| | Ice has characteristics that make it very effective in pre-cooling fruits and vegetables, especially for small-scale market gardeners. These characteristics include its versatility as a cold source for several pre-cooling methods, its thermal storage capacity and its portability.
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Ice may be considered stored refrigeration. A small ice-making machine can produce ice continuously for use at a high rate during a short period. The capital cost of the refrigeration system can therefore be a fraction of the cost of a conventional refrigeration system of the same capacity.
Cooling provided by melting ice is naturally regulated at just above freezing with the humidity of air around the ice near 100 per cent. The ice can be used several ways: directly on the produce, or for chilling the water for hydro-cooling, or in an ice bed to chill the air for forced air cooling.
Ice is also very adaptable to pre-cooling in the field as an integral part of the harvest operation. Residual ice in a mobile ice based cooler can also help maintain the temperature of produce during transport. These characteristics make ice ideal for use in cooling most fresh produce.
Many market gardeners do not pre-cool their produce because of the economics involved. By using ice as a cold source, the fixed cost of refrigeration can be substantially reduced, and the benefits of quality produce realized. The adaptability of ice to provide refrigeration for both wet and dry types of pre-cooling can be the basis for low cost pre-cooling of a wide range of fresh produce grown by market gardeners.
If cube or chunk ice is to be used for direct icing of produce, it should be crushed first. Crushed ice can also be used in a slurry with water for injection into cases of produce.
If ice is to provide refrigeration capacity for a forced air system, the air must be able to flow uniformly and easily around each piece of ice in a bin for the air to be chilled efficiently. Cube or chunk ice is best for this purpose. Flake and crushed ice tend to nest in a way that restricts the flow of air.
Pressure drop through an ice bin can be reduced by increasing the area of the bed. The reduced pressure requires less fan energy and, thus, less heat is generated by the fan.
Data and Rules of Thumb for Developing an Ice-based System
- Air required for pre-cooling is about 2 cubic feet per minute per pound of produce. Allow for at least 2 inches water pressure to move the air through the system.
- Air velocity through the floor should be kept below 200 feet per minute to avoid undue pressure drop.
- Specific heat of air is 0.23 BTUs per pound per degree Fahrenheit.
- 1 pound of air is about 14 cubic feet.
- The bulk density of cube ice is about 32.8 pounds per cubic foot.
- Latent heat of ice is 144 BTUs per pound.
- Latent heat of vaporization of water is about 1000 BTUs per pound.
- Specific heat of water is 1 BTU per pound per degree Fahrenheit.
- Maximum depth of ice should not exceed about 18 inches to avoid undue pressure drop.
- Minimum depth of ice at the start of a cooling cycle should not be less than 8 inches to ensure adequate cooling during the time of high temperature difference across the bed.
- Ice bin floors should be 60 per cent open to contain the ice while minimizing pressure loss.
- The shape and size of ice bin floor openings should be such as to contain the shape and size of ice used.
- Cube or chunk ice should be about 1 inch across to minimize pressure loss and provide sufficient surface area for good cooling.
- It takes about 1 pound of ice to cool 2 to 3 pounds of produce. The actual amount depends mainly on temperature of the produce, how well the cooler is insulated and sealed and the ambient temperature.
- Specific heat of high moisture produce is about 0.85 BTUs per pound per degree Fahrenheit
Example Calculations
Here are sample calculations for an ice-based cooling system. The system will be designed for 700 pounds of produce. The produce will be cooled from 70 degrees F to 34 degrees, a decrease in temperature of 36 degrees F.
- 700 pounds of produce requires 700/3 = 233 pounds of ice.
- 233 pounds of ice occupies 233/32.8 = 7.1 cubic feet.
- 700 pounds of produce requires 700 X 2 = 1400 cubic feet per minute of air.
- the minimum area of ice bed is 1400/200 = 7 square feet.
- if the initial depth of the ice is to be 10 inches (10/12 of a foot), the bin floor area will be 7.1/(10/12) = 8.52 square feet.
Other Interesting Calculations and Information
The area of an ice bin floor is determined by the air velocity through it and the amount of ice required. In sizing an ice bed, determine the total amount of ice required for one load of produce. If 280 pounds of ice are required, this amount will fill a volume of 280/32.8 = 8.53 cubic feet. If the depth of ice is initially 1 foot, this amount will require an area of 8.53/1 = 8.53 square feet. This area could be a bin, for example, that is 2.5' X 3.41' = 8.53 square feet.
Tests done by Ike Edeogu in 1998/99 showed that air can be cooled from 70 degrees F to 34 degrees F, a total drop of 36 degrees, with an 8 inch deep bed of cube or chunk ice using an air flow of about 200 cubic feet per minute per square foot of ice bed area, with a pressure drop of 1.41 inches of water.
To calculate the cooling capacity of 1 square foot of ice bin loaded with cube ice to a depth of 8 inches, make the following calculations. If the maximum 200 cubic feet per minute per square foot of air is used, the air will weigh 200/14 = 14.28 pounds. The amount of heat required to heat 14.28 pounds of air by 36 degrees would be 14.28 X 36 X 0.23 = 118.24 BTUs per minute or about 118.24 X 60 = 7094 BTUs per hour per square foot.
To lower the temperature of 1 pound of produce from 70 degrees F to 34 degrees F, or by a total of 36 degrees, would theoretically require 1 X 36 X 0.85 = 30.6 BTUs. This total would be the equivalent of 30.6/144 = 0.2125 pounds of ice. The ratio of produce to ice would therefore be 1:0.215. Changing this figure to the ratio of ice to produce would theoretically be 1:4.706. A portion of ice, however, is used to offset heat gains from infiltration, fans and lights and through the walls and doors of the facility. The actual ratio of ice to produce used is therefore closer to 1:3. Also, an allowance needs to be made for some ice to cool the unit down.
Index of Manufacturers
Hoshizaki America, Inc.
818 Highway 74 South
Peachtree, Georgia 30269 | Mile High Equipment Company
11100 East 45 Avenue
Denver, Colorado 80239 |
IMI Cornelius, Inc.
One Cornelius Place
Anoka, MN 55303-1592 | Scotsman Ice Systems
775 Corporate Woods Parkway
Vernon Hills, Illinois 60061 |
Manitowoc Ice Inc.
Sub. of Manitowoc Food Service Group Inc.
P.O. Box 1720, Manitowoc, WI 54221 | Kold-Draft
1525 East Lake Road
Erie, PA 16511 |
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