The purpose of this unit is to heat the process fluid from some inlet temperature,
Ti(t), up to a certain desired outlet temperature, T(t). As mentioned,the heating medium is condensing stream.
The energy gained by the process fluid is equal to the heat released by the steam,
provided there are no heat losses to surroundings, that is, the heat exchanger and piping are well insulated.
In this process there are many variables that can change, causing the outlet
temperature to deviate from its desired value. If this happens,some action must be taken to correct for this deviation. That is, the objective is to cntrol the outlet process temperature to maintain its desired value.
One way to accomplish this objective is by first measuring the temperature T(t),
then compareing it to its desired value, and, based on this comparison, deciding what to do to correct for any deviation. The flow of steam can be used to correct for the deviation. This is, if the temperature is above its desired value,then the steam value can be throttled back to cut the stearr flow (energy) to the heat exchange. If the temperature is below its desired value,then the steam value could be opened some more to increase the steam flow (energy) to the exchanger. All of these can be done manually by the operator, and siince the procedure is fairly straightforward, it should present no problem. However, since in most process plants there are hundreds of variables that must be maintained at some desired value, this correction procedure would required a tremendous number of operators. Consequently, we would like to accomplish this control automatically. That is, we want to have instnnnents that control the variables requring intervention from the operator. So this isd what we mean by automatic process control.
To accomplish his objective a control system must be designed and implemented.
A possible control sysatem and its basic components are shown in Fig.2.
The first thing to do is to measure the outlet temperature of the process stream. A
sensor (thermocouple, thermistors, etc) dose this.This sensor is connected physically to a transmitter, which takes the output from the sensor and converts it to a signal strong enough to be transmitter to a controller. The controller then receives the signal, which is related to the temperature, and compares it with desired value. Depending on this comparison, the controller decides what to do maintain the temperature at its desired value. Base on this decision, the controller then sends another signal to fiinal control element, which in turn mainpulates the steam flow.
The preceding paragraph prsents the four basic components of all control systems.
They are
(1) sensor, also often called the primary element. (2) Transmitter, also often called the secondary element. (3) Controller, the ―brain‖ of the control system.
(4) Final control system, often a coontrol value but not always.
Other common final control elements are variable speed pumps, conveyors, and electric motors.
The important of these components is that they perform the three basic operation that must be persent in every control system. These operations are
(1) Measurement (M) : measuring the variable to be controlled is usually done by the combination of sensor and transmitter.
(2) Decision (D) : Based on the measurement, the controller must then decide what to do to maintain the variable at its desired value.
(3) Action (A) : As a result of the controller’s decision, the system must then take an action. This is usually accomplished by the final control element.
As mentioned, these three operations, M, D, and A, must be present in every control system.
PID controllers can be stand-alone controllers (also called single loop controllers), controllers in PLCs, embeded controllers, or software in Visual Basic or C# computer programs.
PID controllers are process controllers with the following characteristics: Continuous process control
Analog input (also known as ―measuremem‖ or ―Process Variavle‖ or ―PV‖) Analog output (referred to simply as ―output‖) Setpoint (SP)
Proportional (P), Integral (I), and/or Derivative (D) constants.
Example of ―continuous process control‖ are temperature, pressure, flow, and level control. For example,controlling the heating of a tank. For simple control, you have two temperature limit sensors (one low and one high) and then switch the heater on when the low temperature limit sensor tums on and then the heater off when the temperature rises to the high temperature limit sensor. This is similar to most home air conditioning & heating thermostats.
In contrast, the PID controller would receive input as the actual temperature and control a value that regulates the flow of gas to the heater. The PID controller automatically finds the correct (constant) flow of gas to the heater that keeps the temperature bouncing back and forth between two points, the temperature is held steady. If the setpoint is lowered, then the PID controller automatically reduces the
amount of gas flowing to the heater. If the setpoint is raised, then the PID controller automatically increases the amount of gas flowing to the heater. Likewise the PID controller would automatically for hot, sunny days (when it is hotter outside the heater) and for cold, cloudy days.
The analog input (measurement) is called the ―process variable‖ or ―PV‖. You want the PV to be a highly accurate indication of the process parameter you are trying to control. For example ,if you want to maintain a temperature of or – one degree then we typically strive for at least ten times that or one – tenth of a degree. If the analogy input is a 12 bit analog input and the temperature rangs for the sensor is 0 to 400 degrees then our ―theoretical‖ accuracy is calculated to be 400 degrees divided by 4, 096(12 bits) = 0.09765625 degrees. We say ―theoratical‖ because it would assume there was no noise and error in our temperature senor, wiring, and analog converter. There are other assumptions such as linearity, etc… The point being -- with 1/10 of a degree ―theoretical‖ accuracy – even with the usual amount of noise and other problems – one degree of accuracy should easily be attainable.
The analog output is often simply referred to as ―output‖. Often this is given as 0~100 percent. In this heating example, it would mean the value is totally closed (0%) or totally open (100%).
The setpoint (SP) is simply – what process value do you want. In this example – what temperature do you want the process at?
The PID controller’s job is to maintain the output at a level so that there is no different (error) between the process variable (PV) and the setpoint (SP).
In Fig.3, the value could be contorlling the gas going to a heater, the chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or any other process control system. What the PID controller is looking at is the difference (or ―error‖) between the PV and the SP.
It looks at the absolute error and the rate of change of error. Absolute error means – is there a big difference in the PV and SP or a little difference? Rate of change of error means – is the difference between the PV or SP getting smaller or larger as time goes on.
When there is a ―process upset‖,meaning, when the process variable or the setpoint quickly changes – the PID controller has to quickly change the output to get the process variable back equal to the setpoint. If you have a walk – in cooler with a PID controller and someone opens the door and walks in, the temperature (process variable) could rise evry quickly. Therefore the PID controller has to increase the cooling (output) to compensate for this rise in temperature.
Once the PID controller has the process variable equal to the setpoint, a good PID controller will not vary the output. You want the output to be evry steady (not changing). If the valve (motor, or other control element) is constantly changing,