MIL-HDBK-1003/6
Tube Design. All tubes in the heat transfer zones should be
4.6.2.3
designed for upflow only. One of the inherent dangers in a HTW generator is
the possibility of developing steam bubbles. Since steam occupies much
greater space than water, steam bubble generation can cause interrupted flow
in a tube or tube circuit with resultant tube failure. When all tubes can
produce only upflow, it is possible for the steam to escape from the generator
and for water flow equalization to be reestablished. Trapped steam spaces
must be avoided. Design of economizers, which are similar in construction to
hot water generators, has shown that upflow is extremely important when the
temperature of the water approaches that of saturation. Where a tube is
designed for downflow of water, it should be removed from the heat transfer
zones.
Equalization of Flow through Tube Circuits. Generator design must
4.6.2.4
provide for equalization of flow through tube circuits. The water circuiting
in the forced circulation generator is a major design consideration. The
circuiting must effectively distribute the water in proportion to the heat
input in any individual circuit. This is especially important in a steam
pressurized system where the water approaches saturation temperature at the
discharge of the generator. The flow of water must be proportioned to the
heat input to prevent the formation of excessive amounts of steam.
Furnace Design. Furnace design and burner installation should be
4.6.2.5
coordinated to ensure that there is no flame impingement on furnace tubes and
walls. Flame clearance is the major criterion when designing oil and gas
combustion chambers.
Blowdown. Continuous blowdown of generators is a carryover from
4.6.2.6
past steam practices of eliminating sludge from mud drums. Blowdown is not
required since there is little, if any, sludge in HTW systems after the
initial fill.
Efficiency.
See Section 9.
4.7
Pressurization. Pressurization is required in HTW systems in order
4.8
to prevent the formation of steam in the flow lines. This can be accomplished
by exerting a pressure on the water greater than the saturation pressure
corresponding to the peak temperature of the system.
Collapse of pressurization must be avoided when system is in
operation. Sudden or complete collapse of pressurization will lead to
flashing of steam, water hammer, and similar phenomena that will set up
stresses and strains that the system is not designed to absorb.
System should minimize fluctuations in system pressure and outgoing
temperature of water from generator. If wide fluctuation in pressure is
allowed, the entire system may be affected. Therefore, those systems that
allow the system pressure or outgoing temperature to rise or fall
significantly are not as satisfactory. It is also possible that pressure
variation will affect the flow rate of water through the generator, especially
with one pump systems. Reduced flow rate may result in vapor locking and
subsequent tube failure.
4.8.1
Acceptable Pressurization Methods
4.8.1.1
Saturated Steam.
The expansion tank is located on the suction side
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