7.2.2 Condenser mode

In many vacuum processes (drying, distillation), large volumes of vapor are released that have to be pumped down. Moreover, significant volumes of leakage air will penetrate into large vessels, and those substances that are being vaporized or dried will release additional air that is contained in pores or dissolved in liquids.

In drying processes, the vapor can always be displaced against atmospheric pressure by a vacuum pump having sufficient water vapor capacity and can then be condensed there. However, this process has the following disadvantages:

• The pump must be very large
• A large volume of gas ballast air will be entrained which, together with the vapor, will carry a great deal of oil mist out of the pump
• It will be necessary to dispose of the resulting condensate from the water vapor and oil mist, which is a costly process

Distillation processes operate with condensers, and the object is to lose as little of the condensing distillate as possible through the connected vacuum pump.

Let us consider a vacuum chamber containing material to be dried, to which enough energy will supplied by heat that 10 kg of water will evaporate per hour. In addition, 0.5 kg of air will be released per hour. The pressure in the chamber should be less than 10 mbar. A pumping station in accordance with Figure 7.2 is used for drying, enabling the steam to be condensed cost-effectively through the employment of a condenser.

Figure 7.2: Drying system (schematic)

The material to be dried (2) is heated in the vacuum chamber (1). The Roots pump (3) pumps the vapor / air mixture into the condenser (4), where a major portion of the vapor condenses. The condenser is cooled with water. The condensing water at a temperature of 25 °C is in equilibrium with the water vapor pressure of 30 mbar. An additional vacuum pump (6) pumps the percentage of air, along with a small volume of water vapor, and expels the mixture against atmospheric pressure. The first step is to calculate gas throughput
Q = P1a · S1 using Formula 1-11: Q = S1 · p1a =
where:

 T = 300 K suction gas temperature R = 8,314 J / (kmol · K) t = 3,600 s time pa = 10 mbar = 1,000 Pa inlet pressure mwat = 10 kg volume of water vapor Mwat = 18 kg/kmol molar mass of water mair = 0.5 kg volume of air Mair = 28.8 kg/kmol molar mass of air,

to find the gas throughput: Q = 397 Pa · m3/s and after being divided by inlet pressure pa = 1,000 Pa to obtain the volume flow rate of the Roots pump: S1 = 0.397 m3/s = 1,428 m3/h.
Gas throughput Q is comprised of 385 Pa · m3/s of water vapor and 12 Pa · m3/s of air.

When evacuating the condenser, the partial air pressure should not exceed 30%, i.e. a maximum of 12.85 mbar. It follows from this that: S2 = · S2 = 0.031 m3/s = 112 m3/h.

We therefore select a Hepta 100 screw pump as the backing vacuum pump. Because its pumping speed is somewhat lower than the calculated volume flow rate, this pump will achieve a slightly higher partial air pressure. And we select an Okta 2000 with the following values as the Roots pump:

 S0 = 2.065 m3/h Δpd = 35 mbar differential pressure at the overflow valve K0 = 28 at pv= 43 mbar.

We estimate the inlet pressure pa to be 1,000 Pa and calculate S1 in accordance with Formula 7-6: S1 = · 1,822 m3/s = 0.506 m3/s.

Using pa = , pa = 785 Pa yields the inlet pressure in the drying chamber which, when again used in Formula 7-6, provides a more precise volume flow rate S1 of 1,736 m3/h at an inlet pressure pa of 823 Pa.

We calculate the condenser for a 10 kg/h volume of vapor to be condensed. The following applies for the condensation surface area:

Formula 7-9:

Calculation of the condensation surface area

where

 Qwat = 2.257 106 Ws/kg specific enthalpy of evaporation mwat = 10 kg volume of water vapor ΔTm = 60 K temperature differential between vapor and condensation surface area t = 3,600 s k = 400 W/(m2·K) thermal transmission coefficient

which yields Ak = 0.261 m2 as the condensation surface area.

The vapor is heated by more than 100 K through the virtually adiabatic compression, however it re-cools on the way to the condenser. So the assumption that ΔTm = 60 K is quite conservative. The thermal transmission coefficient k [17] declines significantly as the concentration of inert gas increases, which results in a larger condensation surface area. Inversely, with a lower concentration of inert gas, it is possible to work with a larger backing pump and a smaller condensation surface area. Particular attention should be paid to small leakage rates, as they, too, increase the concentration of inert gas.

Further technical details can be seen from the literature [18].

Figure 7.3: Roots pumping station for vapor condensation

In the interest of completeness, let us again consider the entire sequence of the drying process: A pressure equilibrium initially occurs in the drying chamber, which results from the water volume that is being vaporized, which is caused by heat-up of the material to be dried and the volume flow rate of the Roots pump.

The Roots pump advances the water vapor into the condenser, where it condenses. Because laminar flow prevails there, the vapor flow advances the inert gas being released by the material to be dried into the condenser.

Were the backing pump to be shut down, the entire condensation process would quickly come to a stop, as the vapor could only reach the condensation surface area through diffusion. As the drying process progresses, the volume of vapor declines and less condenses in the condenser; however the concentration of vapor being extracted by the backing pump will tend to be larger if the concentration of inert gas decreases. If the vapor pressure in the condenser declines below the condensation threshold, the condensate will begin to re-evaporate. This can be prevented if the condensate drains into a condensate storage vessel via a valve and this valve closes when the vapor pressure declines below the condensation pressure.

In the case of large distillation systems, the pumping speed of the backing pump should be regulated on the basis of the condensation rate. This can be accomplished, for example, with the aid of a dosing pump that uniformly discharges the volume of pumped condensate from the storage vessel. When the concentrate level in the storage vessel declines below a given level, the backing pump's inlet valve opens and the inert gas that has collected in the condenser is pumped down. The condensation rate now increases again, the condensate level increases and the backing pump's inlet valve closes again. This arrangement means that the system pumps only when the condensation rate is too low, and only little condensate is lost.

Summary:

When pumping down vapors (drying, distillation), the major pumping effect can be provided by a condenser. Depending upon pressure and temperature conditions, either one or two condensers can be employed (Figure 7.3). The condenser between Roots pump and backing pump is more effective, as the vapor flows into the condenser at a higher temperature and higher pressure, and a small backing pump evacuates only a portion of the vapor. In distillation, condensate loss can be minimized by regulating the pumping speed of the backing pump.

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