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The total refrigeration heat load depends on the design temperature and four load factors:
1. Infiltration load from air entering through doors, openings, and building leaks;
2. Product heat load (the heat removed to bring the product to the design conditions);
3. Internal heat load from motors, lights, people, and other heat sources (i.e., defrosting); and
4. Transmission load (heat transferred through surfaces).
A test facility with a layout that represents all levels of temperatures and numerous production challenges was chosen to illustrate the refrigeration requirements in a typical production-distribution facility. This was a bakery that produces breakfast products frozen in sealed plastic containers.
Spiral freezers at -40 degrees have two inlet and outlet conveyor openings. The inlet side of the freezer is in an unconditioned production area; the outlet side is a 50 degree packaging area where frozen product is loaded into boxes and conveyed into a -15 degree storage freezer. The individual boxes move along a conveyor and exit the storage freezer into an area at 50 degrees, where the boxes are combined and placed on pallets, then stretch-wrapped for storage and shipment.
The skid is loaded back into the storage freezer and held for shipment. Distribution on the skid-mounted product is through a single access door leading to a loading dock designed to be 50 degrees. The access door from the loading dock to the storage freezer has a high-speed, automatic door, plus an airlock with strip curtains on both sides.
ESTIMATING LOADSInfiltration into the spiral freezer and the storage freezers cause major concerns for production efficiency and maintenance. Analyzing the load provides a broader understanding of the proportion of latent to sensible heat.
The load is calculated based on the differential between the low temperature spaces and the adjacent higher temperature areas. The difference for the processing freezer at -40 degrees, compared to the production area at 78 degrees, is significant.
Another concern is the storage freezer, designed at -15 degrees, with access doors and conveyors from a packaging area, pallet area, and loading dock at 50 degrees. The temperature difference is about half that of the processing freezer and the production area, but there are larger and more active access points where leakage and moisture infiltration occur.
There are 10 conveyor openings for moving the product in from packaging and out for pallet. An entry door is used to move the product from packaging to a storage area; an access door is used for full skids to be loaded back into the freezer for storage; and an access door is used at the loading dock for distribution.
To estimate the heat gain from infiltration, the following equation was applied (from ASHRAE’s 1998 Refrigeration Handbook, Chapter 12, “Refrigeration Load”).
Infiltration by direct flow through doorways: Qt = 60 VA (hi – hr) prDt
Qt = average refrigeration load, Btuh
V = average velocity, ft/min (60)
A = opening area, sq ft (100)
hi = enthalpy of infiltration air, Btu/lb (18.62 @ 50 degrees F db/10 degrees C, 47 degrees F wb/8.3 degrees C, 80% RH)
hr = enthalpy of refrigerated air, (-3.26 @ -15 degrees F db/-26.1 degrees C, -15.15 degrees F wb/26.2 degrees C, 90% RH)
pr = density of refrigerated air, lb/cu ft (0.089)
Dt = decimal portion of time doorway is open (0.50)
Equation with applied values: 60 x 60 x 100 x (18.62 + 3.26) x 0.089 x 0.50 = 350,518 Btuh
Load components: 249,569 Btuh of sensible and 100,949 Btuh latent heat.
The latent load is about 29% of the total. The latent portion of the load is actually visible in the form of condensation, fogging, and frost and ice buildup on coils, doors, ceilings, walls, and floors. The largest buildup of frost and ice occurs at the access doors and conveyor openings. High-speed doors and airlocks with strip curtains can be added to help prevent this infiltration.
However, the movement of product will force open the door, conveyor, and strip curtains to allow air exchange. The temperature difference between the two areas causes warm, humid air from the loading dock to enter the freezer at the top of the access door, and cold air from the storage freezer to discharge at the bottom of the door along the floor of the loading dock.
MAINTENANCEMaintenance and safety issues are a concern when floors are wet, icy, or covered with snow. Workers are continuously scraping and sweeping floors. As a result, two full-time people, representing an annual cost of about $50,000, were employed at the facility.
