more of the currently available CAD systems explicitly created for district heating
system design and feasibility studies. If the economic benefits are as great as
indicated by the examples worked here, the incentive for doing so is significant.
Perhaps the most troubling aspect of using optimal design methods for sizing
district heating piping systems is the level of pressure losses resulting. These
pressure losses are rather high when compared to those encountered in operating
systems and designs based on other methods. This result is apparent when we
examine the pressure losses given in Table 1 for both the method presented here and
a common rule of thumb based design. This result has also been observed by others
(Bhm 1986, Koskelainen 1980). It seems that current design practice and the
systems that result are ill-suited to the application of optimal design techniques.
Several possible solutions to this conflict exist. The first is simply to increase the
maximum pressure capability of the piping system used. The logic of this approach
can be seen in European practice where small district heating systems use piping
rated for only 6-bar (600-kPa) maximum pressure and often the connections to the
consumers are direct, i.e., without heat exchangers. For larger systems, piping rated
for 15-bar (1500-kPa) or greater maximum pressure is often used and heat exchang-
ers are used to isolate the consumers' equipment from the high system pressures.
The designer should always find the unconstrained optimum pressure level for the
network first before making a decision of which pressure class of piping to use. It's
quite possible that pressures higher than those used in current practice may be
justified in some instances.
The advent of friction reducing additives (Nordic Council of Ministers 1991),
which are currently being field tested, offers some relief for the problem of excessive
system pressures. Such additives promise to reduce friction and thus pressure losses
by 50% or more. Such a change in something always assumed to be a basic given in
design renders much of what has been learned to date about district heating system
design, optimal or otherwise, nearly worthless. The ability of the method developed
here to rapidly reevaluate either existing or proposed designs clearly illustrates the
value and necessity of such a design tool. For instance, one possibility is that some
of the frictional reducing additives will be relatively short lived when compared to
the life of the district heating system, or even the annual operating cycle. However,
since the maximum flow rate and thus maximum pressure drop are only encoun-
tered over a short period of the yearly cycle, it may be that such friction reducing
additives can be very effective. The method developed here would allow for rapid
evaluation of any possibility to see if they are worthy of further study or consider-
ation for field testing.