ICAR AICE-JRF/SRF (PhD) — Agricultural Engineering subjects 38–43 · India · National Testing Agency for ICAR
Original practice questions written to the published syllabus · not past papersRefrigeration cycles, equipment and cold-store design, from COP to the heat load of a room full of potatoes.
In the exams: GATE AG Section 7 (refrigeration COP/TR in 4 of 8 papers 2019–26; composite cold-store walls recur); ICAR AIEEA-PG Unit VI; ICAR AICE subject 38 Unit 6
ICAR AICE-JRF/SRF (PhD) — Dairy Engineering (38) and Processing & Food Engineering (41)
Subject 38 Unit 6 (refrigeration) and subject 41's storage engineering unit go to PG depth: multistage and cascade systems, volumetric efficiency, absorption COP limits, refrigerant transition under the Kigali Amendment, ice-bank sizing for milk, Plank's equation and its assumptions, and cold-store design choices such as evaporator TD and humidity.
How our questions for ICAR AICE-JRF/SRF spread across the units of Refrigeration, Air-Conditioning & Cold-Chain Engineering. Difficulty: 0 easy, 12 medium, 19 hard.
| Cold-store heat load: transmission, product and respiration | 4 | |
| Ice-bank systems | 4 | |
| Vapour-absorption refrigeration | 3 | |
| Compressors | 3 | |
| Vapour-compression refrigeration cycle | 2 | |
| Refrigerants: ODP and GWP | 2 | |
| Pressure-enthalpy charts | 2 | |
| Condensers and evaporators | 2 | |
| Expansion devices | 2 | |
| Insulation and vapour barriers | 2 | |
| Refrigerated transport and cold-chain logistics | 2 | |
| COP and tonnes of refrigeration | 1 | |
| Psychrometric air-conditioning processes | 1 | |
| Cold-store heat load: infiltration, occupancy and equipment | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A two-stage ammonia system has suction pressure 2.0 bar and condensing pressure 12.5 bar. Using the usual first choice for intermediate pressure, the intercooler pressure is about
D. 5.0 bar
The geometric mean √(p_s p_d) = √(2.0 × 12.5) = 5.0 bar roughly equalises the pressure ratios (2.5 each) and the work of the stages. The arithmetic mean 7.25 bar gives unequal ratios; 6.25 is the overall pressure ratio, not a pressure, and 10.5 bar is the pressure difference.
Further reading: Stoecker and Jones, Refrigeration and Air Conditioning
An ideal absorption refrigerator receives heat at 100 °C, rejects heat to surroundings at 30 °C and refrigerates at -10 °C. Its maximum possible COP is about
B. 1.23
COP_max = [(T_G − T_0)/T_G] × [T_E/(T_0 − T_E)] = (70/373.15) × (263.15/40) = 0.1876 × 6.579 = 1.23. 6.58 is only the Carnot refrigerator part and 0.19 only the engine part; 0.18 uses Celsius temperatures ((70/100) × (10/40) = 0.175).
Further reading: Stoecker and Jones, Refrigeration and Air Conditioning
A carbon dioxide (R-744) refrigeration system in a hot climate usually operates transcritically because
D. Its critical temperature of 31 °C is below usual heat-rejection temperatures
Above the critical temperature no condensation occurs, so heat is rejected in a gas cooler at supercritical pressure. Its critical pressure is high (about 74 bar), not low.
Further reading: ASHRAE Handbook — Refrigeration
For a reciprocating compressor with clearance ratio 0.05 and re-expansion index 1.2, the pressure ratio at which the clearance volumetric efficiency falls to zero is about
D. 38.6
Set 1 + C − C r^(1/n) = 0: r = ((1 + C)/C)^n = (21)^1.2 = 38.6. Forgetting the exponent gives 21; using 1/n gives 12.6. In practice staging is used long before this ratio. 20.0 is 1/C, forgetting both the (1 + C) and the exponent.
Further reading: Arora, Refrigeration and Air Conditioning
A water-cooled condenser rejects 50 kW with refrigerant condensing at 40 °C. Water enters at 30 °C and leaves at 35 °C. With U = 800 W/m² K, the required area is about
B. 8.66 m²
ΔT1 = 10 K, ΔT2 = 5 K. LMTD = (10 − 5)/ln(10/5) = 7.21 K. A = Q/(U·LMTD) = 50,000/(800 × 7.21) = 8.66 m². The arithmetic mean (7.5 K) gives 8.33 m²; using the water rise (5 K) gives 12.5 m²; 0.0087 m² forgets kW → W.
Further reading: Stoecker and Jones, Refrigeration and Air Conditioning
When the load on an evaporator fed by an automatic (constant-pressure) expansion valve increases, the valve tends to
A. Close, starving the evaporator just when more refrigerant is needed
Higher load tends to raise evaporator pressure; the valve acts to hold pressure constant, so it throttles down. That inverse response is why it is used only for small, steady loads.
Further reading: Arora, Refrigeration and Air Conditioning
Spherical fruit pieces of diameter 6 cm (ρ = 1050 kg/m³, L = 280 kJ/kg, freezing point -1.0 °C, frozen k = 1.2 W/m K) are frozen in air at -25 °C with h = 25 W/m² K. Plank's equation gives a freezing time of about
C. 1.79 h
Sphere: P = 1/6, R = 1/24, a = diameter. ρL/ΔT = 12,250,000. Bracket = 0.06/150 + 0.0036/28.8 = 0.000400 + 0.000125 = 0.000525. t = 6,431 s = 1.79 h. Slab constants give 5.36 h and cylinder constants 2.68 h; using the radius gives 0.79 h.
Further reading: Singh and Heldman, Introduction to Food Engineering
Moisture that enters cold-store insulation is harmful mainly because
A. Water and ice conduct heat far better than trapped gas, so insulation value falls and ice builds up
Liquid water conducts heat roughly twenty times better than still air and ice more still; condensation freezes in the cold layers and builds up because the vapour drive is nearly one-way. Weight is a secondary effect.
Further reading: ASHRAE Handbook — Refrigeration
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