There are many ways to use storage in a compressed air system to improve the performance and repeatability of production equipment. No one method is a total solution. Some industry professionals will tell you that storage is not required for certain types of compressors. The system, however, cannot afford the impact on either performance or operating costs.
The alternative to applying storage is to operate at higher pressures with more power all of the time in order to support critical applications and the peak air demand experienced in the system. There are six basic areas where storage should be properly engineered and applied in the system. These are:
- Dedicated storage to improve the speed, thrust, or torque of an application.
- Dedicated storage to protect a critical application from pressure fluctuations.
- Dedicated storage to meter a high rate of flow application into the system.
- General or overhead storage to support applications during the transmission time to the supply side and to create transparency between applications.
- Control storage to support events in the system within an allowable pressure drop.
- Off line, higher pressure air stored to support large system events and reduce peak electrical demand.
Fundamental Principles Which Applies To Storage Tanks
There are a few fundamental principles which must be discussed to understand when and how to apply storage in the system.
First, the article pressure in a system is the terminating pressure at the actual inlet connection to the device. It is not at the regulator or the header, so when someone says they have to have 90 psig for a particular device, it is very important to know where they are monitoring that pressure. This appears to be a small distinction but it makes a huge difference in what is required to support the article.
Second, the purpose of the system is to deliver the required mass of air to the article within the required time. Compressed air travels at a limited velocity inside the system determined by the pressure differential that exists. At 1 psid, this velocity is approximately 250 feet/second which means if the compressors are more than 250 feet away, they won’t see an event which is less than 1 second duration until after it is complete. If you forget to consider time, the value of these storage concepts will be very difficult to grasp.
Third, the primary formula for applying useful storage or capacitance is the capacity to store times the allowable pressure drop. For example, if I have a 660 gallon tank and I can afford to allow the pressure to drop 10 psi then the useful storage is calculated as: (660 gallons / 7.48 gallons/cubic foot) / 14.5 psia = 6.07sscf / psi x 10 psi = 60.7 scf of usable stored air.
With these principles in mind, one can easily apply storage and achieve it’s the benefits as well.
