Dairy Process Engineering

Module 2. Drying

Lesson 13
AIR HEATING SYSTEMS, ATOMIZATION & FEEDING SYSTEM

13.1 Introduction

The success of the drying and obtaining uniform product in spray drier depends very much on the degree of atomization, hot air to spray contact. The distribution of air near the nozzle very much affects the drying process.

(Fig.13.2) Hot Air System and Air Distribution

13.2.1 Air filtration

The quality and condition of the air supplied to the air heating system is to be properly controlled, to avoid contamination and chocking of filters frequently. To achieve this, the following points are to be considered.

(i) The air should be prefiltered and supplied by a separate fan to the fan/filter/heater room. This room must be preferably under pressure to avoid unfiltered air to enter.
(ii) For air to be heated above 120 oC filtration is needed. The filter is generally placed on the suction side of the fan.

13.1.2 Air heating system

The drying air can be heated in different ways:

(i) Indirect : Steam / Gas/ Hot oil
(ii) Direct : Gas / Electricity

Indirect heating

A steam heater is a simple radiator. The temperature to be obtained depends on the steam pressure available. Under normal conditions it is possible to obtain air temperatures 10 °C lower than the corresponding saturation enthalpy of the steam.

Modern steam heaters are divided in sections, so that the cold air first meets the condensate section, then a section with low steam pressure (which is usually the biggest one in order to utilize as much low-pressure steam as possible), and then the air finally enters the high-pressure steam section. The air heater consists of rows of finned tubes housed in an insulated metal case. The heat load is calculated from the quantity and specific heat of the air. The heater size depends upon the heat transfer properties of the tubes and fins and is usually about 50 Kcal/oC x h x m3 for an air velocity of 5 m/sec. Steam-heated air heaters will usually have an efficiency of 98-99%. As the steam boiler is usually placed at some distance from the air heater, 2-3 bar g extra pressure on the boiler should be anticipated due to pressure loss in the steam pipe and over the regulating valve. To avoid corrosion of the tubes in the air heater it is recommended to use stainless steel. Dampers are provided to control the velocity and quantity of air flow to and from the air heaters.

In indirect gas heaters, drying air and combustion gases have separate flow passage. The combustion gases pass through galvanized tubes that act as heat transfer surface for the drying air. The combustion chamber is made of heat-resistant steel. The end cover of the heater should be removable for cleaning of tubes. Heaters of this type will in the range of 175-250°C have an efficiency of about 85% .

Hot oil liquid phase air heaters are used either along, or when high inlet drying air temperatures are required, and the steam pressure is not high enough. The heater system consists of a heater, which can be gas- or oil-fired, and an air heat exchanger. Between these two components a special food-grade oil or heat transfer fluid, which does not crack at high temperature, is circulated at high speed. The main advantage of hot oil liquid phase is the open pressure-less system. (Fig. 13.1)

Direct heating

Direct gas heaters are only used when the combustion gas can be allowed to come into contact with the product. They are therefore not common in the dairy industry, and in some countries it is even not allowed when the powder is to be used for human consumption. The direct gas heater is cheap, it has high efficiency, and the obtainable temperature can be as high as 2000oC. When a plant is designed with an air heater with direct combustion, it is necessary to calculate the amount of vapour resulting from the combustion (44 mg/kg dry air/°C), as this will increase the humidity in the drying air. The outlet temperature has therefore to be increased in order to compensate for this increase in the humidity and to maintain the relative humidity. This type of air heaters are not used in dairy industry.(Fig. 13.2)

The direct heating causes absorption of certain Combustion products in to the milk powder., especially Nitrous oxide. The level of NOx in the process air after the direct fired natural gas air heater will depend on many variable factors, however, with a well adjusted air heater it should be limited. The specially designed CAX low NOx gas burners shown in the (Fig. 13.3 CAX low NOx gas burners)

Electric air heaters are common on laboratory and pilot spray dryers. The heater has low investment costs, but is expensive in operation and therefore not used in industrial size plants.

