Design And Fabrication Of Domestic Deep Freezer

4.1.1     COMPRESSOR EFFICIENCY
Compressor efficiency is measured in terms of its volume in relation to the volume of gas it pumps in a single cycle [Olarewaju, LW. (1995)].
Volumetric Efficiency =Volume of gas pumped per stroke
Volume of compressor cylinder…….(1)[Olarewaju L.
W.(1995)]
Volume efficiency of compressor depends on:
i.     Drop in pressure across the valves due to friction which inturns is governed by the velocity of the gas.
ii.     Temperature of the compression chamber surface which tends to supper heat the gas.
iii.     Loss of compression due to gas i.e. alloyed part, the piston and the valves.
iv.     Recompression of vapor already discharged but which reenter the cylinder during the suction stroke.
v.    Re-expansion of clearance gases.
Therefore, factors that affect the volumetric efficiency of a compressor are:
1.     High-side pressure
2.     Low-side pressure
3.     The size of clearance pocket [Ogundijo, J. A. (1988)]
4.1.2     COMPRESSOR RATINGS
It is rated in terms of three [3] parameters:
i.     The bore diameter D
ii.     The strike diameter L
iii.     The number of cylinder NCw= 2LN ________ iii) [Ogundijo,J.A.1988)].
60

U= nD2 LN x 60 _________ (iii) [Ogundijo, J. A. (1988)]
4
Where
C_w= Piston speed
D   = Bore Diameter (inches)
L    = Length of stroke
U_p  = Suition volume
Compression ratio is the ratio of discharge of high side pressure to the suction low-side pressure i.e.
Compression ratio =high-side pressure¬—  (iv) [Ogundijo J.A. (1988)].  Low-side pressure
 
COMPRESSOR TYPES
Refrigeration compressor may be divided into two types according to their category as follow;
CATEGORY I POSITIVE DISPLACEMENT TYPES
i.     Reciprocating
ii.     Rotary
iii.     Gear
iv.     Screw
CATEGORY II KINETIC DISPLACEMENT TYPE
i.     Centrifugal: Reciprocating compressor is employed in this project     work because of the services for which it is intended and the     refrigerant to be used (Freon — 12 or Hydrocarbon). The following     rules may be used as a guide in selecting the type of compressor:
i.     With refrigerant having a boiling point of 15°F or below; use a     reciprocating compressor or screw compressor type.
ii.     With refrigerant having a boiling point between 10°F and 65°F; use a     rotary compressor
iii.     With refrigerant having a boiling point of 65° or above, use a     centrifugal compressor [Andrew, D. & Alfred F. 13. (1970)].
4.2     THE CONDENSER
The type of condenser is generally characterized by the cooling medium used. Thus there are three (3) types of condenser:
i. Air-cooled condensers
ii. Water-cooled condensers
iii. Evaporative condensers.