A venturi scrubber achieves the required outlet concentration but the running cost is found to be dominated by fan power.
Is this surprising?
Answer: A
The contacting power principle says that efficiency follows the power expended in gas-liquid contact, whatever the geometry. A venturi's excellence on fine particulate and its fan power bill are two descriptions of the same thing.
Please log in or register to report this question.
A cyclone ahead of a baghouse is found to be collecting only the coarsest material.
How should this be judged?
Answer: B
Correct answer: As normal: it is there to protect the filter
A cyclone placed ahead of a baghouse is a pre-cleaner, and collecting only the coarse fraction is exactly what it is for: removing the heavy and abrasive material that would wear the bags, and reducing the dust load the filter must cake and clean. Expecting it to collect fine dust would be to misunderstand its role, and its cut diameter of five to ten micrometres makes clear it cannot.
Please log in or register to report this question.
A precipitator must be improved from 98.2 to 99.5 per cent efficiency at unchanged flow.
What does the Deutsch relationship imply about the cost?
Answer: C
The exponent must move from 4 to about 5.3, which is a proportional increase in area of roughly thirty-three per cent. Each further increment of efficiency costs disproportionately more, which is the characteristic economics of the device.
Please log in or register to report this question.
The particle size at which a cyclone collects half of what enters is its _____ diameter.
Answer: D
Correct answer: cut
The cut diameter, \( d_{50} \), is the particle size collected with fifty per cent efficiency, and it is the single figure used to characterise a cyclone’s performance. Particles well above it are collected almost completely, those well below almost not at all. A conventional cyclone has a cut diameter of five to ten micrometres, a high efficiency one two to five.
Please log in or register to report this question.
The gas flow divided by the total fabric area in a baghouse is the air to _____ ratio.
Answer: A
Correct answer: cloth
The air to cloth ratio is the volumetric gas flow divided by the total fabric area, and despite its name it has the units of velocity. It is the primary sizing parameter of a baghouse: fixing it and the gas flow fixes the fabric area, and hence the number of bags, the size of the casing and the capital cost.
Please log in or register to report this question.
The equation relating precipitator efficiency to plate area, gas flow and migration velocity is the _____ equation.
Answer: B
Correct answer: Deutsch-Anderson
The Deutsch-Anderson equation is
\[ \eta = 1 - e^{-wA/Q} \]with \( w \) the migration velocity, \( A \) the plate area and \( Q \) the gas flow. Its exponential form has an important consequence: each equal increment of plate area removes the same fraction of what reaches it, so successive fields give diminishing absolute returns and the last few tenths of a per cent are very expensive.
Please log in or register to report this question.
A dust layer that holds its charge and discharges back through itself causes _____ corona.
Answer: C
It arises with dust of very high resistivity and collapses the effective field, ruining collection. Conditioning the gas with sulphur trioxide restores the layer's conductivity and with it the precipitator's performance.
Please log in or register to report this question.
The principle that wet scrubber efficiency depends chiefly on the power expended in gas-liquid contact is the _____ power principle.
Answer: D
It holds across scrubbers of quite different geometry, which makes it a remarkably general result. Its practical meaning is that fine particle collection in a wet device cannot be had without paying for it in pressure drop.
Please log in or register to report this question.
Typical performance of five particulate control devices.
| Device | Efficiency on PM10 | Efficiency on PM2.5 | Pressure drop (kPa) |
|---|---|---|---|
| Settling chamber | 20 | under 5 | 0.1 |
| Conventional cyclone | 70 | 25 | 0.8 |
| High efficiency cyclone | 90 | 50 | 2.0 |
| Fabric filter | 99.9 | 99.5 | 1.5 |
| Venturi scrubber | 99.5 | 98 | 9.0 |
Which device offers the best fine particle collection per unit of pressure drop?
Answer: A
The filter achieves the highest fine collection of any of the five while costing a sixth of the venturi's pressure drop. Where the gas condition allows a baghouse at all, that combination is very hard to beat on running cost.
Please log in or register to report this question.
Deutsch-Anderson efficiency for a precipitator at four plate areas, with a migration velocity of 0.10 m/s and a flow of 100 m3/s.
| Plate area (m2) | Exponent Aw/Q | Efficiency (per cent) |
|---|---|---|
| 2000 | 2.0 | 86.5 |
| 4000 | 4.0 | 98.2 |
| 6000 | 6.0 | 99.75 |
| 8000 | 8.0 | 99.97 |
What does the table show about the cost of high efficiency?
Answer: B
The first two thousand square metres buy 86.5 points and the last two thousand buy about a fifth of one. Thinking in penetration rather than efficiency makes the pattern obvious, since penetration simply falls by a constant factor each time.
Please log in or register to report this question.
