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Properly Sizing Your Compressed Air Storage

Compressed Air Storage

No matter where you are, having storage is always nice. It lets you keep the things you need out of the way until it is time to use it. You have the benefit of being able to use that resource whenever you need it, and you won’t have to worry about not having it. Having storage tends only to provide you with benefits, unless you are a pack rat.

Now that we have established that having extra storage is a good thing let’s see how it applies to compressed air storage. Air receivers, also known as a compressed air tanks are an integral part of your compressed air system. The tank acts as temporary storage for your compressed air so that you never have to worry about output, even during peak demand.

By having a tank, you can optimize and improve the efficiency of your air system. These tanks can dampen the pulsations coming from the compressor because the pressure spikes will not affect storage, but it could be detrimental to your hoses and point-of-use equipment. Having tanks in place can ensure that you always have consistent airflow.

Not only do air tanks reduce the load on point-of-use equipment, it benefits the whole system. It reduces the load on the compressor, can prevent your dryer from being over-capacitated, and provide you with a safety net for if your compressor were to fail. So the fix is simple, add some storage to your system.

If only it were as simple as throwing a tank in your air system and going from there. When you are adding compressed air receivers into your system, you need to make sure that your compressor is able to work with your storage air without being overloaded or not holding enough air to the point of the air tank being useless. 

What Makes an Air Receiver Compressed Air Storage

The American Society of Mechanical Engineers, or ASME, has a set of codes and standards that pressure vessels and air tanks must comply with. These rules pertain to welds and joints, tank thickness, and connections and other tank components. During construction the vessels are subject to these rules that determine the design, fabrication, assembly, and allow for inspection during the construction of the pressure vessel.

Here’s a short list of what could cause problems in your tanks if you do not follow their guidelines or if you don’t take proper care of the air tank:

  • Faulty Design (Non-code tanks)
  • Operating Above Maximum Working Pressure
  • Improper Installation
  • Corrosion
  • Cracking
  • Weld Failure
  • Improper Repair
  • Exposure to Extreme Conditions
  • Safety Valve Failure

Now that you know what can go wrong in a pressure vessel, and just how important it is to follow the guidelines laid out by the ASME. In order for a pressure vessel to be considered a tank it must include the following to function properly:

  • Zero Air-Loss Condensate Drain
  • Electronic Auto Condensate Drain
  • Pressure Relief Valves
  • Vibration Pads

If the vessel contains these components, then it was constructed or needs to be constructed following the stringent guidelines in place for compressed air tanks. Once the equipment has passed the inspections it is safe to use in your facility and with proper care these tanks can last 10-15 years under proper maintenance and some can go even longer. 

 

Size Matters With Tanks

Even though it is possible, in theory, to have your compressor system setup and functional without a compressed air storage tank in place, it majorly increases the load on your compressor and will increase uptime and decrease downtime in between. This will greatly decrease the lifespan of your compressor and the cost to buy a new compressor is a bit higher than the cost of the tank.

The ideal set up for your compressed air storage is to have split storage between your wet and dry air. Your split will be ⅓ wet and ⅔ dry air storage. So if you have 600 gallons of storage you will have 200 gallons of wet air storage and then 400 gallons of storage after the dryer. This helps ease the load on your dryer and compressor while also providing you with compressed air available whenever it is needed.

 

How To Size Your Tank

Sizing tanks comes down to following the equation that is laid out below. This basic formula will assist you in finding the volume of compressed air storage that will work with your system.

The formula is as follows:

V = T * (C – Cap) * (Pa) / (P1-P2)

  • V – Volume of receiver tank (cubic feet)
  • T – Time interval (minutes)
  • C – Air requirement of demand (cubic feet per minute)
  • Cap – Compressor capacity (cubic feet per minute)
  • Pa – Absolute atmospheric pressure (PSIA)
  • P1 – Tank pressure (PSIG)
  • P2 = minimum tank pressure (PSIG)

 

If you are more of a visual learner, don’t worry just check out the image below to give you a better idea of how these formulas work and where you might get the variables you need.

 

The reality of these equations is they calculate how much air is being made in a certain amount of time while under demand. You get the desired volume of your compressed air storage by taking the time in minutes air will be needed and multiplying that by the difference of Air demand and compressor capacity which will both be in CFM.

