Mechanical pumps exhaust

Exhaust conditioning requires a precise identification of all the gases and liquids, both residual precursors and by-products. It also requires leak-tight piping and ducts, and suitable mechanical and chemical scrubbers to remove or neutralize dangerous materials and the proper venting of the mechanical pump with a stainless exhaust filter to remove oil mist at the source. 

B. Troubleshooting and Mounting a Mechanical Pump, it will be noted in Fig. 6.2 that gas from a rotary pump is exhausted through an immersed flapper valve. For this valve to make a good seal it must be covered with oil. One common cause of the loss of ultimate vacuum performance in these types of pumps is a low oil level, and this should be the first item to be checked when a pump is not performing well. A telltale sign of low oil is a change in the sound of the pump. 

Aside from the two types of rotary pumps mentioned, you will often see the terms single-stage and double-stage mentioned in the context of pumps. A two-stage pump (also called a compound pump) simply refers to a mechanical pump that has one or two pumps connected in line together. The exhaust of the first one... 

All exhaust from a mechanical pump should be vented to a fume hood regardless of the room s ventilation quality or the type of pumped gases. Each time you bring new samples into vacuum conditions, your system is pumping at atmospheric pressured Because pump oils have low vapor pressures, and pump oils themselves are considered nontoxic, there is little concern for breathing pump oil mist. However, there may be dangers from trapped vapors within the pump oils. Regardless, there is little reason to breathe the pump oil mist if it can be avoided. Check with the manufacturer or distributor of your pump for an oil mist filter for your pump. If you use a condensate trap, be sure you position your exhaust line so that material does not drain back into the pump (see Fig. 7.14). 

Fig. 7.14 The proper orientation of a mechanical pump s exhaust condensate trap. Reprinted from N.S. Harris, Practical Aspects of Constructing, Operating and Maintaining Rotary Vane and Diffusion-Pumped Systems, Vacuum, Vol. 31, 1981, p. 176, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

When changing a mechanical pump s oil, first run the pump for a short time to warm the oil. Warm oil will drain more efficiently from the pump. Open the side valve and let the oil pour directly into a container (use a funnel if necessary). If it is physically possible, tipping the pump may speed oil draining, but it should not be required. The pump may retain small pockets of pump oil within small sections of the pump. These pockets can be emptied out by partially closing the exhaust... 

To prevent (or limit) condensable vapors that reach a pump from affecting the mechanical pump oil, a gas ballast (also called a vented exhaust) is used. The gas ballast allows a small bit of atmosphere (up to 10%) into the pump during the compression stage so that the gas from the system is only part of the gas in the pump at the time of greatest compression. Thus, at the time of compression, the total percentage of condensable vapor within the pump is much less than there would be otherwise. Because the gas prior to being expelled is at a lower pressure, less of the vapor can be compressed into a liquid. Then, as the veins sweep into the vacuum side of the pump, no condensed vapor can expand back into a vapor. 

Traps are supposed to prevent exhausts from being released into the working environment. However, they should not be relied on to the extent of the possible negligence of the useifs) or other possible accidents causing failure. Therefore, all exhausts from mechanical pumps should be vented to fume hoods (see Fig. 7.14 on exhausting mechanical pumps to fume hoods). 

Mist traps limit the amount of the aerosols of mechanical pump oils from leaving the pump and drifting into the room containing the pump. These traps are different from the other traps in that they go on the exhaust of the mechanical pump and do not protect the pump or the system, only the operators. 

Mist traps trap oil aerosols (> 0.3 microns) from the exhaust port of mechanical pumps to minimize exposure to workers in the area. Since they cannot trap gases, vent tubes going to fume hoods are still recommended (see Fig. 7.14). 

Mechanical Pumps. Perhaps the most common form of vacuum pump is a mechanical pump that operates with some sort of rotary action, with moving parts immersed in oil to seal them against back-streaming of exhaust as well as to provide lubrication. These pumps are used as forepumps for diffusion pumps. Other common laboratory applications are the evacuation of desiccators and transfer lines and distillation under reduced pressure. These pumps have ultimate pressures ranging from 10 to 0.05 Torr, and pumping speeds from 0.16 to 150 L s or more, depending on type and intended application. 

A mechanical pump providing even lower vacuum levels is the turbomolecular pump, in which one or more balanced rotors (turbine blades) spin at 20,000 to 50,000 rpm. At these rotation rates, the periphery moves at a speed that exceeds the mean molecular speeds of most molecules, and gas-rotor collisions impart a momentum component to the gas in the direction of the exhaust. Compression ratios up to 10 can be achieved as long as the outlet pressure is kept below about 0.1 Torr by a forepump. 

