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UNIT 4: REFRIGERATIONPRINCIPLE

4.1 Introductioni. Buildings are essentially enclosed spaces designed for the purpose of

accommodating occupants and their activities.ii. Thermal comfort is incorporated with the building design in terms of

architectural design (e.g Buildings designed with transparent atriums toallow natural light to emanate buildings) as well as M & E services(Sufficient lighting, air conditioning, etc).

– Thermal comfort for humans determined by several factors, e.gclothing of occupants, average temperature of locality, etc.

– Average thermal comfort: 27 Degrees Celsius.– Thermal comfort: Crucial in preventing building sickness in occupants

(Especially important for office/working environment, as it will affectproductivity);

iii. Refrigeration cycle: Designed to remove heat from building’s premisesto external environment, as well as heat up building interior duringperiods of cold weather.

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iv. Calculations:a) British Thermal Unit (BTU): Unit of energy to measure power consumption for

air con units;– 1 BTU = amount of heat necessary to raise one pound of water (0.454 kg) y

1 degree Fahrenheit (F) or 0.56 degrees Centigrade;– 1 BTU = 252 calories or 1.055 kilo Joules (kJ);– 1,000 kWh = 3.41 million BTU;– Necessary to design power requirements for cooling;– If BTU is sized too high, humidity of cooled area may be insufficiently

removed. If BTU is too low, insufficient cooling may occur;– Typical methodfor sizing BTU: Area of room (In square feet) x 25;– E.g: If room size is 12 feet by 15 feet, area is 180 square feet x 25 = roughly

4500 BTU;– May include other factors: E.g Temperature and humidity of the locality, no

of occupants (Generally, 1 occupant would represent about 1000 BTU), etc.b) Volumetric Air Flow: Volumetric measurement of airflow per unit time from a

ventilation (Natural or mechanical) device;– Usually in metre cube per hour (CMH) or Liters/Sec;– Used for designing air flow requirements in accordance to Code of Practice

for locality/Country (SS 553: 2009 for Singapore).

Volumetric Flow Rate Conversion Chart

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4.2 Understanding the Refrigeration Cycle

i. Refrigerant cycle functions as a heat exchanger.ii. The medium for heat exchange:

a) Chemical refrigerant: Used for split units, VRV systems andother air conditioning systems;

– Usually chosen for their low boiling points;– Very volatile chemicals, hence have huge capacity of heat

absorption and dissipation. Because of their volatility,copper piping needs to be insulated by armaflex to reducethe effects of condensation;

– Usually stored in canisters complete with nozzles for ease oftopping up in air con systems;

– Environmental Issues: Old refrigerant types using CFCs areknown to threaten the ozone layer, and are usually bannedin most countries (E.g Freon refrigerants manufacturedunder DuPont).

4.2.1 How the Refrigeration Cycle Works

TypicalChemicalRefrigerant forStored inCanister form

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b) Water: Used in water chillers. Transfers heat from a chemicalrefrigerant to a heat dissipation element (i.e Cooling Tower). Usuallymade of tougher materials, e.g stainless steel.

Chilled water piping (Stainless steel)in water chiller system. Connectionto local water supply helps toreplenish losses in system

Copper tubing for refrigerant inA/C system c/w armaflexinsulation

iii. How the RefrigerationCycle Works:

a) Compressor:• Receives low pressure

refrigerant fromevaporator;

• Compresses and pumpsthe refrigerant throughoutthe refrigeration system;

• Provides sufficient highpressure to drive therefrigerant system;

b) Condenser:• Receives high pressure

refrigerant fromcompressor;

• Transfers heat fromrefrigeration system tooutside environment;

HighTemp

LowTemp

LowTemp

Schematic Diagram of Refrigeration Cycle

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c) (Thermal) ExpansionValve:

• Receives highpressure refrigerantfrom Condenser;

• Controls refrigerantflow to evaporator;

• Connected to thesensor bulb, installedat evaporator tomonitor temperature;

• Temperature setting(usually via remotecontrol) will relaymessage to sensor,which will adjustrefrigerant flowaccordingly to meetset temperature.

d) Evaporator:• Receives low pressure refrigerant from

evaporator;• Usually incorporated with a fan coil unit within

the building premises;• Receives heat from building’s premises, which

eventually is sent to the condenser to beexpelled to the external environment.

SensorBulb

Evaporator Unit

Split Unit Air Con FCU

Compressor and Condenser unitincorporated in outdoor condensingunit.

