CPB30004 Process Dynamics and Control Experiment 5: Heat exchanger process control lab report

CPB30004- Process Dynamics and Control Experiment 5: Heat Exchanger Process Control

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1.0 INTRODUCTION Shell and tube heat exchanger is widely used in the industries. The reason that this type of heat exchanger is applicable in chemical industries is because of the thermal stressed that can be applied in it. Other than that, the boiling or condensation can be applied either in shell or in the tube. In addition, the pressure drop can be varied in wide range plus the design of heat exchanger can be done either in vertical or horizontal; suitable with plant layout (Subramanian, n.d). The necessary of heat exchanger to be present in plant is depending on production process. The present of it is in order to transfer liquid with different temperatures throughout the process. From (Max S. Peters & Klaus D. Timmerhaus, 1991), the design and equipment scale up can be describes as follow:

Table 1: Design and equipment scale up for Heat Exchanger Type of

Is pilot

Major

Major variables

Maximum

Approximate

equipment

plant is

variables

characterizing

scale-up

recommended

usually

for operational

size or capacity

ratio based

safety or

necessary?

design (other

on indicated

overdesign

than flow rate)

characterizin

factor, %

g -Temperatures

-Flow rate

variable > l00: l

shell

-Viscosities

-Heat-transfer

> 100: l

heat

-Thermal

area

exchangers

conductivities

Tube-and-

No

15

From the table 1 above, it shows that major variables for operational design (other than flow rate) are varying with temperatures, viscosities and thermal conductivities. By

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application on process control, thermal and temperatures that present in shell and tube heat exchanger can be influenced the process itself. In this heat exchanger process control experiment, the controller that had been used is PID controller (proportional-integral-derivatives controller) which by using this, the offset can be eliminated and it can reduce the error to zero. It also gives a better performance of the process control (National instrument, 2011).

Figure 1: Response of a typical PID closed loop system. Source from National Instrument, http://www.ni.com/white-paper/3782/en/#toc2 Figure 1 shows the response of typical PID in closed loop system. From the figure, it can deduce that by applied this type of controller in process control system, the offset can be eliminate and also the error can be reduce to zero.

2.0 PROCEDURE A. STARTUP PROCEDURES 1. The following steps had been carried out before starting the experiment:

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i.

The tank T62 and the product tank T61 were filled with water up their

ii.

overflow drain pipe. The manual valve was checked at the product downstream pipeline TSV62

was opened to the external drain system. iii. The main power supply at the front cubicle was turned on. 2. The various manual valve was checked as follow: i. All suction and discharge valve pumps (P61, P62 and P611) were opened. ii. Manual pass valve for each pumps and control valve TCV61, TSV61, TSV62 and LSV61 was shut. iii. The manual valve at the inlet of LSV61 was opened. iv. Bottom drain valves of tank T61 and T62 were always shut. 3. The PID controller TIC61 was learned to use i. The TIC61 was displayed in mode Manual (M) mode. The MV was set MV= 100% to open the control. The control valve was checked was open at the ii. iii.

plant. The SV was set to 40 °C. The PID turning page PID1 was accessed and the PID values was change to

iv.

first (I) trial values. a.PB1= 15 % b. T11= 35 sec c. TD1= 8 sec The on/off control (TIC61 and TIC611) at the ‘P/T register page at PO1 and

PO2 were accessed. v. The recorder TR61 was started by pressing On the RCD pushbutton. 4. The control valve was stroke with MV= 25 % , 50 % and 100 % with TIC61 in the Manual (M) mode where the opening TCV61 at the plant was checked.

B. ON/OFF TEMPERATURE CONTROL The heating medium for the heat exchanger was produced by heating water in the tank T62. It maintained constant by the on/off controller TIC62. It controlled with 0.5°C less or more from the set point values. 1. The heating medium circulation was started. i. The valves MV62 and B62 was make sure were opened. ii. Pump P62 was started. iii. The water was ensured flow through the heat exchanger and back to T62. iv. The by pass valve B62 was shut.

