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Wednesday, 16 October 2019

October 16, 2019

PERFORMANCE CHARACTERISTICS OF CENTRIFUGAL COMPRESSOR

We were discussing the working principle of centrifugal compressorvelocity diagram of centrifugal compressorslip phenomenon and slip factor for centrifugal compressorMach number for centrifugal compressorfunction of diffuser in centrifugal compressor and losses in centrifugal compressor in our recent posts, where we were also discussing the various important parts of centrifugal compressor and their functions and we have also secured there the work done on air in centrifugal compressor.  

Today we will be interested here to discuss another very important topic i.e. performance characteristics of centrifugal compressor with the help of this post i.e. performance characteristics of centrifugal compressor. 

Performance characteristics of centrifugal compressor 

Let us first understand here the meaning of performance characteristic curve of a centrifugal compressor and what benefit we will get after studying the performance characteristic curve of a centrifugal compressor. 

Performance characteristics curve of a centrifugal compressor will be normally described with the help of curves of delivery pressure and temperature with respect to mass flow rate for various fixed values of rotational speed at given value of inlet pressure and temperature. 

If we make an experiment and draw the points to form the curve between the pressure ratio and mass flow rate, we will get a performance characteristic curve as displayed here in following figure. 


There are three very important points in the curve i.e. A, B and C. We will find out here the meaning and importance of each point mentioned in the performance characteristic curve. 

As we can see here in the performance characteristics curve, it will be increasing with positive slope up to a point B which will be at maximum value of pressure ratio and further it will be decreasing with negative slope and finally it will touch with abscissa at point C when mass flow rate will be huge and at this point the pressure ratio will be unity. 

Now we must understand the term used here pressure ratio. What does it mean? Pressure ratio mentioned in the curve is basically the ratio of pressure at outlet of compressor and at inlet of compressor. 

Now we will first understand the importance and meaning of point A. What will point A indicate in the following characteristics curve of a centrifugal compressor? 

When mass flow rate will be zero, there will be a pressure ratio and point A indicates this pressure ratio when mass flow rate will be zero. Now, we will have one question in our mind that why there will be a pressure ratio even if mass flow rate is zero. 

So, let us understand it. 

When mass flow rate will be zero i.e. we have closed the delivery valve, impeller will be rotating and energy will be imparted on the working fluid and hence a pressure rise will take place because of the centrifugal action. Hence, fluid may not enter in to the diffuser and there will be a static pressure field which will be due to the churning action of the fluid in the impeller and it will establish a static pressure rise because of the centrifugal action which will be termed as centrifugal head. 

Therefore, the centrifugal head because of the impeller rotation will be imposed on the fluid and hence a pressure ratio will be generated even if the delivery valve will be closed. This pressure ratio is displayed here in the characteristic curve by point A. 

When we will slowly open the delivery valve, flow of working fluid i.e. air will be started and fluid will pass through the vaneless space and diffuser and hence there will be the diffusion process which will result the increase in the pressure. 

Therefore, with increase in the mass flow rate of the fluid, pressure ratio will be increasing and will reach to the point B where pressure ratio will be maximum. It is displayed here in characteristic curve by the line AB. Beyond point B, there will not be any rise in pressure or pressure ratio with increase in mass flow rate due to the losses in the centrifugal compressor. 

Therefore, point B will indicate the maximum pressure ratio point and the efficiency of the centrifugal compressor will be maximum at the point B. Beyond point B, the efficiency of the compressor will fall rapidly as there will be decrease in the pressure ratio with increase in the mass flow rate of the fluid. 

If we are continuously increasing the mass flow rate of the fluid for a fixed rotational speed, there might be one point where mas flow rate will be huge and pressure ratio will be unity i.e.1 and this point is indicated by point C in the characteristic curve. 

Therefore, there will not be any pressure rise and the complete energy which is giving to the compressor will be utilised to overcome the losses (losses in the centrifugal compressor) during handling the huge mass flow rate of the fluid. 

Let us see now the characteristic curve, we will see here two parts i.e. AB and BC. Part AB will have positive slope, as displayed in characteristic curve, and will be usually unstable and it will be very difficult to have this part of characteristic curve in practice. While part BC, which will have negative slope, will be stable. 

Let us consider that compressor is operating at point D on the part AB of the characteristic curve which is having with positive slope as displayed in the curve. Let us consider that the delivery valve is open partial and compressor is discharging the fluid with steady state. 

Let us think that there is reduction in the mass flow rate due to any disturbance or by any chance there is some reduction in the mass flow rate. Due to reduction in the mass flow rate, there will be reduction in the pressure ratio too and it could be understood by observing the point D on the part of AB of the characteristic curve having a positive slop and therefore, delivery pressure of compressor will fall down rapidly. 

