Nozzle atomization requires higher feed temperature (lower viscosity) than
that coming from the evaporator. Preheating is, however, advantageous from an
atomization point of view, also for rotary atomization. Therefore, a
concentrate preheater is recommended of which there are either indirect or
direct preheaters.
DIFFERENT TYPES OF INDIRECT PREHEATERS
- Plate heat exchanger
- Tube-in-Tube heat exchanger
- Scraped surface heat exchanger
PLATE HEAT EXCHANGER
plate heat exchanger system is cheap, but if the concentrate should be
heated to 60-65șC and the solids content is 45-46%, and if a 20 hours run is
the goal, it is necessary to have two interchangeable heaters allowing one to
be cleaned while the other one is being used. Heating mediums can be steam,
warm water or condensate from the first effect of the evaporator.
TUBE-IN-TUBE HEAT EXCHANGER
The HIPEX tube-in-tube heat exchanger a very simple and maintenance-free
concentrate preheater. (see Fig. 46)
The tube in tube heat
exchanger has corrugated tubes providing a high heat transfer rate due to the
turbulence created by the corrugated tubes, which also reduce fouling on the
heat transfer surface. The heating media is typically hot water, and the unit
is designed with counter-current product/double heating media flow with the
product in the middle. A very low temperature difference (< 5șC) allows 20
hours operation between cleaning.
SCRAPED SURFACE HEAT EXCHANGER
(Fig. 47.) In the scraped surface heater the heat transfer surface is
continuously being scraped off by a fast rotating scraper made of food-grade
synthetic material to avoid any product adherence resulting in burnt deposits
with reduced heat transfer as a consequence. The scraped surface heater is
especially suited for products with high solids content. They can operate
continuously for 20 hours and are cleaned together with the remaining feed
system.
The scraped surface heat exchanger is not recommended for
products containing crystals like precrystallized whey or permeates, due to
wear problems.
DIFFERENT TYPES OF DIRECT PREHEATERS
- Direct Steam Injection DSI
- Lenient Steam Injection LSI
DIRECT STEAM INJECTION DSI
In the DSI unit the steam is introduced into the milk concentrate via a
nozzle producing relatively big steam bubbles resulting in a superheating of
some parts of the concentrate which leads to protein denaturation.
LENIENT STEAM INJECTION LSI
In the LSI unit
(patented) the steam is mixed into the concentrate by a dynamic mixer. Very
small steam bubbles are created, and superheating/denaturation is avoided.
Therefore, a higher steam pressure can be used. The LSI unit is often used in
combi-nation with other heat exchangers, if temperatures above 75șC are
required in the concentrate.
Preheating of concentrate is advantageous,
not only from a bacteriological point of view. It also results in a viscosity
decrease, which together with the applied calories results in a capacity
increase of minimum 4% on the spray dryer and an improved solubility of the
produced powder.