Tri-State Technologies - Manufacturers Representative And Distributor Of Equipment And Instrument
Tri-State Technologies - Manufacturers Representative And Distributor Of Equipment And Instrument  
 Tri-State Technologies - Manufacturers Representative And Distributor Of Equipment And Instrument   Tri-State Technologies - Manufacturers Representative And Distributor Of Equipment And Instrument
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DuPont™ Krytox® – Oils & Krytox Greases

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High Vacuum Mechanical Pumps

High Vacuum Pumps Used in the Chemical Industry

Oil-Sealed Mechanical High Vacuum Pumps are increasingly significant application tools for todays's high vacuum technology applications. Their outstanding advantages are quite obvious:

  • High vacuum mechanicle pumps offer constant pumping speed down to ultimate pressure.
  • Low ultimate pressure concomitant with compression against the outside atmosphere.
  • No contaminated service water or waste water.
  • Easily expandable, e.g. by means of Roots (Rotary Lobe) Vacuum Pumps, to ensure high pumping speed at low ultimate pressure.
  • Recovery of pumped vapors easy to accomplish by condensation.

The vacuum required in chemical engineering comprises mainly the low and medium vacuum ranges. The mechanical vacuum pumps used in these vacuum ranges are for the most part sliding vane rotary vacuum pumps, rotary plunger or piston vacuum pumps and liquid ring vacuum pumps. Depending on the required pumping speed high vacuum pumps may be used indipendently or in combination with Roots vacuum pumps, forming a multistage vacuum pumping system.

An operating mode of liquid ring vacuum pumps frequently used in the past is the continuous supply of fresh water in open circuit to remove the heat of compression. Owing to the low operating temperature, the pumped solvent vapors condense during the compression phase and get into the drain water. Increasingly severe environmental protection laws and the involved expenses for waste water reconditioning are the reason why the fresh water supply operating mode is only infrequently used for today's high vacuum technology requirements.

An alternative mode is closed-circuit water supply where the heat of compression is removed via a surface heat exchanger. However, the solvent condensate ensuing from compression gets directly into the operating fluid so that an equivalent amount of liquid has to be withdrawn via an overflow device. Therefore, wherever possible, the process condensate itself is used as the operating fluid.

The inlet pressure of a liquid ring vacuum pump, however, can never be lower than the saturation vapor pressure of the operating fluid (normally water). Use of the process condensate as the operating fluid will therefore only make sense where high-boiling, i.e. low-volatility solvents are concerned. High Vacuum Mechanicle pumps are suggested for other VOC's.

In low-boiling (high-volatility) solvents are pumped, the use of an oilsealed high vacuum pump is preferable. Rotary vane and rotary plunger (piston) pumps of single-stage design operate with their full pumping speed even at working pressures of less than 10 m bar. Two-stage models attain working pressures of less than 0.1 m bar with full pumping speed.

High vacuum pump manufacturers do not normally show (in their catalogs) compatibility of their pumps with substances soluble in the pump's oil, since this characteristic is not defined in the PNEUROP Acceptance Specifications nor in National Standards, like DIN. However, the main function of high vacuum pumps in chemical applications is just the evacuation of oil-soluble vapors, e.g. the elimination of organic solvents from a product to be dried (vacuum drying) or in vacuum distillation and rectification.

Modem sliding vane rotary high vacuum pumps tolerate high, in some cases even boundlessly high partial pressures (e.g. of solvent vapors, such as methylene chloride or acetone) in the pump's intake.

While the solvent vapors are conveyed through the pump without condensing inside the pump, they dissolve in the pump's oil up to equilibrium concentration. This may entail a considerable dilution of the oil, reducing its lubricating properties.

Modern rotary vane vacuum pumps, appropriate for use in chemical production, have a separate bearing lubrication. The oil is therefore only used for internal sealing and dissipation of the compression heat, but not at the same time for lubrication of the rotor bearings or of other susceptible parts. Hence a dilution of the pump's oil does not adversely affect the pump's operation and performance. It must only result in a state of equilibrium given by the amount of gas ballast and the pump temperature and must not exceed a maximum permissible value. This permissible oil dilution has been taken into account in the vapor tolerance for oil-soluble substance indicated in the table above.

In an emission condenser on the outlet/discharge side of the rotary vane high vacuum pump, solvents can easily be condensed and thus recovered.

Oils in positive displacement oil-sealed rotary high vacuum pumps perform three essential functions:

  • Internal Sealing
  • Dissipation of the Heat of Compression
  • Lubrication of Mechanically Moving Parts

They determine the requirements to be met by the oil, especially:

  • Vapor Pressure as Low as Possible
  • High Temperature Resistance
  • Good Lubricating Properties

However, the demands made on vacuum pump oils used in chemical applications may be qualified as follows:

Owing to relevant design features of high vacuum pumps, such as separate lubrication of bearings or intensive water cooling, the requirements on the lubricity or the thermal resistance of the oil may be considerably reduced.

