Compressors and working

details about compressors and its working concepts

1. What are compressors? Compressors are mechanical devices that compresses gases. It is widely used in industries and has various applications

2. How they are different from pumps? Major difference is that compressors handles the gases and pumps handles the liquids. As gases are compressible, the compressor also reduces the volume of gas. Liquids are relatively incompressible; while some can be compressed Centrifugal pump

3. What are its applications? Compressors have many everyday uses, such as in : Air conditioners, (car, home) Home and industrial refrigeration Hydraulic compressors for industrial machines Air compressors for industrial manufacturing Refrigeration compressor

4. What are its various types? Compressor classification can be described by following flow chart:

5. What are dynamic compressors? The dynamic compressor is continuous flow compressor is characterized by rotating impeller to add velocity and thus pressure to fluid. It is widely used in chemical and petroleum refinery industry for specific services. There are two types of dynamic compressors Centrifugal Compressor Axial Flow Compressor

6. Centrifugal Compressor Achieves compression by applying inertial forces to the gas by means of rotating impellers. It is multiple stage ; each stage consists of an impeller as the rotating element and the stationary element, i.e. diffuser Fluid flow enters the impeller axially and discharged radially The gas next flows through a circular chamber (diffuser), where it loses velocity and increases pressure.

7. Axial flow compressor Working fluid principally flows parallel to the axis of rotation . The energy level of air or gas flowing through it is increased by the action of the rotor blades which exert a torque on the fluid Have the benefits of high  efficiency and large mass flow rate Require several rows of airfoils to achieve large pressure rises making them complex and expensive

8. What are positive displacement compressors? Positive displacement compressors causes movement by trapping a fixed amount of air then forcing (displacing) that trapped volume into the discharge pipe . It can be further classified according to the mechanism used to move air. Rotary Compressor Reciprocating compressor

9. Rotary compressors The gas is compressed by the rotating action of a roller inside a cylinder. The roller rotates off- centre around a shaft so that part of the roller is always in contact with the cylinder.  Volume of the gas occupies is reduced and the refrigerant is compressed. High efficient as sucking and compressing refrigerant occur simultaneously.

10. 2.1.2 Rotary compressors These compressors use rotors in place of pistons, giving a pulsating free discharge air. These rotors are power driven. They have the following advantages over reciprocating compressors: They require a lower starting torque They give a continuous, pulsation free discharge air They generally provide higher output They require smaller foundations, vibrate less, and have lesser parts, which means less failure rate

11. Subtypes : Depending on the action and geometry of the rotors, these are classified as follows:

12. Lobe compressor Also called the roots blower, it’s essentially a low pressure blower and is limited to a discharge pressure of 1 bar in a single-stage and 2.2 bar in two stage-design. 12

13. Rotary screw compressors: Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the air into a smaller space (see figure). These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Because of simple design and few wearing parts, rotary screw air compressors are easy to install, operate, and maintain. 13

14. Rotary screw compressors are commercially produced in Oil flooded and Oil free types: Oil flooded compressors are nothing but oil cooled compressors; where oil seals the internal clearances of the compressor. Though filters are needed to separate the oil from the discharge air, cooling takes place right inside the compressor, and thus the working parts never experience extreme operating temperatures leading to prolonged life. The oil free screw air compressors use specially designed air ends to compress air, giving true oil free air. They are water cooled or air cooled and provide the same flexibility as oil flooded rotary compressors 14

15. Rotary vane compressors: One of the oldest compressor technologies, rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. 15

16. The rotor is mounted offset in a larger housing that is either circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. They are well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor. They can have mechanical efficiencies of about 90% 16

17. Scroll compressors: A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress air. Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls (see figure). 17

18. Due to minimum clearance volume between the fixed scroll and the orbiting scroll, these compressors have a very high volumetric efficiency. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range. 18

19. Reciprocating compressor It is a positive-displacement compressor that Uses pistons driven by a crankshaft to deliver gases at high pressure. The intake gas enters the suction manifold, then flows into the compression cylinder It gets compressed by a piston driven in a reciprocating motion via a crankshaft, Discharged at higher pressure

20. RECIPROCATING COMPRESSORS Detailed Analysis

21. Block diagram of reciprocating compressor It is a piston and cylinder device with (automatic) spring controlled inlet and exhaust valves There is a clearance between the piston crown and the top of the cylinder.

22. Construction of Reciprocating Compressors Reciprocating compressors can be divided into two main groups. Gas end. Power end.