13.1.3 Air distribution

The air distribution is one of the most vital points in a spray dryer. There are various systems depending on the plant design and the type of product to be produced. Dryer design falls into three categories: co-current, counter-current and mixed flow. However, as the goal in the dairy industry is to get the best mixture of the hot incoming air and the concentrate droplets in order to obtain a fast evaporation, only co-current dryer design is used.

If the dryer is with a horizontal chamber, the air disperser is arrange like a plenum chamber, and each nozzle will be surrounded by an air stream. However horizontal dryers are not used in dairy industry, the most common is that the air disperser is situated on top of the vertical dryer ceiling, and the atomizing device is placed in the middle of the air disperser thus ensuring an optimal mixing of the air and the atomized droplets. In cylindrical vertical dryers it is also seen that the whole ceiling is perforated plate in order to ensure that the air is cooled by the concentrate. This system, however, makes fines return complicated. It should be noted that an air disperser should have the ability to guide the air and the atomized droplets in the right direction in order to avoid deposits in the drying chamber. (Fig. 13.4)

Two different types of air dispersers are used in spray dryers for food and dairy products. (Fig. 13.5) shows Rotary type ceiling air disperser with adjustable guide vanes. The air enters tangentially into a spiral-shaped distributor housing from where the drying air is led radially and downward over a set of guide vanes for adjustment of the air rotation. This type of air disperser is used for rotary atomizers and nozzle atomizers place in the center of the air disperse. Plug-flow air stream, (Fig. 13.6) The air enters radially through one side and is distributed through an adjustable air guiding arrangement. This type of air disperser is used for nozzle atomizers, where a laminar plug-flow air stream is wanted.

13.2 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

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 °C . 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.

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.

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.

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.

1. 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. Fig. 46. If a total blockage due to deposits on the heat surface occurs, then the cleaning becomes complicated.

2. 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 pre heaters 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 a superheating of the 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 an improved solubility of the produced powder.

Filter

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

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 homogenizer/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.

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.

13.3 Atomization

The aim of atomizing the concentrate is to provide a very large surface, from which the evaporation can take place. The smaller droplets, the bigger surface, the easier evaporation, and a better thermal efficiency of the dryer is obtained. The ideal spray should have same droplet size, to have uniform drying time for all particles and to have an equal moisture content. In practice, however, no atomizing device has yet been designed to produce a completely homogenous spray, although present designs have a high degree of homogeneity. From a powder bulk density point of view a homogenous spray is not wanted, as this would mean powder with low bulk density, and that would mean an increase in packing material.

Function of atomization is:

(i) A high surface to mass ratio resulting in high evaporation rates,
(ii) Production of particles of the desired shape, size and density.

Common types of atomizers

(i) Pressure energy as in pressure nozzles
(ii) Kinetic energy as in two-fluid nozzles
(iii) Centrifugal energy as in rotating discs

(i) Pressure nozzle atomization

The basic function of pressure nozzles is to convert the pressure energy supplied by the high-pressure pump into kinetic energy in form of a thin film, the stability of which is determined by the properties of the liquid such as viscosity, surface tension, density and quantity per unit of time, and by the medium into which the liquid is sprayed.
Most commercially available pressure nozzles, see (Fig. 13.7) & (Fig. 13.8) are designed with a swirl chamber giving the liquid a rotation, so that it will leave the orifice, the second main component of a pressure nozzle, as a hollow cone, In addition to above characteristic design, the obtained spray pattern is a function of the operating pressure. Capacity can usually be assumed directly proportional to the square root of the pressure.

The high-pressure low

Capacity nozzles are mainly used in box dryers and operate at a high pressure 300-400 bar g. Each nozzle will have a capacity of 50-150 kg concentrate, usually with only 40-42% solids, if a reasonable solubility should be maintained in the powder. The actual plant will therefore be equipped with numerous nozzles, all of which are with very small orifices which get easily blocked. Normally, the powder has high bulk density, but tends to be dusty, as it consists of small particles. Due to the low solids requirements the drying becomes at the same time expensive.

