Compressors

The compressor is the heart of the refrigeration system. Acting as a pump, the compressor moves the refrigerant through the refrigeration circuit. The role of the compressor is to increase the pressure and temperature of the vaporised refrigerant

What is a Compressor?

A compressor is a mechanical device that increases the pressure of a gas by reducing its volume.  As can be seen in the diagram below, compressors for air conditioning and refrigeration applications can generally be divided into two main types, positive displacement and dynamic.

Positive Displacement Compressors

– Screw Compressors

History of the Screw Compressor

 

The principle of the screw compressor was first patented in Germany in 1878. At that time it was not possible to develop the technology any further because of the lack of manufacturing capability.

It wasn’t until the 1920s when the original patent had expired, that a Swedish engineer, AJR Lysholm, developed the profile of the screw compressor and tested various configurations and rotor lobe combinations.  Not only was the profile of the rotors significant, but Lysholm also solved the problem and patented the method for accurately machining the rotors.

The 1935 patent clearly shows his asymmetric 5 female – 4 male lobe rotor design, although the shapes have been ‘fine tuned’ over the years, the screw compressor as we know it today was introduced.

Further breakthroughs occured in the 1960s with injection technology leading to the development of the screw compressor without synchronous transmission, but with oil injection cooling where the rotors could counter rotate in a non-contacting manner because of the lubrication provided by the injected oil.

Screw Compressors are used in these Petra systems:

However, screw compressors were still inefficient and expensive to produce.  This changed in the mid 1970s with the introduction of the asymmetric rotor profile which substantially reduced the blow-hole area, the main source of internal leakage, and thereby raised the thermodynamic efficiency of these machines to roughly the same level as that of traditional reciprocating compressors.  Secondly, the introduction of precise thread milling machine tools at approximately the same time made it possible to manufacture items of complex shape such as the rotors, accurately and economically.

From then on, as a result of their ever improving efficiencies, high reliability and compact form, screw compressors have taken an increasing share of the compressor market, especially in the fields of  compressed air production, and refrigeration and air conditioning, and today, a good proportion of compressors manufactured for industry are of this type.

How the Screw Compressor Works

Screw compressor consists essentially of a pair of meshing helical lobed rotors, which rotate within a fixed casing that totally encloses them, as shown in the image to the right.

The space between any two successive lobes of each rotor and its surrounding casing forms a separate working chamber. The volume of this chamber varies as rotation proceeds due to displacement of the line of contact between the two rotors. It is a maximum when the entire length between the lobes is unobstructed by meshing contact with the other rotor. It has a minimum value of zero when there is full meshing contact with the second rotor at the end face.

The two meshing rotors effectively form a pair of helical gear wheels with their lobes acting as teeth. These are normally described as the male or main rotor and the female or gate rotor respectively.

The number of lobes (or valleys) on the rotors will vary from one compressor manufacturer to another. As a rule, the main (male) rotor is made with four; the auxiliary (female) rotor with six. The main rotor is driven by either an electric motor or an engine and transforms about 85-90% of the energy received at the coupling into pressure and heat energy.

 1) View from the top and front        2) View from the bottom and rear

Screw Compressor Rotors

As shown in the image above, gas enters through the suction, or low pressure port presented in light shade. It thus fills the spaces between the lobes, starting from the ends corresponding to A and C.  As can be seen, the trapped volume in each chamber increases as rotation proceeds and the contact line between the rotors recedes. At the point where the maximum volume is filled, the inlet port terminates and rotation proceeds without any further fluid admission in the region corresponding to the dark shaded area.

Viewed from the bottom and the rear (2)), it may be seen that the dark shaded area begins, from the end corresponding to A and C, at the point where the male and female rotor lobes start to re-engage. Thus, from that position, further rotation reduces the volume of gas trapped between the lobes and the casing. This causes the pressure to rise.

