Air-Cooling Evaporatorsokanagancollegefoundationrefrigeration.pbworks.com/w/file/fetch... ·...
Transcript of Air-Cooling Evaporatorsokanagancollegefoundationrefrigeration.pbworks.com/w/file/fetch... ·...
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Air -Cooling Evaporators
• Types of construction
• Circuit Configurations
• Methods of Refrigerant Feed
• Methods of Air Circulation
• Methods of Defrost
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Type of Construction
• Bare tube
• Finned Tube
• Plate-surface
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Bare tube• constructed with bare steel, copper and
aluminum pipe or tubing.
• Bare tube coils are usually custom made for a specific application.
• Common shapes are flat zig -zag or oval trombone and spiral.
• Common bare tube evaporator applications are glycol chiller pack or pre -chilling tank.
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Finned Tube
• Finned tube evaporators are bare tube coils with metal plates or fins added to the coils.
• On bare tube evaporators the majority of the are flowing over the coil passes between the tubes.
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Finned Tube• Adding fins to a bare tube coil increases the
bare tube coil contact surface area with the air passing over the coil, thereby maximizing the heat transfer process.
• The fins must be connected to the tubing in such way to ensure excellent thermal conductivity between the fins and the tubes.
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Plate-surface
• There are several methods of constructing plate type evaporator coils.
• One design employs two aluminum sheets embossed together to form a channel for refrigerant to flow.
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Plate-surface• Another design employs a bare tube
evaporator coil either sandwich between two sheets of aluminum
• Also the bare tube coil can be bonded directly to one side of an aluminum plate or metal rack.
• Commonly used in transport refrigeration, and reserve capacity setups for liquid chilling HVAC systems.
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Circuit Configuration• Single Circuit Evaporators
• Split Circuit Evaporators
• Cross -Flow Evaporators
• Counter-Flow Evaporators
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Single Circuit Evaporators
• Single circuit evaporators employ only one path for the refrigerant to flow.
• The liquid refrigerant enters at the top and the vapor exits at the bottom.
• This setup is commonly found in small commercial refrigerators, freezers and dehumidifiers.
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Split Circuit Evaporators
• This method of construction splits the individual single circuit evaporator into two equal length parallel circuits coils.
• The increasing pressure drop occurring in the single evaporator can be reduced by employing the split circuit evaporator.
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Split CircuitEvaporators
• Splitting the refrigerant flow into two equal parts while reducing pressure drop also reduces refrigerant velocity in the coil circuits.
• Reducing refrigerant velocity can cause oil return issues from the evaporator coil.
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Cross -FlowEvaporators
• The term Cross -Flow refers to the direction of travel the refrigerant takes in relation to the air or fluid flow.
• In Cross -Flow evaporators the refrigerant flows perpendicular (90 °) to the air flow.
• Cross -Flow Heat Exchange is employed in various applications other than just refrigerants and air flow.
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Counter -Flow Evaporators
• The term Counter-Flow refers to the direction of travel the refrigerant takes in relation to the air or fluid flow .
• In Counter-Flow heat exchange design the refrigerant and air flow in opposite directions.
• The coldest refrigerant entering the evaporator is acting on warmest point in the air or fluid stream.
• Counter-Flow Heat Exchange design improves the efficiency of the heat transfer process.
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Liquid Evaporator Headers
• Liquid headers are utilized to simultaneously distribute refrigerant to the individual coil circuits.
• Liquid headers tend to not evenly distribute the saturated refrigerant mixture evenly.
• This causes some circuits to balanced while others are starved of refrigerant.
• Therefore liquid headers are usually only used in flooded evaporator coils.
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Suction Evaporator Headers
• Suction headers are applied to multi-circuit evaporator designs.
• On the suction side of the evaporator where only vapor is present, the concern of unequal refrigerant distribution is negated.
• Suction headers can be implemented in both dry -expansion and flooded type evaporator processes.