Note To determine the photon yield of their bioluminescence, batches of 10 P. noctiluca cells were stimulated mechanically to exhaustion in a laboratory photometer. The batches of P. noctiluca cells were pumped through the bathyphotometer, and the values given assume that the cells were stimulated to exhaustion in the photometer. The mean for all values, stations, and datas is given with its 95% confidence limit. 

Iselin cruise. On all cruises, cells for calibration were collected in nearsurface waters so that cells would have high levels of bioluminescence (34). The dinoflagellates were collected late in the afternoon by net tows, isolated by pipette into filtered seawater, collected from the same depth, and held in 10-mL pipettes with enlarged tips. After several hours in the dark, and at the time of natural darkness, they were gently introduced a few at a time into the sample chamber intake of the bathyphotometer with the pump running. Batches of the same cells were assayed in the laboratory photometer after being isolated into 3 mL of filtered seawater, held several hours in darkness, and stimulated mechanically to exhaustion. 

The mechanical vacuum pump consists of an eccentrically mounted rotor driven inside a cylindrical housing. Two types of mechanical pumps are constructed (a) the rotary piston type, where the rotor comes in close contact with the housing and thus makes the seal between intake and exhaust compartments, and (b) the vane type pump where two vanes, spring-mounted in the rotor, make contact with the walls of the housing and thereby divide the space between the rotor and the housing. Typical construction is shown in Figs. 2 and 3. 

The ventilation system in a laboratory using mercury or mercury compounds should conform to the general recommendation that wet chemistry laboratories involving any hazardous material be provided with 100% fresh air instead of having a portion of the air recirculated. Local ventilation systems, such as the exhausts of mechanical pumps servicing mercury diffusion pumps, should be collected with a local exhaust system and discharged into the fume hood exhaust system in the room or to a separate exhaust duct provided to service such units. The mercury vapor is much heavier than air so it is important that the room exhausts be placed near the floor or at the back of the workbench to collect as much of the vapors as possible. 

FIGURE 7.1 Schematic of a lab-scale 200 mm diameter iCVD reactor system. For a vinyl homopolymerization, a constant flow of monomer and initiator is metered into the pancake -style vacuum reaction chamber. An array of resistively heated wires, suspended a few centimeters above the substrate, heats the vapors. Laser interferometery provides real-time monitoring of the iC VD polymer thickness. The pressure of the chamber is controlled by a throttling value. Unreacted species and volatile reaction by-products are exhausted to a mechanical pump. For copolymerization, an additional monomer feed line would need to be added to the system (top image). Schematic cross-section of the iCVD reactor showing decomposition of the initiator by the heated filaments. Surface modification through polymerization of the monomer occurs on the actively cooled substrate (bottom image). 

Basically the pulsed Laval apparatus consists of one or two pulsed valves suppl3ung gas to a reservoir on which is moimted a Laval nozzle. The pulsed supersonic expansion is generated in the main chamber exhausted by a mechanical pump to pressures of 0.1 1 mbar. The uniformity of the Laval expansion is achieved by the appropriate background pressiue, consisting of the pulsed gases and a slip gas, which collimates the expansion along the axis of the gas flow. After a short transition time due to the valve cycle, the uniform supersonic flow is established in the chamber. As... 

Heating of the gas by compression can pose problems. For example, blower pumps compress large amounts of gas and generate a lot of heat. If the blower pump is exhausted to atmospheric pressure, the pump will overheat and the bearings will suffer. Generally, a blower pump is backed by an oil-sealed mechanical pump so that it exhausts to a pressure lower than atmospheric pressure. 

Booster pump (vacuum technology) A pump used between the high vacuum pump (particularly the diffusion pump) and the backing pump in order to increase the throughput in the medium vacuum range and decrease the volumetric flow through the backing pump. Example Diffusion pump (DP) exhausts into a Roots blower (booster pump) then into an oil-sealed mechanical pump. See also Vacuum pump. 

Demister (vacuum technology) A baffle on the exhaust of an oil-sealed mechanical pump used to condense oil vapors to reduce the loss of oil from the pump. 

Suck-back (vacuum technology) When the mechanical pumps stop, air will suck back from the exhaust side to the low pressure side, bringing with it oil contamination from the mechanical pump. 

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

Mechanically, the pump operates in an oil bath, with the sealing oil lubricating the pump and seals against back flow from the exhaust to the intake/suction. 

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