Section 1: Theory of Heat Unit 3: Refrigeration and Refrigerants

Section 1: Theory Section 1: Theory of Heatof Heat

Unit 3: Unit 3: Refrigeration and Refrigeration and

RefrigerantsRefrigerants

Unit ObjectivesUnit ObjectivesAfter studying this chapter, you should be able to:After studying this chapter, you should be able to: List three common refrigeration temperature List three common refrigeration temperature

rangesranges Describe the term “ton of refrigeration”Describe the term “ton of refrigeration” Describe the basic refrigeration cycle Describe the basic refrigeration cycle Explain the function of the compressor, Explain the function of the compressor,

condenser, metering device and evaporatorcondenser, metering device and evaporator Explain the concepts of subcooling and Explain the concepts of subcooling and

superheatsuperheat List some desired refrigerant characteristicsList some desired refrigerant characteristics Plot a system on a pressure enthalpy chartPlot a system on a pressure enthalpy chart

INTRODUCTION TO INTRODUCTION TO REFRIGERATIONREFRIGERATION

Cooling to preserve products and provide Cooling to preserve products and provide comfortcomfort

1900s were the beginnings of mechanical 1900s were the beginnings of mechanical refrigeration systemsrefrigeration systems

Refrigeration process temperature rangesRefrigeration process temperature ranges High temperature – Air conditioning (comfort)High temperature – Air conditioning (comfort) Medium temperature – Fresh food preservationMedium temperature – Fresh food preservation Low temperature – Frozen food preservationLow temperature – Frozen food preservation

REFRIGERATIONREFRIGERATION Process of transferring heat from a place Process of transferring heat from a place

where it is objectionable to a place where it where it is objectionable to a place where it makes little or no differencemakes little or no difference

Heat naturally flows from a warmer Heat naturally flows from a warmer substance to a cooler substancesubstance to a cooler substance

Heat will flow naturally from a 100°F house Heat will flow naturally from a 100°F house if the outside temperature is 80°Fif the outside temperature is 80°F

Mechanical refrigeration is needed if the Mechanical refrigeration is needed if the house is 80°F and the outside temperature house is 80°F and the outside temperature is 100°Fis 100°F

Inside Temp 100°F

Outside temperature 80°F

Heat flows naturally from a warmer substance to a cooler substance

Inside Temp 80°F


Heat flows naturally from a warmer substance to a cooler substance

Mechanical refrigeration would be needed to cool this house

RATING REFRIGERATION RATING REFRIGERATION EQUIPMENTEQUIPMENT

It takes 144 btu to melt one pound of ice at It takes 144 btu to melt one pound of ice at 32°F32°F

2,000 pounds of ice (1 ton) will require 2,000 pounds of ice (1 ton) will require 288,000 btu to melt (144 btu x 2,000 pounds)288,000 btu to melt (144 btu x 2,000 pounds)

If the melting of 1 ton of ice takes place in one If the melting of 1 ton of ice takes place in one day (24 hours), 12,000 btu must be absorbed day (24 hours), 12,000 btu must be absorbed by the ice every hour (288,000 btu / 24 hours)by the ice every hour (288,000 btu / 24 hours)

12,000 btu/hr = 200 btu/min = 1 ton of 12,000 btu/hr = 200 btu/min = 1 ton of refrigerationrefrigeration

THE REFRIGERATION THE REFRIGERATION PROCESSPROCESS Heat is pumped from a cool box to a warm roomHeat is pumped from a cool box to a warm room

Pumping of heat is similar to pumping water Pumping of heat is similar to pumping water uphilluphill

Air conditioners pump heat from inside to the Air conditioners pump heat from inside to the outsideoutside Inside temperature 75°F, Outside temperature 95°FInside temperature 75°F, Outside temperature 95°F Cooling (indoor) coil temperature 40°FCooling (indoor) coil temperature 40°F Condenser (outdoor) coil temperature 125°FCondenser (outdoor) coil temperature 125°F Indoor heat travels to the indoor coilIndoor heat travels to the indoor coil System heat flows from the outdoor coil to the outside System heat flows from the outdoor coil to the outside

airair

Inside Temp 75°F


Cooling coil at 40°F

Outdoor coil at 125°F

Heat flows naturally from the air inside the house to the cooling coil (evaporator)

Heat also flows naturally from the condenser coil to the ambient air surrounding the coil

PRESSURE/TEMPERATURE PRESSURE/TEMPERATURE RELATIONSHIPRELATIONSHIP

Water boils at 212°F at atmospheric pressure (29.92 in. Water boils at 212°F at atmospheric pressure (29.92 in. Hg)Hg)

Water boils at 250°F if pressure is increased to 15 psigWater boils at 250°F if pressure is increased to 15 psig Water boils at 40°F if pressure is reduced to 0.248 in. Water boils at 40°F if pressure is reduced to 0.248 in.

