How to select a vacuum pump for drying applications

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Fundamentally, we must distinguish between short-term drying and drying processes that can require several hours or even days. Independently of the duration of drying, all drying processes proceed approximately as described on the relevant page.

As an example of an application, the drying of salt (short-term drying) is described, this being an already well-proven drying process.

Drying of salt

First, 881lbs (400kg) of finely divided salt with a water content of about 8% by mass is to be dried in the shortest possible time (about 1 hour) until the water content is less than 1% by mass. The expected water evolution amounts to about 61lbs (28kg). The salt in the chamber is continuously agitated during the drying process and heated to about 176°F (80°C). The vacuum system is schematically drawn in Fig. 2.78.

Fig 2.78 Vacuum diagram for drying of salt. Pump combination consisting of Roots pump, condenser and rotary plunger pump for stepwise switching of the pumping process.

Vacuum chamber with salt filling
Roots pump
Condensers
Throttle valve
Rotary vane pump

During the first quarter of drying time far more than half the quantity of water vapor is evolved. Then the condenser is the actual main pump. Because of the high water vapor temperature and, at the beginning of the drying, the very high water vapor pressure, the condensation efficiency of the condenser is significantly increased. In Fig. 2.78 it is understood that a condenser of 2 m² condensation surface can condense about 3 gallons (15 l) of water at an inlet pressure of 100 mbar in 15 min. However, during this initial process, it must be ensured that the water vapor pressure at the inlet port of the rotary pump does not exceed its water vapor tolerance of 60 mbar. This is ensures by regulation of the rotary vane pumps inlet pressure using the throttle valve. Since the backing pump has only to pump away the small part of the non-condensable gases at this stage, a rotary vane pump SOGEVAC SV65B will suffice. With increasing process time, the water vapor evolution decreases, as does the water vapor pressure in the condenser. After the water pressure in the chamber falls below 27 mbar, the Roots pump is switched in. Thereby the water vapor is pumped more rapidly out of the chamber, the pressure increases in the condensers, and their condensation efficiency again increases. The condensers are isolated by a valve when their water vapor reaches its saturation vapor pressure. At this point, there is a water vapor pressure in the chamber of only about 4 mbar, and pumping is accomplished by the Roots pump with a gas ballast backing pump until the water vapor pressure reaches about 0.65 mbar. From experience it can be assumed that the salt has now reached the desired degree of dryness.
Today’s dry pump technology enables to do the same process without a need for the throttle valve. For example, using a dry screw pump VARODRY VD65, the process could be done without the complex need for pressure regulation. During the first phases of the process, the vapor tolerance of the pump will be short time exceeded. This will cause some condensation of water inside the pump. Those will be pumped out by the VARODRY which will simply dry out later on when the pressure decreases.

Drying of paper

If the pumps are to be of the correct size for a longer process run, it is expedient to break down the process run into characteristic sections. As an example, paper drying is explained in the following where the paper has an initial moisture content of 8%, and the vessel has the volume V.

1. Evacuation

The backing pump must be suitably rated with regard to the volume of the vessel and the desired pump-down time. This pump-down time is arranged according to the desired process duration: if the process is to be finished after 12 –-15 hours, the pump-down time should not last longer than 1 hour. The size of the backing pump may be easily calculated according to the page on that topic.

2. Predrying

During predrying – depending on the pressure region in which the work is carried out – about 75% of the moisture is drawn off. This predrying should occupy the first third of the drying time. The rate at which predrying proceeds depends almost exclusively on the sufficiency of the heat supply. For predrying 1 ton of paper in 5 h, 132lbs (60kg) of water must be evaporated; that is, an energy expenditure of about 40 kWh is needed to evaporate water. Since the paper must be heated to a temperature of about 248°F (120°C) at the same time, an average of about 20 kW must be provided. The mean vapor evolution per hour amounts to 26lbs (12kg). Therefore, a condenser with a capacity of 33lbs (15kg)/h should be sufficient. If the paper is sufficiently preheated (perhaps by air-circulation drying) before evacuation, in the first hour of drying, double vapor evolution must be anticipated.

3. Main drying

(2.37)

(2.38)

If, in the second stage, the pressure in a further 5 h is to fall from 20 to about 5.3 mbar and 75% of the total moisture (i.e., 19% of the total moisture of 33lbs (15kg) is to be drawn off, the pump must, according to equations (2.37) and (2.38), have a pumping speed of

According to equation 1.7, 33lbs (15kg) of water vapor corresponds at 59°F (15°C) to a quantity of water vapor of

Hence the Roots pump would be the suitable pump. The permissible remaining moisture in the product determines the attainable ultimate pressure. The relationship between the ultimate pressure and the remaining moisture is fixed for every product but different from product to product. Leybold has many years of experience to its record regarding applications in this area. Assume that a 0.1% residual moisture content is required, for which the necessary ultimate pressure is 6 · 10^-2 mbar. During the last 5 h the remaining 6% of the moisture content, or 11lbs (5kg) of water, is removed. At a mean pressure of about 0.65 mbar, 2000 m³/h of vapor is evolved. Two possibilities are offered:

a)One continues working with the above-mentioned Roots pump. The ultimate total pressure settles at a value according to the water vapor quantity evolving. One waits until a pressure of about 6.5 · 10-2 mbar is reached, which naturally takes a longer time.

b)From the beginning, a somewhat larger Roots pump is chosen (e.g., one with a pumping speed of 2000 m³/h is suitable). For larger quantities of paper (11,023 lbs or 5000 kg, for example) such a pumping system will be suitable which at a pumping speed for water vapor of up to 20,000 m³/h automatically lowers the pressure from 27 to 10^-2 mbar. The entire time need for drying is significantly reduced when using such pumps.

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