Compressors

Introduction

A refrigerant compressor, as the name indicates, is a machine used to compress the vapour refrigerant from the evaporator and to raise its pressure so that the corresponding saturation temperature is higher than that of the cooling medium. It also continually circulates the refrigerant through the refrigerating system. Since the compression of refrigerant requires some work to be done on it, therefore, a compressor must be driven by some prime mover.

Note : Since the compressor virtually takes the heat at a low temperature from the evaporator and pumps it at the high temperature to the condenser. therefore it is often referred to as a heat pump.

Classification of Compressors

The compressors may be classified in many ways, but the following are important from the subject point of view:

1. According to the method of compression

(a) Reciprocating compressors.

(b) Rotary compressors, and

(c) Centrifugal compressors.

2. According to the number of working strokes

(a) Single acting compressors, and

(b) Double acting compressors.

3. According to the number of stages

(a) Single stage (or single cylinder) compressors, and

(b) Multi-stage (or multi-cylinder) compressors.

4. According to the method of drive employed

(a) Direct drive compressors. and

(b) Belt drive compressors.

5. According to the location of the prime mover

 (a) Semi-hermetic compressors (direct drive, motor and compressor in separate

housings), and

(b) Hermetic compressors (direct drive, motor and compressor in same      housings).

Reciprocating Compressors

The compressors in which the vapour refrigerant is compressed by the reciprocating (i.e. back and forth) motion of the piston, are called reciprocating compressors. These compressors are used for refrigerants which have comparatively low volume per kg and a large differential pressure, such as ammonia (R-717), R-12, R-22, and methyl chloride (R-40). The reciprocating compressors are available in sizes as small as 1/12 kW which are used in small domestic refrigerators and up to about 150 kW for large capacity installations.

The two types of reciprocating compressors in general use are single acting vertical compressors and double acting horizontal compressors. The single acting compressors usually have their cylinders arranged vertically, radially or in a V or W form. The double acting compressors usually have their cylinders arranged horizontally.

Fig. 4.1 shows a single stage single acting reciprocating compressor in its simplest form.

The principle of operation of the compression cycle is as discussed below:

Let us consider that the piston is at the top of its stroke as shown in Fig. 4.1 (a). This is called top dead centre position of the piston. In this position, the suction valve is held closed because of the pressure in the clearance space between the top of the piston and the cylinder head. The discharge valve is also held closed because of the cylinder head pressure acting on the top of it.

When the piston moves downward (i.e. during suction stroke) as shown in Fig. 4.1 (b), the refrigerant left in the clearance space expands. Thus the volume of the cylinder (above the piston) increases and the pressure inside the cylinder decreases. When the pressure becomes slightly less than the suction pressure or atmospheric pressure, the suction valve gets opened and the vapour refrigerant flows into the cylinder. This flow continues until the piston reaches the bottom of its make (I.e. bottom dead centre). At the bottom of the stroke, as shown in Fig. 4.1 (c), the suction valve closes because of spring action. Now when the piston moves upward (i.e. during compression stroke) as shown in Fig. 4.1 (d), the volume of the cylinder decreases and the pressure inside the cylinder Increases. When the pressure inside the cylinder becomes greater than that on the top of the discharge valve, the discharge valve gets opened and the vapour refrigerant is discharged into the condenser and the cycle is repeated.

It may be noted that in a single acting reciprocating compressor, the suction, compression and discharge of refrigerant takes place in two strokes of the piston or in one revolution of the crankshaft.

Notes: 1. In a double acting reciprocating compressor, the suction and compression takes place on both sides of the piston. It is thus obvious, that such a compressor will supply double the volume of refrigerant than a single acting reciprocating compressor (neglecting volume of piston rod).

2. There must be a certain distance between the top of the piston and the cylinder head when the piston is on the top dead centre so that the piston does not strike the cyclinder head. This distance is called clearance space and the volume therein is called the clearance volume. The refrigerant left in this space is at discharge pressure and its pressure must be reduced below that of suction pressure (atmospheric pressure) before any vapour refrigerant flows into the cylinder. The clearance space should be a minimum.

