Heat Transfer Calculator

Heat transfer calculators help you figure out how much heat moves between objects. They use three main formulas based on how the heat travels.

Conduction Formula

Heat conduction happens when heat moves through solid materials. The formula is

Q = k × A × (T₁ - T₂) / d.

Here's what each part means:

• Q is the heat transfer rate in watts
• k is the thermal conductivity of the material
• A is the surface area in square meters
• T₁ - T₂ is the temperature difference in degrees
• d is the thickness of the material in meters

Let's say you have a metal rod. One end is 100°C and the other is 20°C.

If the rod is made of aluminum (k = 205), has an area of 0.01 m², and is 0.5 meters long, the heat transfer would be 328 watts.

Convection Formula

Convection happens when heat moves through liquids or gases. The formula is

Q = h × A × (T_surface - T_fluid)

Breaking down the formula:

• Q is the heat transfer rate in watts
• h is the convection heat transfer coefficient
• A is the surface area in square meters
• T_surface is the temperature of the solid surface
• T_fluid is the temperature of the moving fluid

The convection coefficient depends on many things. Wind speed affects it a lot.

For example, a hot plate at 80°C in 20°C air might transfer heat differently based on conditions. With no wind (h = 5), a 0.1 m² plate transfers 30 watts. With a fan blowing (h = 25), it transfers 150 watts.

Natural convection happens without any forced air movement. Forced convection uses fans or pumps to move the fluid faster.

Radiation

Thermal radiation transfers heat through electromagnetic waves. The formula is

Q = ε × σ × A × (T₁⁴ - T₂⁴)

Here's what each symbol represents:

• Q is the heat transfer rate in watts
• ε is the emissivity of the surface (0 to 1)
• σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸)
• A is the surface area in square meters
• T₁ and T₂ are absolute temperatures in Kelvin

Unlike conduction and convection, radiation works in a vacuum.

Dark, rough surfaces have high emissivity values near 1. Shiny metal surfaces have low values near 0.05.

A black object at 373K (100°C) with area 0.1 m² and emissivity 0.95 radiates about 122 watts to a 293K (20°C) room. A shiny aluminum surface at the same temperature only radiates about 6 watts.