Question

Refrigerant-134a is to be cooled by water in a condenser. The refrigerant enters the condenser with a mass flow rate of 6 kg/min at 1 MPa and 70ºC and leaves at 35°C. The cooling water enters at 300 kPa and 15°C and leaves at 25ºC. Neglecting any pressure drops, determine (a) the mass flow rate of the cooling water required and (b) the heat transfer rate from the refrigerant to water

Answer 1

Part A

Taking the compressed water as a saturated liquid:

Internal energy at $15°C,\>\>h_{f1}=62.982kJkg^{-1}$

Internal energy of water at 25° C,

$h_{f2}$ $=104.83kJ/kg$

The refrigerant enters the condenser as superheated vapor at 70°C and leave as compressed liquid at 35°C.

Internal energy of superheated vapor R -134a at 70°C

$h_{f3}$ $=303.87kJ/kg$

Internal energy of compressed liquid R-134a at 35°C

$h_{f4}=100.88kJ/kg$

Taking $m_w=$ Flow rate of water

$m_w(104.83-62.982)=6(308.87-100.88)$

$m_w=29.1kg/min$

Part B

Change in energy of water $=(104.83-62.982)kJ/kg$

Rate of flow of water $=29.1kg/min$

Heat transfer rate $=29.1×(104.83-62.982)$

$=1217.8kJ/min$