Mechanisms of Heat Loss Heat

Mechanisms of Heat Loss

Heat Loss

How the Rate of Heat Loss is affected by Surrounding Temperatures

Heat transfer occurs whenever there is a temperature differential between a substance and its environment. This differential can be highly significant, or it can be mild and nearly undetectable. The issue is not the strength of the differential, but whether it exists at all. If the substance is at a higher temperature than its surroundings it will lose heat, and if at a lower temperature, it will gain heat. The significance of the differential will affect how quickly the substance gains or loses heat, and can also affect how much heat is lost overall - whether that heat is lost rapidly or whether it dissipates more slowly over time. When heat transfer does occur, it is through one of three separate mechanisms: conduction, convection, and radiation. In order to have a clear understanding of these mechanisms and their relationship to heat transfer, it is necessary to address each one individually. That will allow for a better understanding of how each mechanism works, how they differ from one another, and the ways in which they are similar.

Conduction is a process of diffusion, with the transfer of kinetic energy, in the form of heat, through atomic or molecular interactions between a substance and its environment (Kreith, Manglik and Bohn, 2010). Any solid is characterized by its thermal conductivity (k), which is a function of temperature, and reflects its ability to transfer heat through conduction. Combining Fourier's law for heat transfer and the 1st Law of Thermodynamics (heat transfer across a boundary surface + energy generated within the bound volume = change in storage energy of the bound volume), the general equation for conductive heat loss in Cartesian co-ordinates can be derived as:, where is the rate of energy addition across the volume, ? is the material density and c is the material specific heat. For the more restricted case of steady-state one-dimensional conduction across a homogenous wall, the equation can be reduced to, where T1 and T2 are the wall face temperatures, a is the heat transfer area (assumed to be normal to direction of transfer), L is the wall thickness and k is the wall thermal conductivity (Bejan and Kraus, 2003). Good conductors of heat, such as metals, have high thermal conductivity values, whereas heat insulators, such as non-metals, have low values. In petrochemical or other process industries, conductive heat loss to the surroundings from process piping is a major design issue. The rate of conductive heat transfer through a cylindrical pipe, per unit area, is given by, where k is the thermal conductivity, T2 is the external temperature, T1 is the internal pipe temperature, r2 is the external radius and r1 is the internal radius. The above equation can also be utilized to calculate conduction loss from a human body to ambient air. For example, for a 1.5 tall man wearing dry, insulating clothing, the rate of conductive heat loss on a cold day (ambient temperature at 0oC, normal skin temperature at 37oC) can be calculated as 178W. For the same person wearing wet clothing, however, the equivalent rate is 2,565W. This significant difference in heat losses explains the onset of hypothermia when someone is exposed to ice-cold water or rain (Forinash, 2010). Conduction takes place on a microscopic level as particles of kinetic energy are transferred between two different systems (Abbott, et al., 2005). When atoms and/or molecules heat up, vibrate, or move rapidly, some of their energy gets transferred to other atoms and molecules that are in close proximity. In other words, heat is transferred to the surrounding particles and away from the vibrating particles. Solid objects that are engaged in thermal conduct use conduction as a significant means for heat transfer (Abbott, et al., 2005; Geankoplis, 2003). The vibration is what creates the heat, and the heat is then distributed from one atom or molecule to the other (or others). That distribution results in a net heat loss for some of the atoms or molecules, and a net heat gain for other atoms or molecules. This change in heat will depend on the size of the systems where the atoms or molecules are located, but will also be affected by the rate of vibration and other factors that can account for how quickly (or slowly) the change in heat takes place. The heat loss or gain may be noticeable over time on a level beyond what is seen molecularly, but the heat transfer does not originate at a larger or more detectable level. The molecules and atoms are the beginning of the heat transfer, which expands from that point.