HgHg Refrigerants are substances that boil at low pressures Refrigerants are substances that boil at low pressures

and temperatures and condense at high and temperatures and condense at high pressures/temperaturespressures/temperatures

Saturation temperature – Point at which the addition or Saturation temperature – Point at which the addition or removal of heat will result in a change of stateremoval of heat will result in a change of state

During a change of state, the temperature remains During a change of state, the temperature remains constantconstant

FOUR MAJOR REFRIGERATION FOUR MAJOR REFRIGERATION SYSTEM COMPONENTSSYSTEM COMPONENTS

Evaporator - Absorbs heat from area to Evaporator - Absorbs heat from area to be cooledbe cooled

Compressor – Creates pressure difference Compressor – Creates pressure difference needed to facilitate refrigerant flow needed to facilitate refrigerant flow through the systemthrough the system

Condenser - Rejects system heatCondenser - Rejects system heat Metering device – Regulates refrigerant Metering device – Regulates refrigerant

flow to the evaporatorflow to the evaporator

THE EVAPORATORTHE EVAPORATOR Heat exchange surface used to absorb heatHeat exchange surface used to absorb heat Located on the low-pressure side of the Located on the low-pressure side of the

system between the metering device and system between the metering device and the compressorthe compressor

Operates at temperatures lower than the Operates at temperatures lower than the medium being cooled or conditionedmedium being cooled or conditioned

Absorbs heat by boiling a low temperature Absorbs heat by boiling a low temperature liquid into a low temperature vaporliquid into a low temperature vapor

THE EVAPORATOR (cont’d)THE EVAPORATOR (cont’d) Refrigerant typically enters the evaporator as a Refrigerant typically enters the evaporator as a

liquid/vapor mix (75% liquid, 25% vapor)liquid/vapor mix (75% liquid, 25% vapor) Superheat Superheat

The heating of a vapor above its saturation temperatureThe heating of a vapor above its saturation temperature Ensures that no liquid refrigerant enters the compressorEnsures that no liquid refrigerant enters the compressor Equal to the evaporator outlet temperature minus the Equal to the evaporator outlet temperature minus the

evaporator saturation temperatureevaporator saturation temperature Design superheat is typically between 8°F and 12°FDesign superheat is typically between 8°F and 12°F

Superheated vapor does not follow a Superheated vapor does not follow a pressure/temperature relationshippressure/temperature relationship

AIR CONDITIONING APPLICATION: R-22 EVAPORATOR

Liquid/vapor mixture from the metering device

68.5 psig (40°F from P/T chart)

40°F

Last drop of liquid

41°F 43°F 45°F50°F Superheated

vapor to the compressor

68.5 psig In green region, the P/T relationship does not hold

THE COMPRESSORTHE COMPRESSOR Pumps heat-laden vapor from the evaporator to Pumps heat-laden vapor from the evaporator to

the condenser by increasing the refrigerant the condenser by increasing the refrigerant pressurepressure

Reduces pressure on the low-side of the systemReduces pressure on the low-side of the system Increases pressure on the high-side of the Increases pressure on the high-side of the

systemsystem Common compressor types include the scroll, Common compressor types include the scroll,

reciprocating and the rotaryreciprocating and the rotary Positive displacement compressors require that Positive displacement compressors require that

the compressed gas be moved to the condenserthe compressed gas be moved to the condenser

Refrigerant in the compressor is superheated, so it does not follow a pressure/temperature relationship!