Evaporators require several defrost cycles to keep up with the infiltration load. Frost buildup on coil tubes and fins reduces cooling capacity. Snow on fan guards restricts airflow and overall cooling capacity. When frost builds on evaporator fans, they become unbalanced, causing motor mounts to be torn apart.
Service and part replacement for this storage freezer averaged about $15,000 a year. Personnel were always in danger of slipping, falling, or being harmed by sliding forklifts. One injury a year can be a major cost factor with lost time and insurance claims.
Increasing the number of defrost cycles can help maintain evaporator performance, but this also adds load to the refrigeration system — and anyway, not all the moisture freezes on the coils. The remaining vapor is distributed back into the freezer space, collecting on ceilings and walls or on evaporator fans and grilles that are not defrosted, leading to additional frost.
The power consumption for defrost and recovery can be significant. This wasted energy is proportional to the number of defrost cycles.
SOLUTIONS AND ALTERNATIVESEvaporator coil location: Thoughtful placement of evaporator coils in the storage freezer can relieve some of the frost problem. Positioning the coils further away from the access opening avoids a high negative pressure near the openings. This negative pressure draws warm, humid air in from the adjacent areas.
Even with this approach, if these access openings are active, moisture infiltration can still occur. Often there are access points at both ends of the freezer, where positioning an evaporator in the middle is the only choice.
Doors and airlocks: High-speed doors and airlocks help reduce infiltration. However, the activity of forklifts and containers moving on conveyors cause an opening that allows moisture to enter the freezer. Aggressive loading schedules often cause workers to keep the automatic door open in order to move more products.
Doors and strip curtains break free or are damaged by forklifts, providing gaps where moisture migrates from higher humidity areas to low-temperature freezers. Conveyor openings are a design challenge to prevent leakage. Strip curtains offer very little control of infiltration when containers block open the barriers.
Heaters are often combined with doors to reduce frost on the door panels, elevate the temperature of the infiltrating air, and prevent air from crossing the saturation curve. These heaters may prevent fogging, but the moisture is not controlled and still enters the freezer.
Pressurization: Pressurizing low-temperature areas to prevent infiltration can offer some relief if the pressure is balanced and maintained. However, the balance is delicate; too much pressure causes freezer-temperature air to move into adjacent areas, causing frost on the warm side of the door or conveyor opening. Condensation will also occur at these interim areas, presenting a safety hazard for workers and forklift operators.
Moreover, pressurized air needs to be cooled and dehumidified to avoid placing additional load on the freezer evaporators. This approach only partially reduces the moisture load.
Reducing temperature in adjacent areas: Conditioning the adjacent areas to a lower temperature reduces the differential between the two areas. Dropping the temperature in the loading dock and packaging areas to 35 degrees F can reduce the load by nearly 25%, but is not practical for workers. Also, this strategy will not resolve the wet floor problem in these areas, or totally prevent frost buildup in and around access points to the processing and storage freezers.
Additional concerns can include maintenance costs associated with cleaning up products that fall off food conveyors. Further, in plants regulated by the U.S. Department of Agriculture, condensation must be prevented to eliminate sanitation concerns.
Removing the latent load: Reducing the latent load and allowing the evaporators to remove primarily sensible heat can improve the efficiency of the ammonia refrigeration system. Reducing the latent load to less than 1% of the total load on the evaporator can improve performance and lower the number of defrost cycles. The latent load presents maintenance issues that can be eliminated with dry air.
Establishing dry zones around all access points prevents moisture from entering the freezers. By supplying dehumidified air on the outside of the freezer, problems with fogging and condensation in the loading dock and packaging areas are prevented.
THE DESICCANT METHODOne of the best methods to produce low dewpoints in cold air is through the use of a desiccant dehumidifier. These units remove the latent load directly by attracting the moisture in vapor form instead of condensing (as with refrigeration equipment).
Desiccant dehumidifiers have been applied for more than 50 years in critical industrial processes, manufacturing, automotive testing, ice rinks, and food production, often in concert with ammonia systems.
