Key points
TL;DR
Designing reception filters for dense phase pneumatic conveying is complex due to highly variable surge flows caused by pressurised slugs of material. Undersized filters can lead to costly failures like dust emissions, downtime and even equipment damage. Unlike dilute phase systems, dense phase systems experience fluctuating pressure and airflow, making it harder to predict filter requirements.
Surge flow—caused by the sudden release of compressed gas – is the most critical design consideration. The most reliable method to size filters is full-scale testing with the actual material. If testing isn't feasible, engineers should refer to empirical data for materials with closely matched properties, while using caution. Using accurate surge flow data and filtration velocity from the manufacturer ensures safe and cost-effective filter design.
Introduction
Pneumatic conveying is widely used in process industries. One of the main components of the conveying system is a gas/solid separation unit in which reception filters are used to separate the gas and solid particles.
There are a variety of reception filters available in the market; however, each type and size of filter depends on the type of conveying mode. The design procedure of filters (for the dense phase of conveying) is poorly understood, and unfortunately, there is no dedicated standard methodology for filter sizing.
Most engineers design the reception filter based on their experience and this can either lead to oversizing (expensive for customers) or undersized (but ineffective, potentially hazardous) filters.
Undersized filters not only reduce filtration / conveying capacity and increase emissions but also failure can cause additional downtime and production losses.
There are two critical parameters for reception filter design:
Undersized filters not only reduce filtration / conveying capacity and increase emissions but also failure can cause additional downtime and production losses.
Filtration velocity: This is largely depending on the type of filter fabric and the conveyed material. Filtration velocity can vary between 0.8 to 3.5 m/min Tightly woven fabric (high-pressure drop) and dusty materials (cohesive and non-cohesive) result in lower filtration velocity whereas loosely woven fabric or clean coarse particles result in higher filtration velocity. However, other factors like; material abrasiveness and moisture content also affect filtration velocity.
Filter air flow: This is the value of maximum flow (at any time) through the filter during conveying of material. This is hard to predict for dense phase conveying as material forms dunes and slugs; therefore, both pressure and flow fluctuate depending on the material in the conveying pipe.
With knowledge of the filter airflow and filtration velocity, the filter area can be easily calculated. However, the challenge for dense phase conveying is to find/calculate filter airflow.
Difference between dilute and dense phase reception filter sizing
In the dilute phase, the material always remains suspended in the conveying gas, this results in a stable line flow and pressure, and therefore, the filter flow is close to the system input gas flow. In addition, lean phase systems are designed up to one barg conveying pressure and at a specific pickup velocity. Based on velocity and supply pressure values, the required flow can be calculated.
This flow value can be used for filter area calculations. Whereas dense phase, conveying pressure is higher than in the dilute phase, and maintaining a constant pick-up velocity is also not as critical, so the system input air flow can also be variable. In Fig. 1, at the top, is the dilute phase
(High velocity, low pressure), in the middle, it is an intermittent phase (medium velocity, medium pressure). At the bottom, it is a dense mode of conveying (Low velocity, high-pressure, full-bore slugs, and dunes).
In the dense phase, materials form slugs that require elevated pressure to move. These slugs hold the pressure in the line (upstream) and vessel. At the end of the cycle when the conveying pipe becomes empty, this causes a sudden unrestricted release of pressurised gas in the line which rushes toward the filters (as shown by the high peak of the green line at the end of the cycle in Fig. 2).
This is called surge flow. If filters are undersized, these surges of compressed gas can easily damage the filters, cause unwanted actuation of silo pressure relief devices, and in extreme cases damage silo/filter casings.
There are a few examples of poor design of filters for dense phase conveying.
