Sizing the Expansion Tank
In closed loop systems, expansion 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 heat transfer fluid when the solar loop goes into stagnation. Provided below are the basic sizing rules for selecting and sizing expansion tanks in closed loop solar systems.
Expansion tanks used for potable water are not the same as those used for glycol based non-potable fluids. Potable water expansion tanks are designed to resist corrosion that may occur due to oxygen in the water. Non-potable expansion tanks have bladders designed to resist chemicals typically found in closed loop applications. For closed loop solar systems, the expansion tank must be acceptable for use with propylene glycol or other heat transfer fluid used in the collector loop. In addition, the temperatures and pressures in closed loop solar systems can be significantly higher than found in typical hydronic systems. Care must be taken that the expansion tank selected will meet the requirements of the solar application.
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 closed loop glycol system. The “tank volume” is the total volume of the tank. The “acceptance” is the volume of fluid that the expansion tank can accept i.e. how much fluid can be pushed into the expansion tank when the fluid in the closed loop gets hot and expands. When fluid is pushed into the expansion tank the air cushion is compressed and therefore the pressure in the tank increases. The pressure of the air cushion in the expansion tank regulates the pressure of the fluid in the entire closed loop. When the air cushion is compressed to half its original volume, the absolute pressure in the tank doubles. A large expansion tank will maintain a small pressure swing as the fluid expands and contracts while a small expansion tank will lead to large pressure swings as the system heats up and cools down.
If the expansion tank is undersized, the system pressure may increase sufficiently to open the pressure relief valve, causing loss of glycol and necessitating re-charging the system.
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.
Pressurizing the Expansion Tank
Most non-potable expansion tanks on the US market are designed for use in hydronic heating systems and therefore come pre-charged to 12 psig. Closed loop solar systems are typically charged to 30 – 40 psig, connecting a tank charged to 12 psig to a glycol loop at 30 psig will result in glycol immediately filling 40% of the tank, reducing the effective volume available for additional acceptance. Therefore the air side of the expansion tank should be pressurized to 2 – 5 psi below the intended system fill pressure before the expansion tank is connected to the charged system. Most expansion tanks are fitted with a standard Schrader valve on the air side and can be charged with a bicycle pump and pressure gage.
The WST series expansion tanks available from SunEarth are designed for solar closed loop applications and come with a factory pre-charge of 28 psi, eliminating the need for additional pre-charging.
Calculating the Volume of Fluid in the System
The first step in sizing the expansion tank is to calculate just how much fluid is in the closed solar loop. Once the pipe sizes and the total pipe run lengths have been established, fluid capacities can be calculated using the table below. In addition to the pipe run capacity you should add the capacity of the collectors and the capacity of the heat exchanger as specified in the SunEarth specification sheets. This total will be the closed loop volume, Vloop.
|Pipe Capacity (US Gal/100 ft)|
|Pipe Size||Type M||Type L||Type K|
Calculating the Required Acceptance Volume
The acceptance volume of the expansion tank must be large enough to accommodate the expansion of the glycol under stagnation of the collectors. For a propylene glycol based heat transfer fluid, with temperatures between a minimum of 35oF and a maximum of 200oF, the fluid expansion is slightly less than 5% of the loop volume. Most expansion tank manufacturers and on-line calculators assume that all the fluid remains in the liquid phase and only the volume change due to fluid expansion needs to be accommodated, this is not the case for solar applications where the fluid in the collectors can boil to steam. To accommodate the volume of steam, the expansion tank acceptance needs to include BOTH the volume of the collectors plus any piping above the collectors AND the volume change due to expansion. By sizing the expansion tank this way you will prevent fluid loss due to stagnation and boiling in the collectors. The savings in lost fluid and maintenance cost justify the additional expense of a larger expansion tank. For solar applications, the required acceptance volume, Vacceptance can be calculated from:
Vacceptance = (Vloop * 0.05) + Volume of collectors + Volume of piping above collectors
The expansion tank selected must have a volume no less than Vtank above.
For example: if the expansion tank is pre-charged to 28 psig (42.7 psia) and the pressure relief valve is set to discharge at 75 psig (89.7 psia) the required tank volume would be:
Vtank = Vacceptance / (1 – (42.7 / 89.7)
= Vacceptance * 1.91
The table below lists the total volume, acceptance volume and other specifications for some common expansion tank types available from SunEarth. Allowable volumes for other tank manufactures can be computed with knowledge of overall tank volume and acceptance volume.
Size expansion tank for system with 3 x EP-40 collectors; 16 ft. of 3/4” Type L copper pipe on roof; total pipe run for the system is 80 ft. of 3/4” Type L copper pipe; and the heat exchanger volume is 2.2 gallons. The expansion tank is pre-charge is 28 psi and the PRV is set at 100 psi.
Collector capacity = 3 * 1.2 = 3.6 gallons
Pipe on roof capacity = 2.5 * 16/100 = 0.4 gallons
Steam volume in stagnation = 3.6 + 0.4 = 4.0 gallons
Total volume of system piping = 2.5 * 80/100 = 2.0 gallons
Collector capacity = 3 * 1.2 = 3.6 gallons
Heat exchanger capacity = 2.2 gallons
Total fluid volume = 2.0 + 3.6 + 2.2 = 7.8 gallons
Allowance for fluid expansion = 7.8 * 0.05 = 0.39 gallons
Minimum required acceptance volume of tank = 4.0 + 0.39 = 4.4 gallons
System minimum absolute pressure Pmin = 28 +14.7 = 42.7 pisa
System minimum absolute pressure Pmax = 100 + 14.7 = 114.7 pisa
Minimum required tank capacity = 4.4 / (1 – (42.7 / 114.7) ) = 7.0 gallons
Select Watts Solar WST35 from table above.