In the food processing industry, enough ventilation must be supplied to handle steam, odours and noxious vapours. A safe and comfortable working environment must be coupled with care not to contaminate food, food-contact surfaces or packaging materials. Indeed, unfiltered air, poor HVAC (heating, ventilation and air conditioning) construction and installation practices and negative pressure in food-production processes have been found to be major sources of pathogenic contamination. HVAC systems that are difficult to clean have been found to harbour Listeria and other pathogens. The correct placement of air-blowing equipment, exhaust and intake vents and ducting can help minimize these risks.

An ideal ventilation system for food processing applications will

• Purify air by filtering;
• Maintain appropriate humidity and temperature;
• Keep ducting accessible, but out of the way of processing operations;
• Pressurize the conditioned space;
• Prevent contamination and be easy to clean;
• Supply sufficient volumes of air where needed.

Temperature and Humidity

Cooling and dehumidification are two important elements of the ventilation system. Excessive relative humidity can cause condensation and uncomfortable working conditions, and lead to mould and bacterial growth which tend to cause production losses, poor product quality and increased equipment maintenance costs. Food processing operations such as washing, cooking and cleaning often use a good deal of water that will increase the humidity level. In cold rooms, fog often develops during cleaning because of the use of hot water. This can present a danger to the cleaning staff and results in heavy condensation on surfaces.

The HVAC system may use up to 35% of the energy in many operations, and dehumidification may take up over half this energy. If a refrigerative system is used, proper handling of the condensate is necessary to prevent bacterial and fungal growth. Air reheating might also be necessary. Desiccant wheel systems are another alternative. A proper HVAC system equipped with heating equipment (to be used before cleaning) or desiccant dryers can prevent condensation and reduce cleaning times.

Establishing Appropriate Pressure

In general, positive air pressure zones should be maintained, with the highest positive air pressure in the zone where the product will be exposed to open air for the last time (packaging), typically where packaging and sealing happens and right after the kill step. In the absence of a kill step, this zone will include everything from washing and trimming up to packaging. Progressively lower pressures will be maintained in subsequent and previous stages of processing and handling. This system maintains a hierarchy of contamination risk, with the areas in which the properties of the product are finalised having the lowest risk.

When this system is properly designed, persons entering the building will distinctly feel the airstream, and air will move from high to low pressure zones at roughly 100 m/min. Airflow from contaminated to clean areas will be unlikely. It is especially important for ready to eat (RTE) or near-RTE products. Dust collection systems also benefit from establishing positive pressure zones.

Excessive exhaust fan capacity and use can create negative pressure in food processing plants, which leads to contaminants entering the space whenever there is an opening such as an open door, window, or even a crevice. Without control over the influx of airborne microorganisms, dust and vapours, hygiene and equipment maintenance becomes very difficult and the final product quality may suffer as a result.

Makeup air drawn from outside can be used to establish positive pressure. It is recommended that the air drawn in should exceed the exhausted air by 10% to achieve this. Makeup air and recirculated and purified air from inside the facility can be mixed.

Air Exchange

Air changes should be high enough to remove unwanted steam, odours and other contaminants by filtering while maintaining the proper temperature and humidity conditions. The standards for RTE product processing plants have risen steadily, and today they may keep up more than 25 air changes per hour, with 20 being common. The precise number depends on the processing operations that are taking place and needs to be customised for the factory and process by an experienced ventilation engineer. Stagnant areas can collect fumes, dust and contaminants, so the ventilation system must be set up to avoid them. Where construction is being done on an existing plant, this is especially important, as fumes and oil can affect workers, product and materials.

System Design

Ductwork design is a critical element of the HVAC system. It should preferably be installed outside processing areas and be easy to access for cleaning and maintenance, for example on a walk-on ceiling. Ducting suspended over processing areas is also possible. In this case, round ductwork (that will gather less dust on top) or ducts closely fitted to the ceiling should be installed. Access doors are needed to allow proper cleaning of the inside.

Exhaust stacks should be higher than inlets, and downwind where possible. In any event, the exhaust that reaches inlet ducts should be minimized, and intake vents should also be placed away from potential sources of contamination. While this may seem obvious, it is often neglected in renovating a building, leading to considerable costs. Exhaust stacks also need hoods to keep rain and other precipitation out and prevent unwanted draughts, as well as insect screens and a self-closing damper for when it is not operating.

Insulated ducts must have closed-cell insulation sealed in a double wall. Open insulation must be avoided, as it traps condensation and contaminants, and, being impossible to clean, will spread it through the building. Periodic testing should be done on the ducts and filtration system to ensure cleanliness.

The amount of filtration required depends on the products processed. Recirculated air is often mixed with make-up air in the HVAC unit, then filtered, conditioned and filtered again. The final filtering needs to be very efficient for sensitive ready-to-eat products, with the ability to remove 99% of particles of 1 micron or even smaller. Intakes in addition to the HVAC unit are typically located on the roof, and should also be filtered adequately.