Drive Systems: Considerations with Engine Applications - Sizing
sizing an engine for a specific equipment application, a proper evaluation of
the power requirements, rotational speed (rpm), capacity control or unloading
of the equipment to be driven, along with its operating characteristics need to
be performed. The power rating of the engine is based on the projected use. For
primary power applications, the prime power rating applies; for standby or intermittent
use, the continuous rating for standby power would apply.
Heating Value (HHV) and Lower Heating Value (LHV)
There are two ways to define the energy content of natural gas in common
use -- Higher Heating Value (HHV) and Lower Heating Value (LHV). The difference
can be especially important when reviewing the performance of engine-driven systems.
Heating Values for a fuel include the full energy content as defined by bringing
all products of combustion to 77°F (25° C). Natural gas typically is delivered
by the local gas company with values of 1,000 - 1,050 Btu per cubic foot on this
HHV basis. Since the actual value may vary from month to month, some gas companies
convert to therms. A therm is precisely 100,000 Btu. These measures all represent
higher heating values.
engine manufacturers rate their engines using Lower Heating Values (LHV) which
can be both confusing and potentially misleading to the casual user of their product
literature. Lower heating values neglect the energy in the water vapor formed
by the combustion of hydrogen in the fuel. This water vapor typically represents
about 10% of the energy content. Therefore, the lower heating values for natural
gas are typically 900 - 950 Btu per cubic foot.