instrumentation and control of heat exchanger

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).


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


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


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

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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


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


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.

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TIC

FC

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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.

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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


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


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


Condensate

Accumulator

PIT PIC

FO

FO


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.


Condensate

Accumulator

PIT

PIC

FO

Vent

Reboiler and Vaporizer


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


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