Comparison of Control Mass & Control Volume System | Closed System Vs Open System

Are you looking for a comparison or technical/conceptual difference between control mass system and control volume system? then you are at the right article. This can be also interpreted as the difference between an open system and closed system in classical thermodynamics.

As we all know a system in classical thermodynamics can be classified into a control mass system, control volume system and isolated system. The comparison between control mass system and control volume system are as follows.

Control Mass System (Closed System)

• There should not be any mass interaction. However, there can be energy interactions. Control mass system interacts with the surroundings in the form of work and heat.

• Mass inside the system is constant as well as its identity is fixed. For example, from a system containing water of 100 kg, 10 kg of water is taken out and is replaced with 10 kg of oil. Here even though initial and final mass is the same (constant), that is 100 kg but the identity of mass is changed, it is like 90kg of water and 10 kg of oil instead of 100 kg water. So this cannot be treated as a control mass system.

• In control mass system, the volume of the system may change.

• Example of a closed system or control mass system is piston-cylinder arrangement without a valve.

Control Volume System (Open System)

• In this type of system, there are both mass and energy interactions between system and surroundings. Control volume system interacts with the surroundings in the form of work, heat as well as mass.

• Here identity of mass may not be fixed.

• In an open system, the volume of the system is fixed and is known as control volume.

• The boundary of the control volume is known as the control surface.

Interesting concept: In steady state, control volume system will look similar to control mass system but will differ in terms of identity.  That is, in steady state, mass into the control volume = mass out of control volume and for control mass system, mass is always constant. But in both cases, the identity of mass is different.  

Comparison between Rankine & Reverse Rankine Cycle

This page will give you the basic differences and thermodynamical comparison between Rankine cycle and reverse Rankine cycle. The comparison of the two is easy as one is the reverse of the other. Both are identical cycles but their process diagrams are in opposite direction. The basic idea of Temperature-Entropy (TS) diagram of Rankine and reverse Rankine cycle is a prerequisite for reading this article. The following are the basic comparison between both the cycles.

Rankine Cycle

• It is a work producing cycle and is the basic vapor standard thermodynamic cycle for the steam power plant.

• This concept is centered around Kelvin-Plank statement of the 2nd law of Thermodynamics. This statement gives rise to the idea of work producing devices.

• Steam is the working fluid in this cycle.

• The direction of the various thermodynamic processes in Rankine cycle is in the clockwise direction.

• In Rankine cycle, expansion process in the turbine is considered as isentropic.

• In Rankine cycle, we are interested in finding out the cycle efficiency and net work output and it is desirable to increase both.

• An increase in boiler pressure in Rankine cycle results in the reduction of heat rejection or increment in mean temperature of heat addition which ultimately results in an increase in efficiency. However, dryness fraction decreases and requires a higher capacity feed water pump.

• A decrease in condenser pressure reduces the heat rejection and increases the turbine work output and thus increasing the efficiency. But here also dryness fraction decreases.

• Superheating in Rankine cycle improves the efficiency as well as the dryness fraction of steam.

• Regeneration in Rankine cycle also improves the efficiency.

Reverse Rankine Cycle or Vapour Compression Refrigeration

• It is a work consuming cycle and is the basic vapor standard thermodynamic cycle for the vapour compression refrigeration system.

• This concept is centered around Clausius statement of the 2nd law of Thermodynamics. This statement gives rise to the idea of work consuming devices.

• Working fluid is usually referred to as a refrigerant.

• The direction of the various thermodynamic processes in reverse Rankine cycle is in the counterclockwise direction.

• In reverse Rankine cycle, expansion process in the throttling device is considered as isenthalpic.

• In reverse Rankine cycle, we are interested in finding out the coefficient of performance (COP) and it is desirable to increase it.

• COP decreases with increase in condenser pressure and a decrease in evaporator pressure.

• The variation of COP is unpredicted in the case of superheating. This is because of the increment in refrigeration effect with the same work consumed by the compressor.

• Subcooling increases COP of reverse Rankine cycle.