Centrifugal Compressor

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Centrifugal Compressor

Transcript of Centrifugal Compressor

Unit No. 4 - Turbines 4 - Turbines Unit No.



3.0 3.1

COURSE OBJECTIVES BASIC PRINCIPLES 3.1.1 How a Centrifugal Compressor Works

3 4 6 10 10 11 12 13 13 13 13 15


CAPACITY CONTROL 3.2.1 3.2.2 3.2.3 Speed Control Recycle Loop Adjustable Inlet Guide Vanes


SURGE CONTROL 3.3.1 What Causes Surge? What Does Surge Do? Controlling Surge

Module No. 6 :Module No. 6 : & Turbines Compressors Compressors & Turbines

3.3.2 3.3.3 3.4



COURSE OBJECTIVES This course describes the different types of compressors and turbines found in process plants. The course progresses in Units from basic terminology to the principles of operation of reciprocating and centrifugal compressors and steam and gas turbines. On completion of the course the student will be able to: Explain the basic terminology used in describing compressor and turbine operations.Page 1/16

Explain the functions and classifications of compressors and turbines in process plants, and identify the different types of prime movers used to power compressors. Identify suction scrubbers and explain how they work. Explain the basic principles of operation of reciprocating compressors including capacity control methods, control of suction pressure, and automatic shutdown systems. Explain the basic principles of operation of centrifugal compressors including capacity control methods, surge control, and automatic shutdown systems. Explain the basic principles of operation of gas turbines, steam turbines, and turbo-expanders.

Unit No. 4 - Turbines



Module No. 6 : Compressors & Turbines

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Module No. 6 : Compressors & Turbines

Unit No. 4 - Turbines

Figure 3-1 Centrifugal Compressors

The main working part of a centrifugal compressor is called the impeller. If there is only one impeller in the compressor, the compressor is classed as a single stage compressor. If there is more than one impeller in the compressor, the compressor is classed as a multi-stage compressor.

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In a multi-stage compressor each of the impellers adds pressure to the gas. A multi-stage compressor can be thought of as several single stage compressors joined together inside the same casing. A centrifugal compressor contains only three main parts. These are the casing, the impeller (impellers if it is a multi-stage compressor), and the shaft.Unit No. 4 - Turbines

The casing provides somewhere for gas to enter the compressor. This is the compressor suction, which is sometimes called the suction port. The casing also provides somewhere for the gas to leave the compressor. This is the compressor discharge, which is sometimes called the discharge port. The shape of the compressor casing is important because it must be in the form of a volute. A volute is like part of a spiral. It increases in diameter (and therefore in volume) in the direction of flow. The importance of this will be explained later in this Unit. There are several different designs of impeller, three of which are shown in Figure 3-2.

Figure 3-2 ImpellersModule No. 6 : Compressors & TurbinesThe open impeller is used where a high discharge pressure is required. It can handle small to large flows but is used only in single stage compressors. The semi-enclosed impeller is used for large flows. This type of impeller is usually found in single stage machines although it may be used as the first stage of a multistage compressor. The enclosed impeller is mainly used in multi-stage compressors although it may also be found in single stage machines. The shaft connects the impellers of a multi-stage compressor together and connects either a single stage or a multi-stage compressor to its prime mover.

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How a Centrifugal Compressor Works To understand how a centrifugal compressor works you must first understand something about centrifugal force and about energy. As explained in Unit 1 of this course, when something is turning about a central point, a force pushes outwards from the central point. The faster the thing is turning, the greater the force. This force is known as centrifugal force. In a centrifugal compressor the impeller (impellers if it is a multi-stage compressor) is the thing that is turning. How centrifugal force is used in the compressor will be explained later. Energy cannot be created or destroyed, but it can be changed from one form of energy to another. Think of a hammer which is being used to hit a nail into a piece of wood. The hammer is raised over the head of the nail ready to hit it. At this point the hammer is not moving but it has a form of energy called potential energy. Something has potential energy because of its position. As the hammer moves down to hit the nail, all of the potential energy changes to kinetic energy. Something has kinetic energy because it is moving. As the hammer hits the nail all of the kinetic energy is converted into work, and work is yet another form of energy. The work drives the nail further into the wood. Hammers and nails have nothing to do with centrifugal compressors but they do show how energy changes its form.

