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PROCESS DYNAMICS & CONTROL

CPB 30004

TEMPERATURE CONTROL

LAB REPORT

AHMAD MUZAMMIL BIN IDRIS

55201113653

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1.0 OBJECTIVES

1. To identify the major components of the heat exchanger process control training

system.

2. To systematically start-up the process.

3. To study ON/OFF temperature control of electric heaters.

4. To study temperature control in the heat exchanger using PID controller.

2.0 SUMMARY

The experiment was conducted to determine the main objective which is to identify

the major components of the heat exchanger process control training system and learn how to

start-up the process systematically. Moreover, the experiment were run to study ON/OFF

temperature control of electric heaters and finally the temperature control in the heat

exchanger using PID controller. Based on the theory, the main purpose of a heat exchanger

system is to transfer heat from a hot fluid to a cooler fluid, so temperature control of outlet

fluid is of prime importance. It also functions to control the temperature of outlet fluid of the

heat exchanger system by using a conventional PID controller. The designed controller

regulates the temperature of the outgoing fluid to a desired set point in the shortest possible

time irrespective of load and process disturbances, equipment saturation and nonlinearity.

Hence, there are few types of heat exchanger that works on its own design. The experiment

was begun with the filling up both the tank until it overflows and certain valves were opened

and shut according to the procedure. Next, the set point, PBI, TII and TDI was set and

changed accordingly until the experiment end. (tambah pasal discussion main points) The

result was recorded using the recorder in the PID controller and results were tabulated in

chart paper. Based on the discussion, the green and red pen played an important role. Finally,

the objective of doing this experiment was achieved but some recommendation such as

having good knowledge on control prior of conducting the experiment will be a good value to

avoid misconduct during the experiments.

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3.0 INTRODUCTION AND THEORY

Heat exchanger system is widely used in chemical plants because it can sustain wide

range of temperature and pressure. The main purpose of a heat exchanger system is to

transfer heat from a hot fluid to a cooler fluid, so temperature control of outlet fluid is of

prime importance. To control the temperature of outlet fluid of the heat exchanger system a

conventional PID controller can be used. Due to inherent disadvantages of conventional

control techniques, model based control technique is employed and an internal model based

PID controller is developed to control the temperature of outlet fluid of the heat exchanger

system. The designed controller regulates the temperature of the outgoing fluid to a desired

set point in the shortest possible time irrespective of load and process disturbances,

equipment saturation and nonlinearity. The developed internal model based PID controller

has demonstrated 84% improvement in the overshoot and 44.6% improvement in settling

time as compared to the classical controller.

A shell-and-tube exchanger is used for larger flows, which are very common in

chemical process industries. The design of this exchanger is a shell with a bundle of tubes

inside. The tubes are in parallel and a fluid flows around them in the shell. Each arrangement

allows for a different type of flow such as co-current, counter-current and cross flow. The

tube-side can have one or more passes to increase the energy exchange from the tube-side

fluid. The shell-side may contain baffles, or walls, that channel the fluid flow and induce

turbulence, and thus, increase energy exchange.

Figure 3.1: Shell and Tube Heat Exchanger Controller System in Industry

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Heat exchanger find widespread used in refrigeration, power generation, heating and

air-conditioning, chemical process, manufacturing, and medical application. A heat is

installed in an extension of the double pipe configuration. Instead of single pipe within a

larger pipe, a heat exchanger consists of bundles of pipes or tubes enclosed within a

cylindrical shell. In the heat exchanger one fluid flows through a tubes and a second fluid

flows through within the space between the tubes and the shell.

The outlet temperature of the heat exchanger system has to be kept at a desired set

point according to a process requirement. Firstly a classical PID controller is implemented in

a feedback control loop so as to achieve the control objectives. PID controllers exhibits high

overshoot which is undesirable. To reduce the overshoot and optimize the control

performance, a feed forward controller is used along with a feedback controller. The

combined effect of feedback and feed forward control schemes gives a much better result

than the feedback PID controller.

