PARTICULATE REMOVAL 839
design, it does give a clue as to the influence of operating
conditions of W. The term E o E p is roughly proportional to
the power input to the precipitator, and it is clear that this
should be as high as possible. From the dependence of W
on d p it is clear that precipitators have a higher collection
efficiency for large particles than small ones. This is seldom
taken into account in current design procedures in which an
average particle size is included in the empirical W for an
application.
Dust resistivity can have a very pronounced, although
not quantitatively predictable, effect on precipitator perfor-
mance. If the in situ resistivity of a dust layer collected on
the plate electrode is less than about 10^7 -cm, the electrical
force holding the dust particles to the electrode will be low,
and significant amounts of dust may be reentrained into the
gas stream. If the resistivity is above 2 10 10 -cm, exces-
sive sparking and back corona will occur hurting electrical
performance and efficiency. Resistivity is a function not only
of the dust but also of temperature and gas composition. At
low temperatures surface conduction is predominant due to
a thin layer of adsorbed moisture, and this decreases with
increasing temperature. Volume conduction is important at
high temperatures and decreases with temperature. A maxi-
mum resistivity usually occurs at about 400F due to these
opposing tendencies as is shown in Figure 9.
Gas conditioning by addition of moisture or other sub-
stances is sometimes practiced in order to improve the dust
resistivity and enhance electrical performance of the elec-
trodes. In certain instances addition of 10–20 ppm of ammonia
has dramatically improved precipitator performance. Sulfur
trioxide has also been used successfully, but it is expensive
and creates its own pollution and safety problems. The effect
of resistivity change, caused by conditioning, on precipitator
efficiency is shown in Figure 10. In some gas streams condi-
tioning agents may be naturally present which enhance pre-
cipitator operation. For instance, as indicated in Figure 11,
power plants burning high sulfur fuel have improved perfor-
mance due to the conditioning properties of sulfur combus-
tion products. The present trend towards mandating use oflow sulfur fuels in many urban areas will have an adverse
effect on particulate removal.
Equation (4) shows the dependence of efficiency on gas
flow rate provided flow is uniform. If gas velocities are differ-
ent throughout the precipitator due to maldistribution of flow,
the equation will be applicable only to local efficiency and the
overall precipitator efficiency will be considerably lower. For
instance a precipitator which could operate at 98% efficiency
with even gas distribution would operate at only 96.5% effi-
ciency if the same total gas flow were maldistributed so that
half of the precipitator received 30% more than average and
the other half received 30% less than average. Sharp turns
and rapid expansion and contraction in dust work often make
it difficult to achieve good gas distribution. Very often pre-
liminary flow distribution tests are made on^1 – 6 – 161 scale clear
plastic models using smoke as a tracer. Such model tests are
helpful in calling attention to duct work designs which willTABLE 1
Representative precipitation rates for various applications^27ApplicationPrecipitation rate w ft/secAverage RangeUtility fly ash 0.43 0.13–0.67
Pulp and paper 0.25 0.21–0.31
Sulfuric acid 0.24 0.20–0.28
Cement (wet) 0.35 0.30–0.40
Smelter 0.06 —
Open hearth 0.16 —
Cupola 0.10 —
Blast furnace 0.36 0.20–0.4610101010(^100)
100 200 300 400 500
Temperature, °F
Resistivity, ohm-cm
15
14
13
12
11
Bone dry
1% H 2 O
3% H 2 O
Figure 9 Effect of temperature and
humidity on particle resistivity.^29
99.9
99.8
99.5
99
98
95
90
80
50
10 10 10
Resistivity, ohm-cm
Efficiency, %
NH 3
conditioning
Addition of
fresh catalyst
No conditioning
(NH 4 ) 2 SO 4
conditioning
9 10 11 1012
Figure 10 Effect of gas conditioning on
efficiency of electrostatic precipitators.^26
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