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While W/m² is the SI standard, Btu(IT)/(s·ft²) is convenient in U.S. engineering and historical references, providing an intuitive measure of high-intensity heat flux per square foot per second.

Calorie (IT) per Minute per Square Centimeter [cal(IT)/(min·cm²)]

The unit calorie (IT) per minute per square centimeter, written as cal(IT)/(min·cm²), measures heat flux density or thermal power per unit area using the International Table calorie (1 cal(IT) = 4.1868 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is commonly used in laboratory calorimetry, solar energy experiments, and material testing where small surface areas and the calorie unit are convenient. Typical applications include:

• 
  

  Measuring solar radiation intensity on small surfaces

  
  
• 
  

  Calorimetry experiments with samples

  
  
• 
  

  Thermal testing of materials under concentrated energy flux

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(IT)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

While SI units like W/m² are standard, cal(IT)/(min·cm²) remains convenient in small-scale experiments and historical calorimetric studies, providing an intuitive measure of energy transfer per minute per small surface area.