Classification of Displacement Pumps 2. During the suction stroke, the pump cylinder fills with fresh liquid, and the discharge stroke displaces it through a check valve into the discharge line. Reciprocating pumps can develop very high pressures. Plunger, piston and diaphragm pumps are under these type of pumps. The rotary pump mechanisms consisting of a casing with closely fitted cams, lobes, or vanes, that provide a means for conveying a fluid. Vane, gear, and lobe pumps are positive displacement rotary pumps.
In pneumatic ejectors, compressed air displaces the liquid from a gravity-fed pressure vessel through a check valve into the discharge line in a series of surges spaced by the time required for the tank or receiver to fill again.
Skip to content. Classification of pumps mainly divided into two major categories: 1. Hg maximum. But there are exceptions. Some two stage oil-lubricated designs have vacuum capabilities up to Also see the section on medium-vacuum pumps. The rotary vane design offers significant advantages: compactness; larger flow capaci- ties for a given size; lower cost about 50 percent less for a given displacement and vacuum level ; lower starting and running torques; and quiet, smooth, vibration free, con- tinuous air evacuation without a receiver tank.
Rotary Screw and Lobed Rotor Pumps - Vacuum capabilities of rotary screw pumps are similar to those of piston pumps, but evacuation is nearly pulse-free. Lobed rotor vacuum pumps, like the corresponding compressors, bridge the gap between posi- tive and nonpositive displacement units.
Air flow is high but vacuum capabilities are limited to about 15 in. Capabilities can be improved with staging. The most significant advantage of this design is its ability to provide very-high-volume flow rates-much higher than possible with any of the positive displacement designs. But because of their inherent leakage, these machines are not practical for applications requiring higher vacuum levels and low flow rates.
The principle types of nonpositive displacement vacuum pumps are the centrifugal, axial-flow, and regenerative designs. Single-stage regenerative blowers can provide vacu- ums up to 7 in. Hg with flows to several hundred cfm. Vacuum capabilities of the other designs are lower unless they are multistaged.
Actual pump selection, covered in a separate section, will be based on how these characteristics relate to the intended application. Somewhat less critical are temperature effects and certain other characteristics.
In general, the best pump for a specific job is the one having the greatest pumping capacity at the required vacuum level and operating within an acceptable horsepower range. Vacuum Level - A pump's vacuum rating is the maximum vacuum level for which it is recommended. The rating is expressed in in. Hg and is specified for either continuous or intermittent duty cycles.
Most vacuum pumps can't come near the theoretical maximum vacuum Hg at sea level because of internal leakage. For a reciprocating piston pump, for example, the upper vacuum limit may be 28 or Hg, or roughly 93 to 95 percent of the maximum theoretical value. Internal leakage and clearance volume establish the highest vacuum a pump can pro- duce. For some pumps, this is also the vacuum rating. In other types, however, heat dissipation is a problem. For these, the maximum vacuum rating might be based on allowable temperature rise.
For example, good wear life for some rotary vane pumps requires a maximum F C rise in casing temperature at the exhaust port. Vacuum ratings will be based on this temperature rise. They probably will be higher for intermittent than for continuous duty. The vacuum rating listed for a pump is based on operation at Operating where atmospheric pressure is lower will reduce the vacuum the pump can produce. Effectiveness of the vacuum pump in removing air from the closed system is given by its volumetric efficiency, a measure of how close the pump comes to delivering its calcu- lated volume of air.
Volumetric efficiency for a positive displacement pump is given by the general equation on page In either case, the displacement is the total volume swept by the repetitive movement of the pumping element during the same time period usually one revolution.
With various vacuum pumps having the same displacement, it is the difference in volumetric efficiencies that accounts for the difference in free air capacities. Since these differences exist, pump selection should be based on actual free air capacity rather than on dis- placement. In short, the air removal rate is a measure of vacuum pump capacity. And the capac- ity of standard machines must be determined from the manufacturers' tables or curves showing cfm of free air delivered at rated speed for vacuum levels ranging from 0 in.
Hg open capacity to the maximum vacuum rating. Free air capacity at different speeds for a given vacuum also may be included in the manufacturers' performance curves. As shown in Fig. Hg and will drop rapidly as the vacuum level increases. This reflects a drop in both volumetric efficiency and the volume of air that can be drawn into the pumping chamber. To repeat, a basic characteristic of positive displacement pumps is that capacity drops as the vacuum level increases. The same principle holds for diaphragm pumps.
The dual single-stage unit has twin chambers operating in parallel. In the dual two-stage unit, the twin chambers operate in series. Recall that the staging process produces higher vacuum levels because the first stage exhausts into a second stage already at negative pressure. This causes a reduction in absolute pressure of the air trapped in the clearance volume the space between piston and cylinder head at the time of full compression.
But as Fig. Fluid Power Horsepower - Different techniques have been developed to evaluate the efficiency of energy use by a vacuum pump. Most vacuum pump manufacturers catalog their test results, including brake horsepower actual hp and cfm vs. Fairly accurate evaluations of power needs can be made from such information sources. For example, the relative efficiency of different pumps can be obtained by calculating the cfm of free air removed per horsepower. Or input horsepower can be compared to the "fluid power horsepower" delivered, which is proportional to the product of gauge vacuum and air flow rate.
All comparisons must be made at the same specific vacuum level, usually at 20 in. Hg or above. Drive Power Requirements -The drive unit must be able to meet the pump's peak power requirement. In other words, it must be powerful enough to assure satisfactory operation under all rated operating conditions. This includes providing adequate energy to overcome friction and inertia effects at startup.
