temperature control system...himanshu ramdeo

Project on Temperature

control system

Submitted by-:-Himanshu ramdeo071063electrical

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What is temperature control system?

Whatever the process or the parameter (temperature, flow, speed for example), the principles of control are similar.

Input and output signals are specified as appropriate to the application, usually analog (e.g. thermocouples signal input, solid state output power control) but these may be digital.

Control of a process is achieved by means of a closed loop circuit (power fed to the heater is regulated according to feedback obtained via the thermocouple) as opposed to an open loop in the case of measurement only.

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What is temperature controller ?

To accurately control process temperature without extensive operator involvement, a temperature control system relies upon a controller, which accepts a temperature sensor such as a thermocouple or RTD as input.

It compares the actual temperature to the desired control temperature, or setpoint, and provides an output to a control element.

The controller is one part of the entire control system, and the whole system should be analyzed in selecting the proper controller

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The following items should be considered when selecting a controller:

1)Type of input sensor (thermocouple, RTD) and temperature range.

2)Type of output required (electromechanical relay, SSR, analog output)

3)Control algorithm needed (on/off, proportional,PD, PID)

4)Number and type of outputs (heat, cool, alarm, limit)

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What Are the Different Types of Controllers, and How Do They Work?There are three basic types of controllers:

on-off proportional PD PID. Depending upon the system to be controlled,

the operator will be able to use one type or another to control the process.

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a) On/Off ControlAn on-off controller is the simplest form of

temperature control device. The output from the device is either on or off,

with no middle state. An on-off controller will switch the output only

when the temperature crosses the setpoint. For heating control, the output is on when the

temperature is below the setpoint, and off above setpoint.

Since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below.

Since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below.

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b) 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 heater as the temperature approaches setpoint.

This has the effect of slowing down the heater so that it will not overshoot the setpoint, but will approach the setpoint and maintain a stable temperature.

This proportioning action can be accomplished by turning the output on and off for short time intervals.

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c) Proportional + Derivative (PD) – The Derivative term when combined with proportional action improves control by sensing changes and correcting for them quickly. The proportional is effectively intensified (its gain is increased) to achieve a quicker response.

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d) PID ControlThe 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.

These adjustments, integral and derivative, are expressed in time-based units; they are also referred to by their reciprocals, RESET and RATE, respectively.

The proportional, integral and derivative terms must be individually adjusted or “tuned” to a particular system using trial and error.

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The basic block diagram of a simple control system for maintaining oil temperature in a machine -:

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Some practical application of temperature control system

a)The temperature control system of furnace.

b)The temperature control system of swimming pool.

c)Ultra-precision temperature control system for machine tool.

d)The temperature control of industrial chemical processes.

e) Temperature control of some practically hazardous food.

a) The temperature control of furnace-: The thermostatic switch is used in open loop case when temperature exceeds the desired value (obtained from indicator) connected of the combustion chamber of furnace and no feed back is given to the input.

In closed loop case the sensor detects the temperature and output is given to the switch which is either a power switch or relay which operates only when the difference between input and output is not zero.

b) The temperature control of swimming pool-:

This invention relates to heater temperature control systems, and more particularly, to swimming pool gas, oil or electric heater temperature control systems where the heater is used as a supplemental heat source for a solar heated swimming pool.

Many prior art systems have been developed to control the temperature of conventional gas, oil and electric swimming pool heaters. Basically, these systems include a thermostat which senses the temperature of the pool water and energizes the heater when the water temperature is below a preset temperature level. This temperature level is set by the user to achieve a comfortable swimming temperature in the pool.

Control systems have also been developed in the prior art to adapt the use of solar collectors for heating a swimming pool in an effort to minimize energy consumption. Typically, these systems include means for diverting pool water to the solar collectors whenever the collector temperature exceeds the pool water temperature.

In the block diagram of FIG. the swimming pool water 10 is shown by double solid lines with arrows indicating direction of flow. Electrical connections are shown by single solid lines.

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The thermostat 30 is used to sense the pool water temperature.

When the electrical circuit is completed between the heater control power supply 24 and the controller 26, the heater is energized and begins heating the swimming pool water 10.

The heater 12 will remain energized until the pool water temperature reaches the preset level of thermostat 30 at which point the heater will cycle on and off and maintain the pool water 10 at the desired preset temperature level.

As shown in FIG. the motor 18 used to drive the water pump 16 is energized by means of a time clock 40. The time clock 40 typically consists of a clock motor which makes one full revolution every twenty-four hours

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c)Ultra-precision temperature control system for machine tool-:

In this controls case a sensitive liquid is allowed to flow through the machine tools (cutting tools of lathe machine) . The temperature of liquid is measured with a thermometer which is inserted at the outlet of the liquid.

When the temperature of the liquid is beyond the set limit a signal is sent to the motor of the machine which reduces the speed of the motor .

As a result the temperature of tool is reduced due to less friction losses .

This method of temperature control is highly reliablesimplerlow in costless maintenance requirementhigh precision controlno harm to the working toolssometimes even liquid can be used as a coolant and a sensor

d) Controlled indusrial process: In batch process the reactants are initially charged into the reaction vessel of the batch reactor and are then agitated for a certain period of time to allow the reaction to take place .Upon the completion of reaction the products are discharged.

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For a specific reaction there is an optimum temperature profile according to which the temperature of the reactor mass should be varied to obtain best results.

time

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Automatic temperature control is achieved by providing both steam and cooling water jackets for heating or cooling the reactor mass (cooling is required to remove exothermic heat of reaction during the period the reaction proceeds vigorously).

During the heating phase , the controller the controller closes the water inlet valve and opens and controls the stem inlet valve while the condensate valve is kept open .

Reverse reaction takes place during the cooling phase.

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e) Temperature control of some practically hazardous food :

Food may cause illness because there are high levels of food-poisoning bacteria in the food. The bacteria themselves may make your customers ill or the bacteria may have produced poisons in the food that cause illness. These poisons are called toxins.

A way of preventing or limiting bacteria from multiplying or producing toxins in food is to control the temperature of the food by either keeping it cold or very hot.

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The following foods are examples of potentially hazardous foods:

• Raw and cooked meat (including poultry and game) or foods containing raw or cooked meat such as casseroles, curries and lasagne;

• Small goods such as Strasbourg, ham and chicken loaf;

• Dairy products, for example, milk, custard and dairy-based desserts such as cheesecakes and custard tarts;

• Seafood (excluding live seafood) including seafood salad, patties, fish balls, stews containing seafood and fish stock;

• Processed fruits and vegetables, for example salads and cut melons;

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How to measure the temperature of food? You will need a thermometer that can measure the

internal temperature of food because the surface temperature may be warmer or cooler than the temperature of the rest of the food. This means that you will need a thermometer with a probe that can be inserted into the food.

The thermometer must also be accurate to +/–1C. This means that when the thermometer shows that the food is at a temperature of 5C, the actual temperature of the food will be between 4C and 6C.

Check that your thermometer is accurate by placing the probe in a container of crushed ice that is just melting. The thermometer should read 0C within 1C, that is, between 1C and +1C.

Remember to clean and sanitize the thermometer before inserting it in food.