CPB30004- Process Dynamics and Control Experiment 5: Heat Exchanger Process Control

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The heaters were switch on. The temperature was made sure increased in the

tank T62and the heater is on. vi. The water in the tank T62 was topped up when necessary. 2. The chart recorder was started by pressing the RCD button at the recorder TR61. i. The selector was used and switch to TE62 and TIT62 so that only Green pen ii. iii.

was selected to record. The rise in temperature of water in tank T62 was observed. 15-20 minutes were taken by TE62 reached 55 °C during this time, the recorder was stop and RCD button was pushed and the steps 3 and 4 were

proceed. 3. Annunciator buzzer was pressed ACKNOWLEDGE button to silence the buzzer when it was on. The cause of alarm was rationalize. 4. The parameters of the on/off controller T1C62 were set: i. The menu was accessed and select ‘P/T register’ button to set the high

ii.

temperature limit and dead band of controller TIC62 PO1: high temperature limit= 55°C PO2: dead band= 0.5 °C PO3: high limit for annunciator TAH62 (same as PO1) PO4: dead band for annunciator TAH61 (same as PO2) The menu was accessed and selected I/O data button and the following data

was noted: X2: TE62/TIT62, % of 0-100 °C, input variable to controller DO1: heater status in T62, I(on) and 0 (off) DO2: annunciator TAH62 status, 1(on) and 0(off) 5. The temperature was approaching 55 °C, the recorder TR61 was started by pushing RCD buttons. i. Both recorder TR61 Green pen and the I/O data X2, DO1 and DO2 were ii.

observed. The high temperature limit and dead band data on the chart paper beside

Green pen were wrote down. 6. The temperature TE62/TIT62 at X2 was observed and the following checklist was went through: i. TE62/TIT62 rise and exceeded the high limit 55°C The heater was off The annunciator TAH62 was activated ii. The status of DO1, DO2 abd X2 at I/O data were checked and the data on the iii. iv. v.

chart paper was recorded. The TE62/TIT62 was continue to rise for example the overshoot beyond the high limit The TE62/TIT62 was dropped to high limit (55°C) but the heater was still off The TE62/TIT62 dropped further by an amount equal to dead band (0.5°C) Heater was switched on again The annunciator TAh62 was switched off

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The status DO1, DO2 and X2 at I/O data was checked and data on the vi.

chart paper was recorded The observation was continued to get a full cycle for example 2 temperature

peaks or bottom. vii. The oscillatory nature on/off was noted at the Green pen of the recorder. 7. The record was stopped by pushing the RCD button at the recorder. The chart paper was noted.

C. PID CONTROL OF TEMPERATURE 1. The temperature of heated product at the exit of heat exchanger was measured by TE61/TIT61 which was controlled by PID (Loop1) of TIC61 2. The on/off controller TIC62 was maintained the temperature of tank T62 at 55 °C. the controller TIC was set as follow: i. The setpoint SV was endured SV= 40 °C. ii. TIC61 in Manual (M) mode , the output MV= 100 % was set iii. The PID controller was checked to the trial (I) value PB1= 15 % T11= 35 sec TD1= 8 sec 3. The cold water circulation was started. i. The valve MV61 and B61 were opened ii. Pump 61 was started and ensured the water flew through rotameter FI61 iii. She bypass B61 was shut iv. The manual valve was adjust at MV61 to set the flow rate to 1.3 m3/hr v. The circulating pump P611 was started 4. The TIC61 was switched to Auto (A) mode i. The product flow rate FI61, setpoint, SV and PID value were wrote on the ii.

chart paper beside its response. The response TE61/TIT61 were observed at the recorder RED pen till it

become steady around 40 °C . 5. Test disturbance- setpoint change i. The PID values of controller TIC61 was set to the second (II) trial values PB1= 10 % T11= 30 sec TD1= 7 sec

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ii.

