Question 2.15: A refrigeration process includes a compressor, as explained ...

A) \Delta \widehat{\mathrm{H}}=\widehat{\mathrm{H}}_{\mathrm{out}}-\widehat{\mathrm{H}}_{\mathrm{in}}

Freon 22, saturated vapor and 0.5 bar \rightarrow 383 \frac{\mathrm{kJ}}{\mathrm{kg}}

Freon 22, 2 bar and 20^{\circ} \mathrm{C} \rightarrow 425 \frac{\mathrm{kJ}}{\mathrm{kg}}

425 \frac{\mathrm{kJ}}{\mathrm{kg}}-383 \frac{\mathrm{kJ}}{\mathrm{kg}}=\bf42 \frac{\mathbf{kJ}}{\mathbf{kg}}

B) Freon 22, saturated vapor and 2 bar \rightarrow 396 \frac{\mathrm{kJ}}{\mathrm{kg}}

Freon 22, 8 bar and 60^{\circ} \mathrm{C} \rightarrow 445 \frac{\mathrm{kJ}}{\mathrm{kg}}

445 \frac{\mathrm{kJ}}{\mathrm{kg}}-396 \frac{\mathrm{kJ}}{\mathrm{kg}}=\bf 49 \frac{\mathbf{k J}}{\mathbf{kg}}

C) Freon 22, saturated vapor and 5 bar \rightarrow 405 \frac{\mathrm{kJ}}{\mathrm{kg}}

Freon 22, 20 bar and 80^{\circ} \mathrm{C} \rightarrow 445 \frac{\mathrm{kJ}}{\mathrm{kg}}

445 \frac{\mathrm{kJ}}{\mathrm{kg}}-405 \frac{\mathrm{kJ}}{\mathrm{kg}}=40 \frac{\mathbf{k J}}{\mathrm{kg}}

D) The consistency in compression ratio (always 4) and consistency in \Delta \widehat{\mathrm{H}} (between 40 \frac{\mathrm{kJ}}{\mathrm{kg}} and \left.50 \frac{\mathrm{kJ}}{\mathrm{kg}}\right) leads us to believe that the compression ratio \left(P_{\text {out }} / P_{i n}\right) is a major factor influencing \Delta \widehat{\mathrm{H}} (for Freon 22 vapors). Indeed, in real compressors, the compression ratio is far more important than the absolute values of pressure in determining required work.

But you might wonder, why does the temperature change in a compressor, and are the particular combinations of inlet and outlet temperatures and pressures used in this problem realistic? These issues will be explored in Chapter 4, when we begin to discuss reversible processes and efficiencies.