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:

• 
  

  Heat flux from industrial furnaces or heaters

  
  
• 
  

  Solar radiation incident on small surfaces

  
  
• 
  

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While W/m² is the SI standard, Btu(IT)/(s·ft²) is convenient in U.S. engineering and historical references, providing an intuitive measure of high-intensity heat flux per square foot per second.

Kilocalorie (IT) per Hour per Square Foot [kcal(IT)/(h·ft²)]

The unit kilocalorie (IT) per hour per square foot, written as kcal(IT)/(h·ft²), measures heat flux density or thermal power per unit area using the International Table kilocalorie (1 kcal(IT) = 4186.8 J). It represents the amount of heat energy transferred through 1 square foot of surface in 1 hour.

This unit is commonly used in U.S.-based building construction, insulation, and HVAC studies to quantify energy transfer through walls, floors, or roofs. Typical applications include:

• 
  

  Heat loss or gain through building materials

  
  
• 
  

  Solar energy incident on rooftops or horizontal surfaces

  
  
• 
  

  Design and analysis of heating and cooling systems

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in kcal(IT)/(h·ft²), Q is total heat in kilocalories, A is area in ft², and t is time in hours.

Conversion to SI units:

While modern practice uses W/m², kcal(IT)/(h·ft²) remains prevalent in legacy U.S. building and insulation data, offering a convenient way to evaluate thermal energy transfer over a typical hourly timescale.