Exergy Analysis of a Subcritical Refrigeration Cycle with an Improved Impulse Turbo Expander
">
<p>Schematic and P-h diagram of the economizer vapor injection cycle.</p> ">
<p>Configuration of the conventional ITE.</p> ">
<p>Schematic and P-h diagram of the refrigeration cycle with the conventional ITE.</p> ">
<p>Configuration of the improved ITE.</p> ">
<p>Schematic and P-h diagram of the refrigeration cycle with the improved ITE.</p> ">
<p>Friction losses of the conventional ITE and the improved ITE.</p> ">
<p>COP of investigated cycles at different values of PR.</p> ">
<p>The effect of the ITE efficiency on the value of PR<sub>opt</sub> for the improved ITE cycle.</p> ">
<p>PR<sub>opt</sub> <span class="html-italic">versus</span> evaporator temperature under different condenser temperatures for the improved ITE cycle.</p> ">
Abstract
:1. Introduction
2. The Improved Impulse Turbo Expander and the Corresponding Cycle
- (1)
- A liquid accumulator is provided in the lower part of the housing to collect the liquid working fluid which is fed to the evaporator through outlet 1 arranged at the bottom of the accumulator. Thus if the volume of the accumulator is appropriately determined, the liquid working fluid can remain at a constant level in the accumulator so as not to get in touch with the disk of the expander.
- (2)
- Another outlet (outlet 2), through which the saturated flash vapor is discharged, is provided at the upper part of the housing and communicates with the room inside the housing.
3. Thermodynamic Modeling
- (1)
- There are no pressure losses in pipes and heat exchangers.
- (2)
- The small difference among the intermediate pressure, the discharge pressure of the low-stage compressor and the suction pressure of the high-stage compressor is negligible.
- (3)
- The vapor stream and the liquid stream exiting the ITE are assumed to be saturated.
- (4)
- The exiting working fluid of the evaporator is saturated vapor.
- (5)
- The ITE and the compressor are treated adiabatically.
- (6)
- The numerical simulation modeling of the cycle is based on one unit of the working fluid at the inlet of the ITE.
- (7)
- (8)
- The vapor and liquid separation efficiencies in the turbine and the economizer are negligible.
3.1. Energy Analysis
3.2. Exergy Analysis
4. Results and Discussion
5. Conclusions
- An increase of 20% in the isentropic efficiency can be attained for the improved ITE compared with the conventional ITE owing to the reduction of the friction loss of the rotor.
- Unlike the typical two-stage compression refrigeration cycles, the optimum intermediate pressure for the improved ITE cycle shows a deviation from the square root of the condenser pressure times the evaporator pressure. This deviation mainly depends on the ITE efficiency, and a correlation of the optimum intermediate pressure for the improved ITE cycle is developed.
- As the ITE efficiency increases, the corresponding COPs of the conventional ITE cycle and the improved ITE cycle increase. The improved ITE cycle outperforms the conventional one.
- The improved ITE cycle improves the COP and the exergy efficiency by 1.4%–6.1% over the conventional ITE cycle, 4.6%–8.3% over the economizer cycle and 7.2%–21.6% over the base cycle.
- The total exergy loss in the improved ITE cycle is lower than that in the other three cycles. This reduction is mainly due to the decrease in the expansion process.
Acknowledgments
Nomenclature | |
---|---|
COP | coefficient of performance in cooling condition |
Ex | exergy (kJ/kg) |
h | enthalpy (kJ/kg) |
H | height (m) |
I | specific irreversibility (kJ/kg) |
m | mass flow rate (kg/s) |
N | friction power loss (kW) |
p | pressure (MPa) |
PR | pressure ratio |
q | specific heat transfer rate (kJ/kg) |
Q0 | refrigeration capacity (kW) |
R | radius (m) |
s | specific entropy (kJ/kg K) |
t | temperature (°C) |
T | temperature (K) |
w | specific power (kJ/kg) |
x | vapor quality |
ω | angular velocity (rad/s) |
ρ | density (kg/m3) |
η | efficiency |
Subscripts | |
---|---|
0 | reference environment |
c | compressor |
con | condenser |
e | evaporator |
gm | geometric mean |
int | intermediate |
opt | optimal |
r | refrigerated object |
rot | rotor |
s | isentropic process |
t | turbo |
tot | total |
v | throttle valve |
Author Contributions
Conflicts of Interest
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Zhang, Z.; Tian, L. Exergy Analysis of a Subcritical Refrigeration Cycle with an Improved Impulse Turbo Expander. Entropy 2014, 16, 4392-4407. https://doi.org/10.3390/e16084392
Zhang Z, Tian L. Exergy Analysis of a Subcritical Refrigeration Cycle with an Improved Impulse Turbo Expander. Entropy. 2014; 16(8):4392-4407. https://doi.org/10.3390/e16084392
Chicago/Turabian StyleZhang, Zhenying, and Lili Tian. 2014. "Exergy Analysis of a Subcritical Refrigeration Cycle with an Improved Impulse Turbo Expander" Entropy 16, no. 8: 4392-4407. https://doi.org/10.3390/e16084392