Choose the Right Vacuum Pump Joe Aliasso, Graham Corporation

Reprinted from CHEMICAL ENGINEERING, March 1999, copyright 1999 by the McGrawHill Companies with all rights reserved. Additional reprints may be ordered by calling Chemical Engineering Reprint Department (212) 9043607.

The need to operate under vacuum is Several devices are available for widespread throughout the chemical process industries (CPI). Distillation, drying, flash producing vacuum at a chemical-process plant. cooling, stripping, and evaporation are among Each has its own advantages and drawbacks the unit operations that frequently take place at less than atmospheric pressure. The resultant pressure is higher than the lIn most cases, the need for a cooling liquid suction pressure of the ejector. source to condense the mixture of motive and In many process applications, the overriding process-fluid vapors consideration is the amount of vacuum (or Ejectors use many types of motive fluid. degree of evacuation) required. Of the five Steam is the most common. Other popular Ejectors are especially attractive when the major types of vacuum producing devices choices include ethylene glycol, air, nitrogen, process load contains condensable or corrosive discussed here, the ejector can achieve the and vaporized organic solvents. To avoid vapors, very low absolute pressures are greatest degree of evacuation: down to 5 contamination and other problems, it is needed, or the vacuum producing capacity micrometers of Hg absolute. Dry pumps and important to choose a motive fluid compatible required is very large. However, these devices rotary piston pumps can each evacuate to 10 with the process fluid. are not confined to such applications; they micrometers Hg; once-through oil pumps can should also be evaluated, along with other reach 500 micrometers Hg; and liquid ring Ejectors offer a range of attractions: options discussed below, in other process pumps can go down to 10 mm Hg. lSimple design, with no moving parts and situations. practically no wear Ejectors also work well as boosters upstream Aside from its vacuum producing ability, each lCan be mounted in any orientation of liquid ring pumps (below). This of the five types has its own set of attractions lCan be fabricated of virtually any metal, as combination can minimize capital and utility and drawbacks. Many of these depend on the well as various types of plastics. The latter are costs with no sacrifice in performance. particular application. usually fiber reinforced grades As in any nozzle, the phenomenon known as lLowest capital cost among vacuum critical flow can arise with an ejector. Roughly Ejectors are workhorses producing devices speaking, critical flow prevails when the The simplest and probably most widely used lOffers the largest throughput capacity of any discharge pressure is at least twice the suction vacuum producer is the ejector (Figure 1). vacuum producing device - can handle more pressure. Under these circumstances, a Sometimes called a jet pump, an ejector works than 1,000,000 ft3/min of process fluid standing shock wave is set up. by converting pressure energy of a motive fluid lNo special startup or shutdown procedures A common misconception arises with regard to (which may be the same as or different from required critical flow: that if the discharge pressure is the process fluid) into velocity energy (kinetic lCan handle condensable loads reduced, the suction pressure of the ejector will energy) as it flows through a relatively small lSimple repair and maintenance. decrease and thus a higher vacuum will be converging-diverging nozzle. This lowered created. This is not possible, because the shock pressure of the motive fluid creates suction in a On the other hand, there are also disadvantages wave isolates the inlet conditions from the mixing chamber, into which the process fluid is to ejectors: discharge conditions. An alternative way for drawn from the vessel being evacuated. The lThe requirement of a pressurized motive the vacuum level of the ejector to be increased process fluid mixes with and becomes fluid is by putting less load to it. Usage of ejectors entrained in the motive fluid stream. This lThe inevitable contamination of the motive throughout the CPI is wide. Examples include: mixed fluid then passes on through a fluid by the process gas, and vice versa vacuum distillation in petroleum refineries converging-diverging diffuser, where the lCan be noisy; may require discharge silencers velocity is converted back to pressure energy. or sound insulation and chemical, plastics, pharmaceuticals and the condensed process fluid is available for sensitive process fluids. However, these synthetic fibers plants; refrigeration