Air handling in food manufacturing factories is quite different from air handling in commercial or residential environments. Exposed food products within a manufacturing room require consideration of food safety and quality parameters for the air in the room.

This article will focus on key hygienic evaluation points of an air-handling unit (AHU) and the associated ductwork supplying air to rooms where exposed food products are processed. These key points include risk assessment and verification, hygienic design of AHUs and the associated ductwork, and several important food safety and quality parameters to consider for the air supplied to these rooms. Not discussed in this article are the engineering aspects of sizing, specifications, and capacities of AHUs, ceiling-mounted room refrigeration units, the preventive maintenance of AHUs beyond cleaning and filter changes, AHUs used in nonexposed food product processing areas, air used for product conveyance, or compressed air generation and distribution systems.

Risk Assessment and Verification
The factory’s Hazard Analysis and Critical Control Points (HACCP) risk assessment should define risks to product safety. The factory’s quality system should identify risks to product quality. When capabilities of the AHU can positively influence any of the identified risks, then these capabilities become the basis for the design and maintenance criteria of the AHU. The factory’s hygienic zoning risk assessment should evaluate the adequacy of the AHU system design and maintenance program. Finally, the factory’s internal auditing program should verify the execution of the air-handling system’s maintenance.

The factory’s verification programs should include whether air filter changes are occurring as scheduled. Likewise, the cleaning of AHUs and the associated ductwork must be verified. As with any cleaning activity affecting food safety and quality, these cleaning procedures must be written, proper training of associates performing the tasks must be documented, and the completed work must be properly recorded.

Hygienic Design of AHUs and Associated Ductwork
The engineering, installation, and maintenance of factory AHUs and associated ductwork should be aligned with the risk assessment and verification activities. Identified gaps must be documented and appropriately corrected.

The primary principles of hygienic design, or sanitary design, of food manufacturing equipment also apply to AHUs and the associated ductwork. These principles include cleanable surfaces, accessibility for cleaning and inspection, minimizing of hollow bodies, absence of areas in which accumulations occur, and compatibility of construction materials with product and cleaning compounds. Within the following discussion of these principles, a number of best practices will be shared.

Cleanable Surfaces
At some frequency, all surfaces of AHUs and the associated ductwork must be cleaned. The frequency will depend on conditions specific to the factory, the particular room associated with the AHU, and the risks identified in the HACCP and zoning risk assessments. These cleaning tasks should have documented and detailed procedures, a defined frequency, and adequately trained associates performing the tasks.

Surfaces within the AHUs and associated ductwork should be smooth and nonabsorbent. Exposed fiber insulation, foam insulation, and other such noncleanable surface materials must be avoided for adequate cleaning to occur. Ideally, noncontinuous welds and lap joints should be minimized to avoid harborages for foreign material and microorganisms. The amount of ductwork should be minimized. This will reduce cost of installation and maintenance.

Accessibility for Cleaning and Inspection
The AHU can be located on the roof, in an interstitial space between the production room and the roof, or in an area adjacent to the room. Roof locations are least desirable due to contaminants present as well as snow, ice, and rain. The ease and convenience of accessibility for cleaning and inspection have a direct relationship to efficiency and effectiveness of the process. Employee safety is of utmost importance and must be considered.

The ductwork may be metal or fabric. Without access doors appropriately spaced along the length of the ductwork, the interior surfaces of metal ducts are difficult to clean and inspect. If access doors do not exist, they will need to be installed. These installations create metal shavings and sharp edges that need to be corrected. Finally, cleaning and inspection of these ducts require extremely important safety considerations related to working at heights.

Ductwork is either supplying air to a space from an AHU or returning air from the space to the AHU. The air passing through the supply ducts has been filtered, so these ducts typically remain cleaner than the return ducts. Return ducts will collect airborne contaminants from the room over time. The more dust, fumes, smoke, etc. generated in a room, the more accumulation will occur inside the return ducts.

Fabric ducts offer several advantages over metal ductwork. Foremost, they can be easily taken down and laundered. However, in wet-cleaned environments, they have a tendency to show spots when oversprayed water droplets mix with dust that may be present in or on the duct. Fabric ducts minimize or eliminate drafts that are normally associated with individual vents in metal ducts. Fabric ducts may have vents incorporated into their design or have no vents at all, and the entire duct diffuses the air through the fabric.

