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 (c) High Temperature Water System (HTW). A hot water heating system
operating at temperatures over 350F and pressure of approximately 300
psig. The usual maximum supply water temperature is 400 to 450F.
(d) Selecting type of Hot Water Distribution System. Systems must
maintain adequate pressure and temperature and assure uniform flow of water to
customers. H o t water generators consist of natural circulation boilers or
f o r c e d circulation boilers. Since hot water distribution systems are more
e f f i c i e n t than steam distribution systems, they should be selected whenever
p r a c t i c a l . The lower the temperature required the more efficient the system
should operate due to the lower temperature differential between the hot water
and piping's external temperature. Lower temperature systems are less costly
t o construct as well. A l l projects calling for the replacement or new
i n s t a l l a t i o n of a heating system shall include a life cycle economic analysis
of steam vs LTW, MTW, and HTW distribution systems, and justification, stating
the conditions which prohibits the use of above-ground systems on all MCON and
s p e c i a l projects. This includes repair by replacement type projects where the
existing distribution systems' are steam and/or underground. The following
factors will be among those considered in the analysis:
(1) Economic advantage of thermal storage of the hot water system
in sizing of equipment such as boilers, pumps, and piping.
( 2 ) Operation and maintenance costs of hot water distribution
system versus steam distribution system.
(3) Customer requirements of temperature or pressure served more
economically by steam or hot water.
( 4 ) Replacement or renovation of existing plant and distribution
system compared with construction of new plant and/or distribution system.
Comparison to be on a life cycle basis.
( 5 ) Prevalence of skilled steam plant or hot water plant operators
in area, especially in remote locations.
(6) Complexity of controls and ability of steam to maintain varying
or constant temperature conditions through the assigned or existing heat
transfer equipment.
3.3 Heat Storage Capacity. A useful characteristic of HTW systems is the
large heat storage capacity. This property gives the system a thermal fly-
wheel effect which permits-close temperature control and more rapid response
to changing load demands. In fact, the system acts as an accumulator of the
shows a comparison of the heat storage capacity of water and steam for
different pressures and temperatures. The variations of density and volume of
HTW with changes in temperature are shown in table 4-2.
3 . 4 Temperature Differential. To take full advantage of the high heat
content of HTW, distribution systems are designed for the largest temperature
4-2
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