Compressors I - Learning Objectives

Compressors I - Learning Objectives

• Demonstrate knowledge of compressors, their components, and operation
• Demonstrate knowledge of the fundamental principles of compressors

Heart of the System

• The compressor is the heart of the refrigeration system
• It creates a pressure difference, allowing refrigerant to flow from high pressure to low pressure
• It increases suction pressure to discharge pressure, e.g., 5 PSIG suction pressure to 170 PSIG discharge pressure, a 165 PSIG pressure increase
• Liquid is not compressible; compressors pump vapor

Positive Displacement

• Most compressors are positive displacement type
• A compressor draws suction vapor into a cylinder/chamber
• Reducing chamber volume increases vapor pressure and temperature through compression

Reciprocating Compressor Basics

• Positive displacement
• Piston creates pressure
• Crankshaft drives the piston
• Connecting rod transfers rotational motion to linear motion
• Not the most efficient due to clearance volume

Scroll Compressor Basics

• Fixed scroll stays stationary and attached to compressor body
• Orbiting scroll orbits during operation, creating compression
• Discharge check valve stops refrigerant from entering during the off cycle
• Handles small liquid amounts because the orbiting scroll can lift axially
• Orbiting scroll also separates axially if compression ratio is too high

Rotary Compressor Basics

• Crankshaft fully rotates rollers unlike scroll that orbits
• Vane separates high and low sides in compression chamber
• Positive displacement

Screw Compressor Basics

• Instead of a piston and cylinder, two screw type gears squeeze refrigerant vapor
• Positive displacement
• Used in large commercial/industrial systems

Centrifugal Compressor Basics

• Non-positive displacement
• Compresses refrigerant using an impeller that spins refrigerant outward
• Inlet guide vanes draw gas into the impeller center
• Impeller spins gas outward with centrifugal force

Semi Hermetic Housing

• Semi hermetic- serviceable
• Bolt on heads allow for gasket and plate replacement

Hermetic Housing

• Hermetic compressor housing is welded
• Also known as tin can type compressor
• Usually never repaired, only replaced

Open Drive Compressor

• Open drive compressor is mechanically sealed
• Drive motor is attached to the shaft end
• Designed to be repaired
• Needs shaft seal
• Can be belt or direct drive

Motor Windings Single Phase (PS)

• Start, Common, Run windings
• Start to run reading has highest resistance
• Start to common has second-highest resistance
• Run to common has lowest resistance
• Start to common + run to common = start to run

Motor Windings PSC

• 6 ohms (2nd highest) - Start
• 7.5 ohms (highest) - Common
• 1.5 ohms (lowest) - Run

Internal/External Overloads

• Internal/external overloads break common winding, killing power to motor
• Resistance readings from start to run, but OL from R to C or S to C

Motor Windings 3 Phase

• All resistance values from each winding are equal (e.g., T1 to T2 = 1.8 ohms, T2 to T3= 1.8 ohms, T1 to T3= 1.8 ohms)
• Overloads break each leg of power, single phasing and damage if a leg is removed

Compression Ratio

• Absolute Discharge / Absolute Suction
• Discharge pressure + 14.7 PSI = absolute discharge
• Suction pressure + 14.7 PSI = absolute suction
• Higher ratio requires more work for the compressor to move the refrigerant
• Example: 418 PSIG discharge, 118 PSIG suction, ratio is 3.2:1

Splash Lubrication

• Found in smaller compressors without oil pumps/delivery systems
• Compressor oil is splashed around by using crankshaft/connecting rods to scoop oil from sump

Oil Pump Feed System

• Used in larger compressors
• Pump used to force feed oil
• Lubricates bearings, crankshaft/connecting rods
• Typically mounted to the end of the compressor crankshaft
• Can be electronically driven

Cooling the Compressor

• Suction gas returning to the compressor cools motor
• Air cooled uses a fan blowing air over the compressor housing
• Water cooled uses copper coil piping around the compressor housing with water circulating to cool the compressor

Common Electrical Failures

• Shorted/grounded winding
• Moisture contamination (oil turns acidic and eats motor windings) - proper evacuation is critical
• Overheating of motor windings due to lack of suction gas
• Example: T1 to G = 142 ohms

Common Mechanical Failures

• Oil return issues resulting in mechanical failure
• Springs in reciprocating compressors causing motor knock
• Reed valves faulty, causing high discharge pressure into low-pressure suction chamber
• Scroll plate wearing from lack of oil
• Discharge check valve worn, causing flooded starts

Compressors Recap

• Air Conditioning Compressor Basics
• Diagnosing a Locked Compressor 3D

This quiz covers the fundamental principles and components of compressors, focusing on their operation and significance within refrigeration systems. Understand the functionality of positive displacement compressors and the mechanics behind reciprocating compressors.