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Sometimes a part - or all - of a consumer's load can run on undiluted propane vapor. This normally requires separate distribution piping and modifications to each appliance burner train. Large, centralized loads such as boilers are candidates for this approach.
Propane-air is the more universal approach to standby fuel. By mixing approx. 55% propane and 45% air, a gas is produced that performs well in natural-gas burners without individual adjustment. The mix is injected into the natural-gas system as partial or full replacement.
There are several approaches to mixing propane and air, and many blender (mixer) options can be considered. The mixer type influences selection of other equipment used in the system. Common types of propane-air blenders with related mixed-gas pressure ranges are shown in the diagrams below.
Note that required controls and safety devices are not shown in detail
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RULE OF THUMB...
A propane-air mixture containing 1,470 btu/cf has burning characteristics similar to natural gas containing 1,000 btu/cf with a specific gravity of
.60.
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Venturi style mixing systems use high-pressure propane vapor to entrain air directly from the atmosphere by venturi action. Venturi systems are the most popular choice for applications with mixed-gas pressures up to 15 psig. Higher pressures can be achieved with compressed air.
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Atmospheric Carburetor systems use a "pull-thru" mixing valve and gas booster.
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Pressurized Carburetor systems use a "push-thru" mixing valve and compressed air.
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In Appendix...
See Table 4 for typical propane-air compositions and associated specific gravity and Wobbe values.
See Figure 2 for propane-air dew points.
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Parallel Pipe systems include a variety of control mechanisms (e.g., mass flow control) to proportion compressed air and gas.
At the top end, fully "digital" systems, such as SSi's MFC series blenders, provide very precise control of gas quality under varying process conditions. |
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