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Refrigeration and air conditioning handout for 4th year Thermal Engineering students

Compiled by Derese T.1

DDU IOT Mechanica Enginering Dpt 2005 E.c

PART -I

REFRIGEARTION & AIR CONDITIONING

Basic Concepts in Refrigeration

Heat flows in the direction of decreasing temperature, that is,from high-temperatureregions to low-temperature ones.

The transfer of heat from a low temperature region to a high temperature one requiresspecial devices calledrefrigerators

Refrigerators are cyclic devices, and the working fluids used in the refrigeration cyclesare called refrigerants.

Here QL is the magnitude of the heat removed from the refrigerated space at temperatureTL,

QH is the magnitude of the heat rejected to the warm space at temperature TH, Wnet in is the network input to the refrigerator


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The performance of refrigerator is expressed interms of the COP

In any refrigeration the following components will exist Compressor, Condenser, Evaporator,

Expansion and Refrigerant valve

Refrigerants

Refrigerants are working fluids that are used in referigeration cycle are chemicals used for cooling in automobile air conditioners, window air conditioners,

refrigerators, freezers, water coolers and dehumidifiers

Common refrigerants are:

Freon group (chlorofluorocarbons (CFCs), such as R12, hydrofluorocarbons

. (HCFCs) such as R22),

R-134a,

Air, CO2, ammonia

Requirment for refrigerat

A refrigerant should :

Not cause ozone depletion.

A low global warming potential

Safe (non-toxic and flammable)

High effectiveness of refrigeration cycle

Oil micible

inert

High thermal conductivity

innet

L

W

Q

inputWork

effectCooling

inputquired

outputDesiredCOP

,Re


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Classification of Refrigerator

Refrigerator can be categorized in the following way.

Refrigeration Cycle

Based on the state and type refrigeration used in the refrigeration cycle is classified in thefollowing ways.

a/ Vapor-Compression Refrigeration Cycles


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It is the cycle in which the refrigerantvaporizedand condensed alternatively and is compressed

in vapor phase

Basic four processes taking are:-

Isentropic compression in a compressor Constant-pressure heat rejection in a condenserThrottling in an expansion device Constant-pressure heat absorption in an evaporator


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T-S Diagram


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ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

An actual vapor-compression refrigeration cycle differs from the ideal one in several ways, owing

mostly to the irreversibilities that occur in various components.

Two common sources of irreversibilities are fluid friction (causes pressure drops) and heat transferto or from the surroundings

Comparison of ideal and actual vapor compression refrigeration cycle

S.No

Ideal Vapor

compression

Refrigeration

Actual Vapor

compression

Refrigeration

Should be For AVCR

The refrigerant leaves the

evaporator and enters the

compressor

saturated vapor unknown Vapor should be

slightly super heated

compression process - Internally reversible

- Adiabatic,

- Isentropic

entropy of the

refrigerant may

increase (process1-2) or decrease

(process 1-2)

Exit of condensor Saturated liquid

( Similar to pressure at

the exit of compressor)

May not be due to

pressure drop

Before throttling Saturated liquid May not subcooling


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b. Absorption refrigeration cycleAbsorption refrigeration cycle is a refrigeration system that uses a heat source (e.g., solar,

kerosene-fueled flame etc.) to provide the energy needed to drive the cooling system.

As the name implies, absorption refrigeration systems involve the absorption of a refrigerant by a

transport medium. The most widely used absorption refrigeration system is the ammoniawater

system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium.

In the cycle all processes are similar to vapor compression refrigeration cycle except that the

compressor is replaced by absorption system

Absorption refrigeration systems are often classified as heat-driven systems. Absorption

refrigeration systems:

Are much more expensive than the vapor-compression refrigeration systems


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More complex and occupy more space More difficult to service Much less efficient Requiring much larger cooling towers to reject the waste heat Primarily used in large commercial and industrial installations

The COP of absorption refrigeration systems is defined as

gen

L

inpumpgen

L

Q

Q

WQ

Q

inputquired

outputDesiredCOP

,Re


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c/ Steam and air jet refrigeration system etc

COMMERCIAL, INDUSTRIAL & DOMESTIC REFRIGERATION COMPONENTS

1. COMPRESSOR Considered as the heart of the refrigeration systems. It compressor vapor. Responsible for increasing the pressure on the discharge side of the system Suction gas from the evaporator enters the compressor Refrigerant is discharged to the condenser

Compressers could be more than one for different application in refrigeration system. For

example consider two stage compresser.