Figure 3.2: ON/OFF Controller working scheme to achive Set Point (SV).

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4.0 RESULTS AND DISCUSSIONS

The purpose of this experiment are to identify the major components of the

heat exchanger process control training system, to systematically start-up the process, to

study ON/OFF temperature control of electric heaters and to study temperature control in the

heat exchanger using PID controller. For the first objective, the process consist of two tanks,

T61 and T62, one shell and tube heat exchanger, three centrifugal pump, (P61, P62, P611),

and resistance temperature detector (RTD) as a temperature detector for T61 while

thermocouple is temperature sensor for T62. The heat exchanger used hot water from tank,

T62 as a heating medium to heat cold water from tank, T61. In this experiment also, the green

line indicated the temperature of the flowrate while the red line indicated wall of the heater

temperature.The results obtained while conducting this experiment can be divided into two

parts which were ON/OFF temperature control and PID control of temperature.

4.1 START-UP PROCESS

During the start up procedures (No 3.2 according to Lab Manual), the manual (M)

mode of manipulated value (MV) was 100% has been set with the value of proportional band

(PB1) was 15%, the time integral (TI1) was 35 seconds and the time derivative (TD1) was 8

seconds. The set value (SV) or simply set point was 40C and the chart speed was ensured at

500!!!! . By referring to the graph, green line indicated the temperature of TE62 whilst the red line indicated TE61 temperature.

Basically, the control valve TCV 61 is Air-to-Open and has a current-to-air positioner

(EP), in which it was tagged as TCY61 in the plant. Air to open valves is normally held

closed by the spring and require air pressure (a control signal) to open them - they open

progressively as the air pressure increases. Then, a test on control valve, TCV61 has been

done by adjusting the MV values. When MV = 25%, 50% and 100% opened, the control

valve, TCV61 stem position are also opened according to MV adjustments. Figure 4.1 shows

a pneumatic Actuator, Air to Open and the position of Stem that will drive the position

indicator to shows the value of MV.

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Figure 4.1 : Pneumatic Actuator with Air-to-Open function.

4.2 ON/OFF TEMPERATURE CONTROL

In this experiment, there are two types of controller that we study which is ON/OFF

controller of heater, and the heat exchanger controller using PID controller. Firstly, the

ON/OFF temperature controller of heater experiment is performed. The equipment was tested

by running some trial in order to ensure it runs in a good condition. Besides that, it help a

better understanding on how an ON/OF controller functions; the function of controller in

order to maintain the set point.

For ON/OFF temperature control, the heater will be turned on till it reaches the set

point of 40C. Then, the heater will be switched off when the temperature went above the set

point temperature that was 40C. Due to the tank is an open tank, thus the temperature will

drop. Thus, the heater will then be switched on when the temperature was 0.5C less than the

set point which also known as Deadband. Figure 4.2 explains the theory of deadband.

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The parameters of the ON/OFF controller were then set up at high temperature limit

as shown in Table 4.1 in which (PO1) equal to 55C same as the high limit for annunciator

TAH62 (PO3) and the dead band (PO2) indicated 0.5C same as the dead band for

annunciator TAH61 (PO4). The I/O data showed that the value of X2 was 44.3 with DO2 was

off (0) and DO1 (1) was on position. The data and the graph were shown below as in figure

4.3.

Figure 4.2: Deadband in a temperature operating control system

Table 4.1: The PT Register value set up (According to Lab Manual, 3.3 No. 4)

PT Register Temperature value

PO1 (high temperature limit) 55 oC

PO2 (dead band) 0.5 oC

PO3 (high limit for annunciator) 55 oC

PO4 (dead band for annunciator) 0.5 oC

Table 4.2: The I/O Data recorded

I/O Data

Status X2

TIC62 Controller DO1: 0 55 oC

TAH62, Annunciator DO2: 1 55 oC

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The graph are represented in Figure 4.3. No 1 marked in the graph shows that the

recorder TR61 has been started. During this process (Refer Lab Manual- 3.3 No. 6), TE62 or

