ICAR AIEEA-PG Evaporation & Drying in Dairy Plants: practice questions

1 · Principles of evaporation and boiling-point elevation · easy

10,000 kg of skim milk with 9% solids is concentrated to 45% solids. How much water is evaporated?

1. 8,000 kg
2. 3,600 kg
3. 4,500 kg
4. 2,000 kg

2 · Multiple-effect evaporation and steam economy · medium

An evaporator with a steam economy of 2.7 evaporates 8,000 kg of water. How much steam does it use?

1. 21,600 kg
2. 2,963 kg
3. 2,667 kg
4. 8,000 kg

3 · Spray dryers: atomisers, chamber design, two- and three-stage drying · medium

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?

1. 567 kg
2. 450 kg
3. 420 kg
4. 433 kg

4 · Vacuum pans · medium

Foaming in a vacuum pan is a problem mainly because:

1. Foam raises the boiling point sharply
2. Foam carries product into the condenser
3. Foam sterilises the product
4. Foam crystallises the sugar

5 · Drying fundamentals: drying rate and psychrometrics · easy

During the constant-rate drying period, the surface of a wet material stays near:

1. The dry-bulb temperature of the inlet air
2. 0 °C
3. The wet-bulb temperature
4. The dew point of the outlet air

6 · Spray dryers: atomisers, chamber design, two- and three-stage drying · easy

Which atomiser type tolerates variations in feed rate and viscosity best?

1. Drum film applicator
2. Two-fluid nozzle
3. Rotary wheel
4. High-pressure nozzle

7 · Spray dryers: atomisers, chamber design, two- and three-stage drying · medium

In co-current spray drying, the wettest droplets meet:

1. Air at the outlet temperature only
2. The coldest air
3. The hottest air
4. No air until the fluid bed

8 · Drum dryers · medium

Drum dryers are energy-efficient per kg of water because:

1. They recycle all the vapour as heating steam
2. They use no steam or other heating medium
3. They evaporate water at room temperature
4. Heat passes by conduction from steam to film

9 · Freeze drying and freeze concentration · easy

Freeze drying removes water by:

1. Boiling liquid water at atmospheric pressure
2. Spraying it into hot air
3. Sublimation of ice under vacuum
4. Pressing the product between rollers

10 · Energy and safety in dryers · easy

Why should as much water as possible be removed before the spray dryer?

1. The dryer uses far more energy per kg of water
2. The dryer cannot remove any water at all
3. Evaporators cannot handle concentrated milk
4. Drying dilute milk gives higher bulk density