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You are reading the e-book Milk Powder Technology by Vagn Vestergaard.
The feed system, see Fig. 45, is the link between the evaporator and the spray dryer and comprises:
It is recommended to change between two tanks ever 4 hours of operation of feed tanks. Due to high temperatures of 45-50 ºC, there is a risk of bateria growth in the feed tanks. While one of the feed tanks is in operation, the other is being washed. The size of each tank should correspond to 15-30 min. of the capacity of the spray dryer. The feed tanks are equipped with level indicators and spray nozzles for automatic CIP Cleaning-in-Place. Sometimes, the feed tanks are replaced by vacuum tanks as an integrated part of the last calandria of the evaporator.
The water tank is used during start-up and stop of the plant, and in case of a sudden shortage of concentrate during the run. Level transmitters can be placed in the feed tanks, so that the change-over can be automatic, as a water supply pipe is connected to the feed tank, in which case the water tank is avoided.
If a rotary atomizer is used, the most common feed pump is either the mono type pump or a centrifugal pump. The mono pump requires less energy and can handle concentrates of a higher viscosity than the centrifugal pump, but is more expensive.
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
A plate heat exchanger system is cheap, but if the concentrate should be heated to 60-65 ºC 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 (< 5 ºC) 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
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 75 ºC 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.
An in-line filter is always incorporated in the feed system to avoid lumps etc. passing to the atomizing device.
The first stage is operated at 70-100 bar g, and the second stage at 25-50 bar g. Usually the homogenizer and feed pump are combined in one unit. If nozzle atomization is used then a higher pressure (up to 250 bar g for the nozzles + 150 bar g for homogenizing) is required, and a combined homogenizer/high-pressure pump is chosen to save cost. A variable speed drive for controlling the feed flow and thereby the outlet temperature is preferred, as a return valve tends to give uncontrollable holding time resulting in viscosity problems. The viscosity as function of homogenization pressure and solids content in the concentrate is seen in Fig. 47a.
The feed pipe should naturally be of stainless steel and of course of the high-pressure type, if the atomization is to be carried out by means of nozzles. The dimension should be so that the feed velocity is approx. 1.5 m/sec. In a feed system a return pipe and a device to clean the rotary atomizer, incl. the weel, as well as the nozzle lances, should also be included for the cleaning solution, so that the entire equipment can be thoroughly cleaned.
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