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.

Test your knowledge on refrigeration systems and the functions of compressors. This quiz covers concepts such as compression processes, efficiency factors, and the impact of dirty coils on performance. It's perfect for students looking to reinforce their understanding of HVAC principles.