A Thermal Storage Immersion Hydrocooler
Introduction
Most fresh fruit and vegetables require thorough cooling immediately after harvest to deliver the highest quality product to the consumer. When warm produce is cooled directly by chilled water, the process is known as hydrocooling. Hydrocooling is an especially fast and effective way to cool produce items that are slow to cool because of their large volume to surface area relationship. These items include sweet corn, melons and many large tree fruit.
There is a wide range of hydrocooling equipment available commercially. However, it is designed for high production and is generally very expensive. For example, even a modest unit may cost more than $100,000. To be cost effective, a hydrocooler must be utilized for as long as possible during the harvest season. Therefore, only growers or co-ops with large acreages of one crop or several crops can effectively utilized most commercial hydrocoolers. This publication describes a system designed to provide access to hydrocooling to those growers with produce volume insufficient to take full economic advantage of most commercial hydrocooling equipment.
Capacity
The capacity of a particular hydrocooling system to cool produce depends on a number of factors in addition to the rated capacity of the refrigeration system. These include the nature of the produce to be cooled, the starting and ending temperatures, the method of effecting heat transfer between the produce and the water and the totatl mechanical and heat transfer efficiency of the system. The system described here can hydrocool approximately 2000 crates of sweet corn every 24 hours.
System Description
This thermal storage immersion hydrocooler was built and demonstrated during the 1991-1994 seasons with grant funds from the Energy Division, N.C. Department of Commerce. The systmem in consists of three separate parts as shown in the accopanying drawings.
Refrigeration System
The refrigeration system consists of a standard, commercially available, 20-ton, direct expansion refrigeration system. Assuming a heat transfer efficiency of 80 percent, the maximum thermal output is 192,000 Btu per hour. There are many different commercially available models that would be suitable for this type application. It would be wise to consult a licensed refrigeration contractor when selecting a suitable unit.
Thermal Storage
In response to increasing electrical energy costs and the desire for better lad management, thermal storage technology has recently been developed. Apllied to refrigeration, thermal storage usually involves the nearly constant operation of a refrigeratoin system of a smaller capacity than would ordinarily be required. Cooling capacity, either in the form of chilled water or ice, is stored during periods of low demand and subsequently used, along with a refrigeration system, during periods of peak demand. Thermal storage has been characterized as a kind of thermal battery.
Not only does the application of thermal storage technology reduce the electrical example demand, it can also significantly reduce the capital cost of hydrcooling equipment. For example, if a hydrocooling operation were operating for eight hours per day with a peak load of 60 toans, a thermal storage systm could deliver the same degree of cooling with a 20-ton unit.
The second part of the hydrocooler system si the ice/cold water storage tank (thermal battery.) It consists of an insulated (sprayed polyurethane, R-16) metal tank with a capacity of approximately 5000 gallons. Located inside the tank is approximately 3000 feet of 5/8-inch copper tubing mounted to a rigid frame. The tubing is plumbed to dual inlet and outlet mainfolds through a variable orfice expansion valve. Refrigerant from the refrigeration system passes through the tubing. When filled with water, the tank contains approximately 41,000 pounds of water. The bundles of tubing are positioned inside the tank to allow the formation or up to 35,000 puunds of ice, yet provide for the free circulation of the remaining water. A licensed refrigeration contractor should be consulted concerning the design and specification of this unit.
Hydrocooler Vat
The third par of the system is the hydrocooler vat shown on the accompanying drawings. It is designed for continuous operation, consisting of an open-top insulated vat approximately two feet deep by eight feet wide by 16 feet long. It is designed to be laded at one end with packaged produce (ideally wire-bound crates) and unloaded at the other end. The length of time the produce remains in the water is controlled by the variable speed conveyor built into the bottom of the vat. Five minutes have been shown to be sufficient for fast cooling items such as snap beans. Larger items such as sweet corn, apples or peaches may require as long as 30 minutes.
A continuous hydrocooler such as this is well suited for packing shed operations since produce may be conveniently placed into the hydrocooler directly from the packing line and removed to a refrigerated truck or refrigerated storage. Cold water from the ice/cold water storage tank is pump-circulated to the hydrocooler bat at the rate of approximately 100 gallons per minute through a pair of 1-1/2 inch insulated hoses. A liquid level switch controls the level of water in the vat.
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Prepared by: Michael D. Boyette
Extension Biological & Agricultural Engineering Specialist
Drawings and Figures
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Figure V.
Schematic of Thermal Storage Immersion Cooler
Figure VI.
