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pressure differential of 1 atm. The result for a given fabric–solid combination would be
a“standard cake formation time expressed in minutes.” In gas filtration tests, the most
common method for expressing new fabric resistance is to measure the gas volumetric
flow rate at a 0.5-in. (1.27 cm) H O pressure drop.
2
3. APPLICATION
3.1. General
The use of fabrics as a porous filter medium in both liquid and gas cleaning systems
has been stated, and the separation of solids from liquids will be discussed in detail in
other chapters of this handbook series. The major emphasis of this section is on gas
cleaning, and, in most applications, the gas considered is air.

3.2. Gas Cleaning
Filters used to clean gases are categorized in this section in five different ways
according to the energy required, the fabric employed, the type of cycle, the service, and
the application. The first category includes either high-energy or low-energy filters,
depending on whether the filters are operated at high or low filter pressure drops. For
any given application involving filters, a high-energy system is usually more efficient, but,
ultimately, this depends on the size, size distribution, and type of material being filtered.
Energy and efficiency are not always directly related and will be discussed below.
High-energy systems generally consist of pulse-jet devices, whereas low-energy clean-
ing systems utilize shaking and reverse flow. Note that this classification also describes the
cleaning method used to remove dust from the bags. In the pulse-jet systems, blasts of air are
blown through jet nozzles in pulses to free the dust from the fabric, as shown in Fig. 1. Note
that the cleaning jet is introduced into the Venturi nozzle to expand and clean the bag.
The low-energy systems are split approx 50–50 between continuous and intermittent-
type collectors. Shaking, as the word states, simply implies mechanically flexing the
bag to clean it. Reverse-flow applications consist of introducing air into sections of
the filter system in the opposite direction from normal gas flow to blow the dust off the
bags. There is a third category, in which no cleaning energy is utilized. This applies to
units designed for situations in which the media are disposable.
Fabric filters can be divided generally into two basic types, depending upon the fabric:
felt (unwoven) and woven. Felt media are normally used in high-energy cleaning systems;
woven media are used in low-energy devices. Felt fabrics are tighter in construction
(i.e., less porous), and for this reason, they can be considered to be more of a true fil-
ter medium and should be kept as clean as possible to perform satisfactorily as a filter.
In contrast, the woven fabric is, in general, only a site upon which the true filtering
occurs as the dust layer builds up, through which the actual filtering takes place. In addi-
tion, a third type of fabric filter is nonwoven disposable configuration material, which
is used as a vacuum cleaner with disposable bags.
Filter systems can also be categorized as either continuous or intermittent collectors.
In a continuous collector, the cleaning is accomplished by sectionalizing the filter so
that, while one part is being cleaned, the rest of the filter is still in operation. Under
these conditions, the gas flow through the device and the overall pressure drop across
the device are essentially constant with time. In contrast, there must be an interruption