Figure 4-74.Typical high-temperature hot-water circulation pumps.
Figure 4-75.A typical cascade heater spray nozzle head.
The steam in the space in the expansion tank
provides the pressure or cushion for the system. The
pressure maintained is that of the saturated steam. The
water in the lower portion of the tank will be
approximately saturation (steam or vaporization)
temperature corresponding to this pressure. The water
to be used in the HTHW heating system is drawn from
the lower part of the expansion tank, mixed with the
system return water, and circulated throughout the
system. The mixing is necessary to prevent cavitation
(steam flashing) at the pump suction.
Here are some conditions that are typical of the
saturated-steam cushion design. The expansion tank,
either integral or separate, is a part of the HTHW
system. The entire amount of hot water flowing in the
heating system passes through the expansion tank and
exposes the tank to maximum system heat and any
form of contamination which, in turn, subjects the
expansion tank to thermal stresses and corrosion.
There are explosion hazards typical of a steam boiler in
the system, and good water-level control is important
in maintaining proper operating conditions. Load
variations, causing supply pressure changes, create
flashing of saturated liquid in the system and produce