In rooms with low ceilings, fabric ducts may not be appropriate, as they must be suspended from a ceiling and possibly cause overhead clearance issues. Metal ducts can be installed above the ceiling, very close the ceiling, or have a single vent blowing air a long distance.

Finally, metal ducts should not be painted when installed in environments that are wet-cleaned. In these conditions, peeling paint and corrosion are likely to occur.

Absence of Hollow Bodies
Ideally, framework inside and outside the AHUs and associated ductwork should not contain hollow bodies. These are typically square or round frameworks that support the AHU or components such as the blower, filters, maintenance enclosures, etc. Suitable alternatives include solid square or round stock. Another alternative is angled stock mounted diagonally with the 90-degree angle pointed upward. Threaded rods, also called “all-thread,” should always be avoided as they are not easily cleanable.

Absence of Areas Where Accumulations Occur
Of particular concern is the condensation drain system underneath cooling coils. Mold growth and insect activity are not unusual in drip pans and drainage lines. When properly sloped, these condensation drain systems will be less likely to cause hygiene problems. Accessibility to clean the pans is important, as is proper condensate piping so pans drain without holding water.

Other areas of accumulation include the top surfaces of the AHU and metal ductwork. These horizontal surfaces can be properly sloped so accumulations are minimized. In some instances, the areas underneath the AHU and the associated ductwork are prone to accumulations of condensation and other materials. Clearance, accessibility, and proper slope will make cleaning and inspection much easier.

Compatibility of Construction Materials with Food Product and Cleaning Compounds
Engineering, quality, the AHU and ductwork manufacturer, and the factory’s cleaning chemical supplier can arrive at the correct construction materials during the design phase of the system. This evaluation will need to consider the food product type as well as the cleaning chemistry, methods, frequency, and any safety considerations. These resources must be involved in creating the cleaning procedures to ensure effective, efficient methods achieve optimal results and damage does not occur to the AHU system.

One important consideration for hygienic AHUs and ductwork includes the physical location of AHUs. Systems that supply air to production areas where there is exposed product are best located inside the building or within an enclosed structure located on the roof of the factory. AHUs exposed to the outdoor elements tend to experience more frequent hygiene failures.

Another positive hygienic design is the use of fabric ducting as opposed to rigid ducts. Fabric ducts are less expensive to install and to keep clean. Retrofitting these fabric ducts into existing systems may be cost-effective compared with routine cleaning of metal ducts. The design of fabric ducts may eliminate isolated drafts. Finally, the ease of customizing the duct location within the room serves to create more thorough air exchanges within the production area.

Ductwork installed directly over an exposed product line should be avoided. Condensation and other contaminants could pose a risk to exposed product below the ducts.

Avoid “shortcutting” the supply and return of air by having too short a distance between the two ducts. This is common in roof-mounted systems that simply have straight drops of rigid ducting into the manufacturing space below. Because of the short distance between the supply and the return ducts, much of the air supplied to the room simply travels the short distance to the return duct and does not disperse or circulate throughout the entire room.

Air Parameters for Consideration
If airborne contaminants are identified as a potential risk to product safety and/or quality, then the filtration level must be adequate to control the identified risk. The engineering group at a factory, as well as the AHU and filter suppliers, must help the factory make the best possible decision for the level of air filtration needed based on the specific AHU capability and the identified risk to be controlled.

The most common rating systems for air filters are the European filter class standard (EN779), known by its use of “G” and “F” ratings, and the American filter class standard (ASHRAE 52.2-1999), known by its use of “MERV” ratings. Air filters range from very coarse to very fine. Many AHUs can utilize a coarse prefilter followed by a fine secondary filter. Coarse prefilters can capture insects and many types of foreign materials before they reach the fine secondary filters. These configurations can greatly reduce cost by extending the life of fine secondary filters.

One may think that the finer the filtration, the better the air quality will be within the room. This is not always the case. Well-meaning people may insist on installing finer filters to improve air quality in a space without understanding that finer filtration causes more resistance to the airflow passing through the filters and the AHU. Airflow through the AHU is measured in cubic feet per minute and is determined by the capacity of the fan inside the AHU and the resistance of the recommended filters. A manufacturing room has a set number of cubic feet of space. Therefore, if fewer cubic feet per minute can pass through the AHU because of restriction caused by a finer filter, the fewer times the cubic feet of air within the room will pass through the filters in the AHU. If airborne contaminants are generated in a space faster than they can be removed by air exchanges through the filters, then those airborne contaminants will begin to accumulate. In this case, a finer filter than recommended for the fan capacity may actually create less clean air.

