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As you are aware, the program does not fully
address steam loops. Steam loops consist of a myriad of components, many of
which experience large thermal losses, and are prone to failure (such as steam
traps)
One way to approximate a steam loop is to
model it as a single hot-water loop, but specifying a steam boiler.
The steam boiler default efficiency is reduced to take into account some of
the losses associated with the de-aerator and other parasitic devices; however
the parasitic losses were not well-researched, and the default efficiency
is simply an semi-educated guess. (I came up with the value after thinking
about it for a total of 5 minutes).
Steam loops tend to have HUGE annual losses; I
believe that in most cases they exceed the actual annual end-use
load. Throw in a couple of malfunctioning steam traps and they can be
ridiculous. In spite of the fact that DOE-2.2/PowerDOE/eQUEST can model piping
losses, most people neglect them, thinking they are small. They are NOT
small in most cases.
You should definitely specify the piping UA so
the program can calculate losses. To better account for the UA split between the
hot-water and steam loops, you could model a primary/secondary loop arrangement,
where the primary loop is the "steam" loop, and the secondary loop is the
hot-water loop. To make the pumping energy of the "steam" loop match the
actual pumping energy (condensate, de-aeration, and pressurization),
you can adjust the head loss of the piping, boiler, and valve
modulating flow into the secondary loop.
Hot-water loops can also have very large losses; I
have found that the losses in VAV/hot-water reheat systems
typically exceed the annual reheat load.
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