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Dairy Processing Handbook/Chapter 6.7 153

Fig. 6.7.1The most common type of

sanitary pump in the dairy is the centrifu-gal pump.

Pumps

Pumping demandsDemands on the quality of products, and the profitability of manufacturingprocesses, have grown steadily heavier over the years. Formerly, it wasoften possible to allow liquids to flow through a plant by gravity. Nowadays,

they are forced through long pipelines with many valves, through heatexchangers, filters and other equipment which often have high pressuredrops. The flow rates are frequently high. Pumps are therefore used innumerous parts of a plant, and the need to have the right pump in the rightplace has become increasingly important. Many problems may arise; theycan be summarised under the following headings: Pump installation Suction and delivery lines Type and size of pump required should be selected with regard to:

flow rate product to be pumped viscosity density

temperature pressure in the system material in the pump

Typical dairy pumps are the centrifugal, liquid-ring and positivedisplacement pumps. The three types have different applications. Thecentrifugal pump is the type most widely used in dairies.

The centrifugal pump, shown in Figures 6.7.1 and 6.7.2, is mainly usedfor low-viscosity products, but it cannot handle heavily-aerated liquids. Theliquid-ring pump is used when the air content is high. The positivedisplacement pump is used for gentle treatment and high viscosities.

Fig. 6.7.2Main parts of a centrifugalpump.

1 Delivery line2 Shaft seal3 Suction line

4 Impeller5 Pump casing

6 Back plate

7 Motor shaft8 Motor

9 Stainless steel shroud and soundinsulation

1

2

3

4

5

6

789

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Dairy Processing Handbook/Chapter 6.7154

Suction lineBefore we discuss the pumps themselves, it is important to understand thefacts and problems connected with pumping.

The pump should be installed as close as possible to the tank or othersource from which the liquid is to be pumped, and with as few bends andvalves as possible in the suction line. This should have a large diameter inorder to reduce the risk of cavitation.

Delivery lineAny throttling valve must be fitted in the delivery line, possibly together witha check valve. The throttling valve is used to adjust the flow rate of thepump. The check valve protects the pump from water hammer andprevents liquid from flowing back when the pump has stopped. Normally,the check valve is situated between the pump and the throttling valve.

CavitationCavitation can be detected by a crackling sound in the pump. It occurswhen the pressure drops locally below the vapour pressure and smallvapour bubbles form in the liquid. The pressure increases as the liquid

continues further into the impeller, and the vapour condenses very rapidly.The vapour bubbles collapse at a very high velocity and at a local pressure,which can be as high as 100 000 bar. This is repeated with a highfrequency and can cause pitting damage to the surrounding material,particularly if it is brittle.

Cavitation occurs when the pressure in the suction line is too low relativeto the vapour pressure of the pumped liquid. The tendency to cavitateincreases when viscous or volatile liquids are pumped.

Cavitation in pumps results in reduced head and efficiency. As cavitationincreases, the pump gradually stops pumping.

Cavitation should be avoided. However, should the pumping conditionsbe very difficult, and the pump cavitates slightly but is otherwise operatingwell, it is still possible to use the pump. This is because dairy pumps have

impellers of acidproof steel, which is very resistant to wear caused bycavitation. Some damage to the impeller may occur when the pump hasbeen in operation for a long time.

The possibility of cavitation occurring in a pump can be predicted bycalculation. See NPSH (Net Positive Suction Head) on the following page.

Pump chartPump charts are invaluable for selecting a pump for a given application.Three curves are needed to select the correct pump.

Flow rate and head, QH curve Required motor power, kW

NPSH (net positive suction head)The charts are drawn on the basis of tests with water. The data in the chartmust be recalculated if liquids with other physical properties are to bepumped.

The required flow rate, Q, is usually known when a pump is going to beselected. In the example shown in Figure 6.7.3, the flow rate, Q, is 15 m3/h.The required head must usually be calculated. Here we assume 30 m.

Locate the flow rate on the bottom Q scale. Start from this point andfollow a vertical line upwards until it intersects a horizontal line indicating therequired head, 30 m, on the H scale. This point does not meet any of theQH curves indicating the impeller diameter. The nearest larger impeller size,in this case 160 mm, should be chosen. The resulting head will be 31metres liquid column.

