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Plant Moisture Control

June 12, 2006
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.

Water, water everywhere. That's the normal condition of a refrigerated processing facility. If it's not in puddles or beaded on pipes, it's thick in the air.

At least, that often was the case before USDA started taking its own rules on condensation control seriously. "Until five years ago, the [focus and] emphasis for refrigeration engineers was only on temperature," notes Chuck Taylor, senior vice president of The Stellar Group (www.thestellargroup.com), Jacksonville, Fla. "That has changed. Today, USDA will shut down a plant for condensation problems."

For several years now, USDA has been enforcing long-standing but seldom observed rules on condensation control in refrigerated facilities.

Why the sudden change? In the past, enforcement had been impractical because moisture in many food plants, particularly refrigerated facilities, was extremely difficult to manage.

"We are literally trying to defy the laws of physics in processing plants by trying to maintain a cold room and, at the same time, control a big latent moisture load," explains Taylor.

Sanitation often is a primary source of moisture problems. After two eight-hour processing shifts in a cold environment, hot water is bound to upset the moisture balance. Photo courtesy of Tornado Industries.

But food safety is a growing concern. Furthermore, moisture problems may be increasing in food plants. To reduce bacterial growth, processors are operating their facilities at lower temperatures, driving up energy costs and humidity challenges simultaneously.

"In all food plants, condensation off any part of the building or pipes is a no-no because of potential contamination from drippage onto product," says Roland "Lefty" Leavens, vice president of food processing systems for Food Facility Engineering (www.foodfacility.com), Yakima, Wash.

Beads of moisture can become drips of contamination, with the condensation carrying dirt, microbes and other contaminants. At best, condensation poses a threat to product quality and, at worst, it could lead to consumer illness and a major recall. Moisture also can rot wood and cause other damage to the facility structure, inviting pests and other sources of contamination.

That standard industrial refrigeration equipment can control humidity is a mistaken notion, Taylor maintains. The problem stems from the very nature of the processing plant. Mix hot water, moist carcasses, warm-blooded operators and process heat into a 40°F processing environment, and moisture will accumulate. Guaranteed. The moisture the air cannot absorb results in condensation.

"A standard refrigeration unit simply can't control the water coming off the carcasses, equipment, etc.," Taylor goes on. "The air won't have the capacity to absorb more moisture until the air is warmed, and that is not likely to occur until the clean-up shift."

Condensation means more than slippery surfaces and wet pipes. It can mean big trouble on the food safety front.

The standard solution won't cut it

Condensation in a processing plant is a complex challenge with multiple components: temperature, moisture, pressure and filtration.

"We don't even call it condensation control anymore," says Taylor. "We call it psychrometrics." The term refers to a relatively obscure branch of science - the science of moist air - that is becoming better known to processors by the day.

Telatemp Corp.'s Micro Humidity Logging Thermometer can record up to 10,922 date- and time-stamped temperature and/or humidity measurements and download those readings to a PC.

A greenfield plant today should include systems that deal directly with the conditions, equipment and materials creating or influencing moisture within the facility. To contend with moisture problems in an existing plant, begin with an air balance study.

A study typically starts with analysis of how air is moving in and out of the plant. This is followed by the introduction of a device to record the temperature, pressure and humidity within the plant over a 48-hour period.

"You need empirical data to have a complete idea of what is going on," says Taylor.

Sanitation often is the primary, even sole, source of the problem. After two eight-hour processing shifts in a cold environment, the introduction of hot water during the clean-up shift is bound to upset the moisture balance.

"All processing plants are trying to ‘sweat the equity,'" says Taylor. "They want to run the equipment as long and fast as they can for two shifts. They would stretch it to three shifts if they could, but they must clean the plant. When the cleaning shift hoses down with 140° water in a plant that has been operating at 40° temperatures, you have humidity everywhere. The solution may mean pulling in outside air, conditioning the air to 50° and re-heating the air to 80° and pushing it into the room. Then exhaust the air from the room. Hot air can absorb more moisture."

"Washdown is a challenge," echoes Leavens. "The walls are cold, but you are using hot water. You need air flow that avoids fog and heavy condensation."

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.

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.

"We created a two-foot stratum of dry air," says Taylor. "As the humidity rose, you could see it evaporate. The condensation problem completely went away."

Desiccant or mechanical?

By no means are desiccant systems always the better solution.

