Book - School of Science and Technology

146 Indirect heating systemsTable 6.3 Variation in size of heat emitting equipment with water temperatureType of heatemitting unitComparative length of heat emitting unitfor following temperature differentials (K)water to air at 20 C55 45 35 25 15Primarily radiant 1 1.29 1.79 2.83 5.46Primarily convective 1 1.35 1.95 3.31 7.14increase in size of terminal heat emitting equipment, as Table 6.3. Of equal importance,however, is the reaction of occupants who cannot be convinced that a space is heatedadequately by a radiator, however large, which is `never more than just warm'!As a result of these two similar but conflicting considerations, the concept of bivalencehas emerged. This is a term adopted by the now defunct Electricity Council to representthe use, in parallel, of two energy sources to meet a common requirement. The principalapplication to date has been to demonstrate that a warm water source and a boiler plantmay be arranged to work together throughout a heating season, the former carrying theload in mid-seasons and the latter taking over during periods of extreme weather. Aheating analysis chart has been designed to provide a basis for representation of theproportions of the annual load which may be supported by either of the two energysources and a simplified version is included here in Figure 6.1.Low temperature hot waterUnder this heading fall most of the systems fitted in domestic, commercial andinstitutional premises. The basis for the operating temperature is the traditional practiceof providing that such systems should be connected above the highest point to afeed and expansion cistern open to atmosphere. Thus, a temperature of 82 C (180 F)was chosen for two reasons, first because it provided a reasonably wide margin withrespect to boiling point at atmospheric pressure and second because it left sometolerance within that margin for elevation to perhaps 90 C under sub-normal weatherconditions.The water in a heating system expands when the temperature is raised and the purposeof the feed and expansion cistern is to receive the additional volume when the system ishot and return it when the system cools down. In following this cycle, the water contentremains relatively unchanged and the scaling or corrosion which might arise from theadmission and rejection of fresh water each time is largely avoided.The volume of expansion over any temperature range may be found from Table 6.1since the mass remains constant. Thus, between 10 C and 100 C for example, it is(999:7 958:3)/958:3 ˆ 4:32% or one twenty-third of the original volume. Manufacturers'literature must be consulted for the water content of equipment since this variesgreatly with pattern:Table 6.4 provides that information for pipework.Of recent years, an ever increasing proportion of systems in this category has beenprovided with means, by way of a pressure cylinder as noted later in Chapter 8, todispense with the open cistern. Nevertheless, although an output temperature of about80 C may have originated in an era when hand firing with coke without any form ofautomatic control was the norm, no good reason to abandon it has yet been advanced.Furthermore, for domestic and most commercial premises, where operating skills and