1. Introduction to Food Engineering
Evaporation
Introduction to Food Engineering

2. Evaporation
Removal of water from diluted liquid foods to obtain concentrated products.
Microbiological stability
Reduce transportation costs, storage
Evaporator
Heat exchanger in large chamber
Product under vacuum

3. Evaporator Single-effect evaporator Multiple-effect evaporator
Vapor discarded
Multiple-effect evaporator
Vapor reused as heating medium

6. Boiling-Point Elevation

7. Duhring’s Rule
Linear relationship between boiling-point temperature of solution and boiling point temperature of water at the same pressure.

8. Example
Determine initial and final boiling point of a liquid food. The pressure in evaporator is 20 kPa. The product is being concentrated from 5 % to 25 % solids concentration.
Boiling pt. of water from steam table at 20 kPa = 60 C (333 K)

9. From Duhring’s chart
Initial 60 C
Final 64 C
333 K
337 K

10. Types of Evaporator
1. Batch-type pan evaporator

11. Heat transfer per unit volume is small
=> long residence time, limit capacities

12. Types of Evaporator 2. Natural circulation evaporators
1 – 2 m vertical tubes inside steam chest

13. Types of Evaporator 3. Rising-film evaporator 10 – 15 m vertical tubes
Film of liquid move upward
Need 14 C difference between heating medium and product

15. Types of Evaporator Falling-film evaporator
Thin liquid film move downward
Distribution of liquid in uniform film by spray nozzles
Handle more viscous liquids than rising-film
Less residence time

17. Types of Evaporator 5. Rising/falling-film evaporator
6. Forced-circulation evaporator
Use pump to maintain high circulation rates
7. Agitated thin-film evaporator
Very viscous fluid foods
Feed is spread on heating surface by wiper blades

21. Design of a single-effect evaporator

22. Mass Balance
Flow
Solids

23. Enthalpy Balance kJ/kg feed steam vapor product condensate
Hv (Ts), Hv (T1) , Hc (Ts) from steam table

24. Rate of heat transfer q = rate of heat transfer (W)
U = overall heat transfer coefficient (W/m2K)
A = area, m2

25. Rate of heat transfer U decreases as product becomes concentrated.
increase resistance of heat transfer
Boiling point elevation
But constant U is used -> overdesign

26. Steam Economy
Ratio-rate of mass of water vapor produced per unit of steam consumed
Typically -> 1

27. Example
Apple juice is being concentrated. At steady state, feed = 0.67 kg/s. Concentration of the juice = 11 % total solids. The juice is concentrated to 75 % TS. Specific heats of diluted feed and concentrate are 3.9 and 2.3 kJ/kgC. The steam pressure is measured to be kPa. Inlet feed temp is 43.3 C. The product inside the evaporator boils at 62.2 C. Assume U = 943 W/m2C, negligible boiling-point elevation. Calculate mass flow rate of concentrate product, steam requirements, steam economy and heat-transfer area.

28. Design of multiple-effect evap

31. (7.13)
(7.14)
(7.15)
(7.16)