The setpoint SV= 42 % was changed. The chart paper was marked on the new

iii.

setpoint value. The setpoint values did not increased above 44 °C. The response was watched at recorder till it is almost steady or continued to

oscillate even after 3 cycle. 6. Test disturbance- load change i. The systems were made sure at steady state. ii. The product flow rate was stepped change by opening fully MV61. The flow iii.

rate FI61 was noted and the value at chart paper was written. The relevant portions of the chart paper were watched and the result was

attached in the report. 7. The relevant portions of the chart paper were retrieved and the results were attached to the report. 8. SHUTDOWN PROCEDURE i. The controller TIC61 was switched to Manual (M) mode and MV= 100 % were ii. iii. iv.

switched. Heater was switched off All pumps P61, P62 and P611 were stopped The main power supply was switched off.

CPB30004- Process Dynamics and Control Experiment 5: Heat Exchanger Process Control

3.0 RESULTS & CALCULATIONS 3.1 P&ID of Process

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3.2 Start-up Procedure Report For this process which known as heat exchanger process control, we had been exposed to process plant’s instrument which consists of two tanks that are T61 and T62. We also been manage to identify other major instruments that are involve in this process which are centrifugal pumps that consists of P61,P62 and P63, shell and tube heat exchanger (the most important instrument for this process), control valve TCV 61 (the final control element that function to control the flow rate of water), on / off controller TIC62, PID Controller TIC61(controller used to give better performance of the process system) and the measurement involves in this process which are TE61/TIT61, RTD, and TE62/TIT61,Type K. By following the start-up procedure, the tank 61 and tank 62 had been filled with water up to their overflow drain pipe and the manual valves had been checked at the product downstream line so that it had been opened to external drain system. The next actions that need to be taken is to on the power supply and check various manual valves according to manual given. This is done to make sure all necessary valves had been opened or closed correctly to avoid trouble shooting during conduct the process. After make sure all valves had been opened or closed correctly, the next action to be done during start-up is to test the PID controller of TIC61. This was done by put the TIC61 in the following conditions:    

Manual mode (M) MV=100% SV=40 °C PID values first trial: -PB1=15% -T11=35sec -TD1=8sec Next action in this stage is to access on/off controller (TIC62, TIC611) at “P/T

Register page at PO1 and PO5 and the recorder TR61 is been started by pressing it on the RCD pushbutton. By TIC61 in manual mode, control valve is being stroke with MV=25%,50% and 100%. This is done in stage to learn the PID controller system and observe the graph produce.

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Figure 2: Graph at recorder TR61-PID learning system. Figure 2 shows the result in start-up procedure about learning the PID controller. It been initially put in conditions manual mode with MV=100%, SV=40%, with PID values of PB1=15%, T11=35sec and TD1=8sec. after that, the control valve had been stroke to change the MV to 25%, 50% and 100%. The figure 2 shows the result of this PID controller

3.3 On/Off Controller Results Sheet Table The On/Off control results were tabulated in the table below for different process. Table 3, shows the values of set point for high temperature limit and dead band for controller and annunciator when the heater was OFF.

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Table 3: On/Off temperature control of the electric heaters, when Heater OFF P T Register High Temp Limit

I/O Data Dead Band

Status DO1: 0 (off)

TIC62, Controller

PO1 =__55 °C

PO2 = _0.5 °C

TAH62, Annunciator

PO3 =__55 °C

PO4 =__0.5 °C DO2: 1 (on)

Meanwhile Table 4 shows the values of set point and dead band for controller and annunciator when the heater is ON. Table 4: On/Off temperature control of the electric heaters, when Heater ON P T Register High Temp Limit

I/O Data Dead Band

Status DO1: 1 (on)

TIC62, Controller

PO1 =__55 °C

PO2 = _0.5 °C

TAH62, Annunciator

PO3 =__55 °C

PO4 =__0.5 °C DO2: 0 (off)

The results of this controlled process is shown as in Figure 3 and Figure 4.