The next step is to follow the vertical 15 m3

/h line downwards, until itintersects the power curve for the 160 mm impeller. A horizontal line to theleft of the intersection indicates a power consumption of 2,3 kW. To thisfigure a safety margin of approximately 15 % must be added, giving a total

How to avoid cavitation

The general rule of thumb is: Low pressure drop in the

suction line (large pipe diame-ter, short suction pipe, fewvalves, few bends, etc.)

High inlet pressure to thepump, for example a highliquid level above the pump

Low liquid temperature

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of around 2,6 kW. Therefore, a 3 kW motorcan consequently be used.

If the pump is fitted with a motor of acertain size, always check that the motor isnot overloaded. There should always be asafety margin for excess load.

Finally, the 15 m3/h vertical line is followedto the NPSH curve, to the right in the topdiagram. Following the horizontal line to theright, shows that the required NPSH value is1 metre.

Head (pressure)When selecting a pump, it should beremembered that the head, H, in the flowchart is the head of the pump when the liquidflows into the pump without suction lift or inletpressure.

To obtain the actual pressure after thepump, it is necessary to consider the

conditions on the suction side of the pump. Ifthere is a vacuum in the suction line, thepump must do part of its work before theliquid reaches it. The pressure at the outlet isthen lower than that given in the chart.

On the other hand, if the suction line isflooded to give positive pressure at the pumpinlet, the outlet pressure will be higher thanthat shown in the chart.

NPSH (Net PositiveSuction Head)

As previously mentioned, in planning a pump installation, it is important thatthe suction line is laid out so that the pump does not cavitate. An NPSHcurve is included in the flow charts (Figure 6.7.3). The NPSH of a pump isthe necessary excess pressure above the vapour pressure of the liquidrequired to avoid cavitation. This is called NPSH

req.

Before this can be used, the available NPSH of the suction line inprevailing operation conditions must be calculated. This figure, NPSH

av,

should be equal to or higher than the required NPSH, which is the value inthe chart.

The following formula is used to calculate NPSHav

in the system:p

a= pressure in bar abs at the liquid surface

pv

= vapour pressure in bar absd

r= relative density

hs = static suction lift in metres liquid columnh

fs= pressure drop in suction line, metres liquid column

Note that hsis negative for suction lift and positive for inlet pressure.

Shaft sealsThe shaft seal is often the most sensitive component in a pump, as it mustseal between a rotating part, impeller or shaft, and a stationary part, thepump casing. Normally a mechanical seal is used.

Fig. 6.7.3Pump chart for a centrifugal pump.

40

35

30

25

20

15

10

5

31

110 120 130140

150160163

5 10 15 20 25 30 35 40 45 50 55 60 65 Q (m3/h)

H (m) NPSHreq (m)

1

5

5 10 15 20 25 30 35 40 45 50 55 60 65 Q (m3/h)

10

5

2

1

2,3

110

120130

140

150

160

163

P (kW)

pa

pv

NPSHav

= hs h

fs+ x 10 x 10 m liquid column

dr

dr

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A rotating seal ring has a lapped sealing surface which rotates against alapped stationary seal ring. A liquid film is formed between the sealingsurfaces. The film lubricates the seal and prevents direct contact betweenthe two seal rings. This means minimum wear and long life for the seal. Ifthe pump runs dry, the lubricating liquid film in the seal is destroyed andwear on the sealing rings is increased.

The mechanical seal is usually balanced. This means that it is insensitiveto the pressure in the pump. The sanitary mechanical seal needs noadjustment and causes no wear on the shaft. It is available in single orflushed versions.

Single mechanical shaft sealSingle mechanical seals, Figure 6.7.4, are standard in most sanitary pumpsfor the dairy industry.

In a mechanical seal the stationary seal ring is fastened to the back plateof the pump casing. The rotating ring can be fitted inside or outside thepump and is sealed with an O-ring. The rotating ring can move along theshaft and is pressed against the stationary ring by a spring.

2

1 1

2 2

3

33

4

4

4 5

5

5

1

3

2

Flushing mediaProduct

Flushed shaft sealThe flushed seal, Figure 6.7.5, consists of two seals. Water or steam is

circulated through the space between the two seals to cool or clean theseals or to create a barrier between the product and the atmosphere.

The flushed shaft seal is recommended for the following applications: With barrier steam for pumping sterilised products when reinfection must

be avoided. Water flushing for pumping sticky solutions or products which crystallise,

for example sugar solutions. Water cooling of the seal when matter may be deposited on the shaft at

the seal and burn on because of the higher temperature at the sealingsurfaces. An example is the booster pump in pasteurisers.

