My colleague, Lee Evans, wrote a blog about calculating the size of primary receiver tanks within a compressed air system. (You can read it here: Receiver Tank Principle and Calculations). I would like to expand a bit more about secondary receiver tanks. They can be strategically placed throughout the plant to improve your compressed air system. The primary receiver tanks help to protect the supply side when demands are high, and the secondary receiver tanks help systems on the demand side to optimize performance.
I like to compare the pneumatic system to an electrical system. The receiver tanks are like capacitors. They store energy produced by an air compressor like a capacitor stores energy from an electrical source. If you have ever seen an electrical circuit board, you notice many capacitors with different sizes throughout the circuit board (reference photo above). The reason is to have a ready source of energy to increase efficiency and speed for the ebbs and flows of electrical signals. The same can be said for the secondary receiver tanks in a pneumatic system.
To tie this to a compressed air system, if you have an area that requires a high volume of compressed air intermittently, a secondary receiver tank would benefit this system. There are valves, cylinders, actuators, and pneumatic controls which turn on and off. And in most situations, very quickly. To maximize speed and efficiency, it is important to have a ready source of air nearby to supply the necessary amount quickly.
For calculating a minimum volume size for your secondary receiver tank, we can use Equation 1 below. It is the same as sizing a primary receiver tank, but the scalars are slightly different. The secondary receivers are located to run a certain machine or area. The supply line to this tank will typically come from a header pipe that supplies the entire facility. Generally, it is smaller in diameter; so, we have to look at the air supply that it can feed into the tank. For example, a 1” NPT Schedule 40 Pipe at 100 PSIG can supply a maximum of 150 SCFM of air flow. This value is used for Cap below. C is the largest air demand for the machine or targeted area that will be using the tank. If the C value is less than the Cap value, then a secondary tank is not needed. If the Cap is below the C value, then we can calculate the smallest volume that would be needed. The other value is the minimum tank pressure. In most cases, a regulator is used to set the air pressure for the machine or area. If the specification is 80 PSIG, then you would use this value as P2. P1 is the header pressure that will be coming into the secondary tank. With this collection of information, you can use Equation 1 to calculate the minimum tank volume. So, any larger volume would fit the requirement as a secondary receiver tank.
Secondary Receiver tank capacity formula (Equation 1)
V = T * (C – Cap) * (Pa) / (P1-P2)
V – Volume of receiver tank (cubic feet)
T – Time interval (minutes)
C – Air demand for system (cubic feet per minute)
Cap – Supply value of inlet pipe (cubic feet per minute)
Pa – Absolute atmospheric pressure (PSIA)
P1 – Header Pressure (PSIG)
P2 – Regulated Pressure (PSIG)
If you find that your pneumatic devices are lacking in performance because the air pressure seems to drop during operation, you may need to add a secondary receiver to that system. For any intermittent design, the tank can store that energy like a capacitor to optimize the performance. EXAIR stocks 60 Gallon tanks, model 9500-60 to add to those specific locations, If you have any questions about using a receiver tank in your application, primary or secondary, you can contact an EXAIR Application Engineer. We can restore that efficiency and speed back into your application.