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Refrigerant Distributors
• Distributors are utilized in multi-circuit evaporators.
• Distributors are designed to route the same percentage of saturated refrigerant to each evaporator circuit.
• This cancels out the problem of unequal distribution by the liquid header manifold.
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Method of Refrigerant Feed
• Dry-Expansion Evaporators
• Circulated Refrigerant Evaporators
• Flooded Evaporators
• Thermosyphon Evaporators
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Dry-Expansion Evaporators
• Dry-Expansion evaporators (DX Coils) employ a method of refrigerant feed where both liquid and vapor refrigerant are present through out the coil.
• DX coils utilize a feed method that limits the liquid entering the evaporator to max amount that can be boiled off before the suction end of the evaporator.
• To ensure complete vaporization the refrigerant in the evaporator is allowed to be superheated, thereby ensuring no liquid leaving the coil.
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Circulated Refrigerant Evaporators
• This design of evaporator employs a liquid refrigerant pump. AKA liquid overfeed.
• The circulation pump moves the liquid refrigerant through the evaporator circuits.
• The circ pump draws liquid refrigerant from the bottom of a accumulator or surge drum.
• Utilized in flooded evaporator designed systems.
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Flooded Evaporators• Flooded evaporator have extensive wet
surface areas, the evaporator is nearly filled completely with liquid refrigerant.
• Flooded evaporator systems are very similar to the liquid overfeed system.
• The main difference is liquid overfeed systems utilize a circ pump, while flooded systems utilize gravity to move refrigerant through the evaporator circuits.
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Thermosyphon Evaporators
• Thermosyphon evaporators employ a passive method of heat exchange.
• Thermosyphon systems are based on the natural convection currents occurring in the liquid refrigerant.
• This natural method of circulation eliminates the requirement for complex liquid circulation pump system.
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Method of Air Circulation
• Natural Convection
• Forced Convection
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Natural Convection• Natural Convection Evaporators are
implemented where low air velocities are desired.
• We also use natural convection coil designs where damaging dehumidification can occur.
• Cold air is more dense then warm air, as the air is cooled it falls being replaced by warmer less dense air creating natural air circulation.
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Forced Convection• Forced convection evaporator coils are also
know by unit coolers, fan coils and blower coils.
• These unit designs implement a fan system to force the air across the evaporator coil.
• The air velocity across the coil is a product of it’s application, where high humidity levels are required low air velocity evaporators are utilized.
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Method of Defrost• Off-Cycle Defrost
• Electric Defrost
• Hot Gas Defrost
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Method of Defrost• Heated Air Defrost
• Water Defrost
• Hot Brine Defrost
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Off -Cycle Defrost• This method of defrost is utilized in
applications where the cabinet temperature is maintained above 34 °F (2.8°C)
• During this defrost cycle the compressor is cycle off, while the evaporator fans are kept running to cycle.
• Maintaining airflow while the compressor is off allows the warmer box air circulated over the evaporator to melt the frost build -up.
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Electric Defrosting• Electric defrost is used in applications
where the box temperature is below 34 °F.
• During the defrost cycle evaporator fans are cycled off while the electric heating elements along side the evaporator coil are engaged.
• Also during defrost there are drain pan and drain line heaters to ensure proper exit of condensate from the freezer cabinet.
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Hot Gas Defrosting• Hot Gas defrost routes hot discharge
refrigerant from the compressor directly into the evaporator.
• During the defrost cycle the evaporator fans are cycled off while the compressor continues to run.
• This method of defrost is the fastest, simplest and most efficient of all the defrost methods.
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Heated Air Defrost• Heated air and electric defrost are very similar
in operation.
• The hot air system circulating through the evaporator has to be isolated from the cabinet air.
• During the defrost cycle the compressor is cycled off, the evaporator dampers are closed and the evaporator fans remain running. A heating element or hot gas is energized warming the coil surfaces.
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Water Defrosting• In the water defrost cycle the compressor
and fan motors are cycled off and a water solenoid is opened.
