Sizing the Drainback or Expansion Tank
| In closed loop
systems, expansion and drainback tanks must be appropriately sized for the
system to operate properly. For expansion tanks the acceptance
volume must be sufficient to accommodate expansion of the glycol when the
solar loop goes into stagnation. Drainback tanks must be of
sufficient size that they can store the entire fluid volume of the array
with enough reserve capacity to ensure proper operation of the pump during
steady state operation. Provided below are the basic sizing rules
for determining tank sizing. The first step in sizing drainback or expansion tanks is to calculate just how much fluid is in the closed solar loop. Using pipe run lengths from the pump sizing section, fluid capacities can be calculated using the table below. In addition to the pipe run capacity you should add the capacity of the collectors as specified in the SunEarth spec sheets. This total will be the closed loop volume.
As illustrated below, expansion tanks contain an air
cushion that is separated from the glycol by a butyl bladder. The
compression and expansion of this air cushion is what regulates the
pressure inside the glycol system. The two items we care about are
that the acceptance volume of the tank be large enough to accommodate the
expansion of the glycol under stagnation of the array, and that when the
acceptance volume of the expansion tank is full of glycol, the pressure in
the air cushion does not exceed 100 psig. Vloop=4.2*Vtank The table below provides the maximum closed loop volumes allowable for the common line of Amtrol and Sparco expansion tanks. Allowable volumes for other tank manufactures can be computed with knowledge of overall tank volume and acceptance volume. These equations are valid for either a 12/20 or 20/30 psig ratio of precharge before filling of the solar loop and final loop charged pressure.
Sizing of drainback tanks is quite simple and a standard rule is to select a tank that is twice the loop volume. Therefore, if the closed loop volume were 40 gallons, a 80 gallon drainback tank would be suitable. Standard electric water heaters may be used as drainback tanks and are commonly available in sizes from 30 to 120 gallons. SunEarth CopperStor drainback tanks are available in 5, 7.5, and 10 gallon sizes. Six Rivers Solar manufactures custom drainback tanks in larger sizes. |
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1
Expansion tanks typically come pre-charged at 12 psig from the
factory with the butyl bladder pressed firmly against the fluid
opening
2
When the system is initially charged to a pressure near 20 psig, the
glycol presses against the bladder and fills about 20% of the tank.
3
As the system heats up during operation the glycol expands into the
tank and raises the pressure. When the loop cools, this
pressure will put the glycol back into the system.