The controller senses thermal fluid temperature (known as process variable or PV) from a thermocouple. The controller processes the thermocouple signal and produces a control signal or output that it sends to Honeywell modulating actuator TZ4-1. The actuator responds by either increasing or decreasing the burner firing rate as required to maintain fluid temperature at the set point. Output of the controller is proportional when the thermal fluid temperatures are within a certain range known as the proportional band.
Please see Fig. 6 for a graph that depicts the characteristics of proportional control for a heater that is theoretically the optimum size. It helps in understanding the explanations given below.
In Fig. 6 the output of the controller that maintains the temperature of the thermal fluid at SP is labeled CONSTANT. This is the output of the controller when the error or deviation is zero while the heater is in a steady state. Theoretically, when the set point is the midpoint of the PV range, the constant will be 50 percent. However, the constant for our heaters is usually much less than 50 percent and in rare instances more than 50 percent. It varies because of numerous factors, such as the size of the heater, how well the piping is insulated, etc.
Fig. 7 shows the proportional control for a hypothetical heater that is oversized with an assumed constant of 25 percent. Fig. 8 shows the proportional control for a hypothetical heater that is undersized with an assumed constant of 75 percent.
In all cases, the proportional band is a percentage of the PV (process variable) range. The PV range is determined by what thermocouple is used to sense the temperature of the thermal fluid flowing out of the heater coil. The thermocouple most frequently used with Heatec heaters is a type J with a low range. It is known as “J-LOW” and appears on the controller as JTCL. Its low range can measure temperatures as low as 20 degrees F and as high as 770 degrees F. Accordingly, it has a range of 750 (770 minus 20) degrees and this becomes the PV (process variable) range. Thus, the value you set for the proportional band is a percentage of 750.
Setting the PROP BD to a value of 2 means 2 percent of 750 (or 15 degrees). Thus, a span of 15 degrees F is used as the proportional band. Accordingly, the output of the controller will be proportional over a span of 15 degrees, and this is the full modulating range or proportional band of the controller.
The SP (set point) is always somewhere within that proportional band, usually the center of the band. (The set point is the thermal fluid temperature you wish to maintain.) Heaters at most asphalt plants use a set point of 320 degrees F. So, we will use that as the set point in our explanations. Using 320 degrees F as the SP, the proportional band will range from 312.5 to 327.5 degrees F as shown in Fig. 6, 7 and 8. Controller outputs for temperatures between those two points are proportional.
However, the size or the heater in relation to the heat load affects the firing rate needed to maintain set point and other temperatures within the proportional band. Note the differences in firing rates for a heater of optimum size compared to heaters that are oversized and undersized as shown in Fig. 6, 7 and 8.
Fig. 6 shows the controller output for a heater of optimum size. Its output is at 100 percent when the temperature is at 312.5 degrees F. Its output is at 50 percent when the temperature is at the set point of 320 degrees F. Its output is at zero percent (or low fire) when the temperature is at 327.5 degrees F.
Fig. 7 shows the controller output for a heater that is oversized. Its output is at 50 percent when the temperature is at 312.5 F. Output is at 25 percent when the temperature is at the set point of 320 degrees F. Its output is at zero percent (or low fire) when the temperature is at 327.5 F.
Fig. 8 shows the controller output for a heater that is undersized. Its output is at 100 percent when the temperature is at 312.5 F. Its output is at 75 percent when the temperature is at the set point of 320 degrees F. Its output is at 50 percent when the temperature reaches 327.5 F. Its output is at zero percent (or low fire) if the temperature exceeds 327.5 degrees F.