at pulp reuse in the plant. devices may also prove to be the best choice and paper mills; drying in chemical and The advantages of liquid ring vacuum pumps for other process situations. pharmaceutical plants; drying, flash cooling are as follows: For condensable process fluids, the choice and refrigeration at food plants; and product lSimpler design than most other vacuum between a liquid-ring pump and an ejector degassing in steel mills. A related widespread pumps; employs only one rotating assembly usually depends on the nature of the use is condenser air venting at power plants. lCan be fabricated from any castable metal customer’s business. Ejectors offer the lower lMinimal noise and vibration capital cost, but the liquid-ring models are Liquid ring pumps are lVery little increase in the temperature of the ordinarily less expensive to operate. cool discharged gas Accordingly, for example, a customer with a Another vacuum producing device that can lCan handle condensable loads ready source of inexpensive steam for use as evacuate vessels containing condensable or lNo damage from liquid or small particulates motive fluid might favor the ejector. otherwise “wet” loads is the liquid ring vacuum entrained in the process fluid lMaintenance One-stage (medium vacuum) versions of the pump (Figure 2). In its approximately and rebuilding are simple compared to most liquid-ring pump can evacuate up to 20,000 other vacuum pumps ft3/min. Two-stage (high-vacuum) versions cylindrical body, a sealant fluid under 3 centrifugal force forms a ring against the inside lInherently slow rotational speed (1,800 evacuate up to 7,000 ft /min. of the concentric casing. rev/min or less), which maximizes operating Like ejectors, liquid-ring vacuum pumps are The source of that force is a multi-bladed life widely employed in the CPI. They are found impeller whose shaft is mounted so as to be lCan be started and stopped over and over at petroleum refineries and petrochemical eccentric to the ring of liquid. Because of this lCan use any type of liquid for the sealant plants for vacuum distillation and vapor eccentricity, the pockets bounded by adjacent fluid, in situations where mingling with the recovery, and as an adjunct to vent gas impeller blades (also called buckets) and the process vapor is permissible compressors. Chemical and pharmaceutical ring increases in size on the inlet side of the The drawbacks of liquid-ring vacuum pumps facilities employ them for distillation and pump, and the resulting suction continually are as follows: drying; at pulp and paper mills, they are found draws gas out of the vessel being evacuated As lInevitable mixing of the evacuated gas with for the priming of stock-handling pumps. the blades rotate toward the discharge side of the sealing fluid Plastic and synthetic-fiber facilities apply the pump, the pockets decrease in size and the lRisk of cavitation, which requires that a them for distillation and for venting extruder evacuated gas is compressed, enabling its portion of the process load be noncondensable gases. They dry and flashcool a variety of food discharge. under the pump operating conditions products. Among their environmentally related The ring of liquid not only acts as a seal; it also lHigh power requirement to form and applications are groundwater remediation and absorbs the heat of compression, friction and maintain the liquid ring, resulting in larger sampling, and vacuum filtration of wastewater. condensation. In principle virtually any type motors than for other types of pumps of liquid can be used, so long as it is not prone lAchievable vacuum is limited by the vapor Dry pumps run cleanly to vaporization (and thus to cavitation) at the pressure of sealant fluid at the operating Unlike ejectors and liquid-ring pumps, dry process conditions. Popular choices include temperature The liquid-ring pump is especially vacuum pumps are devices that need no water, ethylene glycol, mineral oil and organic attractive when the process load contains working fluids. Three types are available: the solvents. Assuming that the evacuated process condensable vapors, or if liquid carryover is hook-and-claw, screw and lobe types vapor does not react with or dissolve in the present (due either to normal operation or (Figures 3, 4 and 5). sealant liquid, contamination is minimized and process upsets), or if cool running operation is required due to flammable or temperature CONDENSERS LEND ENCHANCEMENT

