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Modern vapor-compression systems for comfort cooling and industrial refrigeration utilize different types of compressors: reciprocating, rotary, helical (screw type), centrifugal, and scroll.

In some configurations, an external motor drives the compressor (termed as open-drive systems). These systems are easier to service but the seal on the compressor crankshaft drive end can be prone to leaks. Open-drive systems typically use "V" belts or flexible "couplings" to transmit power from the motor to the compressor.

The second major category is the hermetic system, where the motor is placed inside the same housing as the compressor. Here, the motor is cooled by refrigerant vapor rather than outside air, and the crankcase serves as the intake manifold. Hermetic systems have fewer leak problems as they have no crankcase seal, though they are harder to service. However, some components prone to failure are usually placed outside the housing, and connected by leak-proof devices. Motors in hermetic systems cannot emit electrical arcs (thus no brushes) as it would contaminate the refrigerant oil and cause motor burnout.

Hermetic systems are classified as full hermetic or serviceable hermetic (semi-hermetic). Many hermetic compressors have welded housings that aren't serviceable, thus requiring the entire unit to be replaced if the motor or compressor fails. Semi-hermetic systems, commonly used in large reciprocating, centrifugal, screw, and scroll compressors, can be dismantled for major service operations as the housing is bolted and gasket-sealed.

COMPRESSOR COOLING

Compressors generate significant heat during the compression of refrigerant vapor. Most heat travels with the high-pressure vapor to the condenser, but the compressor head also needs to dissipate unwanted heat to remain within safe operating temperatures. This is typically achieved with fins or water passages.

In hermetic and semi-hermetic systems, the suction line feeds cool refrigerant to the cylinder heads, making the temperature and pressure of the suction gas crucial for maintaining proper compressor body temperature. Suction gas should not exceed 65°F (18°C) in low-temperature installations or 90°F (32°C) in high-temperature systems to ensure the vapor is dense enough to pick up heat effectively. The low-pressure cutout control protects the motor from inadequate suction line pressure.

Air-cooled open-drive compressors can be cooled by placing them in the blast of the condenser fan, or by dedicating a fan exclusively for compressor cooling. Water-cooled compressors use jacketed heads to allow water circulation for cooling.

CENTRIFUGAL COMPRESSOR

Centrifugal compressors employ impellers that spin rapidly and use centrifugal force to fling the refrigerant away from the center intake. As each impeller adds little pressure, multiple impellers are often used together to achieve the required discharge pressure. Centrifugal compressors are typically found in large systems, either in semi-hermetic or open configurations. They can operate with positive suction pressure or in a vacuum, depending on the refrigerant and evaporator temperature requirements, and may be shipped pre-charged with refrigerant and oil.

The main moving parts subject to wear in centrifugal compressors are the shaft bearings. Compressor discharge pressure is determined by gas density, impeller diameter and design, and impeller speed. These impellers rotate very rapidly:

• Low speed: 3,600 RPM
• Medium speed: 9,000 RPM
• High speed: above 9,000 RPM

Electric motors or steam turbines supply power. Vapor enters the center of the impeller around the shaft and is directed through the blades. As the impeller accelerates the gas, its kinetic energy transforms into potential energy as it compresses. The gas exits the impeller at high velocity.

Capacity can be controlled by inlet vanes that regulate the amount and direction of refrigerant vapor from the evaporator. If the system has more than three stages, inlet vanes may be omitted.

Centrifugal compressors must be protected from refrigerant flood back, as the high-speed impellers can be damaged. A built-in purge device removes unwanted air, ensuring the system functions optimally.

REFRIGERATION SYSTEM CHILLER COMPONENTS

Figure 6-1: Two-stage centrifugal compressor. Second-stage variable inlet guide vane, first-stage impeller, second-stage impeller, water-cooled motor, base, oil tank, and lubricating oil pump assembly, first-stage guide vanes and capacity control, labyrinth seal, cross-over connection, guide vane actuator, volute casing, pressure-lubricated sleeve bearing (discharge opening not shown).

Figure 6-2: Hermetic centrifugal liquid chiller. Depending on the refrigerant, the system offers flexibility in operations and can be controlled via a microprocessor. Shows the refrigeration cycle in a cutaway view.

HELICAL SCREW COMPRESSORS

Screw-type compressors are typically used in systems with a capacity above 20 tons of refrigeration. They use paired helical screws, or rotors, that rotate together inside a chamber, forcing refrigerant from the intake (low side) towards the high side, compressing it in the process. These compressors do not need valves except at intake and exhaust ports. Continuous rotor spin results in minimal vibration.

