Temperature ControlIn TheProcess Industries

By: Rahat Inayat Ali

Temperature control in the process industries

There are many commercial processes, which may require that the temperature of a material be controlled to produce an acceptable product. This control can be accomplished manually or automatically.

Manual Temperature Control

In manual control an operator periodically reads the process temperature and adjusts the heat or cooling input up or down in such a direction as to drive the temperature to its desired value. Manual control may be used in non-critical applications where small changes in the manipulated variable cause the processes to change slowly and by a small amount.

This is only practical if there are few processes with infrequent process upsets. Manual temperature control is extremely resource heavy, as in order for this to work, the operator must have sufficient time to make a correction before the process temperature exceeds acceptable tolerance.

Automatic Temperature Control System

When accuracy is a must, when several processes are involved, or temperature changes occur too rapidly, it is difficult for the operator to make correction, the use of an automatic control system is indicated. Typically these days the ability to simplify processes and improve efficiencies means that most temperature control systems are automatic.

Temperature Controller Devices:

The temperature controllers are the device used to hold a desired temperature at a specified value.

On/off temperature control devices proportional temperature control devices proportional-integral-differential(PID)

temperature control devices

Temperature Controller Devices:

Working principal of the temperature control devices:

The controlled variable, in this case ‘temperature’ is measured by a suitable sensor such as a thermocouple, RTD, thermistor or infra-red pyrometer and converted to a signal acceptable to the controller.

The controller compares the temperature signal to the desired temperature (setpoint) and actuates the final control device. The final control device alters the manipulated variable to change the quantity of heat being added or taken from the process.

Common manipulated variables in temperature controlled processes are air, water, steam, electricity, oil and gas

Common temperature Control Applications;

Temperature controllers in industry work much the same way they do in common household applications. A basic temperature controller provides control of industrial or laboratory heating and cooling processes. In a typical application, sensors measure the actual temperature. This sensed temperature is constantly compared to a user set point. When the actual temperature deviates from the set point, the controller generates an output signal to activate other temperature regulating devices such as heating elements or refrigeration components to bring the temperature back to the set point.

Common Uses in Industry:

Temperature controllers are used in a wide variety of industries to manage manufacturing processes or operations. Some common uses for temperature controllers in industry include plastic extrusion and injection molding machines, thermo-forming machines, packaging machines, food processing, food storage, and blood banks. The following is a brief overview of some common temperature control applications in industry:

Heat Treat/OvenTemperature controllers are used in ovens and in heat-treating applications within furnaces, ceramic

kilns, boilers, and heat exchangers.

PackagingIn the packaging world, machinery equipped with seal bars, glue applicators, hot melt functions,

shrink wrap tunnels or label applicators must operate at designated temperatures and process time lengths. Temperature controllers precisely regulate these operations to ensure a high quality product output.

PlasticsTemperature control in the plastics industry is common on portable chillers, hoppers and dryers and

molding and extruding equipment. In extruding equipment, temperature controllers are used to precisely monitor and control temperatures at different critical points in the production of plastic.

HealthcareTemperature controllers are used in the healthcare industry to increase the accuracy of temperature

control. Common equipment using temperature controllers includes laboratory and test equipment, autoclaves, incubators, refrigeration equipment, and crystallization growing chambers and test chambers where specimens must be kept or tests must be run within specific temperature parameter

Draft Control The air moving through this process is called the draft.  

Draft control devicesThe draft regulator or the draft controller are typically a round device with a swinging door, which will only swing one way, and with an adjustable weight on it, which will allow you to adjust it to control the draft. You adjust the weight to control the amount of the opening of this damper which in turn controls the amount of excess air that can enter the flue and chimney when the oil burner is operating. In this way you adjust the draft over the fire in the appliance.

Classification Of The Drafts

Classification Of The Draft Controls;

Drafts are produced by the rising combustion gases in the stack, flue, or by mechanical means. For example, a blower can be put into four categories: natural, induced, balanced, and forced.

Natural draft: When air or flue gases flow due to the difference in density of the hot flue gases and cooler ambient gases. The difference in density creates a pressure differential that moves the hotter flue gases into the cooler surroundings.

Forced draft: When air or flue gases are maintained above atmospheric pressure. Normally it is done with the help of a forced draft fan.

Induced draft: When air or flue gases flow under the effect of a gradually decreasing pressure below atmospheric pressure. In this case, the system is said to operate under induced draft. The stacks (or chimneys) provide sufficient natural draft to meet the low draft loss needs. In order to meet higher pressure differentials, the stacks must simultaneously operate with draft fans.

Balanced draft: When the static pressure is equal to the atmospheric pressure, the system is referred to as balanced draft. Draft is said to be zero in this system.

Types Of The Draft Control Devices

AUTOMATIC FLUE DAMPER or FLUE DAMPENER BARMOMETRIC FLUE DAMPER or FLUE DAMPENER COALSTOVE DRAFT CONTROL DRAFT INDUCERS DUCT DAMPERS & ZONE DAMPERS, AUTOMATIC DUCT DAMPERS & ZONE DAMPERS, MANUAL DUCT DAMPERS, SUPPLY AIR REGISTER DUCT VIBRATION DAMPENERS FIRE DAMPERS FIREPLACE DAMPERS FRESH AIR DAMPER CONTROLS KERONSENE HEATER DRAFT CONTROL PELLET STOVE DRAFT CONTROL WOOD-OIL COMBINATION DRAFT CONTROL WOODSTOVE DRAFT CONTROL

Why Draft Control Is Important?

Too much draft will decrease the efficiency of the heating appliance, increasing cost, but it will also increase chimney temperatures (stack temperature) to an unsafe level.  Too little draft can result in incomplete combustion, soot, puff backs, and in an extreme case, carbon monoxide.  Since the chimney draft varies greatly, we need a way to stabilize it.  This is the job of the draft regulator.

Working Principal Of The Draft Control

Static pressure of the cool air (1) Illustration A exerts pressure on the outside of the furnace or boiler, the breaching, and stack. The pressure difference between the room air and heated gas (air) causes products of combustion (2) to flow (draft) through the unit and rise through the breaching and chimney. Room temperature air (3) enters through the barometric draft control (4) in the precise amount needed to overcome the excess drafts caused by temperature variations, wind fluctuations, and barometric pressure changes. Combustion of fuel is complete and the process is stabilized. The velocity of combustion gases through the heat exchanger is slowed so more heat is extracted. The unit operates more efficiently, reliably, and requires lessmaintenance.