Book - School of Science and Technology

114 Survey of heating methodsthey are commonly connected to a cistern, mounted not far above the highest point,the water content of which is open to atmosphere. In some circumstances a diaphragmexpansion cylinder may be used instead of the open cistern but not, in thisapplication, with intent to enable pressure to be raised much above atmospheric level.The operating temperature at the source of heat is thus normally controlled at a levelbelow atmospheric boiling point and the system is described as being at low temperature.In the case of very large individual buildings and of sites with several scattered buildingswhere considerable distances have to be covered by the pipework distribution arrangements,systems are unlikely to be open to atmosphere. By arranging that they are sealed,the pressure and thence the operating temperature of the water may be raised aboveatmospheric boiling point, so increasing the quantity of heat which is carried by a givensize of pipe and reducing the physical dimensions of heat emitting equipment. Dependingupon the level to which these operating criteria are raised, the description mediumtemperature or high temperature is applied to such a system.Refer to Chapter 6 for typical operating temperatures for water heating systems.Terminal equipmentAll hot water heating systems, whatever the method or temperature of distribution,require some form of terminal heat emitting equipment for installation in the spacesto be served. A later chapter will consider this subject in more detail but, for thepurpose of comparison in the present context, the following are the more commonusages:Low temperature systems only. exposed piping. radiators. pipe coils embedded in the structure. metal panels in suspended ceilings.Low, medium and higher temperature systems. metal radiant panels and strips. natural convectors. forced (fan) convectors. skirting heaters. unit heaters. air/water heat exchangers for ventilation systems.Indirect systems ± vapour mediaDistribution of heatIn earlier editions of this book, the use of steam as a distribution medium was consideredin some detail. During the intervening period, in particular the last three decades, it haslargely been superseded for that purpose by either medium or high temperature hot water.More recent practice, therefore, is to use steam only in circumstances where it is either