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CHAPTER 2

Numerous types of fluid machines are used in the chemical process p rocess industries. Their basic objective is to transfer mechanical energy to a flow medium. medium. This chapter discuses the classification, characteristic features, operating principles and applications of the the common types of fluid machines used in the chemical and process industries.

2.1 CLASSIFICATION OF FLUID MACHINES

Fluid machines are mostly classified based on the flow medium they handle and the principle of energy transfer. In the first case they are classified into pumps and gas movers. In the second case they are classified into dynamic and positive displacement machines. In each category the classification goes some steps further as discussed below.

Pumps are fluid machines designed to transfer energy to liquids while fans, blowers and compressors are designed to transfer energy to gases; the latter three are commonly known as gas movers. The classification of the gas movers into fans, blowers and compressors is mainly based on the discharge to suction pressure ratio: Pr =P D/PS. Table2.1 summarizes this classification.

In calculations involving pumps and fans the density of the fluid does not change appreciably and incompressible flow theory can be assumed in the analysis. In analyzing blowers and compressors, however, there will be significant change in density between the suction and discharge ends of the machine therefore compressible flow theory should be employed. As the discharge to suction pressure ratio increases the temperature rises beyond the safety limit and artificial cooling (commonly with cooling water or air) is

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Chapter 2 Characteristic Features Features of Common Fluid Machines

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used and with still further increase in PD/PS it becomes more advantageous to use multiple stage compression with separate cooling between stages.

Table 2.1 Characteristics of Fluid Machines Fluid Machine

Type of Fluid

Pr=PD/PS

Pumps

Liquid

Fans

Gas

<1.15

No

Blowers

Gas

1.15
No

Compressors

Gas

>3

Yes

-

Artificial Cooling

No

The other classification is based on whether the energy is transferred mainly in the form of pressure energy or velocity energy. While by mechanical energy of a flow medium we mean the sum of the velocity energy, pressure energy and geodetic (potential) energy of the flow medium it is usually required to limit the velocity energy due to the energy loss accompanied with fluid friction, which increases significantly with the velocity of the flow medium. On the other hand too small flow velocity inside fluid machines results in bigger sizes or limited capacity. Hence fluid machines, which transfer energy mainly in the form of velocity energy, are generally less efficient but more compact.

Based on this principle of energy transfer, fluid machines are classified into dynamic and positive displacement. In dynamic fluid machines energy is added to the flow medium mainly by increasing its velocity in excess of what is required at the discharge end and converting this excess velocity energy into pressure energy at the discharge portion of the machine. Centrifugal and axial flow machines are the most commonly used dynamic fluid machines in process industries.

In positive displacement machines energy is added to the flow medium by a direct application of force on one or more movable boundaries of the flow medium. This application of force directly increases the pressure energy and no conversion from velocity to pressure energy is required. There are reciprocating reciprocating and rotary types of positive displacement machines. In the reciprocating machines the part of the machine that transfers the energy moves fro and back horizontally (up and down if the machine is


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arranged vertically). vertically). Piston, plunger, and diaphragm are the most common reciprocating pumps and compressors in process industries. In rotary positive displacement machines the part that directly transfers the energy to the flow medium is one or more rotating elements called rotors. Gear pumps, screw pumps and compressors, lobe pumps and compressors, vane pumps and compressors are some of the most common rotary positive displacement machines used in chemical and process industries. Figure 2.1 describes the classification discussed above. Fluid Machines

Dynamic

Turbomachines

-Centrifugal -Axial flow -Mixed flow

Positive Displacement

Special Effect

-Jet pumps -Gas lift

Reciprocating

-Piston -Plunger -Diaphragm

Rotary

-Screw -lobe -vane - Gear*

*gear pump Figure 2.1 Classification of fluid machines

2.2 CHARECTERISTICS FEATURES AND OPERATING PRINCIPLES

In the section below the characteristic features, operation principles, applications and ranges of the most common fluid machines are discussed. Even though the basic features of most pumps, fans compressors and blowers are the same, because of the differences in construction and in attaining compression and pumping, machines involving compression are discussed separately from those that involve pumping.

Pumps: Both dynamic and positive displacement pumps are used in process industries.