3. The low capacity compressors are air cooled. The cylinders of these compressors usually have fins to provide better air cooling. The high capacity compressors are cooled by providing water jackets around the cylinder.

Work Done by a Single Stage Reciprocating Compressor

We have already discussed that in a reciprocating compressor, the vapour refrigerant is first sucked, compressed and then discharged. So there are three different operations of the compressor. Thus we see that work is done on the piston during the suction of refrigerant. Similarly, work is done by the piston during compression as well as discharge of the refrigerant. A little consideration will show, that the work done by a reciprocating compressor is mathematically equal to the work done by the compressor during compression as well as discharge minus the work done on the compressor during suction.

Here we shall discuss the following two important cases of work done:

1. When there is no clearance volume in the cylinder, and

2. When there is some clearance volume.

Hermetic Sealed Compressors

When the compressor and motor operate on the same shaft and are enclosed in a common casing, they are known as hermetic sealed compressors. These types of compressors eliminate the use of crank shaft seal which is necessary in ordinary compressors in order to prevent leakage of refrigerant. These compressors may operate on either reciprocating or rotary principle and may be mounted with the shaft in either the vertical or horizontal position. The hermetic units are widely used for small capacity refrigerating systems such as in domestic refrigerators, home freezers and window air conditioners.

The hermetic sealed compressors have the following advantages and disadvantages:

Advantages

1. The leakage of refrigerant is completely prevented.

2. It is less noisy.

3. It requires small space because of compactness.

4. The lubrication is simple as the motor and compressor operate in a sealed space with the lubricating oil.

Disadvantages

I. The maintenance is not easy because the moving parts are inaccessible.

2. A separate pump is required for evacuation and charging of refrigerant.

Rotary Compressors

In rotary compressors, the vapour refrigerant from the evaporator is compressed due to the movement of blades. The rotary compressors are positive displacement type compressors. Since the clearance in rotary compressors is negligible, therefore they have high volumetric efficiency. These compressors may be used with refrigerants R-12, R-22, R-114 and ammonia. Following are the two basic types of rotary compressors:

. Single stationary blade type rotary compressor. A single stationary blade type rotary compressor is shown in Fig. 9.14. This consists of a stationary cylinder, a roller (or impeller) and a shaft. The shaft has an eccentric on which the roller is mounted. A blade is set into the slot of a cylinder in such a manner that it always maintains contacts with the roller by means of a spring. The blade moves in and out of the slot to follow the rotor when it rotates. Since the blade separates the suction and discharge ports as shown in Fig. 9.14, therefore it is often called a sealing blade. When the shaft rotates, the roller also rotates so that it always touches the cylinder wall

Fig. 4.3(a) to (d) shows the various positions of roller as the vapour refrigerant is compressed. Fig. 4.3(a) shows the completion of intake stroke (i.e. the cylinder is full of low pressure and temperature vapour refrigerant) and the beginning of compression stroke. When the roller rotates, the vapour refrigerant ahead of the roller is being compressed and the new intake from the evaporator is drawn into the cylinder, as shown in Fig.4.3 (b). As the roller turns towards mid position as shown in Fig. 4.3 (c), more vapour refrigerant is drawn into the cylinder while the compressed refrigerant is discharged to the condenser. At the end of compression stroke, as shown in Fig. 4.3 (d), most of the compressed vapour refrigerant is passed through the discharge port to the condenser. A new charge of refrigerant is drawn into the cylinder. This, in turn, is compressed and discharged to the condenser. In this way, the low pressure and temperature vapour refrigerant is compressed gradually to a high pressure and temperature.

2. Rotating blade type rotary compressor. The rotating blade type rotary compressor is shown in Fig. 4.4. This consists of a cylinder and a slotted rotor containing a number of blades. The centre of the rotor is eccentric with the centre of the cylinder.

The blades are forced against the cylinder wall by the  centrifugal action during the rotation of the motor.

