Evaporative Cooling's New Twist

Although it's old (and proven) technology, vacuum/evaporative cooling evolves to meet today's needs.

By Lou Decker, Contributor

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Vacuum systems are also used for chilling water for various applications and processes. In these cases, due to the above function, the water is its own refrigerant eliminating the need for chemical refrigerant.

The amount of water evaporated from the food product typically ranges from 1-5 percent. The longer the product is exposed to the vacuum or the lower the temperature/pressure, the more moisture is removed. But this results in surface drying of the product; therefore, care must be taken to control the surface texture and perhaps color. During cooling of potatoes in a retort, for example, moisture is controlled by a fine mist of water sprayed onto the product which absorbs the moisture maintaining texture and color.

Processing of sauces, soups and viscous liquids, mixing or agitation is required. Since it is important to minimize temperature layers in the product, the pattern of agitation and speed is important. Horizontal agitation is recommended for most viscous the materials, whereas vertical agitation is used for low-viscosity products. Occasionally, regardless of the type of agitation or speed, the product becomes concentrated and additional water or another liquid must be used for taste enhancement, either before or after processing.

Vacuum cooling of light fluids/products is not without some inherent problems. Milk, chicken, broths, some soups and high protein-content products tend to foam and sometimes erupt when the vacuum is first turned on. This eruption/foaming results in "carry-over" into the vacuum system, affecting its performance and, most importantly, decreasing the yield. This eruption usually stops when a certain pressure/temperature is reached. With proper control, the reaction can be minimized and perhaps eliminated.

Evaporation source

There are basically two different methods of creating the required vacuum for cooling, depending on the ultimate temperature to be achieved and the time required to reach the temperature. An additional deciding factor in the process design may be the quantity of product to be cooled. The two methods to be considered are Steam Ejector Systems and Hybrid Systems.

  • Steam Ejector Systems. These use the simplicity of steam jets, which have no moving parts and therefore are basically maintenance-free. They are motivated by steam using water in inter-stage condensers, usually shell & tube design. With operating temperatures in the range of 35- 80 degrees Fahrenheit, multi-stage systems are typically more efficient, larger capacity, simple to operate and more cost effective. Steam jet systems are based on the principle of using steam as the motive fluid, entering at a high pressure and low velocity and passing through a converging/diverging nozzle, which decreases the pressure and increases the velocity.

An increase in velocity of the steam entrains the process gas (e.g., air, water vapor, etc.) creating a vacuum. This mixture is then recompressed within the diffuser by conversion of the velocity energy back to pressure energy. After recompression the gases are considerably less than the motive pressure, but higher than the suction pressure. The combined mixture flow is the total of the two flow rates, motive plus suction.

The mixture then discharges into a condenser operating at an intermediate pressure, allowing for the majority of the process vapor and motive steam to be condensed before passing on to the next stage. The required product temperature and pressure determine how many stages are required. Typically a product temperature of 35o F requires a three-stage vacuum system; for temperatures of 80o F and above a single or two stage unit may be adequate.

  • Hybrid System: To reduce the steam consumption and also facilitate a low level installation of the vacuum system, a hybrid unit is recommended. A hybrid vacuum system is a combination of steam ejectors, discussed above, an intercondenser and a liquid ring vacuum pump. The steam jets operate at motive pressures as low as 5-10 psig achieving the same low suction pressures or product temperatures. The liquid ring pump then handles the discharge from the intercondenser operating at an intermediate pressure, discharging to atmosphere.

The liquid ring vacuum pump is a non-pulsating rotary vacuum unit whose only moving part, the rotor, is mounted on a shaft offset from the center axis of the pump housing. The rotor turns, throwing the liquid against the outer wall due to centrifugal force. A liquid ring pump forms against the housing wall. Since the rotor is offset, each revolution results in a chamber between the rotor blades being filled with liquid then pushed out a port. As this rotation continues and the liquid decreases, the void formed is filled with a gas as the chamber exposes an inlet port. The liquid and gas are then compressed and discharged through another port into an atmospheric tank where they are separated.

The vacuum achieved by a liquid ring vacuum pump depends on the number of stages -- typically, one or two stages -- and most importantly the temperature of the sealant liquid used. The colder the liquid temperature, the lower the vacuum. However, as the temperature approaches the operating pressure, contraction will occur.

An all-ejector-type vacuum system typically requires installation at a height suitable for draining the condensate from the condenser. The recommended height is typically 36 ft. minimum. To facilitate a low-level installation of an all ejector system, the addition of a condensate receiver or a pressure powered pump can be added, which means an additional component is required. The hybrid system can be installed at floor level since the liquid ring vacuum pump has the multiple function of a condensate removal pump and final vacuum stage.

Due to the advantages of evaporative cooling consideration should be given to this technology to enhance product quality, maintenance free ease of operation while using an efficient automated approach.

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