Among these centrifugal and axial flow pumps are the most commonly used ones. Axial flow pumps are used when the flow rate required is high and the head is low. Centrifugal pumps are used in high headlow flow rate applications. Although not as common, jet


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pumps are also used. Positive displacement d isplacement pumps pum ps are used for low flow rate high head applications and when the flow rate is required to remain constant irrespective of variation in head. Rotary positive displacement pumps are also effective in handling viscous liquids. They are also preferred in high head – low low flow rate applications. Both reciprocating and rotary pumps are in use.

Fans:

Fans are limited to centrifugal and axial flow. In processes requiring low to

medium flow rate and high-pressure centrifugal fans are used. In high flow rate-low pressure applications axial flow fans are used.

Blowers and compressors: Both dynamic and positive displacement blowers and

compressors are common in industrial applications. From the dynamic, centrifugal and axial flow compressors are the most commonly used. Their basic features and operation principles are similar to centrifugal and axial flow pumps. Both rotary and a nd reciprocating positive displacement compressors are used in process industries. industries.

2.2.1

Dynamic Fluid Machines

Turbomachines

Turbomachines include centrifugal, axial and mixed flow machines. Turbo pumps, fans and compressors are very widely used fluid machines in almost all types of industries. Drive(Motor ) Discharge

Pump Suction Base plate

Figure 2.2 Centrifugal pump


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They are specially preferred because of their long maintenance free operation. Centrifugal machines consist of an impeller/impellers mounted on a shaft that is driven by a motor, diesel engine, gas or steam turbine. The impeller is covered by a casing also called housing. The essential components of the impeller are its blades (See Figure 2.4 to 2.6 ). ). The number and design of the blades directly affects the performance characteristics of the machine. Figure 2.2 shows a centrifugal pump with a motor drive. Figure 2.3 shows an axial flow pump without the motor. Pump discharge

Pump suction

Shaft

Impeller

Figure 2.3 Axial flow pump without the drive

Turbomachines are commonly divided into radial flow, mixed flow and axial flow machines. The classification is based on the direction of flow of the fluid inside the machine. Figure 2.4 depicts radial, mixed and axial flow impellers.

Radial Flow Impeller

Mixed Flow Impeller

Axial Flow Impeller

Figure 2.4 Types of impellers

The different parts of a centrifugal impeller are indicated in Figure 2.5. The outer covers of the impeller are called shrouds. The purpose of the shroud is to guide the fluid through the impeller and it is not directly related to the energy transfer, hence it is not an essential


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part. Axial flow impellers do not have shrouds while radial and mixed flow impellers may or may not have shrouds.

Front shroud Hub Blade

Back shroud

Impeller Eye

Figure 2.5 Components of a centrifugal impeller

An impeller that has both front and back shroud is called closed impeller and one that has only the back shroud is called semi-open impeller. An impeller that does not have both shrouds is called open impeller. Figure 2.6 shows a semi-open impeller. Back shroud Blades

Figure 2.6 Semi open impeller

In centrifugal (radial flow) machines (Figure 2.7) the flow medium from the suction nozzle enters the impeller through the impeller eye and is distributed radially outwards between blade channels. While passing through the blade channels the blades transfer energy from the shaft to the flow medium by increasing its torque. When the fluid leaves the impeller its velocity increases significantly. In the casing this excess velocity energy is gradually converted to pressure energy.


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In axial flow machines the flow medium flows axially, parallel to the centerline of the Figure 2.7 Section of a centrifugal pump

shaft. While passing through the impeller the blades increase the velocity by imparting a rotational component to it, which later may be converted to pressure energy using stationary or moving vanes.

The main features of centrifugal fans and compressors are the same as centrifugal pumps. Their working principles are also the same; their theoretical analysis is discussed in chapter 4. Below you find some images of centrifugal and axial flow fans and compressors.

Figure 2.8a Multistage Centrifugal Compressor

Figure 2.9a Centrifugal Fan

Figure 2.8b Multistage Axial Compressor

Figure 2.9b Axial flow fan

Jet Pumps

Jet pumps are dynamic pumps that use the momentum of one fluid (motive fluid) to fluid) to move another. Jet pumps do not have any moving part. They consist of two main parts, the nozzle and diffuser. Figure 2.10 shows a general-purpose jet-pump jet-pump and its major parts.


