ICAR AICE-JRF/SRF (PhD): Dairy Chemistry, Dairy Microbiology, Dairy Technology, Dairy Engineering · India · NTA for ICAR
Original practice questions written to the published syllabus · not past papersEvaporators and dryers for milk and whey: design, energy economy, operation and powder engineering.
In the exams: ICAR AICE subject 38 Unit 2; ICAR AIEEA-PG group 17 Unit II (evaporation, vacuum pan, drum dryer, spray dryer, fluidisation); GATE XL-U Section 4
ICAR AICE-JRF/SRF (PhD) — Dairy Engineering (38)
Unit 2: evaporation techniques and evaporators, drying fundamentals and dryer types, spray dryers, separation and product recovery. Unit 1: freezing, freeze drying and freeze concentration. Expect mass and energy balances, steam economy, TVR and MVR principles, psychrometric calculations and spray-dryer design questions.
How our questions for ICAR AICE-JRF/SRF spread across the units of Evaporation & Drying in Dairy Plants. Difficulty: 4 easy, 45 medium, 20 hard.
| Spray dryers: atomisers, chamber design, two- and three-stage drying | 9 | |
| Multiple-effect evaporation and steam economy | 7 | |
| Drying fundamentals: drying rate and psychrometrics | 7 | |
| Freeze drying and freeze concentration | 7 | |
| Principles of evaporation and boiling-point elevation | 6 | |
| Energy and safety in dryers | 6 | |
| Falling-film evaporators | 6 | |
| Thermal and mechanical vapour recompression | 6 | |
| Separation and recovery of powder: cyclones and bag filters | 5 | |
| Fluid-bed dryers and agglomeration | 4 | |
| Vacuum pans | 3 | |
| Drum dryers | 3 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
An effect evaporates 2,000 kg/h of water at 70 °C (latent heat 2,330 kJ/kg). With U = 2.5 kW/m² K and ΔT = 10 K, what heating area is needed (ignoring sensible heat)?
C. 51.8 m²
Q = 2,000 × 2,330 ÷ 3,600 = 1294 kW; A = 1294 ÷ (2.5 × 10) = 51.8 m². 518 m² ignores ΔT; 129.4 m² ignores U; 186,400 m² forgets to convert hours to seconds.
Further reading: Singh and Heldman, Introduction to Food Engineering
1,000 kg/h of whey concentrate at 55% solids is spray-dried to powder of 97% solids. How much water is evaporated per hour?
D. 433 kg
Powder = 550 ÷ 0.97 = 567 kg; water = 1,000 − 567 = 433 kg. 450 kg is the water in the feed, ignoring moisture left in powder; 567 kg is the powder; 420 kg subtracts percentages (97 − 55) of the feed.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
How much heat is needed to sublime 10 kg of ice in freeze drying (latent heat of sublimation about 2,830 kJ/kg)?
A. 28.3 MJ
10 × 2,830 = 28,300 kJ = 28.3 MJ. 2.83 MJ slips a decimal; 22.6 MJ uses the latent heat of vaporisation; 3.34 MJ uses the latent heat of fusion.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Why is skim milk usually not concentrated much above about 50% solids before spray drying?
C. Viscosity rises steeply and atomisation and heat transfer suffer
High viscosity limits both evaporator performance and atomisation.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Why do MVR evaporators need large heating areas?
A. The compressor raises vapour temperature only a few degrees
Small ΔT means more area for the same duty.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Final powder moisture is governed largely by:
A. Equilibrium moisture at outlet air temperature and humidity
Powder approaches equilibrium with the exhaust air.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Which atomiser type tolerates variations in feed rate and viscosity best?
C. Rotary wheel
Wheels handle changes well but need wide chambers.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Why are fines from cyclones returned to the atomisation zone?
B. To build agglomerates and recover product
Fines stick to moist droplets.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Why are vibrating fluid beds used for dairy powders?
A. Cohesive, sticky powders fluidise poorly in static beds
Vibration aids movement and fluidisation of cohesive powder.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
Pulse-jet cleaning in bag filters:
C. Removes the powder cake from the bags
Reverse air pulses dislodge the cake into the hopper.
Further reading: Westergaard, Milk Powder Technology: Evaporation and Spray Drying
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