The proportional band (for an asphalt plant) is usually from 7-1/2 degrees below set point to 7-1/2 degrees above set point. Thus, when the set point is 320 degrees F, the controller causes the burner to modulate or vary proportionally while thermal fluid temperatures vary from 312.5 to 327.5 degrees F. Thermal fluid temperatures above 327.5 cause the controller to go to zero output, which sets the burner to its low-fire position. Thermal fluid temperatures below 312.5 cause the controller to fire the burner at its maximum rate.
Burner low-fire control. This function of the Modulating Controller causes the burner to change from low-fire to high-fire when the temperature of the thermal fluid reaches a predetermined value. It is normally set manually at 200 degrees F. Thus, the burner will remain at low-fire until the thermal fluid temperature reaches about 230 degrees F. At that point it goes to high fire and remains there until the temperature reaches the modulating range.
Burner on-off control. This function of the Modulating Controller shuts off the burner completely by shutting off its main fuel valve. This happens if system heat demand decreases, allowing thermal fluid temperature to reach a limit equal to the set point plus a predetermined value. This value is normally set manually at 20. It is known as a floating value since it creates the limit based on the set point. Thus, if its value is 20 and the set point is 320 degrees F, the limit is 340 degrees F (the sum of 320 plus 20). After the thermal fluid cools down about 30 degrees the controller initiates a new burner cycle.
Please see separate Tec-Note on programming and re-tuning the Honeywell UDC3300 for additional explanation of how the controller works.
HIGH MEDIA TEMP CONTROLLER (Fig. 2)
Note: the term media means thermal fluid. Thermal fluid is commonly called hot oil.
The temperature controller normally used is a Honeywell UDC2300. It limits maximum temperature of thermal fluid to a set point (SP) value that the operator presets in the controller. (Do not confuse the set point of this controller with the set point of the Modulating Controller.)
The high media temperature controller senses fluid temperature from a thermocouple (Fig. 6) in the outlet side of the helical coil. If the temperature of the thermal fluid exceeds preset limits (SP) the controller de-energizes the high media temperature relay, opening its contacts and shutting off the burner. The controller must be manually reset in order to restart the burner. The controller cannot be reset until the temperature of the thermal fluid has fallen below the set point value.
The preset temperature limit (SP) should always be set to a value higher than the modulating range of the burner. Otherwise, normal modulation of the burner may cause the burner to shut off prematurely.
The maximum limit for a single line heater is not the same as the limit for a heater with a manifold. On single line heaters set the maximum limit to at least 50 degrees F above setpoint of the modulating controller. On heaters with manifolds set the limit to at least 80 degrees F above the setpoint of the modulating controller. Remember, the temperature settings in this example are typical and may need to be reset to different values in the field for actual operating conditions.
HEATER ALARM horn (Fig. 2)
This horn denotes the status of the heater alarm system. When the horn is sounding the heater is in an alarm state. A switch is provided in the horn circuit so that the horn can be switched off. The alarm is controlled by the flame monitor, which turns on the alarm when it detects a variety of abnormal conditions. The monitor displays one of several messages to report the condition that caused the alarm.
Door handle
Use this handle (Fig. 2) to open the control panel. Do not open the door unless you are a qualified technician familiar with the hazards of electricity.
As a safety feature the handle shuts off electric power inside the control panel when the panel is opened. A keyed rod from the main circuit breaker (Fig. 3) inside the panel engages a slot on the back of the handle. The rod switches the breaker on and off as the handle is rotated. The handle is marked to indicate its functions.
Figure 3. Main disconnect circuit breaker with keyed rod.
Be aware that certain heater control panels have electrical circuits that are not shut off by the rotary door-mounted handle. Panels that combine heater controls and plant controls may include live circuits controlled by circuit breakers elsewhere in the plant. Consequently, shutting off the main breaker inside the panel does not remove all power to components within the panel.
A screw on the left side of the handle allows you to open the panel without shutting off the main circuit breaker. To do so, use a screwdriver to turn the screw about 1/4 turn counter-clockwise and then pull the handle to open the door. The purpose of this feature is to allow you to open the panel without shutting down the heater. Use extra caution when working inside the panel while the power is on. Always use a voltage tester to make sure there is no voltage on terminals or bare wires you may touch.