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Jet pumps work by the Bernoulli principle. When the motive fluid passes through the nozzle, the static pressure energy is converted to velocity energy with the maximum velocity being around the discharge end of the nozzle. At this point the static pressure is lower than any part in the pumping system. Hence, liquid is drawn into the pump and gets mixed with the motive liquid. In the first part of the diffuser, the two liquids are mixed and the velocity is in excess of that is required in the pipe system at the end of the pump the area of the diffuser increases and the velocity energy is converted back to pressure energy. Diffuser Motive fluid

Discharge

Suction

Nozzle

Throat

Figure 2.10 A general-purpose jet pump

Ejectors and injectors are the two types of jet pumps common in chemical industries. The ejector, which is also known as siphon or eductor, is used for low-pressure applications. The discharge static pressure is lower than the static pressure of the motive fluid. Injector is a type of jet pump, which uses a condensable gas to entrain a liquid and discharge against a pressure higher than either motive or suction pressure. It is used for boiler feed with steam as the motive fluid.

Jet pumps are sometimes used in combination with centrifugal pumps. In such cases the centrifugal pump is the motive pump and is mainly used to circulate the motive fluid. This jet pump- centrifugal pump combination is commercially known simply as a jet pump. It is specially used for pumping water from wells. wells.


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Discharge

Discharge

Suction

Suction (a)

Motive fluid recirculation

(b)

Figure 2.11 Shallow-well (a) and Deep- well (b) jet pumps (Jet-centrifugal)

In shallow well jet pumps both the centrifugal pump and eductor (jet pump) are closely attached and the circulation of the motive fluid takes place inside the casing of the surface pump, hence there is only one inlet pipe that goes to the well, the suction-pipe.

Centrifugal Pump

Eductor Foot valve

Strainer Figure 2.12 Jet pump-centrifugal pump-centrifugal pump used used together (deep well)

In deep well applications the centrifugal pump is at the surface and the eductor is at subsurface (Figure 2.12), hence wee see two pipes inside the well and at the end of the surface pump. One of the pipes is to circulate the motive fluid while the other is for moving the mix upwards. Jet pumps are less efficient than most mechanical pumps but


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their simple construction, absence of moving parts, their light weight, compact design and low cost makes them attractive for many applications.

2.2.2

Positive Displacement Machines

I.

Reciprocating Pumps and Compressors

The most common reciprocating machines are piston, plunger and diaphragm machines. The characteristic feature and operating principles of these reciprocating pumps and compressors are discussed below.

A. Reciprocatin Reciprocatin g Pumps

Piston Pumps

Figure 2.13 shows the schematic diagram of a piston pump. Piston pumps are reciprocating positive displacement pumps that consist of a piston (5) moving fro and back in a cylinder (1). Check valves are provided at the suction and discharge ports (3) and (4). 7 1 4 2 5

3 6

Figure 2.13 Piston pump

At the start of the backward (towards right in Figure 2.13 ) ) motion of the piston the pressure in the cylinder decreases and when it is sufficiently low the suction valve opens. As the piston moves backwards liquid is drawn into the cylinder until it reaches the end of the suction stroke at the extreme right.

As the piston starts starts the forward forward motion

(towards left in Figure 2.13) the pressure inside the cylinder grows and the discharge valve opens and the liquid starts to discharge. Discharge takes place until the liquid drawn during the suction stroke is removed from the cylinder. There are single acting and double acting piston pumps.


Figure 2.14a Single acting piston pump

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Figure 2.14b ouble acting piston pump

In single acting pumps suction and discharge processes take place at different times and only one face of the piston is used. In double acting piston pumps both faces of the piston are used and both suction and discharge take place at the same time in the same cylinder at different sides of the piston head. Figures 2.14a and 2.14b show a single acting and a double acting piston pumps respectively.

The capacity of crank actuated piston pumps varies cyclically with time. This is because of the variation of the speed of the piston head at different crank angles. This variation of capacity is called pulsation. There are different ways of reducing the pulsation. One of the common ways is using multiple cylinders in parallel, driven by the same crankshaft. The details will be discussed in chapter 5. Piston pumps are typically used in high headlow flow rate applications and where significant variations of capacity with system head are not tolerable.

