2 Most Common Wet & Dry Storage Tank Misconceptions

Throughout my career in the compressed air industry I’ve noticed that there are a lot of misconceptions floating around regarding “Best Practices” by the end user.  Whether they’ve not been educated by their service provider(s) throughout the years, or they simply have forgotten over time here are a few topics that everyone should understand.

Wet & Dry Receiver Tanks – Importance of Storage and the Differences Between the Two?

This seems to be a hot topic when having the discussion with end users that are either in the process of purchasing a new system, or they simply have no storage in their existing system at all.  The two main questions that I regularly get asked are what are the differences between a Wet and Dry Tank, and what is their importance to the air system?

Importance of Storage

The main importance of storage is to provide a large volume of air so that the air compressor can react accordingly to a stable demand.  Both a Wet Tank and Dry Tank help attribute to the solution.

Without proper storage (basic rule of thumb is 3-5 gal. per cfm generated dependent on capacity control of the air compressor) plant demand can fluctuate rapidly causing an air compressor’s controls to cycle in a way that can cause harm to the compressor.  For a simple comparison, imagine driving a vehicle where you need to quickly push the pedal to the floor and then release continuously.  As you can imagine big time problems will occur as the motor rpm’s are rapidly fluctuating from the highest point to the lowest point which is something that we all don’t want.  Trying to eliminate this as much as possible is key to prolonging the life of the air compressor, and that is by having the proper storage.

Proper storage also leads to energy efficiency as it will allow your compressor to run at lower pressure set points.  The key variable here is that for every 2 PSI that is reduced the energy costs will decrease by 1%.

Wet Tanks

A Wet Tank is located downstream of the air compressor and upstream of the system’s air dryer.  No matter the style of dryer that’s implemented within the system (desiccant or refrigerated) Wet Tanks are extremely important, and arguably the most important of the two tanks. The main purposes/advantages of a wet tank are the following:

• To be able to supply a steady volume of air to the compressor in order to react to plant demands that fluctuate rapidly.  As previously discussed, this will allow the compressor to properly load/unload as needed and will eliminate rapid cycling.  Rapid Cycling is prevented as the tank fills up completely, and then that air is drawn from the tank due to the plant demand.   As this happens, the compressor will react to the volume of air being pulled and filled as needed.
• Since most air compressors have a discharge temperature of 100 Deg. F. above ambient temperatures its always best to cool down that air as much as possible before entering the dryer as most dryer’s max inlet temperatures range from 110-120 Deg. F. This can of course be achieved by a compressor’s aftercooler, but a Wet Tank can also help reduce the temperature as well by allowing time to pass before the air gets to the dryer.  The lower the temperature, the happier the dryer will be, and the longer it will last.
• Wet Tanks are great for handling the initial bulk of oil carry over and water from the air compressor as long as a suitable drain is installed on the tank.  By eliminating these loads of oil and water the inline filtration downstream along with the dryer will have their life extended greatly.  Not only that, it will help eliminate the water and oil from getting downstream of the system and into the plant’s lines.

Dry Tanks

A Dry Tank is located downstream of the systems air dryer and is also is important to the system.  The main purposes/advantages of a dry tank are the following:

• Even though throughout a system that implements “Best Practices” which, in order, includes a compressor with an aftercooler, moisture separator, wet tank, dryer, and inline filtration prior to entering a dry tank it can still act as a supplemental option to eliminate oil and water.  This is beneficial if there is massive oil carry over, or if either the drains on the wet tank, dryer, and filtration fail.
• As discussed, proper storage is key, and since a dry tank can be installed downstream of a dryer prior to entering a plants production area they can typically be sized much bigger than a wet tank primarily due to space constraints.  This will allow most to be able to achieve the recommended 3-5 gallons of storage per cfm output of their compressor.
• The main and most important key to a dry tank is to provide that last volume of complete dry air to the production floors.  As long as all pieces of equipment prior to the dry tank does their job the goal of having a sufficient amount of clean dry air will be achieved.

By understanding the importance of storage to a compressed air system along with the benefits of utilizing a wet tank and dry tank properly the end user should be ultimately satisfied with their system. Not only will they obtain the goal of clean dry air on the production floor, but the compressor and dryer (the most expensive pieces of equipment in the system) will have a longer shelf life and so will the items downstream on the production floor.

– Cole Garland
Branch Manager