The impulse connection must also be installed correctly. The tap into the combustion air pipe should not protrude into the pipe or else the tap might sense velocity pressure or turbulence in addition to the impulse pressure. Since the design of the system depends on an accurate signal, poorly installed impulse taps can destroy the accuracy and may also affect burner stability.
Piping into and out of the regulator must not be reduced in size with bushings, etc. Bushings affect the flow turbulence and, accordingly, affect the gas pressure sensed by the regulator itself. If the regulator sees a false pressure reading, it cannot control accurately.
Proper fuel pressure at the inlet to the regulator must be maintained. Normally, this means that the gas pressure should be at least eight in. water column (w.c.) above the highest pressure that the impulse line will apply to the regulator. If the gas pressure is too low, the regulator will not be able to “keep up” with the impulse signal at high fire, causing the fuel flow to deviate from the proper value.
If the system requires an exceptional range of operation—higher than 7:1—then two additional guidelines apply. Use one regulator per burner, particularly if high back pressure, high velocity burners are installed. High velocity burners typically have a large combustion block pressure, and this pressure is impressed on the incoming gas line. As the combustion takes place, this back pressure varies slightly. Without a regulator on each burner, the varying back pressure causes the burner fuel flow to vary, leading to burner instability or even flame out.
Additionally, use a bypass around the regulator to set minimum firing rate. This consists of a small needle valve (1⁄8 in.) and a tubing connection upstream and downstream of the regulator. All regulators, especially those with low output, have some level of hysteresis. The bypass line can be used to set a consistent, low level of fuel flow to assure stable minimum firing rates and ease of lighting.
The system setup is most accurately achieved in either case by adjusting the fuel and air input with the burner(s) at the maximum firing rate. With the air at maximum, simply adjust the gas limiting orifice for the correct fuel flow. When this is completed, it is wise to check the fuel and air flow at intermediate firing levels to be sure that the regulator is performing properly.
Additionally, NOx levels are typically higher at reduced outputs due to limited furnace air entrainment and somewhat higher effective flame temperatures. If NOx is a serious issue, alternatives do exist in the selection of the burner type. Pulse firing systems also hold great promise for NOx reduction.
In summary, the proportional firing system is one of the simplest systems available for burner operation. Using the guidelines mentioned here will assure accuracy, stability and reduced energy consumption. In the next column, we’ll look at alternative systems, including excess air and pulse firing.