EXPERIMENT NO. 3Air Temperature Process Control (AT 922)

OBJECTIVES1. To identify the important components of the air temperature control system and to

mark them in the P&I Diagram.2. To carry out the start-up procedures systematically. 3. To determine the values of parameters for a first order plus dead time transfer function

model of a thermal process.4. To control the air heater process using PID controller.

KEYWORDSAir temperature system, PID controller, Dead time, FOPTD transfer function model

1.0 INTRODUCTIONThis model uses air to simulate a gas or vapor phase temperature process. The temperature process is a multicapacity lag/dead time process with no noise. It uses a thyristor to adjust the heat flow to the heater. The air flow rate is measured using a rotameter. A selective control technique is employed here that automatically select only a less heat demanding output to manipulate only final control element (the thyristor/heater). This system requires a high gain PID controller.

2.0 EXPERIMENTAL EQUIPMENTThis equipment used in this experiment is the air temperature process control training system, Model AT 922.

2.1 AIR TEMPERATURE PROCESS PLANT (AT 922)The process plant consists of a forced convective annulus electric heater to heat the incoming process air from an external air supply system. The process air is connected to a air pressure regulator (AR90) and flows into the heater and then to the process vent, VT or to another air process control training system. By varying the air flow rate at the discharge manual valve downstream of the rotameter, FI90, the air flow load changes through the heater can be implemented. The purpose of the the PID control is to maintain the air temperature (TE91/TIT91) at the heater exit, at the operator setpoint without burning out the heater.

3.0 EXPERIMENTAL PROCEDURE

3.1 IDENTIFICATION OF MAJOR COMPONENTS IN THE PLANT1. Walk around the experimental equipment, IDENTIFY and MARK the following major

components of the system in the P&I Diagram provided at the end the lab manual. AR90 Air pressure regulator at the process inlet FS90 Flow switch (cut-off the heater power when very low air is detected) MV90A Manual valve at the process inlet PRV90 Pressure relief valve to prevent excessive pressure from AR90 TCY90 Thyristor (installed inside the cubicle, below the control panel) TE91 Resistance temperature detector (RTD), measures the temperature of the hot air at the exit of the heater TIT92 Temperature indicating transmitter for TE92 TR91 Recorder TIC90 ON/OFF temperature controller TIC91 PID temperature controller - Loop 1 TIC92 PID temperature controller - Loop 2 FI90 Variable area flow meter VT Process vent HV90 Heater by-pass

3.2 START-UP PROCEDURES1. The following steps constitute the start-up procedure. Go through these steps before

starting any experiment. 2. Switch the "PANEL, SCADA/DDC" selector switch at the front of the cubicle to

"PANEL,SCADA"position. Switch ON the main power supply at the front of the cubicle. All the panel instruments will lit up. (Make sure the heater is switched OFF at this point)

3. If any annunciator gets activated, press the ACKNOWLEDGE button to silence the buzzer.

4. Shut the manual valve MV90A and set air supply regulator AR90 to the pressure indicated at the air regulator (45 psig).

5. Make sure the heater by-pass valve HV90 and manual valve MV90B are shut. Open the vent valve VT fully to discharge air to the atmosphere.

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6. The followings should be verified:i) Annunciator FAL90 is activated and should be

“ACKNOWLEDGED” (Reason - No air flow because MV90A is shut at the moment)

ii) The pressure gauge PG90 reads as atmospheric pressure. If it reads more, then check and open the vent valve VT.

iii)The variable area flow meters FI90 shows no air.iv) Check the temperature at recorder TR91 is not rising because the heater is

still OFF.7. Manually open fully the inlet valve MV90A so that the air flow rate at flow meter FI90

reads about 45 Nm3/hr. (Do not alter the air pressure that was previously set at air supply regulator AR90)

8. Check the pressure gauge PG90 should reads slightly above atmospheric pressure.9. Now, press the “RCD” button on the recorder TR91 to start recording the temperature

response.

