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.

CHU/Hour/Square Foot

The unit CHU/hour/square foot is used to measure Cooling Heat Units per hour per square foot, commonly applied in air conditioning and refrigeration calculations. It indicates the amount of cooling energy, in CHUs, that passes through or is required for one square foot of a surface area over an hour. This measurement helps engineers and designers assess cooling loads, select appropriate air conditioning systems, and ensure thermal comfort in buildings. A higher CHU/hour/square foot value shows greater heat gain, meaning more cooling is needed, while a lower value indicates better insulation or lower cooling requirements. It is essential in designing energy-efficient HVAC systems and optimizing equipment sizing, helping reduce operational costs and maintain desired indoor temperatures. By using CHU/hour/square foot calculations, building designers can balance comfort, energy efficiency, and sustainability effectively.