Vacuums are used primarily to reduce the temperature
at which operations take place thereby reducing potential deterioration in the quality of the material being processed or, in special cases (i.e. edible oil
processing), to avoid unwanted oxidation of the product during processing at higher temperature.
Field of application
A vacuum is applied to many unit operations in the food industry (drying, evaporation, deodorisation/distillative neutralisation, filtration etc).
Techniques, methods and equipment
There are three basic systems for producing a vacuum. These are:
- steam-jet ejectors, which can produce absolute pressures down to 1 or 2 mm Hg
- reciprocating pumps, which can produce absolute pressures down to 10 mm Hg
- rotary vacuum pumps, which can produce absolute pressures as low as 0.01 mm Hg.
Steam jet ejectors
A steam jet ejector consists of a steam nozzle that discharges a high velocity jet across a suction chamber connected to the equipment to be evacuated. The gas is entrained in the steam and carried into a venturi-shaped diffuser that converts the velocity energy of the steam into pressure
energy or possibly that is recirculated over cooling
towers, e.g. under high pH conditions for saponification of free fatty acids (e.g. edible oil processing). The steam and vaporised material from the ejector are condensed either directly in a spray of water
(barometric condensers) or indirectly with surface type condensers or especially condensed under low-temperature conditions (ice condensation: < - 20 ˚C). With barometric condensers, the cooling
water can be used on a straight-through basis or recirculated over cooling towers, e.g. under high pH conditions for saponification of free fatty acids (e.g. edible oil processing). With indirect condensers, the condensate can be recovered. The size of the condenser depends on the cooling temperature used, this also controls the amount of steam required. Chilling or freezing
systems can be used to enable operation at low temperature, thereby reducing steam usage.
The reciprocating pump is of the positive displacement type. Air is drawn into the pump chamber and then compressed by means of a piston before being discharged. Reciprocating vacuum pumps can be either single or multistage devices. The number of stages is determined by the compression ration. The compression ratio per stage is generally limited to four.
Rotary vacuum pump
Rotary vacuum pumps are again of the positive displacement type, i.e. essentially constant volume with variable discharge pressure. The discharge pressure will vary with the resistance on the discharge side of the system. The widely used water-ring vacuum pump has the inlet and outlet ports located on the impeller hub. As the vaned impeller rotates, centrifugal forces drive the sealing liquid against the walls of the elliptical housing, causing the air
to be successively drawn into the vane cavities and expelled with the discharge pressure.