Question

Refrigerant 134a enters a tube at a rate of 0.07 kg/s as saturated liquid at 70 C and leaves the tube as saturated liquid as well at -8 C. It loses 10 kW of heat to the surroundings in the process. If the surroundings are at a temperature 10 C, find the total entropy generation in this process. Assume steady conditions

Answer #1

a tube-within-a-tube heat
exchanger operating at steady state is composed of one pipe
containing
Refrigerant 134a and another pipe containing an ideal gas with
constant specific heat at constant
pressure of 1.2 kJ/(kg∙K). The refrigerant 134a enters the heat
exchanger in a saturated liquid state
and exits the heat exchanger in a saturated vapor state. The
temperature and mass flow rate of the
refrigerant 134a are -20° C and 3 kgs/s, respectively, at both its
inlet and outlet. The ideal...

A vapor-compression refrigeration cycle operates at steady state
with Refrigerant 134a as the working fluid. Saturated vapor enters
the compressor at 2 bar, and saturated liquid exits the condenser
at 8 bar. The isentropic compressor efficiency is 80%. The mass
flow rate of refrigerant is 7 kg/min. Determine: (a) the compressor
power, in kW, (b) the refrigeration capacity, in tons, (1 ton =
3.5168 kW) and, (c) the coefficient of performance, (d) rate of
entropy production in kW/K, for the...

Consider an air-cooled condenser in which a stream of R-134a
enters at 12 bar and 60degC, and leaves as a saturated liquid at 12
bar, and air enters as a separate stream at 1 bar, 35degC and
leaves at 1 bar, 45degC. The two streams do not mix but heat
transfer from one stream to the other stream occurs at the rate of
6.94 kW.
Determine
(a) the mass flow rates for the R-134a and the air (kg/s),
and
(b)...

A commercial refrigerator with refrigerant-134a as the working
fluid is used to keep the refrigerated space at −30°C by rejecting
its waste heat to cooling water that enters the condenser at 18°C
at a rate of 0.32 kg/s and leaves at 26°C. The refrigerant enters
the condenser at 1.2 MPa and 65°C and leaves at 42°C. The inlet
state of the compressor is 60 kPa and −34°C and the compressor is
estimated to gain a net heat of 460 W...

Refrigerant-134a enters a diffuser steadily as saturated vapour at
600 kPa with a velocity of 160 m/s, and it leaves at 700 kPa and
40°C. The refrigerant is gaining heat at a rate of 2 kJ/s as it
passes through the diffuser : determine
(a- the exit velocity
(b- the mass flow rate of the refrigerant.
If the exit area is twice the inlet area (A2=2A1),

Water is used to cool R-134a in the condenser of a heat
exchanger. The refrigerant enters the counter-flow heat exchanger
at 800 kPa, 80 0C and a mass flow rate of 2 kg/s. The refrigerant
exits as a saturated liquid. Cooling water enters the condenser at
500 kPa and 18 0C and leaves the condenser at 30 0C. Determine the
necessary mass flow rate of water. Each fluid is assumed to flow at
constant pressure.

An ice-making machine operates on the ideal vapor-compression
cycle, using R-134a. The refrigerant enters the compressor as
saturated vapor at 140 kPa and leaves the condenser as saturated
liquid at 600 kPa. Water enters the ice machine at 13oC and leaves
as ice at -4oC, while removing heat at 393 kJ per kg of water.
Estimate the mass flow rate of the refrigerant and the power input
to the ice machine for an ice production rate of 7 kg/h.

A saturated vapor of refrigerant 134a at 1000 kPa enters a
nozzle with a negligible velocity. It leaves the nozzle at 500 kPa.
What is the maximum velocity it could have, in m/s?
Hint: Remember that a nozzle does an energy
conversion. The maximum velocity occurs when this is being done
with the highest possible efficiency. What did you learn about
entropy when efficiency is at a maximum?

A piston cylinder device contains 5 kg of Refrigerant 134a at
800 kPa and 70 C. The refrigerant is now cooled at constant
pressure until it reaches a saturated vapor state. How much heat
was lost in the process? Express your result in kJ and you may
ignore the negative sign.

Refrigerant 134a enters a well-insulated nozzle at 14 bar, 60°C,
with a velocity of 37 m/s and exits at 1.2 bar with a velocity of
460 m/s. For steady-state operation, and neglecting potential
energy effects, determine the exit temperature, in °C.

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