Often people who are installing new compressed air systems and those who use compressed air find themselves debating whether to purchase an air receiver tank.
In the following article there are a few topics that hopefully answer any questions that you may have about compressed air receiver tanks. They include the following:
- The primary role of a compressed storage tank.
- Why air receiver tanks manage efficiency.
- Is it advantageous to put a receiver tank outdoors?
- Advantages of wet storage tanks.
- Advantages of dry storage tanks.
- How to determine how much storage capacity you need.
- Options for internal lining.
- Accessories for compressed air tanks.
- Making sure your tank is ASME certified.
- Compressed air storage.
- Provides a steady air signal to the compressor controls.
- To act as a second heat exchanger.
The main purpose of air receiver tanks, are to give you the air storage capacity to meet high demand events that last for short periods of time (up to 30 seconds). This could be anything from a production worker sandblasting to someone using a blowgun to quickly dust themselves off. Air receiver tanks work in a similar manner to a battery. They allow you to use a smaller horsepower compressor to complete a larger task by utilizing stored energy.
Receiver tanks also help steady the compressor controls to eliminate short cycling and over-pressurization. If receivers are too small or not present, the compressor will rapidly cycle which will lead to a variety of issues.
The air tank acts as a second heat exchanger because as the air passes through it, its temperature is lowered approximately another 10 degrees below what the first heat exchanger has already cooled it to.
- They reduce cycle counts,
- They allow you to lower the pressure on your compressor because of stored energy,
- The stored air approaches the dryer at a lower temperature, thus increasing the efficiency of the dryer.
Efficiency can be increased with a properly sized air receiver tank. Every time a rotary screw air compressor unloads, the sump tank (oil tank) is vented. Over time, the compressed air that is wasted during venting, adds up to thousands of cubic feet of compressed air. With appropriately sized storage, the frequency of cycles is reduced, therefore reducing or, in some cases even eliminating, unnecessary waste.
Compressed air storage allows users to reduce the peak pressure at which they operate. This increases efficiency because for every 2 PSI that you increase your pressure, 1% in energy is wasted. to protect these users from short cycling, pressure bands are set.
Lastly, since the air passes slowly through the receiver tank, the temperature is reduced because it has time to cool before it goes into the piping system. This allows moisture to condense and drain out of the valve at the bottom of the tank. The dryer becomes more efficient because less moisture and particulate is being passed through it.
Is it advantageous to put a receiver tank outdoors?
- Dependent on the climate.
- Tank will be more effective as a secondary heat exchanger if it is outdoors.
- Can eliminate heat from being radiated into the compressor rooms.
Often compressed air users will ask if they can put their tank outside, as it will save them space in their facility. This will only work if the surrounding area’s climate is warm and temperatures never get below freezing. In freezing temperatures, outdoor tanks may ice up and rupture unless they are insulated, monitored, and heated (only necessary if whatever heat is generated by the air in the tank is not sufficiently heat already).
If the surrounding climate is in your favour and you can put your tank outside, it is beneficial because the air is most likely cooler outside than it is in the compressor room. This will help the effectiveness of a wet tank by further reducing the approach temperature to the dryer.
Another advantage of leaving your tank outside, is that the heat radiated from the air inside the tank is released to the outdoors rather than inside your factory or compressor room. This helps to reduce the temperature indoors.
Advantages of wet storage tanks
- Prolongs the life of the pre-filter element.
- Helps the dryer efficiency.
- Wet tanks provide a clean signal to the compressor.
As previously mentioned, wet tanks have major benefits and are located between the compressor and the dryer. The wet compressed air enters the receiver tank through a lower port and exits out of the uppermost port. Because the compressed air travels through the tank at a very slow pace, the air cools and moisture inside of it condenses and falls to the drain port. This reduces slugging of the pre-filter (located between the wet tank and the dryer), which in turn prolongs the filter’s life and reduces pressure drop caused by slugging.
All this drastically improves the efficiency and lifetime of the dryer because it means that cleaner, drier air enters the dryer.
Lastly, there is no pressure drop in a wet tank because the air has not gone through any filtration. This provides a steady pressure signal to the compressor controller. If a system doesn’t have a wet tank and the controller registers up to 10 PSI of pressure drop in the dry tank (because of the dryer and filtration), the pressure band is reduced significantly because of the inaccurate pressure signal
Advantages of dry storage tanks
The ideal ratio of compressed air storage in an air system is ⅓ wet to ⅔ dry capacity. Without a dry tank, the air dryer is at risk of being over capacitated during periods of high demand because the air system tries to pull more air through the dryer than it is rated for. This could lead to water in the system because the dryer is underperforming. The only instance where a dry tank is not needed is in systems that have a steady air flow – Read the next section for more details.
For example: If we are installing 3 gallons per operation CFM on a 100 HP compressor producing 400 CFM, we would need a total of 1,200 gallons storage of which 400 would be wet storage and 800 gallons dry. Dry storage needs to be greater than wet storage because otherwise, in periods of high demand, air will be drawn from the wet tank’s storage, thus over capacitating the dryer. This will cause water in the air lines.
How to determine how much storage capacity you need
- 3-4 gallons per CFM of online compressor with ⅓ of total storage as wet storage and ⅔ of total storage as dry storage.
- The variations to the rules.
- Consistency in airflow.
- The amount of CFM required at your points of highest demand.
- Does your piping have a 2” diameter or greater?
Although the above points are a good place to start, there are other factors to consider before you can be sure how much storage you need.
Compressed air users must keep in mind that air flow consistency makes a difference in how much storage you need for your compressed air system. In many manufacturing environments today, there is a very steady airflow. In instances like robotics, it could be argued that 1.5 gallons of storage per CFM of compressor capacity is plenty, if it’s all wet storage. In cases like this where the air flow is steady, there is no need for a dry tank because the air would just constantly flow through the tank instead of being stored up.
The amount of CFM needed at your point of highest demand is the most important factor when determining how much air storage you need. Proper testing should be carried out to determine what this volume is.
It is also important to note that systems with pipework of 2” diameter or greater should take the additional storage provided by the pipe into consideration. This may slightly reduce the amount of tank storage needed.
Options for internal lining
- Bare steel with primer exterior.
- Epoxy coated or galvanized.
Most of tanks that are installed in compressed air systems are bare steel on the inside with a primer coating on the exterior. This exterior paint is commonly matched with the compressor equipment. In some instances, the inside of the tank is sprayed with an epoxy coating or galvanized to reduce corrosion and maintain the quality of the air put into it. Although this is advantageous, it also adds to the cost of the tank. Sometimes (very rarely), where high purity air is required, a stainless-steel tank is preferred. Applications for these epoxies lined, galvanized, or stainless-steel tanks include: hospitals, labs, electronics, chemical aerospace, etc.
Accessories for compressed air tanks
- Electronic auto condensate drainsfor automatic condensate drainage
- Zero air loss condensate drainsfor automatic drainage plus energy savings
- Pressure relief valves for safety
- Gauges for a visual indication of pressure levels
- Vibration pads for applications where the compressor is mounted on top of the tank to eliminate fatigue
Making sure your tank is ASME certified
- ASME Website
- Non-code should never be considered. There is no standard guiding its manufacturing press or testing procedure.
- The fire-marshall
Certified receiver tanks are the only tanks that should ever be considered. Frighteningly enough, many of the compressors from Big Box stores have non-code tanks. This can pose a life-threatening problem if something were to go awry with it. If you are not sure if your tank is certified, it is recommended that you get in touch with your local fire Marshall. They will test your tank with metal thickness testing technology. Please pay heed to this testing – your life, and possibly even your co-workers’ lives may depend on it!