| | System Description
.
The ice-based cooling system described in this section is intended for use in the field as an integral part of harvest operations. It is designed to be mounted on a pick-up truck, but could be modified to mount on a trailer if required.
The unit consists of an enclosure divided into produce, ice bin and engine/fan compartments. A fan forces air down through a bed of ice, along a plenum on the floor, up through containers of produce in the produce compartment and back through the fan. Air exits the ice bed at or below 35 degrees Fahrenheit and near 100 per cent relative humidity, making it ideal for chilling fragile produce. Once the ice starts to melt at 32 degrees Fahrenheit, the air temperature remains relatively constant, making temperature controls unnecessary.
Figures 1 through 17 provide detail on each step of the horizontal ice bin system construction.
Figure 1. Ice-based cooling system.
Figure 2. Produce/ice bin access view.
Figure 3. Engine/fan side view.
Figure 4. Truck box insulation and drains.
Figure 5. Ice bin frame.
Figure 6. Assembled ice bin.
Figure 7. Installation of ice bin.
Figure 8. Produce frame.
Figure 9. Produce mask installation.
Figure 10. Installation of produce floor.
Figure 11. Cooler frame.
Figure 12. Bulkhead wall.
Figure 13. Engine compartment.
Figure 14. Installation of cladding.
Figure 15. Fan and engine installation.
Figure 16. Door installation.
Figure 17. Installation of insulation.
An alternative to the horizontal ice bin system is a vertical wall ice bin. This system may be appropriate for batch cooling in a central facility. Vertical ice walls require less air pressure due to the distance the air travels through the ice, so the temperature is optimized and consistent.
Because the ice must be loaded in at the top of the wall, an ice machine can be located to load directly into a feed hopper from above, if the unit is in a central facility. Also, regular agitation of the ice in the wall can be done more easily in a central facility. Ice left in the wall can more likely be utilized with greater effectiveness in a central facility as well.
System Specifications
The horizontal ice bin system unit is designed to handle a maximum of about 700 pounds of product. This amount of product will require about 300 pounds of ice to lower the temperature by 50 degrees Fahrenheit.
A generous ice bin area of about 12 square feet is available in the system described. The porous floor should be at least 60 per cent open. The openings should be about 3/8" across to minimize ice loss. The material should be corrosion resistant or have a corrosion resistant coating.
An air flow of about 1400 cubic feet per minute at 2 inches water pressure is provided for cooling by a centrifugal fan driven by a gasoline engine. The fan has an inlet on one side only. The engine is rated at about 4 horse power. The belt drive should be carefully considered. The belt drive could have a ratio, for example, of about 2.5 to 1 for the fan to run at 1200 RPM with the engine running at 3000 RPM. At this speed, small light engines run fairly smooth under load.
All areas are insulated with 2 inch extruded rigid polystyrene foam. Separate exterior doors provide access to the produce area, engine compartment and ice bin. The tail gate of the truck is left on to provide a standing area for loading and unloading the produce. Drains for water from the melting ice are provided in the ice bin area, which requires appropriately located drain holes in the truck box.
Containers used in the system must accommodate the vertical flow of air through them. This approach means that the holes are only in the bottom of the containers and in the top when applicable. A mask on the floor plenum directs chilled air up through the bottoms of the containers. A mask is a sheet of material, such as plywood, that has cut-outs that the containers are stacked on to direct the flow of air through only the bottoms of the containers.
A relatively air tight seal is also required between stacked containers to ensure a positive flow of air through the produce in all the containers. This flow may be achieved with just the containers if a relatively tight seal can be made between the top and bottom of the stacked containers. A second method makes use of a mask similar to the one on the floor. This mask is then installed between each layer of containers.
Fabrication
Fabrication of the cooler requires good basic shop skills. Plans for the cooler have been kept general because of the wide range of truck box configurations, fans, engines, fabrication methods and construction materials that could be used. Basic concepts of the system should be adhered to so the unit will function efficiently. The unit requires the following:
- A recirculating and positive vertical flow of cooling air from the fan, through the ice bin, through the produce and back through the fan.
- Fan located so air is supplied to it from the produce area and not the ice bin area. Heat is added to the air by the fan in the process of compressing and moving the air. It is also more difficult to achieve a uniform flow of air through the produce if the high velocity air from the fan discharge is close to the produce air inlet.
- Insulate and seal refrigerated compartments and fan to minimize heat gain and optimize cooling effect of ice. A generous ice floor area to minimize air pressure required.
- Containers that are easy to handle, provide for vertical air flow only and can be easily cleaned and stacked.
Construction of the cooler is described in Figures 1 through 17. Materials open to the ice bin and produce compartments should be selected to avoid contamination of the produce. Pressure-treated wood or exposed fiberglass insulation are to be avoided. Consider cedar for ice bin and produce frames. Clean and coat all metal components to minimize rust.
Modifications may be required to the top portion of the unit if the unit is to be removed and installed often. For example, corner jacks or lifting brackets may be installed.
Operation
- Load empty containers into produce compartment.
- Determine the amount of ice required and load into ice bin. Use cube or cylindrical ice. Flake ice and fine granular types of ice restrict the flow of air. Use about 1 pound of ice cubes for every 2 to 3 pounds of produce - more on hot days less on cold days.
- Move cooler to a location in the field that is central to harvest operations.
- Unload containers.
- Ensure ice bin and produce doors are closed; start fan to cool down the unit.
- Harvest produce and fill containers.
- Load containers of produce into cooler as they are filled, so they can be cooling as soon as possible.
- Monitor ice bin and adjust ice levels if required. Some areas may melt faster than others. Stack the ice deeper in these areas.
- Continue loading until unit is full or harvest is done.
- Continue cooling until the top containers are chilled. At this point, the ice should be nearly used up. Air pressure drop through the system at the start will be due to the depth of ice, while near the end of loading, the pressure drop will be due mainly to the depth of product.
- Use the cooler unit to transport and store the cooled produce. If the produce is to be held in the cooler for a substantial time, more ice will be required, and the fan will need to be run periodically to maintain the temperature. Be aware that some circulation of air will result from convective forces.
- For high harvest rates where a central pre- cooler is available, only a portion of the cooling may be done in the mobile cooler. The mobile cooler can be unloaded into the central facility, allowing the mobile unit to return to the field to support harvest operations.
Maintenance
After use, clean and dry out the mobile cooler. Store the unit inside if possible, so the doors can be left open to dry and air out. When the unit is to be stored for long periods, make sure that the gasoline is drained completely from the tank and that the engine has clean oil in it. |
|