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The heat transfer coefficient defines the ease with which heat passes from one material to another, usually from a solid to a fluid or gas, or from a fluid or gas to a solid. 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.
Heat transfer 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 heat 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 heat 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 heat transfer coefficient of each of its interfaces. Insulation is important for 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.
Quantitatively, the heat transfer 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 equations determine the amount of heat that is transferred per unit area, per degree temperature difference between the two adjoining materials, and per time period. 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. In this example, 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.
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