Air -Cooling Evaporators

• Types of construction

• Circuit Configurations

• Methods of Refrigerant Feed

• Methods of Air Circulation

• Methods of Defrost

Type of Construction

• Bare tube

• Finned Tube

• Plate-surface

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.

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.

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.

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.

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.

Circuit Configuration• Single Circuit Evaporators

• Split Circuit Evaporators

• Cross -Flow Evaporators

• Counter-Flow Evaporators

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.

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.

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.

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.

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.

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.

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.

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.

Method of Refrigerant Feed

• Dry-Expansion Evaporators

• Circulated Refrigerant Evaporators

• Flooded Evaporators

• Thermosyphon Evaporators

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.

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.

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.

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.

Method of Air Circulation

• Natural Convection

• Forced Convection

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.

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.

Method of Defrost• Off-Cycle Defrost

• Electric Defrost

• Hot Gas Defrost

Method of Defrost• Heated Air Defrost

• Water Defrost

• Hot Brine Defrost

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.

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.

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.

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.

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.

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.

Liquid -Chilling Evaporators

• Double -Pipe Coolers

• Baudelot Coolers

• Tank-Type Coolers

• Shell-and -Coil Coolers

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.

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.

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.

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.

Liquid -Chilling Evaporators

• Shell-and -Tube Chiller Bundles

• Flooded Chiller Barrels

• Dry-Expansion Chiller Bundles

• Spray -Type Chiller Barrels

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.

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.

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.

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.

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.