Foot-Pound per Minute per Square Foot [ft·lb/(min·ft²)]

The unit foot-pound per minute per square foot, written as ft·lb/(min·ft²), measures energy flux density or power per unit area in the Imperial system. One foot-pound (ft·lb) is the energy required to move a force of 1 pound-force over a distance of 1 foot, equivalent to 1 ft·lb ≈ 1.35582 joules. Dividing by time (minutes) and area (ft²) gives the rate of energy transfer per unit area over time.

This unit is commonly used in U.S.-based engineering, building physics, and solar energy studies where energy flux through surfaces is measured. Applications include:

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  Heat transfer through walls, floors, or roofs

  
  
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  Solar radiation incident on building surfaces

  
  
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  Mechanical or thermal energy flux in industrial processes

  
  

Mathematically, the flux is expressed as:

where q is flux in ft·lb/(min·ft²), E is energy in foot-pounds, A is area in ft², and t is time in minutes.

Conversion to SI units:

While modern practice favors W/m², ft·lb/(min·ft²) remains relevant in legacy U.S. building and thermal engineering literature, providing an intuitive measure of energy transfer per square foot per minute.

Calorie (th) per Hour per Square Centimeter [cal(th)/(h·cm²)]

The unit calorie (th) per hour per square centimeter, written as cal(th)/(h·cm²), measures heat flux density or thermal 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 in 1 hour.

This unit is often used in solar energy studies, calorimetry experiments, and building physics, especially when small surface areas and long durations are considered. Typical applications include:

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  Solar radiation incident on small surfaces

  
  
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  Thermal testing of materials over time

  
  
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  Calorimetry experiments

  
  

Mathematically, heat flux is expressed as:

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

Conversion to SI units:

While SI units such as W/m² are standard, cal(th)/(h·cm²) remains convenient in small-scale laboratory and historical studies, providing an intuitive way to quantify energy transfer per hour per unit area.