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v. Condensation Issues for Air Can Evaporator/Fan Coil Units Condensation occurs in fan coil units when the cold surface of the

evaporator meets the warm air within the building proper; The situation worsens if the fan coil unit is located near sources of air, e.g

near the entrance of the lobby, where the constant opening and closing ofdoor causes constant warm air to come into contact with the evaporator;

Drain pan is usually located beneaththe evaporator coil;

Condensate is collected at the pan,which is connected to PVCcondensate piping (See picture onright);

Condensate piping is usuallyconnected to a drain or waste trapand flows away as waste water in thesanitary system;

Regular maintenance is required toprevent backflow of condensatewater due to chokage from debris,such as dirt particles.

Typical design for condensatepiping.

Condensate piping is usually designed with a downward gradient, i.e thehighest point from the condensate pan must be directed downwards viapiping towards the drainage point. Concept similar to sanitary drainagesystem;

Should the condensation piping follow a irregular travel pattern, i.e thepiping travels upwards and downwards before reaching the drainagepoint, the water may not be able to flow to the drainage point. This willcause water to floor backwards to the pan, causing leakage at the FCU;

In this case, a condensate pump needs to be installed to pump away thecondensate.

Typical drainage pump installed topump away condensate to highergradient

Typical condensate pump

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vi. Compressors

• Compressors: Device which increases pressure and decreases volume of afluid (Hence compressing the fluid) to drive it through a piping system;

• Electrically driven: Needs a constant power source to drive it;• Difference from pumps: Pumps do not compress fluids as their main

function is to transport fluids via pressurization (e.g condensate pumpused to transport a/c condensate to drainage).

Compressor Categories

Single Stage CentrifugalCompressor

• Types of A/C Compressors:a) Reciprocating Compressors Range: Up to 180kW; Typical for small localized systems, e.g

window sill A/C units; Positive displacement type;How it Works– Piston moves downwards, creating

vacuum in cylinder space above thepiston. This draws refrigerant into thespace from the intake valve (Outletvalve is in closed position during thisprocess);

– When piston reaches bottom, it springsup and compresses the refrigerant.Compressed refrigerant is forced intothe exhaust valve. At this point, theintake valve is closed;

– Cycle is repeated.

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b) Rotary Screw Compressors Mid-Range: Up to 2MW; Suitable for mid-range chiller

systems; Compact, smooth running,

limited vibration; Spring suspensions not

required; Consists of Dry Type and Oil

Flooded Type.Dry Type:– Two rotors (1 is driver, the

other is driven) or helicalscrews used to compress gas;

– Timing gears are used toensure rotors maintain precisealignment;

– Oil-free, suitable for sensitiveenvironments, e.g Hospitals.

Dry Type Rotary Compressor

Helical Screws for compressing Fluids

Oil-flooded type:– Similar design,– Oil is injected into

compression cavities for: Sealing purposes to prevent

backflow of refrigerant; Cooling sink for refrigerant

discharge; Transferring mechanical

energy between driver anddriven rotors;

– Refrigerant gas enterscompressor, mixes with oil andexits compressor;

– Oil is mixed with refrigerantand separated in gas-oilseparator before compressedgas is discharged.

– Separated oil is returned tomicro oil filter for filtering andthen to oil cooler to be cooled.

Oil-flooded Rotary CompressorSystem

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c) Rotary Vane Compressors Rotary Motion: Utilizing a

series of vanes and slots in therotor;

Rotor is actually offset fromcenter of the housing;

How it Works:– Rotating shaft rotates and

shifts, turning the rotor as well;– Vanes move in and out of the

slots, causing refrigerant fromsuction port to be compressedbefore it leaves thecompressor.

Cross Section of Rotary VaneCompressor

Schematic of Rotary VaneCompressor

d) Centrifugal Compressors High Range: 3.5MW; Incorporated with large,

centralized chiller systems;How it Works:– Rotating disk/impeller

rotates in housing, drivingrefrigerant to force it to theimpeller rim;

– Velocity increases, convertsto high pressure and isdiverted to condenser.

Centrifugal Compressorincorporated in Centralized AirConditioning System

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4.3 Air Conditioning & Mechanical VentilationTypes• Various types of air conditioning and mechanical ventilation systems are

available in the market;• While functions may differ in accordance to their allocated functions (e.g Type

of refrigeration, Centralized and localized cooling, etc), the basic design issimilar to the vapor compression system: Heat absorption (Evaporator),temperature control apparatus (Expansion Valve), heat dissipation(Condenser), propulsion of heat transfer medium (Compressor).

4.3.1 Localized Cooling Systems

• Designed for localized cooling in small rooms or areas, e.g rooms, offices,etc;

• Cooling element (Evaporator) is usually incorporated with mechanicalfan/blower;

• Types of localized cooling systems include:a) Window sill air conditionersb) Split systemsc) Multi split systemsd) Variable refrigerant flow split systems.