3.4 Comparison for Different Response on Different PID Values Part 1: On / Off Temperatures control Set point values

= 40oC

High temperature limit = 55oC Dead band

= 0.5oC

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Figure 3: Graph at recorder TR61-heater off. DO1: Heater status in T62

= 0 (off)

DO2: annunciator TAH62 status

= 1 (on)

X2=TE62/TIT62, % of 0-100oC, input variables to the controller = 55.1oC From the figure 3 above, the graph indicates the increase in line and then constant steady line (green pen). The increase in line indicates the temperature is keep rising until the temperature constant at 55oC with dead band of 0.5oC. For the constant steady line, during this time, the input variables; X2, is not exceed the high limit temperatures and also in the range of dead band. The process is run smoothly without any disturbance applied on it.

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Figure 4: Graph at recorder TR61-heater on. DO1: Heater status in T62

= 1 (on)

DO2: annunciator TAH62 status

= 0 (off)

X2=TE62/TIT62, % of 0-100oC, input variables to the controller = 54.3oC From the figure 4 above, the graph indicates the change in line (green pen) which now it oscillates with half cycle. During this time, the input variables; X2 is now below the dead band range; which by that, the heater at DO1 need to be switch on while the annunciator at DO2 is to be switch off. Here is the disturbance; that is heater, is being switch on to make the process back to the steady state as shown in the graph. By switch the heater to on, it increases the temperature in shell and tube heat exchanger and maintain the dead band around 0.5oC. Part 2: PID control of temperature

Part 2: Manual Mode and Auto Mode Control

Figure 5: Graph at recorder TR61-manual / auto mode. At controller TIC61-First (I) trial values: 1. Manual mode

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 MV: manipulated values (output)= 100%  SV: Set point values= 40oC  PB1=15%  T11=35sec  TD1=8sec 2. Switching to auto mode  MV: manipulated values (output)= 47.5%  SV: Set point values= 40oC  PV=41.4  FI61: Product flow rate= 1.3 m3 / hr From figure 5 above, it shows the changes from manual mode process to auto mode process. At manual mode; by referring to red pen, the line kept at steady line. This indicates the process is run smoothly without any disturbance occurred. By switching the process to auto mode, by referring to red pen, the line become decline drastically due to disturbance occurred. By comparing the manipulated values; MV from both manual at auto mode, the values are 100% and 47.5% respectively. From the MV values of both modes, there is 52.5% big difference of them. By such a great difference, that is the reason why the red line decline drastically when converting the process mode from auto to manual. Part 3: Test Disturbance-set point change

Figure 6: Graph at recorder TR61-set point change.

At controller TIC61-Second (II) trial values:

CPB30004- Process Dynamics and Control Experiment 5: Heat Exchanger Process Control

   

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SV: Set point values= 42oC PB1=10% T11=30sec TD1=7sec

From the figure 6 above, for the set point change from 40oC to 42oC, the line for green pen oscillates continually. But for red pen, the line after changing the set point oscillates at the beginning and then after a few minutes, it keeps in steady state line. The reason that the line keep oscillates for green lines is due to changing of set point in the process. The disturbance was introduced in the process. Supposedly, the set point is to be set at 40 oC but after changing it to 42 oC, the process needs to reduce it to initial set point so that a steady state line can be achieved. The PID controller keeps changing because it needs to reduce the set point so that the zero error can be reduced and the offset can be eliminated. That is the reason why the green pen line keeps oscillate throughout the process.

Part 4: Test Disturbance

Figure 7: Graph at recorder TR61-load change. By opening fully MV61;   

FI61: Product flow rate= 2.7 m3 / hr PV=41.6 SV=42oC

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

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MV=76.0%

From the figure 7 above, after changing the load by opening fully MV61, the line shows continually oscillations which indicates this process not steady and the disturbance is kept been applying in this process. The PID controller need to minimize the zero error during the disturbance takes place in the system.