If the pressure of fluid at downstream side of the compressor, where compressor is discharging the fluid, does not fall quickly then there will be generated a pressure gradient where pressure of the fluid at the downstream side will be higher than the delivery pressure of the compressor. 

Therefore, fluid will start to flow back to the compressor from the downstream side of the compressor. Hence, net flow from the compressor will be reduced due to opposing flow and hence there will be further drop in pressure and finally point A will be secured where there will not be any flow but there will be a pressure ratio as discussed above. 

Due to reduction in mass flow rate, pressure at the downstream side of the compressor will be reduced sufficiently and therefore again there will be a pressure gradient and flow will start to flow from delivery end of compressor to the downstream side of the compressor i.e. positive flow will be developed again.  Therefore, there will be flow reversal again. 

Hence, A small disturbance or reduction in the mass flow rate has been resulted to have the repeating  cycles or where flow reversal will be there. 

We must note it here that this type of flow reversal will take place when the compressor is on stiffer side of the part AB of the characteristic curve and that will be termed as surging of the compressor.
Therefore, this part of the characteristic curve AB will be unstable and other part i.e. BC will be stable. 

As we can see from the curve that part AB of the characteristics curve is stiffer initially but becomes flat near point B. Therefore there might be one point, over the part AB which is having positive slope, just left to the point B where surging of compressor will not take place. 

Surging of compressor will take place if the operating point of compressor is some distance away from point B. Point B indicates the maximum pressure ratio point where the efficiency of the compressor will be maximum. 

Now we will consider the part BC of characteristics curve which is having with negative slope. We will find out here the importance and meaning of point E which has been displayed in characteristics curve. 

When mass flow rate will be increased, the pressure ratio will be decreased and hence the delivery pressure will be decreased. Density of fluid will be decreased due to decrease in the delivery pressure. 

Velocity of flow will be directly proportional to the mass flow rate and inverse proportional to the density of the fluid. Hence there will be huge increase in the flow velocity due to increase in mass flow rate and decrease in the density of the fluid. 

Therefore for a fixed value of rotational speed, there will be a point where sonic velocity will be achieved and compressor sonic condition will be there. At this point, there will not be any further increase in the mass flow rate of the fluid even if we keep opening the delivery valve. 

This particular point will be termed as a point of maximum delivery obtainable at the particular rotational speed for which the characteristics curve is drawn. Point E indicates this point of maximum delivery obtainable. We must note it here that at point E, there will not be any possibility of increase in the mass flow rate of the fluid and then it will not matter that how much wider delivery valve is opened. 

Therefore, we have seen here the characteristic curve of a centrifugal compressor and we have also  seen here the various important features associated with the characteristic curve of a centrifugal compressor. 

Do you have any suggestions? Please write in comment box and also drop your email id in the given mail box which is given at right hand side of page for further and continuous update from www.hkdivedi.com.  

Further we will find out, in our next post, . 

Reference:  

Fluid Machines, By Prof S.K. Som 

Sunday, 13 October 2019

October 13, 2019

BASICS OF CENTRIFUGAL COMPRESSOR

In our recent posts, we were focused on the basics of centrifugal compressor. We have seen the various posts based on the working principle, velocity diagram of centrifugal compressor, slip phenomenon and slip factor, Mach number for centrifugal compressor, function of diffuser and various losses in centrifugal compressor and these posts are as mentioned below.

Posts based on basics of centrifugal compressor 

There are the desired pages links which will provide the complete information about te centrifugal compressor.

We were discussing the following posts. 
  1. Working principle of centrifugal compressor
  2. Velocity diagram of centrifugal compressor 
  3. Slip phenomenon and slip factor for centrifugal compressor
  4. Mach number for centrifugal compressor
  5. Function of diffuser in centrifugal compressor  
  6. Performance characteristics of centrifugal compressor
Do you have any suggestions? Please write in comment box and also drop your email id in the given mail box which is given at right hand side of page for further and continuous update from www.hkdivedi.com

Further we will find out, in our next post, main components of reciprocating pump.   

Also read 

October 13, 2019

DIFFERENT TYPES OF LOSSES IN CENTRIFUGAL COMPRESSOR

We were discussing the working principle of centrifugal compressorvelocity diagram of centrifugal compressorslip phenomenon and slip factor for centrifugal compressor, Mach number for centrifugal compressor and function of diffuser in centrifugal compressor in our recent posts, where we were also discussing the various important parts of centrifugal compressor and their functions and we have also secured there the work done on air in centrifugal compressor.  

