Laboratory Vacuum Pumps
/Most Laboratory vacuum applications operate at partial vacuum levels between 1 Torr and 277 torr(29.88"HgV & 19"HgV). This pressure range is where most common lab applications operate and also represents the rough range of vacuum.
Common lab applications can be defined as vacuum filtration, liquid aspiration and similiar applications that use vacuum to move liquids and are best served in the 60- to 277 Torr range. Partial vacuum below 60 torr can be detrimental to filtration and aspiration applications as vapor pressure concerns come into play. If the partial vacuum is allowed to move as low as the vapor pressure rating of the material this will result in the evaporation of the filtrate and can prove unfavorable for both the application and the vacuum pump. These common laboratory applications are typically served by a "House Vacuum System" or HVS, and can be designed to operate anywhere in the 60- to 277 Torr range.
Laboratory applications including evaporation, distillation, gel drying, and vacuum ovens require a partial vacuum range of 1- to 60 Torr. HVS can also be designed to accommodate these requirements. This pressure range crosses into the use of smaller individual laboratory vacuum pumps and is a perfect fit for dry diaphragm pumps.
Drying/freeze drying, molecular distillation, and glove box applications all require a partial vacuum from 1- to 10-3 Torr. These low pressure applications cannot be served by the typical HVS but require the use of 2-stage oil flooded rotary vane vacuum pumps.
Gas chromatography/mass spectrometry (GC-MS) type lab applications require a partial vacuum as low as 10-8 Torr and are served by turbo-molecular vacuum pumps.
Definition:
SCFM is measured at standard conditions (Air at 68F, 29.92"Hg or 14.pais)
ACFM is measured at actual inlet conditions
When sizing for typical medium sized labs, as a rule of thumb, use 1.0 scfm per outlet and a 50% usage factor. Because the majority of vacuum pump manufacturers rate their vacuum pumps capacity in ACFM a simple method of converting these flow rates has been developed in the lab vacuum industry. The method is based on Boyle's gas law which describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a closed system.
Example: A small laboratory vacuum system requires 20scfm of air at a partial vacuum level of 25"HgV at sea level, convert the mass flow rate to a volumetric flow rate and select a vacuum pump. If the pressure in the system decreases then the volume of the gas occupied will increase proportionally according to the following formula: P1V1=P2V2
V2= 29.92x20/4.92 = 121.6acfm @ 25"HgV
V1= 4.92x121.6/29.92 = 20scfm @ 25"HgV
Pump Selection: After determination of the pressure and flow required for the house vacuum system, specific application requirements need to be considered before final pump and system selection including;
- A close look at the process constituents may include simple water, saline or biological media thats non-corrosive but very wet. Another application may include more corrosive material such as bleach and require a thorough examination of the pump system construction, its operating fluids if applicable, pre-filters or condensers/traps may may have to be evaluated. Pre and discharge manifold arrangements including discharge drains may require consideration.
- Where is the vacuum pump system to be installed? Is there an area classification? It's hot in Houston and it's well known that heat is a major enemy of mechanical and electrical equipment. That mechanical room that used to be 75 degrees back when there were only 3 or 4 pieces of equipment in it keeps growing, and the ambient temperature right along with it. Consider the type of vacuum pump technology, can the technology be water cooled? does the facility offer chilled water? It's estimated that for every 15 to 18 degrees F above the maximum rated operating temperature (180F) of oil in a rotary vane vacuum pump, the oil degradation rate doubles. As a rule of thumb regarding electric motors, for every 18 degrees F rise above the highest allowable stator winding temperature reduces the motor insulation useful life in half.
- Other factors such as space requirements, operating sound levels and operating budgets must also be considered.
Common HVS Vacuum Pump Types:
- Oil sealed liquid ring vacuum pumps use light weight oil in the pumping chamber to create compression. They are one of the most reliable vacuum systems available and be configured for air or water cooling. Liquid ring pumps operate with the lowest sound pressure compared to any other vacuum pump available in the market today. Pump operating temperatures can be selected form 165F to 190F in order to match the expected gas load to be handled. Since the oil in the compression chamber is not used as a lubricant, it can easily last for one year or more before an oil change is necessary. Oil can be selected to include anti-wear, anti-oxidants, and corrosion inhibitors to tolerate many mild corrosive gas and vapors. Liquid ring pumps are capable of producing pressures as low as 28.9"HgV (25.9 Torr) and can discharge up to 30psig with an atmospheric inlet depending on the pumps configuration. Pumps are available as either single or 2-stage designs depending on the pressure/capacity requirements and use single acting mechanical shaft seals.