Plunger Pump

The moving element in plunger pumps is known a plunger. The main difference between a plunger and a piston is that the plunger moves past a stationary packing, whereas a piston carries its packing packin g with it. For pumping liquids that contain solids plunger pumps are more appropriate than piston pumps. Plunger

Cylinder Figure 2.15 Plunger pump


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Diaphragm Pumps

The diaphragm pump consists of a flexible membrane, a pumping chamber, suction and discharge check valves and a driving mechanism. Air bleed valve Relief valve Plunger

Discharge

Diaphragm

Pumping chamber

Hydraulic fluid Relief valve

Suction

Figure 2.16 Diaphragm pump

The driving mechanism may be a mechanical system (eccentric mechanism), mechanism), a hydraulic system or a pneumatic system. system.

Figure 2.16 shows a diaphragm pump driven by a

hydraulic system.

The hydraulic system is driven by piston-cylinder mechanism in which the piston moves the hydraulic fluid and the hydraulic fluid moves the diaphragm. When the diaphragm moves to the left a partial vacuum is created in the pumping chamber and the suction port opens and liquid is drawn in to the pump. When the diaphragm is pushed to the right, the pressure inside the pumping pump ing chamber grows and the discharge valve opens op ens and the liquid is pushed through the discharge port.

The driving mechanism of diaphragm pumps is completely isolated from the pumped fluid by the diaphragm. For this reason, the diaphragm pump is applied where leakage or contamination of the process fluid cannot be tolerated. The diaphragm in a mechanically driven pump cannot be supported over its entire area. Stresses place a limit on the pumping pressure. The fluid-driven type, which dose not have this drawback, is used for higher pressure.

Diaphragms are fabricated from from elastomers, plastics and metals. Fluid driven pumps with with metal diaphragms may be used up to 45,000 psi (3103 bar). With plastics such as Teflon,


29 0

operation is usually restricted to below 1,500 psi (103.4 bar) and 280 F. Elastomer 0

materials are usually limited to 750 psi (51.7 bar) and 212 F.

B. Recipr ocatin g Compress Compressors or s

The most common types of reciprocating compressors are piston, plunger and diaphragm compressors. The operating principles and major parts of plunger and piston compressors are the same. Hence piston compressors co mpressors and diaphragm compressors are discussed.

Piston Compressors

The basic components of a piston compressor are: piston, cylinder, suction and discharge check valves (Figure 2.17). During the suction stroke, the piston moves down and the pressure inside the cylinder decreases and when it reaches the suction pressure the suction valve opens and gas starts to be drawn into the cylinder. At the end of the suction stroke the piston changes it direction and starts to move up and the suction valve gets closed.

Figure 2.17 Piston compressor

The volume of the gas inside the cylinder decreases and the pressure increases. When the pressure inside the cylinder c ylinder exceeds the spring force of the discharge valve, it opens and the gas is forced out of the cylinder. The piston, however, is not allowed to hit the head of the cylinder and some amount of gas is always always left inside the cylinder. This leftover gas expands during the suction it reduces the capacity of the compressor hence its effect should be considered during analysis. When higher pressures are required multiple compressors are arranged in series with cooling of the gas in between.


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Piston compressors are available in single acting and double acting configurations. In sing acting compressor compression takes place only on one side of the piston head while in double acting compression takes place on both sides; suction on one side while compressing at the other side at the same time. Both single-acting and double acting compressors are available in lubricated or non-lubricated designs. Non-lubricated cylinder designs utilize rider and wear rings on the pistons. Non- lubricated cylinder designs are used in applications that cannot tolerate compressor lubricants in the process or product.

Diaphragm Compressor

A diaphragm compressor consists of a set of diaphragms, a piston, a hydraulic cylinder and a hydraulic oil system.

The structure of diaphragm compressors compressors is similar to to

diaphragm pumps. The piston moves a volume of hydraulic oil and the oil bends a set of diaphragms up and down, and the diaphragm compresses the gas. A diaphragm compressor is used when little or no leakage is tolerable. In such cases ordinary piston compressor cannot be used due to leakage or gas contamination. Some of the applications are:

Table 2.2 Application of Diaphragm Compressor

II.