TIT62 rises and exceeds the High Limit, 55C. When the temperature go above 55C, the

heaters are noted to be switched off and the Annunciator TAH62 is activated (with alarm

sounds). This indicates the temperature that already exceeds the highest point that has been

set in the system, 55C. Then, temperature drops to the High Limit Temperature, 55C. The

temperature drops further by an amount equal to deadband which is 0.5C (drops 0.5C

below high limit), this is representated as No. 2 in the graph. In this stage, the heaters are

switched on again. The temperature will rise till it reaches the High Limit of 55C and

exceeds further. When it exceeds the high limit, (56C) the annunciator will sounds and

TAH62 is switched OFF by pressing the acknowledge button on the control panel. The

status and the I/O Data has been recorded in Table 4.2.

The process continues in the same mode as shown in graph, a full of 3 cycles with

decay ratio (the measure of the amount by which the controlled variable exceeds the set-point

in successive peaks) is achieved and indicated as No. 3 (56C), No. 4 (57C) and No. 5

(56C) on the graph in Figure 4.3. It produces an oscillatory curve which is the nature of

ON/OFF controller as shown in Figure 3.2.

Basically, an On-Off control is like operating a switch. This type of temperature

controller will turn on the heat when the process variable is below the set point and turn it off

when the process variable is above the set point. These controllers normally include a delay,

hysterisis and or a cycle time to reduce the cycling or "hunting" when the process variable is

close to the set point.

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Figure 4.3: The graph produced for ON/OFF controller.

4.3 PID CONTROL OF TEMPERATURE

For this experiment, the temperature of heated product at the exit of the heat

exchanger measured by TE61/TIT61 is controlled by the controller PID of TIC61.

On the temperature controller using the PID, the auto mode is being selected. The first

point is set at 40C and the PID values are recorded in Table 4.3.

Table 4.3: First Controller Parameters Values

PID PB1 (%) TI1 (seconds) TD1

(seconds)

Product

Flowrate

(FI61) (!!!") Setpoint

(SV1) (C)

First Trial

PID

15 35 8 1.3 40

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By referring to the graph in Figure 4.4, noted as No. 1 was the time during the chart

start to record. Where as, No. 2 indicates the observation on response of TE61/TIT61 as it

becomes steady around 40C when the Auto mode has been activated with the PID values

from Table 4.3. The process response smoothly in short period of time.

Since this process use all three control algorithms (PID) together, this process can be explained:

! Achieve rapid response to major disturbances with derivative control

! Hold the process near setpoint without major fluctuations with proportional control

! Eliminate offset with integral control

Then, the process maintain at 40C steadily as indicated No. 3 on the graph in Figure 4.4.

For the second temperature controller in heat exchanger, initially the cold water

circulation was started up. Then, the mode will be changed into Auto (A) mode. Two test

disturbances were being conducted; set point change and load change.

Set point change has been done according to Lab Manual, 3.4 No. 5. The second Set

point and PID trial values has been key in on the control panel as shown in Table 4.4. Where

as, by referring the graph in Figure 4.4, its noted as No. 4. Since this is a temperature

operating process, thus a small dead time is encountered at No. 4 before it become steady at

new setpoint, 42C at No.5 on the graph in Figure 4.4.

Table 4.4: Second Controller Parameters Values

PID PB1 (%) TI1 (seconds) TD1

(seconds)

Product

Flowrate

(FI61) (!!!") Setpoint

(SV2) (C)

Second Trial

PID

10 30 7 2.8 42

Then, as the system maintained in steady state, the step change of the product flow

rate by opeing fully the MV61 has been done. The flowrate at FI61 has been noted was at 2.8

m3/hr. This was noted as No. 6 in the graph in Figure 4.4. The temperature drops to almost

40C before becomes steady again as indicated as No. 7 in the graph in Figure 4.4.

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One of the temperature sensor elements used in this system was, Platinum thin film

RTD temperature sensor elements. The surface of thin film RTD temperature sensor element

is coated with ceramic, so the element can withstand high voltage and show high insulation

resistance.

