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36 Lawrence K. Wang et al.

8.3. Gas Coolers
To maximize the collection of HAPs by electrostatic precipitators and fabric filters,
the gas stream volume can be reduced by utilizing a gas cooler. Control devices, such
as the Venturi scrubber, are less sensitive to high gas-emission-stream temperature,
because these types of device cool the gas emission stream prior to particle collection.
As a result of the temperature decrease of the gas emission stream, HAPs in vapor
form will also decrease. When cooling the gas emission stream, care must be exer-
cised to ensure that the temperature of the gas emission stream is maintained above
the dew point temperature. A good standard of practice is to maintain a 50–100ºF
cushion above the dew point to account for process fluctuations. Procedures for
calculating the dew point temperature for a gas emission stream were presented in
previous sections.
Coolers are available as dry or wet types. The dry-type cooler cools the emission gas
stream by radiating heat to the atmosphere. Spray chambers are utilized by wet-type
coolers to add humidity and cool the emission gas stream with evaporating water.
Another way to cool the emission gas stream is to add cooler dilution air. The cost, dew
point temperature, and downstream control device must be considered when selecting
gas-cooling equipment. If the downstream control device is a fabric filter, then a wet-
type cooler would not be appropriate for cooling, as this type of cooler would increase
the possibility of condensation with the fabric-filter system.
When a gas cooler is used, the gas stream parameters will have to be recalculated
using standard industrial equations. For instance, when a wet-type gas cooler is used,
then a new actual gas flow rate and moisture content for the emission gas stream will
have to be calculated.

8.4. Gas Preheaters
The temperature of the emission gas stream can be increased by using a gas pre-
heater. Increasing the temperature of the emission gas stream reduces the likelihood of
condensation. In fabric filters, condensation can plug or blind fabric pores. Additionally,
condensation can increase the corrosion of metal surfaces in a control device. To over-
come these problems, a gas preheater can be used to increase the temperature of the
emission gas stream above the dew point temperature. Three methods are commonly
utilized to raise the gas emission temperature: direct-fired afterburners, heat exchang-
ers, and stream tracking. Direct-fire afterburners preheat the gas stream by using a flame
produced from burning an auxiliary fuel. Additionally, the flame also combusts organ-
ic constituents in the emission gas stream that might otherwise blind the filter bags in a
downstream control device. An shell-and-tube arrangement is used by a heat exchanger to
preheat the gas emission stream. The stream-tracking method runs an emission gas stream
line inside of a steam line to preheat the emission stream. This method is typically only
employed when a steam line is available at the site.
When the emission streams contain HAPs, the preheating of the stream should be
raised to only 50–100ºF above the dew point temperature to minimize the vapor com-
ponents of the HAP. This allows the downstream control devices, such as a bag house
or an electrostatic precipitator, to control the HAP as effectively as possible. Emission