Type of gas

Examples

Toxic

Boron triflouride

Corrosive

Fluorine

Flammable

Hydrogen

Pyrophoric

Silane

Oxidizer

Chlorine

Rotary Pumps and compressors

Rotary pumps and compressors are positive displacement fluid machines in which energy is transferred from the rotating element (rotor) of the pump to the fluid by direct application of force on the boundary of the fluid. This boundary is defined by the rotating and stationary components of the pump. The inlet and outlet ports of rotary fluid


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machines are separated by the action and position of the pumping elements and the close running clearance of the fluid machine. Hence unlike reciprocating machines rotary fluid machines do not need check valves. The following most common rotary pumps used in process industries are gear pumps (External Gear and Internal Gear Pumps), l obe obe pumps, vane v ane pumps pu mps and screw pumps. The most common rotary compressors in process industries are: sliding vane compressors, screw compressors, lobe compressors. The characteristics feature, operation principle, application and range of operation of the common rotary machines are discussed in the following section.

A. Rotar y Pumps

External Gear Pumps

External gear pumps work by the action of two rotating gears, which are cut externally that mesh and unmesh to cause the pumping action. In the most common design only one of the gears is directly driven by motor, the other one is idler gear, it is driven by the former gear. For severe services, however, both pumping gears are driven by timing gears. The two gears are supported on both sides by bearings. Figure 2.18 shows typical external gear pump with side cover removed. External spur gears

Figure 2.18 External gear pump with side cover removed (spur gear)

The gears of external gear pumps may be of type spur, helical or herringbone (double helical) gears. For small duty spur spur gears are invariably used because it is difficult to cut small helical and herringbone gears. Figure 2.19.shows a herringbone gear pump. Helical and herringbone gear pumps deliver uniform flow with insignificant pulsation as compared to spur gear pumps.


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Herringbone gear

Figure 2.19 A herringbone gear pump

Figure 2.20 shows the schematic diagram of a gear pump. When the gears unmesh at the suction side of the pump a partial vacuum is created and the liquid is drawn into the pump. This liquid is then trapped between the moving gear teeth and the fixed casing and is carried along to the discharge side of the pump. While rotating the gear teeth transfers energy to the fluid by directly applying force on the surface of the entrapped liquid. The meshing of the gear teeth and small clearance between the teeth and the casing prevents return of liquid from discharge to the suction side.

The suction and discharge ends of the pump are determined by the direction of gear rotation. A basic external gear pump can discharge liquid either way, depending on the direction of rotation. However, in most gear pumps the direction of rotation is fixed and is clearly marked on the pump.

Figure 2.20 External gear pump

External gear pumps can be designed with close tolerance, which makes them convenient for precise flow control. Due to this, external gear pumps are common in processes that require precise flow rates like in polymer processing and handling of expensive liquids. External gear pumps are lightweight, require small space and can handle liquids that


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contain gasses and vapors. The liquid should be relatively clean. They depend on the liquid pumped to lubricate internal moving parts and can be damaged if run dry. 3

Spur gear pumps have low capacity, commonly less than 200 gpm (45.4 m /hr). /hr). Helical 3

and herringbone gear pumps however can handle up to 5000 gpm(1135.6 gpm(1135.6 m /hr). /hr). Rotor speeds up to 1800 rpm can be used. External gear pumps can work against a pressure of 5000 psig (344.7 bar).

Internal Gear Pumps

In internal gear pumps the driving gear has internally cut gear teeth that mesh and umesh with the teeth of an externally cut idler gear to cause a pumping action. The idler gear is set off-center from the driving gear (see Figure 2.21) and the resulting space on the side opposite the meshing point is filled with a stationary crescent-shaped piece attached to the casing.

Figure 2.21 Internal gear pump

Unlike external gear pumps the gears of an internal gear pump rotate in the same direction. This results in a lower relative velocity between the two gears, which reduces friction, wear and turbulence. It also permits pumping of liquid that are more shear sensitive.