Window Sill Air Con Installationat Residential Unit

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i. Window Sill A/C:• As the name suggests, the

apparatus is mounted onwindow sill;

• Follows the vaporcompression/refrigerant cyclemethod;

• All components inside cabinetunit;

• Evaporator facing unit interior,condenser facing outsideenvironment;

• Evaporator: Warm air is drawnfrom room into filter, thenpasses back to room as cold air;

• Condenser: Expels heat frombuilding via blower.

ii. Single Split Unit A/Cs• Similar to Window Sill A/C;• Evaporator and Expansion Valve

(Fan Coil Unit, or FCU) housedseparately from Condenser Unit(CU: Combination of compressorand Condenser);

• Comes with Ducting andDuctless units.

Ductless Split UnitA/C

Ducted Split UnitA/C

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iii. Multi-Split Unit Air Conditioning

• Similar to single split unit A/Cs;• One CU unit is linked to more than 1

no of FCU units;• Advantages:

– Saves space, without the need tofree up floor space to install moreCU units;

– Energy Saving Features.

Comparison between single splitunits and multiple split units

iv. Variable refrigerant flow (VRF)Split Unit Systems

• Also known as VRV(VariableRefrigerant Volume)for Daikinprototypes;

• Similar to split units,with up to 40 FCUsconnected toexternal modular CUunit;

• Each FCU iscontrolled by itsown solenoid valvekit, able to regulaterefrigerant flow (andtemp)individuallyfor each FCU;

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• Prototypes may includeheating and coolingelements;

• Advantages:a) Does not need huge

floor space forinstallation, unlikechiller systems;

b) Flexibility of use, astemperature forvarious FCUs can beadjusted individualunlike centralizedsystems;

c) Energy savingproperties;

d) Can be used for avariety of settings, e.gIndustrial premises,retail, offices, etc.

4.3.2 Reverse Cycle Heat Pump (HVAC)i. In countries where the 4 seasons are

prevalent, HVAC (Heating Ventilationand Air Conditioning) systems arerequired to provide both cooling andheating functions within buildings.

ii. Reverse Cycle Heat Pumps can beemployed to serve a HVAC function:– System is similar to vapor

compression cycle;– Uses refrigerant similar to standard

split unit A/C;– Can be used for cooling and heating

purposes by reversing refrigerantcycle flow;

– Requires a heat sink to transfer heat(Similar to CU unit of Vaporcompression cycle):

• Outside environment;• Ground.

Outdoor Units for HeatPumps

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iii. How it works:a) For cooling purposes:• Outdoor unit serves as condenser, expelling heat via fan (Similar to split

unit a/c);• Internal unit serves as a evaporator, receiving heat and directing it to the

outdoor unit;

b) For heating purposes:• Outdoor unit serves as evaporator, as heat is received by the outdoor unit

and directed to the indoor unit via the heated refrigerant;• Internal unit serves as a condenser, receiving heat and directing it to the

indoor unit to heat the building interior.

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iv. Geothermal Heat Pump

• Similar to normal heat pumps, but usesground as heat sink;

• During winter, heat is pumped from theground and used to heat building;

• During summer, the reverse happens, andthe building is cooled.

• Advantages: Cheap: Electricity is required only to

drive the compressor; Environmentally friendly: Significantly

low CO2 emissions; Highly efficient, as it utilizes the

ground as the heat sink (Especially sofor heating purposes).

4.3.3 Absorption Cooling

i. Using heat (Derived from a heatingelement, e.g gasoline, solar power, etc)to drive heat exchange medium (insteadof compressor in typical vaporcompression cycle) as a driving force;

ii. Can be localized or centralized A/Csystems;

iii. Preferable to vapor compression typesin terms of:

• Lower operating costs (Because nocompressor is used);

• Unreliable electricity supply in locality;• Where there is a supply of surplus heat

source (e.g economizer from a nearbyboiler can use recycled heat to be used forabsorption cooling);

• Can utilize solar power and other forms of“clean energy” to provide the heatingelement.

A localized domesticAbsorption CoolingRefrigerator

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iv. Basic Absorption Cooling Cyclea) Heat is the driving force for the absorption

cooling cycle;b) Evaporation Process:• Liquid refrigerant enters evaporator;• Absorbs heat from evaporator’s

surroundings, becomes gaseous form;c) Absorption Process:• Gaseous refrigerant enters absorber;• Low pressure gaseous refrigerant dissolved

in another fluid;• This allows for more refrigerant to

evaporate in evaporator due to reducedpartial pressure;

d) Regeneration Process:• Dissolved refrigerant in absorber heated,

allowing refrigerant to evaporate (Similar tocompressor function);

• Evaporated refrigerant condenses via heatexchanger, replenishes refrigerant inevaporator (Similar to condenser function).

v. Single Pressure Absorption Cycle

• Capacity range: 5kW up to12MW;

• Utilizes absorbent (Lithium-bromide) and refrigerant(Water).