4.4 CONTROL ALGORITHMS FOR DIFFERENT PROCESS VARIABLES

Figure 4.5: Control Algorithms for a few Controlled Variables.

The heat transfer process is generally a slow and a low gain process compared to flow

or level processes. This is also can be explained that, the processes with heat transfer has a

high chance to produce dead time and decay ratio. Because of the time required to change the

temperature of a process fluid (time taken for the heat to be transferred from the heating

medium to the fluid), temperature loops tend to be relatively slow. Feedforward control

strategies are often used to increase the speed of the temperature loop response. RTDs or

thermocouples are typical temperature sensors. Temperature transmitters and controllers are

used, although it is not uncommon to see temperature sensors wired directly to the input

interface of a controller.

Where as, flow and level control loops are regarded as fast loops that respond to

changes quickly, since they are dealing with heat transfer, rather only with mass transfer.

Therefore, flow control equipment must have fast sampling and response times.

Furthermore, by referring to Figure 4.5, its stated that the one and only process that

uses all three algorithm was temperature process/controlled variable. Nevertheless, only

temperature variable uses derivative algorithm and not the flow or level control. This is due

temperature variable are very slow response. Thus Derivative algorithm are needed to apply

an immediate response that is equal to the proportional plus reset action that would have

occurred in the process.

Controller Algorithms and Tuning

Controller Algorithms

Fundamentals of Control 45 2006 PAControl.com

ActivitiesProportional, PI, and PID ControlBy using all three control algorithms together, process operators can: Achieve rapid response to major disturbances with derivative

control Hold the process near setpoint without major fluctuations with

proportional control Eliminate offset with integral control

Not every process requires a full PID control strategy. If a small offset has no impact on the process, then proportional control alone may be sufficient.

PI control is used where no offset can be tolerated, where noise(temporary error readings that do not reflect the true process variable condition) may be present, and where excessive dead time (time after a disturbance before control action takes place) is not a problem.

In processes where no offset can be tolerated, no noise is present, and where dead time is an issue, customers can use full PID control. Table 7.2 shows common types of control loops and which types of control algorithms are typically used.

ControlledVariable

Proportional Control PI Control PID Control

Flow Yes Yes No

Level Yes Yes Rare

Temperature Yes Yes Yes

Pressure Yes Yes Rare

Analytical Yes Yes Rare

Table 7.2: Control Loops and Control Algorithms

14. What type of control is used in an application where noise is present, but where no offset can be tolerated?

P onlyPDPIPID

1

2

3

4

COMPLETE WORKBOOK EXERCISE - CONTROLLER ALGORITHMS AND TUNING

www.PAControl.com

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With integral action, the controller output is proportional to the amount of time the

error is present. Integral action eliminates offset.

Figure 4.6: P, I and D controllers to show the response against Process Variable.

It can be noted that the offset (deviation from set-point) in the time response plots is

has gone. Integral action has eliminated the offset. The response is somewhat oscillatory and

can be stabilized some by adding derivative action. (Graphic courtesy of ExperTune Loop

Simulator.)

Integral action gives the controller a large gain at low frequencies that results in

eliminating offset and "beating down" load disturbances. The controller phase starts out at

90 degrees and increases to near 0 degrees at the break frequency. This additional phase lag is

what you give up by adding integral action. Derivative action adds phase lead and is used to

compensate for the lag introduced by integral action.

Where as, with derivative action, the controller output is proportional to the rate of

change of the measurement or error. The controller output is calculated by the rate of change

of the measurement with time. Some manufacturers use the term rate or pre-act instead of

derivative. Derivative, rate, and pre-act are the same thing.

DERIVATIVE = RATE = PRE ACT

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Derivative action can compensate for a changing measurement. Thus derivative takes

action to inhibit more rapid changes of the measurement than proportional action. When a

load or set-point change occurs, the derivative action causes the controller gain to move the

"wrong" way when the measurement gets near the set-point. Derivative is often used to avoid

overshoot. Derivative action can stabilize loops since it adds phase lead. Generally, if

derivative action was used, more controller gain and reset can be used.

