Steam (hf = 632.2, hfg = 2113.2, hg = 2745.4 kJ kg⁻¹) at 150 °C is used to sterilize milk by direct steam injection. Milk is initially at 90 °C and after sterilization, the blend of resultant milk and condensed steam (hf = 567.6, hfg = 2159.2, hg = 2726.7 kJ kg⁻¹) are at 135 °C. Specific heat capacity of milk is 3.8 kJ kg⁻¹ K⁻¹. Assuming no energy loss, the amount of milk (in kg) sterilized per kg steam supplied is
Options:
(A) 21.57
(B) 12.74
(C) 9.73
(D) 4.48
Official Answer Key:
B
Step-by-Step Solution & Explanation
• Per kg of injected steam, the heat released as it condenses and cools from the 150 °C steam state to the 135 °C liquid (condensate) state = hg(150°C) – hf(135°C) = 2745.4 – 567.6 = 2177.8 kJ/kg steam.
• This heat raises 1 kg of milk from 90 °C to the final blend temperature 135 °C: heat required per kg milk = Cp×ΔT = 3.8×(135–90) = 3.8×45 = 171 kJ/kg milk.
• Energy balance: (kg milk) × 171 = (kg steam) × 2177.8 ⇒ kg milk per kg steam = 2177.8/171 ≈ 12.74.
• Final Answer: (B) 12.74
Appendix A — Consolidated Answer Table
Q. No. | Section | Topic | Type | Marks | Official Key
Appendix B — Section-wise Weightage
Preparation observations
• General Aptitude (15%) and Farm Power together with Irrigation & Drainage Engineering (14% each) carry the single largest blocks of marks in this paper — together these three areas account for 43% of the total marks, so they reward disproportionately more study time than their single-section label suggests.
• Engineering Mathematics (13%) is concentrated in a predictable set of tools — matrices/eigenvalues, basic vector calculus, elementary calculus, probability, and Simpson’s rule — all of which appeared as direct, formula-recall questions rather than multi-step applied problems; this makes Section 1 comparatively high-yield for the preparation effort it requires.
• Numerical-answer type (NAT) questions dominate the 2-mark question set across Farm Machinery, Farm Power, Irrigation & Drainage, Agricultural Process, and Dairy & Food Engineering — these consistently reward being fluent with unit conversions (kN·m, kPa, L/min·m³/s, kg/min·kg/s) since a units slip is the most common way to lose marks on an otherwise correctly set-up NAT question.
• Several questions blend two syllabus lines from the same section (e.g., Q30 blends Machine Design with Farm Machinery tillage-tool forces; Q41 blends Hydrology instrumentation with Groundwater Hydrology aquifer-testing) — candidates should not prepare each syllabus line in isolation but practice recognizing when a single numerical problem draws on more than one topic heading.
• Heat-and-mass-transfer and food-preservation kinetics (Section 7, 10%) consistently used two recurring frameworks — lumped-capacitance transient conduction and D-value/z-value thermal death time — so mastering these two derivations covers a disproportionate share of that section's marks.
• Question 35 (kingpin torque) is flagged in this document as a case where independent first-principles derivation did not exactly reproduce the official key range; candidates should treat the officially published key as authoritative for scoring purposes even where a textbook derivation gives a close but not identical value.
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