Btu (th) per Second per Square Inch [Btu(th)/(s·in²)]

The unit Btu (th) per second per square inch, written as Btu(th)/(s·in²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 J). It represents the amount of heat energy transferred through 1 square inch of surface every second.

This unit is commonly used in high-intensity thermal applications, engineering experiments, and solar energy studies where energy flux over small surfaces is significant. Typical applications include:

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  Heat flux from industrial furnaces or radiative heaters

  
  
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  Solar radiation on concentrated panels or small surfaces

  
  
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  Thermal testing of materials under high-intensity conditions

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in Btu(th)/(s·in²), Q is energy in Btu(th), A is area in in², and t is time in seconds.

Conversion to SI units:

While SI units like W/m² are standard, Btu(th)/(s·in²) is useful in U.S. engineering, laboratory experiments, and historical references, providing an intuitive measure of high-intensity heat flux per square inch per second.

Metric Horsepower per Square Foot [hp(metric)/ft²]

The unit metric horsepower per square foot, written as hp(metric)/ft², measures power or energy flux density per unit area using metric horsepower. One metric horsepower is defined as 735.5 watts, slightly less than the mechanical (Imperial) horsepower of 745.7 W. Dividing by square feet gives the rate of power delivery or absorption per unit area.

This unit is used in engineering, industrial design, and energy flux studies where power distribution per unit surface area is analyzed. Applications include:

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  Mechanical equipment power output per platform area

  
  
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  Energy flux in industrial processes

  
  
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  Assessment of high-power machinery energy delivery

  
  

Mathematically, the power flux can be expressed as:

where q is power per unit area (hp(metric)/ft²), P is power in metric horsepower, and A is area in ft².

Conversion to SI units:

While W/m² is the SI standard, hp(metric)/ft² is convenient in regions using metric horsepower, especially in European engineering and historical industrial literature, providing an intuitive measure of high power density per unit surface area.