UPHESC Assistant Professor Agricultural Engineering PYQs

Concept: FAO design formula for micro-catchment water harvesting: $\frac{C}{CA}=\frac{CWR-DR}{DR\times RC\times EF}$, where C = catchment area and CA = cultivated area.

1. CWR − DR = 575 − 380 = 195 mm.
2. DR × RC × EF = 380 × 0.13 × 0.5 = 24.7 mm.
3. C : CA = 195 ÷ 24.7 ≈ 7.9, i.e. about 8 : 1 — the catchment must be about 8 times the cultivated area.
4. The question asks for catchment : cultivated, but the options give 1 : 5, 1 : 8, 1 : 7, 1 : 3 — each makes the catchment smaller than the cultivated area. '1 : 8' is the reverse of the correct 8 : 1, so as printed no option is correct.

Short trick: Rainfall short of crop need → catchment must be LARGER than the field, so C : CA is always more than 1.

Concept: Area of a trapezoidal section is $A=(b+zd)d$.

1. b = 1 m, z = 1, d = 1 m.
2. A = (1 + 1×1) × 1 = 2 m².

Short trick: Top width = b + 2zd = 3 m; mean width 2 m × depth 1 m = 2 m².

Concept: Surface drainage layouts are chosen according to land slope.

1. Bedding system – flat lands (< 1.5%).
2. Random drains – flat land with scattered depressions.
3. Cross-slope (interception) drain system – sloping lands up to about 4%; drains run across the slope to intercept runoff.

Short trick: Cross-slope drains → gently sloping land up to ~4%.

Concept: Brake power = $\frac{\rho gQH}{\eta }$, or in hp = $\frac{Q(\text{l/s})\times H(\text{m})}{75\eta }$.

1. Water hp = 7.6 × 50 ÷ 75 = 5.07 hp.
2. Brake hp = 5.07 ÷ 0.70 ≈ 7.24 hp.
3. The next standard motor size is 7.5 hp.

Short trick: hp = QH/75η (Q in l/s).

Concept: Water distribution efficiency: $E_d=100(1-\frac{y}{d})$.

1. d = average depth of water stored along the run; y = average numerical deviation from d.
2. Storage efficiency = water stored in root zone ÷ water needed in root zone.

Short trick: Deviation (y) in the formula → uniformity → distribution.

Concept: Excess sodium disperses soil clay, reduces permeability and creates sodic (alkali) soils.

1. It is measured by SAR = $\frac{Na}{\sqrt{(Ca+Mg)/2}}$.
2. Ca and Mg improve soil structure; K is a nutrient.

Short trick: SAR has Na on top → sodium is the villain.

Concept: Volume needed in field = area × depth; supply needed = field volume ÷ conveyance efficiency.

1. 5 cm over 1 ha = 0.05 × 10,000 = 500 m³ = 5,00,000 L.
2. With 10% loss, efficiency 90%: 5,00,000 ÷ 0.9 = 5,55,556 L.
3. Time = 5,55,556 ÷ 16,000 ≈ 34.7 h.

Short trick: 1 ha-cm = 100 m³ = 1 lakh litres.

Concept: Motor power for a pump = $\frac{QH}{75{\eta }_p{\eta }_m}$ hp, with Q in litres (kg) per second and H in metres.

1. The question gives the discharge as '500 metres per minute' — metres is a length, not a volume, so no discharge Q can be taken from the printed data.
2. Only if the figure were 500 litres per minute: Q = 8.33 l/s, water hp = 8.33 × 50 ÷ 75 = 5.56 hp, input = 5.56 ÷ (0.7 × 0.9) ≈ 8.8 hp → a 10 hp motor.
3. That needs changing the printed unit, so as printed no option can be marked correct.

Short trick: Check units first: hp = (litres/s × m) ÷ (75 × overall efficiency).

Concept: Hammer mills are impact grinders used for feed and grain grinding.

1. Agricultural-processing texts list commercial hammer mills in capacities of about 400–1500 kg/h, driven by roughly 5–25 hp motors.
2. Capacity depends on screen size, hammer tip speed (about 50–110 m/s) and the moisture of the material.

Short trick: Hammer mill: impact grinder, roughly 0.4–1.5 t/h.

Concept: Henderson's equation (1952): $1-RH=e^{-cT{M}_e^n}$.

1. It relates equilibrium moisture content $M_e$ to relative humidity and temperature.
2. Chung–Pfost and Chen–Clayton are other EMC models.

Short trick: EMC ↔ Henderson.

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.