Steaming Rates
The Steaming Rate (the rate at which a boiler produces steam, normally expressed in terms of Lbs/Hr or Kg/h) is an item that is frequently misunderstood and such a misunderstanding can lead to the purchase of the wrong size boiler. It is, therefore, essential that the Steaming Rate be qualified when selecting a boiler size. The three common Steaming Rate terms are:
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From and at 212 ºF (100 °C) Steaming Rate
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Gross Steaming Rate
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Net Steaming Rate
From and at 212 ºF (100 °C)Is the Steaming Rate for a boiler producing steam, at the outlet flange, at 212 ºF (100 °C), and 0 PSIG, with feedwater at the inlet flange at 212 ºF (100 °C) and 0 PSIG. This is the most common steaming rate term and is used most often in brochures, etc. when steaming rate information is provided. One Boiler Horsepower (BHP) is, by definition, equivalent to 34.5 pounds of steam per hour, from and at 212 ºF (100 °C).
Gross Steaming Rate
Is the rate at which a boiler
produces steam, at the outlet flange, based on application
specific feedwater conditions at the inlet flange and
application specific steam conditions. The Gross Steaming
Rate, typically, differs from the From and at 100 °C
Steaming Rate because both the feedwater inlet and the steam
conditions are different than 100 °C and 0 barg. A
typical application may, for instance, have feedwater at 88 ºC
and produce saturated steam at 6.9 barg 170 ºC. Since the
inlet temperature is less than 100 °C and the outlet
temperature is greater than 100 °C, the amount of heat
needed to produce a kilogram of steam, at these conditions, is
greater than the amount needed to produce a kilogram of steam with
inlet and outlet temperatures of100 °C. The Gross
Steaming Rate is, therefore, frequently, less that the From and
at 100 °C. It may, however, actually be greater, if the
feedwater receiver is a pressurized deaerator that heats the
feedwater to a temperature above 100 °C.
Net Steaming Rate
Is the steaming rate at which a boiler
produces steam, to your plant or process and, thus, is
the most important steaming rate to consider. Net steaming rate,
differs from Gross Steaming Rate in that it takes into account
the amount of steam needed to heat the feedwater in the
feedwater receiver (deaerator or hotwell): specifically, the Net
Steaming Rate equals the Gross Steaming Rate minus the steaming
rate to the feedwater receiver. Except for some very unusual
applications, the Net Steaming Rate is less than the Gross or
From and at 100 °C Steaming Rate.
Take, for example, a 100 BHP (981kW) boiler operating with
100% make-up water at 15.6°Cand producing steam at 8.6 barg. In this case,
the From and at 100 °C Steaming rate is 1,565 kg/h
but the Net Steaming Rate is only 1,304 kg/h - 17% less
than the From and at 100 °C Steaming Rate.
The effect of feedwater heating is applicable in all
applications and, thus, should always be considered. There is
another factor that has an effect and can be significant in some
applications. That factor is the amount of "blowdown" that is
required in order for the boiler to operate effectively. In this
case, "blowdown", refers to the amount of water that must be
removed from the boiler system, on a regular basis, in order to
control the level of Total Dissolved Solids (TDS) in the boiler.
Water that is removed to control TDS has been heated and the
amount of energy needed to heat this water reduces the amount of
energy that is available to produce steam. (See the Blowdown
Considerations article).
In summary, users should be certain to qualify steaming rates
when using them to define the size of a boiler. Boiler
Horsepower is a specific term and no further information is
needed to select the size of a boiler. However, if a steaming
rate is used to specify boiler size then the steaming rate must
be qualified - From and at 100 °C, Gross, or kg, per hour.
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