The low-pressure high-capacity nozzles with a capacity of up to 1000-1500 kg/h have gained more and more use after the development of the two-stage drying process, where the particle temperature is much lower. The solids content can therefore be increased to 48% and the pressure decreased (150-200 bar g) without affecting the solubility, thus making nozzle atomization interesting also from an economical point of view.

The advantages of pressure nozzles can be summarized as follows:

• Powder with low occluded air
• Powder with high bulk density
• Improved flowability, especially in whole milk
• Tendency to give less deposits in the drying chamber when difficult products are produced
• Ability to produce big particles

Two-fluid nozzle or pneumatic atomization

The energy available for atomization in two-fluid atomizers is independent of liquid flow and pressure. The necessary energy (kinetic) is supplied by compressed air. The atomization is created due to high frictional shearing forces between the liquid surface and the air having a high velocity even at sonic velocities and sometimes rotated to obtain maximum atomization. (Fig. 13.9) and (Fig. 13.10). Two-fluid atomization is the only successful nozzle method of producing very small particles, especially from highly viscous liquids.

Rotary atomization

In rotary atomizers (Fig 13.11) the liquid is continuously accelerated to the wheel edge by centrifugal forces, produced by the rotation of the wheel. The liquid is distributed centrally and then extends over the wheel surface in a thin sheet, discharged at high speed at the periphery of the wheel. The degree of atomization depends upon peripheral speed, properties of the liquid, and feed rate. (Fig. 13.12 Atomizing disc with curved vanes)

The wheel should be designed, so that it will bring the liquid up to the peripheral speed prior to the disengagement. Very often the wheels are therefore with vanes of different design to prevent liquid slippage over the internal surface in the wheel. The vanes also concentrate the liquid at the disc edge, producing there a liquid film analogous to the one considered in pressure nozzles.
In spite of intensive investigations into the mechanism of atomization from rotating atomizer wheels, the prediction of spray characteristics still remains uncertain. The effect of individual variables has been established over a limited range and there is only a few dealing with high capacity, high speed industrial atomizers.

The peripheral speed is widely accepted as the main variable for adjustment of a specified droplet size. However, it has been shown that droplet size does not necessarily remain constant, if equal peripheral speeds are produced in wheel designs of various diameter and speed combinations, and there is a tendency that bigger wheels produce bigger particles all other things being equal. However, in the choice of wheel diameter one should rather look at the reliability of the atomizer, as the differences in spray characteristics are negligible. Further, smaller wheels are easier to handle when cleaned.

The rotary atomizer has been known and used in the dairy industry for many years, the main advantages are:

• Flexibility as to through-put
• Ability to handle large quantities
• Ability to handle highly viscous concentrates
• Different wheel designs giving different powder characteristics
• Ability to handle products with crystals
• Higher solids content in the feed is possible, therefore better economy

The use of pressure nozzle or rotary wheel is affect the physical properties of final product.

13.4 Drying Chamber

All the drying chambers are intended to achieve complete mixing of the milk droplets with the hot air, followed by as rapid drying as possible in a space of reasonable dimensions without heat degradation and unwanted wall deposits. Product discharge must be continuous and the method of discharge conducive to the desired form of dried product. Drying chambers are designed to discharge the majority of the product at the base or to convey all the product with the exhaust air to a product separation and recovery unit. The drying chamber may be of different designs. (Fig. 13.13)

The drying chamber is the most space demanding part of the dryer and requires heavy investment to house a large drying chamber. The drying chambers can be designed to erect in the open space without a building. The insulation of the chamber is increased from 100 mm to 200 mm. The insulation is provided by cladding of plastic coated, mild steel sheets. The cladding is extended downwards to cover the support for the chamber and upwards to cover a light steel structure on the top of the chamber to accommodate the atomizer.