This is the first part of the equation, now if you are doing it by hand you will want to find the answer of time multiplied by difference in CFM. If you are just plugging it all into a calculator then you won’t need to do that additional step. This answer, or group of variables, will then need to be multiplied by the difference in pressure.

The difference in pressure is calculated by subtracting the ending tank pressure from the initial tank pressure. This pressure will be the difference between your maximum operating pressure and your minimum operating pressure of your compressed air system.

And that’s all there is to it to sizing your compressed air tank. It may seem intimidating as a big clump of letters but when you break it down into the individual components it becomes easier to understand and easier to find your own volume needs.

 

But Wait, There’s More To Consider

Before you go out and buy an air tank there are a few big points to remember about your compressed air storage tank. Don’t worry these aren’t more worries to add to your plate but rather the list of benefits the tank provides for you by simply having it properly sized and in place.

 

Minimizing Pressure Drops

The quality of your end product can often be affected by pressure fluctuations and be detrimental to the production process as a whole. By attaching an air tank you can minimize these fluctuations as the tank will absorb the pressure fluctuations and continuously output a steady flow.

When you are picking your compressed air storage, you need to be mindful of two different pressures. The pressure being output from the compressor and the pressure needed at point-of-use. If the pressure being output does not meet the requirements then the tank is not sufficient for your operation.

This is why it is important to know the duration your tank will be under demand to make sure you have enough air.

 

Meeting Short Term Peak Air Demands

If your demand experiences spikes and dips during production it is important to make sure spikes do not cause the tank to dip below operating pressure by using too much air. The compressed air storage acts as a buffer during peak demand.

This helps your system meet demands that it might not be able to with the compressor alone. It is important to fully understand your compressed air needs so that your system can reduce the load on your compressor during peak demand. This will help you avoid shortages of air as well.

 

Energy Considerations

By including compressed air storage in your system you can reduce the energy consumption of your air compressor. By storing the compressed air, it allows your fixed speed compressor to experience longer cycles with tighter pressure bands without wearing out the compressor. By properly sizing your tanks it will reduce pressure fluctuation and frequent motor starts. 

This will not only save you money on your energy bill but it will also extend the life of your compressor by maintaining a consistent output pressure. 

 

Safety Considerations

If the need arises, compressed air storage can act as an emergency buffer.  It allows you to shut down the compressor during an emergency situation. You will be able to continue the current project until supply runs out, but if you have enough storage it can allow you to repair the compressor without running out of air.

 

The Not So Fun Considerations

Just a few other things to keep in mind with your compressed air storage is the amount of water made by compressed air and to account for pressure drop.When it comes to figuring out the amount of water in your system it is dependent on compressor run time and the daily humidity.

When it comes to your compressed air storage, you want to make sure there is not a pressure drop over 1.45PSI between the air compressor outputting the air and the most remote point of demand.

As long as you keep these additional concerns in mind, you will be perfectly fine following the formula to decide the size of the tank you need. If you know you live in an area with excess humidity or have long run times you will want to keep that in mind when sizing your compressed air storage.

 

Before You Buy

A good rule of thumb for compressed air storage is having 3-4 Gallons of storage for every CFM you have, if you use metric that would be 10-15 liters for each CFM of compressed air. This should not be what you use to determine your exact air needs but it is good to give you an idea of what sized tank you might need. 

When it comes to sizing your compressed air storage, you need to know how hefty your demands are and the pressure you need. With this information and the formula above you will be able to find the answer for your needs. And never undersize. If your measurement comes in between two different sizes of tanks always go up and never go down.

By having a tank installed in your compressed air system it will ease the load on your compressor and provide you with many different benefits. The tank can act as a buffer between your compressor and individual components to make sure that they do not become overwhelmed with demand and are able to do their individual jobs properly.

Additional storage will also reduce your energy consumption by allowing your compressor more downtime and less direct flow needed on start up. These also prevent your compressor from being overworked during peak demand periods and gives you reserves of air during emergency shutdown.

The benefits of compressed air storage make the implementation of one into your system well worth the cost of a new tank. Your compressed air system and your sanity will benefit from having compressed air storage put in place if you do not already have it.

 

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