Today we will be interested here to discuss another very important topic i.e. losses in centrifugal compressor with the help of this post i.e. different types of losses in centrifugal compressor. 

Different types of losses in centrifugal compressor 

When fluid flows through the centrifugal compressor, there will be some losses in the mechanical energy or we can also say that there will be drop in static pressure energy at the outlet of compressor. This losses or drop in static pressure energy at the outlet of centrifugal compressor will be termed as losses in centrifugal compressor. 

There will be basically three types of losses in the centrifugal compressor and these losses are as 
mentioned here.
  1. Frictional losses
  2. Incidence losses
  3. Clearance losses

Now let us discuss each type of loss in centrifugal compressor in detail. 

Frictional loss

Frictional loss is the major portion of losses in centrifugal compressor and it is basically due to the friction. Working fluid i.e. air will flow through the compressor blade passages and will come in contact with solid surfaces and hence fluid will face resistance i.e. friction while flowing through the compressor blade passages. Due to viscosity, there will be friction between fluid layers also. Therefore, there will be skin friction. 

There will also be boundary layer separation losses and loss due to shock.
Frictional losses, hence, will be the combined effect of losses due to skin friction and boundary layer separation. 

Frictional losses basically depend over the friction factor, square of the fluid flow velocity and length of the flow passage. We can see here the variation of frictional losses with respect to the mass flow with the help of following curve as displayed here in following figure. 

Incidence losses 

Incidence losses occur when the compressor will not work at its design condition. When flow velocity of working fluid does not follow the vane angle while flowing through the vane, incidence losses will be there. 

When compressor will work on off-design condition, the direction of relative velocity of flowing fluid at inlet will not match with the inlet blade angle and hence working fluid i.e. air will not enter the blade passage smoothly by gliding along the blade surface. 

We can see here the variation of incidence losses with respect to the mass flow with the help of following curve as displayed here in following figure. 

Clearance losses

There must be minimum clearance between impeller shaft and casing of the centrifugal compressor and between the outlet periphery of the impeller eye and the casing. 

Clearance losses could be reduced by providing proper sealing arrangement. 

We must note it here that the contribution of clearance losses in the total losses in the centrifugal compressor will be very less. Hence, in order to avoid the losses in the centrifugal compressor, we must have to reduce the losses due to friction and incidence losses. Frictional losses and incidence losses are the main parts of total losses in the centrifugal; compressor. 

Losses in the centrifugal compressor Vs Mass flow 

Let us draw here the curve between the losses and mass flow as displayed here in following figure.
If we see the curve, we can easily conclude that frictional losses will be increasing with respect to mass flow. 

Incidence loss is also displayed here in following figure and we can conclude that incidence losses will be minimum at design condition only. 

Curve which is showing the total loss is basically the combined effect of frictional losses and incidence losses. Therefore this curve indicates the total losses of energy or static pressure head in the centrifugal compressor.  

Therefore, we have studied here the various types of losses in the centrifugal compressor and we have also seen here a curve between mass flow and losses. 

Do you have any suggestions? Please write in comment box and also drop your email id in the given mail box which is given at right hand side of page for further and continuous update from www.hkdivedi.com.  

Further we will find out, in our next post, performance characteristics of centrifugal compressor. 

Reference:  

Fluid Machines, By Prof S.K. Som 

Image courtesy: Google   

Also read 

Friday, 11 October 2019

October 11, 2019

FUNCTION OF DIFFUSER IN CENTRIFUGAL COMPRESSOR

We were discussing the basic principle of operations, functions of various parts and performance characteristics of different types of fluid machines such as hydraulic turbinescentrifugal pumps and reciprocating pumps in our previous posts. We have seen there that working fluid for above mentioned fluid machines was liquid such as water.  

Now it’s time to discuss few other types of fluid machines such as centrifugal compressors, axial flow compressors, fans and blowers where working fluid will be steam, air or gases.  

We have already seen the working principle of centrifugal compressorvelocity diagram of centrifugal compressor, slip phenomenon and slip factor for centrifugal compressor and Mach number for centrifugal compressor in our last posts, where we were also discussing the various important parts of centrifugal compressor and their functions and we have also secured there the work done on air in centrifugal compressor. 

Today we will be interested here to discuss a very important part of centrifugal compressor in detail i.e. Diffuser with the help of this post i.e. Function of diffuser in centrifugal compressor.  

Function of diffuser in centrifugal compressor 

Impeller and diffuser are two very important parts of a centrifugal compressor. Energy will be added to the impeller and when working fluid i.e. air will come out from the impeller i.e. at the impeller outlet tip, it will gain the energy by transfer of angular momentum by the rotating action of impeller blades. 