Water barrier to exclude air from the product when pumping at a verylow inlet pressure, e.g.from a vacuum vessel.

The barrier steam pressure must not exceed the atmospheric pressure at

100 C, as the steam may then become dry. This would result in the sealrunning dry and the sealing surfaces being damaged. The steam and watersupply is regulated at the inlet to the seal, and there must be no

Fig. 6.7.4Single mechanical shaft

seal.

1 Shaft 5 Impeller

Fig. 6.7.5Flushed mechanical shaft

seal.

Fig. 6.7.6Double mechanical shaft

seal with flushing media.

2 Stationary ring 3 Rotating ring 4 Back plate

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obstructions in the outlet pipe. The barrier is always supplied through thelower connection.

Double mechanical shaft sealThe double mechanical seal, Figure 6.7.6, is similar to the flushed seal.However, the lip seal is replaced by a stationary/rotating seal arrangementsimilar to the single seal and the primary sealing parts of the flushed and

double mechanical seals hence the name double mechanical.The double mechanical seal can be used instead of a flushed seal and is

recommended for the following applications: As a steam barrier for pumping sterilised products to avoid possible

contamination For cleaning of abrasive products that may damage a lip seal

arrangement To handle aggressive flushing fluids, such as those used in certain

pharmaceutical or chemical processes, as these may damage a lip sealarrangement

As a high-pressure flushing barrier (up to 5 bar) if a lip seal arrangementis not suitable

Internal shaft sealMost pumps have external shaft seals, as the design is simple and they arethe optimum solution from a hygienic point of view. The external seal issuitable for most applications.

For the external seal, the processed product is located inside the sealand product pressure forces the product out between the seal faces. Thismeans that the external seal has a limitation regarding maximal productpressure, typically 10 bar.

Therefore, high inlet pressure and multi-stage centrifugal pumps need aninternal shaft seal.

For the internal seal, the processed product is outside, surrounding theseal. This principle, together with heavy-duty designed seal parts, meansthat the internal seal can handle inlet pressures of up to 40 bar, see Figure6.7.7.

The internal seal is available as a single seal or a flushed seal. Handlingand suitability of the flushed seal is the same as for the external flushedseal.

Material for shaft sealsA commonly used combination of materials is carbon for the rotating sealring and stainless steel for the stationary ring. A better combination is siliconcarbide against carbon. For abrasive liquids, seals with very hard faces arerecommended. Silicon carbide against silicon carbide is commonly used forsuch applications.

Centrifugal pumps

Pumping principleThe liquid entering the pump is directed to thecentre (eye) of the impeller and is set in circularmotion by the impeller vanes, as in Figure 6.7.8. As aresult of the centrifugal force and the impeller motion, the liquidleaves the impeller at a higher pressure and velocity than at theimpeller eye. The velocity is partly converted into pressure in the pumpcasing before the liquid leaves the pump through the outlet connection.

The impeller vanes form channels in the pump. The vanes are normally

curved backward, but may be straight in small pumps.

RotationRotation

Fig. 6.7.8 Flow principle in a centrifugal

pump.

Fig. 6.7.7 Internal shaft seal.1 Shaft

2 Stationary ring

3 Rotating ring4 Back plate

5 Impeller

1

2

4

35

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Centrifugal pump typesDifferent types of centrifugal pump are available, depending on theapplication requirements.

These types are: Standard centrifugal pump High inlet pressure centrifugal pump Multi-stage centrifugal pump

Self-priming centrifugal pump

Standard centrifugal pumpThis is the cheapest and most commonly used centrifugal pump, as it issuitable for most non-viscous applications.

The standard pump has some limitations regarding high inlet and systempressures, as well as aerated applications. In these cases other centrifugalpump types should be used.

High inlet pressure centrifugal pumpThis pump is specially designed for applications with high inlet pressurerequirements, such as filtration systems.

The special-purpose parts in this pump are a specialised motor, heavy-walled pump casing, thick backplate and a hygienic internal mechanicalshaft seal to withstand the high inlet pressure, Figure 6.7.9.

Multi-stage centrifugal pumpThis pump is specially designed for high outlet pressure requirements at

relatively low capacities. Pumps of this type are typically used as booster

pumps.

The pump consists of several stages and it works in a similar way

to several pumps coupled in series.

Its special-purpose pump design includes several impellers and

intermediate casings, thick backplate and a hygienic internal

mechanical shaft seal, Figure 6.7.10.

The motor is either standard or special purpose, depending on the levelof inlet pressure.