3.3 DETERMINATION OF PROCESS TRANSFER FUNCTION PARAMETERSThe following procedures are conducted for determining the process transfer function parameters.1. Make sure the air flow rate at variable area flow meter FI90 is 45 Nm3/hr. If it is not

read as 45 Nm3/hr, adjust inlet valve MV90A.2. Wait for the temperature response to be almost stable. (Red pen - TE91/TIT91 and

Green pen - TE92/TIT92 are almost steady)3. Note the average air flow rate at the variable area flow meter FI90.4. With TIC91 in Manual (M) mode, adjust its MV = 30%.5. Switch ON the heater and mark on the recorder chart paper the instant the heater is

switched ON.6. Wait until the temperature response become steady. 7. Stop the recorder TR91 by pressing “RCD” button and take out the chart paper and

examine the response.8. Draw the steepest tangent for the temperature response to intersect the baseline to

calculate the deadtime (DT). (Measure the distance (in mm) between the point of intersection and the instant when heater was switched ON)

The dead time is given by:

9. The slope of the steepest tangent gives the Response Rate (RR).10. The process gain, Kp is equal to the maximum change in the air temperature (∆ PV)

at the exit divided by the % change in the manipulated variable (∆ MV%).11.The time constant, θ is equal to the Response Rate (RR) divided by the maximum

change in the air temperature at the exit (∆ PV).*The air temperature process is governed by a first order plus time delay (FOPTD) transfer function. FOPTD transfer function,

K = process gain θ = time delay/dead time

τ= time constant

3.4 CONTROL OF AIR HEATER SYSTEM1. Check the vent VT is fully opened.2. The process air supply and flow rate should remain as previously set in Section 3.3.3. Set a suitable setpoint, SV (i.e. range from 60°C to 120°C) at the temperature PID

controller, TIC91, say 10°C higher than the current setpoint SV. (TIC91 is still in Manual (M) mode)

4. Set “SSW” (at the “PID2” page) to “1” so that only TIC91 is used/activated. This step makes PID 2, Loop 2 disabled.

5. Access the PID values in controller TIC91/TIC92 and set the first (I) trial values. PB1 = 10%, TI1 = 100 s, TD1 = 25 s6. Transfer TIC91 to Auto (A) mode and make sure the heater is ON.7. Observe patiently the temperature response of both the heater surface temperature

(TE90/TIC90, Green pen) and the heated air temperature (TE91/TIC91), Red pen). Wait until the temperature response is steady at its setpoint value, SV.

DO NOT FORGET TO MARK THE PID VALUES AND THE SET-POINT ON THE CHART PAPER WHENEVER THE VALUES ARE CHANGED. ALSO MARK ON THE CHART THE BEGINNING OF EACH PART OF THE EXPERIMENT.

8. Note that the heated air temperature (TE91/TIC91), Red pen) overshoots the setpoint, SV even when the heater surface temperature (TE90/TIC90, Green pen) has started decreasing.

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Deadtime = Dis tance(mm)Recordchartspeed 500mm hr( ) ×

3600s1hr

Y s( )X s( ) =

K .e−θs

τ s +1

9. Repeat the experiment with the following disturbances (using first (I) PID trial values):i) Load disturbance Switch the controller TIC91 to Manual (M) mode. Increase the control output, MV at about 10% to 20%. Then, quickly switch the controller TIC91 to Auto (A) mode. Observe the temperature response.ii) Setpoint change Switch the controller to Manual (M) mode. Step increase the temperature setpoint, SV at controller TIC91 so that it is about the 10 - 15°C above the current setpoint, SV (do not exceed 120°C). Then, quickly switch the controller TIC91 to Auto (A) mode. Observe the temperature response.10. Repeat Step (9) for the following PID trial values: Second (II) trial value: PB1 = 20%, TI1 = 70 s, TD1 = 18 s Third (III) trial value: PB1 = 10%, TI1 = 22 s, TD1 = 5 s

4.0 SHUTDOWN PROCEDURES1. Stop the recorder chart by pressing ”RCD” pressbutton at recorder FPTR91.2. Switch TIC91 to Manual mode with MV = 0%.3. Switch off the heater and let the air flows to cool down the heater for sometime. 4. Switch off the main power supply.5. Shut off the process air supply at AR90.6. Shut off the instrument air supply.

5.0 RESULTS1. The recorder FPTR91 records the results of the experiments. Suitable portions of the

recorder chart paper should be submitted as RESULTS of the experiment.2. The report should contain:

i) The P&I Diagram with all major components marked clearly.ii) Using data/results from Section 3.3, determine the parameters of the first

order plus time delay and its transfer function.iii) The PID temperature control responses for load disturbances and setpoint

changes for different controller settings.iv) Discussion on the response characteristics for different controller settings.v) Temperature process is typically slow compared to others (level, flow and

pressure). Discuss the statement.vi)Temperature process usually exhibits overshoot. Discuss on this matter as

well.

Figure 3.1: An example of process response for PID controller

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Figure 3.2: P&I Diagram for Air Temperature Process Control Plant

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