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Temperature control system

1. Introduction
A control system is a device, or set of devices, that manages, commands, directs or regulates the behavior of other device(s) or system(s). A kind of the system is temperature control system which is a device which the change of temperature (and objects collectively within) are measured or otherwise detected, and is adjusted to achieve a desired average temperature.

2. How does it work?
A temperature control system for a HVAC system can range from the simplest room thermostat to the most complicated computerized system, has four basic elements: sensor, controller, controlled device and source of energy.
•  Sensor measures the actual value of controlled variable such as temperature, humidity or flow and provides information to the controller.
•  Controller receives input from the sensor, processes the input and then produces an output signal for controlled device.
•  Controlled device acts to modify a controlled variable as directed by controller.
•  A source of energy is indeed needed to power the control system.

3. Benefits
Temperature control system are required for one or more of the following reasons:
• Maintain thermal comfort conditions
• Reduce energy use
• Safe plant operation
•  To reduce manpower costs
•  Identify maintenance problems
•  Monitoring system performance

4. Applications
There are three basic types of controllers: on-off, proportional and proportional – integral – derivative (PID)(1). Depending upon the system to be controlled, the operator will be able to use one type or another to control the process.

On-Off Control
An on-off controller is the simplest form of temperature control device. An on-off controller will switch the output only when the temperature crosses the setpoint. For cooling control, the output is on when the temperature is above the setpoint, and off below setpoint.
On-off control is usually used where a precise control is not necessary, in systems which cannot handle having the energy turned on and off frequently, where the mass of the system is so great that temperatures change extremely slowly, or for a temperature alarm.

Proportional Control
Proportional controls are designed to eliminate the cycling associated with on-off control. A proportional controller decreases the average power supplied to the A/C as the temperature approaches setpoint.
One of the advantages of proportional control is the simplicity of operation. It may require an operator to make a small adjustment (manual reset) to bring the temperature to setpoint on initial startup, or if the process conditions change significantly.

PID Control
The third controller type provides proportional with integral and derivative control, or PID. This controller combines proportional control with two additional adjustments, which helps the unit automatically compensate for changes in the system. It is recommended in systems where the load changes often and the controller is expected to compensate automatically due to frequent changes in setpoint, the amount of energy available, or the mass to be controlled.

Copyright
(1) http://www.omega.com/prodinfo/temperaturecontrollers.html

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