Helical (screw) compressors may be configured as open-drive or hermetic systems. The male rotor drives, and the female rotor follows. Capacity control is achieved through a slide valve that allows some vapor to exit without compression. Some units can operate efficiently at just 10% capacity.

Figure 6-4: Basic operation of screw compressor. Includes stages showing vapor compression through rotor interlobe spaces.

RECIPROCATING COMPRESSORS

Reciprocating compressors use pistons sliding inside cylinders to compress refrigerant vapor. This mechanism is shown in basic construction diagrams, illustrating how refrigerant vapor moves through intake and exhaust valves during piston strokes.

Reciprocating compressors vary in size from small, two-piston systems to large industrial multi-cylinder compressors. The compressor crankcase, usually made of cast iron with fins, dissipates generated heat. In hermetic and semi-hermetic compressors, suction line refrigerant cools the system. Larger compressors have separate oil and compression rings for optimal performance.

The crankcase shaft converts rotary motion to reciprocating motion, driving the pistons. Different linkages connect the rod to the crankshaft:

• Conventional connecting rod (most common)
• Eccentric crankshaft (eliminates the need for caps or bolts)
• Scotch yoke (moves the piston without a rod, found mainly in domestic systems)

Open-drive systems commonly face issues with crankcase seals but are mitigated in hermetic designs. Misalignment during motor-to-compressor shaft connection can stress seals, leading to leaks detectable by refrigerant leak detectors. Properly align the shafts to prevent chronic issues.

RECIPROCATING COMPRESSOR HEADS AND VALVE PLATES

Cylinder heads, primarily made of cast iron, ensure gaskets seal between components. Head passages allow gas suction, and cap screws hold heads in place. Valves manage refrigerant intake and discharge, seated on the valve plate. Intake valves operate cooler with constant oil vapor lubrication, while discharge valves handle higher temperatures and require precise fitting to avoid issues like carbon buildup.

The compressor's efficient operation depends on proper valve function, with design considerations for opening and closure sizes to manage refrigerant flow and maintain system performance.

Figure 6-7: Reciprocating compressor valve plate assembly diagram.

Figure 6-8: Hermetic reciprocating compressor. Showcasing banks of cylinders and bolted design for servicing ease.

ROTARY COMPRESSOR

Rotary compressors use blades within cylinders for compression, featuring rotating and stationary configurations:

1. Rotating blades (vane)
2. Stationary blade (divider block)

Both designs allow gas compression without pistons, using centrifugal force and eccentric shaft revolutions. Larger intake (suction) ports compared to discharge ports facilitate efficient gas movement, with check valves preventing oil and high-pressure vapor backflow when the compressor is off.

ROTATING BLADE (VANE) COMPRESSOR

In a rotating vane setup, a non-centered rotor (shaft) within the cylinder compresses gas by sweeping it with sliding vanes. The gas volume decreases as it compresses, achieving high compression efficiency due to minimal clearance volume.

Rotary vane compressors are commonly used in cascade systems, with designs varying from two to eight vanes depending on system size. Precision-engineered blade edges and slots ensure smooth operation and minimal leakage.

Figure 6-9: Rotary blade compressor diagram with vapor flow directions.

STATIONARY BLADE (DIVIDER BLOCK) ROTARY COMPRESSOR

This system uses a sliding blade to separate low and high-pressure vapor within the cylinder. An eccentric shaft's impeller rubs against the cylinder wall, trapping and compressing vapor until it discharges.

Figure 6-10: Diagram of rotary compressor with stationary blade or divider block.

Figure 6-11: Hermetic, single stationary-blade rotary compressor.

SCROLL COMPRESSOR

Kehong

Scroll compressors use two scroll elements for compression: an orbiting and a fixed scroll. The fixed scroll remains stationary while the orbiting scroll moves in a circular path around it, generating compressive pockets. As these pockets move toward the center, compression increases until gas discharges at the center port. Multiple pockets operate simultaneously for nearly continuous compression.

Figure 6-12: Scroll compression process with orbiting and stationary scrolls.

The continuous process results in smooth operation without conventional valves. A check valve prevents reverse operation post-power-off, ensuring reliability.

Figure 6-13: Cross-section of a swash plate reciprocating compressor.

Suction from the outer portion and discharge from the inner portion create a continuous process with smooth operation.

