Hot Water Tank Experiment

Aims

The aims of this experiment are

To show how a control system can be set up
To introduce the use of on-off control

This example is based on an experiment in the control laboratory. It is proposed to control the temperature of the water in a tank using on/off feedback control. This is the simplest and most commonly experienced type of control. It is possible to determine the relationship between the precision obtained and the amount of oscillation.

Theory

There are two theory sections associated with this experiment:

Setting up a Control System, discussed in Module 1.2
Operation of the Control Loop

Pre-Experimental Questions

Below are a number of pre-experimental questions. Please think about the answers while you are running the experiment. The answers will be given after the simulation has been run.

What is the effect of altering the flowrate through the system?
What is the effect of altering the deadtime of the controller?
What are the disadvantages in using on/off control?
Can you think of any situations where on/off control could be used and others where it definately can't?

Experimental Apparatus

Below is a diagram and photograph of the experimental apparatus.

Note that it consists of

A tank of water. Cold water enters at the bottom of the tank and the hot water overflows out the top thus keeping a constant volume. The flowrate can be altered as part of the experiment. The tank is insulated to reduce heat losses to the environment.
A temperature measurement device.
An electric heater.
A controller. This adjusts the power to the heater either turning the supply on or off.

Procedure

Before the experiment is run, it is necessary to give some information about the system:

The temperature of the cold water entering the tank is 20^oC.
The setpoint temperature of the hot water leaving the tank is 40^oC.
There is a fixed heat input of 200W.
The flowrates in and out are equal hence the mass in the tank stays constant at 10kg.

To carry out the experiment simply choose appropriate values for the hot water demand rate and the dead zone for the controller and click on Run Experiment. This is done by clicking on the button using the mouse and dragging the cursor to the desired value.

The computer then takes these values, runs a program and produces the results. These come in the form of a graph showing

The measured temperature
The setpoint temperature
An indication of when the heater is on

It is possible to run the experiment any number of times. Please try different hot water demand rates e.g. flows of 0.5 and 1.5 kg/s and different precision of control e.g. dead zones of 1.0 and 20 %.

Answers

Click here for the answers

Summary

In summary therefore, this is the simplest and most commonly experienced type of control, although not the most commonly used in chemical processes. Advantage of this type is that it is inexpensive and extremely simple. The disadvantage lies mainly in the oscillatory nature of the control with oscillations becoming more rapid as precision is increased. Therefore this type of control is suitable only for those applications where it can be used alone and close control is not essential. It is also applicable only to systems which have a significant capacity as will be seen in a later example.