As the milk to be evaporated has a temperature of 5-10șC it has to be heated to the boiling temperature of the first effect in order to save steam for the evaporation. The milk is therefore first passed through a vapour cooler/preheater, placed between the last effect's separator and the condenser, thereby saving cooling water as well. From the vapour cooler the milk is passed through the preheating section of the last effect and then backwards to the first effect, before it enters the boiling section of the first effect.
The preheating system can technically be carried out in two different ways:
- Spiral-tube preheaters
- Straight-tube preheaters
Spiral-Tube Preheaters
The spiral tubes are placed inside the heating rooms in the calandria surrounding the falling-film tubes, thus being heated by vapour, see Fig. 20. The system is simple, but does not offer the possibility of inspection for deposits or leakage.
Fig. 20 Spiral-tube preheaters
Straight-Tube Preheaters
The straight-tube preheaters are placed vertically outside the evaporator and like the spiral tubes heated by vapour from the corresponding calandria. See Fig. 21. The vapour connection is at the top of the calandria, so that uncondensable gasses can easily be extracted. This ensures an optimum utilization of the heating surface of the evaporation tubes. With this system inspection and manual cleaning are possible, if in rare cases it should prove necessary. The heat transfer surface in the preheater is arranged in groups of parallel tubes with small diameter resulting in a large surface. Each group of tubes is connected by normal dairy fittings at the end. Due to the parallel flow the holding time is very much shorter than that of the spiral tube preheater. The viscosity of the final concentrate is therefore lower in evaporators equipped with straight-tube preheaters.
Fig. 21 Straight-tube preheaters
The large surface of the preheaters and the temperature level prevailing during operation (40-65șC) offers, however, optimal growth conditions for thermophilic bacteria. After 14-16 h of operation a bio-film is formed on the inner surface of the preheaters, where they can form spores. Unless special attention is paid, as discussed on page 49 in the chapter "Low Thermophilic Preheating Equipment" one cannot expect a 20 hours production without increase of thermophilic bacteria and their spores during the last 4-5 hours of a 20 hour production.