Module No. 6 : Compressors & Turbines

Unit No. 4 - Turbines

Figure 3-3 What an Impeller Does Gas enters the casing of a Centrifugal compressor through the suction port. Inside the casing the gas passes to the inlet of the impeller. This is called the eye of the impeller. The impeller is spinning round very fast. The gas is forced outwards to the outer rim of the impeller by centrifugal force. As the gas moves towards the outer rim of the Page 5/16

impeller its velocity is increased. How does an object which is spinning (the impeller) increase something which is passing through it? Look at Figure 3-4. the speed of

Figure 3-4 shows a circular object which can rotate round its centre point. In other words, it can spin. The object could be anything, including the impeller of a compressor, but we will simply call it a wheel.

Unit No. 4 - Turbines

Two points, Point A and Point B, have been marked on the wheel. For our purposes the wheel does not need to turn a full circle It will move only from Position 1 to Position 2. The wheel makes this move in a certain amount of time. The amount of time does not really matter. What is important is that Point A and Point B both move from Position 1 to Position 2 in the same amount of time.

Module No. 6 : Compressors & Turbines

Figure 3-4 Velocity Increase Now look at how far Point A and Point B travel. You can see from Figure 3-4 that Point A travels about two times as far as Point B. Use a ruler to measure the dotted lines on the drawing. You will find that Point A moves about 5 centimetres and Point B moves about 2.5 centimetres. Remember that this movement has Page 6/16

taken place in the same amount of time for each of the points. Let us say that the wheel has moved from Position 1 to Position 2 in 1 second. Point B has moved at a velocity of 2.5 centimetres per second (2.5 cm/s) while Point A has moved at a velocity of 5.0 cm/s. Imagine now that instead of Points A and B there is a solid object on the wheel. As the wheel spins centrifugal force makes the object move out from the centre of the wheel to the outer edge. As already shown, when this happens the velocity of the object is increased. Even a molecule of gas is a solid object.

Unit No. 4 - Turbines

Inside the compressor the gas has passed through the impeller and its velocity has been increased. Because the gas is moving, its kinetic energy has been increased also. As the gas leaves the impeller it passes into a passageway called the diffuser. When the gas enters the diffuser the impeller is no longer acting directly on the gas. The diffuser has a larger diameter than the impeller. Because the flow path of. the gas is now longer, and there is no direct action on the gas by the impeller, the velocity of the gas decreases. As the velocity of the gas decreases so does its kinetic energy. Remember that energy cannot be either created or destroyed, it can only be converted from one form to another. In this case the kinetic energy is converted into pressure energy. Gas passes from the diffuser into the volute. In the volute the conversion from velocity (kinetic energy) to pressure continues.

Module No. 6 : Compressors & Turbines

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Figure 3-5 Diffuser and Volute

Unit No. 4 -No. 4 - Turbines Unit Turbines

To sum up what happens; the work put in to the compressor by the prime mover is used to increase the velocity of the gas inside the compressor. The velocity (kinetic energy) is then converted into pressure energy and the pressure of the gas is increased. 3,2 CAPACITY CONTROL_ In a -continuous gas process the flow of gas through the process may not be constant. - There may be changes in either the supply of gas or the demand for gas. If a compressor is installed as part of a continuous gas process we need to be able to control the capacity of the compressor to suit changes in gas flow. Capacity is the amount of gas flowing through the compressor at any one time. One of the easiest ways to do this is to control the speed of the compressor. The lower the speed, the lower the capacity and the higher the speed, the higher the capacity. However, the prime mover of the compressor may not be suitable for speed control. In this case other methods of capacity control are used. These are; A recycle loop, which is sometimes known as a hot g