23. Different Parts Of Gas End Various parts of gas end are: Cylinder & liner Piston Piston rod Piston rod packing Piston rings Valves

24. Different Parts of Power End Various parts of power end are Crank and Crankshaft Connecting rod crosshead Crank and crankshaft crosshead Connecting rod

25. Widespread use, many applications Gas is accelerated outwards by rotating impeller Can be built for operation as low as 5 psi, or operation as high as 8,000 psi (35 kPa or 55,000 kPa ) Sizes range from 300 hp to 50,000 hp Centrifugal compressors Single Case Compressor Centrifugal Impeller IMPELLERS Picture of horizontal split Cross section of barrel type compressor Picture of barrel type compressor Cross section of bull gear compressor Picture of (bull) gear and impellers Picture of bull gear compressor Cross section of horizontal split DIFFUSERS

26. Compressor inlet nozzle Thrust bearing Journal bearing Shaft and labyrinth seal Impeller inlet labyrinth seals Discharge volutes Impellers Drive coupling Casing (horizontally split flange) Compressor discharge nozzle Cross section of horizontal split

27. Picture of horizontal split

28. Cross section of barrel type compressor

29. Picture of barrel type compressor

30. Compressor volutes Gear casing Pinion shafts Journal bearing Impellers Drive coupling Labyrinth seals Main gear Inlet guide vanes Cross section of bull gear compressor

31. Picture of bull gear compressor

32. Axial compressors Gas flows in direction of rotating shaft Can be built for lower pressures only 10 to 100 psi (0.7 to 6.8 Bar) High flow rate Efficient Not as common as centrifugals Rotor Blades Casing Stator Blades Stator Blades Rotor Blades Casing Shaft

33. Compressor outlet nozzle Rotor blades Labyrinth seals Guide-vane actuator linkage Compressor rotor Compressor inlet nozzle Thrust bearing Adjustable guide vanes Cross section of axial compressor

34. Picture of axial compressor

35. Graph showing operating regions of various compressors

36. Table showing operating conditions of various compressors

37. Advantages and Disadvantages of dynamic compressors Advantages Disadvantages Dynamic Compressors Centrifugal Wide operating range High reliability Low Maintenance Instability at reduced flow Sensitive to gas composition change Axial High Capacity for given size High efficiency Heavy duty Low maintenance Low Compression ratios Limited turndown

38. Advantages and disadvantages of positive displacement type compressor Advantages Disadvantages Positive displacement compressor Reciprocating Wide pressure ratios High efficiency Heavy foundation required Flow pulsation High maintenance Diaphragm Very high pressure Low flow No moving seal Limited capacity range Periodic replacement of diaphragm Screw Wide application High efficiency High pressure ratio Expensive Unsuitable for corrosive or dirty gases

39. Stable zone of operation Developing the compressor curve Minimum speed Power limit Maximum speed Process limit Q s, vol Adding control margins Stonewall or choke limit Surge limit R c Actual available operating zone

40. Developing the compressor curve P d Discharge Pressure (P 2 ) D P c Differential Pressure (P d - P s ) or (P 2 - P 1 ) R c Pressure Ratio (P d /P s ) or (P 2 /P 1 ) H p Polytropic Head R c Q s, normal Q s, mass Q s, vol Compressor curve for a specific speed N1 R process,1 Q 1 R c1 R process,2 Q 2 R c2 O.P.

41. Surges in centrifugal compressors What are surges? Surge is defined as the operating point at which centrifugal compressor peak head capability and minimum flow limits are reached. When the plenum pressure behind the compressor is higher than the compressor outlet pressure, the fluid tends to reverse or even flow back in the compressor. As a consequence, the plenum pressure will decrease, inlet pressure will increase and the flow reverses again. This phenomenon, called surge, repeats and occurs in cycles with frequencies varying from 1 to 2 Hz.  47

42. Effects on performance Surging can cause the compressor to overheat to the point at which the maximum allowable temperature of the unit is exceeded. Also, surging can cause damage to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side. This is defined as the surge cycle of the compressor. Surge points for centrifugal compressors running at varying speeds 48

43. Anti surge control systems These systems detect when a process compression stage is approaching to surge and subsequently take action to reverse the movement of the operating point towards the surge line (SL). This decreases the plenum pressure and increases the flow through the compressor, resulting in stable working conditions. It is normally achieved by opening an Anti-Surge Control Valve (or ASCV), returning the discharge gas to the inlet of the compressor via a suction cooler. resulting increase in compressor inlet volume flow moves the operating point away from surge. Shifting of operating point away from surge point 49

44. Surge description Flow reverses in 20 to 50 milliseconds Surge cycles at a rate of 0.3 s to 3 s per cycle Compressor vibrates Temperature rises “Whooshing” noise Trips may occur Conventional instruments and human operators may fail to recognize surge

45. Some surge consequences Unstable flow and pressure Damage in sequence with increasing severity to seals, bearings, impellers, shaft Increased seal clearances and leakage Lower energy efficiency Reduced compressor life

46. Rapid flow oscillations Reversal flow leads to reversal thrust Potential damage FLOW PRESSURE TEMPERATURE TIME (sec.) 1 2 3 TIME (sec.) 1 2 3 TIME (sec.) 1 2 3 Major Process Parameters during Surge Rapid pressure oscillations with process instability Rising temperatures inside compressor which can be seen at the Discgarge Operators may fail to recognize surge