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RESEARCH AND DEVELOPMENT OF FLOW PART OF CENTRIFUGAL COMPRESSORS OLDŘICH Jiří, Head of thermodynamic design and R&D, ČKD NOVÉ ENERGO, a.s., Klečákova 1947, 190 02 Praha 9, Czech Republic, jiri.oldrich@ckdenergo.cz Abstract

The goal for next years is to produce machines with high efficiency and low energy consumption. One way how it can be reached is to extend research and development. The goal of this contribution is to give short information about development of radial compressor stages in company ČKD NOVE ENERGO, a.s. by using CFD methods. An intention of development is to find interdependence of flow part details shape and molecular mass of compressed gas, inlet and discharge parameters by using CFD methods. The new centrifugal stages suitable for compression of both light and heavy gases with higher efficiency and higher-pressure ratio will be designed.

1. Introduction

Company ČKD NOVE ENERGO, a.s. is one of traditional manufacturers of compressors. Compressors are produced for more than 100 years. Centrifugal compressors of own design of ČKD have been manufactured since 1916. Though ČKD produces various types of compressors like piston, centrifugal and screw, centrifugal compressors are a significant part of all production of compressors. This is a main reason why it is necessary to pay attention to research and development of centrifugal compressors. There are many various problems that must be solved. For example material selection, solving of strength problems, bearing, seals and many another technical problems. 2. Centrifugal compressor

First of all it is necessary to revise some basic information about centrifugal compressors. Compressor is a machinery designed to increase gas pressure. This increase of the pressure is used to perform some work and to overpower friction loss in pipelines and friction loss of flow gas in equipment of some chemical technology. Centrifugal compressors have many advantages in comparison to piston compressors. For example centrifugal compressor has not any parts that perform reciprocating movement, consequently centrifugal compressor has very quiet run. In view of its quiet run and because centrifugal compressor has not any part that appose, except of bearings, centrifugal compressor is very reliable. Next advantage is the fact that specific mass of centrifugal compressor is decreasing with its increasing capacity.

Design of centrifugal compressor and generally design of all machines working with gaseous mixtures is interdisciplinary process including aerodynamics, thermodynamics, stress analysis, vibration analysis, selection of materials and so on. During design stage possibilities and requirements of manufacturing must be taken into account.

Pressure ratio, that can be reached in one stage of centrifugal compressor, depend on molecular mass of compressed gas, this implies that available pressure ratio considerably depend on physical properties of compressed gas. Because usually reached pressures are higher than barometric pressure, it is necessary to respect real behavior of gases or gaseous mixtures. 3. What is centrifugal compressor stage

Every compressor stage consists of several basic parts. Schematic sketch of common used compressor stage of multistage centrifugal compressor is given on Figure 1. Sucked gas flows through

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suction part of compressor 0’-0-1, goes to impeller, where its velocity is due to centrifugal forces increases (it means that rotating impeller transmit part of its kinetic energy to the compressed gas), at the same time pressure of gas is also increased. Then the gas flows into diffuser where part of its kinetic energy is changed to pressure energy. To get into the next stage the direction of flow must be changed in return channel.

On Figure 2 there is photography of real stage of multistage compressor for compression of refrigerant R134a (this compressor was manufactured in CKD in 2005). Changes of energy in particular parts of compressor stage are shown on Figure 3. When we carefully watch Figure 3 we can see that highest increase of pressure occurs in the impeller. From this point of view the impeller is main part of the compressor and its quality influences quality of the whole compressor. Nevertheless other parts of the compressor have also great importance on function and quality of designed compressors.

The design of compressor consists in finding of acceptable compromise between several seemingly contradictory requirements. The main requirements are high efficiency, high pressure ratio in one compressor stage, minimization of number of compressor stages, simplicity of manufacturing, operational reliability, low weight and low price. Sometimes, for example in the case of compressor for compression of hydrogen gas, gas

with variable composition (it means gas with variable molecular mass) must be compressed.

Compressors are designed for various gases and their mixtures. Thermodynamic properties of all compressed gases are calculated by using of real gas equation of state. Very good experience is with BWR equation of state, which is in CKD used for many years. In database of real gases properties, it means constants for BWR equation of state, constants for equation of specific heat, constants for calculation of viscosity and thermal conductivity there are data for more than eighty individual gases. This databank was created in cooperation with Czech chemical university in Prague. When some gas that we need is not found in our databank we are able to get its properties in short time. Subroutines for calculation of real gases properties and thermodynamic processes based on above mentioned equations and relations are included in our firm software for thermodynamic design of compressors.

