Feeding System Dairy Plant- Dairy Engineering- Technical Superintendant Milma

Contents

Feeding System

The feed system, is the link between the evaporator and the spray dryer and comprises:

(1) Feed tanks
(2) Water tank
(3) Concentrate pump
(4) Preheating system
(5) Filter
(6) Homogenizer/high-pressure pump
(7) Feed line, including Return line for CIP

1.Feed Tanks

It is recommended to use two feed tanks and change from one to the other at least once an hour.

This is due to the risk of bacteria growth in the food normally having a temperature of 40-45 oC .

One is therefore in use while the other one in being washed. The size of each tank should correspond to 15-30 min. of the capacity of the dryer and provide with a lid to avoid contamination from the air.

Very often the feed tanks are equipped with spray nozzles for automatic CIP cleaning.

2.Water Tank

The water tank is used during start and stop of the plant, and if during the run there will be a sudden shortage of concentrate.

Level controls can be placed on the feed tanks, so that the change-over can be automatic.

3.Concentrate Pump

If a rotary atomizer is used, the most common feed pump is either the mono type 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.

4.Pre heating System

Nozzle atomization requires higher feed temperature than that coming from the evaporator.

Therefore, a concentrate pre heater is necessary. Two types can be used, either indirect or direct.

(A) Indirect pre heaters

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 20 hours run is aimed at, it is necessary to have two interchangeable heaters allowing one to be cleaned while the other one is being used.

Steam, warm water or condensate from the first effect of the evaporator can be used as heating medium.

Spiral Tube Heat Exchanger
The spiral tube heater is able to heat concentrate with a higher solids content to a higher temperature without frequent cleaning due to high product velocity and low Δt, but it is more expensive. If a total blockage due to deposits on the heat surface occurs, then the cleaning becomes complicated.
Scraped Surface Heat Exchanger
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 if high temperatures are required. They can operate continuously for 20 hours and are cleaned together with the
remaining feed system.
(B) Direct preheaters are of the type:
(i) Direct Steam Injection DSI
(ii) Lenient Steam Injection LSI
(i) 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 superheating of some parts of the concentrate which leads to protein denaturation.

(ii) Lenilent 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 much higher steam pressure can be used. The LSI unit is often used in combination with the scraped surface heat exchanger if temperatures above 75oC are required in the concentrate.

Preheating of concentrate is advantageous, not only from a bacteriological point of view. It also produces viscosity decreases, which together with the applied calories results in a capacity increase of minimum 4% on the spray dryer and improved solubility of the produced powder.

5.Filter

An in-line filter is always incorporated in the feed system to avoid lumps etc. passing to the atomizing device.

6. Homogenizer/High-Pressure Pump

If whole milk powder should be produced, it is recommended to incorporate a homogenizer in order to reduce the free fat content in the final powder. A two-stage homogenizer is preferred.

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 omogenizer/high-pressure pump is chosen.

A variable speed drive for controlling the outlet temperature is preferred, as a return valve tends to give uncontrollable holding time resulting in viscosity problems.

7.Feed Line

The feed pipe should naturally be of stainless steel and of course of the high-pressure type, it 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 should also be included for the cleaning solution, so that the entire equipment can be thoroughly cleaned.

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