large heat storage available in the HTW system, but these heat exchangers will
accumulate solids and must be periodically cleaned.
Indirect High Temperature. Heat exchangers should be designed to
effect maximum temperature drops consistent with the pressure or temperature
of the medium produced. A reduction in primary operating temperatures may
effectively nullify the operation of these heat exchangers unless they are
specifically designed for varying inlet-temperature conditions. There are two
basic types of indirect heat exchangers:
a) Flash or instantaneous types. These are vessels which store a
small amount of water with respect to the heating capabilities of the coil.
Since no water is stored, the fly wheel effect of heavy demands is not
available, and any demand in excess of the capabilities of the coil results in
rapid temperature drops.
b) Storage or inertial types. These are classified as vessels
which store a large amount of water with respect to the heating ability of the
coil. The large amount of stored water is able to meet unusually heavy
demands without much difficulty. If load requirements are periodic, standby
losses may be excessive.
Makeup Water and Water Treatment
a) Makeup water is required due to small leaks, leakage around
packed joints of valves, relief valves, pump shafts, etc.
b) Two makeup pumps are required, each sized for 0.5 percent of
the total distribution system flow. In case of a coal-fired generator, an
additional emergency pump rated at 5.0 percent of the distribution system flow
c) A treated water storage tank should be provided for emergency
pumping requirements. This tank is usually sized for approximately 20 minutes
makeup pump demand except, in coal-fired plants, the tank shall be sized for
40 minutes of emergency flow capacity.
d) A qualified water consultant should be retained on every HTW
installation to advise on water treatment for the primary and secondary water
e) The initial filling of the HTW system or large water loss
replacements should be through a water treatment source other than the plant
f) Generally deaeration is not required.
Oxygen is usually
removed during the initial startup of the system.
A demineralization unit is usually not required for a HTW