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Refrigeration

Components

Refrigeration and Air Conditioning

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Content

Evaporators

Types of Evaporators

Chillers

Condensers

Types of Condensers

Cooling Towers

Expansion devices

Thermostatic Expansion Valve

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Vapor-Compression Refrigeration Components

expansiondevice

A

F condenser

p r e s s u r e

B

CE D

compressor

enthalpy

evaporator

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Evaporators An evaporator is any heat transfer surface in which a volatile

liquid is vaporized for the purpose of removing heat from a

refrigerated space of product.

Evaporator may be classified in a number of different ways such

as: Type of construction

Method of liquid feed

Operating condition

Method of air (or liquid) circulation,

Type of refrigerant control

Applications

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Types of construction There are normally 3 types of evaporator construction:

- Bare-tube - Plate-surface - Finned

Both bare-tube and plate-surface evaporators are classified as prime-surface

evaporators in that the entire surface of these types is in contact with the

vaporizing refrigerant inside.

Bare-tube: Flat zigzag coil Bare-tube: Oval trombone coil

Bar-tube evaporators are usually constructed of either steel pipe or copper tubing.

6Plate evaporator Finned evaporator, FCU

For the finned evaporator, the fins are not filled with refrigerant and are only

secondary surface in that they pick up heat from the air and conduct it to the

refrigerant-carrying tubes. The fins have the effect of increasing the outside

surface area of the evaporator, thus improving its efficiency for cooling air.

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Finned-Tube Evaporator A finned-tube evaporator includes rows of tubes passing through sheets

of formed fins.

Cool, liquid refrigerant flows through the tubes, cooling the tube and fin

surfaces.

As air passes through the coil and comes into contact with the cold fin

surfaces, heat is transferred from the air to the refrigerant, causing the

refrigerant to boil and leave the evaporator as vapor.

Refrigerant vapor

Liquid/vapor

refrigerant

Airflow

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Evaporator Capacity

Evaporator capacity is the rate at which heat will pass through the

evaporator walls from the space or product to the vaporizing liquid

inside by conduction and is influenced by:

– Temperature difference between refrigerant and air or water being cooled

– Flow rate of air or water through evaporator

– Flow rate of refrigerant through evaporator

Q = U x A x LMTD

Q : Quantity of heat transferred (evaporator capacity)

A : Outside surface area of the evaporator

U : Overall conductance factor

LMTD: Logarithmic mean temperature difference

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Assuming that the drop in temperature

occurs at a constant rate as the air

passes through the coil, thetemperature reduction of the air is

represented by a straight line (dashed

line).

Practically, the drop in air temperature

is greatest across the first row of the

coil and reduce as the air passes across

each succeeding row. Thus the actualdrop in air temperature is shown by a

solid curved line.

Logarithmic Mean Temperature Difference

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Mean temperature difference of both lines may be calculated by:

2

)TT()TT(MD RLRE −

=

)TT(

)TT(ln

)TT()TT(LMTD

RL

RE

RLRE

−

−

−=

TE

: Temperature of the air entering the coil

TL

: Temperature of the air leaving the coil

TR

: Temperature of the refrigerant in the tubes

MD : Arithmetic mean temperature difference

LMTD : Logarithmic mean temperature difference

or Mean effective temperature difference (METD)

Note that MD is slightly different from LMTD

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To avoid unnecessary losses in compressor capacity and efficiency, it is

desirable to design the evaporator so that the refrigerant experiences a

minimum drop in pressure.

A certain amount of pressure drop is required, however, to flow the refrigerant

through the evaporator.

The drop in pressure must be sufficient to ensure refrigerant velocities high

enough to sweep the tube surface free of vapor bubbles.

Good design of the evaporator circuiting is required to provide the minimum

necessary pressure drop to produce sufficient refrigerant velocities.

