UFC 3-440-01
14 June 2002
4.4.1.4.3
Determination of Design Pressures. The air-side of closed expansion
tanks are normally required by the manufacturer to be precharged to some pressure
above atmospheric. This initial or precharged pressure (Pi) must be determined, along
with the collector loop fill pressure (Pf) and the maximum relief pressure allowed in the
system (Pr). As discussed previously, the maximum pressure in the collector loop
should be 125 psi (862 kPa). The system fill pressure should result in a +10 to +15 psi
(+69 to +103 kPa) pressure at the highest point of the system. The expansion tank
precharge pressure should be equal to the fill pressure at the expansion tank inlet,
minus 5 to 10 psi (35 to 69 kPa). This initial condition allows fluid to be contained within
the expansion tank at the time of filling and will provide positive pressure in the event of
the system operating at temperatures below that occurring when the system is filled.
4.4.1.4.4
Sizing and Specification. Once the acceptance volume and the design
pressures have been determined, the total (fluid plus air) expansion tank volume VT can
be calculated by using Equation 4-2.
Vcoll
VT =
(eq. 4-2)
Pi Pi
-
P
f Pr
where Vcoll is the total volume of the collectors and piping above the collectors. This
equation is plotted graphically in Figure 4-9. Manufacturers provide expansion tank
sizes by either the total volume of both the air and fluid, or by separate specification of
the acceptance volume and design pressures. When the manufacturer supplies both
the acceptance and total tank volumes, the designer should specify the tank that
satisfies both conditions. The volume data given by the manufacturer in these cases
may not coincide exactly with those calculated by the methods shown above. The
values should be close, however, since variations should only be due to slightly
different types of fluid/air separation mechanisms. The manufacturer should supply
literature on their particular requirements for initial charge (if any) and temperature and
pressure limits. Careful attention should be given to the bladder materials. EPDM
rubber is the recommended material for use with propylene glycol. As in other parts of
the system, the propylene glycol based heat transfer fluid should not be allowed to
come in contact with ferrous materials, especially galvanized steel.
4-18
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