UNIT 1

Introduction

Welcome to ISA's Independent Learning Module on Temperature Control. The first unit provides an overview of the applications, distinguishing characteristics, and control system considerations for temperature control.

Learning Objectives When you have completed this unit you should:

1. Appreciate the significance of improving temperature control.
   
   
2. Recognize the four common sources of temperature control problems.
   
   
3. Anticipate sensor, controller tuning, and control strategy options for different applications.

1-1. Importance of Good Temperature Control

Temperature is one of the four most common types of loops. While the other common loops (flow, level, pressure) occur more often, temperature loops are generally more difficult and important. It is the single most frequently stated type of loop of interest to users, and the concern for better control extends to the widest variety of industries.

Temperature is a critical condition for reaction, fermentation, combustion, drying, calcination crystallization, extrusion, or degradation rate and is an inference of a column tray concentration in the process industries.

Tight temperature control translates to lower defects and greater yields during seeding, crystal pulling, and rapid thermal processing of silicon wafers for the semiconductor industry.

For boilers, temperature is important for water and air preheat, fuel oil viscosity, and steam superheat control. For incinerators, an optimum temperature often exists in terms of ensured destruction of hazardous compounds and minimum energy cost. For heat transfer fluids such as cooling tower, chilled water, brine, or Therminol , good temperature control minimizes upsets to the users.

Temperature control in cold rooms is used to reduce the contamination and degradation rate in pharmaceutical, biochemical, beverage, and food research and production.

Temperature control in plant growth chambers is important for studying the effects of hybridization, genetic engineering, and plant growth regulators.

The great impact of temperature control on conductivity and pH loops was not widely realized until recently. For aqueous solutions of acids, bases, and salts, the inferred concentration from conductivity and the pH of the solution often changes at the rate of about twenty percent and four tenths of a pH unit, respectively, per ten degrees centigrade. These changes, which reflect a fundamental alteration in the solution due to different ion dissociation and mobility, should not to be confused with temperature effects on the signal developed by the electrode, which are corrected for by conventional temperature compensators. Thus, temperature control can be important for raw, cooling tower, deaerator, boiler, and wastewater treatment.

Good temperature control is important during the research, reaction, separation, processing, and storage of products and feeds and is thus a key to product quality It is also of importance for environmental control and energy conservation.

Tight temperature control can extend the life of process equipment (e.g., reactor glass lining, scrubber fiberglass trays, or furnace firebrick) by prevention of excursions beyond the temperature rating. However, abrupt changes in coolant or steam flow can shock equipment and upset other utility users. Thus, it is also important to monitor the controller output and use methods (e.g., set point velocity limits and split-range, criss-cross prevention logic) to prevent rapid changes or oscillations. Since temperature control is typically achieved by the direct or indirect manipulation of heat flow into or out of the system, a reduction in the overshoot and oscillation of temperature loops can also correspond to a decrease in energy consumption.