EVAPORATING EQUIPMENT 1. Open pan 2. Natural circulation evaporator 3. Horizontal tube 4. Vertical tube 5. Rising film evaporator 6. Falling film evaporator 7. Rising-falling film evaporator 8. Forced circulation evaporator 9. Agitated thin film evaporator 10. Plate evaporator
1. OPEN PAN a)
Heating is non-uniform causing fouling as liquid is stationary and A/V ratio is low for heat transfer.
b)
Not suitable for heat-labile products as the residence time is long at high temperature.
c)
Product boils at 100oC (atm pressure), thus vacuum is reduced to the boiling temperature
d)
The simplest but most expensive.
Need further explanation?â–ź https://www.youtube.com/watch?v=6FSdoqbk6QA
2. NATURAL CIRCULATION EVAPORATOR a)
Consists of short tube bundles within the batch pan or external shell and tube heater outside the vessel.
b)
The external heater has the advantage that its size is not dependant upon the size and shape of the vessel itself.
Need further explanation?â–ź https://www.youtube.com/watch?v=8d_d5gI0iQo
3. HORIZONTAL TUBE (STEAM INSIDE TUBE) a)
The shell carries a bundle of internally steam heated, horizontal tubes.
b)
Large and long space above the heater is to permit a gravity separation of liquid droplets carried off with the vapour.
c)
The baffles are fitted to reduce entrainment of droplets.
d)
The U is low because the tube bundles interfere with the circulation.
e)
For non-scaling, non-foaming and low viscosity product.
Need further visualisation?â–ź https://www.youtube.com/watch?v=nNneoIStja4
4. VERTICAL TUBE (STEAM OUTSIDE TUBE) a)
Standard evaporator in food industry.
b)
Steam condenses on the outside of the tubes.
c)
Tube length are from 0.5 – 2 m and diameters of 25-75 mm in an assembly of tubes – called calandria.
d)
Calandria has a central downcomer which gives the natural circulation currents effect.
e)
Current – due to temperature difference between central down stream and heated up stream in the tubes.
f)
Tubes should be kept covered with liquor to minimize fouling.
g)
This unit can be fitted with ‘basket calandria’ which can be easily removed for maintenance.
h)
‘basket calandria’ types give good rates of evaporation with non-corrosive, mildly scaling liquor of moderate viscosity.
i)
Application: concentration of solutions of beet sugar, glucose, malt extract, salt and fruit juices.
Need further visualisation?▼ https://www.youtube.com/watch?v=wN-TvorOwIg
5. RISING FILM EVAPORATOR a)
Feed liquid, preheated near boiling, is introduced at the bottom of tube assembly.
b)
The feed liquid will rise up the tubes as a thin film on the inner walls of the pipe with vapour filling the centre of the tubes.
c)
The temperature difference between steam and product must be > 14oC to create the film effect.
d)
Residence time in the heating zone is short (3 sec for 4 m high tube), the heat transfer is rapid and the U is very high, making it suitable for heat sensitive product.
Need further explanation?â–ź https://www.youtube.com/watch?v=wTLBb_bBhJs
6. FALLING FILM EVAPORATOR a)
Feed liquid, preheated near boiling is introduced at the top of tube assembly.
b)
The feed liquid will fall down as a thin film in which the central and high velocity valour dragging a film of liquid with it.
c)
No hydrostatic head in the tubes so a uniform, low boiling temperature may be maintained.
d)
The difference temperature between steam and product must not necessarily be > 14oC so more effects can be accommodated.
VIDEO 1
e)
The residence time is 20-30 sec so is excellent for heat sensitive materials.
f)
High rate of evaporation with boiling temperature as low as 10-16oC under vacuum for citrus fruit juices.
g)
Uniform flow is difficult to achieve due surging.
Need further explanation?â–ź https://www.youtube.com/watch?v=JVJ81J05_pk
7. RISING-FALLING FILM EVAPORATOR a)
The feed enters at the bottom of the calandria (which is divided into two sides), rises up one set of tubes, then falls down a second set, exiting from the base of the calandria.
b)
It is a common configuration for a single effect evaporator.
8. FORCED CIRCULATION EVAPORATOR a)
Liquid is pumped and circulated at a high rate (2-6 m/s) through a calandria.
b)
The liquid does not boil in the calandria due to maintained hydrostatic pressure but flashes to vapour in a lower absolute pressure of a separator.
c)
It is suitable for products susceptible to scaling.
d)
More expensive for the needs of more recirculation pipe works and pumps.
Need further explanation?â–ź https://www.youtube.com/watch?v=Z9cf205f6ZA
9. AGITATED THIN FILM EVAPORATOR (WIPED FILM) a)
Also a SSHE used to concentrate very viscous material and to strip off the solvents.
b)
Feed is introduced at the top and spread on the inner surface by a blade.
c)
Evaporation of the solvent will take place as the thin film moves down evaporator wall.
d)
The heating medium is normally high pressure steam or oil.
e)
It requires high pressure heating medium to increase the evaporation rate since the heat transfer surface is small.
f)
Disadvantages: e) Small heat transfer surface f) Maintenance for moving parts g) High capital cost.
Need further explanation?â–ź https://www.youtube.com/watch?v=o0HjkMSgn6k
10. PLATE EVAPORATOR i.
This utilizes the climbing falling film principles within a PHE.
ii.
PHE unit consists of 2 steam plates and 2 product plates. 1st product passage is a rising pass 2nd – a falling pass.
iii.
The steam plates are arranged alternately between each product passage.
iii.
Application: juice and dairy products at operating temperature between 38oC and 93oC.
iv.
Advantages: Extended capacity Less headroom Low suspended solids High product quality
Need further visualisation?▼ https://www.youtube.com/watch?v=Jv5p7o-7Pms