subtracting amounts of any load condition. The pressure within the expansion
tank, however, will not remain constant since the water level is constantly
changing with heating load change. As the heating load decreases and the
return water becomes hotter, the water expands and flows into the expansion
tank raising its level. This rise in level will compress the nitrogen to a
higher pressure. In addition to these thermal expansion pressure changes,
loss in system water volume will also reduce tank level and subsequently the
pressure. Nitrogen is automatically added to maintain the minimum system
pressure. Any operating experience that shows steam formation is taking place
in the system requires a reevaluation of pressure settings. Because the
system is designed to make pressure and temperature independent of each other,
the pressure levels may be raised without affecting temperature control.
Minimum pressure should be 25 psig (103.5 kPa) above saturation pressure
corresponding to supply water temperature.
It is important to note that the HTW generator burners should not
be fired until the minimum system pressure has been established. Since the
HTW generator will steam even with cold water at low pressure, tube failure is
likely. When steam bubbles begin to form in the generator tubing, they
insulate the tubes and further speed up tube burnout. Scaling of tubes is
also accelerated with steam formation. Nitrogen should be added automatically
to the expansion tank through a pressure reducing valve from the nitrogen
storage tank. Do not use compressed air for pressurization because air will
be absorbed by the HTW system and consequently cause oxygen corrosion in metal
Unacceptable Pressurization Methods
Elevated Tank. This is theoretically the simplest method of
pressurization. The tank or pressurizing vessel is placed on a hillside or
suitable elevation, sufficient to provide proper hydraulic pressurization.
Pressure change with volume is automatic as it varies with the rise and fall
of temperature. However, the practicability of this system depends on
suitable terrain, which is rare.