2 Pump types and performance data
2.3 Pump types and their applications
2.3.4 Special pump types
Vortex pumps: In vortex pumps according to Fig. 2.17, there is a large axial dis- tance between the casing front wall and the open impeller so that the flow path from the suction to the discharge nozzle is nearly unrestricted. The impeller, equipped with radial or backward-curved blades, generates a recirculating (or
“vortex”) flow due to centrifugal forces. The incoming fluid is thus forced into a strong rotation. The centrifugal pressure field developed in this way transports the liquid, together with possible inclusions of solids or gas, into the discharge nozzle.
As the fluid rotating outside the impeller is moved only by exchange of momen- tum, there are additional losses due to turbulent dissipation in comparison to cen- trifugal pumps which transport the fluid in a more regular flow through the impel- ler channels. Consequently the efficiencies of vortex pumps are about 30% lower than those of normal centrifugal pumps of similar sizes and specific speeds, see also Chap. 7.4.
Typical parameters of vortex pumps are: specific speeds nq = 10 to 80; effi- ciencies ηopt = 0.34 to 0.55; pressure coefficients at BEP ȥopt = 0.2 to 0.6; at shut- off ȥo = 0.8 to 1.3; impeller blade outlet angles β2B = 30 to 65° or radial blades with 90°; ratio of casing width to impeller outlet width bk/b2 = 0.58 to 1.5; ratio of casing diameter to impeller outer diameter da/d2 = 1.25 to 2.1, data from [2.10].
As their impeller is only slightly exposed to contact with the medium, vortex pumps are fit for pumping liquids with all sorts of impurities including textiles, fi- bers, abrasive solids and gases (without danger of choking or clogging). These pumps are also convenient for transporting delicate products like fish, vegetables, or crystals. Small vortex pumps are applied quite commonly for pumping sewage.
Friction pumps: The impeller of a friction pump is composed of smooth disks ar- ranged close together by means of bolts and held apart by spacers (see the sketch in Table 2.3). The disks are set into rotation by the driver. The pumping action is due to the centrifugal forces and shear stresses created within the boundary layers on these disks.
The fluid adhering directly to the disks moves with the circumferential speed of the rotor (in the absolute reference frame). Because of laminar and turbulent shear stresses, the liquid between the disks is dragged along, set in rotation and trans- ported outwards by centrifugal forces. The flow regime between the disks may ei- ther be laminar or turbulent. The transition takes place in the range of Reb = 1200 to 2300 (Reb = r×b×ω/ν with b signifying the distance between the disks). The head coefficient reaches nearly ψ = 2 at very low flow close to shut-off. With in- creasing flow rate it drops rapidly, because the momentum transferable by shear stresses is limited. The relation ψ = 2 - 0.06 nq gives a rough approximation for the value of the head coefficient, though this depends on the distance between the disks. The efficiencies are in the region of 25 to 35%.
Friction pumps are built with one or several disks operating in parallel. Be- cause of the smooth flow without blades these pumps are very quiet and largely free from vibration and cavitation problems. For that reason they may be consid- ered when quiet operation is of prime importance. Friction pumps are rather in-
2.3 Pump types and their applications 65
sensitive to abrasive wear by solid particles contained in the pumpage. More de- tails concerning friction pumps and literature may be found in [2.4 to 2.6].
Side channel pumps: In side channel and peripheral pumps the energy transfer from the impeller to the fluid results from exchange of momentum between the flow in the impeller and in the stator channel. Due to centrifugal forces in the im- peller, a circulating flow is generated between the impeller and the casing chan- nel, see Fig. 2.18.
A A
a
b
c
Flow stopper įR
Fig. 2.18. Side channel pump. a meridional section; b section A with flow stopper;
c development of side channel
This circulation strongly intensifies the exchange of momentum because the circumferential speeds of fluid and impeller are nearly the same, but significantly higher than in the channel. The fluid flows back from the side channel into the impeller in a spiral path. This happens several times over the perimeter; the more often this occurs, the higher are energy transfer and pressure rise. The number of spiraling loops grows when the flow rate is reduced. The energy transfer therefore rises strongly at partload. As the fluid travels through the side channel, the pres- sure increases from the inlet opening to the outlet port. Both are separated by a flow stopper, which makes the pressure build-up possible. The spiraling flow in- duces an intensive exchange of momentum resulting in far higher head coeffi- cients than could be obtained with radial impellers (Table 2.3). At partload, head coefficients above ψ = 10 are common. The above energy transfer mechanisms can be described as a “partial admission” pump. Detailed flow studies by laser ve- locimetry confirm this flow model, [2.11].
The optimum number of impeller blades is between 22 and 26. The straight blades are radial, or they may be inclined backward or forward (β2B = 70° to 140°); they are profiled and may be raked as in Fig. 2.18 (where the rake angle is įR < 90°). However, commonly radial blades without a rake are used. The side channel wraps around 270 to 320°of the circumference.
