Btu (IT) per Second per Square Foot [Btu(IT)/(s·ft²)]

The unit Btu (IT) per second per square foot, written as Btu(IT)/(s·ft²), measures heat flux density or thermal power per unit area in the Imperial system using the International Table British Thermal Unit (1 Btu(IT) = 1055.06 J). It represents the amount of heat energy transferred through 1 square foot of surface every second.

This unit is commonly used in high-intensity heat transfer applications, solar energy studies, and engineering where energy flux per small surface area is important. Typical applications include:

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  Heat flux from industrial furnaces or heaters

  
  
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  Solar radiation incident on small surfaces

  
  
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  Thermal testing of materials

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in Btu(IT)/(s·ft²), Q is energy in Btu(IT), A is area in ft², and t is time in seconds.

Conversion to SI units:

While W/m² is the SI standard, Btu(IT)/(s·ft²) is convenient in U.S. engineering and historical references, providing an intuitive measure of high-intensity heat flux per square foot per second.

Btu (th) per Second per Square Foot [Btu(th)/(s·ft²)]

The unit Btu (th) per second per square foot, written as Btu(th)/(s·ft²), 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 foot of surface every second.

This unit is commonly used in high-intensity heat transfer applications, industrial processes, solar energy studies, and material testing, especially where energy flux over relatively large surfaces is significant. Applications include:

• 
  

  Heat flux from furnaces, radiators, or heaters

  
  
• 
  

  Solar radiation received by panels or flat surfaces

  
  
• 
  

  Thermal testing of building or industrial materials

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in Btu(th)/(s·ft²), Q is energy in Btu(th), A is area in ft², and t is time in seconds.

Conversion to SI units:

Although W/m² is the SI standard, Btu(th)/(s·ft²) remains useful in U.S. engineering, thermal studies, and historical literature, providing an intuitive measure of high-intensity heat flux per square foot per second.