Book - School of Science and Technology

Steam systems - principles 251Figure 9.13 Connections to a radiator from a single-pipe circuitworks produced before 1950. The very fact that systems having connected loads of up to3 MW were designed on this basis may come as a surprise when viewed in the context ofpressure availabilities of not much more than 4 or 5 Pa/m in comparison with the 100±500available today with pumped systems. It is no wonder that pipes of 200 mm diameter wererequired in boiler houses serving loads of this order and that the mass of calculationsnecessary was equally formidable!Steam systems - principlesAs will be obvious, no external motive force is required in any normal circumstances toproduce steam flow since pressure is inherent in the state. An argument can be developedas to the practical unit to be used for steam mass, as distinct from that which best suits thepurist approach. The most recent edition of the Guide Section C4 has retained kg/s inpreference to the far more useful unit of g/s listed in the 1970 edition. The latter equates toan energy supply unit of 2.25 kW and, moreover, permits working in whole numbersrather than with three or four decimal places.While reasonable care must be taken in the calculations leading to selection of pipe sizesfor steam service, it must be remembered that a number of imponderables exist in thatheat losses from the pipework, whether insulated or not, will lead to formation ofcondensate. In consequence, the steam pipe will be carrying a mixture of vapour andliquid, the proportions of the two components varying from time to time dependent uponthe external ambient air temperature and fluctuations in the rate of mass steam flow. Thevariables relating to flow of condensate are no less obscure since air and flash steam willaccompany the boiling water in pipes which are often only half full and are under achanging mixture of pressure influences.Calculation routinesFor selection of steam pipe sizes, the following information must be available:. The mass flow required, in g/s. For heating service this will equate to the energyrequirement in watts divided by the latent heat at the pressure of utilisation. Forkitchen or other equipment, the manufacturers' data must be consulted.. Where the steam main pipework is comparatively short, heat loss from it may beignored but, for extensive runs, use must be made of Tables 6.14 and 6.15 (p. 171)and the net requirement then adjusted to suit.. The initial pressure available at the steam source.. The minimum pressure required at the point of consumption.. Lengths of the various pipeline sections, with details of all single resistances.