al zami Posted June 10, 2013 Share Posted June 10, 2013 in case of turbine it is said the work is done by the system and for compressor it is said that the work is done on the system.in case of turbine steam moves the blades and in case of compressor air moves the piston.then where is the difference in regard to work done or work by??? Link to comment Share on other sites More sharing options...
swansont Posted June 10, 2013 Share Posted June 10, 2013 It's a matter of defining your system and "keeping the books". Energy is conserved. If your system transfers energy out, then we say work is done by the system. If energy comes in, work is done on it. Link to comment Share on other sites More sharing options...
al zami Posted June 10, 2013 Author Share Posted June 10, 2013 how in case of compressor "work is done on it"//??? Link to comment Share on other sites More sharing options...
swansont Posted June 10, 2013 Share Posted June 10, 2013 how in case of compressor "work is done on it"//??? For the compressor I assume that's the gas itself. Work is done on it, because when you compress the gas you are adding energy to it. Link to comment Share on other sites More sharing options...
studiot Posted June 10, 2013 Share Posted June 10, 2013 how in case of compressor "work is done on it"//??? Well it's fairly simple. Every compressor has an input shaft that must be turned by an engine, which does work turning the shaft. This is true of a piston or fan bladed compressor. Conversely a turbine has an output shaft which can drive another machine ie do work when gas or liquid passes through the machine. So from the point of view of the machine, work is input to a compressor and output from a turbine. From the point of view of the working fluid, A compressor outputs work to the fluid and a turbine extracts work from the fluid. As Swansont said, you need to define your system and your book regards the machine as the system, not the working fluid. Does this help? Link to comment Share on other sites More sharing options...
daniton Posted June 10, 2013 Share Posted June 10, 2013 (edited) For the compressor I assume that's the gas itself. Work is done on it, because when you compress the gas you are adding energy to it. i believe to define work it is better to consider the change in energy rather than adding or losing energy(not the energy by itself).so if you are compressing you are doing 'work on the gas' the gas since the change occurs on you and if the gas is expanding 'work is done by' the gas since the gas is paying for what it did. Edited June 10, 2013 by daniton Link to comment Share on other sites More sharing options...
swansont Posted June 10, 2013 Share Posted June 10, 2013 i believe to define work it is better to consider the change in energy rather than adding or losing energy(not the energy by itself).so if you are compressing you are doing 'work on the gas' the gas since the change occurs on you and if the gas is expanding 'work is done by' the gas since the gas is paying for what it did. Yes, as I said earlier, it all depends on how you define the system and the sign of the work. Book-keeping from conservtion of energy. Link to comment Share on other sites More sharing options...
studiot Posted June 10, 2013 Share Posted June 10, 2013 Note it is not a good idea to only consider the work done on the gas since no machine is 100% efficient. Some of the work you have to input to the compressor in order to achieve compressed fluid is lost in the compressor as friction etc, some may be employed in changing the temperature. Link to comment Share on other sites More sharing options...
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