The power requirements of a vacuum pump are relatively low, compared with those of an air compressor. The primary reason is the low compression work requirement. Both the volume flow rate and the pressure difference across the machine are much lower than in a compressor. When the pump operates near atmospheric pressure, for example, the mass flow rate cfm free air pumped is at its highest, but pressure differences between inlet and outlet are very small.
The amount of work that must be added per pound of air is therefore very low. At higher vacuum levels, the amount of work that must be done increases because of the larger difference between inlet and discharge pressure. The mass flow rate or cfm free air pumped drops progressively, however. The total amount of compression work thus remains very low.
Drive Speeds -in addition to actual brake horsepower required for various vacuum levels, catalogs will generally show the speed required to develop various rated capacities.
At higher vacuum levels, there is very little air flow through the pump. Most of the air has been exhausted. There is thus very little transfer of internal heat to this remaining air.
Much of the heat generated by friction must be absorbed and dissipated by the pump. Since some pumps generate heat faster than it can be dissipated, a gradual rise in pump temperature results, drastically reducing service life. One solution is to give careful consideration to pump ratings. For example, a continu- ous-duty pump should have a high maximum vacuum rating. On the other hand, an inter- mittent-duty pump may be specified for high vacuum levels if the off period is adequate for effective cool-down of the pump.
Complications may arise if the maximum on period greatly exceeds the off cooling period. When a pump is unloaded, the atmospheric-pressure air being drawn through it carries the accumulated heat away rapidly. When a pump is shut off with vacuum inside, however, heat loss is much slower because it occurs only through the outside of the casing.
Vacuum Pump Selection The previous section describes how the designer evaluates performance based on vacuum level, air flow, power requirements and temperature effects. This section covers the factors involved in applying the basic characteristics to particular operation and appli- cation needs. In short, we wish to narrow down the selection process to a single type, size, and horsepower for the vacuum pump and related system components.
Vacuum Level Factors Basically, the selection of the appropriate type of vacuum pump is determined by compar- ing the application's requirements with the maximum ratings of available commercial pumps Table 7. But how are required working vacuum levels determined? When a mechanical force is required, the necessary working vacuum is determined in a way similar to establishing pressure requirements of air-operated devices.
Increasing the size of the device to increase its area reduces the working vacuum required. The requirements of specific vacuum devices in the line can be determined by calculations based on handbook formulas, theoretical data, catalog data, or performance curves and tests made with prototype systems.
When this level is relatively low about 15 in. Hg , the designer has a large variety of different types and models of pumps from which to choose. But as vacuum levels rise, the designer has fewer and fewer options, sometimes to a choice of one.
Maximum Vacuum Rating -There are practical limits on the degree of vacuum that can be economically produced to accomplish work. These limits represent the maximum vacuum capabilities of the mechanical pumps used to remove air from the system. Depending on the type of pump involved, this limit ranges from 20 to Very sophisticated and costly equipment is required to obtain higher vacuum levels.
Representative of available commercial vacuum pumps, Table 7 summarizes the Gast line. The Diaphragm and rocking piston types provide the highest vacuum ratings.
Also, they are generally preferred for continuous-service applications. Capabilities of oil- lubricated rotary vane pumps, however, approach these levels. System Control Factors If the system's vacuum level is controlled by a relief valve, the maximum vacuum re- quired should be selected on the basis of the highest working level of any single air device in the system. Temperature Factors Temperature is an important consideration from two standpoints: Environmental Temperature -For ambient temperatures above o F 38O C , select a pump rated for higher vacuum operation and provide some external cooling.
Internal Pump Temperature -Ope ration at higher vacuums increases pump tem- perature and can be the most severe limiting factor on pump operation. Heavy-duty pumps with cooling can operate continuously. But light-duty pumps can operate at maxi- mum vacuums for only short periods; they must be allowed to cool between cycles.
Miscellaneous Type Selection Factors After the basic step in matching vacuum level requirements with the maximum vacuum ratings of available pumps, the selection process proceeds by determining if any of several factors may influence the decision. More often it applies to the exhaust portion of a system, for example, in a food processing plant where dirty air or oil vapors can contaminate products or materials. The most straightforward solution is the selection of an oil-less vacuum pump.
Maintenance-Free Operation -Nothing mechanical is absolutely "maintenance-free. Pulse-Free Air Flow -Rotary vane and nonpositive displacement machines have smooth, continuous air removal characteristics without the extra cost and space require- ments of a receiver tank.
The regenerative blower is also basically vibration-free, but the impellers may generate high-pitched noise. Space Limitations -Again, the rotary vane design is often selected because of its relative compactness. If higher vacuums are required, the rocking piston could be suitable.
Vacuum Capacity Factors The optimum pump size for an application is determined by comparing the rate at which air must be removed from the system with the capacities of various commercial pumps avail- able Table 8.
In general, small capacity and large capacity pumps that have the same maximum vacuum capabilities will pull the same vacuum on a closed system. The small pump will simply require more time to reach maximum vacuum. To directly compare vacuum pump and compressor rating data, the air removal rate is calculated in cubic feet of free air per minute just as in pressure systems.
To determine the free air that must be removed, the volume is multiplied by the vacuum level in atmo- spheres. The latter is obtained by dividing the gauge vacuum in in. Hg by standard atmo- spheric pressure This value is multiplied by the number of work cycles per minute, and the requirements for all the work devices are totaled. Generally, to accommodate possible leaks, the selected vacuum pump should have a capacity rating 10 to 25 percent above the air removal rate actually required.
The capacity of a vacuum pump is generally given by manufacturers' curves or perfor- mance tables showing cfm of free air pumped at rated speed against inlet conditions ranging from 0 in. The capacity rating at the operating vacuum level is generally used to actually select the size.
Keep in mind that there is some flexibility in sizing selection. If a required type of pump is not available in the required size, then two or more smaller pumps can be teamed to provide the necessa ry capacity.
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