Figure 4.7: Phase degree and Amplitude Ratio for Derivative and Integral

With a PID controller the amplitude ratio now has a dip near the center of the

frequency response. Integral action gives the controller high gain at low frequencies, and

derivative action causes the gain to start rising after the "dip". At higher frequencies the filter

on derivative action limits the derivative action. At very high frequencies (above 314

radians/time; the Nyquist frequency) the controller phase and amplitude ratio increase and

decrease quite a bit because of discrete sampling. If the controller had no filter the controller

amplitude ratio would steadily increase at high frequencies up to the Nyquist frequency (1/2

the sampling frequency). The controller phase now has a hump due to the derivative lead

action and filtering.

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Valve positioners compare a control signal to a valve actuators position and move the

actuator accordingly. They are used with both linear valves and rotary valves. Valve

positioners are used when the 0.2 to 1 bar pressure in the diaphragm chamber is not able to

cope with friction and high differential pressures. The positioner is fitted to the yoke of the

actuator and is linked to the spindle of the actuator by a feedback arm in order to monitor

valve position. When a control signal differs from the valve actuators position, the valve

positioner sends the necessary power to move the actuator until the correct position is

reached. This uses a high air supply.

Figure 3: shows the temperature and heat output in a room controlled by an ON/OFF

controller. In practice the use of ON/OFF control can cause problems. As can be seen in

Figure 2, the heating system rapidly switches ON an OFF leading to inefficient system

operation and increased mechanical wear.

Figure 2: ON/OFF control of air temperature.

upper set point OFF

lower set point ON

oC

ON

oC

ON

OFF

ON

OFF

OFF

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Figure 3: ON/OFF control with a dead band

To address this deficiency a 'dead band' may be introduced. Effectively this defines an upper

and lower set-point. The control mechanism is now as summarised in Figure 3:

if the sensed temperature is below the lower set-point then the heating system is ON;

if the sensed temperature rises above the lower set-point but is still below the upper set-

point then the heating system is ON;

if the sensed temperature is above the upper set-point then the heating system is OFF; and

if the sensed temperature falls below the upper set-point but is still above the lower set-

point then the heating system is ON.

The addition of the upper and lower set-points acts to reduce the frequency of the plant

switching at the expense of poorer control of the controlled variable (here temperature).

ON/OFF control offers a crude means of controlling conditions in a building and is typically

employed where close control is not required, e.g. temperature control of domestic boilers.

Qmax

heat output room air temperature

set point temp.

oC

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5.0 CONCLUSION

The experiment conducted meet the objective as were discussed earlier. The entire

four objectives were achieved and discussed with the accordance in theory of process control.

6.0Recommendation

As a recommendation, using more advance or intelligent machine can help to get

accurate and better result. Especially the recorder, by using more computational and visualize

to show the pattern of the response it would be much easier; the response can be record using

software or cds so that the students do not have to use paper to record because the papers got

stucked. Besides, students that handle the machine should get the overview how the machine

running and also should have some basic knowledge to obtain a correct reading. Mainly, the

chart paper should be placed correctly in order to avoid error during the recording.

Furthermore, during the changes of set point and other related parameters the student should

jot down for discussion purpose and their understanding. Ensure all the valves are closed and

opened as mentioned in the methodology. Set Point change for ON/OFF controller should be

added in the experiment change as well as the PID Controller system, and both types of

control system should be discussed to show the major difference of both control systems.

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REFERENCES

Yuvraj, B.K, and Yaduvir, S. (2010). PID Control of Heat Exchanger System, International

Journal of Computer Applications, 22-23.

Mark, J.W. (1999). Some Conventional Pocess Control Schemes, Department of Chemical

and Process Engineering University of Newcastle, 2-5.

Process Control, 2012. [pdf].

Available at: [Accessed 4

September 2012]