Wednesday, January 11, 2012

Types of Heat Transfer

By Doug Hargrave
Mid Atlantic Masonry Heat
Radiant heat is transmitted from a warm object to a cooler object through infrared radiation. This is the same type of heat transfer that takes place when the rays of the sun shine on the earth. The distance between objects, their surface area and their temperature difference affect the rate of the radiant heat exchange. A good example of radiant heat transfer is the warmth you feel when you sit close to another person. Another example would be the way a radiant heater warms the surfaces and objects inside a home rather than the air.
When the distance between two solid objects of differing temperatures goes to zero and they come into direct contact, the heat exchange between them is then called conduction. Conduction between solid objects results in a faster rate of heat exchange than that of radiation. A good example of this difference is the amount of heat one would feel holding their hand just above a hot stove (radiation) versus actually touching the stove (conduction).
Heat transfer within solid objects is accomplished through conduction. The heat storage capacity and the transfer rate will vary with different solids. For example, the higher density of soapstone allows it to absorb and then radiate more heat per unit volume than common brick which has a lower density. The heat transfer rate within metals is much faster than the transfer rate within masonry materials.
Heat transfer within gases is quite different from heat transfer within solids. Gases have relatively little mass (weight) and very little density (weight per unit volume) when compared to solids. Unlike solids gases can dramatically expand or contract their density. Their density expands when they are heated and contracts when they are cooled. Warm gases that are expanded are lighter than cool gases that are contracted. The difference in weight causes warmer gases to rise and cool gases to fall creating movement within the body of gas. This movement is called convection. The speed of the convection (movement) is largely determined by the how much and how quickly heat is introduced into the body of gas. For example, a 600 degree wood stove causes much more convection (air movement) than a 200 degree masonry heater in the same living area.

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