COMPRESSOR UNIT:

Three types of compressors aroused : (i) Reciprocating, (ii) Rotary and (iii) Centrifugal. The compressors may be further catagorised, according to their construction, as open, hermetic or semi - hermitic.

in an open type, one end of the crankshaft extends through the crankcase house for connection, via a coupling or pulley to an external drive motor. In case of hermetic compressors, the compressor and its electric motor are entirely housed in a gas - tight casing. The advantage of hermetic in comparison with the open - type, is that the shaft seal, a potential source of refrigerant leakage, is eliminated. Semi - hermitic units provide access to the compressor and motor components for maintenance and repairs.

RECIPROCATING COMPRESSORS:

Most refrigerating Compressors, from the smallest to the largest units, are the reciprocating type. Cylinder housing and crankcase are usually made of iron casting, the piston may be aluminium or cast iron and the crankshaft of steel or cast iron. The suction and discharge valves may be of the spring loaded ring - plate type or reed valves (end clamped or free floating type) depending on compressor size. Most valve assemblies have spring - loaded safety heads to prevent damage in the event of incompressible liquid refrigerant or oil entering the cylinder.

Ammonia (R 717) and R 22 Compressors are some - times fitted with water cooled jackets around the upper parts of the cylinders to prevent excessively high discharge temperatures. The use of water - jackets also improves efficiency of the compressor.

Open type Compressors are fitted with shaft seals which normally consist of a spring loaded, self- lubricated hard carbon ring, bearing onto a steel collar, the seal to the crank - shaft or crankcase housing being provided by synthetic rubber gaskets. Shaft seals are a common cause of leakage and should be regularly inspected and leak tested.

Large compressors are generally provided with an unloading system which enables the compressor to start easily with no vapour pressure load in the cylinder, permitting the use of electric motors with low starting torques. Unloading is effected by holding the suction valves open or by opening a bypass valve between the discharge and suction sides during starting. The unloading mechanism may be actuated hydraulically, mechanically or by solenoid valve.

The unloader system may also be used for capacity control by successively cutting in or out cylinders or cylinder groups. Other methods of capacity control include varying the compressor speed and 'hot gas bypass' which involved passing a proportion of the discharge gas from the compressor directly to the evaporator by - passing the condenser.

Compressors are sometimes fitted with crank - case heaters as a safe guard against oil foaming. The heater keeps the oil warm during standstill periods, thus preventing refrigerant vapour migrating from the evaporator and condensing in the crankcase. If liquid refrigerant is allowed to accumulate in the crankcase, it will vaporise at start - up and causing foaming of the oil which results in an increased amount of oil being pumped out of the compressor and loss of oil pressure. In extreme cases, oil foaming may also result total loss of oil from crankcase and liquid refrigerant and oil entering the cyclinder, causing severe damage to the pistons, valves etc.

As a rule, two stage or compound compressors are used in preference to single stage compressors in plants where the difference between the condensing and evaporating temparatures is of the order of 50K to 60K. Two stage compression combined with intermediate cooling has the following advantages : (a) lower cylinder pressures hence lower bearing loads and wear ; (b) lower discharge gas temparatures, preventing oil decomposition and valve damage ; (c) increased volumetric efficiency and (d) increased refrigerating capacity and the coeff. of performance of the compressor.

SCREW COMPRESSORS :

Two types of screw compressors are used in marine plants, double screw compressors and mono - screw compressors. A double screw compressor consists of two rotors with matched helical grooves, a male rotor with four lobes which mesh with the corresponding six flutes on a female rotor. In modern designs, the motor drive can be directly connected to the male or female rotor, the driving force being transmitted to the mating rotor by the thin layer of oil sealing the clearance space between the two rotors. The male and female rotors trap and compress the gas as they mesh and turn together. Suction gas is drawn into the compressor as the interlobe spaces of the rotors pas the inlet port. As they continue to rotate, a lobe of the male rotor progressively fills up the space which is available for the gas between the female lobes, and the gas is forced towards axially and compressed. The lobes continue to intermesh until the opposite end of the rotor passes the outlet port and the compressed gas is discharged. In the majority of screw compressors, capacity control is effected by means of a regulating slide valve mounted underneath the rotors, connecting via a by pass port with the suction inlet. When open, the valve allows some of the gas in the interlobe spaces of the rotors to return to the suction inlet. The regulating slide movement is controlled by a hydraulic piston giving continuous regulation from 100% down to about 10% of full output.

CENTRIFUGAL COMPRESSORS :

One or more impellers are mounted on a steel shaft and enclosed in a volute casing, usually of cast iron, the number of impellers depends on the compression ratio required and the number of stages necessary.

The use of centrifugal compressors are limited generally to large air - conditioning systems requiring large refrigerating capacities.

Capacity control may be achieved by means of adjustable inlet vanes, speed variation, hot gas by pass or by the use of a damper vlave in the suction line.

INDEX ADV ENG                                                                                         NEXT