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For this explanation, consider a theoretical area of the plant to help paint a picture of controlling plant air pressure. Figure 1 shows a single production area, Zone A, with another space right next to it, Zone B. To ensure Zone A stays pressurized, a makeup air unit supplying 1,000 CFM into the room while the exhaust fan is removing 800 CFM. In theory, Zone A should always remain under constant pressure until you consider the opening and closing of the doors. As the rollup door between Zone A and Zone B opens, the air pressure in Zone A briefly falls as the pressure is pushed to Zone B. Once the volume of Zone B matches that of Zone A, both will briefly be in a positive pressure and Zone A will reach its intended pressure setting. Understand that the construction of the building allows it to “breathe” as the arrow on the left of the diagram shows. Having a completely airtight building is not only not feasible, but also not desirable. Construction material expands and contracts like any other materials. As such, there will inevitably be infiltration some engineers call “infiltration.” To give you an idea of how much air can escape due to open doors, imagine the roll up door between Zone A and Zone B is a 10 x 10-ft opening and the air pressure is set for 0.05 in. wc. That doesn’t sound like much pressure at all, but at that pressure, and the size of the opening, we’re approaching some 80,000 cfm of air movement from the higher pressure to the lower. As pressure begins to equalize, that volume slows down considerably. The measure 0.05 in. wc is on the high end of the pressure range so this was just to give you an example. The goal should be to achieve enough air pressure to reach the desired air balance at the lowest pressure possible. One extremely cheap way of checking to see if air is flowing the right way is to use a hair net while standing in the doorway. The direction the hair net moves is the direction of the airflow. Ideally, a digital pressure gauge or “micromanometer” would be used, which measures pressure differ ences. MEASURING INEFFICIENCY It is one thing to calculate how much energy is required to cool or freeze a given area that is enclosed. It is something else entirely to calculate not only that cost, but the additional cost of treating the air,

Figure 1 Source: The University of Wisconsin, Madison, Industrial Refrigeration Consortium “The Cold Front,” Vol. 5, No. 4, 2005.

whether heated or cooled, to the condi tions desired by using outside air to help pressurize the building/plant. These costs vary depending on the: temperature and humidity; seasons of the year; climate of the area where the plant is located; and the efficiency of the plant itself. Other factors may include environmental remediation such as the storage of dry ice pellets. Dry ice is comprised of solid CO 2 and is formed at roughly -190°F. Some believe that storing dry ice pellets in containers in a cooler or freezer will not have any negative impacts. However, without mitigation, CO 2 levels can quickly reach hazardous levels as the solid CO 2 begins to gas off at any temperature above its freeze point. Remediation is typically accomplished using an RMAU where the outside air damper is not used for pressure so much as it is modulated by active CO 2 readings in the space. The higher the CO 2 , the more outside air is used to dilute the concentration of CO 2 . In other cases, the use of peracetic acid (PAA), an organic peroxide, is used for antimicrobial intervention in the form of dip stations where product is dipped into the solution (PAA and water) or used in the main bird chillers. Because PAA is an oxidizer, it gases off much like chlorine. PAA, however, has an extremely low threshold in airborne concentrations of less than 1 ppm. Rare is the case, when this chemical is introduced to a process, that air balance is a consideration during the upfront costs of implementation. Concen trations of PAA vary in the water from day to day based upon several factors but it should be noted that the higher the

concentration of PAA in the water, the more it will gas off into the air. Communi cation between operations and refrigera tion (or whoever takes care of air balance) is paramount to success. A simple heads up of “hey, we’re increasing PAA today,” or something along those lines, goes a long way. This would give refrigeration a heads up that airborne PAA may become an issue, and they can plan to increase air pressure to dilute the PAA to safe levels. There are also processing plants that also have dip stations or spray applications. Air balance is much like any safety program in that one of the very first things to understand is what hazards exist and how to properly mitigate those risks whether it is temperature, condensation potentials, chemical hazards, etc. Therefore, an opportunity exists to build relationships with quality assurance, maintenance and operations to identify hazards before they become an issue, put a plan in place and execute those plans. One way to do this could be to perform a “management of change” when any new equipment is proposed for installation, changes to line layout, plant expansion, or any other change that “could” impact ventilation or refrigeration to have these discussions early on. As they say, two sets of eyes are better than one to make sure we are covering all details. This feature was an abstract originally prepared for the RETA National Confer ence 2024. Victor Dearman, CIRO, CRST is Director of Process Safety Management for the Beef Division of JBS USA in Three Rivers, MI.

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