Page 55 - Air Pollution Control Engineering

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Air Quality and Pollution Control 35

h = h = Btu/scf
e d
Q = Q = scfm
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8. GAS STREAM CONDITIONING
8.1. General

Gas conditioning equipment is installed upstream of the control device, to ensure
that the control device operates efficiently and economically. This equipment is used
to temper or treat the gas stream prior to its entering the control device. A mechanical
dust collector, wet or dry gas cooler, and gas preheater can be used as precondition-
ing equipment. Typically, when the control device is a fabric filter or electrostatic pre-
cipitator and the gas stream contains significant amounts of large particles, a mechanical
dust collector is installed upstream of the control device to remove large particles
(20). To protect the fabric used in the fabric-filter control device from damaging high
gas stream temperatures, a gas cooling device could be installed upstream of this
device to lower the gas stream temperature to temperatures within the operating tem-
perature of the fabric. The gas cooler device can also be used to reduce the volume of
gas stream or maximize the collection of HAPs by electrostatic precipitators or fab-
ric filters. The elimination of moisture condensation problems can be accomplished
by using a gas preheater, which will increase the temperature of the gas stream entering
the control device. This manual presents discussions on gas conditioning equipment,
but not equipment designs. The latter are readily available from vendors and common
literature sources.
8.2. Mechanical Collectors
The removal of heavy dust particles from a gas stream can be accomplished with a
mechanical dust collector, such as a cyclone. This device utilizes centrifugal force to sep-
arate the dust particles from the gas stream. The efficiency of the cyclone is dependent on
the gas velocity entering the cyclone and the diameter of the inlet of the cyclone. In the-
ory, as the inlet velocity increases or as the diameter of the inlet decreases, the greater the
collection efficiency is and the greater the pressure drop through the cyclone is. Particles
above 20–30 µm in size can be effectively removed for the gas stream by the cyclone. This
removal will reduce the loading and wear on downstream control equipment.
In evaluating the use of a cyclone as a control for an emission stream, one must
first determine the size distribution for the particulates in the gas stream. If the gas
stream has significant amounts of particles above 20–30 µm, the installation will
require that filters or electrostatic precipitators be installed upstream of the cyclone
to reduce the loading to the cyclone. When utilizing a wetted Venturi scrubber as a
control for an emission stream, a preconditioning device is not generally required
even when the gas stream contains large particles (20–30 µm in size). When using a
nonwetted Venturi scrubber as the control device for an emission stream, the emis-
sion stream must be free of particles that could clog the nozzles. Therefore, the emission
stream either has to be free of particulates or a pretreatment device such as mechani-
cal dust equipment must used to treat the emission prior to the nonwetted Venturi
scrubber (20,21).