The **key difference between thermal conductivity and heat transfer coefficient** is that thermal conductivity is related to the spatial molecular diffusion of the heat throughout the fluid, whereas heat transfer coefficient is a proportionality constant between the supplied heat and the thermodynamic driving force of heat flow through unit area.

Thermal conductivity is the ability of a particular material to conduct heat through itself. Heat transfer coefficient, on the other hand, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat.

### CONTENTS

1. Overview and Key Difference

2. What is Thermal Conductivity

3. What is Heat Transfer Coefficient

4. Thermal Conductivity vs Heat Transfer Coefficient in Tabular Form

5. Summary – Thermal Conductivity vs Heat Transfer Coefficient

## What is Thermal Conductivity?

Thermal conductivity can be described as the ability of a particular material to conduct heat through itself. We can use three ways to denote this term: k, λ, or κ. Generally, a material consisting of a high thermal conductivity displays a high rate of heat transfer. For instance, metals usually have high thermal conductivity and are very efficient in conducting heat. In contrast, insulating materials such as Styrofoam have low thermal conductivity and show a low rate of heat transfer. Therefore, we can use materials with high thermal conductivity in heat sink applications and materials with low thermal conductivity in thermal insulation applications. Moreover, “thermal resistivity” is the reciprocal of thermal conductivity.

Mathematically, we can express thermal conductivity as q = -k∇T, where q is the heat flux, k is the thermal conductivity, and ∇T is the temperature gradient. We call this “the Fourier’s law of heat conduction.”

We can define thermal conduction as the transport of energy due to random molecular motion across a temperature gradient. We can distinguish this term from energy transport through convection and molecular work because it does not involve any microscopic flows or internal stresses that are work-performing.

When considering the units of measurement for thermal conductivity, the SI units are “Watts per meter-Kelvin” or W/m.K. However, in imperial units, we can measure thermal conductivity in BTU/(h.ft.°F). BTU is a British thermal unit, where h is time in hours, ft is the distance in foot, and F is the temperature in Fahrenheit. Furthermore, there are two major ways of measuring the thermal conductivity of a material: steady-state and transient methods.

## What is Heat Transfer Coefficient?

Heat transfer coefficient is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat. It is also known as film coefficient or film effectiveness in thermodynamics. Usually, the overall heat transfer rate for some systems is expressed in terms of an overall conductance or the heat transfer coefficient, which is denoted by U.

The heat transfer coefficient is useful in calculating the heat transfer by convection or phase transition between a fluid and a solid. When considering the SI units, the heat transfer coefficient has the units W/(m2K) (watts per square meter Kelvin).

Moreover, the heat transfer coefficient can be described as the reciprocal of thermal insulance. We can use the heat transfer coefficient for building materials and for clothing insulation.

## What is the Difference Between Thermal Conductivity and Heat Transfer Coefficient?

Thermal conductivity and heat transfer coefficient are important terms in physical chemistry. The key difference between thermal conductivity and heat transfer coefficient is that the thermal conductivity is related to the spatial molecular diffusion of the heat throughout the fluid, whereas heat transfer coefficient is a proportionality constant between the supplied heat and the thermodynamic driving force of heat flow through unit area.

The following table summarizes the difference between thermal conductivity and heat transfer coefficient.

## Summary – Thermal Conductivity vs Heat Transfer Coefficient

The key difference between thermal conductivity and heat transfer coefficient is that thermal conductivity is related to the spatial molecular diffusion of the heat throughout the fluid, whereas heat transfer coefficient is a proportionality constant between the supplied heat and the thermodynamic driving force of heat flow through unit area.

##### Reference:

1. “Overall Heat Transfer Coefficients.” *Engineering ToolBox*.

##### Image Courtesy:

1. “Simple definition of thermal conductivity” (CC0) via Commons Wikimedia

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