Page 73 - Chemical Process Equipment - Selection and Design

3.3. EQUIPMENT CONTROL 45
(a)

HTM

Adjustable Adjustable
Collar

(d)

Weight Sensing

Fgwe 3.7. Solids feeders with variable speed drives. (a) Rotary Y ‘VALVE
vane (star) feeder with variable speed drive. (b) Horizontal screw
feeder. (c) Belt feeder taking material frlom a bin with an adjustable
underflow weir. (d) Rotary plate feeder: Rate of discharge is I I
controlled by the rotation speed, height of the collar, and the WTM
position of the plow. (e) Continuously weighing feeder with variable
speed belt conveyor.

Figure 3.8. Heat exchangers without phase change. PF = process
fluid, HTM = heat transfer medium. (a) Feedback control of PF
outlet temperature. Flow rate of HTM is adjusted as the PF outlet
temperature is perturbed. The valve may be in either the input or
output line. (b) Feedforward control. IPF outlet setpoint T-2 and
perturbations of PF input flow and temperature are fed to the Dowthei
Boiler
monitor which adjusts the flow rate of the HTM to maintain I ’ Dowtherm circulation
constant PF outlet temperature T2. (c) Exchanger with bypass of
process fluid with a three-way valve. ‘The purpose of TC-2 is to
conserve on that fluid or to limit its temperature. When the inherent
leakage of the three-way valve is objectionable, the more expensive
two two-way valves in the positions shown are operated off TC-1.
(d) A two-fluid heat transfer system. The PF is heated with the
HTM which is a closeld circuit heated by Dowtherm or combustion
gases. The Dowtherm is on flow control acting off TC-2 which is on
the HTM circuit and is reset by TC-1 on the PF outlet. The HTM
also is on flour control. Smoother control is achievable this way than
with direct heat transfer from very high temperature Dowtherm or
combustion gases. (e) Air cooler. Air flow rate is controllable with adjustable louvers
adjustable lowers or variable pitch fan or variable speed motors.
The latter two methods achieve some saving of power compared variable pitch fan
with the louver design. Multispeed motors are also used for change
between day and night and between winter and summer. The variable speed motor
switching can be made automatically off the air temperature.