Page 255 - Boiler plant and distribution system optimization manual

240 Boiler Plant and Distribution System Optimization Manual

perature from 100°F to 110°F. the cycle is repeated.
The tray type tower has a resistance to gas A thermosyphon type heat pipe does not uti-
flow between 0.5 and 1.0 inches of water column lize a wicking material, instead the pipe is placed
and an outlet temperature from 130°F to 140°F. in a vertical position with the condenser placed
The packed tower type has excellent heat above the evaporator section. As the vapor con-
transfer but also has a high pressure drop in ex- denses, it returns to the evaporator section by
cess of 5 to 10 inches water column, but the res- gravity. Even with wicking material it is advan-
ervoir temperature is as high as 150°F depending tageous to locate the evaporator -section-- below
on conditions. the condenser so condensed fluid is aided by
gravity.
Heat Pipe Heat pipes have the advantage that they can
A heat pipe is composed of a sealed pipe par- be constructed of corrosion resistant materials.
tially charged with water, Freon, ammonia or oth- They are mostly applied to transfer heat from one
er suitable substance. It can be divided into three gas stream to another, but have been used suc-
sections (Figure 12.15), the evaporator section, the cessfully in fluid streams also. The gas streams
adiabatic section and the condenser section. must be adjacent to each other because of the loss
In the evaporator section, heat is absorbed of performance from routing streams of working
by the internal working fluid by evaporation. fluid and vapor through an unusually long adi-
Pressure differential causes the vapor to abatic section. Heat pipes can be compact, oper-
flow to the cooler condensing section where the ate at low temperature differentials and have no
latent heat is given up to the cooler environment moving parts to malfunction.
condensing the working fluid.
The internal circumference of the heat pipe Plate Type Heat Exchanger
is lined with a thin layer of wicking or mesh type Plate type gas-to-gas heat exchangers can be
material and the working fluid migrates by capil- used in counterflow or crossflow through adja-
lary action back to the evaporator section where cent passages separated by heat conducting walls

Figure 12.13—Direct contact flue gas condensing heat recovery tower with heat exchanger.