Heat Energy Transfer
By E. Jess Tudor
Conduction is direct heat flow through matter (molecular stimulation). It results from contact of a warm area of an object with a cooler area or with another object. The greatest flow of heat possible between materials is where there is direct conduction between solids. Heat is always conducted from warm to cold and always moves via the shortest and easiest route. In general, the denser a substance the better conductor it is. Because air has low density the percentage of heat transferred by conduction through air is minimal. A less dense mass has less flow of heat by conduction resulting in better insulation. The use of structural materials to strengthen insulating barriers typically results in, “Thermal bridging” conduction, reducing the value of that insulation.
Convection is the transport of heat within a gas or liquid, caused by flow of the material itself (mass motion). Natural convection heat flow is largely upward and somewhat sideways. This is called "free convection". A warm stove loses heat by conduction to the cooler air in contact with it. This added heat activates (warms) the molecules of the air which expand, becoming less dense, and rise. Cooler, heavier air rushes in from the side and below to replace it. Convection when mechanically induced by a fan is called, "forced convection".
Radiation is the transmission of electromagnetic rays (infrared) through space. Infrared rays occur between light and radar waves. Radiation refers only to infrared rays. All objects radiate infrared rays from their surfaces in all directions in a straight line until they are reflected or absorbed by another object. Traveling at the speed of light these rays are invisible and they have no temperature only energy. Heating an object excites the surface molecules causing them to give off infrared radiation. When these infrared rays strike the surface of another object the rays are absorbed and heat is produced in the object. This heat spreads throughout the mass by conduction. The heated object then transmits infrared rays from exposed surfaces by radiation as they are exposed directly to an air space. Radiation, which accounts for 65% to 80% of all heat transfer, will pass through air with ease just as radiation travels the 93 million miles that separate the earth from the sun.
Water vapor is the gas phase of water and as a gas it will expand or contract to fill any space it may be in. In a given space with air at a given temperature, there is a limited amount of vapor that can be suspended and any excess will result in water. The point just before condensation commences is called 100% saturation.
- The warmer the temperature the more vapor the air can hold.
- A larger space holds more vapor.
- The more vapor in a given space, the greater its density.
- Greater vapor density flows to areas of lower vapor density.
- Permeability is a prerequisite for vapor transmission, less permeable, less vapor transfer.
R value or resistance to heat flow is a measure of insulation's ability to slow conductive heat flow however doesn’t consider radiation loss or convection loss. EPS foam becomes a conductor of heat and loses its insulating efficiency when the relative humidity increases because of moisture retention. Insulation with mass normally contain at least the average amount of moisture in the air and can’t compare to lab rated R values. The R value of 4” to 2” EPS foam spa covers is reduced by over 35% with typical ambient moisture content. As energy loss follows the least form of resistance, it is accurate to measure from the 2” dimension (from R-7.4 rated to R-4.8 effectual). As EPS spa covers continue to gain water weight from condensation, they increase energy costs exponentially.
Summary In both winter and summer, 65% to 80% of heat is lost through radiation. Aluminum foil has low thermal emissivity (.05%) and when surrounded by the low thermal conductivity of air it is possible to significantly reduce heat transfer by radiation and convection. Aluminum foil can reflect 95% of all radiant energy and is impervious to water vapor and convection currents. The performance of most aluminum insulation is unsurpassed for winter heat and adds efficiency for summer by minimizing convection currents. Aluminum inhibits infrared rays from penetrating its surface while reflecting that energy back.
"Dead air space” does not exist as far as heat transfer is concerned even with an air-tight compartment such as a Thermos mug. Convection currents are inevitable when differences in surface temperatures exist and since air has some density, there will be some heat transfer by conduction when space is heated.
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