The unmeshing of the internal teeth of the rotor and the external teeth of the idler creates a partial vacuum in the suction side. This causes liquid to flow into the pump. The liquid is then carried to the discharge side of the pump between the teeth of both the rotor and idler, and the fixed casing. The crescent part of the fixed casing divides the liquid flow between the rotor and idler gear. Internal gear pumps are available with capacities c apacities up to


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3

1,100 gpm (249.8 m /hr). /hr). In general, discharge pressures are limited to 100 psig (6.89 bar).

Lobe Pumps

Lobe pumps consist of a pair of rotors with two to four lobes that operate similar to external gear pumps. Unlike gear pumps the two rotors are driven independently using timing gears and usually a small clearance is maintained between the lobes. The flow is more pulsating than gear pumps and the pulsation decreases as the number of lobes increases. These pumps are simple for cleaning and are preferable where sanitary is a priority like in food, beverage, pulp and paper and pharmaceutical industries. They can handle solids, slurries, and pastes.

Discharge

Two lobe rotors Suction

Figure 2.22 Lobe pump

Lobe pumps tend to wear less than external gear pumps. In some pumps, the lobe ends are fitted with replaceable packing strips to protect the lobes from wear. Since the rotors movement is controlled by timing gears and not one-driving-the-other, the alignment is not as critical as for external and internal gear pumps.

Suction

Discharge

Three lobe rotor

Figure 2.23 Schematic drawing of a lobe pump


35 3

Lobe pumps are readily available in capacities up to 600 gpm (136.3 m /hr). /hr). However, 3

lobe pumps up to 2000 gpm(454.2 gpm(454.2 m /hr) /hr) capacities have been made. The commonly available pumps operate against discharge pressure up to 400 psig (27.6 bar).

Vane Pump

Vane pumps are available in a number of vane configurations including sliding vane, flexible vane, swinging vane, rolling vane and external vane. Each type of configuration offers unique advantages. For example external vane can handle large solids. Flexible vane pumps, on the other hand can only handle small solids but create good vacuum. Despite the different configurations, most vane pumps operate under the same basic principle. A slotted rotor is eccentrically supported in a cycloidal cam. Vanes are fitted inside the slots of the rotor. The rotor is sealed into the cam by side plates. As the rotor revolves, centrifugal force, hydraulic pressure, and/or pushrods push the vanes to the walls of the fixed casing to form a seal.

Figure 2.24 Sliding vane pump

The eccentricity of the revolving rotor produces a partial vacuum at the suction side of the pump, causing an inflow of liquid. The fluid is trapped in the pockets created by the vanes, rotor, and cam and side plates. As the rotor rotates around, the vanes sweep the fluid to the opposite side of the crescent where it is squeezed through the discharge end. The tight seal among the vanes, rotor, cam and side plates is the key to the good suction characteristics common to the vane pumping principle. p rinciple.

Vane pumps have negligible pulsation and give constant flow rate independent of the discharge pressure. Their vanes are self-compensating for wear, and the original capacity is not affected until the vanes are critically worn out. Replaceable vane liners are


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available. They are light in weight and can handle liquids containing vapors and gasses. Vane pumps are not suitable for high viscosity liquids and liquids containing abrasive solids and foreign matter in liquid can damage the pump. 3

Vane pumps are available in capacities up to 2000 gpm (454.2 m /hr). /hr). They can operate at temperatures up to 450 F and against discharge pressure up to 150 psig (10.3 bar). Such pumps can also be made to operate up to 2000 psig (137.9 bar). The commonly available pumps handle liquids with viscosity up to 500,000 SSU. Rotor speeds up to 960 rpm are used.

Screw Pumps

The rotating element in screw pumps is a helical screw that revolves in a fixed casing. When the screw rotates cavities are formed at the suction and move towards the discharge side of the pump.

Figure 2.25 Single-rotor screw pump

The formation of this cavity produces partial vacuum that causes liquid to flow into the pump and moves along the progressing cavity. At the discharge side, the shape of the fixed casing causes the cavity to close. This generates an increase in pressure, forcing the liquid into the outlet line. The discharge pressure required determines the length and pitch of the helical screw rotor.