To maintain the air quality within the room, the frequency of the filter changes must be determined. The appropriate frequency will depend on variables such as whether prefilters are utilized as well as the amount of airborne contaminants trapped by the filters. In some AHUs, air pressure monitors are installed inside the AHU. Filter changes are due when air pressure reaches a defined point.

Airflow restriction caused by waiting too long to change the filters can have two possible results. First, the filter may deform and bend inward. This will pull the edges of the filter away from the mounting frame and allow airflow to follow the path of least resistance around the filter instead of through it. When this happens, unfiltered air will enter the production room. Second, the restriction in airflow will reduce the air exchange in the room and airborne contaminants within the room may accumulate as discussed above.

Well-engineered filter holders are very important. This is a common deficiency in AHUs as poorly engineered holders may allow air to bypass the filter panels.

Be mindful of the possibility of unauthorized substitutions of filters with a different level of filtration. This change could happen for any number of reasons such as availability, cost savings, inattention, etc. Changes to filtration levels must be controlled and reviewed appropriately at the factory.

Temperature Set Points
Temperature set points will not be discussed in detail, as these are highly specific to food product types as well as regional and seasonal influences. If temperature control in a food processing area is necessary for food safety, it should be identified in a factory’s HACCP plan and controlled appropriately for the risks identified. Regulatory requirements may also need to be considered. Cooler temperatures in a manufacturing space will slow the growth of microorganisms within that space. Temperature may also have an influence on the quality of specific products. Finally, temperature within a space affects employee comfort. All these considerations may require specific controls.

Humidity Set Points
Humidity control should be considered for product safety, product quality, and product handling when caking and clumping is a concern. These parameters will be specific to the product type and should be addressed in the product development process and the associated engineering studies.

The problems of condensation formation, mold growth, and slower drying time after wet-cleaning processes are problems that can also be improved through humidity control. A humid environment can also be uncomfortable for employees.

Air Balance
In general, the highest air pressure in a food manufacturing factory should be supplied to the area with the most-sensitive exposed product. Progressively lower pressures should be supplied to the lower-risk areas within the factory. The lowest pressure should be outside of the factory. The higher air pressure should be sufficient to keep airborne contaminants from entering from adjacent lower-pressure areas. These pressure differences throughout a factory are referred to as “air balance.” The factory’s engineering group and AHU supplier can make recommendations to achieve and maintain this air balance.

Once air balance is achieved, maintaining it is important. The easiest strategy is simply keeping doors closed throughout the factory. Also very helpful are dual interlocked doors, also known as air locks, wherein two doors with a vestibule between them are installed at a room access point and both doors cannot be open at the same time.

Air Exchanges
The number of times air in a room is circulated through an AHU in a given time is called air exchange. As described above, this air exchange becomes critical for many of the air parameter considerations. If heat, cold, dust, smoke, fumes, etc. are being generated in a room, the number of air exchanges must be sufficient for the AHU to hold the chosen air parameters within the established range. The established range must be agreed upon by the factory stakeholders as well as be compliant with applicable regulations.

Air exchanges can also be considered as fresh air intake into a room versus air exhausted from a room. This particular air exchange is exceptionally helpful in exhausting warm, moist, and steamy air during wash-down activities in wet-cleaned rooms. The exhausting of this air reduces conditions affecting employee safety, condensation formation, and excessive drying times after wet cleaning.

Air handling in a manufacturing factory where exposed food products are present is very different from air handling in any other commercial or residential environment. Air in these exposed food product environments must be controlled according to parameters determined through a risk assessment. Once the important controls are determined, the AHUs and associated ductwork can be designed, maintained, and cleaned in the most appropriate way to achieve the desired control. Finally, verification activities will ensure that the controls are maintained to the standards set by the factory stakeholders.   

Duane Grassmann is the corporate hygiene manager at Nestlé USA. Over the past 40 years, Duane has made food manufacturing sanitation and hygiene his primary focus. In his career, he has worked with countless factories in the U.S. and Canada, and has significant hygiene-related involvement in hundreds of factory engineering projects. In the last 11 years, he has held corporate roles at Nestlé USA and Schwan’s Company. Prior to these corporate positions, he worked for 30 years with numerous companies in various sanitation management positions.