OIL SYSTEMS FOR COMPRESSORS

Reciprocating compressors generally use two lubrication systems:

1. Splash system (less effective, can cause noise)
2. Oil pressure system (uses a pump for efficient lubrication)

Rotary compressors require oil for cylinders, blades, and rollers. Centrifugal compressors, running at high speeds, often have elaborate oil control systems, including pumps, oil separators, reservoirs, filters, and coolers. Helical screw compressors need oil for cooling, sealing, and silencing, often using a positive displacement pump for independent operation. Scroll compressors use oil for cooling and sealing, driven by centrifugal action.

Industrial refrigeration systems commonly include an oil separator, oil level regulator, and oil reservoir. Regular oil tests are essential for detecting acidity and maintaining system health.

Promoting oil return ensures efficient operation, needing appropriate refrigerant velocity in evaporator tubes. Inclining refrigeration lines towards the compressor, using right-sized suction lines, and monitoring oil viscosity and refrigerant solubility are crucial. Proper design reduces oil return issues, especially in low-temperature evaporators.

DISCHARGE LINE

The discharge line, connecting the compressor to the condenser, often uses copper tubing and includes components like vibration absorbers, mufflers, oil separators, and controls. Effective vibration absorption reduces noise and enhances longevity. Mufflers mitigate reciprocating compressor discharge pulsations, and oil separators collect and recycle oil efficiently, ensuring clean operation.

CONDENSER

The condenser, a high-side refrigeration component, expels latent heat from refrigerant gas, converting it to high-pressure liquid. It expels roughly 1.25 times the heat absorbed in evaporators due to inefficiencies. Condensers can be water-cooled or air-cooled, each with pros and cons. Water-cooled units are efficient but may face water scarcity or chemical issues, while air-cooled units avoid water-related problems but need proper maintenance for optimal performance.

AIR-COOLED CONDENSER

Air-cooled condensers use fans to move air across tubes and fins. Designed for the hottest conditions, they must be kept clean for effective heat transfer. Outdoor units in cold weather need special designs to maintain head pressure and prevent compressor oil dilution.

For optimal operation in cold conditions, systems may require:

1. Weatherproof housing
2. Compressor short cycle prevention methods
3. Winter head pressure control
4. Compressor oil dilution prevention techniques

For more information, please visit 30P refrigerating machine factory.

Disclaimer - While Berg Chilling Systems Inc. ("Berg") makes reasonable efforts to provide accurate information, no representations or warranties are made regarding any content accuracy. We assume no liability for any errors or omissions. Content is subject to modification without notice.

Oldrich Bocek (1939-2003)
Thermal Management Expert
Berg Chilling Systems Inc.

HVAC Refrigeration Cycle: How It Works

The HVAC refrigeration cycle isn’t limited to refrigeration systems. It applies across various cooling systems, from small home AC units to large industrial chiller systems.

Understanding this cycle is crucial for HVACR technicians, as it underpins the cooling processes in all these systems.

Four Components of the HVAC Refrigeration Cycle

The refrigeration cycle involves four key components:

• Compressor
• Condenser
• Metering device
• Evaporator

The refrigerant circulates between these components, changing pressure and state to absorb and release heat.

The Science Behind the Refrigeration Cycle

The following physics principles are essential for understanding the cycle:

• Heat transfers but isn’t created or destroyed; cold is the absence of heat.
• Heat flows from warmer to cooler areas.
• Vapor pressure and temperature rise and fall together.
• Pressure affects the boiling point of liquids.
• Evaporation occurs at high temperature and pressure, condensation at low temperature and pressure.

Applying Physics in the Refrigeration Cycle

In an AC system, refrigerant boils at low temperatures. Pressure changes increase or decrease refrigerant temperature, facilitating its cycle through liquid and vapor states for heat transfer.

Roles of the Components in the Cycle

• Compressor: Moves refrigerant, creating high and low-pressure sides.
• Condenser: Cools superheated refrigerant, turning it into a liquid.
• Metering device: Drops pressure, creating a cold liquid-vapor mix.
• Evaporator: Absorbs heat, cooling air and boiling refrigerant back to vapor.

Each component’s function helps technicians analyze and diagnose system performance issues.

HVACR Technician Knowledge

A complete understanding of the refrigeration cycle, including auxiliary components like accumulators and refrigerant lines, is crucial for HVACR technicians. This awareness aids in the effective maintenance and troubleshooting of cooling systems.

To learn more about technical aspects of HVACR, check out our training videos.

HVACR Career Connect NY was created to promote the benefits of a career in HVAC and Refrigeration service, providing a resource for employers and technicians in the New York City metro area. For more information, visit 30P refrigerating machine factory.