29
Chapter
Basic Refrigeration Cycle
11
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Objectives
• Draw a diagram of the basic refrigeration cycle.• Describe the condition of the refrigerant in each
component.• Explain the purpose of each component of the
refrigeration system.• Identify refrigeration system component
variations.
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Basic Refrigeration System
• All systems control refrigerant flow• Basic refrigeration cycle
– Refrigerant absorbs unwanted heat at one location and releases it at another location
• Refrigerants absorb heat by evaporating and release heat through condensing
• A thorough understanding of system operation is needed to be a technician
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Refrigeration System Components
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Evaporator
• A heat-exchanging device located in the conditioned space
• Performs the main purpose of the system: refrigeration
• Liquid refrigerant entering the evaporator leaves as a vapor
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Superheat
• Temperature above saturation point
• Normal superheat at the evaporator outlet is 10°F (5.5°C)– Provides maximum
cooling effect and prevents liquid from leaving the evaporator
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Evaporator Temperature Difference
• Refrigerant inside evaporator tubing is in a low-temperature, low-pressure saturated condition
• Evaporator temperature difference (td) must exist for heat to transfer from hot to cold
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Suction Line
• Usually made of copper tubing• Connects evaporator outlet to compressor inlet• Insulated to prevent condensation and additional,
unwanted superheat• Contains low-temperature, low-pressure
superheated vapor
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Compressor
• Separates low-pressure from high-pressure side
• Removes vapor from evaporator to maintain its low boiling point
• Compresses and moves gas through system
• Reciprocating type is most common
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Hot-Gas Discharge Line
• Copper tubing that carries discharged hot gas from the compressor to the condenser
• Smaller than suction line because the gas has been compressed to a smaller volume and higher pressure
• Hottest part of the system outside the compressor
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Condenser
• A heat exchanger, designed to release heat
• Purpose is to remove heat from the superheated refrigerant vapor– Causing vapor to
condense back to a liquid
• Can be air-cooled or water-cooled
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High Pressure in Condenser
• Raises the saturation point of the refrigerant so that ambient air can:– Remove superheat to the saturation point (sensible
heat, lowering temperature)– Remove enough heat to cause the refrigerant to
condense back to a liquid (latent heat)– Remove enough heat to partially subcool the liquid
before it leaves the condenser (sensible heat, lowering temperature again)
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Liquid Line
• Copper tubing line connects the condenser outlet to the refrigerant metering device
• Should contain only liquid refrigerant (no vapor) subcooled 10°F (5.5°C)
• Liquid refrigerant is still under high pressure, and tubing is slightly warm to the touch
• Refrigerant inside is a high-temperature, high-pressure, subcooled liquid
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Refrigerant Metering Device
• Controls the flow of liquid refrigerant into the evaporator to ensure that all liquid is boiled off before it enters the suction line
• Second division point: between the high- and low-pressure sides of the system
• Various types of refrigerant metering devices available for different applications
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Condition of Refrigerant inEach Component
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Evaporators
• Manufactured in many shapes and sizes to fill specific design or operational needs
• Can operate on the principles of conduction or convection
• Convection can be natural or forced• Classified into five types
– Shell, Shelf, Wall, Plate, Finned-tube with forced convection
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Evaporator Types
• Shell-type– Tubing run along the inner wall of freezing
compartment to form a shell• Shelf-type
– Aluminum tubing forms shelves inside a freezer• Wall-type
– Used in chest-type domestic freezers– Tubing attached to the surface of inner cabinet
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Evaporator Types (Cont.)
• Plate-Type– Not commonly used– Refrigerant flows through plates– Uses natural convection
• Finned tube with forced convection– Most widely used in all applications– Allows much refrigeration with a small evaporator– Fins increase surface area
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Suction Line
• Factors determining size of suction line– Refrigerant velocity, pressure, volume, density, and
pressure drop• Very long line or having many bends can increase
pressure drop• Line should be installed to help all circulating oil
return to the compressor
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Suction Line Insulation
• To limit uncontrolled superheating of gas that would reduce compressor efficiency
• To eliminate condensation that could cause frost and ice problems
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Compressor Styles
• Open– Belt-driven or directly driven (using a coupling) by
an external electric motor• Semihermetic
– Electric motor and compressor sealed within– May be disassembled and opened for service
• Hermetic– Motor and compressor sealed in a solid steel body– May not be opened for service
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Condensers
• Must remove heat from refrigerant at same rate heat is being absorbed
• For proper condenser operation, compressor must increase refrigerant pressure to saturation temperature– 30°F to 35°F (17°F to 19°C) higher than ambient air
entering the condenser (for air-cooled)– 20°F (11°C) higher than temperature of the water
exiting the condenser (water-cooled)
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Condenser Cooling Stages
• First, the superheat of the gas is removed, cooling the gas down to the saturation point– Sensible heat removed
• Second, the gas is condensed to a liquid– Latent heat removed
• Third, the liquid is subcooled below its saturation point– Sensible heat removed
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Domestic Condensers
• Finned-tube, forced convection– Most commonly used condenser– Fan creates airflow for convection
• Wire static– Natural convection with wires attached to tubing
• Wall static– Natural convection with a box inside a box, having
condenser tubing mounted along outer wall
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Commercial Air-Cooled Condensers
• Heat is removed through airflow
• Most common type is forced convection
• Vary greatly in size, style, and capacity
• Some mounted vertically, some horizontally
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Commercial Water-Cooled Condenser
• Heat is removed through conduction into flowing water
• Very efficient at cooling refrigerant
• Water-related problems• Types available
– Tube-in-a-tube– Tube-in-a-shell– Coil-in-a-shell
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Cooling Tower
• Lowers water temperature for reuse in a water-cooled condenser
• Usually located outdoors where ambient air can cool the water
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Water-Regulating Valve
• Controls amount of water traveling through the condenser
• A bellows and diaphragm assembly connects by capillary tube to the high-pressure side of the compressor
• As head pressure increases, the valve opens more to permit more waterflow
• Spring tension is adjustable and permits control of head pressure