At the position where the trapped volume is sufficiently reduced to achieve the required pressure rise, the ends of the rotors corresponding to D and B are exposed to an opening presented in light shade, which forms the high pressure or discharge port. Further rotation reduces the trapped volume causing the fluid to flow out through the high pressure port at approximately constant pressure. This continues until the trapped volume is reduced to virtually zero and all the gas between the lobes is expelled.  The process is then repeated for each chamber.

Advantages of Screw Compressors

Screw machines have a number of advantages over other positive displacement types.  Firstly, unlike reciprocating machines, the moving parts all rotate and hence can run at much higher speeds.  Secondly, unlike vane machines, the contact forces within them are low, which makes them very reliable. Thirdly, and far less well known, unlike the reciprocating, scroll and vane machines, all the sealing lines of contact which define the boundaries of each cell chamber, decrease in length as the size of the working chamber decreases and the pressure within it rises, this minimises the escape of gas from the chamber due to leakage during the compression or expansion process.

Reference: Stosic N, Smith I K, Kovacevic A and Mujic E, 2011, Geometry of screw compressor rotors and their tools, Journal of Zhejiang University-SCIENCE A (Applied Physics & Engineering)

Methods Of Capacity Control

Screw compressors can include stepped capacity control systems or continuous (stepless) capacity control.  Both of these capacity control systems consist of a modulating slide valve and unloading piston connected by a piston rod.  Oil pressure is used to drive the piston in the cylinder.

The more the slider is moved to the discharge (outlet) side (right side as shown in the adjacent diagram), the smaller becomes the resultant profile volume. Less refrigerant is taken in and therefore the mass flow is reduced along with the cooling capacity.

Stepped capacity control system

Solenoid valves are installed on the compressor that control the compressor capacity from minimum capacity to full load (100%).  There are two/three normally closed (NC) solenoid valves used to control the various required capacity steps.  If all solenoid valves are closed, the piston will move the slide to its limit away from the discharge port and therefore provides maximum capacity.

Opening of each of the solenoid valves (16,15 & 14) in succession decreases the pressure on the piston and the cooling capacity is reduced accordingly.

 

Unloaded Starting

When starting the compressor solenoid valve 14 is energised, so that the unloading piston/slide valve is in the minimum capacity position. This ensures the compressor starts in an unloaded condition reducing any start-up start up torque and current draw (amps).

1. Suction Filter5. Suction Bearings9. High Pressure Gas Outlet13. Capillary
2. Low Pressure Gas6. Male Rotor10. Lubricant14. Solenoid Valve – min%
3. Motor7. Discharge Bearings11. Oil Separator15. Solenoid Valve – 50%
4. Oil Filter Cartridge8. Oil Separator Baffle12. Gas Discharge16. Solenoid Valve – 66-75%
17. Slide Valve

 

Stepless capacity control system

Solenoid valves are installed on the compressor that control the compressor capacity from minimum capacity to full load (100%).  There are two/three normally closed (NC) solenoid valves used to control the various required capacity steps.  If all solenoid valves are closed, the piston will move the slide to its limit away from the discharge port and therefore provides maximum capacity.

Opening of each of the solenoid valves (16,15 & 14) in succession decreases the pressure on the piston and the cooling capacity is reduced accordingly.

 

Unloaded Starting

When starting the compressor solenoid valve 14 is energised, so that the unloading piston/slide valve is in the minimum capacity position. This ensures the compressor starts in an unloaded condition reducing any start-up start up torque and current draw (amps).

1. Suction Filter5. Suction Bearings9. High Pressure Gas Outlet13. Capillary
2. Low Pressure Gas6. Male Rotor10. Lubricant14. Solenoid Valve – min%
3. Motor7. Discharge Bearings11. Oil Separator15. Solenoid Valve – 50%
4. Oil Filter Cartridge8. Oil Separator Baffle12. Gas Discharge16. Solenoid Valve – 66-75%
17. Slide Valve