For oil-sealed rotary vane high vacuum pumps, used in the chemical industry, a very great variety of oils with very different properties to choose from is available. Thanks to modern high vacuum pump design, of the originally three functions of the oil there remains only the internal sealing, vacuum pump oil is increasingly referred to as sealing fluid. The special requirements to be met by the sealing fluid in chemical applications are manifold. Some of the most important are:

  • Inert to Chemical Attack (e.g. by acids, bases, halogens and halogen-induced ageing)
  • Resistance to Strong Oxidants Like Oxygen, Fluorine and Chlorine
  • Good Solvent Power for Problematic, Mostly Organic Constituents of Pumped Media, such as Oligomers and Polymers, Resins, Crystalline Decomposition or Reaction Products.

Oil-sealed rotary vane and rotary plunger (piston) high vacuum pumps, like every other machine using oil a lubricant, need periodic oil changes The chief purpose of changing the oil is the removal of particles originating from ageing of the oil or generated by the process (dust, decomposition products).

Oil aging and particulate formation are inevitable and so oil change must be carried out. However, the endurance time of the oil can be greatly extended and thus the wear on the pump be minimized through the use of oil filtering devices.

Basically, partial-flow (bypass) a full-flow oil filtration is possible. Bypass oil filtration has the disadvantage that the oil supplied to the active part of the pump is a mix We of purified and unpurified oil taken from the pump's oil reservoir. In the case of full-flow oil filtration only filtered (purified) oil is fed via an oil circulation pump to the active part of the high vacuum pump. The filter elements used are either a paper filter, candle (mechanical filter) or a filter cartridge filled with activated alumina (chemical filter). This chemical filter effects adsorption of oil aging products as well as chemisorption, e.g. of acids and bases, and acts additionally as a good mechanical filter (like the paper filter). The efficiency of oil filtration may be improved and optimized using a combination filter, consisting of paper filter plus activated alumina.

Typical Application of High Vacuum Mechanical Pumps in Vacuum Drying. The drying of products under reduced pressure is an ideal field of application for mechanical high vacuum pumps. Vacuum drying is advisable wherever temperature-sensitive products have to be dried at low temperature or the residual moisture is difficult to eliminate from the product.

An important advantage of vacuum drying, especially today, is the facility to recover in a simple way solvents removed from the product. The proportionally small amount of permanent gases in the pumped medium allows virtually complete condensation of the solvent vapors, thus minimizing emissions.

Intermittent vacuum drying processes are distinguished by the fact that process data, such as mass flow rate, temperature and working pressure, may vary in a very short time over a wide range. In most cases, the whole drying process can be subdivided into two phases.

  • Main or primary drying at constant working pressure.
  • Final or secondary drying at permanently decreasing working pressure.

During the primary drying phase, the "free" i.e. not capillary-bound solvent is mainly evaporated. The working pressure, therefore, corresponds to the saturation vapor pressure of the respective solvent at the prevailing product temperature. Under such conditions the solvent vapors released from the product can be extracted almost completely by condensation. The high vacuum pump has only to remove the leakage air penetrating through leaks in the process system or equipment.

The secondary drying phase starts a soon as all unbound solvent has bee removed from the product surface by evaporation. Now the evaporation of solvent is continued within the capillaries or other cavities of the product. The vapor transport to the product surface goes along with a reduction of the vapor pressure which becomes lower the more the liquid level within the capillaries is decreasing.

The vacuum generating system is expected to meet the following requirements:

  • The working pressure, i.e. the process pressure in the drier, should be as low as possible to ensure complete drying of the product even where residual moisture is difficult to eliminate.
  • High pumping speed at low working pressure is required. The reduction of the vapor pressure, on the one hand, gives rise to an extremely increasing vapor volume; on the other hand, the condensation temperature is so much reduced that condensation before the pump inlet becomes impossible.

During the primary or main drying phase, the inlet-side condenser is active making a high pumping speed for solvent vapor available. The main pipe to the high vacuum pump is closed by the vacuum valve. By means of an aperture plate in the bypass line the pumping speed of the rotary vane high vacuum pump is reduced so far that only the leakage air, saturated with solvent vapor, is conveyed through the pump. At constant temperature the amount of solvent needed to saturate the leakage air flow decreases, while the total pressure is increasing. Additional condensation on the outlet of the high vacuum pump will thus further reduce the amount of solvent in the exhaust air. Fig. 7, illustrates that it is possible to operate a rotary vane high vacuum pump with gas ballast without increasing the total emission. To do this, a partial flow of the exhaust air depleted of solvent at the condenser outlet is tapped and fed back as gas ballast to the Pump.

The secondary drying phase is characterized by a permanent decrease of the working pressure. As long as the working pressure is higher than the saturation vapor pressure associated with the minimum condensation temperature attainable, the inlet-side condenser remains effective. As soon as the working pressure drops and reaches the value of this saturation vapor pressure the condensate collector must be isolated by means of valve, in order to prevent re-evaporation of solvent from this receiver. At the same time, the main pipe to the rotary vane high vacuum pump is opened through valve V 3 making the full pumping speed avail able. The following-on start of the Roots vacuum pump P 1 increases additionally the volume flow rate of the pumping system, thus answering to the increased demand for pumping speed in the final drying phase.

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