Vacuum cooling works by rapid evaporation of water from certain vegetables or other products under very low atmospheric pressures inside a vacuum chamber. Energy in the form of heat is required to change water from a liquid to a vapor state as in the boiling of water. At reduced atmospheric pressure in a vacuum chamber water boils at a lower than normal temperature.
(See PRESSURE / TEMPERATURE CHART at the end of this article.)
Heat is required to boil or evaporate water. In vacuum cooling this heat comes from the product being cooled. This includes field heat and heat from its respiration. Evaporation of 1 pound of water from the product will cool about 1,000 pounds of product by 1 F. Therefor the amount of water removed is roughly 1% of the initial product weight for each 10 F that it is cooled. Generally the moisture comes equally from all parts of the product being cooled and localized wilting is not a problem.
Evaporation of water begins when the atmospheric pressure in the vacuum chamber is reduced to the boiling point corresponding to the initial temperature of the product being cooled. This is sometimes called the flash-point. As the atmospheric pressure in the vacuum chamber is further reduced, evaporation continues until the desired product temperature is achieved. It is possible to freeze product in a vacuum chamber if the atmospheric pressure goes to low.
The vacuum cooling process consist of two phases, the pump down phase and the cooling phase. No significant cooling takes place during the pump down phase. The pump down time should be somewhat constant depending on the initial product temperature. The rate of cooling during the cooling phase depends primarily upon the ratio of the amount of surface area of the product to its weight (its surface to mass ratio) and the ease with which a product gives up water from its tissues. In some cases where product has lost moisture prior to vacuum cooling, it becomes more difficult to cool. Wetting a product prior to or during vacuum cooling can be beneficial in certain cases.
It is impractical to pump all the vapor generated during the vacuum cooling process using a mechanical vacuum pump. For example a pound of water from a product at 32 F in vapor form occupies 3,304 cubic feet. In liquid form this same amount of water accuses only 0.016 cubic feet. Therefor water vapor is condensed on a cold surface or vapor condensing coil in order to efficiently reduce the pressure inside the vacuum chamber. The heat absorbed by the coil when condensing the water vapor determines the load for the refrigeration system. The load on the refrigeration system begins when the atmospheric pressure inside the chamber reaches the flash-point.
The vapor condensing coil of the vacuum system is the evaporator coil for the refrigeration system. The coils can be mounted inside the vacuum chamber or outside in a separate chamber. Coil configuration and construction depends on the overall system design requirements. (The coil shown in the adjacent picture is stainless steel for use in a combination vacuum cooler and hydro cooler.)
Temperature management of a product is important after it has been vacuum cooled. After it is removed from the vacuum cooler it should be placed in a cold room for storage or directly into a refrigerated shipping vehicle.
Safety and maintenance are important to a vacuum cooling operation. Operators should follow safety rules and proper operating and maintenance procedures. To increase safety, operating efficiency and reduce down-time, 4 Star Ag Refrigeration offers Remote Monitoring on a local network or the internet.
Ag Refrigeration research and development – New product utilizing vacuum based temperature limited high speed bulk material moisture level conditioning.