When a single refrigerant condenses and evaporates at a constant pressure betwee

Question

When a single refrigerant condenses and evaporates at a constant pressure between saturated liquid and saturated vapor, the temperature remains constant. A binary solution, on the other hand, experiences a change in temperature. When a refrigeration system cools a fluid stream through a large temperature range or the system rejects heat to a fluid that increases in temperature as it passes through the condenser, it may be possible to conserve compression power by using a binary solution. Some applications of this concept have been made in cryogenic and petrochemical industries.

The refrigeration cycle adapatable to refrigerant mixtures employs the usual components (compressor, condenser, evaporator, and expansion device) and a heat exchanger as shown in the figure below. A mixture of R-12 and R-114 will be explored.

When compressing a mixture, the work of compression is found by proportioning according to the mass fraction of the constituents in the mixture. The mixture is assumed to be ideal, and Dalton's and Raoult's laws apply. The molecular weights of R-12 and R-114 are 120.93 and 170.94, respectively.

ASSIGNMENT

For this specified refrigeration duty (approximately 20 kW), determin the composition of the mixture that results in minimum power requirements at the compressor.


p.s.
I believe that the answer is somewhere in the "Chegg Guided Solutions for Design of Fluid Thermal Systems 3rd Edition", but I don't have that book, so I can't figure out which answer is for this question

Explanation / Answer

PROJECT TITLE.
Energy conservation by using a refrigerant mixture

Description
When a single refrigerant condenses and evaporates at a constant pressure between saturated liquid and saturated vapor, the temperature remains constant. For specified refrigeration duty the study is directed towards determining the composition of the mixture that results in minimum power requirements at the compressor.
THE REFRIGERATION SYSTEM








When a single refrigerant condenses and evaporates at a constant pressure between saturated liquid and saturated vapor, the temperature remains constant. A binary solution, on the other hand, experiences a change in temperature. When a refrigeration system cools a fluid stream through a large temperature range or the system rejects heat to a fluid that increases in temperature as it passes through the condenser, it may be possible to conserve compression power by using a binary solution. Some applications of this concept have been made in cryogenic and petrochemical industries.
The refrigeration cycle adapatable to refrigerant mixtures employs the usual components (compressor, condenser, evaporator, and expansion device) and a heat exchanger as shown in the figure below. A mixture of R-12 and R-114 will be explored.



Data
:
Design
UA value of the evaporator 5 kW/K
UA value of the condenser 5 kW/K

UA value of the heat exchanger 0.3 kW/K
Condenser cooling water (entering) 25

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