Rotating Wheel Systems

According to Speltz, when the required dewpoint is low, or when very low relative humidities are needed, desiccant-based dehumidification is a cost-effective technology. The equipment uses difference in vapor pressure to remove moisture from the air.

“The surface of the dry desiccant has a very low vapor pressure compared to the much higher vapor pressure of humid air,” said Speltz. “Water vapor naturally moves out of the humid air onto the desiccant surface to eliminate the difference in vapor pressure.

“Eventually, the desiccant surface collects enough water vapor to equal the vapor pressure of the humid air,” she continued. “Then the desiccant must be dried,” or reactivated, “by applying heat before it is recycled to remove more moisture from the humid process airstream.”

The source of this airstream may come from gas, steam, or electric. “There are many ways of presenting a desiccant to an airstream,” she said, “but in most modern, atmospheric pressure industrial dehumidifiers, the desiccant is held in a lightweight matrix in the shape of a wheel, which rotates between two separate airstreams,” the reactivation and process streams.

The process air passes through the desiccant wheel, “giving off its water vapor to the desiccant contained in the walls of the air passages,” she said. The dry air leaves the wheel and is carried to the point of use by fans or blowers. “While that is happening, part of the wheel is rotating through a second, smaller airstream with the reactivation air, which has been heated.” This hot air heats the wheel and drives water vapor out of the desiccant.

As each section of the wheel rotates out of the reactivation air, its desiccant is dried and can once again remove moisture from the process air.

“As air is dried, the temperature of the process air rises in proportion to the amount of water removed,” said Speltz. “Drier air means warmer air.” This is the reverse of the evaporative cooling process; “When water is evaporated into air, the heat needed for evaporation comes from that same air,” said Speltz, “so its sensible temperature falls. Conversely, when air is dehumidified, the heat needed to evaporate the water originally is liberated, raising the temperature of the airstream.

“Because a desiccant dehumidifier removes water from the air as a vapor rather than as a condensed liquid,” she continued, “there is no risk of freezing condensate. So this type of equipment is most often used for applications where dewpoints below 50°F air required.”

Refrigeration Applications

Food production: Many food production processes use a lot of fluids, either for the product itself, its preparation, or leftover from sanitation. Spot washdowns during the production day add still more moisture; this needs to be addressed in order to meet industry hazard compliance rules (see “Tightening Up the Chicken Outfit,” Dec. 4, 2000), as does condensation that forms on cool surfaces (colder than the dewpoint of the surrounding air mass).

Desiccant dehumidification systems, available in a range of capacities and configurations, can be used for localized control or for an entire production processing area, said Speltz.

Sanitation washdowns: Keeping food production areas within code compliance requires thorough washdowns using steam-injected water at the end of the production day and/or week. However, “Extended use of steam-injected water and cleaning fluids often results in fog, which reduces visibility and creates a safety hazard,” Speltz pointed out.

“Fog prevention and the recovery time required after sanitation can be critical, since inspectors prohibit production from resuming until the area is dry.”

Figure 1 shows a sample sanitation washdown application. At the onset of the sanitation shift, the conditioned temperature in the production area is at 45°, 68% rh. “This cold air is so close to saturation, it cannot absorb the large amounts of water vapor being sprayed into the room,” Speltz said. “As a result, water being sprayed precipitates out, immediately forming fog and condensation throughout the room.”

“During sanitation and throughout the recovery period, a column of 9,000 scfm of return air is delivered from the conditioned production area to the dehumidifier,” said Speltz. “This full volume of air then passes through the desiccant wheel and is supplied back to the production area at a dried condition of 77°, 4% relative humidity. To maximize airflow through the production area, the return and supplies are located at opposite ends of the room.”

Answering an attendee’s question, she added that there are no problems with desiccants resulting from caustics sometimes used during washdown.

Process freezers: “Even small amounts of moisture cause ice in the extreme conditions of process freezers,” Speltz said. “This occurs because very cold air can hold only a fraction of the amount of moisture than warmer environments.”

Unwanted ice in a process freezer can lead to belt jams and control malfunctions, which, in turn, can lead to a halt in production, product loss, and unscheduled breaks for the workforce. “Even when production does not come to a complete stop, plant efficiency is affected,” Speltz said.

Desiccant dehumidifiers help fight snow and ice during this application by delivering extremely dry air at the conveyor opening(s), or directly inside the freezer near the coils.

Determining which area to deliver the dry air to depends heavily on the sources of the moisture load:

Infiltration moisture, where warm packaging or production air entering the freezer carries the largest moisture load; air enters continuously through openings around the conveyors; or

Product moisture, where water vapor from the product is relatively easy to quantify by weighing the product entering and leaving the process freezer, Speltz pointed out. However, if the product is wrapped or packaged before freezing, the product moisture load will be close to zero, “unless a sanitizing spray is present.”

Packaging, palletizing, and staging: These food-processing areas are often located just outside of a wet production room, close to cold storage rooms. This leads to moisture and fog, which can affect package and adhesive integrity.

“It is most effective to remove the moist, humid air at its source, before it can enter the cold palletizing area from production,” pointed out Speltz. “Therefore, a duct is positioned above the conveyor opening to capture and return the moist air to the desiccant dehumidifier.”

Coolers, storage freezers: According to Speltz, 95% of the wet floors, ice, fogging, and frost found in most common cold storage and loading dock areas result from moisture infiltration through truck doors. “This moisture heads right for the cold surfaces of the freezer evaporators, typically located over freezer entrances.

“This requires frequent defrosting of the coil, which can sometimes necessitate shutdowns to thoroughly clean the coil with expensive labor,” she said.

“Ice and snow formation can also create malfunctions of sophisticated rapid-action freezer entrance doors. Ice forming on door frames and on the floor below can jam doors, prevent them from sealing properly, and cause delays in moving product.” Who has to chip the ice away? Employees, who could be more efficiently employed doing other work.

Desiccant dehumidifiers help solve these problems by addressing the moisture at its source, Speltz said, and delivering dry air directly toward the evaporator coils inside the freezer or at the freezer entrances.

Loading docks: “Getting product out the door is what food distribution is all about,” Speltz said. Fork trucks, pallet movers, and foot traffic are slowed by fog and wet or icy floors, which also create safety hazards.

High humidity also reduces refrigeration efficiency. “Moisture freezing on coil surfaces restricts free airflow, reducing its ability to cool and dehumidify,” she said. “Ice builds until a defrost cycle is initiated to melt it so the ice can drain away as water. During defrost, the coil no longer cools or dehumidifies; instead, it adds heat to the space and some of the melting water re-evaporates into the air. This process is, of course, expected and compensated for by the capacity of the other evaporators serving the area.

“However, when moisture loads are higher than expected, an inability to maintain the proper dock temperature can result as coils attempt to keep up with the load. This may prohibit using the dock as a staging area for fear of sacrificing product quality.”

Again, the solution is to capture the moisture before it can affect the space.

Publication date: 01/08/2001

Not Sold On Savings

According to Speltz, desiccant systems are not so much sold for their energy savings in commercial-industrial refrigeration processes, as for safety and product-production saving issues.

However, “It may decrease the defrost,” she added. “It does take a significant load off the coils.”

There are greater benefits to be reaped from overall plant efficiency. There’s also a payback in worker efficiency. Many food processing plant workers are often aware of adverse conditions, but Speltz said they are also too often not listened to or believed.

Publication date: 01/08/2001