The challenge in the reception filter design for the dense phase
The big challenge in the filter design is finding the surge flow value. Even approximation of surge flow is a challenge because this also depends on various factors:
- Conveying pressure: Surge flow is proportional to the conveying pressure. The higher the conveying pressure, the higher the surge flow. Some materials like sand (clean and granular) can be conveyed at any regulator pressure (low or high) but usually, to get a high conveying rate, sand gets conveyed at high pressure. This high-pressure stores more compressed air in the volume of the system and causes high surge flows when slugs of material discharge from the conveying pipeline.
- Vessel and pipeline sizes: The size of the vessel and pipeline impacts surge greatly. Larger vessel and pipeline dimensions (diameter and length), store more pressurised conveying gas. At the end of conveying all the stored gas (at high pressure) causes a sudden increase in surge flow.
- Material Properties: Material properties or types of material play a major role. Fine and fluidisable materials are easy to convey and therefore, require low conveying pressures. Slugs / dunes of these materials tend to be permeable and not hold gas at higher pressures, whereas heavy, granular, and less permeable materials are challenging materials and require high pressures. This high pressure (behind the slug) suddenly expands and causes a huge surge of air, when slugs enter into the silo/hopper. The maximum surge occurs at the end of the cycle, where compressed gas from the vessel and pipeline also adds up with conveying gas and exits through filters in a short time period.
If filters are undersized, these surges of compressed gas can easily damage the filters, cause unwanted actuation of silo pressure relief devices, and in extreme cases damage silo/filter casings.
As surge flow is dependent on many factors, it is difficult to calculate or predict, especially without empirical data or experience as a starting point.
The best approach to design the filters for the dense phase
There are mainly two ways to get the surge flow value for the material
- Based on similar materials conveyed in the past
Surge flow can be found from the test data of similar materials. However, similar materials means
- If the materials have similar or close particle size distributions, density, moisture content, and particle behaviours (on Geldart chart for example).
However, some materials having similar properties can sometimes still differ in conveying behaviour, resulting in different surge flows. Engineers need to be careful while adopting this approach.
- Second approach which is always the best, is to conduct testing on a full-scale test rig to obtain accurate surge flows. This eliminates all possibilities of wrong design of filters. Testing of materials not only helps to get accurate surge flows but also eradicates the need for high safety factors and gives an optimised reliable design.
After measuring the surge flow, filtration velocity can be obtained from the reception filter manufacturer. These parameters (filter velocity and surge flow) can be used to get the accurate design of the filters.
For the most optimum filter size, the filter supplier must consider that the surge flow may only last for a short period. When calculating the cloth area for a dense phase system using the surge flow, a higher-than-normal filtration velocity may be acceptable.
FAQs
What is a reception filter in dense phase conveying
A reception filter separates solids from gas at the end of a pneumatic conveying line helping to contain dust and protect downstream equipment.
Why is dense phase filter design more difficult than dilute phase
In dense phase systems material forms slugs and pressure fluctuates greatly causing unpredictable airflow and dangerous surge flows that are hard to model.
What is surge flow and why is it important
Surge flow is a sudden high-volume gas release caused when dense material slugs exit the pipeline. It can damage filters and silos if not properly accounted for.
How can engineers estimate surge flow
Engineers can use test data from similar materials or conduct full-scale tests. Full-scale testing is the most reliable method.
What happens if the filter is undersized
An undersized filter can rupture during surge flow cause dust leakage activate pressure relief systems or damage the filter housing.
What are key factors affecting surge flow
Conveying pressure vessel and pipeline size and material properties all impact surge flow intensity.
How is filter area calculated for dense phase systems
Filter area is determined by dividing the surge airflow by the acceptable filtration velocity provided by the filter manufacturer.
Is it safe to use high filtration velocity for surge flow
Yes if the surge duration is short a higher-than-usual filtration velocity may be acceptable for sizing.
Why is full-scale testing recommended
It provides real surge flow data reduces safety factors and ensures reliable filter performance tailored to the material.
Can two similar materials have different surge flows
Yes even with similar properties two materials can behave differently in conveying conditions due to differences in permeability or flow characteristics.
