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In the last column (CI May 2002, p. 19 [http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,77528,00.html]), we reviewed basic fluid flow calculations and the operation of a standard ratio regulator as used in a proportioning system. One of the simplest systems available to automatically proportion the flow of fuel and air, this regulator has been used for decades and continues to achieve wide acceptance because of its simplicity, low cost and accuracy (as good as ±3% with quality components and the correct design). Performance of the system can suffer badly, however, if proper design guidelines or setup parameters are not used.
Important GuidelinesThe impulse line that provides the signal air must be a “pure” signal, i.e., it must be reflective of the air flow. If the impulse line is not installed properly or is placed in the wrong location, the ratio accuracy can deteriorate severely. The location of the impulse line connection in the combustion air line must be an area of minimal turbulence. To insure this, no valves or fittings should be located within 10 pipe diameters upstream, and five pipe diameters downstream, of the impulse point.
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.
Accurate MeteringSet up of the proportional system is simple and straightforward, provided that the system has the correct measurement devices. Ideally, metering orifices are placed in the gas and air line in order to accurately measure fuel and air flow. Alternatively, some burners—particularly high velocity burners—may have metering orifices built into them. These can be adequate if the inlet piping to the burner has sufficient straight runs of piping into the air and fuel ports. Unfortunately, this is typically overlooked by most system installers.
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.
DisadvantagesThe proportional control system offers energy efficiency when used in ceramic firing because no excess air is used. But there are also disadvantages. If a high velocity burner is used with the proportional system, high velocity will only be achieved when the burner inputs are near maximum. Since this will not always be the case, circulation and temperature uniformity may suffer.
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.