Book - School of Science and Technology

4 FundamentalsSpecific heat capacity (gases). Whereas, for practical purposes, solids and liquids eachhave a single specific heat capacity which does not vary significantly, gases have onevalue for a condition of constant pressure and another for a condition of constantvolume. Both values will vary with temperature but, for a given gas, the ratio betweenthem will remain the same. For example, with dry air at 20 C, the value at constantpressure is 1.012 kJ/kg K and the value at constant volume is 0.722 kJ/kg K. At 100 C,the value at constant pressure is 1.017 kJ/kg K.Thermal expansion (solids and liquids). With very few exceptions, materials expand whenheated to an extent which varies directly with their dimensions and with the temperaturedifference. The examples in Table 1.1, which are averages that are only validbetween about 15 and 100 C, show that metals expand more than most buildingmaterials, with the result that, for example, care must be taken to accommodate thedifferential movement between a long straight pipe and a supporting building structure.For a temperature change from 10 to 80 C (70 K), a 10 m length of steel pipe willincrease in length by 7.9 mm (70 10 0:0113).Coefficients for superficial (area) and cubic (volume) expansion are taken,respectively, as twice and three times those listed for linear movement.Thermal expansion (gases). A perfect gas will conform to Boyle's and Charles's lawswhich state that the pressure (P), the volume (V), and the thermodynamic temperature(T) are related such that PV/T is a constant. The coefficient of cubic expansion istherefore temperature dependent and does not have a single value. Most gases conformvery closely to the properties of a perfect gas when at a temperature remote from that atwhich they liquify.Vapour pressure. Dalton's law of the partial pressures states that if a mixture of gasesoccupies a given volume at a given temperature, then the total pressure exerted by themixture will be the sum of the pressures exerted by the components.Criteria which affect human comfort ± definitionsThe ancients taught that humans had seven senses,* but it is no more than coincidencethat the principal influences which affect human comfort are also seven in number:. temperature. conduction, convection and radiation. air volume and movement. activity and clothing. air purity. humidity. ionisation.TemperatureThe direction of heat flow from one substance to another is determined by the temperatureof the first relative to that of the second. Thus, in that sense, temperature is akin to apressure potential and is a relative term:the temperature of boiling water is higher thanthat of water drawn straight from a cold tap and the temperature of the latter is higher than* Animation:feeling:hearing:seeing:smelling:speaking:tasting.