A long horizontal copper pipe with a 1 cm inside diameter has R-134A entering as

Question

A long horizontal copper pipe with a 1 cm inside diameter has R-134A entering as a saturated vapor at 8°C. It exits the pipe at 12.5 bar and 30C. It has a mass flow rate of 0.05 kg/s 4. Calculate the velocity at the inlet and outlet of the pipe. Answers: about 10 m/s and 0.5 m/s. a. b. Calculate the heat transfer from the pipe. Hint: Analyze this as an open system. Answer: about 9 kW Note: this is a realistic description of what is going on in the outdoor part of a home air-conditioner called the "condenser" because inside the coil, the refrigerant is condensing from vapor to liquid).

Explanation / Answer

(a) we know that mass flow rate = 0.05 kg/s

mass flow rate = pAV
where p is den sity , A is area of cross section and V is the velocity
pi = 0.3184 kg/m3 at 48oC (from table)
A = Pi*(0.5*10-2) = 0.0157 m2
At inlet
0.05 = 0.3184*0.0157*Vi
Vi = 10 m/s
At outlet
po = 6.369 kg/m3 at 12.5 bar and 30oC (from table)
0.05 = 6.369*0.0157*Vo
Vo = 0.5 m/s
(b) We know that in open system
h1 + Q = h2
where h1 and h2 is enthalpy at inlet and outlet
from tablle h1 = 372.44 kJ/kg
h2 = 241.7 kJ/kg
Q = h2 - h1 = -130.74 kJ/kg
-ve sign means heat is lost by system
Power = m*Q = 0.05*130.74 = 6.537 kW
It seems quite different then your answer it is because of the values which I have taken from the table.
Please refer the table which is given with the question or in your book.
According to those values you will get your answer.

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