The symptoms you describe could be caused by a number of factors. However, based on direct conversation with the person who submitted the question, the difference in lengths between the airlocks and the convey line does not appear to be the main cause, although in some instances this could contribute to performance differences due to flow and dispersion characteristics of the material below the airlocks and ease of pickup in the convey line.

We learned the system has two bins and airlocks in series (sharing a common convey line), with the airlock above the longer chute being closest to a vacuum receiver feeding packaging line, with a third airlock underneath this receiver, and positive displacement blower as the prime mover, pulling system vacuum upstream of the receiver.

Based on this additional information, the most likely cause is insufficient conveying air flow when operating the bin or airlock directly connected (short) above the convey line:

a. If one or more of the 3 system airlock(s) is (are) leaking (worn) excessively, this can cause insufficient flow in the convey line. Most likely, the airlock with the 10 ft line may be leaking, leading to insufficient convey pick-up velocity in the other airlock discharge point, causing capacity limits.
b. Dirty filters, leaks in piping, connection, gates, or flanges could cause or contribute to consuming available convey energy.
c. The blower capacity could be less than the system is requiring, due to a slow speed or increased blowers slip (loss in efficiency) due to wear or damage, or possibly an undersized blower.
d. A combination of the above, with the cumulative effect resulting in a system capacity limit.

Ideas for resolution:
1. The ideas presented could be tested by isolating the various components (e.g., bypassing or blocking off suspected leaks) and seeing if capacity improvements occur. External leaks may be heard, felt, or tested.
2. Check all airlocks for wear and maintain as required.
3. Check conveying lines for material buildup. Clean out as required.
4. Thoroughly check system for leaks and repair as required
5. Check filters for plugging, and clean out as required.

One common cause of noise in rotary airlocks is rotating blades making contact with the stationary housing. Each airlock is designed so rotor/housing clearances can operate without contact at some maximum temperature. If this temperature is exceeded, the thermal expansion of the rotor can exceed that of the housing, at some point leading to metal-to-metal contact and noise. If the temperature can be reduced to the design point, this is the easiest answer. Another solution is to have a qualified shop machine the rotor diameter/length for this higher temperature and eliminate this noise and a potentially serious mechanical problem. Contact may also happen for process temperature conditions that are changing (e.g., start-up), where the external housing may not heat up as quickly as the rotor. The simplest solution is often to reduce the rate of heat up to avoid the rotor-to-housing contact. Secondly, some materials (e.g., lime, plastics, sugar) under certain conditions (temperature, shear stress, moisture) can buildup on the inner dimensions of the airlock housing and can make noise as the blades shear through this hardened thin material layer. If material buildup is causing the problem, a modification to your process conditions may be recommended. Contact your process engineer or airlock supplier to assist you in these reviews.

When defining bulk density, the key is to understand all materials have a range of possible densities and some materials can have a broad range. Bulk density and flow rate determine the size and speed of a rotary airlock. When sizing equipment, one must understand the range of densities and related material properties (particle size distribution and shape, stickiness, etc.). Different processes can increase or decrease the bulk density of a material through compaction or aeration. Different materials fill the airlock pockets to different volumetric capacities (i.e., fill factor) as a function of rotational speed and as a function of how the product’s bulk density actually changes as it is aerated by the airlock’s normal leakage. As an example, we had a request for sizing an airlock processing a specific rate (lb/hr) of wood waste. Wood byproduct density may vary from 10 to 30 lb/ft3, depending on particle size and shape, moisture content, and the level of compaction vs. aeration. If a relatively high value for the bulk density is used (e.g., 20-25 lb/ft3), a 10 in. diameter airlock would be selected, but conversely, using 10-15 lb/ft3 would result in a 12 in. diameter airlock size selection.

We worked with one customer processing silicates that had used rotary airlocks for a long time to do this. They were changing out airlocks/components every couple of months due to blade and housing wear, even with wear resistant components. We displaced the rotary airlock with a double flap-gate with wear resistant components. This lasted about twice as long, but still meant unwanted downtime. A third solution was then tried: a solids screw pump. Although more expensive than the other two technologies, the customer was able to get over one year between replacement part /maintenance intervals. The technology choice is based mainly on the abrasiveness of the materials being processed, the process condition, and the cost of downtime. We have other customers in similar applications where rotary airlocks and double flapgates last a long time on relatively abrasive materials.