4.0 ANALYSIS In this experiment, two types of sensor is used which are red pen and green pen. Red pen indicates the wall of the heater temperature which it represent RTD by ON/OFF controller. Green pen indicates the temperature of the flow rate and it represented RTD by PID controller. This experiment is to operate ON/OFF temperature control and RTD is used because of its sensitivity. Table 3 and 4 show the P T register and I/O Data input for ON/OFF temperature control of the heater. The setting of the value will keep the temperature stable around 55°C. The ON/OFF controller will always switch the output only when the temperature has crossed the set point. The heater will automatically turn off when the temperature of the water reached above the set temperature which is 55°C. The heater will automatically turn on in order to keep the temperature at the desired temperature when the temperature drop below the set temperature.

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Green pen represented PID controller. Based on the response observed in the chart paper, it can be seen that there is an increase in the amplitude of an oscillation compared to the red pen. To be concluded, from the response observed, the ON/OFF controller has higher damping compared with PID controller where damping is the decrease in amplitude of an oscillation. The experiment continued with test disturbance with setpoint change and load change respectively. It is shown on the chart paper that both of the pen show the fluctuate reading of the oscillations as the set point changed. PID controller is more fluctuated that ON/OFF controller. The flow rate was changed by opened fully the valve under steady state system. The oscillation ON/OFF controller is more stable compared to PID controller. The oscillation of PID controller response is quite slow compared to before disturbance. If there is a disturbance, the reading of the response are easily fluctuate. The temperature increased slowly and in time it fluctuate and went down a little bit before going up again when the mode is change from Manual [M] mode to Auto [A] mode. Annunciator is initiated when the temperature exceed the desired value as the heater is not turn off because of the heater will automatically turn on when the temperature is lowered than wanted temperature. 5.0 DISCUSSION The Heat Exchanger Process Control objectives were to study the On/Off temperature control of electric heaters and to study the temperature control in heat exchanger using PID controller. Based on the results obtained as shown in Figure 3 and Figure 4, the chart shows that the High Temperature Limit of the process being controlled by the heat exchanger perfectly. The curves in Figure 3 shows that the temperature increasing gradually until reach the temperature limit, and it stopped increasing afterward. It also shows that the process was able to run smoothly when there is no disturbance applied to the process.

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The disturbance might interfere with the process thus resulting a decreasing or increasing in temperature if applied to the process. By referring to Figure 4, the process curves show that it oscillates in half cycle. In this step, the disturbance has cause the process output variable, X2 reading below the dead band range. To control this condition, the heater is on and able to regulate the temperature in shell and heat exchanger and maintain the dead band around 0.5 °C. The experiment was continued to the second part, to study the Manual Mode (M), and Auto Mode (A) control of process. According to the chart paper retrieved, when the process was in Manual mode, the process able to run smoothly because there is no disturbance when the process is being control in Manual mode. Differ from the Auto mode, when the process was conducted automatically, the process shows that there is disturbance in the process, where can be seen in Figure 5, the curve line decrease and decline drastically. Big difference can be seen for the manipulated value, MV for Manual and Auto mode where for Manual mode, the MV is at 100% while in Auto mode, the MV is at 47.5%. Auto mode in PID control, the output is calculated by the controller using the error signal the difference between set point and the process variable. Meanwhile, manual mode is when the output is set manually (Common terms in the process control terminology, n.d) Auto mode of PID control have a better response compared to Manual mode because Auto mode able to regulate the condition of the process to the set point compared to Manual mode where the process need to be controlled manually by human to regulate the operating condition to original set point. So, Auto mode is much better compare to Manual mode. However, based on the results obtained, its shown that Manual mode results is much better than Auto mode controller. For the next part, the process was continued with test disturbance-set point change. The set point value is 42 °C, PB1 is 10%, TI1 is 30 secs and TD1 is 7 secs. The curve as shown in Figure 6, where the disturbance applied to this process shows an oscillation where the process control was reducing the disturbance to produce a stable process to achieve a steady state line. The controller tries to eliminate the disturbance to achieve zero error of a process.

