One of the challenges of plotting an R-407C system on a pressure/enthalpy chart is that this refrigerant
A. is a single-compound refrigerant with one pressure for each temperature.
B. has a significant temperature glide.
C. is a binary blend of R-32 and R-125, making it unstable.
D. is easily mistaken for R-134a and R-410A.
The correct answer and explanation is :
The correct answer is B. has a significant temperature glide.
Explanation:
R-407C is a refrigerant blend that consists of three components: R-32 (difluoromethane), R-125 (pentafluoroethane), and R-134a (1,1,1,2-tetrafluoroethane). Unlike single-component refrigerants, which typically exhibit a constant temperature at a given pressure (a characteristic of pure refrigerants), R-407C is a zeotropic mixture, meaning that it has a significant temperature glide.
Temperature glide refers to the phenomenon where the temperature of the refrigerant mixture changes as it evaporates or condenses at a constant pressure. This occurs because the components of the mixture have different boiling points, leading to different rates of evaporation and condensation at the same pressure. As a result, when plotting R-407C on a pressure/enthalpy chart, the line representing the phase change (evaporation or condensation) is not vertical but has a slope, indicating that the temperature changes over the course of the phase transition.
Why the other options are incorrect:
- A. is a single-compound refrigerant with one pressure for each temperature: This is incorrect because R-407C is not a single-compound refrigerant. It is a blend of multiple components with varying boiling points, leading to a temperature glide.
- C. is a binary blend of R-32 and R-125, making it unstable: While R-407C is indeed a blend of R-32 and R-125, it is not a binary blend; it also contains R-134a. Moreover, R-407C is not inherently unstable. The instability of a refrigerant typically refers to chemical instability under certain conditions, which is not a characteristic of R-407C.
- D. is easily mistaken for R-134a and R-410A: While R-407C may share some similarities with R-134a and R-410A in terms of performance and applications, it is a distinct blend and is not typically mistaken for these refrigerants when proper identification is made.
Thus, the key challenge when plotting R-407C on a pressure/enthalpy chart is its significant temperature glide, which makes it more complex to represent compared to single-component refrigerants.