Storage Engineering Questions and Answers

Answer: C

Correct answer: 4

Respiration doubles for each ten degree rise, so cooling by twenty degrees is two halvings:

\[ \frac{r_{30}}{r_{10}} = 2^{(30-10)/10} = 2^2 = 4 \]

The distractor \( 2 \) accounts for only one ten-degree step, and \( 8 \) would correspond to a thirty degree drop. This exponential dependence is the whole justification for precooling and the cold chain: a fourfold reduction in metabolic rate translates directly into a several-fold extension of storage life.

Answer: D

Correct answer: 960 g

Multiply the required dosage by the volume to be treated:

\[ m = 2\ \mathrm{g/m^3} \times 480\ \mathrm{m^3} = 960\ \mathrm{g} \]

The volume is the whole enclosed space, not just the grain, since the gas must fill it. And the dosage is only half the requirement: the structure must be sealed well enough to hold that concentration for the full exposure period, typically several days for phosphine, or the surviving insects will include the resistant ones.

Answer: A

Correct answer: 240 days

Doubling the storage life gives

\[ 120 \times 2 = 240\ \mathrm{days} \]

The underlying reason is that the processes limiting storage life - respiration, mould growth and insect development - all slow markedly with temperature, so a ten degree reduction typically doubles the safe period. The same relation compounds: cooling to 10 degrees would extend it further again, which is why aeration to cool a bulk is among the cheapest interventions available.

Answer: B

The four loads sum to \( 42.8 \) kW, and adding fifteen per cent gives \( 49.2 \) kW as the duty to specify. Omitting the respiration and infiltration terms is much the commonest way such plants end up undersized in service.

Answer: C

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. Aeration does not dry grain; it cools it and evens out the bulk. Grain must already be at a safe moisture before it is stored, because aeration cannot correct grain put away too wet.

Statement II is correct and is precisely why. At \( 0.1 \) cubic metres per minute per tonne, the air flow is one to two orders of magnitude below what a dryer uses - enough to move a cooling front slowly through the bulk, but nowhere near enough to carry away significant moisture.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. Storage insects breed rapidly in the 25 to 33 degree range typical of an unaerated tropical store, and their development slows sharply below about 15 degrees.

Cooling the bulk by aeration therefore controls infestation directly, without chemicals and without residues, and it simultaneously slows mould growth and respiration. Where night air is cool enough to be useful, this is among the cheapest and most durable protections available.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second describes the mechanism. Temperature differences within a stored bulk drive slow convection currents in the interstitial air. In a warm store cooling from outside, air warmed near the centre rises, carrying moisture with it, and on reaching the cold underside of the roof it is cooled below its dew point and deposits that moisture.

The result is a wet layer at the grain surface where moulds and insects establish, even though the bulk average moisture is unchanged. Aeration counters it by removing the temperature gradients that drive the circulation.

Answer: B

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. Hermetic storage does not work by keeping the grain dry - the grain must still be dry enough to store, but that is not the mechanism.

Statement II is correct and gives the actual principle: in a sealed structure the respiration of the grain, insects and moulds consumes the oxygen and raises the carbon dioxide, and the resulting atmosphere kills the insects and suppresses the moulds. The infestation destroys its own environment. The requirement is therefore an airtight seal, and any leak that admits fresh air defeats it.

Answer: C

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. Respiration heat cannot be neglected - 40 tonnes of produce respiring at \( 45 \) mW/kg generates \( 1.8 \) kW continuously, comparable with the transmission load of a well-insulated room and, unlike the product load, present for the whole storage period.

Statement II is correct: respiration roughly doubles for every ten degree rise. That relation is exactly why the load must be included and why it should be evaluated at the warmest temperature the produce will occupy, which is during pull-down rather than at the final storage temperature.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. A thin steel wall has little thermal mass and follows the ambient temperature closely, so the grain adjacent to it swings with the day and the season while the core of the bulk lags far behind.

Those steep and changing gradients drive the convection currents that carry moisture from warm regions to cold surfaces, where it condenses. A concrete silo's thermal mass damps the swings and so reduces the driving force. Metal silos are cheap and quick to erect, which is why they are common despite needing more careful aeration.