How to make sure you select the rightdry vacuum pumpBy Joe Aliasso. Unlike steam jet ejectors and liquid ring vacuum pumps, dry vacuum pumps do not requireany working fluids to create a vacuum. They operate by either gas compression, or a combination of gascompression and mechanical compression (volume reduction).

The definition of a dry vacuum pump is apump that does not use any fluids to create avacuum or contact the process gas and can alsodischarge to atmosphere.

All dry vacuum rumps run hot, since there areno liquids to remove the heat created bycompressing the gas. They all employ a coolingjacket to help regulate the internal temperaturein the pump, preventing excessive thermalgrowth, which would lead to failure. Timinggears with oil reservoirs and close clearancesbetween the rotors and housing are additionalattributes that make up a dry vacuum pump.

Since dry vacuum pumps run hot, they caneasily handle corrosive vapours with standardiron construction, as corrosion normally occursin the presence of moisture. Cast or ductile ironis inexpensive to manufacture in, and limitsthermal growth. Dry pumps are notmanufactured in stainless steel or other highalloy materials because of the excessive thermalgrowth characteristics associated with thesematerials This would require larger clearancesto allow further proper thermal growth, thus

resulting in a lower efficiency (morebackslippage of gas). Also, because of the

increased clearances, the ultimatevacuum would be degraded.Another consideration would be themanufacturing costs due to thecomplex parts that make up a dryvacuum pump. Stainless steelnormally is two to three times theprice of iron. With the properselection and correct operatingprocedure, dry vacuum pumpsconstructed in iron negate the use ofstainless steel or other high alloymaterials.The similarities of the three types ofdry vacuum pumps end here. Eachmanufacturer includes uniquefeatures to differentiate their pumpfrom others. The end user shouldunderstand what features areavailable and then choose the drypump that best fits the application.

Rotary lobe design

The rotary lobe style was the firstdry pump used in the chemicalindustry. It is based on the Rootsblower principal. This design, whenproperly selected, works well withfew problems. Maintenance istypically moderate to high on thisdesign as lip seals are employed thatfrequently leak, and cleanout of thelast stage is required as buildup willoccur here. When the lip seals leak,gas can contaminate the bearing oil.If left unattended, the bearings willfail and the rotors can make contact.Rotary lobe type pumps canemploy heat exchangers to reducethe operating temperature of thepump. If condensables are handled,then these heat exchangers cancondense under vacuum. This wouldrequire a barometric leg or a complexcontrol scheme to drain out anyliquid in the pump.

The rotary lobe and rotary screw type aredifficult to re-time when repairing. Setting theclearances on a rotary screw type is difficult asthe maintenance person will need to performthis on a curved surface. The rotary lobe typewill require trained personnel to set theclearances and re-time the pump also.

Rotary screw

The rotary screw type vacuum pump can reachextremely high vacuum levels (low absolutepressures). It is in the range of 10 to 100microns. Even though it can reach these levels,it cannot be used for every application.Because the rotary screw pumps run hot,actually the hottest of any of the designs,temperature sensitive materials and severeservice applications will cause problems forthis design. Polymerization can occur becauseof the operating temperatures.

Rotary screw pumps move the gas along thelength of the screws. Gas compression andmechanical compression does not occur untilthe final half turn of the screws. Here, immenseheat is produced. Discharge temperatures of660°F can be produced in the pump.Temperature sensitive products and flammablegases are difficult to handle with this design. Inaddition, if any wear occurs on the screws,they need to be replaced in matched sets. Otherdry pump designs allow for individual partreplacement.

Some manufacturers offer a spray coating onthe screws. The purpose of this coating is forinitial fit up of the screws. This spray-oncoating is extremely thin and allows for tighterclearances to produce a better ultimate pressureand higher efficiency of the pump. As thepump is operated, the spray-on coating willwear off and affect performance.

Hook and claw

Figure 1: Lobe-type pump.

There are two types of hook and claw pumpsin the market, a two-stage and a three-stage.The three stage produces vacuum levels lessthan 1 mmHgA by a combination of mechanicaland gas compression. The two stage producesvacuum levels down to 5 mmHgA by gascompression only.

The three stage, with a combination ofmechanical and gas compression, producesdischarge temperatures of 430°F or greater.One manufacturer has solved this temperatureproblem by offering a pump that has dischargetemperatures less than 100°F

A hook and claw type requires a largerfootprint and runs slightly noisier than therotary lobe and rotary screw type pumps.

Most types of dry vacuum pumps use lip sealswith an N2 purge. Lip seals require frequentmaintenance with difficult replacement. Onedesign comes standard with single mechanicalseals. This eliminates the maintenanceproblems associated with lip seals and alsoeliminates N2 requirements.

When handling corrosive and wet vapours, thehot running type of dry vacuum pump isdesirable. This will prevent corrosion of theiron body components. Also, for handling gasesthat can solidify at room temperature, the hotrunning type is also required to prevent anyproduct solidification inside the pump.

The cool running hook and claw vacuum pumpdesign allows safe pumping of explosive andtemperature sensitive gases.The cool running hook and claw pump uses aunique discharge gas heat exchanger thatprovides for direct gas cooling. Once the inletport is isolated and the pumping chamber is atits maximum volume, then cooled discharge gasis injected in. The gases mix and compress,increasing density (shrinking the volume). The

discharge port is then opened andthe compressed gas is merely sweptout. No mechanical compression, byreducing the volume of the pumpingchamber, occurs inside this pump.

On this design, all the discharge gaspasses through a heat exchanger atatmospheric pressure where it iscooled. This heat exchanger can alsoact as a condenser and be used forsolvent recovery. Since it operatesat atmospheric pressure, simpledraining of the condensed liquids isaccomplished. No barometric leg orcomplex control scheme is required.

The two-stage and cool runninghook and claw is the only dryvacuum pump that does not use

mechanical compression toachieve vacuum. Because of thisunique design, it can be startedcompletely flooded with liquid.This provides a distinctadvantage over other types ofdry pumps. If the pump isstarting to accumulate productbuildup during operation, then itcan be stopped and filled with asolvent to dissolve the buildup.The pump can then be startedflooded, pumping out the

liquids. This also reduces downtime,as the pump would not need to betaken apart and cleaned.

Handling liquidcontaminants

All dry pumps have difficultyhandling slugs of liquids. Knockoutpots are generally required. A liquidring pump is the only type ofvacuum pump that can safely handleslugs of liquids. If the processresults in the pump continuouslybeing taken out of service forrepairs, then a simple to rebuild drypump is desired. As mentionedpreviously, timing, setting ofclearances and replacement of lipseals requires a major effort. Themanufacturer with the cool runningdry pump offers simplified re-building. This pump can be takenapart and re-built in 2 to 4 hours,with no timing requirements. All

other types usually take days to repair.

All dry pumps will be sensitive to processupsets and should be carefully selected toinsure maximum run time. Inlet filters andknockout pots, along with discharge silencersare normally required for a complete operatingsystem.

Contact:, Graham Corporation, 20 FlorenceAvenue, PO Box 719, Batavia, NY 14021-0719,USA. Tel: +l 716 343 2216; Fax: +1 716 3431097; E-mail: equipment@graham-mfg.com;www.graham-mfg.com

Graham Precision Pumps Limited, The Forge,Forge Lane, Congleton, Cheshire, England,CW12 4HQ, UK. Tel: +44 1260 274721; Fax:+44 1260 276965

Figure 2: Screw-type pump.

Figure 3: Claw-type pump.