Air Heating Systems Of Dairy Plant- Dairy Engineering-Technical Superintendant Milma

Contents

Air Heating Systems of Dairy Plant

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.

1. Hot Air System and Air Distribution

1.1 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.

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 10oC
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 passages.

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 175-250oC 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 irculated at high speed.

The main advantage of hot oil liquid phase is the open pressure-less system.

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/oC), 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

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

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.

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 arranged 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.

Two different types of air dispersers are used in spray dryers for food and dairy products:

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,

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.

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