- Water sealed liquid ring vacuum pumps use water in the pumping chamber to create compression. They are one of the most robust vacuum systems available and can tolerate up to 1/4" soft solids and large condensable loads. These systems can be configured as once through or total recirculated seal fluid to meet the application and customers requirements. Liquid ring vacuum pump systems also offer very low operating sound pressures. Water sealed liquid ring systems offer the coolest operating temperatures compared to any other vacuum pump in its class featuring near isothermal compression. Pumps are available as single or 2-stage designs to meet application requirements and use single acting mechanical shaft seals. Water sealed liquid ring vacuum pumps are capable of producing pressures as low as 28.9"HgV (25.9 Torr). Pump construction is available in cast iron, CI/stainless or all 316 stainless steel to handle very corrosive gas and vapors.
- Oil flooded rotary vane single stage vacuum pump systems can produce a partial vacuum as low as 1 Torr. Due to their internal design these pumps should only be selected for applications that require lower pressures from 75 Torr to 1 Torr. When operating oil flooded vacuum pumps above 75 Torr you can expect the pumps operating temperature to be hotter than normal. The elevated operating temperature leads to a shorter oil and oil mist eliminator life, shorter bearing, seal and vane life expectancy along with oil blowing out the pumps discharge port. In special cases the pump can be water cooled and secondary mist eliminators can be applied to manage the situation. In other situations an auto-purge system may be utilized to help control condensable pump loads from condensing inside the pump and creating vane and cylinder damage. Rotary vane vacuum pumps are best applied to clean dry applications but over the years have been adapted to operate beyond their intended capabilities. We want to pay close attention to the pumps rated water vapor handling characteristics before applying this machine to wet applications. Pump construction is cast iron with ductile iron rotor using 3 phenolic vanes, needle bearings and viton lip seals.
- All of the above vacuum pump technologies can be used in conjunction with a backing blower to achieve a lower working pressure. Rotary claw and liquid ring vacuum pumps can utilize a VFD.
- Dry rotary vane single stage vacuum pumps can produce pressures as low as 27"HgV. They are constructed of cast iron with ductile iron rotor using up to 6 carbon vanes, sleeve bearings and lip seals. These pumps offer low initial cost but higher life cycle cost than their oil flooded counterparts as vanes must be replaced every 1-2 years of operation. These pumps should not be used in wet applications due to the carbon dust accumulation in the pumping chamber. When the carbon dust comes into contact with water and vapor mist it has a strong tendency to transform into what looks like tiny BB shaped balls that can interfere with the pumps normal operation creating internal leak paths or catastrophic vane failure. Great care should be considered when specifying this technology to protect the pump from wet or dirty pump loads. Dry rotary vane vacuum pumps will produce one of the highest sound levels of any mechanical vacuum pump.
- Rotary claw vacuum pump systems offer no fluids in the pumping chamber and up front seems to allow a more environmentally friendly atmosphere compared to pumps that use oil or other fluids in the pumping chamber. The issue here is that the labyrinth seals are not 100% leak proof and over time oil from the gear box makes it’s way into the pumping chamber. Because there is no “clean up equipment” on the discharge side of the pump hot oil vapor is allowed to discharge directly into the atmosphere. The claw vacuum pump offers an ultimate vacuum level of 27"HgV and can discharge up to 32psig with an atmospheric inlet. The claw pumps operating temperature is hotter (350F> range) compared to pumps with fluids (190F<) in the pumping chamber and generally exhibits a higher sound level as it operates at 3500 rpm. The claw pumps internals are coated with molybdenum disulfide (MoS2) and can tolerate wet and mild corrosive gas and vapors. Because of the claws unique design it performs with the greatest Hp to cfm efficiency (depending on the operating vacuum level) than any other vacuum pump in its class.. Maintenance cost is about the same compared with pumps that use oil in the pumping chamber and still require regular oil changes in the gear box. The claw pump is only available in an air cooled configuration and uses labyrinth shaft seals.
Other HVS pump technologies are available to fit special lab requirements including the scroll vacuum pump
(0.1 Torr) and the twin screw dry vacuum pump (.075 Torr). The screw is available with several types of
protective coatings to handle corrosive gas and vapor. Fixed pitch screw pumps offer high discharge
temperatures that lend itself to handling many acids with low vapor pressures. Variable pitch dry screw
vacuum pumps offer lower discharge temperatures and can successfully pump potentially heat sensitive
reactive gases. Sound levels are high and require discharge silencers.