The conditioning of air in a food facility is critical to maintain a hygienic operation and food safe environment. Air flow throughout a food facility requires air handling systems designed to provide the proper environment based on the temperature and filtration needs of each area they serve. Temperature control and filtration are important not only for the safety of food products, but also for employee comfort and productivity.
When air is removed from a space in a facility and replaced with outdoor air, this is considered a ventilation system. The replacement air, called make-up air, should be filtered and may also be treated for temperature and/or humidity prior to entering the space. Most typical ventilation systems provide only heat treatment of the make-up air, but this is dependent on the application. For example, a space that needs to maintain a specific temperature will need to have make-up air that is cooled in addition to being heated.
When air is recirculated within an area or facility, this is considered an air conditioning system. This system may also bring in outside air. However, the primary function of an air conditioning system is adding or removing heat to the recirculated air. These systems also filter the air in the room through the recirculation process.
A facility ventilation system must balance the air that is removed from the space with air from another space or from outside. If more air is removed (exhausted) from a space than is brought in (make-up air), the space will have a negative air pressure condition. This condition will result in air entering from other spaces. If the air balance for the entire facility has not been considered, this can lead to unconditioned air entering the facility.
In general, good system design should prevent air from uncontrollably entering the facility and each room. To accomplish this, the air flow in the entire facility must be accounted for by considering each space and its air balance relative to each adjacent space. Many times, only the air flow in each room is considered without respect to how it affects the entire facility. Each point of exhaust and supply from the facility must be accounted for so that the building is in proper balance. For example, air balance coordination between high- and low-hygienic areas needs to be planned so air cannot flow from low-hygienic spaces into high-hygienic spaces.
Exhaust fans are very simple components of ventilation systems that remove excessive heat and moisture generated in a facility. They are typically located in the area around a cooking operation where heat, smoke, and steam vapors are generated and in equipment wash areas or washrooms where heat and steam vapors are generated. Exhaust fans are also used to help ventilate areas such as interstitial (attic) spaces, dry warehouses, and mechanical rooms. Air that is exhausted from these fans can be made-up by putting wall louvers in the space or by using supply fans.
Supply fans are used to provide make-up air for areas where treatment of the incoming air temperature is not needed or desired. These fans can also have filters added to them for low-level filtration of the air. However, supply fans typically cannot provide filtration above low levels.
Make-up air units are used to replace the air exhausted through fans. MUAs can heat, cool, and filter air before it is delivered to the space. Units that provide heated air can be direct fired (combustion gas is burned directly in the air stream) or indirect fired units (combustion gas is burned in a heat exchanger).
MUAs that also cool the air come in a variety of configurations—with all configurations having the common design of passing air through a cooling coil. The coil can be cooled using direct expansion halocarbon systems or chilled water systems. In some cases, the units can utilize ammonia as the refrigerant.
Air conditioning systems in food plants are typically used to reduce air temperature and control humidity for processes or products that are affected by it. For example, baking facilities need air conditioning systems to provide a consistent environment so that formulations do not need seasonal adjustments. Dry blending operations are another example of when humidity can affect the flowability and mixing of products. Air conditioning is also important for employee comfort and improved productivity.
Packaged rooftop units are designed as self-contained cooling and heating units. RTUs are placed on a roof with ductwork routed from the unit to serve the space below. The units have a supply air section into which the ductwork is connected to distribute the conditioned air to the space below. Air returns to the unit on the return air side, where it passes through a filter and a cooling coil or heat exchanger for conditioning.
The return air for these units is combined with fresh air from the outside. This combined airstream passes through dampers, filters, and heating/cooling coils (depending on the internal configuration/device type) before entering the room through the supply ductwork. Depending on the unit configuration, air can be completely exhausted from the RTU or recycled for heating/cooling.
The cooling for these units is accomplished using a direct expansion halocarbon system. This system provides cooling in the coil by directly expanding the refrigerant in the coil. The capacity of these units is preset by the amount of refrigerant that is designed to expand in the coil. The temperature of the coil—and thus the temperature of the space—is controlled by turning the refrigerant being fed to the coil on and off. Heating in these units can be accomplished using an electric coil or a gas fired heat exchanger.
A chilled water/glycol system contain two primary components: the chiller unit and the air handling unit. This system uses one or more central chiller units as it cools either water or, typically, a glycol-water solution. The glycol-water mixture leaves the chiller unit, and it is pumped through pipes connected to the air handling units. Within the air handling units, the glycol-water solution passes through cooling coils. Air from the room is directed through the coil and becomes conditioned.
Compared to a RTU system, the coil temperature of a chilled water system is controlled by adjusting the temperature of the glycol-water solution feeding the coil. This allows for greater control over the room humidity, as well as greater diversity in the sizing of the unit. These units can better adapt to the conditions in the room and provide better overall temperature and humidity control. However, chilled water systems require piping, control valves, and pumps in addition to the chiller and air handling units. This may create more system maintenance, but these systems can be more energy efficient (which can offset the additional maintenance costs).