(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

0.8kg of saturated liquid R-134a with an initial temperature of -5 °C is contained in a well-insulated, weighted piston-cylinder device. This device contains an electrical resistor to which 10 volts are applied causing a current of 2 amperes to flow through the resistor. Determine the time required for the refrigerant to be converted to a saturated vapor, and the final temperature.

2. Relevant equations

Assumption: The system is at constant pressure throughout the process since both the atmospheric pressure and the weight of the piston remain constant throughout.

[tex]Q_{in}+W_{e,in}-W_{b}=\Delta U+\Delta KE+ \Delta PE[/tex]

3. The attempt at a solution

[tex]Q_{in}=\Delta KE= \Delta PE=0[/tex]

[tex]W_{e,in}=\Delta U+W_{b}=\Delta H[/tex]

[tex]VI\Delta t=m(h_{2}-h_{1})[/tex]

[tex]\Delta t=\frac{m(h_2-h_1)}{VI}[/tex]

State 1

P_{1}=P_{sat}@ -5 °C = 243.5kPa

Saturated Liquid

Thus, h_{1}=h_{f}@ 243.5kPa = 45.143 kJ/kg

State 2

P_{2}=P_{1}=243.5kPa

Saturated Vapor

Thus, h_{2}=h_{g}@ 243.5kPa = 247.49 kJ/kg

T_{2}= T_{sat}@ 243.5 kPA = -5 °C

Rearranging the energy balance equation fortyields 135 minutes for the time it takes to change the saturated liquid refrigerant into saturated vapor state.

I am having doubts about my calculation for T_{2}. It seems like the temperature would increase as that is the nature of a resistance heater. Any thoughts?

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# Homework Help: Electrical work in a piston-cylinder filled with refrigerant

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