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Application of Conditioned Air

When the air has been filtered to the required standard (particulate and microbiological control), heated, cooled and distributed through a duct system, control of the air in the manufacturing space or direct product contact is the next step.

The application of air for food manufacture is divided into two groups namely environmental (room) and process air. A breakdown into specific use groups is illustrated as follows:

Not all these areas may require a supply of conditioned air and the selection is based on the type of food process and the layout of the factory. The choice of air filtration level can vary from one application to another.

Generally process air systems are more critical in the degree of air filtration required, and they are often dedicated to a single application. A combination of environmental and process air requirements may also be serviced with a single air handling and filtration system.

Once the conditioned air enters the manufacturing environment there will be a gradual loss of benefit – cleanliness, temperature, directional flow – due to the operating conditions prevalent within the space. The number and effectiveness of the air changes can be critical in making best use of the supply air. Air entering the manufacturing space over the process with extract behind the operators is a useful consideration. Extract points at wash areas, personnel entry, tray and utensil wash rooms and over dust generating processes will assist in maintaining clean conditions in the manufacturing space. The higher the air change rate of a correctly designed system the better the air quality control. Specific design features to suit various manufacturing layouts have evolved and a minimum of five air changes per hour is a good rule to adopt.

An economical way of overcoming an issue of room air quality control is to isolate the "high risk" air requirement, thus limiting the amount of highly cleaned air required. This is now a process air system and is referred to as a controlled or mini-environment which is discussed later. Tertiary air quality control remote from the process may be required, especially if the supply environmental air is also to be used in less critical areas. Total control of a contained space is easier to monitor - personnel and other factors which disrupt the controlled space are reduced if not eliminated. Fan assisted terminal systems which take air from the general pre-cleaned environment are also an effective way of installing an isolated facility. Care must be taken to ensure that the air filtration system within the process space is not subjected to a high level of airborne contamination.

For non "high care" requirements the use of good quality air filtration with efficient ceiling diffusers is usually sufficient to ensure acceptable room cleanliness. A positive room air movement condition is important for most types of food manufacturing facilities, to prevent atmospheric pollution entering the working space from the environment.

Process air systems, where the treated air is delivered directly to the food product are usually of the "high risk" type. Examples are spray and fluidised bed dryers, pneumatic conveying, compressed air supply, tank venting and some tunnel cooling systems.

Although it is technically correct to operate a process air system under pressure with the final filters installed after the fan, this is not always possible, especially with high pressure blower systems. In this case the risks should be assessed and necessary design changes implemented.

When the correct air movement system is in place, it is time to consider the control of the food manufacture environment as part of the factory layout. Positive air movement from critical food process rooms can only be maintained if air loss restrictions are installed and other access (if any) to the food process rooms is carefully controlled. Operating personnel may well require specially cleaned sterile garments and, although careful air control in the process space may help, a "clean" (air quality controlled) working condition can easily be lost if procedures are not strictly enforced. Air passing to low care should be kept to a minimum on the basis of at least 1.0 - 1.5m/s at openings. Adding up the fixed open area access points from the manufacturing room and multiplying the nett area by (say) 1.5 will give you the air loss volume which must be replaced with outside air at the air handling system. It is expensive to cool this air and loss of air should be minimised. Differential pressure is a result of air movement at openings, and a robust air movement of, say, 1.5m/s is usually more important than a high differential pressure, in view of the cost of maintaining, for instance, a 15 pascal condition which would require a velocity (air speed) at openings of 5m/s.

Temperature and humidity control are two areas where operating problems often occur, many of which cannot be fully appreciated at the factory design stage. Correct thermal barriers and access control to high and low temperature areas are most important. Air volume at the correct temperature and direction across the process area is often the answer to condensation problems. In fact air movement is the key to the majority of complaints about variations in temperature and humidity. At the design stage therefore an additional 10% - 15% capacity in the air supply system can be most useful for balancing out these problems.