Food Applications In The Vacuum Pump Industry
It seems there are as many applications for vacuum pumps in the food industry as there are different foods to eat. From meat packaging that evacuates the air out of a plastic package (4 torr) to improve shelf life to protein dryers that process meat byproducts to isolate their proteins for use as food or dietary supplements. The product which has been broken down using various chemicals, is heated and vacuum is applied to dehydrate it for storage and packaging. Because the product is degraded by excess heat, vacuum is key in efficient processing (28”HgG).
In the poultry industry the product is first eviscerated using vacuum (25-28”HgG), can then be marinated under vacuum 28”HgG, the partial vacuum applied opens the pores of the meat allowing the marinate to completely penetrate in as little as 15 minutes, and then finally packaged under vacuum.
Evaporation is a very common process in the food industry. It describes a process where water is evaporated or “flashed off” of various liquids in order to condense them or ultimately convert them from liquids into solids, usually in the form of a powder. By controlling the temperature and pressure (or absence of) in the evaporator, the vapor pressure of the water in a product can be reached, causing some of the water to change phase from a liquid to a vapor. This water vapor is then pulled out of the evaporator, through a condenser, and into the vacuum pump. When this occurs, the solids are suspended in the remaining water, but now in a more concentrated solution. In some types of evaporators, called multi-effect, this concentrate is evaporated several times resulting in a very concentrated end product.
There are several types of evaporators including the falling film, rising film, plate and circulation. A falling film evaporator (Pictured below) is used in the production of dairy & juice products, milk concentrates and powders, whey products, sweetened and unsweetened condensed milk and evaporated milk.
Some basic concepts that can be learned with this example are that evaporation & drying are most energy intensive, i.e. the addition of heat. Vacuum accelerates the evaporation process, i.e. the boiling point of the solution is reduced by the lower pressure & resulting vapor. In the case of processing cheese whey a multi-effect evaporator is used in converting the watery liquid into a thick slurry which can then be spray dried into a powder and packaged in snacks. The whey solids are very high in protein and certain nutrients, and are resold for use in candy, cattle feed and nutritional supplements.
Pump selection is based on a common operating pressure of 27-28”HgG, and capacity is usually selected by two factors; what a desirable time frame is for hogging down the evaporator, and what the gas load will be once the evaporator is at normal operating conditions. Many times, these considerations result in a dual system, with two identical vacuum pumps which can be run together for hogging, or individually for holding. Common pump sizes are 10-25Hp two-stage liquid rings.
Pump down formula does not consider any type of friction loss from the vacuum pump to the chamber:
S=V/T In (P1/P2) Where S is pump speed in acfm, V is volume of chamber in cu/ft, T is time in minutes, In is the natural log, P1 is the starting pressure and P2 is the final operating pressure measured at the vacuum pump inlet. What size pump would be required to evacuate a 20cu/ft chamber from 760torr to 50 torr in 3 minutes;
20/3 In (760/50) = 18.14acfm