Compressors and its applications
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The document explains the function and types of compressors used in refrigeration systems, detailing their mechanisms, components, and efficiency considerations. It covers the importance of compression ratios and methods of compression, particularly highlighting two-stage compression in industrial applications. Additionally, various compressor types such as reciprocating, screw, rotary, and scroll compressors are described, alongside their specific uses and maintenance concerns.
Compressors and its applications
- 2. OBJECTIVES • Explain the function of the compressor • Discuss the concept of compression ratio • List common compressors found in refrigeration systems • Describe four different methods of compression • Describe the component parts of reciprocating compressors
- 3. 3 © UNEP 2006 Two Basic Compressor Types Types of Compressors Type of compressor
- 4. FUNCTION OF THE COMPRESSOR Considered the heart of the refrigeration systems Compressors are vapor pumps Responsible for lowering the pressure on the suction side of the system Responsible for increasing the pressure on the discharge side of the system Suction gas from the evaporator enters the compressor Refrigerant is discharged to the condenser
- 5. COMPARISON OF COMPRESSORS • Efficiency at full, partial and no load • Noise level • Size • Oil carry-over • Vibration • Maintenance • Capacity • Pressure
- 6. COMPRESSION RATIO Compares pumping conditions for compressors Defined as the high side pressure divided by the low side pressure High compression ratio can lead to overheated compressor oil High compression ratio leads to reduced refrigerant flow through the system Reduced refrigerant flow reduces system capacity
- 7. COMPRESSION RATIO EXAMPLES R-12 compressor 169 psig high side, 2 psig low side 183.7 psia high side, 16.7 psia low side 183.7 psia ÷ 16.7 psia = 11:1 compression ratio R-134a compressor 184.6 psig high side, 0.7 in. Hg. vacuum low side 199.3 psia high side, 14.35 psia low side 199.3 psia ÷ 14.35 psia = 13.89:1 compression ratio
- 8. TWO-STAGE COMPRESSION Lowers the compression ratio Utilizes two compressors One compressor discharges into suction of the other Also referred to as compound compression Often used when the compression ratio of a single compressor system exceeds 10:1 Often used in low-temperature commercial and industrial storage applications
- 9. TWO-STAGE COMPRESSION FIRST STAGE SECOND STAGE 21 psig 100 psig 169 psig Suction Discharge Discharge Suction
- 10. TYPES OF COMPRESSORS Reciprocating Fully welded, hermetic compressors Semi-hermetic compressors Open-drive compressors Belt-driven and direct-drive compressors Screw compressors Rotary compressors Scroll compressors Centrifugal compressors
- 11. WELDED HERMETIC RECIPROCATING COMPRESSORS Motor and compressor contained in a welded shell Cannot be field serviced Typically a “throw-away” compressor Considered to be a low-side component Cooled by suction gas from the evaporator Lubricated by the splash method
- 12. SEMI-HERMETIC COMPRESSORS Bolted together, can be field serviced Housing is made of cast iron Has a horizontal crankshaft Smaller compressors are splash lubricated Larger compressors use pressure lubrication systems Often air cooled Piston heads are located at the top of the compressor
- 13. OPEN DRIVE COMPRESSORS Can be direct drive or belt-driven compressors Must have a shaft seal to prevent leakage Bolted together, can be field serviced Belt-driven compressors have the compressor and motor shafts parallel to each other Belt-driven compressors use belts and pulleys Direct drive compressors have the compressor and motor shafts connected end to end
- 14. RECIPROCATING COMPRESSOR COMPONENTS Crankshaft Transfers motor motion to the piston Creates the back and forth motion of the piston Connecting rods Connects the crankshaft to the pistons Pistons Slide up and down in the cylinder Used to compress and expand the refrigerant
- 15. RECIPROCATING COMPRESSOR COMPONENTS (CONT’D) Refrigerant cylinder valves (suction) Durable, flexible steel Located on the bottom of the valve plate Open when refrigerant is introduced to the pump Refrigerant cylinder valves (discharge) Durable, flexible steel Open when refrigerant is discharged from the pump Located on the top of the valve plate
- 16. OTHER COMPRESSOR TYPES Screw compressor Used in large commercial/industrial applications Uses two matching, tapered gears, and open motor design Rotary compressor Used in residential and light commercial applications – primarily in domestic refrigerators Scroll compressor Uses a matched set or scrolls to achieve compression Becoming more popular for their ability to handle liquid refrigerant without compressor damage Centrifugal compressors Used extensively for air conditioning in large structures