Self-priming centrifugal pumpThis self-priming pump is specially designed for aerated applications, suchas CIP return systems.

The pump is a standard centrifugal pump that is equipped with a tank,two non-return valves and a tee.

If it is only pumping fluids, the pump works as a normal centrifugalpump. However, if air/gases enter the pump, the special tank/non-return

valve design will create a vacuum and separate and expel the air/gasesthrough the pump discharge until only fluids remain. The pump will

then resume work as a normal centrifugal pump.

Centrifugal pump applicationsThe centrifugal pump is the most commonly used pump in the

dairy industry and should be selected if it is suitable for theapplication in question. The reason for this is that a centrifugalpump is usually cheaper to purchase, operate and maintain, andis also the most adaptable pump for different operatingconditions.

The centrifugal pump can be used for pumping of all liquids ofrelatively low viscosity which do not require particularly gentle treatment. Itcan also be used for liquids containing relatively large particles, provided ofcourse that the particle size does not exceed the dimensions of the impeller

channel.A disadvantage of the centrifugal pump is that it cannot pump aeratedliquids; it loses prime and stops pumping. It must then be stopped and

Fig. 6.7.10Multi-stage centrifugal pump

for high outlet pressure.

Fig. 6.7.11Self-priming pump equipped

with tank and valves.

Fig. 6.7.9Centrifugal pump adapted for

high inlet pressure.

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primed filled with liquid and started again before it can continuepumping. Consequently, the centrifugal pump is not self-primingand thesuction line and pump casing must be filled with liquid before it can operate.The installation should therefore be carefully planned.

Flow controlIt is seldom possible to select a standard pump that fits the required capa-

city exactly. Some sort of adaptation must therefore be made by: throttling highly flexible but uneconomical reducing the impeller diameter less flexible but more economical speed control flexible and economicalThe three alternatives are illustrated in Figure 6.7.12.

Fig. 6.7.13 Flow reduction when theimpeller diameter is reduced from D

to D1.

H1

H

H

O Q1 QQ

A

B

D1

D

Throttling

Impellerreduction

Speedcontrol

Fig. 6.7.12 Methods of flow control in a centrifugal pump.

D1 = D x

H1

H

ThrottlingThe most simple flow control is to fit a throttling valve in the pump outletline. It is then possible to adjust the pump exactly to the required pressureand flow rate. This is the correct method if the pump is used for varyingpressures and flow rates. The disadvantage is that throttling isuneconomical when pressure and flow are constant.

Throttling can be carried out with orifice plates in the pipe, with manualor automatic control valves or with a mechanical flow controller, which isoften fitted in milk treatment lines.

Reducing impeller diameterA lower pump curve than the maximum curve is obtained by reducing theoriginal impeller diameter D to D

1(Figure 6.7.13). The new diameter D

1can

be roughly determined by drawing a straight line from O on the chartthrough the required operating point A to the standard curve B, for impellerdiameter D. Read pressure H and the required new pressure H

1. The new

impeller diameter D1is obtained from the formula:

The most economical pump installation is obtained if the impeller diameteris reduced to diameter D

1. Most pump charts have curves for different

impeller diameters.

Speed controlChanging the speed will change the centrifugal force created by theimpeller. Pressure and capacity will then also change up for higher speedand down for lower.

Speed control is the most efficient way of regulating a pump. The speedof the impeller is always exactly right for the performance of the pump, andtherefore also the power consumption and the treatment of the liquid.

A frequency converter can be used, together with standard three-phasemotors. They are available for manual or automatic control of flow andpressure.

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Pumps for 60 HzMost centrifugal pumps are designed for 50 Hz, which means 3 000 rpm(revolutions per minute) for a two-pole motor. The power supplies in somecountries operate at 60 Hz, which means that the speed increases by 20 %to 3 600 rpm. Pump curves for 60 Hz are available from pumpmanufacturers.

Head and pressureDensityThe head in metres liquid column is independent of the density of the liquidbeing pumped. However, the density is of great importance to thedischarge pressure and for the power consumption.

If the pump and the viscosity of the liquid are the same in the differentcases, the liquid column will be lifted to the same height (10 metres in theexample), regardless of the density. The pump head in metres liquid columnis the same. However, as the density the mass of the liquid varies, thepressure gauge readings will also vary (Figure 6.7.14).

