Bag House Operation Problems
Bag House Operation Problems
Filter bags are a physical barrier and therefore the cleaning efficiency does not show pronounced dependency on gas and dust properties like in electrostatic precipitators (ESPs). Lower emissions can be maintained in a more economical way and no CO shut downs are necessary. On the other hand filter bags are more susceptible regarding upset conditions (in particular temperature peaks, condensation and abrasion), therefore process control has to focus on other parameters than for ESPs. The bags are responsible for troubles with bag filter in just some cases but are typically the component which is finally failing.
Lab analyses of failing bags are a way to determine the mechanisms (mechanical/chemical) responsible for the damage. Initial failures are often resulting in secondary problems - it is also important to know about the distribution of failing bags within the bag house. For a detailed determination of the cause for damage, and subsequent decision on corrective measures, information on condition of e.g. cages, cleaning facilities and sealing is essential.
Some typical failure patterns will be presented (operating problems, lab analysis and plant performance after corrective measures) and successful solutions to solve bag related problems discussed.
Cost Overview - Bag House Operation and Maintenance
The operation of a bag house results in expenses for fan power, pressureised air and bag material as well as cost for manpower and the loss of production during maintenance. Bag costs are typically the most obvious and therefore best evaluated cost factor as it is a big investment at the start-up and at the time of complete replacement.
In fact it is typically just number 2 after the fan power. With its effect on all other operating costs the choice of bags has to be made properly, including the evaluation of its effect on other costs. In Fig. 2 the operating costs for a bag house are displayed, split in cost for new bags, bag change and disposal and ID fan power. Major differences and therefore a wide range of each single cost factor has to be considered for different applications and bag house layout.
Fig 2: Major bag house operating/maintenance costs.
Experienced Operating Problems - Causes
The operator experiences 2 different types of typical problems:
- increased pressure drop of the filter unit
- increased dust emission
Premature bag failures and therefore high emissions are often experienced after problems with high pressure drop. In general it is often a concatenation of problems which do not always originate from the bags but bag failures are what is finally experienced.
The Pressure Drop in the Filter
The pressure drop across a bag house consists of the pressure drop of the housing (valves, ductwork,..) the pressure drop of the filter sleeves and the pressure drop of the dust cake upon the sleeves. An increased pressure drop is not always indication for problems, but are due to fundamental physical laws.
The Pressure Drop of the Dust Cake
Influences on the pressure drop of the dust cake are:
- Air to cloth ratio: The air flow in the dust cake and in the filter sleeve are both laminar and therefore the pressure drop can be considered to develop app. linear with the filter face velocity (air to cloth ratio) if cleaning is time controlled.
- Dust load: With time controlled cleaning the pressure drop of the dust cake increases app. linearly with the dust load. The development may be slightly progressive as the dust cake may be more compact when exposed to higher pressure drop.
- Dust properties: Finer dust results in a denser dust cake with finer pores and higher specific pressure drop. Some dust components may result in a dense blocking layer. One example is ammonium sulphate. The particular problem is that salts decompose at temperatures around 300°C and crystallisation takes place on the filter media (Fig. 3). This results in much stronger bonding between dust and filter material than from inert dust. Membranes appear to have a more pronounced susceptibility in this regards, leading to immediate and permanent problems with increased pressure drop.
- Operating conditions: Excessive humidity, (if condensation takes place) can cause sticky behaviour of dust. Condensation in capillary, like between particulate, takes place above the dew point, hygroscopic dust properties further increase the critical temperature. Below a certain humidity the pressure drop will also increase (just slightly, typically without reasonable problems) as binding forces between particles are reduced without contribution of monomolecular water layers on the surface.