The drop in pressure through any one evaporator circuit will generally depend

on :

The side of the tube

The length of the circuit

The circuit load (the time rate of heat flow through the tube walls of

the circuit).

Evaporator Circuiting

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The circuit load determines the quantity of refrigerant that must pass through

the circuit per unit time.

The greater the amount of refrigerant flowing through the circuit, the greater

will be the pressure drop.

For a given tube size, the greater the load on the circuit, the shorter the circuit

must be in order to avoid excessive pressure drop.

To reduce the pressure drop through the evaporator, the appropriate circuitarrangement is required.

Single series circuit Split refrigerant circuit

Refrigerant distributor and

suction header (widely used)

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Parallel Circuits

To provide uniform heat transfer throughout the coil, the liquid refrigerant is

distributed to the coil tubes in several parallel circuits.

A distributor is used to ensure uniform refrigerant distribution through these

multiple coil circuits. The refrigerant vapor leaves the coil tubes and collects in a suction header.

Liquid/Vapor refrigerant

Refrigerant vapor

Liquiddistributor

Airflow

Suction header

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Face-Split Arrangement

When an evaporator contains more than one liquid-refrigerant distributor, it

is split into independently-controlled sections, each being served by its own

expansion valve.

The three common arrangements for splitting finned-tube evaporator coils

include: Face-split

Intertwined

Row-split

– The face-split coil configuration, also called horizontal-split or parallel-

flow, is split into parallel sections.

– The intertwined coil configuration splits the coil sections by alternating

the tubes fed in each row between two distributors.

– the row-split coil configuration, also called vertical-split or series-flow,

places the independently-controlled coil sections in series in the

airstream.

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Coil Arrangements

Distributors

Face-split or Horizontal-split

Row-split or Vertical-split

Intertwined

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Methods of Refrigerant Feed

Based on the methods of liquid feed, the evaporators can be classified as:

- Dry-expansion - Liquid overfeed - Flooded

With the dry-expansion evaporator, the amount of liquid refrigerant fed into theevaporator is limited to that which can be completely vaporized by the time it

reaches the end of the evaporator.

Dry-expansion

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A liquid overfeed evaporator is one wherein the amount of liquid

refrigerant circulated through the evaporator is considerably in

excess of that which can be vaporized.

The excess liquid is separated from the vapor in a low-pressure

accumulator and recirculated to the evaporator while the vapor is

drawn off to the compressor suction.

Liquid overfeed

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The full-flooded method is operated completely filled with liquid refrigerant,

thus, proving the greatest amount of interior wetted tube surface and the

highest possible heat transfer rate.

– An accumulator serves as a reservoir from which the refrigerant is circulated

by gravity through the evaporator circuits.

– A low-side or high-side float control maintains the liquid level in the

accumulator.

– The vapor generated by the boiling action of the refrigerant in the tubes is

separated from the liquid in the upper part of the accumulator. Therefore, the

flash gas resulting from the reduction of pressure never enters the heat

transfer portion of the evaporator.

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Liquid-Chilling Evaporators

There are 5 general types of liquid chillers that are commonly used:

Double-pipe cooler

Baudelot cooler

Tank-type cooler Shell-and-coil cooler

Shell-and-tube chiller (the most widely used type)

Double-Pipe Cooler Shell-and-Tube Chiller

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Direct Expansion Evaporator

Refrigerant vapor

Liquid

refrigerant

Chilled

water

return

Chilled

water

supply

Tube bundle

Baffles

For the direct expansion (DX) shell-and-tube evaporator, low-pressure liquid

refrigerant flows through the tubes and water fills the surrounding shell.

As heat is transferred from the water to the refrigerant, the refrigerant boils

inside the tubes and the resulting vapor is drawn to the compressor.

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Flooded Shell-and-Tube Evaporator

Low-pressure liquid refrigerant enters the distribution system inside the shell and is

distributed uniformly over the tubes , absorbing heat from relatively warm water that

flows through the tubes.

