humidity) charts such as those of (9.7)
Figures 9.1 andi 9.2, but tabulated data and equations also are
available for gIeater accuracy. A computer version is available Vapor pressure of water is given as a function of temperature by
(Wiley Professional Software, Wiley, New York). The terminal
properties of a particular adiabatic humification of air are located ps = exp(11.9176 - 7173.9/(T + 389.511, atm, "F. (9.8)
23 1