The low pressure and temperature vapour  refrigerant from the evaporator is drawn through the suction port. As the rotor turns, the suction vapour refrigerant entrapped between the two adjacent blades is compressed. The compressed refrigerant at high pressure and temperature is discharged through the discharge port to the condenser.

Note: The whole assembly of both the types of rotary compressors is enclosed in a housing which is filled with oil. When the compressor is working, an oil film forms the seal between the high pressure and low pressure side. But when the compressor stops, this seal is lost and therefore high pressure vapour refrigerant will flow into low pressure side. In order to avoid this, a check valve is usually provided in the suction line. This valve prevents the high pressure vapour refrigerant from flowing back to the evaporator.

Centrifugal Compressors

The centrifugal compressor for refrigeration systems was designed and developed by Dr.Willis H. Carrier in 1922. This compressor increases the pressure of low pressure vapour refrigerant to a high pressure by centrifugal force. The centrifugal compressor is generally used for refrigerants that require large displacement and low condensing pressure, such as R-11 and R-113. However, the refrigerant R-12 is also employed for large capacity applications and low-temperature applications.

A single stage centrifugal compressor, in its simplest form, consists of an impeller to which a number of curved vanes are fitted symmetrically, Inlet as shown in Fig.. 4.5. The impeller rotates in an air tight volute casing with inlet and outlet points.

The impeller draws in low pressure vapour refrigerant from the evaporator. When the impeller rotates, it pushes the vapour refrigerant from the centre of the impeller to its periphery by centrifugal force. The high speed of the impeller leaves the vapour refrigerant at a high velocity at the vane tips of the impeller. The kinetic energy thus attained at the impeller outlet is converted into pressure energy when the high velocity vapour refrigerant passes over the diffuser. The diffuser is normally a vaneless type as it permits more efficient part load operation which is quite usual in any air-conditioning plant. The volute casing collects the refrigerant from the diffuser and it further converts the kinetic energy into pressure energy before it leaves the refrigerant to the evaporator.

Notes: 1. In case of a single stage centrifugal compressor, the compression ratio that an impeller can develop is limited to about 4.5. But when high compression ratio is desired, multi- stage centrifugal compressors with intercoolers are employed.

2. The centrifugal compressors have no valves, pistons and cylinders. The only wearing parts are the main bearings.

Advantages and Disadvantages of Centrifugal Compressors over Reciprocating Compressors

Following are the advantages and disadvantages of centrifugal compressors over reciprocating compressors:-

Advantages

I. Since the centrifugal compressors have no valves, pistons. cylinders, connecting rod etc., therefore the working life of these compressors is more as compared to reciprocating compressors.

2. These compressors operate with little or no vibration as there are no unbalanced masses.

3. The operation of centrifugal compressors is quiet and calm.

4. The centrifugal compressors run at high speeds (3000 r.p.m. and above), therefore these can be directly connected to electric motors or steam turbines.

5. Because of the high speed, these compressors can handle large volume of vapour refrigerant, as compared to reciprocating compressors.

6. The centrifugal compressors are especially adapted for systems ranging from 50 to 5000 tonnes. They are also used for temperature ranges between - 90° C and + 10° C.

7. The efficiency of these compressors is considerably high.

8. The large sizes centrifugal compressors require less floor area as compared to reciprocating compressors.

Disadvantages

I. The main disadvantage in centrifugal compressors is •surging. It occurs when the refrigeration load decreases to below 35 percent of the rated capacity and causes severe stress conditions in the compressor.

2. The increase in pressure per stage is less as compared to reciprocating compressors.

3. The centrifugal compressors are practical below 50 tonnes capacity load.

4. The refrigerants used with these compressors should have high specific volume.

Capacity Control of Compressors

There are many refrigeration applications in which the refrigeration load is not constant. It is, therefore, necessary to provide some means to control the capacity of a compressor according to the load. It may be noted that the compressors operating under partial loads and low back pressure creates a condition where the coil may freeze or damage may result.

References

1. Refrigeration and air conditioning                  -           Ballaney P. L.
2. Refrigeration and air conditioning                  -           Stocker W. F.