Where temperatures are below freezing, the elimination or significant reduction of the latent load provides improved ammonia coil performance. The same reasoning applies in this application, as equipment configurations are readily available to match any type of requirement.
The test facility offered an opportunity to demonstrate the application of desiccant dehumidifiers and to determine the benefit to the existing ammonia system. Significant improvements were documented.
The desiccant dehumidifier uses a rotor that attracts moisture from the process airstream. As the media rotates from the process stream to an opposing airstream (reactivation), the water vapor is transferred. This reactivation air is drawn from outdoors and heated to increase the surface vapor pressure, liberating the moisture from the desiccant to the airstream. This moisture vapor is exhausted outdoors. The desiccant rotates back into the process airstream where it is cooled to collect more moisture.
The temperature gain in the dry process air adds some sensible load to the ammonia evaporator coil. However, without the latent load, the ammonia coil is now more efficient at removing this sensible heat.
In every application where the dehumidifier is installed for moisture control in a processing or storage freezer, the evaporators produce colder temperatures than the original design. This confirms that there is a net gain in ammonia coil performance when the latent load is less than 1% of the total load on the evaporator.
INSTALLATION POINTERSThe success or failure of desiccant dehumidifiers lies in the design and installation. In the bakery, equipment placement and dry air distribution are designed to serve access openings. To maximize performance of the dehumidifier, return air is drawn from near the evaporators located in the loading dock and packaging areas. The colder air temperature provides ideal conditions for maximum moisture removal with the desiccant rotor.
The dehumidified air provides a dry zone at all openings to prevent humid air from entering, including all doors and/or conveyors that access the processing and storage freezers. The dry air is first introduced directly toward the opening.
Conveyor openings are tunneled to contain the dry air. This dry air is drawn into the freezer in place of the warm, humid air. The tunnel also prohibits any leakage into the opening other than the dry air.
Doorways require some combination of automatic door and/or airlock to act as a barrier between loading docks, packaging, and production areas. The dry air is applied to the door entry to allow flow into the freezer. This overflow, when the door is not in use, provides a drying effect on adjacent floors and walls to prevent condensation. Dry air can also be supplied to the airlock between the strip curtains.
During the loading process, pallets often block the entry. Automatic doors are sometimes kept open to speed up product movement from storage to trucks. Whatever the circumstance, moisture is prevented from entering the freezer.
The dehumidifier is normally installed on the roof, with return and supply air connections through a curb. Reactivation energy is usually gas, but manufacturers offer steam, electric, and/or combinations with hot water.
Thirty-percent filters are standard; but with certain critical applications on processing freezers, 85% to 95% or HEPA 99.97% (food-grade) filters must be furnished to satisfy USDA requirements. Filters should be checked every 30 days and/or replaced to satisfy USDA standards.
Desiccant rotor life in this application is 10 to 12 years. Even at that point, performance is only diminished by 10% to 20% from system startup. Ducts must be sealed on the supply and return side to prevent leakage or loss of the dry air before reaching the freezer access openings.
Opportunities for applying dehumidification in concert with ammonia refrigeration should begin in the design phase of the project. This allows for compatible sizing and convenient equipment placement. Retrofitting existing facilities can be accomplished with equipment mounted on the roof. Installing the distribution system often requires off-shift scheduling.
Even with these concerns, the gains in performance, reductions in maintenance, and improved safety offer opportunities for reducing plant-operating costs.
Each facility layout should be reviewed, along with the production and distribution operation, to determine specific design requirements. Repairing the truck door seal and access doors to the freezer and conveyors should be coordinated at the same time.
Consulting engineers, design-build contractors, and dehumidifier manufacturers offer experience in the design and retrofit of these systems. Many successful dehumidifier installations can now serve as models for further application of the technology.
Parkman and Bradley are engineers with Concepts and Designs MS, Rochester, NY. The article is based on a presentation they made at the 2001 IIAR Conference in Long Beach, CA. Questions can be directed to the company’s corporate office in Wixom, MI, at 248-344-7236.
Publication date: 12/02/2002