# Is the mass flow rate constant in a thermodynamic cycle?

Is the mass flow rate constant in a thermodynamic cycle? More specifically a Rankine or Carnot cycle.

For example if the mass flow rate of the turbine is 100 kg/s, is the mass flow rate for the boiler / condenser / compressor or pump also 100 kg/s?

Is the mass flow rate constant in a thermodynamic cycle?

For general:

Depends.

Here is how;

If one of your components effect the pumping/boiling etc. of course your mass flow rate will change, it may change periodicly or it may not. Also there can be leakage or something that interacts with your fluid, of course that will cause to decrease in your mass flow rate with time.

But mostly, in general thermodynamic cycles, you don't need to worry. Because they build with simple components (boiler/pump/turbine/condenser) and they are not a factor that mass flow rate depends.

If we be specific about Rankine and Carnot cycles; your assumptions is actually TRUE!

The working fluid in a Rankine cycle follows a closed loop and is reused constantly. (Wikipedia)

That "reused constantly" means the total mass of the fluid is constant in cycle, therefore you have to use the same fluid in the same system. Unless there is a malfunction at one of your parts, you can be sure that you mass flow rate is constant.

The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. It provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference (e.g. refrigeration) by the application of work to the system. It is not an actual thermodynamic cycle but is a theoretical construct. (Wikipedia)

So we know it is theoretical. But again, if we speaking about "a simple closed system (control mass analysis)", yes, the Carnot Cycle will be running with constant mass flow rate because of the same reason of Rankine Cycle.

Don't forget to read Mark's comment about the real life application issues that happens while trying to hold mass flow rate constant.

• This is approximately correct, but in real-world applications there can be differences. Examples include boiler blow-down, which is a constant drain from the boiler; turbine extractions which draw steam out of the turbine somewhere in the middle and may be used for things like soot blowing, air heating, feed water heating, vacuum pumps, etc. There can also be reheating in which steam is extracted from the turbine, sent through a reheater in the boiler, and then returned to the turbine.
– Mark
Aug 24, 2016 at 14:38
• I am trying to say what you say while "Unless there is a malfunction at one of your parts", but I forgot to mention about examples or real life application issues, so I think it is better with your comment, Thanks!
– kfb
Aug 25, 2016 at 5:31
• Thanks for the extensive answer and example, clear up alot! Aug 25, 2016 at 12:41