This transfer of heat boils the film of liquid refrigerant on the tube surfaces and the

resulting vapor is drawn back to the compressor.

Chilled

water

return Liquid level sensor

Tube bundle

Refrigerant vapor

Liquid

refrigerant

Chilled

water

supply

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Packaged Chillers

Centrifugal Water-Cooled Water Chiller

Condenser

Compressor

Control

panel

Evaporator

Starter

Motor

Air-Cooled Water Chiller

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Coolers

CPU Refrigeration Cooler

Thermoelectric Cooler

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Condensers The condenser, like the evaporator, is a heat transfer surface.

Heat from the hot refrigerant vapor passes through the walls of the

condenser to the condensing medium.

The refrigerant vapor is first cooled to saturation and then condensed

into the liquid state.

The major condensing medium employed is either air or water, or a

combination of both.

Condensers are of three general types:

Air-cooled Condenser

Water-cooled Condenser

Evaporative (using both air and water)

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Condenser Capacity

Condenser capacity is influenced by:

– Temperature difference between refrigerant and cooling media

(air, water, or both)

– Flow rate of cooling media through condenser

– Flow rate of refrigerant through condenser

Q = U x A x LMTD

Q : Quantity of heat transferred (condenser capacity)

A : Outside surface area of the condenser

U : Overall conductance factor

LMTD: Logarithmic mean temperature difference

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Quantity and Temperature Rise of Condensing Medium

In both air-cooled and water-cooled condensers, the heat given off by

the condensing refrigerant increases the temperature of the condensing

medium.

The temperature rise of the condensing medium (∆T) is computed by

Qc: Heat rejected at the condenser (kW)

m : Mass flow rate of air or water (kg/s)

c : Specific heat of the condensing medium (kJ/kg oK)

)c)(m(

QT c=Δ

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Centrifugal

fan

Condenser

coil

Centrifugal Fan Air-Cooled Condenser

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Water-Cooled Condensers

The shell-and-tube is the most common type of water-cooled condenser. With

this design, water is pumped through the tubes while the refrigerant vapor fills

the shell space surrounding the tubes.

As heat is transferred from the refrigerant to the water, the refrigerant vapor

condenses on the tube surfaces.

The condensed liquid refrigerant then falls to the bottom of the shell, where it

flows through an enclosure that contains additional tubes (the subcooler).

More heat is transferred from the liquid refrigerant to the water inside these

tubes, subcooling the refrigerant.

Cooling water

Subcooled, liquid refrigerantSubcooler

Hot, refrigerant vapor

85ºF

[29ºC]

95ºF

[35ºC]

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Subcooler

Liquid

refrigerant

Baffle

Tube bundle

Refrigerant vapor

Cooling

tower

water

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Cooling Towers

Sprays

from

condenser

to

condenser

Sump

Propeller

fan

Outdoor

air

85ºF

[29ºC]

95ºF

[35ºC]

Filler

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A cooling tower is a device commonly used to cool condensing water.

Warm water is sprayed over fill in the cooling tower while a propeller fan

draws outdoor air upward through the fill.

The movement of air through the spray causes some of the water to

evaporate, a process that cools the remaining water. This cooled water

then falls to the tower sump to be returned to the condenser.

The evaporation process uses up water to dissipate heat. As the water

evaporates, the dissolved minerals and water treatment chemicals become

concentrated in the sump.

To prevent this solution from becoming concentrated and possibly

corrosive, water is periodically bled from the sump and an equal amount

of fresh water is added.

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The effectiveness of the cooling tower depends upon:

The web bulb temperature of the entering air

The amount of exposed water surface and the length of time of

exposure

The velocity of the air passing through the tower

The direction of the air flow with relation to the exposed water surface

- Parallel - Transverse - Counter

The temperature of the water leaving the tower will usually be 7-10 oC

above the web bulb temperature of the entering air.

