Question

Seven lbm of ammonia in a piston-cylinder assembly, initially at p1 = 60 lbf/in2 and T1 = 120 (one hundred twenty) oF, undergoes an isobaric process to a final state. The work done on a system is 90 Btu. At the final state, determine the temperature (in oF) and the quality of ammonia.

Answer 1

From given data

i) Mass of ammonia is

ii) Initial state properties are and

iii) Work done on the system is

From property tables of ammonia, for and

i) specific enthalpy of ammonia is

ii) Ammonia is in superheated vapour state.

Let us consider the final specific enthalpy of ammonia is .

Then, the workdone is given by

  

substitute all the known values,

Thus, the specific enthalpy of ammonia at final state is

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Since the process is constant pressure process,

For , saturated specific enthalpy of ammonia vapour is .

Here we get the specific enthalpy at final state .

Thus, the final state of ammonia is also in superheated vapour region only.

Hence there won't exist dryness fraction (or quality) of ammonia at final state.

Now, for a constant pressure process, Change in enthalpy is given by

   ...... (1)

here is specific heat capacity of ammonia at constant pressure.

is temperature of ammonia at final state.

For , specific heat capacity of ammonia at constant pressure is

Substitute all the known values in (1),

  

Thus, the final state temperature of ammonia is