Many times, when planning a new compressed air system, there is a debate about whether or not to buy a receiver tank. Even operators of established systems sometimes wonder… what is the function of air receiver tank? We collected several questions that we often get about this vital part for an efficient compressed air system.
What is the primary role of a compressed air storage tank?
- Compressed air storage
- Steady air signal to the compressor controls
- Secondary heat exchanger
The main purpose of air receiver tanks is to provide air capacity to meet high-demand events that last for short periods of time (30 seconds or less). This could be anything from sandblasting being done by a production worker, to someone using a blowgun to dust off their work bench. Air receiver tanks work in the same way a battery works on a car. They enable a small horsepower compressor to complete a larger task by utilizing stored energy.
Receiver tanks also help regulate the compressor controls to prevent “short-cycling” and “over-pressurization”. If a receiver is too small or not present at all, the compressor will rapid-cycle, leading to a variety of control issues.
The air tank also acts as a heat exchanger. As the air passes through it from the bottom port upwards to the exit port at the top, the temperature can drop more than 10°F when compared to the temperature that the compressor heat exchanger cooled it to.
How do air receiver tanks manage efficiency?
- Air tanks reduce cycle counts and the stored energy allows for lower stress on the compressor. When an air receiver tank is the right size for the system, this also increases efficiency.
- Every time a rotary screw compressor unloads, the sump tank (oil tank) vents. Over time, the wasted compressed air that is released during ventilation adds up to thousands of cubic feet of compressed air. When a receiver tank is correctly sized for the system, there is a reduction in cycles, which reduces unnecessary waste.
- Compressed air storage allows the reduction of the peak pressure that the compressor operates at. For every 2 PSI in increased pressure, there is an energy waste of 1%. Users can also eliminate against short-cycling by properly setting the load set point and the unload set points. This is done by determining the exact minimum and maximum pressures required by the air-using devices in the system.
- The stored air approaches the dryer at a lower temperature which reduces the workload on the dryer. Since the air passes through the receiver tank at a slow and steady rate, the temperature of the air drops a few degrees. 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. This allows the dryer becomes more efficient since less moisture and particulate is being passed through it.
Is there an advantage to placing the receiver tank outdoors?
- Depends on the climate
- Becomes a secondary heat exchanger
- Reduces heat in the compressor room
Often times 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 climate is non-freezing. In freezing temperatures, outdoor tanks may ice up and rupture unless they are insulated and monitored. Auxiliary heating is also necessary, unless the heat generated by the compressed air from the compressor is warm enough to maintain the compressed air temp above freezing.
If the climate in your area does not involve temperatures that are below freezing temperatures, putting the tank outside is a great idea. It is beneficial because the ambient air is most likely cooler outside than it is in the compressor room. This will help the effectiveness of a wet tank by reducing the approach temperature to the dryer.
Another advantage of placing the receiver tank outside is the heat from the tank is released to the outside instead of inside the facility or compressor room. This helps to reduce the temperature in the compressor room and the demand on the air conditioning in the facility.
What are the advantages of wet storage tanks?
- Prolongs the life of the pre-filter element
- Lowers the workload on the dryer
- Provides a clean signal to the compressor
Wet tanks are located between the compressor and the dryer. The wet compressed air enters into the receiver tank through a port near the bottom of the tank and exits out of a port near the top of the tank. When using a properly sized receiver, compressed air travels through the tank at a very slow pace. As the air cools and the moisture condenses, it falls to the bottom of the tank and out of the drain port. This reduces water loading of the pre-filter between the wet tank and the dryer, prolongs its life, and reduces pressure drop. A water-logged filter is basically a clogged filter and is inefficient.
“A wet tank improves the efficiency and lifetime of the dryer because cleaner, drier air is coming into it.”
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 does not have a wet tank and the controller registers a 10 PSI of pressure drop or more (because of the dryer and filtration) in the dry tank, the pressure band is reduced significantly because of the inaccurate pressure signal.
What about dry storage tanks?
- Balances the flow through the dryer when there is a wet tank on the system
- Helps with dryer efficiency during peak demand
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 overworked during periods of high demand. The system tries to pull more air through the dryer than it is capable of handling. This could lead to water in the system because the dryer is underperforming. The only instance where a dry tank is not needed is where systems have a steady air flow.
Example: If there is 3 gallons per operational CFM on a 100 HP compressor producing 400 CFM, a total of 1200 gallons storage would be needed. 400 gallons would be wet storage and 800 gallons dry. Dry storage needs to be greater than wet storage. In periods of high demand, air will be drawn from the wet tank, which will over-capacitate the dryer, causing moisture in the air lines.
How Do I Determine How Much Storage Capacity I 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. (There are exceptions to this rule of thumb.)
- It can be calculated when the high demand CF and its duration are known.
Although these points are a good place to start, there are other factors to consider before you can accurately determine 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 system. In many manufacturing environments today, there is a very steady airflow. In instances like robotics, 1.5 gallons of storage per CFM of compressor capacity is enough, as long as it is all wet storage. When the air flow is steady, there is no need for a dry tank because the air would simply flow directly through the tank instead of being stored.
The amount of CFM needed at the point of highest demand is the most important factor when determining how much air storage is needed. Proper testing should be done in order to determine what this volume is.
It is also important to note that systems with pipework of 2” diameter or greater should consider the storage provided by the pipe itself. This may reduce the amount of storage tank capacity that is needed.
What are the options for protective coatings?
- Bare steel with primer exterior
- Epoxy coated or galvanized – internal & external
- Stainless Steel
The majority of tanks 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 incoming air. Although this is a benefit, it also adds to the cost of the tank. In rare instances, where there is a need for high purity air, a stainless steel tank is required. Applications for these epoxy lined, galvanized, or stainless steel tanks include: hospitals, labs, electronics, chemical, and aerospace.
What accessories are commonly used on compressed air tanks?
- Electronic auto condensate drains for automatic condensate drainage
- Zero air loss condensate drains for automatic drainage plus energy savings
- Pressure relief valves for safety
- Gauges for a visual indication of pressure levels
- Fatigue-eliminating vibration pads for applications where the compressor is mounted on top of the tank.
Is it important that my tank is ASME certified?
YES! A non-code, uncertified tank should NEVER be used. If a tank is not certified there is no standard guiding its manufacturing process or testing procedure. Visit the ASME website for more information.
Certified receiver tanks are the only tanks that should ever be considered. Frighteningly, many compressors from the “big box” stores or online retailers have non-code tanks. This can pose a life threatening risk if things go wrong. If you are not sure if you have a certified tank, we recommend that you contact your local fire marshal. They will test your tank with technology that measures the thickness of the metal. Please heed this warning – your life, and even the lives of your co-workers may depend on it!