The difference between the temperature of the water leaving the tower

and the wet bulb temperature of the entering air is called the Approach

of the tower

The temperature difference between the entering and leaving water is

called the Range of the tower .

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Vary Condenser Water and Air Flow Rates

Cooling tower

Condenser

Variable-speed

drive

Cooling tower

Condenser

Variable-

speed drive

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Evaporative Condensers

sump

Pump

Condenser coil

Refrigerant vapor

Subcooler

Fan

Liquid refrigerant

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A modification of the air-cooled condenser is the evaporative condenser.

Within this device, the refrigerant flows through tubes and air is drawn or

blown over the tubes by a fan. And water is sprayed on the tube surfaces.

As the air passes over the coil, it causes a small portion of the water to

evaporate. This evaporation process absorbs heat from the coil, causing the

refrigerant vapor within the tubes to condense. The remaining water then

falls to the sump to be recirculated and used again.

Subcooling of the refrigerant can be accomplished by piping the condensed

liquid back through another few rows of coil tubing, located either in the

condenser air stream or in the water sump, where additional heat transfer

reduces the temperature of the liquid refrigerant.

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Expansion Devices

An expansion device is used to maintain a pressure difference between

the high-pressure (condenser) and low-pressure (evaporator) sides of the

system established by the compressor.

This pressure difference allows the evaporator temperature to be low

enough to absorb heat from the air or water to be cooled, while also

allowing the refrigerant to be at a high enough temperature in the

condenser to reject heat to air or water at normally available

temperatures.

There are several types of expansion devices, including expansion valves

(thermostatic or electronic), capillary tubes, and orifices.

Thermostatic expansion valves (TXVs) are commonly used and perform

essentially the same function as other expansion devices.

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Thermostatic Expansion Valve (TXV)

In addition to maintaining a pressure difference, the thermostatic expansion

valve controls the quantity of liquid refrigerant entering the evaporator.

It ensures that the refrigerant will be completely vaporized within the

evaporator (A) and maintains the proper amount of superheat in the system.

thermostatic

expansion valve

(TXV)

liquid/vapor

mixture

refrigerant

vapor

evaporator

A

liquid

refrigerant

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TXV Operation

external equalizer

Remote

bulb

Evaporator

Spring

Suction

line Distributor

Diaphragm

The outlet of the valve is connected to the distributor. A remote bulb is attached to

the suction line, where it senses the refrigerant vapor temperature leaving the

evaporator. This bulb is charged with refrigerant and as heat is transferred from

the suction line to the bulb, the refrigerant inside the bulb vaporizes. The resulting

refrigerant vapor pressure is transmitted through a tube to the space above a

diaphragm in the TXV.

The pressure of the refrigerant vapor leaving the evaporator is transmitted to the

space beneath the diaphragm through an external equalizing line that is tapped

into the suction line downstream of the bulb.

Finally, the valve contains an adjustable spring that applies a force to the lower

side of the diaphragm.

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valve pinsuction

line

spring94ºC

0.54 MPa

0.67 MPa

0.54 MPa 0.13 MPa

valve diaphragm

Assuming the 9.4°C refrigerant vapor leaving the evaporator boils the refrigerant in the

bulb, generating 0.67 MPa of pressure within the remote bulb. This pressure is transmitted

to the top side of the valve diaphragm, creating a force that pushes down on the diaphragm.

The 0.54 MPa evaporating pressure, on the other hand, is transmitted to the bottom side of

the valve diaphragm, producing an opposing force.

Since the difference between the evaporator pressure and the pressure within the remote

bulb is due to superheat, the tension of the spring is adjusted to provide the difference inorder to balance the forces and produce the desired amount of superheat. In this example,

the spring tension is adjusted to produce an 0.13 MPa pressure difference, which

corresponds to 6.7°C of superheat.

Any variation in evaporator pressure causes these forces to vary from this equilibrium and

move the pin up or down, thus closing or opening the valve.

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