In situations where the process fluid being evacuated consists largely or fully of condensable vapor, the performance and economics of a vacuum-producing system can be optimized by including a condenser. Indeed, the condenser by itself functions as an efficient vacuum-producing device. A conventional vacuum pump, as described in the main text of this article, is required immediately downstream to remove noncondensable gases, including air that leaks into the process system. However, the load on this pump is far smaller than it would be without the condenser. For environmental and other reasons, the condenser of choice is usually a shell-and-tube exchanger. It can be configured for either shellside or tubeside condensation. In either case, the process-fluid pressure drop through the condenser must be low, to prevent overloading the downstream pump. In that regard, a common mistake is to simply install a heat exchanger designed for liquid-to-liquid service in the same throughput range. Such exchangers always incur excessive pressure drop, due to the presence of baffles throughout the shell and the absence of any aircooling section. What’s more, they usually lack enough surface area to serve for condensing. When sizing a vacuum system with a condenser, the engineer must realize that for steady state conditions handling condensable vapors, the condenser reduces load to the downstream equipment. When this same The OTO option system is used during startup, running on air only, the condenser cannot reduce the load to the downstream A once-through-oil (OTO) vacuum equipment. This startup condition may determine the size of the equipment or warrant a special startup pump is a sliding-vane type that uses procedure. A well-written specification for a condenser includes the following information: once-through oil to seal clearances ·Operating pressure and temperature and lubricate moving parts. The ·Discharge pressure vanes are in slots in a rotor, mounted · Mass flowrate, molecular weight and relevant chemical properties (such as corrosiveness) of each eccentrically to the pump chamber. component in the process stream ·Type and inlet temperature of available cooling fluid As the rotor assembly rotates, The choice as to type of condenser (fixed tubesheet, U-tube, removable bundle, other) and the materials of centrifugal force pushes the vanes out construction may be included in the specification, left to the equipment vendor, or discussed between the of the slots and against the chamber two parties. walls, creating pockets whose size These pumps work by either of two lMay require a gas purge for cooling, varies similarly to those of the liquid- mechanisms, volumetric reduction or the or to protect the bearings and seals ring pump. Because of this variation, mixing of lower-pressure gas with higher- from the process gas suction draws process gas into the pressure discharge gas (as in a Roots blower). lLimited choice of materials of con- pump from the vessel being Some types use a combination of both struction for the pump evacuated, and compression occurs as mechanisms, the particular combination lDue to the high operating the vanes rotate toward the discharge depending on the pump manufacturer. temperature, some process gases may side of the device, decreasing the area Unlike liquidring pumps, dry pumps normally have a tendency to polymerize and forcing the gas and lubricating run hot, because there is no liquid to absorb the oil against the discharge valve. The heat of compression. This increases the Dry pumps are more expensive than discharge valve opens slightly above temperature of the process gas. To either ejectors or liquid-ring pumps. atmospheric pressure. counteract this, some designs provide They should be used when for precompression by recycling contamination of the process fluid is The attractions of the OTO pump discharged gas that has been cooled. to be avoided, when solvent recovery include these: is the main objective, or when lCan handle acidic or otherwise Advantages of dry-running pumps emissions must be particularly low. corrosive vapors, because the once- include the following: In cases where there is risk of liquid through oil continuously flushes the lNo contamination of evacuated gas, carryover from the evacuated vessel, vapors out which thus can be recovered readily the use of knockout pots, filters or lCan be fabricated of cast iron lDue to lack of condensation (which other similar devices is required so lMinimal vibration is assured because the device runs the pump will not be damaged. lCan handle high inlet temperature hot), pump can be fabricated of Single-stage dry pumps range in (typically, up to about 250°F) standard, inexpensive cast iron capacity from 70 to 1,500 ft3/min. But However, the OTO concept also has lCan discharge to the atmosphere when the pump is used as part of a some disadvantages: The drawbacks associated with dry two-stage system, capacities can be as