One of the most important aspects of a Ready to Eat (RTE) food facility is cleanliness. Product handling procedures, methodical consistent clean up, and good construction details are of tremendous value in avoiding any type of infestation or product contamination. In addition, any Balance of Plant (BOP) system that may have food contact, or the ability to spread bacteria, mold, moisture or product fines etc. requires attention from the engineers and designers of the facility. One of the primary BOP systems that demands thoughtful design is the HVAC systems. Airborne particles of food, spores, bacteria, yeast, etc. can initiate or spread contamination quickly. The Air Handling Systems themselves can also be a place where bacteria can grow and be spread in the internals of the Air Handling Units (AHU) and ductwork.
Outdoor Air Handling and Filtration
The first step in HVAC system design is to prevent the intake of bacteria and other contaminants from the outside into the plant. Outdoor air is abounding in bacteria, pollens, spores and dusts which can initiate contamination or provide growth sites. Filtering all incoming air can remove the vast majority of invasive particulate. However, even if all the air handling intakes are well filtered, there can still be a major issue if the overall plant is negatively pressurized. Many food plants have more exhaust than make-up air, and any opening, especially doorways will draw outdoor air into the plant where it will not be filtered.
The following is a table with a short list of contaminants and products that can become a core site for bacterial growth, and their sizes.
Sizes of Various Particles in Air
(1000 microns = 1 millimeter)
|Particle Type||Size (microns)|
|Milled Flour or Corn||1-100|
As one can see, with the exception of viruses and fine sugars, most of these potential offenders can be largely filtered out. Standard design of most filtration systems use a rough filter followed by a fine filter. Rough filters should be rated for MERV 7 or 8 (which remove 70% of the particles in the 3-10-micron range) while a good final filter that will take out most bacteria would be a minimum of a MERV 15 (which removes 85% of the 0.3-1-micron particles and over 90% in the 1-10-micron range). A MERV 16 or MERV 17 (HEPA) filter will improve the efficiency of removing the smallest particles (HEPA filters are rated to remove 99.97% of 0.3-micron particles and above.) There are various charts available that compare the filtration efficiency and particle size.
Another tip is to require filters have the “A” rating, such as MERV 15A. MERV ratings dissipate over the life of the filter if it does not have the “A” rating. This rating is explained in ASHRAE 52.2 appendix J.
Filters also need to be kept dry, so a good mist eliminator may be desired. Additionally, regularly checking and replacing the filters is a must to maintain filter efficiency and acceptable pressure drop across the filters.
The air turnover rate also requires attention from the designer of the HVAC system. The higher the air change rate the more the air passes through the filters and thus the cleaner the air. However, larger fans and equipment will increase the capital cost. Most areas will require 4-6 air changes minimum, with cleaner areas using up to 20 air changes. Higher turnover rates usually use return air so that energy use does not become excessive.
Positive pressurization of the entire plant sounds straight forward but each individual space needs to be reviewed for pressure requirements. It is advantageous to make the wet areas of the plant with slight negative pressure to keep humidity from migrating into adjacent areas. So, any doors or openings to the outside into a wet area may require a vestibule or some type of air curtain. In addition, the most hygienic rooms in the facility, such as a packaging area where there may be open product after a kill step, will require the highest positive pressure, whereas in other sensitive areas it may still be advantageous to have a slight positive pressure. The overall pressurization of the facility should be looked at in a comprehensive way and in a room by room relationship basis, otherwise there could be the possibility of spreading contaminants from a less clean or Basic Hygiene area to a High Hygiene area.
There are also many situations where Transition Rooms are also a necessity. Even if one has provided the High Hygiene area with positive pressure, a frequently opened door or permanent open passageway directly into a less hygienic area may still allow for migration of air from the less Hygienic to the High Hygiene area. Transition rooms, while requiring clothes and shoe changes etc. should also provide the proper air pressure and filtration, forcing air out towards the less Hygienic areas will provide a buffer zone compared with a single door or passageway.
Good Manufacturing Practice (GMP)
There are many general HVAC system design and construction issues to consider when designing and specifying the equipment and ductwork. Good Manufacturing Practice necessitates no small cracks or crevices in construction, and cleanable surfaces in and around food process areas.
What does this mean for the internal liner of the HVAC unit, where there is air passing through continuously? Do the internals of the HVAC unit require washdown? One factor is whether the unit uses 100% outside air or if there is a considerable flow of return air. If there is return air, while it saves the cost of energy which may allow for more air turnover and filtration, it also will allow for higher possibility of spreading contamination from one area to another. If there is minimal-to-no return air, the risk of spreading a contaminant from one room to another is reduced, but it will reduce the amount of filtration and increase the energy spent conditioning the air.
High quality HVAC systems for food plants incorporate stainless liners with welded seams and rounded corners (no cracks) with the floors sloped to drain. This allows for periodic washdown. Regardless of the type of liner, HVAC unit construction should allow for sloped drain pans to collect condensate so that there is no possibility for water to stand or accumulate within the unit and become a site for bacterial growth to occur.
HVAC unit coils should also limit the number of fins per inch, so the coils are cleanable and/or UV lighting can be employed to keep wet surfaces on cooling coils sterile.
In general ductwork within a processing area should be avoided. Where possible, route ducts on roofs or outside the main processing areas and penetrate the process area with only the supply discharge or return grilles. Provide ductwork with doors that allow for cleaning of the inside of the duct.
Other considerations include, using round duct to avoid dust shelf, and even consider filters at return inlets to keep the duct from accumulating food debris or dust.
Air Conditioning and Humidity Control
Air conditioning the process areas may be a good option to reduce contamination risk, because a colder environment is less likely to breed organisms. Keeping a space in the 65°F-75°F range (or even cooler) is less likely to be a breeding ground for bacteria than say a humid 90°F environment. Obviously, there are energy costs to consider and if the decision is made to condition the space, the cooling coil and resulting condensate must be drained properly and immediately (avoid stagnant water). Here, UV lights are a good option for preventing growth on wet coils or in drain pans. Some situations may call for controlling humidity when air conditioning temperatures in the process area may not be required or even desired. In that case using desiccants or using a cooling coil to reduce the humidity of a portion of the air-stream then remixing may be good alternatives to completely air conditioning the room.
The controls and operating schedule of the HVAC systems also need consideration. Airflow should be continuous, so not to allow for stagnant conditions that will serve to multiply bacteria. If there are unoccupied periods where production is shut down, how to operate the air handling system during these periods needs careful consideration.
DX cooling circuits, if over-sized, or during low load conditions, may cycle off when the space temperature is satisfied allowing the humidity to increase. Using a cooling coil followed by reheat such as Hot gas reheat on a DX system, is a good option to mitigate this issue.
In wet process areas that require hose down, HVAC units can be outfitted with a Clean-up mode that introduces additional fresh air and exhaust during wash down periods to help avoid too much mist in the air for extended periods. Remote panels with timers can automatically initiate clean-up mode per operator needs then resume normal operation when timed out.
There are many ways to provide good hygienic HVAC to a food plant through good filtration, fresh outside air, and proper design. The above ideas need to be thoroughly considered and reviewed with the client’s GMP procedures and process to ensure a sanitary design and ultimately a clean RTE product.
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