B Pumping sugar solution

of relative density = 1,210 mliquid col = 12 m

water col = 1,2 bar

CPumping alcohol of

relative density 0,810 mliquid col = 8 m

water col = 0,8 bar

APumping water ofrelative density = 1,0

10 mliquid col

10 m water col = 1,0 bar

Fig. 6.7.14Comparison of liquid and water columns for products with different densities.

10 m 10 m 10 m

1,0 bar 1,2 bar 0,8 bar

A CB

The pump pressure in metres water column is consequently obtained if

the pressure in metres liquid column is multiplied by the relative density.The pump must do more work with the heavier liquid than with thelighter. The power required changes proportionally to the density. If, inexample A, the figure requires 1 kW, then example B will require 1,2 kW andexample C only 0,8 kW.

ViscosityLiquids of higher viscosity create higher resistance to flow than liquids oflower viscosity. When liquids of higher viscosity are pumped, the flow rateand head are reduced and power demand increases because of increasedflow resistance in the impeller and pump casing.

Centrifugal pumps can handle liquids of relatively high viscosities, but arenot recommended for viscosities much above 500 cP, because the power

demand rises sharply above that level.

Note:

In the pump flow charts, thehead is always in metres liquidcolumn and the powerconsumption for water withdensity 1,0. This means that forpumping liquids of higherdensity, the power in thecurve must be multiplied bythe density.

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Liquid-ring pumpsLiquid-ring pumps (Figures 6.7.15 and 6.7.16) are self-priming if the casingsare at least half-filled with liquid. They can then handle liquids with a highgas or air content.

The pump consists of an impeller with straight radial vanes (4)rotating in a casing, an inlet, an outlet and a drive motor. From theinlet (1), the liquid is led between the vanes and accelerated out

towards the pump casing, where it forms aliquid ringwith essentially thesame speed of rotation as the impeller.

There is a channel in the wall of the casing. It is shallow at (2) andbecomes progressively deeper and wider as it approaches (3) and thengradually becomes shallow again to point (6). As the liquid is transported bythe vanes, the channel is also filled, increasing the volume available for theliquid between the vanes. This results in a vacuum in the centre, whichcauses more liquid to be drawn into the space from the suction line.

The deep channel (3) has been passed, the volume between the vanes isreduced as the channel becomes shallower. This gradually forces the liquidtowards the centre and increases the pressure and liquid is dischargedthrough port (7) to pump outlet (5).

Air in the suction line will be pumped in the same way as the liquid.

ApplicationsLiquid-ring pumps for the dairy industry are used where theproduct contains large quantities of air or gas, and wherecentrifugal pumps therefore cannot be used. Theclearances between impeller and casing are small, andthis type of pump is therefore not suitable for handlingabrasive products.

A common application is as a CIP return pump for cleaningsolution after a tank, as the CIP solution contains normally largeamounts of air.

Positive displacement pumps

Pumping principleThis group of pumps works on the positive displacement principle. They aredivided into two main categories: rotary pumps and reciprocating pumps.Each category includes several types.

The principle of a positive displacement pump is that for each revolutionor each reciprocating movement, a definite net amount of liquid is pumped,regardless of manometric head, H.

However, at lower viscosities there may be someslip(internal leakage) asthe pressure increases. This will reduce the flow per revolution or stroke.

The slip is reduced with increased viscosity.Throttling the outlet of a positive displacement pump will

increase the pressure dramatically. It is therefore important that:1 No valve after the pump can be closed2 The pump is fitted with a pressure-relief valve, built into the

pump or as a by-pass valve.

Flow controlThe flow of a positive displacement pump is normally controlledby regulating the speed. Adjustment of the stroke of a reciprocat-ing pump is another possibility.

Pipe dimensions and lengthsGreat care must be taken in dimensioning the pipework when high-viscosityproducts are pumped. The pumps must then be placed close to the

Fig. 6.7.17 Positive displacement pumpof the lobe-rotor type with geared motor

assembled on a frame.

Fig. 6.7.16 Working principle of a self-priming liquid-ring pump.

1 Suction line2 Shallow channel

3 Deep channel

4 Radial vanes5 Pump outlet

6 Shallow channel7 Discharge port

Fig. 6.7.15Liquid-ring pump.

1

2

3

4

5

6

7

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feeding product tank and the pipe dimensions must be large. Otherwise thepressure drop will be so high that the pump will cavitate.

The same applies to the outlet side. The pressure will be very high if thepipes are long and narrow.