In twin screws (Figure 2.26) two rotors with involute helical surfaces are used in place of a single helical – – screw screw rotor. On entering the suction chamber, liquid is divided and flows to opposite ends of the pump body. At these points, liquid enters the twin rotors and is conveyed to the center of the pump where it is discharged. Timing gears lubricated by the liquid being pumped prevent contact between the twin screw rotors


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Screw pumps have negligible pulsation and can handle liquids containing vapors and gasses. They can be used for liquids containing a substantial amount amount of solids. solids. They produce relatively low wear on moving mov ing parts. They are known for their exceptional long life. Their disadvantages are that they are heavy and need large space. Variations in viscosity and discharge pressure cause significant change in capacity. Screw pumps are 3

available in capacities up to 3000 gpm (681.4 m /hr). /hr). The commonly available pumps operate against discharge pressure up to 1000 psig (68.9 bar). Screw pumps can pump liquids with viscosity as high as 5 million SSU. Discharge

Suction Figure 2.26 Twin screw double end pump

B. Rotar y compr essors

Sliding Vane Compressors

Sliding vane compressors consist of a cylindrical housing in which the rotor is eccentrically mounted. Vanes are placed in longitudinal slots cut into the rotor. Upon rotation the vanes slide out of the slots due to centrifugal force and form a seal against the internal wall of the housing. The gas is trapped in the space between the vanes, side plates and housing and is carried over the discharge side. In the process its volume gradually reduces and its pressure increases. Finally it is exhausted at higher pressure at the discharge end of the machine.


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Figure 2.27 Sliding vane compressors

Vane compressors exist in oil lubricated and oil flooded designs. In both cases, the compressor oil serves not only for lubrication but also as a sealant between individual vanes and the housing inner wall. For oil flooded vane compressors, a considerable quantity of oil is introduced into the compression chamber. This quantity of oil serves to remove the heat generated due to compression. This type of compressor can produce up to 50 psig (3.4 bar) per stage and is available in two two stage arrangements for pressures pressures up 3

to 125 psig (8.6 bar). Capacities range up to 1,500 to 2000 ft /min. The injected oil is filtered through separators after compression and channeled back to the circuit after separation. Screw Compressors

The screw compressor consists of two helical rotors fitted inside a casing. Bearing sets are fitted to each rotor to absorb axial and radial loads that develop during normal operation. The main rotor (male) rotor is driven through a shaft extension by an engine or electric motor. The other rotor (female) is (female) is driven by the main rotor. Screw compressors are available in both lubricated and oil-free configurations.

Figure 2.28 Section of a screw compressor

In lubricated units, the male rotor drives the female and oil is injected into the cylinder serving as both a lubricant and coolant, and also as an oil seal to reduce back slippage. On non-lubricated types, timing gears are used to drive the rotors and multi-staging is


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necessary to prevent gas temperatures from going too high. The popularity of rotary compressors is due to the relatively simple design, ease of installation, low routine maintenance requirements, ease of maintenance, long operating life and affordable cost. Oil Lubricated Screw Compressor

In oil lubricated screw compressors, lubricating oil is injected directly into the compression chamber to seal the rotors and to cool the compressed air or gas. This cooling of the compressed air allows gas allows gas to be compressed be compressed to higher pressures in a single stage configuration (around 14 bar for air). air). Fig. 2.29 shows the compression process at different stages in a screw compressor. The air enters through the inlet opening into the screw grooves of the rotor, which are open towards the suction side. As the rotors mesh the gas is trapped between the rotor groves and the casing. In the course of rotation, the male and female rotors generate a v-shaped space for the air drawn in, which becomes smaller and smaller right up to the end, between the rotor lobes and the cylinder walls.

Figure 2.29 Compression process in screw compressor

Oil Free Screw Compressors

Process industries in the manufacturing of textile products, foods and beverages, chemicals, medical and plastics usually require that the lubricant should not contaminate the process gas. In such cases oil free compressors are require since any contact with compressor can reduce the product quality. Oil-free rotary screw compressors offer many of the same performance, reliability and cost savings of lubricated rotary designs but without compressor lubricant lubricant in the compression chamber.