lRequires the use of immediately pumps are as follows: high as 25,000 ft s/min. upstream knockout pots to prevent lCannot handle particulates, nor In the CPI, dry pumps appear in liquid from hitting (and thus large slugs of liquid pharmaceutical, fine-and-specialty- breaking) the vanes lMay require a silencer chemical and other chemical plants lContamination of the discharged lMay discharge gases at high in conjunction with distillation, process gas by the oil (typically to the temperatures, in some situations as evaporation, and drying. They are extent of 0.12 to 2 gal/d) high as 600°F wide spread in the semiconductor lCannot handle particulates lIn most models, dificult to repair or industry. And, food processors employ rebuild them for vacuum distillation. Advantages are as follows: detailed, well-written specification. lSolvent recovery not possible, due to oil lMinimal vibration, due to balanced Regardless of the type of vacuum-producing contamination configuration device, the vendor needs to know the mass lCostly to repair or rebuild lRugged design, fostering long life flowrate and molecular weight of each lCannot readily accommodate condensable lCan handle small particulates component present in the process fluid to be vapors The disadvantages, many of them in evacuated, as well as the pressure and lRequires constant monitoring of the oil common with those of the OTO pump, are temperature at the inlet to the vacuum- system as follows: producing system. If an ejector is under OTO pumps are usually found in lCannot handle liquids; requires the use of consideration, the customer should furnish pharmaceutical plants and fine-and- knockout pots the type, temperature, and maximum and specialty-chemical plants, for drying, lThe condensable vapors are not easily minimum available pressure for the motive evaporation, distillation or other vessel handled fluid, as well as the vapor pressure for each evacuation tasks. Typical capacities are lThe discharge gas is contaminated with oil component in the process fluid. If the about 450 ft3min. lSolvent recovery not possible, due to oil motive fluid is steam, its quality should be In practice, the OTO pumps have often contamination specified. been misused, the aforementioned lSome designs are noisy The pressure against which the device will limitations being ignored. Today, this type of The big attraction of the rotary piston pump discharge is important for all types of pumps pump is being replaced by dry pumps, which is its ruggedness. Its major markets are covered in this article. If a liquid-ring pump incur lower lifecycle costs. outside the CPI: automotive, aerospace and is under consideration, the type and Rotary-piston models heat treating. properties of the available sealant fluid A rugged type of vacuum-producing device Rotary piston pumps are often combined in should be indicated in the specification. is the rotary-piston vacuum pump. Its piston series with another vacuumproducing The available power supply is a key input is attached to a cam that is mounted device upstream, for greater pumping for liquid-ring, dry, OTO and rotary-piston eccentrically to the main bore of the pump speeds and lower operating pressures. pumps. And for the last named three types, cylinder. Singlestage capacities are in the range of the customer must tell the vendor about At the start of the cycle, the volume 750 ft3/min. possible liquid carryover, likelihood of between the piston and cylinder increases These vacuum pumps should be used when polymerization or other solidification, and as the shaft rotates the piston cam the process load is dry and contains only temperature limits due to autoignition or assembly. Gas is drawn in through a noncondensables. Contamination of the other phenomena. channel in the piston, until this volume is at process gas cannot be avoided with this Edited by Nicholas P. Chopey its maximum. At that point, the pocket design. Furthermore, liquid slugs can becomes sealed from the inlet as the inlet damage the pump. channel in the piston closes off. Lubricating Preparing a good oil helps seal the clearances. specification The shaft then further rotates the piston- Companies that manufacture and offer the and-cam assembly, in a way that complete line of vacuum-producing devices compresses the sealed-off gas against the are in a particularly good position for pump cylinder and the discharge valve. The choosing the best system for a given discharge valve opens when the gas customer application. In any case, however, pressure is slightly above atmospheric. the vendor customer interaction is greatly The gas and lubricating oil is then forced enhanced if the customer prepares a out and the cycle repeats itself.