Dust resistivity affects an electrostatic precipitator because
Answer: C
There is a window of about ten to the seventh to ten to the tenth ohm centimetres within which collection works well. Outside it the dust either holds its charge and discharges back through the layer, or loses it at once and is swept away again.
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Flue gas conditioning with sulphur trioxide is used to
Answer: D
A thin conductive film on the particle surface brings the resistivity back into the workable window and restores collection. It became common when low sulphur coals were adopted and precipitator performance fell away unexpectedly.
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A venturi scrubber collects particles chiefly by
Answer: A
The high relative velocity between gas and injected liquid shatters the liquid into fine droplets and drives particles into them. It is the most effective wet device for fine particulate and correspondingly the most demanding in pressure drop.
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The contacting power principle for wet scrubbers states that
Answer: B
Scrubbers of quite different geometry achieve similar efficiency when they expend similar power, which is a remarkably general result. It means there is no way to obtain fine particle collection in a wet device without paying for it in pressure drop.
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The principal disadvantage of a wet scrubber compared with a fabric filter is
Answer: C
The captured dust leaves as a slurry that must be thickened, dewatered and disposed of, and the water itself often needs treatment. Against that, a scrubber handles sticky, hot and flammable streams that would defeat a baghouse entirely.
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Fabric filters are unsuitable for gas streams that are
Answer: D
Heat destroys the fabric and condensation blinds it with a paste that cleaning cannot remove. Bag material selection and keeping the gas above its dew point are therefore the two crucial operating disciplines for a baghouse.
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A multiclone or multicyclone consists of
Answer: A
Small diameter tubes give a smaller cut diameter than one large body would, and putting many in parallel restores the throughput. Uneven distribution between the tubes is the usual reason a multiclone underperforms its design figure.
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Collection efficiency of all inertial devices falls off for fine particles because
Answer: B
Inertia is what separates a particle from the flow, and it scales with mass, so the finest particles simply follow the gas wherever it goes. It is why cyclones and settling chambers are pre-cleaners and why filters and precipitators finish the job.
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Pressure drop matters in selecting a control device because
Answer: C
Correct answer: It determines the fan power required
The fan must move the whole gas stream against the device’s resistance, and
\[ P = \frac{Q\Delta p}{\eta} \]so pressure drop translates directly into shaft power and therefore into the running cost, which over a plant’s life usually exceeds the capital cost. This is why a venturi scrubber at 10 kPa is expensive to run and a precipitator at a few hundred pascals is cheap.
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A precipitator is often built with several fields in series because
Answer: D
The Deutsch relationship means each field removes a similar fraction of what reaches it, so three or four in series achieve what one very long field could. Sectionalising also lets each field run at the voltage its own dust loading allows.
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