NCEES PE Agricultural and Biological Engineering Engineering Properties of Agricultural Produce: practice questions

1 · Volume and density: bulk and true · medium

A cylindrical bin 6.0 m in diameter is filled with grain to a level depth of 8.0 m. Using a bulk density of 760 kg/m³ and ignoring compaction and the surface cone, how much grain does it hold?

1. 171.9 t
2. 687.6 t
3. 43.0 t
4. 226.2 t

2 · Porosity · medium

When a bin is filled from a central spout without a spreader, an aeration fan later fails to cool the centre of the bin. The most likely cause is that

1. whole kernels collect in the centre while fines roll to the walls, so the centre has too little resistance
2. the centre of the bin has a higher porosity, so the air bypasses it and travels up along the walls instead
3. kernel density is lower at the centre, so the grain there holds more heat and cools more slowly
4. fines and broken kernels collect in the central core, lowering porosity and raising airflow resistance there

3 · Frictional properties: angle of repose and coefficient of friction · medium

A hopper-bottom grain bin must empty completely by gravity. How should the designer set the slope of the hopper floor?

1. Steeper than the emptying angle of repose of the grain at its highest expected moisture
2. Equal to the filling angle of repose measured on dry grain poured onto a flat floor
3. Flatter than the emptying angle of repose, so that grain slows down and is not damaged
4. At 45 degrees for every crop, since the angle of repose does not affect hopper flow

4 · Frictional properties: angle of repose and coefficient of friction · hard

Using Janssen's equation, the lateral pressure on the wall of a deep bin approaches L_max = ρ_b g R / μ', where R is the hydraulic radius (area/perimeter). For a 6.0 m diameter bin storing grain of bulk density 780 kg/m³ with a wall friction coefficient μ' = 0.40, L_max is

1. 57.4 kPa
2. 4.6 kPa
3. 114.8 kPa
4. 28.7 kPa

5 · Frictional properties: angle of repose and coefficient of friction · medium

Why do bin design standards apply an overpressure factor to the static Janssen lateral pressure?

1. Because Janssen's equation overstates wall pressure, so the factor reduces it to a safe level
2. Because grain bulk density falls during filling, which the static equation does not include
3. Because flow of grain during emptying produces wall pressures above the static values
4. Because the factor converts lateral pressure into vertical floor pressure for the foundation

6 · Thermal properties: specific heat, conductivity and diffusivity · medium

Two spherical fruits of the same variety are hydrocooled under identical conditions with internal resistance controlling. Fruit B has twice the diameter of fruit A. The time for B's centre to reach the same fractional temperature change is

1. twice that of A, since cooling time is proportional to diameter
2. the same as A, since both have the same thermal diffusivity
3. eight times that of A, since cooling time scales with volume
4. four times that of A, since the Fourier number αt/L² must be equal

7 · Water activity · hard

A dryer receives 2000 kg of maize at 24% moisture (wet basis) and dries it to 14% wet basis. The mass of water removed is

1. 232.6 kg
2. 200.0 kg
3. 263.2 kg
4. 1767.4 kg

8 · Water activity · medium

At a fixed relative humidity of the drying air, raising the air temperature changes the equilibrium moisture content of grain in which direction?

1. It raises the EMC, so warmer air at the same RH leaves the grain wetter at equilibrium
2. It leaves the EMC unchanged, since EMC depends on relative humidity alone
3. It lowers the EMC, so warmer air at the same RH dries grain to a lower final moisture
4. It raises the EMC above 100% dry basis whenever the air exceeds 60 °C