Horsepower per Square Foot [hp/ft²]

The unit horsepower per square foot, written as hp/ft², measures power or energy flux density per unit area in the Imperial system. One horsepower (hp) is defined as 745.7 watts, representing the rate of doing work or energy transfer. Dividing by square feet gives the amount of power delivered or absorbed per unit area.

This unit is used in engineering, HVAC, and industrial applications to describe concentrated power distribution. Typical uses include:

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  Power output of engines or motors per area of a platform

  
  
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  Mechanical energy flux in manufacturing equipment

  
  
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  Evaluation of energy delivery systems on surfaces or panels

  
  

Mathematically, power flux can be expressed as:

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

Conversion to SI units:

Although W/m² is the SI standard, hp/ft² is convenient in U.S.-based applications and historical literature, providing an intuitive measure of high power density over a surface in familiar Imperial units.

Calorie (th) per Second per Square Centimeter [cal(th)/(s·cm²)]

The unit calorie (th) per second per square centimeter, written as cal(th)/(s·cm²), measures heat flux density or power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface every second.

This unit is commonly used in laboratory-scale heat transfer experiments, solar energy studies, and focused radiation applications, especially where the energy is concentrated over a small surface area. Applications include:

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  Solar radiation intensity measurements

  
  
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  Laser or concentrated light experiments

  
  
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  Calorimetry experiments on small samples

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in cal(th)/(s·cm²), Q is energy in calories, A is area in cm², and t is time in seconds.

Conversion to SI units:

While W/m² is the SI standard, cal(th)/(s·cm²) remains useful in laboratory and historical references, providing a practical measure for high-intensity, small-area heat transfer.