Book - School of Science and Technology

Water systems ± characteristics 149In view of the elevated surface temperature, the types of heat emitting equipment whichmay be fitted are restricted to those which may be touched with safety by occupants.Thus, natural and fan convectors are used at working levels but unit heaters of variouspatterns, radiant strip heaters and radiant panels may be fitted at higher levels where theyare out of reach.High temperature hot waterThe father of all high temperature systems was Perkins who filed a patent in 1831. In thissystem, the wrought iron piping was about 22 mm bore (32 mm outside diameter, jointedwith right- and left-hand threads) and each circuit formed one continuous coil, part of itinside the boiler with the remainder forming the heating surface in the building, Figure6.2:one such coil was measured recently as being over 180 m in length! Partial allowancefor expansion of the water content was provided by means of closed vessels. Operatingtemperatures of 180 C were common and, firing being by hand with no real control,frequently reached 280 C (7 MPa!) near the boiler. Examples of the system remain in use,some converted to oil firing, in churches and chapels built towards the end of the 19thcentury.The basic concept of water circulation at elevated temperatures and at pressures aboveatmospheric boiling point was revived during the years between the wars and applied, inparticular, to large industrial premises. The essential differences between this applicationand the Perkins system were that circulation was by pump, through conventional pipecircuits arranged in parallel, and that the working pressure and temperature were controlledat the boiler plant to more specific levels. Prior to the reintroduction of this system,the type of building to which it was particularly suited had been supplied by steam plantand arguments among advocates of the alternative methods, as to the superiority of each,continued for 20 years. The flexibility of the water system, both in the potential availableto vary the output temperature to suit weather conditions and in the physical freedom toExpansion vesselCoilCoilFurnace coilsPressuregaugeDouble widthcoil formingradiatorCharging connectionFigure 6.2 Diagram of Perkins' high temperature hot water system