CONDENSED AND DRIED MILKS

Module 13. Technology of dried milks

Lesson 30
AIR HEATING

30.1 Introduction

In the process of spray drying, air is playing a major role which enters the drier through air filter, air heater and finally drying the milk.

30.2 Air Filter

Different types of air filters may be used. Close woven filters are unsuitable because of the heavy, extra load which their resistance places upon the suction fan. In some plants, the incoming air is washed by drawing it through a spray of water or through a series of screens of wire mesh or gauge over which a film of water trickles. The water spray or film of water is limited in its efficiency as an air filter. It removes the coarser mechanical impurities but it fails to purify the air from the grease and turbidity of city atmosphere.

The filter, most commonly used is a type in which oil is used as filtering element. It embodies the adhesive impingement principle of removing dust from the air. This type of filter consists of multiple cells filled with expanded metal. The cells are dipped in oil which coats the entire filter with adhesive film consisting of odourless oil that is sufficiently heavy to prevent being blown off. A plant with an average capacity of ~ 100 kgs of dried skim milk per hour requires about 230.5 m³ of air per minute.

In the presence of air of average purity, it is a good practice to wash the cells at least every two weeks. In order to maintain a satisfactory average condition of the filter, the rotation system of cleaning is generally preferred. The washing is done by soaking the cells overnight in hot water, containing an alkaline cleaning compound followed by blowing live steam through the cells for a period of 15 to 30 minutes. The clean cells are recharged by dipping them in oil and allowing them to dry in12 hours.

30.3 Air Heater

The air may be heated directly or indirectly or semi-direct such as by means of a furnace with hot air or by indirect means whereby the air passes over banks of fin type steam coils.

30.2.1 The direct and semi direct heating systems

Advantages

(1) These systems have greater thermal efficiency provided the escape of the furnace gases at uneconomically high temperature is prevented.

(2) They attain higher temperature than are possible with indirect heating.

(3) It consumes much of the oxygen (O₂) that is contained in the air and to that extent lessens the tendency of tallowy flavour in the milk powder.

Disadvantages

(1) There is danger of getting black specks due to the presence of particles of extraneous material such as entrained furnace spot and dust, and scales of rust breaking loose from the flues.

(2) Less temperature control.

30.2.2 The indirect heating system

This is the system commonly used in spray drying of the milk. The incoming current of fresh air passes over fin type of steam coils installed in the heating system. Heat control is facilitated by separate valve for each coil section. Fuel is saved by using exhaust steam in the first coil section and high pressure steam in subsequent section. Other media that may be used for heating the air in the spray drying system are hot oils and electric current.

Advantages

(1) Easy control of temperature

(2) Thermal loss can be avoided by placing insulated lines between the heating devices and drying chambers.

30.3 Drying Chambers

The drying chambers vary in shape and size. Shape and size of the chamber varies according to the capacity of the unit and manufacturing firms. Main aim is to remove the powder as soon as it is dried. Some drying chambers are rectangular, other cylindrical, some have straight vertical sides, and others are cone shape slopping downward. Drying chambers are usually made of stainless steel and provided with observation glass. In modern times, dryer has been developed to produce instant milk powder, which will have tall tower type chamber.

For a reason of economic efficiency, the relationship of position of milk inlet into the drying chamber and air inlet and discharge should be such as to accomplish maximum saturation of drying air and maximum utilization of heat contained therein. For best preservation of important marketable properties of the resulting powder (quality and solubility), milk inlet and air inlet and discharge should be so located as to accomplish maximum rapidity of evaporation and uniformly low moisture content of the finished product.

30.4 Counter Current versus Parallel Current Flow

In general, the milk and air either enter at opposite side (at top and bottom respectively) or flow in countercurrent toward each other or they both enter in the same region to get in parallel or co-current flow. In counter-current principle the points of inlet and direction of flow are so organized as to cause the incoming air (which is driest and hottest at this point) to meet the milk particles at the point of their lowest humidity and to cause the outgoing air (which is most nearly saturated) to leave the drying chamber at the point where the milk is wettest (which is near the atomizer).

30.5 Effect of Parallel Flow on Uniformity of Particle Size

In the parallel flow principle, milk and air enter in the same region and travel concurrently. The seeming logic of the counter current principle, in which the drying milk passes successively through zones of dry air increasingly and finishes in the incoming air which is hottest and driest at the point, suggests maximum dryness of the finished product. It is conceivable, however, that the head on collision between air current and milk fog causes countless numbers of atomized milk particles to collide and fuse, forming big droplets, dissipating uniformity of particle size and impeding the speed of drying. In the parallel flow principle, milk fog and air currents move in the same direction, eliminating the danger of violent collision between milk particles and preserving the uniform fineness of particle size.

30.6 Effect of Parallel Flow on Rapidity of Drying

T he parallel flow of milk fog and air current gives maximum rapidity of drying. The set up here is based on the theory that the high temperature and very low humidity of the incoming heated air in the presence of moisture content and tremendous surface area of the freshly atomized milk particles, cause maximum rapidity of the evaporation. The dry hot air at this point is at its maximum capacity for instantaneous absorption of the vapours released by the vast surface area of the drying milk. Simultaneously, the dried milk particles drop to the bottom while the saturated air currents exhaust through the top of the chamber.

It is of interest to point out here the tendency of the newer designs of air drying chambers is definitely in the direction of the parallel flow intake of milk dryers. Maximum rapidity of drying favours high solubility and keeping quality of finished product.