14 June 2002
on all piping.
Operation. An expansion tank is required in the collector circulation loop.
In a closed-loop system, the expansion tank must serve two purposes: to protect the
system from overpressure due to thermal expansion of the fluid at high temperatures
and to maintain the required minimum pressure when the fluid in the loop is cold.
Expansion tanks are closed and initially charged with a gas (usually air) at some given
minimum pressure. As the temperature increases in the loop and thermal expansion
takes place, increasing amounts of displaced fluid enter the expansion tank and
compress the air within it. There are three common types of closed expansion tanks:
non-bladder, bladder, and diaphragm. In the non-bladder expansion tank, the
expanding fluid is in direct contact with the air charge. Bladder tanks are fitted with a
flexible balloon-like surface that separates the air from the expanding fluid. Usually,
bladder tanks require an initial fluid volume and air pressure, and do not permit the fluid
to come in contact with the metal tank surface. Diaphragm tanks are initially charged
with air also, but allow some fluid-metal contact as they fill. These mechanisms prevent
the air charge from being absorbed into the expanding fluid, with a resulting decrease
of corrosion problems and periodic venting maintenance. Because bladder tanks are
widely available and they prevent any metal-fluid contact, their use is required for solar
preheat systems. The expansion tank should be located in the equipment room on the
suction side of the pump.
Determination of Acceptance Volume. Determination of the collector
loop expansion tank acceptance volume is similar to that for a conventional hydronic or
boiler system tank sizing, with one important variation. Typical expansion tank sizing
routines account only for the variation of fluid volume with temperature change in the
liquid phase. While this is the condition existing within the solar collector loop during
normal operation, a more critical condition exists in the event of system stagnation that
requires a much larger volume than the conventional sizing routines. A detailed
account of stagnation and over-temperature protection of the system is discussed in
APPENDIX F. Solar energy systems are quite capable of boiling during stagnation, and
the expansion tank must be sized to account for the displacement of all of the fluid
contained in the collector array that is subject to vaporization. Since the stagnation
condition requires far greater volume than that needed for the conventional liquid-phase
expansion case and these two situations will never occur at the same time, the
conventional temperature-based expansion term is not needed. Experience shows that
during stagnation conditions, only the volume of fluid located in the collector array and
associated piping above the lowest point of the collectors is subject to vaporization.
Thermal stratification prevents fluid below this point from vaporizing to any significant
degree. The required acceptance volume of the collector loop expansion tank is thus
determined by adding the total volume of all collectors plus the volume of any piping at
or above the elevation of the collector inlets. When properly applied, this procedure
provides fail-safe pressure protection of the system, and prevents the loss of the
propylene glycol solution from the pressure relief valves. The result is a large decrease
in the number of failures and resulting maintenance calls.