Question

Figure P4.95 provides steady-state data for a throttling valve in series with a heat exchanger. Saturated liquid Refrigerant134a enters the valve atT₁=36 degrees C with a mass flow rate of 0.26 kg/s and is throttled to T₂ = -8degrees C. The refrigerant then enters the heat exchanger, exiting as saturated vapor with no significant decrease in pressure. In a separate stream, liquid water enters the heat exchanger at T₄ = 20 degrees C and exits as a liquid atT₅= 10 degrees C. Stray heat transfer and kinetic and potential energy effects can be ignored. Determine (a) the pressure at state 2, in kPa, and (b)the mass flow rate of the liquid water stream, in kg/s.

Answer 1

Answer 2

Draw the diagram showing the throttling valve and heat exchanger:

Obtain properties of R-134a at from table A-10, “Properties of saturated refrigerant 134a” in the text book.

Specific enthalpy,

In throttling expansion, enthalpy remains constant.

Note the properties of R-134a at state2.

Obtain properties of R-134a at from table A-10, “Properties of saturated R-134a tables” in the text book.

Saturation pressure,

Specific enthalpy of saturated liquid,

Specific enthalpy of saturated vapor,

Enthalpy at state 2 is in between saturated liquid and saturated vapor.

Since the state 2 is in wet condition, the pressure at state 2 is equal to saturation pressure at .

Therefore, the pressure at state 2 is .

(b)

Obtain specific enthalpy at state 3 at from table A-10, “properties of saturated R-134a tables” in the text book.

Obtain enthalpy of liquid water at temperature from table A-2, “Properties of saturated water” in the text book.

Similarly, Obtain enthalpy of liquid water at temperature from table A-2, “Properties of saturated water” in the text book.

Apply energy balance to the heat exchanger.

Here, is mass flow rate of R-134a and is mass flow rate of liquid water.

Substitute 0.26 kg/s for , for , for , for , and for .

Therefore, mass flow rate of liquid water is.