When air will come out at the impeller tip, it will have energy in the form of kinetic energy and static pressure energy. 

Practical application of compressed air will be usually found in engines, gas turbine plants, turbojet engines etc., where this compressed air will be used in a combustion chamber to burn the fuel. 

When fuel will be burnt, there will be generation of high temperature or air will be heated to a high temperature by energy generated due to the burning of fuel. In order to secure the smooth and efficient burning of fuel, there will be requirement of the compressed air at high pressure with very small velocity as low as possible. 

High velocity in the combustion chamber will cause the instability of combustion and combustion could not be made efficiently. Therefore, we must note it here that we want air at the outlet of compressor with relatively very low velocity and high static pressure. 

Excess kinetic energy of the air, available at the outlet of impeller tip, will be converted in to static pressure energy in the diffuser. Therefore, diffuser converts the kinetic energy of the working fluid i.e. air in to static pressure energy and at the outlet of diffuser air will have relatively lower velocity and high static pressure. 

As we can see in figure, there will be vaneless space beyond the impeller tip and partial diffusion process will take place here in this vaneless space. Further a number of vanes will be available to execute the final diffusion process. 

Number of vanes will be provided to divide the air stream in to several channels to make effective control of the flow and simultaneously we will be able to secure the final diffusion i.e. rise in static pressure by conversion of kinetic energy in to static pressure within a short length. Therefore, number of vanes will be provided to direct the air stream in to different channels. 

Why are vaneless space in centrifugal compressor provided? 

This is a very important question that why vaneless space are required in a centrifugal compressor and why not these diffuser vanes are provided just after the outlet of impeller tip so that fluid coming out at the outlet of the impeller may go to diffuser directly. 

But it is not done ever and vaneless space are always provided between the impeller outlet and diffuser vanes. 

There are two basic and very important reasons behind the provision of vaneless space between the impeller outlet and diffuser vanes. 

First reason is that the mach number of flow need to be reduced before it enters to the diffuser vanes. In other words, we can say that velocity of fluid flow need to be reduced before fluid enters to the diffuser vanes. 

Therefore, there must be some space between the impeller outlet and diffuser vanes in order to reduce the velocity of flow of fluid before it enters to the diffuser vanes. This task of reduction of velocity of fluid flow, coming out from the impeller outlet tip, will be carried out by this vaneless space. 

Second reason is that if we will not provide this vaneless space, there will be an excessive circumferential variation of static pressure. If all diffuser vanes will be provided very close to the impeller outlet tip, there will be an excessive variation of circumferential stress and this stress will be propagated radially upstream inward to the impeller and will cause the vibration. 

If vaneless space will not be provided, impeller blades of centrifugal compressor might be damaged due to this vibration which will be caused by radially upstream inward propagation of circumferential stress. 

This vibration will be a function of relative velocity of fluid flow and number of impeller vanes. 

So, we have seen here the basic need and function of diffuser in the operation of a centrifugal compressor. We have also discussed here the importance of vaneless space in diffusion action in the operation of a centrifugal compressor. 

Do you have any suggestions? Please write in comment box and also drop your email id in the given mail box which is given at right hand side of page for further and continuous update from www.hkdivedi.com

Further we will find out, in our next post, different types of losses in centrifugal compressor.  

Reference:  

Fluid Machines, By Prof S.K. Som 
Image courtesy: Google   

Also read 

October 11, 2019

MACH NUMBER FOR CENTRIFUGAL COMPRESSOR

We were discussing the basic principle of operations, functions of various parts and performance characteristics of different types of fluid machines such as hydraulic turbinescentrifugal pumps and reciprocating pumps in our previous posts. We have seen there that working fluid for above mentioned fluid machines was liquid such as water. 

Now it’s time to discuss few other types of fluid machines such as centrifugal compressors, axial flow compressors, fans and blowers where working fluid will be steam, air or gases. 

We have already seen the working principle of centrifugal compressorvelocity diagram of centrifugal compressor and slip phenomenon and slip factor for centrifugal compressor in our last posts, where we were also discussing the various important parts of centrifugal compressor and their functions and we have also secured there the work done on air in centrifugal compressor. 

Today we will be interested here to discuss a very important criteria i.e. Mach number for centrifugal compressor with the help of this post. 

Mach number for centrifugal compressor 

Mach number is basically a dimensionless number which is used to specify the flow in all types of machines that handles the compressible fluid. 

Mach number is basically defined as the ratio of flow velocity to the velocity of sound related to the fluid flow at the state of the fluid. It is indicated by Ma. 

