Basic Refrigeration Cycle 11 Chapter Basic Refrigeration Cycle

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.

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

Refrigeration System Components

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

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

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

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

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

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

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

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)

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

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

Condition of Refrigerant in Each Component

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

Evaporator Types Shell-type Shelf-type Wall-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

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

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

Suction Line Insulation To limit uncontrolled superheating of gas that would reduce compressor efficiency To eliminate condensation that could cause frost and ice problems

Compressor Styles Open Semihermetic Hermetic 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

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)

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

Domestic Condensers Finned-tube, forced convection Wire static 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

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

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

Cooling Tower Lowers water temperature for reuse in a water-cooled condenser Usually located outdoors where ambient air can cool the water

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