• Opening the water solenoid directs water to the evaporator and is sprayed over the surfaces quickly removing the frost.
• This defrost strategy becomes less acceptable as the cabinet temperature decreases below freezing.
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Hot Brine Defrosting• This method of defrost is often used in
industrial brine cooling applications.
• During the defrost cycle the brine in the evaporator coil circuits is heater.
• The warm brine within the tubes melts the ice accumulated on the coil surfaces.
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Liquid -Chilling Evaporators
• Double -Pipe Coolers
• Baudelot Coolers
• Tank-Type Coolers
• Shell-and -Coil Coolers
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Double -Pipe Coolers• The double -pipe liquid cooler contains two
different diameter tubes arranged with the smaller in the centre of the larger tube.
• The chilled liquid circulates through the centre pipe while the refrigerant flows through space between the smaller and larger pipes.
• The main advantages of this system are service access to the fluid coils for cleaning and repairs.
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Baudelot Coolers• A Baudelot cooler is similar in appearance to
the double -pipe setup, the main difference is there is not fluid circulating through a centre pipe.
• The refrigerant circulates through the piping system while the chilled fluid is passed over the outside of the refrigerated piping.
• The chilled liquid open to the air, this type of system is implemented where aeration is required.
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Tank-Type Coolers• Tank type coolers utilize a bare tube
evaporator coil suspended inside a large tank, containing the liquid to be cooled.
• An agitator circulates the liquid over the cooling coil.
• Baffles are utilized to ensure proper fluid mixing and distribution throughout the storage tank.
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Shell -and-Coil Coolers
• This design of evaporator utilizes a bare tube coil enclosed in a welded steel shell.
• These systems are typically operated as dry -expansion type evaporator coils, but can also be a flooded type system.
• The volume of liquid in the shell is greater than the volume of bare tube coil, giving additional holdover capacity for applications having high peak loads.
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Liquid -Chilling Evaporators
• Shell-and -Tube Chiller Bundles
• Flooded Chiller Barrels
• Dry-Expansion Chiller Bundles
• Spray -Type Chiller Barrels
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Shell -and-Tube Chiller Bundles
• This design is implemented in large capacity commercial, industrial and institutional applications.
• A shell-and -tube bundle consists of a cylindrical steel outer shell in which tubes are suspended, the tubes are held in place by baffle supports.
• The tube configuration is a function of the refrigerant feed and the refrigerant type.
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Flooded Chiller Barrels
• In these applications, the refrigerant is contained within the shell and the chilled liquid passes through the tubes.
• The arrangement of the end -plate baffling determines the number of passes the chilled liquid makes through the tubes before leaving the tubes.
• In some flooded chiller barrel designs, the shell is only partially filled with tubes. This design provides a large vapor-disengaging area.
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Dry-Expansion Chiller Bundles
• The principal advantages of the DX chiller bundle over the flooded is it’s smaller refrigerant charge, positive oil return, and reduced chance of tube damage in the event of freeze-up.
• DX chiller barrels can be divided into circuits to maintain refrigerant velocities.
• The refrigerant circuits are produced using baffles cast into the end -plates.
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Spray -Type Chiller Barrels
• In this design the refrigerant contained in the chiller barrel is sprayed over the outside surfaces of the liquid tubes.
• The refrigerant that does not vaporize off the tubes drips into a sump at the bottom of the barrel.
• The principal advantages of this design are it’s high efficiency and relatively small refrigerant charge.
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Direct and Indirect Cooling Systems
• Direct cooling systems transfer heat directly to th e volatile refrigerant.
• Indirect cooling systems incorporate an intermediat e fluid to transport chilled liquid between the refri geration system and the process.
• In many system designs, it is not economical to cir culate direct-expansion refrigerant to all of the zones.
• Long refrigerant lines are seldom practical because they are expensive to install, increase refrigerant char ge, reduce system efficiency and cause oil return probl ems.