Bag Filters

Comparison of Pulse Jet And Reverse Air Filters For High Temperature Applications

Choice of Filter

For both temperature applications utilising suitable fabric materials up to 250oC, both pulse jet and reverse air systems are suitable.

For temperatures above 250oC rigid ceramic or metallic elements are used and these can only be served by a pulse jet filter suitably modified to suit element type application.

Constructional Comparisons

Both the pulse jet and reverse air filters are available in modular or custom built construction. Externally both pulse jet and reverse air filters are similar. The reverse air filter requires two access levels for maintenance, while pulse jets filters require a single access point. The reverse air is inherently of walk-in plenum design, whilst the pulse jet may be of walk-in plenum or hatch cover design. With hatch cover design, for large filters it is common to install a penthouse to provide weather protection. The walk-in plenum arrangement is recommended for high temperature applications to minimise casing cold spots.

For the pulse jet filter a compressed air system is required for bag cleaning, for the reverse air filter, a small fan is required. A separate duct conveys this air to each module.

Pulse jet filter bags are suspended from a tube sheet in the upper part of the casing requiring an inserted cage to maintain bag shape. In a reverse air filter, the tube sheet is at hopper level and the bags are suspended from a frame at the top of the filter. The bag has several circumferential rings encased in fabric to minimise rubbing on the fabric of the bag.

In operation the dust is deposited on the outside of the fabric in a pulse jet filter and on the inside of the fabric in a reverse air filter.

Cleaning Method Comparison

The life of a bag is determined by the number of times it is cleaned and the aggressiveness of the cleaning action. The cleaning frequency is directly related to the filtration velocity (old term was air to cloth ratio), the higher the filtration velocity the quicker the dust is deposited on the fabric and the more often the bag has to clean to maintain an acceptable pressure drop.

In a pulse jet system the pulse air (or cleaning air) is required to overcome the forward velocity of the process air. The typical reverse pulse jet pressure is 3 to 4.5 kPa overcoming the forward pressure of up to 1.5 kPa.

The reverse jet entrains additional air from the clean air plenum preventing the flow of process air up the bag as it cleans down. The reverse pressure generated overcomes the forward pressure and the bag balloons outwards from the cage, thus shedding the agglomerated dust from the outer surface of the fabric. Upon completion of the pulse, the bag forms back into the cage under the action of the forward process air flow. Continual working of the bag on the cage from the above described cleaning process causes wear points in the fabric.

In a reverse air system, each module is isolated in sequence during the cleaning cycle, thus allowing two actions:

  • The forward process air flow is stopped, the effort required to remove the dust is reduced as it does not have to overcome the forward pressure.
     

  • The bag “relaxes” and allows heavier dust particles to disengage.

 
For cleaning, a reverse flow of air is passed through the bag. This action tends to collapse the bag structure enabling all remaining particulate to fall into the hopper. The action is gentle and does not exhibit the snap action inherent in the pulse jet filter. As a result, reverse air filter bags tend to last longer. Bleeding of the bag during the cleaning cycle is minimised with the reverse air filter.

Advantages / Disadvantages:

  • The footprint of a pulse jet system is less than that of the reverse air system.
     

  • Initial (and subsequent replacement) bag costs less of an expense with the reverse air filter.
     

  • For high temperature applications, expensive fabrics are required in a pulse jet filter. The more economical fibre glass bags are used in a reverse air filter.
     

  • Initial (and subsequent replacement) cage costs do not apply to a reverse air filter.
     

  • The reverse air flow reduces stress on the fabric and extends the effective life of the bags.
     

  • Operating costs are similar for both systems. The reverse air fan motor, as it only services one module at a time, may be lower kW than the compressed air requirement motor which has to service all modules concurrently.
     

  • Bag replacement time is comparable for both systems.
     

  • The capital outlay of a reverse air filter is usually higher due to a larger plant size/layout, however in very large installations reverse air can be more economical.
     

  • Both systems are suitable for modular construction allowing off line maintenance.
     

  • Cleaned hot process gas is utilised as the reverse air flow. Hence, the reverse air filter is ideal for conditions where the treated gas is near dew point. 

 

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