Plant Moisture Control

Once-ignored USDA requirements for condensation control are compelling processors to take a closer look at humidity problems. Also: tips on choosing between desiccant and mechanical dehumidifiers.

By Mike Pehanich, Plant Operations Editor

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Still, many different scenarios can play out within the almost endless mix of variables in food plants. Heating rooms is easy. Getting moisture out can be difficult. The constant generation and movement of hot air and cold air creates countless opportunities for condensation and associated airflow challenges.

Bakeries require vapor barriers to prevent condensation caused largely by oven exhaust and the moisture given off by baked product.

An IQF freezer poses a different challenge, yet the same principles apply. "A nitrogen freezer is a tunnel," says Leavens. "A plant has to exhaust the nitrogen. You must make up for the cubic feet of nitrogen being exhausted."

One guiding principle suggests leaving no cold pipe exposed in an area where warm air can cause condensation to occur. Pipes may require insulation with a PVC covering and a sealed vapor barrier. This is particularly important during washdown. If warm moist air gets to the pipe, it may leak through the insulation.

"Each project must be looked at from the standpoint of temperature requirements, people requirements and humidity requirements," says Leavens.

"On the modern cut floor, the air may be changing every 2-1/2 minutes," says Taylor. "If you have a source of contamination, it will be easy to spread. But if the air is properly filtered, you can scrub the air, filter out bacteria. You are sanitizing the room. So filtration is an important part of psychrometrics, too."

Controlling condensation can be especially difficult in a plant constructed before the department began its crackdown on condensation.

Plants typically have attempted to resolve condensation problems either with fans to draw warm air in or with exhaust fans to pull cold air out. But this creation of negative air pressure also can lead to condensation and assorted problems. In most cases, solutions require some type of dehumidification technology.


Controlling Moisture in the Plant: Rooftop dehumidification unit
Rooftop dehumidification units are an efficient way to remove moisture. Courtesy of the Stellar Group.

"In a refrigerated process area, you need to cool the air, but you also need to have air changes to meet employee requirements," says Leavens. "You need outside air, but it must be chilled air before it enters the room or you will create clouds and condensation and drippage."

Neither fans nor conventional refrigeration units will control humidity. The job generally falls to dehumidifiers, which come in two types: mechanical and desiccant.

Mechanical dehumidifiers set two coils in series, passing cold air first through a cooling coil, then through a reheat coil. "We first want to overcool the air," says Taylor. "The cool air is colder than needed to meet the room space and is at 100 percent humidity. When the air passes through the second coil, it is re-heated five degrees. It comes off at 85 percent humidity (because warm air has greater moisture carrying capability) and is now capable of absorbing moisture once again."

A desiccant dehumidifier takes plant air and runs it over a desiccant wheel, which absorbs much of the moisture from the air. It can bring the relative humidity down as low as 15 percent.

But a desiccant wheel offers its own set of challenges. As the wheel passes through a hot section (ovens, fryers) of the processing plant, it picks up heat. When the hot wheel re-enters the air stream, it passes heat to other sections of the plant, elevating the temperature of ambient air. "You can't put 95-degree air back into the room," says Taylor. "That is why using a desiccant wheel costs more than a mechanical system. Still, it does more dehumidifying than a mechanical system."

Processor No. 1: Controlling pressure

Problem: Psychrometric analysis of a pork processor found negative air pressure in the plant. One part of the plant operated in a cold environment while another part of the plant operated in an ambient condition. The unrefrigerated section drew the cold air from the refrigerated section. Filling the void of the displaced refrigerated air, the plant drew in air from outside the plant, causing condensation to form.

Solution: Engineers prescribed supply fans running on variable frequency drives in the ambient area of the plant. A pressure sensor in the room was able to control air pressure, reducing the need to draw ambient air.

"The solution was simple," says Taylor. "But only because we were able to understand the problem first."

Moral: It's expensive to get rid of condensation, but much easier to control the conditions that cause it.

Processor No. 2: Mechanical dehumidification

Problem: Excessive water use in a poultry evisceration operation created a hostile working environment and uncontrollable condensation. So bad were conditions that USDA was prepared to shut down the plant.

"Plant management wanted to operate the plant at a 75°F temperature during processing, but we determined they would have to dehumidify the plant first so the air could absorb moisture," explains Taylor.

To control condensation, engineers had to either heat the surfaces (of equipment, floor, ceiling and walls) or lower the dew point of the room.

Solution: Heat tape on doors and the bottoms of drain pans helped to elevate surface temperatures. To lower the dew point, the plant explored both mechanical and desiccant dehumidifying systems. Engineers finally decided on a desiccant wheel positioned over the top of the evisceration room and added exhaust fans. The wheel absorbed the moisture, and the exhaust fans drew it out of the plant.

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