Figure 1 Compressor stage

Figure 3 Changes of energy in particular parts of centrifugal compressor

Figure 2 Compressor stage

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Very important is wide flow range. Industrial compressors must very often deliver gas in wide flow range at constant pressure. For example in case of refrigeration compressors control range of flow from 15% to 100% is required. It can be accomplished by using of both adjustable inlet guide vanes in inlet of first stage and adjustable diffuser blades of all stages.

In similar way like other centrifugal compressor manufacturers CKD also uses to design of flow part of the compressors modular system based on aerodynamic similarity. This system is based on developed and good tested compressor stages whose dimensionless characteristics serve to design stages of produced compressor. Some of the main parts of compressors like for example stages, inlet and outlet parts and many others parts are unified. Every newly designed compressor stage was tested on test stand with air and or with some another gas with higher molecular mass (for example with some refrigerant) Measured dimensional quantities like a pressure, temperature and flow rate were used to drawing of dimensional performance curves. Dimensional parameters taken by tests were converted into dimensionless parameters. These dimensionless parameters served to get dimensionless performance curves (characteristics). Advantage of these characteristics is that they can be used to design new compressor for some industrial gas.

To satisfy aerodynamic similarity must be some conditions kept. Basic presumption is similarity of velocity triangles (see Figure 4.).

obrázek 1

Next requirement is compliance of equality of tip speed Mach numbers

0

22 a

uM = (1)

where ao [m/s] is sound velocity in an inlet of the stage u2 [m/s] is tip speed of impeller

There are some next conditions that are not mentioned here. Dimensionless parameters are: Flow coefficient related to outlet of impeller

222

2

2

2

ubDQ

uc m

πϕ == (2)

Figure 4 Velocity triangles (c2 –absolute speed, u2 – tip speed, w2 – relative speed)

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where c2m [m/s] is radial speed Q2 [m3/s] is volume flow in discharge of impeller

22 2RD = [m] is diameter of the impeller b2 [m] is discharge width of the impeller

Flow coefficient (coefficient of volume flow) related to inlet of impeller

222

04uD

Qπ

=Φ (3)

Isentropic head coefficient

22uhs

s∆

=χ (4)

Work input coefficient

22

2 uh∆

=σ (5)

Isentropic stage efficiency

Polytropic stage efficiency

hhp

p ∆=η

(7)

By using the above mentioned parameters dimensionless characteristics )( 0φχ −s , )( 2ϕχ −s , )( 02 φσ −

and )( 22 ϕσ − are prepared. These characteristics are implemented into databank of compressor stages that is used by our firm software for design of commercial compressors.

Design of centrifugal compressor is an activity that interferes in many branches of engineering. Design of centrifugal compressors begins by thermodynamic design, which determines how many of compressor stages will be used, type of used stages, basic parameters of used stages like a diameter of each impeller, its discharge width and rotational speed. Other parameters like for example blade inlet and exit angles, numbers of impeller blades, shape of blades and some others depend on selected stage type.

The goal is to design a compressor that will fulfill all requirements of costumer with maximal efficiency and with an optimal number of stages. The design of compressor is optimization procedure that leads up to finding an acceptable compromise between requirements to fulfill parameters of compressor and technical possibilities. As a result of this design we receive basic conception of the compressor. After thermodynamic design, preparation of technical drawings comes. Next step is selection of materials and stress and dynamic analysis. 4. Main goals of research

To prepare well designed centrifugal compressor, which is able to fulfill required parameters with high efficiency, designer must have wide database of well designed and tested compressor stages. Goal of described development and research is to prepare and add new compressor stages into existing database. Main demand is to receive better possibility of calculation and design of compressors in very wide range of parameters.

hhs

s ∆=η (6)

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Because of growing pressure to save environment, technologies where compressors are used are changed in order to reduce energy consumption. In consequence of it changes in design of compressors must be done. Main demand is to increase efficiency and decrease compressor mass. Both tasks can be executed by innovation of compressor stages. If designer has more stages at disposal, he can design compressor with high efficiency and at the same time with small number of stages. That means saving of energy consumed for compression of gas, saving of material and together saving of energy consumed for producing of material from which is compressor made. 5. Research in CKD

First must be determined what compressor stages we need and which one will be designed first. On the basis of analysis of our commonly used compressor stages and requirements of our customers the stages with low flow number were chosen. These stages are very often used for design of compressor with small capacity intended for compression of hydrogen gas or natural gas. There are such stages in our databank of stages but it is necessary to improve them.

