Feedback temperature control

Figure 2.28. Simple feedback temperature control system. M-motor with stirrer, PV-pneumatic valve, Tx - temperature measurement, - temperature controller.

Handling of disturbance in the inlet process stream temperature is passive. Any changes in this load variable will affect the furnace temperature. The change in furnace temperature is measured by the outlet temperature transducer (TT) and sent to the feedback temperature controller (TC). The primary controller then acts accordingly to reduce the deviation in the furnace temperature. 

If a proportional feedback temperature controller is used, calculate the con> troller gain that yields a closedloop damping coefficient of 0.707 and calculate the closedloop time constant of the system when (u) Jacket water only is used. 

Modify your feedforward controller design of Prob. 11.10 so that it can handle both feed temperature and feed flow rale changes and uses a feedback temperature controller to trim up the steam flow. 

Figure 1.2 Feedback temperature control for a tank heater.

Temperature Change with Time-Adjusted Power in Feedback Temperature Control... 

Li et al. (2010) investigated the microwave-assisted convective drying of apple slices with time-adjusted power in feedback temperature control. Three desirable temperatures of 75, 65, and 55 °C were chosen, and the corresponding maximum power requirements were set at 400, 300, and 240 W, respectively. The relationships of power versus time and of power versus moisture content (d.b.) can be described by the following equations, respectively ... 

Where sudden load changes are encountered and close control is necessary, feedforward systems have proven effective. The heat-balance equation is similar to that solved for the heat exchanger in Fig. 8.4. The only difference is that fuel flow is manipulated instead of steam and heat of combustion takes the place of latent heat of vaporization. Although the loss of heat out the stack may be significant, it yaries directly with load and can be readily accommodated by the action of the feedback temperature controller, as is done in Fig. 8.17. 

In a typical experiment, the test sample and a suitable reference material are contained in two separate, identical ampoules kept at constant temperature in separate, identically constructed wells of the calorimeter. Ideally, the reference material is identical or very similar to the test sample in mass, heat capacity and thermal conductivity, but, unlike the test sample, it is thermally inert (i.e. the reference material will not undergo changes that result in heat production or absorption under the conditions of the experiment). One example is a small quantity of ordinary glass beads in air at room temperature used as reference for the same amoimt of a hydrated ceramic material which is expected to lose water under the same conditions. Consequently, most of the noise arising from temperature fluctuations is removed when the reference data are subtracted. A feedback temperature control system between the wells (a) serves to ensure that the temperature difference between the weUs is zero and (b) provides an output that measures any difference in electric power requirement of one well relative to the other, needed to keep the temperature of both weUs the same. This power difference, as a function of time, is the output from the calorimeter, which is recorded continuously or intermittently over the duration of the test. 

FIG. 8-53 The reactor temperature controller sets coolant outlet temperature in cascade, with primary integral feedback taken from the secondary temperature measurement. 

Regulatory Control For most batch processes, the discrete logic reqmrements overshadow the continuous control requirements. For many batch processes, the continuous control can be provided by simple loops for flow, pressure, level, and temperature. However, very sophisticated advanced control techniques are occasionally apphed. As temperature control is especially critical in reactors, the simple feedback approach is replaced by model-based strategies that rival if not exceed the sophistication of advanced control loops in continuous plants. 

Case study example 4.6.2 (Temperature Control) %Use of feedback... 

Example 10.2 Consider the temperature control of a gas furnace used in heating a process stream. The probable disturbances are in the process stream temperature and flow rate, and the fuel gas flow rate. Draw the schematic diagram of the furnace temperature control system, and show how feedforward, feedback and cascade controls can all be implemented together to handle load changes. 

B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lube oil cooler is the plant in this example, and its controlled output is the lube oil temperature. The temperature transmitter is the feedback element. It senses the controlled output and lube oil temperature and produces the feedback signal. 

Fig. 2.7 Temperature profiles for a 30mL sample of 1-methyl-2-pyrrolidone heated under open-vessel microwave irradiation conditions [19]. Multimode microwave heating at different maximum power levels for 6 min with temperature control using the feedback from a fiber-...

Fig. 2.8 Temperature (T), pressure (p), and power (P) profiles for a 3 mL sample of methanol heated under sealed-vessel microwave irradiation conditions [12]. Single-mode micro-wave heating (250 W, Q-30s), temperature control using the feedback from IR thermography...

Fig. 4.4 Temperature and power profiles for a Biginelli condensation (Scheme 4.24.a) under sealed quartz vessel/microwave irradiation conditions (see Fig. 3.17). Linear heating ramp to 120 °C (3 min), temperature control using the feedback from the reference vessel temperature measurement (constant 120 °C, 20 min), and forced air cooling (20 min). The reaction was performed in eight quartz vessels...

Fig. 5.2 Temperature profile for a 30 ml sample ofwater heated under sealed-vessel conditions. Multimode microwave heating with 100 W maximum power for 8 min with temperature control using the feedback from a f ber-optic probe ramp within 120 s to 70 °C hold for 120 s at 70 °C ramp within 120 s to 100 °C hold for 120 s at 100 °C.

Fig. 5.3 Heating profile for a typical Biginelli condensation in AcOH/EtOH (3 1) under sealed-vessel microwave irradiation conditions microwave flash heating (300 W, 0-40 s), temperature control using the feedback from IR thermography (constant 120 °C, 40-600 s), and active cooling (600-660 s).

For accurate temperature monitoring when conducting a temperature-controlled program, a minimum filling volume of the vessels is crucial. In the case of IR temperature measurement from the bottom of a vessel, only a very small amount of reaction mixture (ca. 50 pL) is sufficient to obtain a precise temperature feedback in a monomode instrument (CEM Discover series). On the other hand, a rectangular mounted IR sensor, as used in Biotage instruments (see Section 3.5) requires a certain minimum filling volume (200 pL for the smallest reaction vials see Fig. 3.21). 

The temperature sensor is located in the microhotplate center (J2x, 10 kO nominal). This polysilicon resistor is biased with a temperature-independent current source (/bias)- The voltage-drop across the polysilicon temperature sensor provides the feedback signal for the temperature controller. 

Production of the API begins with the selection of a synthetic route, as determined in the development program. Raw materials are added into a reaction vessel. These raw materials as reactants are heated or cooled in the reaction vessel (normal range is from -15 to 140 °C purpose-built vessels are needed for extreme reactions that require lower or higher temperature controls or pressurization of reaction processes). The chemical synthesis reactions are monitored and controlled via sensor probes (pH, temperature, and pressure) with in-process feedback controls for adjustments and alarms when necessary. Samples are withdrawn at dehned intervals for analysis to determine the reaction progress. Catalysts, including enzymes, may be added to speed up and direct the reaction along a certain pathway. 

If all the state variables are not measured, an observer should be implemented. In the Figure 14, the jacket temperature is assumed as not measured, but it can be easily estimated by the rest of inputs and outputs and based on the separation principle, the observer and the control can be calculated independently. In this structure, the observer block will provide the missing output, the integrators block will integrate the concentration and temperature errors and, these three variables, together with the directly measured, will input the state feedback (static) control law, K. Details about the design of these blocks can be found in the cited references. 

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