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Lastly, the process was tested with test disturbance to observe the ability of the process controller in reducing the disturbance being apply to the process. based on Figure 7, the curves in the chart portion retrieved shows that it keeps oscillating, indicates that the controller try to reduce the disturbance applied throughout the whole process.

6.0 ADDITIONAL QUESTIONS 1. Based on the results, discuss the difference/characteristics of measurement taken by Thermocouple and RTD. No. 1.

Differences/ characteristics Thermocouple Thermocouples are temperature sensors

RTD RTDs are temperature sensors that use the

that use two different metals in the sensor changes in the electrical resistance of metals to produce a voltage that can be read to

to measure the changes in the local

determine the local temperature.

temperature.

CPB30004- Process Dynamics and Control Experiment 5: Heat Exchanger Process Control

2.

3.

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Thermocouple has better cost, cheaper,

RTD has high cost, low measurement

measurement speeds and range of

speeds and narrow range of temperature.

temperature than RTD Thermocouples are designed to be more

RTDs are superior to thermocouples in that

durable and react faster to changes in

their readings are more accurate and more

temperature because of that same design.

repeatable which over multiple trials, the readings of temperature produce same

4.

5.

Thermocouple is produced to be sturdier

results. RTD react slower than thermocouple to

and react quickly to changes in

changes in temperature in same design.

temperature in same design. Thermocouple accept less strong signals

RTD continue bearing constant readings

and it is complicated to alter and read the

longer than Thermocouple. RTD accept

readings.

more strong signals and it is easier to mechanically alter or read the readings due to the design produced

2. Temperature is considered as a process variable with a slow response. Discuss the statement. Temperature is considered as process variable with slow response because it is parameter that takes time to increase or decrease the readings. It is hard to see any different in responding curve when the temperature takes a longer time to response.

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7.0 CONCLUSION There are two main objectives in the Heat Exchanger Process Control experiment. They are to study on/off temperature control of electric heaters and to study temperature control in heat exchanger using PID controller. By controlling the on/off of the heater, the temperature in shell and tube heat exchanger was managed to increase or and the dead band was maintained around 0.5˚C. The heater was switched on so that the process can go back to its steady state as shown in the graph. Except on figure 6, the green line was oscillating perfectly but for the red line it oscillates at the beginning and kept a steady line after a few minutes. This happened because the set point was changed or a disturbance was introduced. The PID controller keeps changing because it needs to reduce the set point so that the zero error can be

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reduced and the offset can be eliminated. By seeing all the important findings throughout the experiment, the objectives of the experiment was achieved and it was a success.

8.0 SUGGESTIONS & RECOMMENDATIONS To make sure the experiment was running smoothly, all members must follow the instructions in the lab manual very carefully. The result that we obtained could have been better if we remember to close the by-pass valve B62 during on/off temperature procedure. This is an important step to obtain more accurate and desirable result and data of this experiment in the future. It is necessary to make sure manual valves is in correct whether open or shut and the manual valve. Besides that, make sure the result or chart is recorded subsequently the data is set up in monitor so that the output response in the chart will stable. Other than that, make sure to know the differences between oscillatory and steady. This is important to test whether the set point was set up correctly or not. If not, the objective to study temperature control using the PID controller cannot be achieved.

9.0 REFERENCES

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1. Max S. Peters & Klaus D. Timmerhaus. (1991). PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS. New York: McGraw-Hill Chemical Engineering Series: Fourth Edition. Retrieved November 17, 2016, from McGrawHill Chemical Engineering Series: Fourth Edition.

2. National instrument. (2011, March 29). PID Theory Explained. Retrieved November 17, 2016, from National Instrument: http://www.ni.com/white-paper/3782/en/#toc2

3. Subramanian, R. S. (n.d). Shell-and-Tube Heat Exchangers. Retrieved November 17, 2016, from http://web2.clarkson.edu/projects/subramanian/ch302/notes/shelltube.pdf

4. Process Control and Common Terms. (n.d.). Retrieved November 17, 2016, from http://www.engineeringtoolbox.com/process-control-terms-d_666.htm