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Oil-Free rotary screw compressors are of multi stage designs. This is done to gain the improved efficiency of an intercooler and to minimize the final final discharge temperature. Cooling of the intercooler can either be air or water-cooled. Water-cooling is normally the preferred method since the lower operating temperatures will reduce the moisture load on downstream purification equipment. Flow, pressure and horsepower ratings for oil free compressors are listed below (table 2.3) Table 2.3 Range of, single stage oil free screw compressors

Quantity Discharge pressure (bar)

7-8.5

Flow rate (m3/min)

5.5 – 5.5 – 12.5 12.5

Brake Power (kW)

35-75

Lobe Blowers (Compressors)

Lobe blowers are also popularly known as Roots Blower on the name of its inventor. They consist of 8-shaped rotor inside an oval shaped casing, closed at ends by side plates. One of the rotors is driven by motor and the other is driven by a pair of equal ratio timing gears. The two rotors rotate at the same speed but in opposite direction.

Figure 2.30 Lobe blower

As the rotors rotate, air is drawn into inlet side of the cylinder and forced out the discharge side against the system pressure. With each revolution, four such volumes are displaced. Because of the very small clearance between the lobes and between the lobes and the casing the air in the discharge side will not come back to the suction side except a very small amount called called ‘slip’.

There is no actual compression inside the machine it merely displaces the air from the suction end to the discharge end, against the discharge system resistance. Since the lobe runs with finite clearance, no internal lubrication is required. Only the timing gears are


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lubricated and they are external to the casing. The gas or air, thus delivered is 100% oil free. These blowers deliver, practically, a constant flow rate independent of the discharge pressure conditions. The flow rate is dependent, largely on the operating speed.

REVIEW QUESTIONS

1.

What is the difference between pumps and gas movers?

2.

What is the difference between fans, blowers and compressors?

3.

Why is artificial cooling is required in compressors, while it is not required in fans, blowers and pumps?

4.

What is the difference between positive displacement fluid machines and dynamic fluid machines?

5.

What are the two types of positive displacement machines, what is their difference?

6.

Discuss the working principle, their characteristic features and applications of the main types of dynamic fluid machines in the chemical industry?

7.

In what situations are diaphragm compressors commonly used? Why?

8.

What in the difference between lobe compressors and most rotary positive displacement compressors.

9.

Discuss the main difference between oil free, oil lubricated and oil-flooded compressors and their application.

10.

Explain why there are no positive p ositive displacement fans?


42

REFERENCES [2.1]

Matley,J., Fluid Movers: Pumps, Compressors, Compressors , Fans and Blowers, 1979,McGraw-Hill

[2.2]

nd

Karassik, I.J., Krutzsch,W.C., (etal ), Pump Handbook, 2 Edition, 1986, McGraw-Hill.

[2.3]

Karassik, I.J., McGuire, T., Centrifugal Pumps, Chapman &Hall, 1998

[2.4]

Perry, H.R., Green,D., Perry’s Chemical Engineers’ Handbook , 6

th

editions,1984, editions,1984, McGraw-Hill. [2.5]

Sheer,W., Turbo Mahinery (AAU Teaching Material), 1976

[2.6]

Bloch, H.P., Soarres, C. , Process Plant Machinery, 2 ndedition, 1998, Butterworth-Heinemann

[2.7]

Cherkassky: Pumps, Fans and Compressors, Mir Publisher,1977. Publisher,1977 .

[2.8]

Hanlon, P.C., Compressor Handbook, McGraw-Hill, 2001

[2.9]

Eck, B., FANS : Design and Operation of Centrifugal, Axial-Flow and Crossth

Flow Fans, Pergamon Press, 5 Ed. [2.10]

Bleier,F.P., Fan Handbook, McGraw-Hill,1998 McGraw-Hil l,1998

[2.11]

Bathie,W.W., Fundamentals of Gas Turbines, John Wiley & Sons,Inc.,2 nd Edition, 1996.

[2.12]

Lazarkiewicz, S,Impeller Pumps, Pergamon Press, 1965.

[2.13]

Banga,T.R., Sharma,S.C., Hydraulic Machines, Khanna, Publishers, 1972.

[2.14]

Rao,N.S.G., Fluid Flow Machines, Tata McGraw-Hill, McGraw-Hill , 9183.

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