Page 240 - Chemical Process Equipment - Selection and Design

210 HEAT TRANSFER AND HEAT EXCHANGERS

Noncondensoble
gas outlet
Cooling
water inlet

metric
ensei
Recircu Io t ton

-Condensate
Circulating pipe

I I+, Product
Circdlation & ~~~~sion Feed discharge
inlet

vapor , /ZJ Cooling water
Steam
liquor
inlector Circulation Mot her
Evopototor

suspension

Evaporator crystol lizer Vacuum cooiing crystal lizer Coohg crystal ker
(9)
Figure I.lb(continued)

THERMAL ECONOMY achieved. Jet compressor thermal efficiencies are 20-30%. A
possible drawback is the contamination of condensate with
Thermal economy is a major consideration in the design and entrainment from the evaporator. When electricity is affordable, the
operation of evaporators. This is improved by operating several pressure of the vapor can be boosted mechanically, in compressors
vessels in series at successively lower pressures and utilizing vapors with efficiencies of 70-75%.
from upstream units to reboil the contents of downstream units. Because of the elevation of boiling point by dissolved solids,
Figure 8.17 shows such arrangements. Thermal economy is ex- the difference in temperatures of saturated vapor and boiling
pressed as a ratio of the amount of water evaporated in the solution may be 3-10°F which reduces the driving force available
complete unit to the amount of external steam that is supplied. For for heat transfer. In backward feed [Fig. 8.17(b)] the more
a single effect, the thermal economy is about 0.8, for two effects it is concentrated solution is heated with steam at higher pressure which
1.6, for three effects it is 2.4, and so on. Minimum cost usually is makes for lesser heating surface requirements. Forward feed under
obtained with eight or more effects. When high pressure steam is the influence of pressure differences in the several vessels requires
available, the pressure of the vapor can be boosted with a steam jet more surface but avoids the complications of operating pumps
compressor [Fig. 8.16(c)] to a usable value; in this way savings of under severe conditions.
one-half to two-thirds in the amount of external steam can be Several comprehensive examples of heat balances and surface