- 17. RECIPROCATING COMPRESSOR • Used for air and refrigerant compression • Works like a bicycle pump: cylinder volume reduces while pressure increases, with pulsating output • Many configurations available • Single acting when using one side of the piston, and double acting when using both sides
- 18. Suction line Discharge line Valve plate Head Discharge valve Suction valve Piston Rings CrankshaftConnecting Rod
- 19. RECIPROCATING COMPRESSOR COMPONENTS (CONT’D) Compressor head Holds the top of the cylinder and its components together Contains both high and low pressure refrigerant Mufflers Designed to reduce compressor noise Compressor housing Encases the compressor and sometimes the motor
- 20. BELT-DRIVE MECHANISMS Motor pulley is called the drive pulley Compressor pulley is called the driven pulley Pulleys can be adjusted to change compressor speed Drive size x Drive rpm = Driven size x Driven rpm Shafts must be properly aligned Pulleys with multiple grooves must used matched sets of belts
- 21. DIRECT-DRIVE COMPRESSOR CHARACTERISTICS Direct drive compressors turn at the same speed as the motor used Motor shaft and compressor shaft must be perfectly aligned end to end Motor shaft and compressor shafts are joined with a flexible coupling
- 22. RECIPROCATING COMPRESSOR EFFICIENCY Determined by initial compressor design Four processes take place during the compression process Expansion (re-expansion) Suction (Intake) Compression Discharge
- 23. COMPRESSION PROCESS - EXPANSION Piston is the highest point in the cylinder Referred to as top dead center Both the suction and discharge valves are closed Cylinder pressure is equal to discharge pressure As the crankshaft continues to turn, the piston moves down in the cylinder The volume in the cylinder increases The pressure of the refrigerant decreases
- 24. Suction valve closed Discharge valve closed Piston moving downward in the cylinder Refrigerant trapped in the cylinder Pressure of the refrigerant in the cylinder is equal to the discharge pressure
- 25. COMPRESSION PROCESS – SUCTION As the piston moves down, the pressure decreases When the cylinder pressure falls below suction pressure, the suction valve opens The discharge valve remains in the closed position As the piston continues downward, vapor from the suction line is pulled into the cylinder Suction continues until the piston reaches the lowest position in the cylinder (bottom dead center) At the bottom of the stroke, suction valves close
- 26. Suction valve open Discharge valve closed Piston moving downward in the cylinder Pressure of the refrigerant in the cylinder is equal to the suction pressure Suction gas pulled into the compression cylinder Most of the energy that entering the compressor in the suction cylinder is latent heat.
- 27. COMPRESSION PROCESS - COMPRESSION Piston starts to move upwards in the cylinder The suction valve closes and the discharge valve remains closed As the piston moves upwards, the volume in the cylinder decreases The pressure of the refrigerant increases Compression continues until the pressure in the cylinder rises just above discharge pressure
- 28. Suction valve closed Discharge valve closed Piston moving up in the cylinder Pressure of the refrigerant in the cylinder is equal to the suction pressure Volume is decreasing, compressing the refrigerant
- 29. COMPRESSION PROCESS - DISCHARGE When the cylinder pressure rises above discharge pressure, the discharge valve opens and the suction valve remains closed As the piston continues to move upwards, the refrigerant is discharged from the compressor Discharge continues until the piston reaches top dead center
- 30. Suction valve open Discharge valve closed Piston moving up in the cylinder Pressure of the refrigerant in the cylinder is equal to the discharge pressure Discharge gas pushed from the compression cylinder What do you think may happen if both of the valves remain closed on an upstroke? The gas leaving the compressor is very warm.
- 31. LIQUID IN THE COMPRESSION CYLINDER If liquid enters the cylinder, damage will occur Liquids cannot be compressed Liquid slugging can cause immediate damage to the compressor components Common causes of liquid slugging include an overfeeding metering device, poor evaporator air circulation, low heat load, defective evaporator fan motor and a frosted evaporator coil
- 32. SYSTEM MAINTENANCE AND COMPRESSOR EFFICIENCY High suction pressures and low discharge pressures keep the compression ratio low Dirty evaporators cause suction pressure to drop Low suction reduces compressor pumping capacity Dirty condensers increase head pressure Compression ratio is increased by dirty or blocked condenser and evaporator coils