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52
District heating
District medium- or high-pressure hot-water heating, employing two-,
three- or four-pipe underground distribution systems, will provide heat primar-
ily to the largest and most consistent users, such as hospitals, factory estates and
city centres. Further custom will be won from existing buildings by straight
price competition. Flow and return pipes will be well insulated and may be in-
stalled inside one large-diameter pipe which will form the structural duct and
moisture barrier.
The CHP plant generates electricity for the locality and is connected into
the national grid. It should also incinerate local refuse, utilize the heat pro-
duced, and recycle materials such as metals and glass. It will provide hot water
for sanitary appliances and air conditioning and, as these will be summer as
well as winter heat loads, a method of separating them from the heating system
will be used. The supply of heat to each dwelling will be controlled by an elec-
tric motorized valve, actuated by a temperature sensor in the heat exchanger,
which will enable existing low-pressure hot-water systems to be connected. A
heat meter, consisting of a water flow meter and flow and return temperature
recorders, will continuously integrate the energy used, and quarterly bills could
be issued through a directly linked computer.
Medium- and high-temperature hot-water heating systems are sealed from
the atmosphere. Pressurization methods involve restraining thermal expansion,
charging with air or nitrogen, or making use of the static head of tall buildings.
As the boiling point of water increases with increasing pressure, high flow tem-
peratures can be used. This permits a large drop in temperature from flow to re-
turn (5 °C or more), and water flow rates can be reduced compared with low-
pressure hot-water open systems. Pipe sizes are smaller and the system is more
economical to install when used on a large scale.
District cooling from a central refrigeration plant serving air-conditioning
units in commercial buildings can be developed alongside a CHP scheme. Un-
derground chilled water pipework will be separate from the heat network, and
space, cost and acoustic advantages could be gained in comparison with indi-
vidual systems. A higher standard of service should be available from central-
ized services, with fewer breakdowns and closer control of pollution.
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