Lobe-rotor pumpsThe lobe-rotor pump (Figure 6.7.17) has two rotors, usually with 2 4 lobes

each. A vacuum is created at the inlet when the rotors rotate. This vacuumdraws the liquid into the pump. It is then moved along the periphery of thepump casing to the outlet. There, the volume is reduced and the liquidforced out through the outlet. The course of events is illustrated in Figure6.7.18.

The rotors are independently driven by a timing gear at the back of thepump. The rotors do not touch each other or the pump casing, but theclearances between all parts in the pump are very narrow.

ApplicationsThis type of pump has 100 % volumetric efficiency (no slip) when theviscosity exceeds approximately 300 cP. Because of the sanitary designand the gentle treatment of the product, this type of pump is widely used

for pumping cream with a high fat content, cultured milk products, curd/whey mixtures, etc.

Eccentric-screw pumpsThis pump is tighter than the lobe rotor pump for lower viscosity products. Itis not considered quite as hygienic as the lobe-rotor pump, but handles thepumped product gently. The range of application is the same as that of thelobe-rotor pump.

The eccentric-screw pump (Figure 6.7.19) cannot be run dry, even for afew seconds, without being damaged.

Fig. 6.7.19The eccentric-screw pump.

Product inlet

Pressurised product

Cooling water

Piston pumpsA piston pump normally has 1, 2, 3 or 5 pistons, Figure 6.7.20. A rotatingcrankshaft drives the pistons backwards and forwards via a piston rod.Check valves on both the suction- and pressure side regulate the flow in the

right direction.A 5-piston pump gives a less pulsating flow than a 3-piston one.Piston pumps are normally used when high pressure and low energy

consumption are required. The high-pressure homogeniser is an example ofa piston pump followed by a homogenising device.

Volumetric efficiency is also close to 100 % at low viscosity and varying

Fig. 6.7.18Lobe-rotor pump principle.

Fig. 6.7.20Piston pump.

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back pressure. A piston pump can therefore be used as an accuratemetering pump. The capacity is proportional to the crankshaft speed.

For metering purposes, a special type of piston pump can be used. Thecapacity of each piston is adjusted via its stroke length. This type of pumpis used when several different components are mixed in given proportions.Each piston handles one component.

Diaphragm pumpsAir-powered diaphragm pumps, one of which is illustrated in Figure 6.7.21,are used for gentle treatment of the product. There are pulsations in theoutlet pressure and the capacity will change with changing product pres-sures, as the air pressure is constant. These pumps are therefore mainlyused to transport products and not used very often within the processesthemselves.

Mechanically powered diaphragm pumps are often used as meteringpumps.

Working principleDiaphragm pumps are double-acting positive displacement pumps with twoalternating pump chambers. The compressed air required for driving the

unit is admitted through a control valve to the rear of each diaphragm inturn. This displaces the medium from alternate pump chambers.

The diaphragm has the additional function of separating the pumpedproduct from the compressed air. Since the same pressure prevails in boththe compressed air and pumping chambers during each stroke, thediaphragms themselves are not subjected to pressure differences. This isone reason for the long life of the diaphragms.

A vacuum is created by the retraction of the diaphragm, and thepumped product flows into the chamber. The volume in the oppositechamber is simultaneously reduced, and the product is discharged throughthe outlet check valve.

The two diaphragms are connected with a common piston rod, andsuction therefore always occurs in one chamber while product is

discharged from the other. The compressed air serves a dual purposeduring each phase: the actual discharge process and the intake of furthermedium to be conveyed.

Peristaltic pumps (hose pumps)This type of pump (Figure 6.7.22) can be used for transportation as well asfor relatively accurate metering of products.

The rotor rotates in the lubricant-filled pump housing and compressesthe hose with the rollers. The suction and discharge sides are hermeticallysealed from each other.

During rotation, the medium (liquid or gas) inside the hose is transportedto the lower outlet connection. This creates a vacuum on the suction side,

and the product is drawn into the pump. The pump isself-primingand istherefore suitable for emptying barrels with juice concentrates andanhydrous milk fat (AMF).

The volume between the rollers is equal to half the volume transportedper rotation. This amount is constantly pumped to the outlet connectionduring rotation, while the same amount is drawn in on the suction side.

Fig. 6.7.22 Pumping sequence of a

peristaltic pump.

Fig. 6.7.21The diaphragm pump.1 Open ball valve during sucking

2 Sucking diaphragm

3 Pumping diaphragm4 Closed ball valve

1

2

3

4

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06.07 Pumps

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