How to make sure you select the right dry vacuum pump

By Joe Aliasso. Unlike steam jet ejectors and liquid ring vacuum pumps, dry vacuum pumps do not require any working fluids to create a vacuum. They operate by either gas compression, or a combination of gas compression and mechanical compression (volume reduction).

The definition of a dry vacuum pump is a increased clearances, the ultimate pump that does not use any fluids to create a vacuum would be degraded. The rotary lobe and rotary screw type are vacuum or contact the process gas and can also Another consideration would be the difficult to re-time when repairing. Setting the discharge to atmosphere. manufacturing costs due to the clearances on a rotary screw type is difficult as complex parts that make up a dry the maintenance person will need to perform All dry vacuum rumps run hot, since there are vacuum pump. Stainless steel this on a curved surface. The rotary lobe type no liquids to remove the heat created by normally is two to three times the will require trained personnel to set the compressing the gas. They all employ a cooling price of iron. With the proper clearances and re-time the pump also. jacket to help regulate the internal temperature selection and correct operating in the pump, preventing excessive thermal procedure, dry vacuum pumps Rotary screw growth, which would lead to failure. Timing constructed in iron negate the use of gears with oil reservoirs and close clearances stainless steel or other high alloy The rotary screw type vacuum pump can reach between the rotors and housing are additional materials. extremely high vacuum levels (low absolute attributes that make up a dry vacuum pump. The similarities of the three types of pressures). It is in the range of 10 to 100 dry vacuum pumps end here. Each microns. Even though it can reach these levels, Since dry vacuum pumps run hot, they can manufacturer includes unique it cannot be used for every application. easily handle corrosive vapours with standard features to differentiate their pump Because the rotary screw pumps run hot, iron construction, as corrosion normally occurs from others. The end user should actually the hottest of any of the designs, in the presence of moisture. Cast or ductile iron understand what features are temperature sensitive materials and severe is inexpensive to manufacture in, and limits available and then choose the dry service applications will cause problems for thermal growth. Dry pumps are not pump that best fits the application. this design. Polymerization can occur because manufactured in stainless steel or other high of the operating temperatures. alloy materials because of the excessive thermal growth characteristics associated with these Rotary lobe design Rotary screw pumps move the gas along the materials This would require larger clearances length of the screws. Gas compression and to allow further proper thermal growth, thus The rotary lobe style was the first mechanical compression does not occur until Figure 1: Lobe-type pump. dry pump used in the chemical the final half turn of the screws. Here, immense industry. It is based on the Roots heat is produced. Discharge temperatures of blower principal. This design, when 660°F can be produced in the pump. properly selected, works well with Temperature sensitive products and flammable few problems. Maintenance is gases are difficult to handle with this design. In typically moderate to high on this addition, if any wear occurs on the screws, design as lip seals are employed that they need to be replaced in matched sets. Other frequently leak, and cleanout of the dry pump designs allow for individual part last stage is required as buildup will replacement. occur here. When the lip seals leak, gas can contaminate the bearing oil. Some manufacturers offer a spray coating on If left unattended, the bearings will the screws. The purpose of this coating is for fail and the rotors can make contact. initial fit up of the screws. This spray-on Rotary lobe type pumps can coating is extremely thin and allows for tighter employ heat exchangers to reduce clearances to produce a better ultimate pressure the operating temperature of the and higher efficiency of the pump. As the pump. If condensables are handled, pump is operated, the spray-on coating will then these heat exchangers can wear off and affect performance. condense under vacuum. This would resulting in a lower efficiency (more require a barometric leg or a complex backslippage of gas). Also, because of the control scheme to drain out any liquid in the pump. Hook and claw discharge port is then opened and Figure 3: Claw-type pump. There are two types of hook and claw pumps the compressed gas is merely swept in the market, a two-stage and a three-stage. out. No mechanical compression, by The three stage produces vacuum levels less reducing the volume of the pumping than 1 mmHgA by a combination of mechanical chamber, occurs inside this pump. and gas compression. The two stage produces vacuum levels down to 5 mmHgA by gas On this design, all the discharge gas compression only. passes through a heat exchanger at atmospheric pressure where it is The three stage, with a combination of cooled. This heat exchanger can also mechanical and gas compression, produces act as a condenser and be used for discharge temperatures of 430°F or greater. solvent recovery. Since it operates One manufacturer has solved this temperature at atmospheric pressure, simple problem by offering a pump that has discharge draining of the condensed liquids is temperatures less than 100°F accomplished. No barometric leg or complex control scheme is required. other types usually take days to repair. A hook and claw type requires a larger footprint and runs slightly noisier than the The two-stage and cool running All dry pumps will be sensitive to process rotary lobe and rotary screw type pumps. hook and claw is the only dry upsets and should be carefully selected to vacuum pump that does not use insure maximum run time. Inlet filters and mechanical compression to knockout pots, along with discharge silencers Figure 2: Screw-type pump. achieve vacuum. Because of this are normally required for a complete operating unique design, it can be started system. completely flooded with liquid. This provides a distinct Contact:, Graham Corporation, 20 Florence advantage over other types of Avenue, PO Box 719, Batavia, NY 14021-0719, dry pumps. If the pump is USA. Tel: +l 716 343 2216; Fax: +1 716 343 starting to accumulate product 1097; E-mail: [email protected]; buildup during operation, then it www.graham-mfg.com can be stopped and filled with a solvent to dissolve the buildup. Graham Precision Pumps Limited, The Forge, Forge Lane, Congleton, Cheshire, England, The pump can then be started CW12 4HQ, UK. Tel: +44 1260 274721; Fax: flooded, pumping out the +44 1260 276965 liquids. This also reduces downtime, Most types of dry vacuum pumps use lip seals as the pump would not need to be with an N2 purge. Lip seals require frequent taken apart and cleaned. maintenance with difficult replacement. One design comes standard with single mechanical seals. This eliminates the maintenance Handling liquid problems associated with lip seals and also

eliminates N2 requirements. contaminants

When handling corrosive and wet vapours, the All dry pumps have difficulty hot running type of dry vacuum pump is handling slugs of liquids. Knockout desirable. This will prevent corrosion of the pots are generally required. A liquid iron body components. Also, for handling gases ring pump is the only type of that can solidify at room temperature, the hot vacuum pump that can safely handle running type is also required to prevent any slugs of liquids. If the process product solidification inside the pump. results in the pump continuously being taken out of service for The cool running hook and claw vacuum pump repairs, then a simple to rebuild dry design allows safe pumping of explosive and pump is desired. As mentioned temperature sensitive gases. previously, timing, setting of The cool running hook and claw pump uses a clearances and replacement of lip unique discharge gas heat exchanger that seals requires a major effort. The provides for direct gas cooling. Once the inlet manufacturer with the cool running port is isolated and the pumping chamber is at dry pump offers simplified re- its maximum volume, then cooled discharge gas building. This pump can be taken is injected in. The gases mix and compress, apart and re-built in 2 to 4 hours, increasing density (shrinking the volume). The with no timing requirements. All