TITLE.PM5

750 ENGINEERING THERMODYNAMICS

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\M-therm\Th14-3.pm5

(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. Suitability Suitable for handling large Suitable for low discharges
   volumes of air at low pressures of air at high pressure.


2. Operational speeds Usually high Low


3. Air supply Continuous Pulsating


4. Balancing Less vibrations Cyclic vibrations occur


5. Lubrication system Generally simple lubrication Generally complicated
   systems are required.


6. Quality of air delivered Air delivered is relatively Generally contaminated
   more clean with oil.


7. Air compressor size Small for given discharge Large for same discharge


8. Free air handled 2000-3000 m^3 /min 250-300 m^3 /min


9. Delivery pressure Normally below 10 bar 500 to 800 bar


10. Usual standard of compression Isentropic compression Isothermal compression


11. Action of compressor Dynamic action Positive displacement.

(iii) Using property table of R-12 :

h 2 = 344.927 kJ/kg

h 4 = h 1 = 228.538 kJ/kg

(cp)v = 0.611 kJ/kg°C

s 2 = s 3

or 1.56323 = 1.5434 + 0.611 loge
t 3 273
30 273

+

+

L

N

M

O

Q

P

or t 3 = 39.995°C
h 3 = 363.575 + 0.611(39.995 – 30)
= 369.68 kJ/kg.
Rn/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. = R

W

n=116.389

24.753

= 4.702. (Ans.)

Assuming cp for ice = 2.0935 kJ/kg°C

Heat to be removed to produce ice

=

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) =
11.3409
4.702
= 2.4 kW. (Ans.)
Example 14.17. A R-12 refrigerator works between the temperature limits of – 10°C and

• 30°C. The compressor employed is of 20 cm × 15 cm, twin cylinder, single-acting compressor
  having a volumetric efficiency of 85%. The compressor runs at 500 r.p.m. The refrigerant is

Fig. 14.27

p

h

2

4 30°C 3

(^1) Evap.
Cont.
Th.

• 15°C Comp.