Refrigeration Systems and Compressors Quiz

Compressor Overview

• A compressor is crucial in refrigeration systems, driving refrigerant throughout the entire cycle.
• Its primary function is compressing low-pressure, low-temperature refrigerant gas from the evaporator into a high-pressure, high-temperature gas.
• This increase in pressure and temperature allows refrigerant to release heat more efficiently in the condenser, facilitating effective heat absorption (cooling) and rejection.

Compressor Function

• The compressor drives the refrigerant through the refrigeration cycle, increasing its pressure and temperature.
• It moves the refrigerant from the evaporator, then releases heat in the condenser to maintain the desired cooling within the system.

Compression Ratio

• Measures how much the compressor increases the refrigerant pressure from the low side (evaporator) to the high side (condenser).
• Formula: Compression Ratio = High-Side Pressure (psia) / Low-Side Pressure (psia)
• Note: Add 14.7 to gauge pressure (psig) to get absolute pressure (psia).

Importance of Compression Ratio

• Ensures efficient system performance.
• High ratios can lead to overheating of compressor oil, reduced refrigerant flow, and lower system capacity.

Example Compression Ratio Calculation

• Given: Low-side pressure = 49 psig, High-side pressure = 278 psig
• Converting to psia: Low-side = 64 psia, High-side = 293 psia
• Calculating Compression Ratio: 293 psia / 64 psia = 4.6

Specific Compression Ratios

• Air conditioning (AC) compressor at 40°F evaporator and 125°F condensing = 3.5:1 ratio
• Freezer compressor at -20°F evaporator and 125°F condensing = 11.8:1 ratio

Difference in Compression Ratios

• Refrigeration systems require more significant pressure increases because they operate at lower temperatures compared to air-conditioning systems, leading to higher compression ratios.

Two-Stage Compression

• Two-stage compression is a process where refrigerant compression occurs in two steps, separated by an intercooling stage.
• First stage: Compresses refrigerant from low pressure to an intermediate pressure.
• Intercooling: Cools the refrigerant to reduce its temperature before the second stage to minimize work required by the second compressor.
• Second stage: Further compresses the refrigerant to the condenser pressure.

Types of Compressors

Reciprocating Compressor

• Also known as a piston compressor.
• Uses pistons driven by a crankshaft to compress air.

Welded Hermetic Reciprocating Compressors

• Commonly used in refrigeration and air conditioning systems.
• Fully sealed housing, typically made of steel, welded shut to prevent refrigerant leakage, ensuring long-term reliability.

Semi-Hermetic Compressors

• Refrigeration or air conditioning compressor where the motor and compressor are housed together in a single shell but not completely sealed.
• Unlike hermetic compressors, these can be opened for maintenance or repairs due to the use of gaskets and bolts to seal the casing.

Belt-Driven Compressor

• Used for refrigerating and heating cycles in refrigeration and HVAC systems.

Direct-Driven Compressor

• Air compressor type where the motor or engine is directly connected to the compressor pump, typically through a single shaft.

Screw Compressor

• A type of gas compressor that uses a rotary mechanism, specifically a pair of interlocking helical rotors or screws.

Rotary Compressor

• Also known as an air compressor or refrigerant compressor.
• Accepts low-temperature refrigerant gas, then uses a revolving mechanism to compress it into high-pressure gas, which then passes through an accumulator/

Scroll Compressor

• Two spiral-shaped components compress refrigerant via rotating interlocking action, resulting in quiet and efficient operation.

Open-Drive Compressor

• Uses an external motor to drive the compressor through a direct connection or a belt system.
• Primary function is compressing gases (like air or refrigerants) for various industrial, commercial, or residential applications.

Centrifugal Compressor

• A type of dynamic compressor used to increase the pressure of a gas by converting its kinetic energy into potential energy
• Commonly used in industrial applications to compress gases (air, natural gas, refrigerants).
• Used extensively for air conditioning in large structures

Reciprocating Compressor Components

• Crankshaft, Connecting Rod, Pistons, Refrigerant cylinder valves (suction), Refrigerant cylinder valves (discharge), Compressor head, mufflers, compressor housing, Suction line, Discharge line, Discharge valves, Rings

Belt-Drive Mechanisms

• System used to transmit mechanical power or motion between two or more rotating shafts using a flexible belt.

Compression Process - Stages

Compression

• Piston starts to move upwards in the cylinder.
• Suction valve closes and discharge valve remains closed.
• As the piston moves upward, the volume in the cylinder decreases; pressure of the refrigerant increases.
• Compression continues until pressure in the cylinder rises just above discharge pressure.

Discharge

• When cylinder pressure rises above discharge pressure, the discharge valve opens and the suction valve remains closed.
• As the piston continues to move upwards, refrigerant is discharged from the compressor.
• Discharge continues until piston reaches top dead center.

Suction

• The compression process-suction is the stage in a compressor where the intake valve opens, allowing low-pressure gas or air to enter the compression chamber before being compressed.
• 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 (bottom dead center).
• At the bottom of the stroke, suction valves close.

Liquid in the Compression Cylinder

• If liquid enters the cylinder, damage will occur.
• Liquid slugging can cause immediate damage to the compressor components.
• Liquids are incompressible, unlike gases that are compressible.
• Compressors are designed to handle gases which contract and expand as pressure is applied, while liquids cannot compress resulting in pressure buildup.

Reciprocating Compressor Efficiency

• Determined by the initial compressor design.
• Four processes occur in the compression process: Expansion/Re-expansion, Suction (intake), Discharge, and Compression.

System Maintenance and Compressor Efficiency

• High suction pressures and low discharge pressures keep the compression ratio low.
• Dirty evaporators cause suction pressure to drop, reducing compressor pumping capacity.
• Dirty condensers increase head pressure.
• Compression ratio is increased by dirty or blocked condenser and evaporator coils.