The plan of the research of centrifugal stages for next three years is divided into several steps. In the first stage of development, existing stages will be improved and optimized. In second step new stages will be designed. First step of design, improving and optimization of existing stages is already in progress. Second step will be begun in next year.

Both in the first part and in the second part specialized software will be used. Concept NREC software enable in first step made some changes in geometry of existing stage. In second stage of development, i.e. design of new compressor stages software Concepts NREC will be used to design suitable new flow part . Parts of this software are program unit for optimization that enables to find better shape of all parts of compressor stage and program unit for rapid CFD analysis. CFD served to confirm that there are no regions of separated or reversed flow. All this extensions will accelerate and improved effectiveness of design of stages.

In second step software ANSYS CFX will be used. This software enables to do detailed and very exact CFD analysis of optimized or newly designed stages. Software ANSYS CFX will be used as “virtual test room”. Only some selected stages will be tested on our test stand.

Because most of compressors produced in CKD compress various gases and its mixtures, new stages must be designed or modified for these gases. CKD NE has his own databank of properties of real gases. The data from this databank will be implement into above mention software.

Outcome of development will be dimensionless characteristics and detailed design of each new stage. When development of each compressor stages and its design will be done, this dimensionless characteristic and design will be transferred into digital form and added in database of compressor stages in order that they can to be used both in our firm software for compressor design and in CAD software. 6. Testing

Selected stages will be tested. Measurement of prototype stage (impeller, diffuser and return channel) will be performed in single stage open-loop test stand. These tests will be carried out only with air. New test blower will be driven by variable speed electric motor with high speed without gearbox. This solution enables to carry out measurements for various impeller tip speed (it also means for various tip speed Mach numbers).

Within the scope of measurement of compressor performance curves will be measurement of pressures and temperatures, measurement of capacity and measurement of internal power consumption. Rotational speed, barometric pressure, ambient temperature and humidity will be measured too. To find out mechanical losses in blower, bearing temperature and oil flow rate will be measured.

All data founded by this way will be processed and evaluated. Results of it will be dimensionless characteristics that will complete existing database of properties of compressor stages.

Date received from calculation by ANSYS CFX and data received by measurement will be compared. On base of this comparing reliability of ANSYS CFX as the virtual test room will be checked.

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7. Conclusion Development of flow part of centrifugal compressors in CKD NE was reopened after several years break.

CKD bought software for analysis of flow through centrifugal compressors ANSYS CFX and software for thermodynamic and aerodynamic design of compressors Concepts NREC. In this year CKD received grant of Ministry of Industry and Trade for project “Research and development of flow part of centrifugal compressors”. Works on development will be in cooperation with the Faculty of Mechanical Engineering (CTU)

References

§ MISÁREK, D.: Turbokompresory, SNTL, Praha 1963

§ OLDŘICH, J. - NOVÁK, J. - MALIJEVSKÝ, A.: Adiabatický děj v technické praxi (Adiabatic process in technical practice) in Czech, Strojírenství, 40, 1990, č.2, str. 69/76

§ OLDŘICH, J., - MALIJEVSKÝ, A.: The Isothermal Change of a Real Gas, Technolzgz Today, 1992, No.2, pp. 56/60

§ OLDŘICH, J.: Polytropická komprese reálného plynu, přednáška na konferenci CHISA‘95, Kouty nad Desnou, 1995 ( Polytropic compression of real gas, contribution on conference CHISA‘95) in Czech

§ OLDŘICH, J.: Some methods of solution of thermodynamic problems connected with compressors and cryogenics, Příspěvek ve Sborníku konference Kryogenika ‚96, Praha 1996

§ STRAKA, F.,:Bioplyn, GAS 2003 - DĚDEK, J., MALIJEVSKÝ, A., NOVÁK, J., OLDŘICH, J.: Kapitola 4 –“Fyzikální a chemické vlastnosti bioplynu“ (Physical and chemical properties of biogas) in Czech

§ OLDŘICH, J., NOVÁK, J. P., MALIJEVSKÝ, A., DĚDEK, J.: Calculation of work and heat at thermodynamic processes using the Bender equation of state, Příspěvek ve sborníku konference ESAT 2000, Kutná Hora 2000

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Table 1 Schedule of optimalization of existing compressor stages