UPHESC Assistant Professor Agricultural Engineering PYQs

Concept: The stoichiometric A/F for petrol is about 15 : 1, and maximum power comes with a slightly rich mixture of about 12–12.5 : 1.

1. A rich mixture makes sure all the air is used and burns fastest.
2. Maximum economy needs a lean mixture of about 16–17 : 1. 9 : 1 is too rich (used at starting or idling).

Short trick: Power → Rich 12 : 1; Economy → Lean 16 : 1.

Concept: Dry matter stays constant during drying.

1. Dry matter = 2000 × (1 − 0.22) = 1560 kg; initial water = 440 kg.
2. Final water at 13% d.b. = 0.13 × 1560 = 202.8 kg.
3. Water evaporated = 440 − 202.8 = 237.2 kg.

Short trick: Use dry matter as the anchor; d.b. → multiply by dry matter.

Concept: Drying behaviour is described by drying curves and by thin-layer models.

1. Drying rate vs time and drying rate vs moisture content show the constant-rate and falling-rate periods.
2. Page's model $MR=e^{-k{t}^n}$, fitted to the data, gives the drying constants. All three are used together.

Short trick: Curves + model = full dehydration characteristics.

Concept: Pasteurisation is a mild heat treatment that destroys pathogens and most vegetative cells, but not all microbes or spores.

1. Sterilisation kills all microorganisms, including spores.
2. Blanching mainly inactivates enzymes in vegetables.

Short trick: Pasteurise = partial kill; Sterilise = total kill.

Concept: Oilseeds such as soybean are stored safely at about 10–12% (w.b.) or less, i.e. within 8–12%.

1. Cereals are usually stored at about 12–14%.
2. Above 13% soybean is prone to mould and heating; 3–6% is far drier than needed.

Short trick: Oilseeds ≈ 10%, cereals ≈ 12–14%.

Concept: Rice husk has a high ash content (about 18–22%) and a calorific value of about 13–15 MJ/kg ≈ 3000–3500 kcal/kg.

1. Wood is about 4000–4500 kcal/kg and coal about 5000–6000 kcal/kg.
2. Diesel is about 10,000 kcal/kg.

Short trick: Husk ≈ 3000 kcal/kg (lots of silica ash).

Concept: Zenith angle θ_z is the angle between the sun's rays and the vertical; θ_z = 90° − solar altitude α.

1. At sunrise or sunset the sun is on the horizon, so α = 0°.
2. Therefore θ_z = 90°.

Short trick: Horizon sun → altitude 0 → zenith 90°.

Concept: In a co-current (downdraft) gasifier the gas leaves through the hot oxidation/reduction zone, so the tars crack.

1. The gas is nearly tar-free and suits IC engines.
2. The counter-current (updraft) gasifier gives tar-rich gas; cross-draft and fluidised-bed gasifiers give moderate tar.

Short trick: Downdraft = tar goes down through fire → cracked.

Concept: Most PV cells are made of semiconductor silicon (mono- or poly-crystalline, or amorphous).

1. A p–n junction of doped silicon converts light directly into DC electricity.
2. Silver is used only for the contact grid lines.

Short trick: PV = Silicon p–n junction.

Concept: Biomass heating boilers are grouped by thermal output.

1. Small (domestic) boilers – up to about 100 kW.
2. Medium-power boilers – about 100–350 kW (farms, schools, small dryers).
3. Large boilers – above this range (MW scale).

Short trick: Small < 100 kW; medium ≈ 100–350 kW.

Concept: Air-blown gasification gives producer gas: CO ≈ 18–22%, H₂ ≈ 15–20%, N₂ ≈ 45–50%, plus some CH₄ and CO₂.

1. CO and H₂ are the combustible parts; N₂ comes from the air and dilutes the gas.
2. Calorific value is low, about 4.5–5 MJ/m³.

Short trick: Producer gas = CO + H₂ + lots of N₂.

Concept: The sun (≈ 5800 K black body) radiates almost all its energy in the short-wave band of about 0.2–4.0 µm.

1. About 7% is UV (< 0.38 µm), 47% visible and 46% near-infrared.
2. Earth's long-wave emission lies at about 4–100 µm.

Short trick: Solar = short-wave 0.2–4 µm; terrestrial = long-wave > 4 µm.

Concept: Daily global radiation on a horizontal surface in India is about 14.5 MJ/m²/day in the winter months and rises to about 23 MJ/m²/day in summer (April–May).

1. Clear skies and a high sun in summer give the maximum.
2. Monsoon cloud lowers it in July–August.

Short trick: Winter ≈ 14.5, summer ≈ 23 MJ/m²/day.

Concept: In pressure-nozzle atomisation, concentrated milk is pumped through a fine nozzle at about 140–210 kg/cm² (2000–3000 psi).

1. This breaks it into fine droplets for rapid drying in hot air.
2. Centrifugal (spinning disc) atomisers are the other type and need no high pressure.

Short trick: Pressure nozzle ≈ 2000–3000 psi ≈ 140–210 kg/cm².

Concept: Pasteurisation (63 °C for 30 min, or HTST 72 °C for 15 s) destroys all pathogenic organisms and most spoilage organisms.

1. Fat and moisture are not removed.
2. It is designed around destroying Coxiella burnetii, the most heat-resistant pathogen in milk.

Short trick: Pasteurise → pathogens die, milk stays.

Concept: 1 TR is the heat removed to freeze 1 ton of water at 0 °C in 24 h, = 3.5 kW ≈ 211 kJ/min ≈ 3024 kcal/h.

1. 3024 kcal/h ÷ 60 ≈ 50 kcal/min.

Short trick: 1 TR ≈ 3.5 kW ≈ 50 kcal/min ≈ 12,000 BTU/h.

Concept: Homogenisation breaks fat globules from about 3–4 µm down to below 1 µm.

1. The number of globules and their total surface area rise many times.
2. More surfaces scatter more light, so the milk looks whiter (greater whitening power).

Short trick: Smaller globules → more fat surface → more light scattering.

Concept: Rennet (chymosin/rennin from the calf stomach) coagulates casein to form the curd in cheese making.

1. Pepsin and ficin can act as substitutes but give bitterness; trypsin is not used.
2. Microbial and fermentation-produced chymosin are modern alternatives.

Short trick: Cheese curd ← Rennet (chymosin).

Concept: Centrifugal force on a particle is $\frac{m{v}^2}{r}$, so separation improves with higher inlet velocity and smaller radius.

1. Increasing the inlet air velocity raises the centrifugal force → finer particles are caught (within limits of re-entrainment).
2. A larger cyclone diameter or a larger exit duct lowers efficiency.

Short trick: Fast and narrow spin = better separation.

Concept: The flow diversion valve (FDV), worked by the temperature recorder-controller, sends milk forward only if it has reached 72 °C.

1. Under-heated milk is diverted back to the balance tank for reheating.
2. The heat exchanger only heats or cools the milk; it does not route the flow.

Short trick: FDV = safety gate of HTST.