Optimal Design of Piping Systems for
District Heating
GARY PHETTEPLACE
CHAPTER 1: INTRODUCTION
District heating is the practice of heating multiple buildings from a single heating
plant. Heat is conveyed to the buildings by means of hot water or steam. District
heating systems offer enormous potential for energy conservation, in addition to the
advantages of fuel flexibility and reduced environmental impact. For these reasons
district heating has been used extensively in Europe with favorable results. For
example, in Denmark district heating serves 42% of the demand for space and hot
tap water heating (NRC 1985).
In the United States district heating is much less widespread, accounting for
about 4% of the space and hot tap water heating (NRC 1985). A few cities have
systems, as well as a number of college campuses and other large institutions. With
approximately 6000 miles (10,000 km) of district heating piping in place (Segan and
Chen 1984), the Department of Defense is the single largest user of the technology
within the United States. A major barrier to more widespread use of district heating
in the United States is the high capital cost of the piping required to convey the heat
to the buildings. The piping system is most often the major cost of district heating.
However, the lack of development of district heating in the United States is often
attributed to "institutional barriers." Such barriers, where they truly exist, would be
significantly weakened if not removed should the economics become more favor-
able.
CURRENT DESIGN PRACTICE
Because the hot water or steam piping networks represent such a major portion
of the capital costs, they also represent an opportunity for significant cost savings by
optimal design. Despite this, in practice little effort is expended either here or in
Europe to ensure that proposed designs reduce costs as much as possible. Currently,
most designs are based on previous experience and often may be far from optimal.
Rules of thumb are commonly used, as are design guides developed for other
purposes, such as for plumbing within buildings.
To achieve an optimal design with minimum life cycle costs, all major costs
associated with constructing and operating the system must be considered. Capital
costs for piping and installation vary widely and must be determined for each case.
Operating costs strongly depend on the nature of the load and the load management
strategy adopted. For these reasons, it is impossible to develop guidance that can be
applied universally to obtain designs that are sufficiently close to the lowest life
cycle cost.
As with most areas in the practice of engineering, computer-aided design
methods are becoming more widespread in district heating system design. The use
of such methods allows the rapid evaluation of many alternate designs, a formidable
task if carried out without such methods. A number of computer-aided design
methods are available for thermal piping networks (Reisman 1985, Rasmussen and
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