Tunnel dryers are commonly used in brick manufacture because they
Answer: A
Cars of green ware move slowly against a counter-current of air that becomes hotter and drier as they advance. The profile matches the schedule the ware needs rather than imposing one condition on everything. Counter-current flow uses the air twice over.
Please log in or register to report this question.
Waste heat for a ceramic dryer is most often taken from
Answer: B
Correct answer: the cooling zone of the kiln
Ware leaving the firing zone must be cooled, and the air used to cool it emerges hot and clean - exactly what a dryer needs. Ducting it to the dryer recovers heat that would otherwise be vented and is the single largest energy saving available in a ceramic works, which is why kiln and dryer are usually designed and sited as one system.
Please log in or register to report this question.
State whether the following is true or false.
Drying and firing are usually treated as entirely separate operations with no connection.
Answer: C
Kiln cooling air commonly supplies the dryer, and a poorly dried piece will fail in the kiln. The two are coupled both thermally and in terms of what can go wrong. A drying fault frequently only becomes visible in the kiln.
Please log in or register to report this question.
Consider the following statements about drying.
1. Shrinkage occurs mainly during the constant rate period.
2. The critical moisture content marks the start of the falling rate period.
3. Drying can be accelerated safely from the very beginning.
Answer: D
The first two are correct. The third is the opposite of good practice, since the early stage is exactly when shrinkage stresses are generated and the schedule must be gentle then. Rate is raised progressively as the piece becomes less vulnerable.
Please log in or register to report this question.
Microwave or dielectric drying differs from conventional drying in that it
Answer: A
Correct answer: heats the wet interior preferentially rather than the outer surface
Microwave and radio-frequency energy is absorbed by water molecules throughout the volume rather than being conducted in from a hot surface, so the wettest parts of the piece heat most. That is the opposite of conventional drying, in which the surface is always the driest and hottest region.
Please log in or register to report this question.
Assertion (A): Dielectric drying can reverse the usual moisture gradient in a ceramic piece.
Reason (R): Energy is absorbed by the water itself, so the wettest regions heat most strongly.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the mechanism. Because the water itself absorbs the energy, the wet interior heats faster than the dry surface, and the vapour pressure generated inside drives moisture outward. The gradient therefore runs the useful way - wettest and hottest at the centre - which allows much faster drying without the surface-to-core stress that cracks conventionally dried ware.
Please log in or register to report this question.
State whether the following is true or false.
Dielectric drying is limited by the same moisture gradients as conventional drying.
Answer: C
It heats the water directly wherever it is, so the wettest interior dries fastest and the gradient is reversed. Its limitations are cost and the difficulty of applying it uniformly to complex shapes. Hybrid dryers combine it with conventional air drying.
Please log in or register to report this question.
A dryer removes 400 kg of water an hour. If evaporating water needs about 2.3 megajoules per kilogram, what is the minimum heat input?
Answer: D
Correct answer: about 920 megajoules an hour
\[ 400\ \mathrm{kg/h} \times 2.3\ \mathrm{MJ/kg} = 920\ \mathrm{MJ/h}. \] This is a minimum, since it accounts only for the latent heat of the water. A real dryer must also heat the ware and the air, and must vent moist air carrying sensible heat with it, so the actual input is typically half as much again or more - which is what makes waste heat recovery worth the ducting.
Please log in or register to report this question.
The moisture content at which a body is safe to fire is usually
Answer: A
Any appreciable free water turns to steam in the kiln and can burst the piece during the early heating. Complete dryness is neither achievable nor necessary, since a small residue escapes safely if the initial heating is gradual. A slow start to the firing curve exists partly for this reason.
Please log in or register to report this question.
State whether the following is true or false.
A green piece can be placed directly into a hot kiln without prior drying.
Answer: B
Free water flashing to steam would shatter it almost immediately. Drying is a separate operation precisely because the rates that a wet body tolerates are far lower than kiln heating rates. The two operations run on quite different timescales. Drying is measured in hours and kiln heating in degrees per hour.
Please log in or register to report this question.
