Book - School of Science and Technology

Criteria which affect human comfort ± definitions 5Table 1.2 Temperature ScalesDegreesAbsolutezeroFreezingpointBoilingpointCentigrade C 273 200 100 18 0 10 16 20 28 40 60 71 82 100 200Fahrenheit F 460 328 200 0 32 50 61 68 82 104 140 160 180 212 392Kelvin K 0 ± ± ± 273 ± ± ± ± ± ± ± ± 373 ±that of ice. Ice, however, may be said to be hot, or at a high temperature, relative to liquidair at 190 C. Following the adoption of the SysteÁme International, temperature ismeasured in units of C or K. The Celsius scale has a convenient false zero (0 C), whichcorresponds to the temperature at which water freezes, and has equal intervals above thisto the temperature at which water boils under atmospheric pressure (100 C). The thermodynamicor absolute scale, established from the study of pressure effects upon gases, usesthe same intervals as the Celsius scale but with a true zero corresponding to the minimumpossible temperature obtainable. The intervals here are kelvin (K) and, with this scale,water freezes at 273 K.In order to avoid confusion in terminology, it was the accepted convention in Imperialunits that temperature level (or potential) should be expressed in terms of F, whereastemperature difference (or interval) was in terms of deg. F. Similarly, under strict SI rules,temperature level is expressed in C and difference in K and, although this usage is notobligatory, it is an aid to clarity and will be adopted throughout this book. For the benefitof those readers who were educated prior to the adoption of metric units, a conversiontable, Celsius to Fahrenheit, will be found in Appendix I. Table 1.2 shows the relationshipbetween the various scales and some notable conversions. Reference will be made later inthis chapter to a variety of different temperature notations: dry-bulb; wet-bulb; globe andradiant. These relate to methods of measurement for particular purposes and all use theCelsius or absolute scales noted above.Conduction, convection and radiationThese three terms have already been mentioned as being associated with heat transfer andit is necessary, for the discussions which follow, that there should be a clear perception ofthe difference between them.ConductionThis may be described as heat transfer from one particle to another by contact. Forexample, if two blocks of metal, one hot and one cold, were to be placed in contact, thenheat would be conducted from the one to the other until both reached an intermediatetemperature. If both blocks were of the same metal, then this temperature could becalculated by the simple process of relating the mass and temperature of one to those ofthe other:but if the materials were not the same, it would be necessary to take account ofthe different specific heat capacities as noted earlier.Conductivity is a measure of the quantity of heat that will be transferred through unitarea and thickness in unit time for a unit temperature difference (J m/s m 2 K ˆ W/m K).Table 1.1 lists values of this property for various materials and it will be noted thatmetals have a high conductivity, whereas, at the other end of the scale, materials known