• Heat source: Steam, hot water;• Coefficient of Performance

(COP): <1;How it Works:a) Generator:• Heat source heats up the

lithium-bromide solution;• Water evaporates and enters

condenser;• Concentrated lithium-bromide

solution flows into absorber, tobe pumped back to generatorafter being cooled by coolingwater;

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b) Condenser:• Receives vaporized water

(Refrigerant) from generator;• Cooling water system: Transfers

heat from vaporized water tocooling tower for externalcooling;

• Vaporized water condenses,pressure increases, temperaturedecreases and flows toevaporator;

c) Evaporator:• Specialized nozzles spray the low

temperature refrigerant onchilled water piping systemcirculating between the indoorcooling unit (Usually an airhandling unit) and theevaporator;

• Cooled chilled water circulatedback to indoor unit;

• Heated refrigerant is absorbedinto solution in absorber.

d) Absorber• Continuous water-lithium bromide

solution constantly receives heatedwater vapor from evaporator, as wellas lithium bromide solution fromgenerator;

• Solution is pumped back to generatorto complete cycle via solution pump.

vi. Two-Stage/Double Effect Absorption Chillers

• More efficient than single stage/single absorption cycles/units;• Requires higher temperature from heat source at generator (>140

degrees Centigrade);• Uses 2 generators:a) Main generator (steam fed or direct gas fired burners): Generates heat

for the system;b) Low temperature generator: Recovers heat which would otherwise be

lost to cooling tower to drive the refrigerant.c) Triple effect absorption chillers also available as well, similar principle as

two stage absorption chillers.

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vii. Water Spray Absorption Refrigeration

• Inclusive of de-humidifying function;

• How it works:1) Warm, moist air

enters ducting systemfan air inlet;

• Salt solution sprayed;• Humidity reduced;• Temperature relatively

unaffected;2) Warm, dry air enters

evaporative cooler;• Pure water sprayed;• Temperature reduced;• Humidity increases;

1

2

3

4

3) Cool, moist air enters fan air outlet;• Cool moist air is sprayed with salt solution at fan

air outlet to reduce humidity;4) Cool dry air is exits the system.• Refrigeration system (In Blue): Salt solution tank

collects spray from both pure water and saltsolution, where it is heated, condensed and purewater is collected to complete refrigerant cycle.

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4.3.4 Chilled Water Air Conditioning Systemsi. Designed for centralized cooling

over large properties, e.gshopping malls;

ii. How it works:a) Condensing Water System:• Uses water as refrigerant;• Transfers heat from compressor

to cooling tower;• Heat is dissipated from cooling

tower;• Condensing pump: Provides

energy for water to transversesystem;

b) Chiller: Acts as a heat exchangerbetween chilled water systemand condenser water system;

• Vapor compression cycle;• Chemical refrigerant used;

Typical Chilled Water A/C Systemincorporating Air Handling Unit Systemand Exhaust System

c) Chilled water system:• Uses water as refrigerant;• Transfers heat from air handling unit to

chiller;• Chilled water pump: Provides energy for

water to transverse system;d) Air handling unit: Acts as a evaporator

for centralized cooling;• Ducting air vents split to various

locations throughout designatedlocations for cooling purposes;

• Cooling fans installed to provemechanical ventilation;

• Filters installed to filter out dust fromoutlets;

• Dehumidifier installed to reducehumidity;

e) Exhaust system: Mechanicalventilation system designed to expelstale air from building proper.

Typical Air Handling Unit

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Tutorial Questions1) Explain in detail, with the aid of illustration, how the Refrigeration Cycle (Vapor

Compression Cycle) works (20 marks).2) Condensation issues in Fan Coil Units (FCUs) are a common problem in many air

con installations, causing FCU units to leak, or “sweat” .i. Briefly describe how condensation occurs (2 points).ii. Explain, with the aid of a simple sketch, how condensation piping should be

installed so as to prevent leakage in air cons (10 points).iii. Explain, with the aid of a simple sketch, the condition(s) whereby air con

condensation pumps should be installed in condensation piping system (8 points).3) Illustrate with the aid of a simple sketch, on how the Absorption Cooling Cycle

works (20 marks).4) Heat Ventilation Air Conditioning (HVAC) systems are common in some

countries.i. Identify the type of environmental conditions which will necessitate the use of

HVACs (2 points);ii. Identify the basic functions of the HVAC systems (2 points);iii. Describe in detail how Reverse Cycle Heat Pump works(9 points).iv. Briefly describe how the Geothermal Heat Pump works, and list down the

advantages of using the geothermal heat pump (7 points).