Conversion of Boiler HP to lb/hr

Understanding the definition of boiler HP and its impact on its conversion to lb/hr

Steam Flow (lb/hr) = Power (HP) x 34.5 (lb/hr·HP)

The standard definition of boiler horsepower (HP or boiler horsepower) is based on the production of 34.5 lb/hr of saturated steam, but under specific conditions: at a reference pressure of 0 psig, i.e., atmospheric pressure. This standard was established to simplify comparisons, but it does not reflect the actual operating conditions of industrial boilers.

Limitations of This Definition

In practice, no boiler operates at atmospheric pressure. Operating pressures are generally much higher, which alters the thermodynamic properties of steam, notably its enthalpy (the energy contained). Consequently, applying the standard conversion of 1 HP = 34.5 lb/hr directly can lead to errors when trying to assess the actual performance or capacity of a boiler.

Nominal Value vs. Actual Value

To address these differences, boiler manufacturers introduce two key concepts:

Nominal Value: This corresponds to the theoretical capacity of the boiler according to the standard definition (34.5 lb/hr per HP at 0 psig).
Actual Value: This reflects the real steam flow produced, calculated based on the specific operating pressure and temperature conditions.

These two values allow for better comparison of performance between boilers and account for discrepancies due to actual operating conditions.

Transition Between Nominal and Actual Values

To convert from nominal value to actual value, or vice versa, it is essential to use the energy properties of steam. This involves working with thermodynamic data, such as the enthalpies of saturated or superheated steam.

Subsequently, a thermal and mass balance of the boiler must be performed. This analysis takes into account the energy entering the system, such as the feedwater, and the energy exiting, particularly in the form of produced steam. It is also necessary to include losses associated with continuous (blowdown) and intermittent drains, which remove impurities but also result in the loss of hot water and energy. By balancing these different flows, it is possible to accurately determine the actual steam flow under operating conditions.

Given that the latent heat of vaporization at 0 psig is 970.2 BTU/lb:

Energy output (BTU/hr) = Power (HP) x 34.5 (lb/hr·HP) x 970.108 (BTU/lb)

Energy output (BTU/hr) = Power (HP) x 33,475 (BTU/hr·HP)

Difference Between Energy Input and Output: Understanding Efficiency

The definition of boiler HP is based on energy output, i.e., the heat transferred to the steam. To produce this energy, the boiler consumes input energy, primarily in the form of fuel. The difference between the two represents losses—such as those from flue gases or radiation—and defines the boiler’s efficiency.

Energy output (BTU/hr) = Energy input (BTU/hr) x Efficiency (%)

Ressources :
  • Excel file for calculations converting HP to lb/hr under actual conditions (Coming soon for download. Contact us to receive a copy by email.)
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