Two-stage compression

Utilizes two compressors

One compressor discharges into suction of the other

Also referred to as compound compression

Often used in low-temperature commercial and industrial storage applications

Types of compressors1. Resprocating compressor

The piston follows resprocating motion

i. Welded hermetic reciprocating compressors

Motor and compressor contained in a welded shell Cannot be field serviced


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Typically a throw-away compressor Cooled by suction gas from the evaporator Lubricated by the splash method

ii. Semi-hermetic compressors

Bolted together, can be field servicedHousing is made of cast iron

Has a horizontal crankshaft

Larger compressors use pressure lubrication systems

Often air cooled

2. Screw compressor

Used in large commercial/industrial applicaons Uses two matching, tapered gears, and open motor design

3. Rotary compressorUsed in residential and light commercial applications

The four processes take place during the compression process are: Expansion (re-expansion),

suction (Intake), compression and discharge.

Compression Process Expansion

Piston is the highest point in the cylinder referred to as top dead center Both the suction and discharge valves are closed Cylinder pressure is equal to discharge pressure As the crankshaft continues to turn, the piston moves down in the cylinderThe volume in the cylinder increasesThe pressure of the refrigerant decreases

Compression Process Suction

As the piston moves down, the pressure decreases When the cylinder pressure falls below suction pressure, the suction valve opensThe discharge valve remains in the closed position As the piston continues downward, vapor from the suction line is pulled into the cylinder Suction continues until the piston reaches the lowest position in the cylinder (bottom dead

center)

At the bottom of the stroke, suction valves close

Compression Process - Compression

Piston starts to move upwards in the cylinderThe suction valve closes and the discharge valve remains closed


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As the piston moves upwards, the volume in the cylinder decreasesThe pressure of the refrigerant increases Compression continues until the pressure in the cylinder rises just above discharge

pressure

Compression Process Discharge When the cylinder pressure rises above discharge pressure, the discharge valve opens

and the suction valve remains closed

As the piston continues to move upwards, the refrigerant is discharged from thecompressor

Discharge continues until the piston reaches top dead center

2. CONDENSERIt is the heat exchanger surface that rejects system heat

Rejects heat

-Desuperheating vapor refrigerant from the compressor


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-Subcools refrigerant at the outlet of the condenser

The greatest amount of heat is transferred during the change of state. It is on the high pressure

side of the system. Conders could be divided into two based on the cooling medium.

2.1 Water-Cooled Condensers

More efficient than air-cooled condensers. Water temperature directly affects system pressures.

Three types of water-cooled condensers are:

i. Tube within a tube condenser

ii. Shell and coil condenser

iii. Shell and tube condenser

i. Tube within a tube condenser

Heat exchange takes place between the fluids in the inner and outer tubes

Refrigerant flows in the outer tube Water flows in the inner tube Refrigerant and water flowin opposite direcons to maximize the heat transfer rate Depending on the construcon, the condenser can be cleaned mechanically or chemically

ii. Shell and coil condensers

Coil of tubing enclosed in a welded shell Water flows through the coil Refrigerant from the compressor is discharged into the shell When refrigerant comes in contact with the cool coil, it condenses and falls to the bottom


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This condenser must be cleaned chemically

iii. Shell and tube condensers

Compressor discharge gas is piped into the shell Water flows through the tubes in the condenserThe shell acts as a receiver Most expensive type of condenser


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2.2 Air-cooled condensers

Uses air to absorb heat rejected by the system

Used in locations where water is difficult to use

Hot gas enters the condenser from the top

3. EVAPORATORS Responsible for absorbing heat into the refrigeration system The evaporator is maintained at a temperature that is lower than the medium

being cooled

Removes the heat from the air in the refrigerated boxHeat exchange characteristics of the evaporator

Common materials used are copper, aluminum, steel, brass and stainless steel

Corrosion factor and application determines the materials used for a particular

evaporator

Rapid heat transfer rate between two liquidsSlower heat transfer rate between two vapors

Film factor

Relationship between the medium giving up heat and the heat exchange surface

Related to the velocity of medium passing over the coil


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Temperature difference between two mediums

Large temperature difference = high heat transfer rate

Small temperature difference = low heat transfer rate

EVAPORATOR TYPESNatural convecon evaporators

Made up of bare tubes or pipes Physically large, very low air velocity Located high, near the ceiling of a walk-in cooler

Mechanical evaporators

Use blowers or fans to move air across the coil Improved heat transfer rate Physically smaller than natural dra evaporators

On the other hand Evaporators can be divided as:

Stamped plate evaporators

o Creates a large heat transfer surfaceo Made of metal plates stamped with grooves that provide a path for the refrigerant to

take

Finned tube evaporators Increased surface area

Mulple circuit evaporators

o Parallel circuits are created in the coilo Results in is reduced pressure drop across the coil

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