Building Services Engineering 5th Edition Handbook

Heating 109
12%
Flue
High-pressure steam
Turbine
Three-phase 50 Hz
30%
Fuel 100%
Alternator
Boiler
Heat
exchanger
Condenser
District heating
Pump
38%
20%
Feed pump
4.27 Combined heat and power plant.
Pump
Cooling tower
District heating
District medium- or high-pressure hot-water heating, employing two-, three- or four-pipe
underground distribution systems, will provide heat primarily 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. The street distribution layout is indicated in Fig. 4.28.
Flow and return pipes will be well-insulated and may be installed 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, utilizing the heat produced, 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. This can be done with the three- and four-pipe arrangements
shown in Figs 4.29 and 4.30 to economize on pump running costs and pipe heat losses during
the summer.
The supply of heat to each dwelling will be controlled by an electric 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 temperatures can be used. This permits a large drop in temperature from
flow to return (50 K 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. Underground chilled-water pipework will