It is extremely important to maintain negative pressure at all times, at all locations within the heater, and at all firing rates. Positive pressures anywhere in the heater can result in severe damage or complete heater destruction.
Heater draft is always monitored but seldom automatically controlled. (Section 200 of this manual gives additional information on draft.) However, if the need arises to replace a local draft pressure indicator (PI) with a draft transmitter, always dedicate the point to the transmitter; do not share the original PI function with the new transmitter.
To prevent plugging, be sure the line slopes upward from the sample point to the transmitter. Install a local PI (3 to 15 psig or 4 to 20 mA electronic indicator) for the transmitted signal near the original PI location, and so that the PI is visible from the heater damper actuator. Usually, the point chosen for transmission is the point of lowest draft (highest pressure); that is, the bottom of the convection section.
If relocation of draft measurement points is needed, refer to the heater manufacturer for assistance. Relocating the draft sensing location needs careful attention in order to avoid having part of the firebox under positive pressure.
Note that the Company has had several difficult experiences operating heaters with multiple fire boxes discharging flue gases into shared stacks, or through ducting to shared stacks. Generally, the damper operation of one box affects the backpressure and the draft in other fireboxes. Consequently, this has led operators to open all dampers further than necessary, resulting in lost efficiency.
When designing a new heater and if multiple boxes are proposed to discharge into a common stack, be sure to examine the long-term operating cost as a result of lost efficiency. This cost may offset any capital saved by conducting flue gases to a common stack.