Question

(a) A water pump has an inlet conditions of 4.55 MPa and 20oC. The outlet of the pump is at a pressure of 18.00 MPa. What is the enthalpy change across a reversible pump in kJ/kg?

(b) For a real system, which of the following pumps might be considered for the job?

12.80 kJ/kg
13.48 kJ/kg
14.18 kJ/kg

(c) If a real pump for the states given in (a) has and efficiency of 0.78, what would be the enthalpy change of water through the pump?

Answer 1

A) Here we will make use of water property table to evaluate enthalpy and any other required data

At inlet conditions of 4.55 MPa and T = 20^oC

Hin =88.1911575850 KJ/Kg

Now energy transfer through pumps is generally an isothermal process and also for liquid there is hardly any increase in temperature due to pressurization

at outlet at P = 18 MPa and T =20 ^oC

Hout =100.7207 KJ/Kg

Enthalpy transfer = Hout - Hin = 100.7207 -88.1911 = 12.5296 KJ/Kg

b) From the option provided, the first option is closest to 12.5296 KJ/Kg,

Hence first option 12.80 KJ/Kg pump can be selected

c) Now efficiency of the pump is 0.78

so only 78 % of the enthalpy has been transferred through the pump

Hence enthalpy change =0.78*12.526 =9.77 KJ/Kg