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INDUSTRIAL PROCESS CONTROL:CONTROL OF HEAT EXCHANGERNikhilbinoy.CAssistant Professor, ICE Department, N.S.S. College of Engineering, Palakkad.
Variable and Degrees of Freedom Control loops function on the basis of
controlling one variable by manipulating the same or some other process variable.
Maximum number of independently acting automatic controllers that can be placed on a process is called degrees of freedom.
Terminology
Types:1) Liquid-to-liquid heat
exchanger2) Steam heater3) Condenser4) Reboiler
Shell: - Filled with hot fluidTube: - Through which cold fluid is
flowing
Process Medium: -Process liquid
Process Vapour
Heat Transfer Medium: -Cooling liquid
Cooling vapour
Heating liquid
Heating vapour
Liquid-to-Liquid Heat Exchanger
Instrumentation and Control
There are four temperature and two flow variables with one defining equation. DOF = 6 – 1 = 5
Cooler Heater
The fluid, whose
temperature is relatively low,
is flowing through the
tube.
Here, the temperature of both process fluid and heat transfer fluid is changed. But the phase
of these two fluids remains constant (as liquid).
Instrumentation and Control
Location of CV is desirably at heat transfer medium side. Mounted on the low temperature side. Provided positioner to minimize valve friction effect. Equal percentage valve is recommended, which will maintain
control system gain under changing throughput condition. Keep the relationship between valve opening and temperature
change constant.
TIT
TIC
TIC TIT
Instrumentation and Control
PID controller is used. ‘D’ is essential for long time lag, or sudden changes. ‘P’ is essential for relatively slowly nature systems. ‘I’ is essential to correct the temperature offset.
TIT
TIC
TIC TIT
Instrumentation and Control
Selection and location of thermal element is important. Be located far enough from the exchanger for
adequate mixing. Close enough so that the introduced time delay will
not be substantial.
TIT
TIC
TIC TITContradictory statements.
Location of the thermal element is in between these two contradictory positions.
Three Way Valves
The limits within which process temperature can be controlled are function of the nature of the load changes expected and speed of response of the whole unit. In case the process time lag is too great, to allow for effective
control during load changes, partially by-pass is used.
TIT
TIC
TIT
TIC
Diverter Valve
Mixing Valve
If the process medium temperature is high, the TIC
with respect to the measurement TIT opens the three way control valve, so that the flow rate of process
medium through cooler is increased and flow rate of
process medium through by-pass is decreased. So the
temperature of process fluid is decreased, because more fluid is flowed through the cooler.
Three Way Valves
Three way valves are unbalanced designs. Normally provided with linear ports.
Linear ports prevents the deviation from the relationship between valve movement and temperature changes.
Equal percentage valve is used. Misalignment or distortion in a control valve installation cause binding,
leakage at the seats, high dead band and packing friction. Due to high temperature service in three way valve.
TIT
TIC
Diverter Valve
Flow
100
0 100 Valve Opening
Three Way Valves
Due to high temperature service in three way valve, uniformity of valve temperature must be ensured. Temperature of fluid at all points of valve must be same. Diverted valve is favoured.
TIT
TIC
TIT
TIC
Diverter Valve
Mixing Valve
Balancing the Three Way Valve
A manual balancing valve is installed in the exchanger by-pass line. This valve is so adjusted that the resistance
offered by heat exchanger to flow equals the resistance offered by this hand valve.
TIT
TIC
Two Two-Way Valves If it is unable to use three-
way valve due to temperature and other considerations, two two-way valve is used to improve the performance. Linear plugs are used to
get the same performance of three-way valve.
Cost is increased. Price of three-way valve is
65% of two two-way valves.
Capacity of three-way valve is equal to capacity of single ported two-way valve (equal to 70% of capacity of double ported two-way valve).
TIT
TIC
FO
FC
Loop components, accessories, sensor location and time lag considerations discussed in liquid-to-liquid heat exchanger is also applicable here.
Use of equal percentage valve is more pronounced here. High rangeability is required due to high variation in condensate
pressure.
Steam Heater
Instrumentation and Control
Used to increase the temperature (to heat) of process fluid.
Phase (dynamics) of process fluid is not changed.
Phase (dynamics) of heat transfer medium is changed from steam to water. Due to the change in dynamics, the
outlet flow rate is almost constant, only the inlet flow rate varies with respect to the performance of steam heater. So control valve is located at the inlet
side (except at some special conditions).
Due to the change in dynamics, the temperature of heat transfer medium is independent of the heat transfer process. Therefore the number of variables are
4. DOF = 4 – 1 = 3
Instrumentation and Control
Condensate line is a function of load when the temperature is controlled by steam inlet.
Low load and low operating temperature results in a below atmospheric condensing pressure. This condensate pressure is not
sufficient to discharge the condensate (liquid) through the steam tap, which accumulates inside the exchanger.
More and more heat transfer area will cover up, resulting in a corresponding increase in condensate pressure.
When this pressure rises sufficiently to discharge the trap, the condensate is suddenly blown up (a big upset). The control of temperature is
impossible in this special case.
TIT
TIC
FC
Control Valve in the Condensate Line
Reduces the blown up problem. Cost is the another
advantage, because the size of the control valve is small here.
Has no effect on steam pressure.
Accurate temperature control is not possible. Due to change in
dynamics.
TIT
TIC
FC
TIT
TIC
Level Controller
Low condensing pressure is a result of the combination of low load and high heat transfer surface area. Reducing heat
transfer area will prevent vacuum. Level controller is used
instead of steam trap.
TIT
TIC
FC
LT
LIC
By-pass Control
Advantages and disadvantages of this system are similar to liquid-to-liquid heat exchanger.
By-pass gives additional degrees of freedom. Flow rate of liquid
through the by-pass line is the additional variable.
PIT
PIC
TIT
TIC
FO
Cascade Loop on Steam Heater
The change in dynamics has great effect on the steam pressure. Dynamics may
change with respect to steam purity, etc.
To reduce the effect of load changes, steam pressure is also controlled.
PIT
PIC
SP
TIT
TIC
Condenser
Instrumentation and Control
Used to condense the process vapour to liquid.
Phase (dynamics) of the process fluid changes from vapour to liquid.
Phase (dynamics) of the heat transfer medium remains constant (as liquid).
The number of variables are four. DOF = 4 – 1 = 3
Instrumentation and Control
Both of these throttle the cooling water flow through the condenser. For efficiency, water velocity through the condenser should
be such that its residence time does not exceed one minute.
Process Vapour Source
Condensate
Accumulator
TIT TIC
Process Vapour Source
Condensate
Accumulator
PIT PIC
FO
FO
Instrumentation and Control
When it is not desirable to throttle the cooling water, this system is used.
Exposed condenser surface is varied to control the rate of condensation. When recondensables are
present, a constant purge is used to remove inert.
Non-symmetricity.
Process Vapour Source
Condensate
Accumulator
PIT
PIC
FO
Vent
Reboiler and Vaporizer
Instrumentation and Control
Used to vaporize the process liquid by using steam.
Phase (dynamics) of the process fluid changes from liquid to vapour.
Phase (dynamics) of the heat transfer medium changes from vapour to liquid. Both medium have the
change of phase. The number of
variables are two. DOF = 2 – 1 = 1
Instrumentation and Control
Only variable to control is the rate of steam.
TIT TIC
FC
Even if not so perfect, the inclusion of by-pass will increase the number of degrees of freedom.
Effective for the processes which have a very small DOF.
Remember??!
Thank You