by Dan Fried and Devon DeJong
Does your mind go straight to the scene of your favorite Syfy movie when someone mentions airlocks – where the hero (or villain) is trapped between safety and being sucked out into deep space at the press of a button? We like a good science fiction movie, too, but airlocks in pneumatic conveying get us just as excited!
An airlock is a compartment with doors that can be sealed against pressure which permits the passage of people and objects between environments of differing pressure or atmospheric composition while minimizing the change of pressure in the adjoining spaces and mixing of environments. The airlock consists of a relatively small chamber with two airtight doors in series which do not open simultaneously. (Source)
In pneumatic conveying, airlocks are the passageways that bulk materials use to move between environments consisting of different pressures, gases, or both, with the goal being to minimize pressure loss and to prevent the gases from mixing. (In pneumatic conveying, airlocks are used for passage between various environments of different materials like gases, or different pressures, or both, to minimize pressure loss or prevent the gases from mixing). Airlocks are a critical component of pneumatic conveying designs. When applied to conveying, the objective is to move bulk material across a pressure differential; low to high (Pressure Conveying) or high to low (Vacuum Conveying).
How airlocks work can sound complex; however, the design is simple. When a product drops into a pocket on the top, the rotor spins and drops the product out the bottom. The rotor has VERY tight tolerances with the housing to prevent air or the product from blowing past the rotor tips. (A typical airlock will come with a rotor tip-to-housing clearance of between .004” and .007”; Once that clearance is above .0079″ it is no longer NFPA compliant). An airlock prevents air leakage across the pressure differential.
Airlocks Used in Bulk Material Handling
Drop-through airlock. What is beneficial about this airlock is that the cost is lower because it is more widely available. The downfall to the drop-through airlock is the head height.
Drop-through airlock with rails. Customers appreciate this airlock because it is operator and maintenance-friendly. The drawback to this airlock is the cost on the application’s front end.
Blow-through airlock. Compared to the drop-through airlock, the blow-through lock is preferred because of its head height and the feature that no pick-up shoe is needed. The blow-through airlock is avoided at times because of the cost and the few options it provides.
Airlocks and NFPA Compliance
Airlocks are one of the most important components of an equipment’s success. Knowing the application will determine whether the airlocks need to be NFPA Compliant.
The National Fire Protection Association is an organization that is devoted to eliminating death, injury, property, and economic loss due to fire, electrical and related hazards. The NFPA Codes & Standards states that the airlock has a minimum of 8 vanes. At least two vanes must always stay in contact with each side of the housing, and the vanes must be at least 3mm thick. This helps keep flames contained to the rotor pocket if there is an explosion. Plastic or rubber tips are not NFPA compliant, as they will not hold up to the heat of an explosion. It’s also important to know that rotor-to-housing clearance must stay below .0079″ (0.2mm).
SELECTING AN AIRLOCK
Many factors go into selecting the right airlock for a project. Knowing the environment to which the airlock will be exposed is a vital consideration.
- Temperature is a key factor, so it is important to know if the airlock will be indoor or outdoor, and the temperature of the material being conveyed.
- Pressure above or below the throughput needs makes a difference.
- Product details surrounding the airlock need to be considered to select the proper airlock; i.e. bulk density, angle of repose, flowability, abrasiveness and explosiveness are all product characteristics that can affect the airlock.
When questioning which airlock is best suited for a project, consider the airlock itself; body type, body material, rotor style/material, inboard vs. outboard bearings, and gland vs. air purge shaft seal play into what airlock will perform the best.
There are many options to rotary airlocks.
- The shallow pocket types reduce the volume per requirement and are generally used on filters, cyclones and silos.
- A staggered pocket rotor has a continuous and uniform flow to provide a more accurate rate of material flow.
- Fine Dosing rotors are beneficial when feeding or discharging fine powders, materials or granules that are contained in hoppers and silos.
- A scalloped rotor is best suited for sticky materials or food-grade applications.
- Flexible tip rotors are helpful when trying to avoid jams in the valve and are more suited for dust filters than food-grade applications.
- Consider an adjustable rotor when handling more abrasive material.
Airlocks can also offer a variety of features.
A low-cost feature with a high reward is a shaft seal air purge. A shaft seal air purge will prevent material from entering the seals and bearings.
A Zero Speed Switch is a valuable feature where it alerts the system if the rotor stops spinning, jams, or breaks.
Although they have a high price point, Slider Rails allows access for cleaning and maintenance.
The material and the coating of an airlock is critical. Nickel plating, chrome plating, and tungsten coating all help prevent premature wear of the airlock.
Factors, options, and features of airlocks can be overwhelming. Working with a skilled team of knowledgeable engineers in the field can help. With over 25 years of experience, HaF Equipment is that team Ready To Connect!
Summary:
HaF Equipment is here to assist you with which application would benefit from pneumatic conveying; and seeing is believing. So, contact a HaF account manager today to see what options we can provide you.
CONTACT US today to discuss your pneumatic conveying equipment needs.