A heat transfer coefficient is a concept in physics and thermodynamics that details how and how easily heat energy passes from one material to another. In many instances heat is transferred most readily as the subject materials shift from solids to fluids to gasses; heat can also pass from a fluid to a gas or or vice versa, such as is the case of cool air above a warm lake. Heat will always flow from hot to cold for materials in direct contact, and the transfer coefficient is one way of expressing this shift tangibly and in mathematical terms. It’s an important concept for manufacturers and builders in many industries. It helps engineers design better cooking pots, for instance, and helps make machinery and things like combustion engines in cars safer; it’s also used to make insulation in homes and offices more efficient. Setting out the basics of the coefficient is usually pretty straightforward, but the specifics of how its core formula both works and applies to changing thermodynamic scenarios can be somewhat complicated.
Quantitatively, the coefficient is a function of the two materials in contact; the temperature of each, which determines the driving force; and factors that enhance or detract from the heat transfer, such as convection or surface fouling, respectively. The standard calculation rubric is typically expressed as h = q / ∆t, where “h” is the overall heat transfer coefficient, “q” is the amount of heat transferred per unit area, and "∆t" is the temperature difference between the adjoining surfaces or surfaces in question.
There are also equations to determine the amount of heat that is transferred per unit area, per degree temperature difference between the two adjoining materials, and per time period that can help influence the more basic formula. Calculations for sizing industrial equipment, such as heaters and heat exchangers, usually solve for heat transferred per hour because plant production capacity is usually determined on an hourly basis.
An overall heat transfer coefficient, such as is often used in heat exchanger equations, would need to consider a number of factors. For example, in a steam engine scenario, the saturated steam at a given temperature, the steam to tube interface, conductivity through the tube wall, the interface to the liquid inside the tubes such as oil, and the temperature of incoming oil would all need to be considered. Information from these factors could help determine how large a heat exchanger would be needed, and what design and materials strategy would work best.
These coefficients are always considered when designing equipment that is specifically intended to transfer heat — or to not transfer heat. Cooking pots, cooling fins on a motorcycle engine, blowing on a spoonful of too-hot soup, or one person warming another’s cold hands are all instances of enhancing the heat transfer coefficient. The greatest single contributor to better transfer coefficients, given the material constraints, is rapid movement of the fluid phase of the components. Blowing air through a radiator, inducing turbulent flow in a heat exchanger, or rapidly moving air in a convection oven effect much higher transfer coefficients than still conditions. This is because more molecules to absorb heat are presented to the hot surface in a shorter amount of time.
On the other hand, the search for highly effective insulation also considers the calculated heat transfer of each of its interfaces. Insulation is important for all sorts of things, including refrigerators and freezers, picnic coolers, winter clothing, and energy efficient homes. Dead air spaces, voids in foam, and materials with low conductivity all help provide insulation.