COMMON THERMAL ELEMENTS

This chapter discusses common thermal elements. It explains that an exponential lag causes a time lag accompanied by a change in magnitude; this was characterized by the transfer function that related its output to its input. A transfer function that causes a time lag without a change in magnitude is the transit delay, which is also called the pure-time lag, distance–velocity lag, dead-time delay, or finite delay. The transit delay is frequently encountered in thermal processes. A typical example is when temperature cannot be measured at the desired point in a process because of the difficulties associated with the installation of the temperature transducer. Then, temperature must be sensed from a transducer located later in the process, when transducer installation is easier. The measurement so made is correct in magnitude but delayed in time when compared with the temperature at the desired location. Another common example is the time delay between the initiation of an actuating signal, such as close valve A, and its achievement Valve A closed. The magnitude of this transit delay depends upon the valve construction and the type of actuating signal—pneumatic, hydraulic, and electric. If the actuating signal is electrical, for example, the transit delay of a solenoid-operated valve will be less than that of a motor-operated valve.