Karl Meixsell | Choice Bagging Equipment, Ltd.
An air packer requires a reliable source of low-pressure, high-volume air to fluidize and convey material during bagging. This article describes the three most common air sources used by air packers and compares the advantages and disadvantages of each.
An air packer (also called a pressure packer or forced air packer) is a commonly used pneumatic packaging machine designed to fill multiwall valve bags with capacities between 20 and 110 pounds (9 to 50 kilograms). Air packers work well with a range of powders and granular materials and mixes, including concrete, sand, cement, barite, bentonite, gypsum, limestone, plastic pellets, iron oxide, titanium dioxide, zinc oxide, calcium chloride, salt, calcium carbonate, starch, corn flour, sugar, and hard grains.
An air packer’s basic components include a pressurizing chamber with a feed inlet at the top, an air disseminator inlet to allow for pressurizing air to be injected into the chamber, and a porous fluidizing pad at the bottom. A discharge tube called a fill spout is located near the bottom of the pressurizing chamber with a cut-off valve to control material flow into the spout. A bag clamp mounted over the fill spout holds a valve bag over the spout during filling, and a bag chair equipped with a mechanical (counter balance) scale or digital load cell weighing scale monitors the bag weight.
In operation, the material to be bagged is fed into the pressurizing chamber through a butterfly or slide-gate valve, and an empty valve bag is placed onto the bag chair with its opening over the fill spout. The feed inlet closes, sealing the pressurizing chamber, and air flows into the chamber through the air disseminator at the top of the chamber and the fluidizing pad at the underside of the chamber. The pinch valve (referred to as a pinch tube cutoff) opens and allows material to flow through the spout into the bag. When the bag reaches the proper fill weight, the cut-off valve closes, the bag clamp releases, and the bag chair tilts to discharge the filled bag.
An air packer requires high-pressure, low-volume compressed air to activate the machine’s pneumatic controls and low-pressure, high-volume air to fluidize and convey the material from the pressurizing chamber into the bag. The three most common methods for providing this low-pressure, high-volume air are: high-pressure plant compressed air, a regenerative blower, and a positive-displacement (PD) blower. In this article, we’ll discuss the advantages and disadvantages of each.
Plant compressed air
Compressed air is so widely used in manufacturing that it’s often considered a fourth utility after electricity, natural gas, and water. A typical manufacturing plant uses compressed air for dozens of power, process, and control applications, including operating pneumatic controls on processing equipment such as an air packer. Since compressed air will already be available at the air packer to operate the controls, using the same air for fluidizing and conveying the material may be a logical and economical solution in some applications.
Advantages. Using the plant’s existing high-pressure compressed air system is the quietest method of supplying conveying air to an air packer. This method requires no additional blowers, motors, pumps, or filters beyond those already being used to compress the plant air. This can reduce equipment and maintenance costs, since, theoretically, an air packer connected directly to the high-pressure plant air line may fill the bags without any special accommodations. In practice, however, you’ll get better results by feeding the plant air into a reservoir tank via a regulator so that when the air packer calls for air, sufficient pressure and volume are always available to fill the bag instantaneously.
Disadvantages. Plant compressed air is notorious for containing liquid condensation. Depending on the amount of moisture and the application, this condensed moisture can cause material flow or product quality problems if pumped directly into the air packer’s pressurizing chamber. Compressed air also frequently contains oil, rust, or other contaminants from the lubricants used in the compressor or from rust inside the piping between the compressor and the air packer.
When using plant compressed air, the volume of air entering the pressurizing chamber is limited by the size of the high-pressure line feeding the machine. This can decrease bag-filling speed and reduce production rates, particularly when not using an air reservoir tank. Optimizing material flow into the bag requires the correct balance between air pressure and volume. Maintaining this balance is difficult when converting from high-pressure, low-volume plant compressed air to low-pressure, high-volume conveying and fluidizing air, which can limit the air packer’s ability to consistently fully open and fill the bags.
Finally, using plant compressed air carries the risk of over-pressurizing the air packer. Plant air is typically compressed to between 80 and 120 psi, while most air-packer pressurizing chambers are rated for a maximum of 30 psi and often for much less. Applying too much pressure to the pressurizing chamber can create an explosion hazard.
A regenerative blower (also called a side-channel blower or ring blower) uses an impeller with numerous blades rotating inside a ring-shaped housing to generate high-volume, low-pressure airflow. The blower housing has inboard and outboard channels that, along with the impeller blade design create a circular or “regenerative” airflow pattern inside the housing that increases the blower’s pressure-generating capacity.
Advantages. A regenerative blower can provide increased production capability compared to plant compressed air. When sized correctly, a regenerative blower is capable of creating the volume-pressure balance that’s required for optimal material flow into the bags. Regenerative blowers are typically mounted directly to the air packer, allowing all electrical and pneumatic connections to be prewired and plumbed at the factory, reducing installation costs. While regenerative blowers are louder than plant air, they’re typically quieter than PD blowers.
Disadvantages. Since regenerative blowers are mounted on or very close to the air packer, the blower noise is close to the workers operating the machine. This also means that the blower is exposed to the dirt and dust of the bagging operation, which can increase blower maintenance requirements and the likelihood of expensive, unplanned downtime.
Regenerative blowers typically require more horsepower to generate the same airflow compared to PD blowers, resulting in higher energy consumption for a given air volume output. Also, regenerative blowers build pressure more slowly, which can slow down bag filling compared to the same machine using a PD blower.
Positive displacement blower
A PD blower uses two counter-rotating rotors, each with two or three lobes, to draw air into the intake side of the blower housing and discharge the air out through the blower’s discharge port. The clearances between the rotor lobes and the housing are minimal, preventing backflow and ensuring that the volume of air moved by the blower remains consistent regardless of upstream line pressure or conveying load resistance.
Advantages. PD blowers are designed to output a constant amount of air pressure and volume simultaneously. An air packer requires a specific ratio of air volume and flow to fluidize the material and move it into the bag. The correct air pressure is also required to fully open the bag and deliver the material at a sufficient velocity to prevent it from forming a mound at the filling-spout discharge, which can cause inaccurate bag filling, increased fill time, spillage when the bag is removed, and other negative side effects. By optimizing the air pressure and volume, the PD blower is capable of maximizing the air packer’s production output.
While a PD blower is typically louder than a regenerative blower, a PD blower can be located up to 100 feet away from the air packer, which keeps the noise away from the machine operator. This can also keep the blower away from the dirt and dust of the working area. Also, PD blowers use standard motors and drive components that can be purchased locally, allowing the blower to be easily repaired or rebuilt when necessary.
Disadvantages. A PD blower typically has a higher initial installation cost than either plant air or a regenerative blower. The PD blower requires a 2- to 2½- inch supply line between the blower and the air packer, along with electrical to wherever the blower is installed. Also, while the blower can be installed away from the air packer, blower controls must be mounted at the air packer for ease of access. PD blowers require some minimal periodic maintenance, including checking the oil every 6 months and cleaning the air filters once a month.
Depending on the situation, any of the three air sources discussed in this article can deliver the low-pressure, high-volume air needed to power an air packer. However, for most applications, a PD blower tends to deliver greater production output, better performance, and lower overall operating costs than plant compressed air or a regenerative blower.
For further reading
Karl Meixsell is the founder and owner of Choice Bagging Equipment. He’s been servicing, designing, and manufacturing bagging equipment and systems for more than 20 years. He currently focuses on problem solving and consulting on a wide range of bulk solids bagging and handling applications.
Choice Bagging Equipment • Taylor, TX
512-352-3694 • www.choicebagging.com
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