Mach number is very much dependent over the state of fluid. State of fluid indicates the density and temperature of the fluid.   
Mach number = Flow velocity /velocity of sound

Ma = V/a 
Where,
Ma = Mach number

V = Flow velocity

a= Velocity of sound 

Now we will classify the types of flow on the basis of Mach number 

When flow velocity is equal to the velocity of sound, Mach number will be one and such flow will be termed as sonic flow. 

When flow velocity is less than the velocity of sound, Mach number will be less than one and such flow will be subsonic flow. 

When flow velocity is more than the velocity of sound, Mach number will be greater than one and such flow will be supersonic flow.  

Things to remember – 

Sonic flow, V = a or Ma = 1 
Subsonic flow, V < a or Ma < 1 
Supersonic flow, V > a or Ma > 1 
Incompressible flow, Ma < 0.3 

We have mentioned above incompressible flow. Incompressible fluid flow indicates that density will not be changing with respect to pressure. In case, if density is changing with respect to pressure then it will be negligible and could be neglected and considered that density of fluid is constant with respect to pressure.  

If flow is supersonic i.e. Mach number is greater than one, there will be some losses in the static pressure of the centrifugal compressor and these losses will not be desirable and therefore a compressor designer has to be very careful to avoid the unnecessary increment in Mach number.  

Important Notes:  

In above equation of mach number i.e. Ma = V/a, V is the velocity of flow. Velocity of flow here is the velocity related to the solid surface. In a turbo machine, solid surface is also moving and therefore relative velocity of the fluid will be considered to determine the Mach number in turbo machine i.e. centrifugal compressor. 

Mach number = Relative velocity of fluid flow /velocity of sound 

Ma = Vr/a  

Mach number must be less than 1 to secure the subsonic flow in order to avoid the losses in the static pressure of the centrifugal compressor. A centrifugal compressor designer will determine the Mach number for each and every section of the compressor.  

Fluid flowing through centrifugal compressor will gain the kinetic energy from the impeller and therefore velocity of fluid flow will be high at the outlet of the impeller. Hence, Mach number will be maximum at the outlet of the impeller tip. 

In order to restrict the Mach number at the outlet of impeller tip where it will be maximum, we need to check the mach number at the inlet of the impeller tip. Therefore inlet design of centrifugal compressor will be carried out in such a way that mach number at the inlet of the impeller tip will be relatively low.   

Do you have any suggestions? Please write in comment box and also drop your email id in the given mail box which is given at right hand side of page for further and continuous update from www.hkdivedi.com.    

Further we will find out, in our next post, function of diffuser in centrifugal compressor.   

Reference:  

Fluid mechanics, By Prof S.K. Som 
Image courtesy: Google  

Also read  

Thursday, 10 October 2019

October 10, 2019

DIFFERENT TYPES OF THERMODYNAMIC CYCLES

In our recent posts, we were focused on different types of thermodynamic cycles. We have seen various posts based on the different types of thermodynamic cycles and these posts are as mentioned below. 

Here we have written each post based on the different types of thermodynamic cycles for easy access and to secure complete information about the different types of thermodynamic cycles. 

Posts based on different types of thermodynamic cycles

  1. Reversed Carnot heat engine cycle
  2. Classification of thermodynamic cycles
  3. Difference between Rankine and Carnot cycle
  4. Rankine cycle
  5. Carnot cycle
  6. Otto cycle
  7. Diesel cycle
  8. Actual Rankine cycle versus ideal Rankine cycle
  9. Basic concept of an ideal regenerative Rankine cycle
  10. Regenerative Rankine cycle with feedwater heater
  11. Rankine cycle with reheat and regeneration
  12. Otto cycle: the ideal cycle for internal combustion spark ignition engine
  13. Diesel cycle the ideal cycle for compression-ignition engines
  14. Working principle of open cycle gas turbine
  15. Working principle of closed cycle gas turbine
  16. Brayton cycle the ideal cycle for gas-turbine engines
  17. Dual cycle, limited pressure cycle or mixed cycle
  18. Effect of regeneration on Brayton cycle 
  19. Effect of inter-cooling on Brayton cycle
  20. Brayton cycle
  21. Brayton cycle with reheating
  22. Stirling cycle PV and TS diagram
  23. Ericsson cycle PV and TS diagram 


Do you have any suggestions? Please write in comment box and also drop your email id in the given mail box which is given at right hand side of page for further and continuous update from www.hkdivedi.com

Further we will find out, in our next post, main components of reciprocating pump.  

Reference: 

Fluid Machines, By Prof. S. K. Som
Image courtesy: Google 

Also read