Question 12.16: (i) What are the advantages of using an expansion valve inst...
(i) What are the advantages of using an expansion valve instead of an expander in a vapour compression refrigeration cycle ?
(ii) Give a comparison between centrifugal and reciprocating compressors.
(iii) An ice-making machine operates on ideal vapour compression refrigeration cycle using refrigerant R-12. The refrigerant enters the compressor as dry saturated vapour at – 15°C and leaves the condenser as saturated liquid at 30°C. Water enters the machine at 15°C and leaves as ice at – 5°C. For an ice production rate of 2400 kg in a day, determine the power required to run the unit. Find also the C.O.P. of the machine. Use refrigerant table only to solve the problem. Take the latent heat of fusion for water as 335 kJ/kg.
(i) If an expansion cylinder is used in a vapour compression system, the work recovered would be extremely small, in fact not even sufficient to overcome the mechanical friction. It will not be possible to gain any work. Further, the expansion cylinder is bulky. On the other hand the expansion valve is a very simple and handy device, much cheaper than the expansion cylinder. It does not need installation, lubrication or maintenance.
The expansion valve also controls the refrigerant flow rate according to the requirement, in addition to serving the function of reducting the pressure of the refrigerant.
(ii) The comparison between centrifugal and reciprocating compressors :
The comparison between centrifugal and reciprocating compressors is given in the table below:
| S. No. | Particulars | Centrifugal compressor | Reciprocating compressor |
| 1.
2. 6. 7. |
Suitability
Operational speeds Quality of air delivered Air compressor size |
Suitable for handling large volumes of air at low pressures Usually high Continuous Less vibrations Generally simple lubrication systems are required. Air delivered is relatively more clean Small for given discharge 2000–3000 m³/min Normally below 10 bar Isentropic compression Dynamic action |
Suitable for low discharges of air at high pressure. Low Pulsating Cyclic vibrations occur Generally complicatedGenerally contaminated with oil. Large for same discharge 250–300 m³/min 500 to 800 bar Isothermal compression Positive displacement. |
(iii) Using property table of R-12 :
h_{2} = 344.927 kJ/kg
h_{4} = h_{1} = 228.538 kJ/kg
(c_{p})_{v} = 0.611 kJ/kg°C
s_{2} = s_{3}
or 1.56323 = 1.5434 + 0.611 log_{e} [\frac{t_{3} + 273}{30 + 273} ]
or t_{3} = 39.995°C
h_{3} = 363.575 + 0.611(39.995 – 30)
= 369.68 kJ/kg.
R_{n} /kg = h_{2} – h_{1} = 344.927 – 228.538
= 116.389 kJ/kg
W/kg = h_{3} – h_{2} = 369.68 – 344.927 = 24.753
C.O.P. = \frac{R_{n}}{W} = \frac{116.389}{24.753} = 4.702.
Assuming c_{p} for ice = 2.0935 kJ/kg°C
Heat to be removed to produce ice
= \frac{2400}{24 × 3600} [4.187(15 – 0) + 335 + 2.0935(0 – (– 5))]
= 11.3409 kJ/s = Work required, kJ/s (kW) × C.O.P.
∴ Work required (Power) = \frac{11.3409}{4.702} = 2.4 kW.