An engineering team faced with condensation and temperature problems in a slaughterhouse, for example, concluded that a mechanical dehumidifier would be most cost-effective. They added refrigeration equipment with reheat coils to solve the problems.

Desiccant systems cost more than mechanical dehumidification systems, but they also absorb more moisture. So which system is more cost-effective? It depends on the conditions of the plant.

Count on one constant: every situation is different.

Ironically, the psychrometric principles that are solving condensation problems in refrigerated facilities today have been long employed by HVAC engineers. Why there hasn't been more rapid carryover into cold processing engineering is puzzling to many, but, as they say, better late than never.

Today's engineer must understand the tools available to apply the appropriate solution to a moisture or condensation problem.

Whether you are building a new facility or attempting to solve a moisture problem in an existing plant, understanding the dynamics of airflow, heat and humidity of your process and plant will be critical to containing condensation and moisture problems.

Note to management, engineering and quality assurance

USDA is tightening the safety belt with its policing of facility condensation. The cost and challenge of modifying existing plants to meet long-standing but seldom enforced requirements can be considerable. But the impact on new plant designs is likely to be quick, effective and not nearly so painful.

Building condensation into a plant from the blueprint stage should be relatively simple, given what we have learned in recent years. The important thing to impress upon your project team is that facility engineers, whether on payroll or for hire, are aware of rules governing condensation control and put the necessary elements for control into the design.

Keep in mind that the long-term result of effective moisture control is a safer plant, safer product and a more responsible and respected industry.


Water pooling on the plant floor is not just a food safety concern. It also creates slippery floors and stairwells, causes plant traffic to spread moisture and creates pedestrian hazards in other sections of the plant.

"Floors should have a quarter-inch per foot of slope," says Roland "Lefty" Leavens, vice president of food processing systems for Food Facility Engineering (www.foodfacility.com), Yakima, Wash. "That used to be a USDA requirement. Today it has been changed to a Good Manufacturing Practice and the requirement is simply that the floor ‘must drain.' If the slope is 3/16-inch instead of ¼-inch, it is still OK. The important thing is that the floor drains."

The products run in the plant should largely determine floor material. What kind of cleaning materials will be used? Does the floor need to be chemical-resistant? Will the floor material be subjected to sugars or acids?

One response to slippery grates, steps, floor plates, catwalks, metal ladders and stairwells is SlipNot Metal Safety Flooring (www.slipnot.com) products. The Detroit-based firm applies its patented super-hard molten metal alloy plasma stream deposition at over 400 psi to the surface of aluminum, stainless steel, galvanized steel and other steel materials subjected to plant foot traffic. The "random hatch matrix surface" results in a hardness of better than 55 on the Rockwell "C" scale, providing slip-resistance and durability. The material - available in fine, medium and coarse grades - exceeds OSHA and NFPA safety requirements. USDA and FDA have approved the material for incidental food contact.

"A lot of food plants are using SlipNot now," says sales rep Jeff Baker, who includes Campbell Soup among its oldest and most satisfied customers. Its customer base includes frozen food manufacturers, cheese makers, poultry operations, slaughterhouses and a wide range of processed food segments.

SlipNot materials provide sure footing even under wet or oily conditions. "The material is like walking on sandpaper," says the plant manager of a dairy product plant in Wisconsin, who asked that his plant not be identified. "Yet it is not so rough that it poses a tripping hazard. We have had no accidents - no slips or falls - since it was installed."

He also notes that the material cleans easily with a high-pressure hose and, with good ventilation, dries quickly.

The product line includes plates, grates, stair tread and landing flats. But the company also customizes materials for plant space and specific application.

Designing your plant for condensation control

If you are planning a new processing facility, eliminate condensation problems from the get-go by adhering to these principles:

  • Keep critical areas clean.

  • Control the flow of contaminants.

  • Pressure packed near packaging.

  • The plant needs air.
Have a constant supply of air in the plant. Measure exhaust from the room and add sufficient outside air to maintain desirable air pressure in the plant. The highest pressure in your plant should be at the juncture of the processing line and the packaging area. If you have negative air pressure in your packaging area, contaminated air may migrate from a raw product area to the packaging area. Make your plant a "positive pressure" plant so that air flow dynamics do not bring air with potential contaminants into the processing area.
    Identify areas that need to be the cleanest, such as the ready-to-eat area.

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