Disadvantages include the relatively low efficiency and the high noise level, the blade passing frequency (rotational frequency times number of blades) being particularly noticeable. Both disadvantages are caused by the recirculating flow and the associated losses. The shape of the flow stopper has an important influ- ence on the noise level. These pumps are also sensitive to cavitation damage. For this reason the tip speed is limited to about 35 m/s for pumping water.
The axial clearances between impeller and casing should not exceed 0.05 to 0.15 mm. Therefore, the impeller can move axially on the shaft in most designs.
Because of the small clearances these pumps are sensitive to abrasive wear.
In addition to the layout with one side channel, shown in Fig. 2.18, there are al- so pumps with side channels placed on both sides of the impeller in order to dou- ble the flow rate.
The vortex flow created within the impeller and the casing channel is able to entrain gases. Side channel pumps are therefore self-priming, i.e. they can evacu- ate the suction pipe, provided some liquid is left in the pump when starting. Side channel pumps are thus capable of pumping liquids with relatively high gas con- tent generating pressure ratios of 1.6 < pd/ps < 35.
Peripheral pumps according to Fig. 2.19 are similar to side channel pumps as far as the design and working principle are concerned. The difference between both types is essentially limited to the annular chamber which in peripheral pumps is placed not only laterally next to the impeller but also around its outer diameter.
Peripheral pumps are built with up to three sets of blades machined on different diameters of the same disk, forming thus a “3-stage” design with heads up to 2000 m and flow rates up to 80 m3/h. The impeller being symmetric, axial thrust is balanced. More details concerning side channel and peripheral pumps can be found in [B.8] and [B.13].
Flow stopper
Suction nozzle Discharge nozzle
Fig. 2.19. Peripheral pump [2.7]
2.3 Pump types and their applications 67
Liquid-ring pumps are self-priming if sufficient liquid is available in the pump casing. A rotor with radial blades is placed in the pump casing in eccentric posi- tion, Fig. 2.20. The rotation of the impeller causes a fluid ring to form at the outer perimeter of the casing due to centrifugal forces. As the fluid ring is concentric with the casing, the cell volumes formed by the blades vary along the circumfer- ence. The fluid ring seals the clearance between casing and impeller. Where the cells are largest, gas is drawn in via channels connected to the suction nozzle. The gas exits via slots into the discharge nozzle, where the cells are smallest. The volumetric flow rate depends on the cell size and the speed. Considering its work- ing principle, this machine is a positive displacement rather than a centrifugal pump.
Liquid-ring pumps are useful for processes requiring the pumping (at least spo- radically) of liquids containing gas. According to [2.8] they are employed most often as vacuum pumps in the chemical process industry. Liquid-ring pumps also can transport pure liquids. However, they perform poorly in this application be- cause the cell volumes get narrower towards the exit.
Fig. 2.20. Working principle of a liquid-ring pump. 1 gas inlet, 2 ring fluid entry, 3 impel- ler, 4 ring fluid, 5 sickle form cavity, 6 gas and ring fluid outlet, [2.8]
Pitot tube pumps, Fig. 2.21: The liquid to be pumped enters via a suction nozzle into a rotating casing. Consequently, the fluid rotates with the casing as a forced vortex. Due to centrifugal forces, the static pressure in the casing increases with the square of the circumferential speed according to Eq. (1.27). Some “slip” oc- curs between the circumferential speed of the casing and the fluid. The slip in- creases with growing flow rate. A stationary Pitot tube within the rotating casing draws the pumped liquid from the casing and conducts it to the discharge branch.
At the inlet of the Pitot tube, the fluid has the absolute velocity c2u = Ȗ×u2, the static head c2u2/(2g) and the dynamic head c2u2/(2g); Ȗ is the slip factor. The fluid is decelerated in a diffuser built into the Pitot tube and led to the discharge nozzle.
The theoretical head coefficient would thus be ȥ = 2.0; due to the slip and losses ȥopt = 1.9 may be achieved.
The casing side walls may be equipped with rudimentary vanes, slots or chan- nels to decrease the slip and to improve shut-off head, head rise and stability. The rotating casing eliminates the need of a mechanical seal on the discharge side;
there is only one mechanical seal on the suction side.
Since the casing rotates in ambient air, disk friction losses are low. Therefore efficiencies are relatively high in spite of the low specific speeds. Typical per- formance ranges can be described by the following data: shut-off head up to 2000 m; efficiencies: 50% for nq = 2; 62% for nq = 5; flow rates at BEP up to 100 m3/h; head rise Ho/Hopt = 1.2 to 1.3.
Most likely the stationary Pitot tube, which is exposed to the circumferential speed in the rotating casing, is profiled in order to reduce losses and vortex shed- ding. Since there is no bladed impeller, pressure pulsations are low. Partload re- circulation does not occur and minimum flow control valves are not needed.
Fig. 2.21. Pitot-tube pump, Sterling Fluid Systems