Suction lineDischarge line

Low pressure, low temperature superheated

vapor enters the compressor

High pressure, high temperature superheated

vapor leaves the compressor

Refrigerant from the evaporator

Refrigerant to the condenser

THE CONDENSERTHE CONDENSER Rejects sensible and latent heat from the Rejects sensible and latent heat from the

system that was absorbed by compressor and system that was absorbed by compressor and evaporatorevaporator

Located on the high-pressure side of the Located on the high-pressure side of the systemsystem

The refrigerant condenses from a high The refrigerant condenses from a high temperature vapor to a high temperature liquidtemperature vapor to a high temperature liquid

Condensing temperature is determined by the Condensing temperature is determined by the high side pressure in the system high side pressure in the system

Refrigerant is subcooled at the outlet of the Refrigerant is subcooled at the outlet of the condensercondenser

THE CONDENSER (cont’d)THE CONDENSER (cont’d) SubcoolingSubcooling

The cooling of liquid refrigerant below its The cooling of liquid refrigerant below its saturation temperaturesaturation temperature

Standard air-cooled systems are designed to Standard air-cooled systems are designed to operate with a minimum of 10°F of subcoolingoperate with a minimum of 10°F of subcooling

High efficiency condensers operate with more High efficiency condensers operate with more subcooling than standard efficiency systemssubcooling than standard efficiency systems

Determined by subtracting the condenser Determined by subtracting the condenser saturation temperature from the condenser saturation temperature from the condenser outlet temperatureoutlet temperature

AIR CONDITIONING APPLICATION: R-22 CONDENSER

Subcooled liquid to the metering device

110°F

First drop of liquid

110°F 130°F 185°F 200°FSuperheated vapor from compressor

226 psig In green regions, the P/T relationship does not hold

110°F106°F 101°F 98°F 95°F

Refrigerant is subcooling

Refrigerant is desuperheating

THE METERING DEVICETHE METERING DEVICE Controls the flow of subcooled liquid from Controls the flow of subcooled liquid from

the condenser to the evaporatorthe condenser to the evaporator Creates a pressure drop between the high Creates a pressure drop between the high

and low pressure sides of the systemand low pressure sides of the system About 25% of the liquid leaving the About 25% of the liquid leaving the

metering device immediately vaporizes metering device immediately vaporizes (flash gas)(flash gas)

Three commonly used metering devices are Three commonly used metering devices are the capillary tube, automatic expansion the capillary tube, automatic expansion valve and the thermostatic expansion valvevalve and the thermostatic expansion valve

PUTTING IT TOGETHER: A WINDOW UNIT EXAMPLE

Compressor Condenser

Metering deviceEvaporator

Indoors: 75°F

Outdoors: 95°F

R-22


Outdoors: 95°F

200°F

75% liquid 25% vapor

69 psig (40°F)

Saturated refrigerant

100% vapor 69 psig

50°F

R-22

278 psig

125°F

110°F

278 psig


Outdoors: 95°F

200°F

69 psig (40°F)

69 psig

50°F

R-22

278 psig

125°F

110°F

Superheat = 50°F - 40°F = 10°F

Subcooling = 125°F - 110°F = 15°F

REFRIGERANTSREFRIGERANTS R-12 used primarily for high and medium R-12 used primarily for high and medium

temperature refrigeration applications temperature refrigeration applications (manufacture banned in 1996)(manufacture banned in 1996)

R-22 used primarily in air conditioning R-22 used primarily in air conditioning applications (slated for total phase-out in 2030)applications (slated for total phase-out in 2030)

R-500 and R-502 banned in 1996R-500 and R-502 banned in 1996 R-134a – replacement for R-12 with retrofitR-134a – replacement for R-12 with retrofit Replacements for R-22 include R-410a and R-Replacements for R-22 include R-410a and R-

407c407c

REFRIGERANTS MUST BE REFRIGERANTS MUST BE SAFESAFE

Designed to protect people from Designed to protect people from sickness, injury and deathsickness, injury and death

Proper ventilation is requiredProper ventilation is required Refrigerants can displace oxygen if Refrigerants can displace oxygen if

permitted to accumulatepermitted to accumulate Modern refrigerants are non-toxicModern refrigerants are non-toxic When burned, toxic/corrosive gases are When burned, toxic/corrosive gases are

createdcreated

REFRIGERANTS MUST BE REFRIGERANTS MUST BE DETECTABLEDETECTABLE

Soap bubble solution – pinpoints leaksSoap bubble solution – pinpoints leaks Halide leak detector – uses an open flameHalide leak detector – uses an open flame Electronic leak detectors – general area Electronic leak detectors – general area

leaksleaks Ultraviolet leak detectors – pinpoints leaksUltraviolet leak detectors – pinpoints leaks Ultrasonic leak detectors – uses sound Ultrasonic leak detectors – uses sound

waveswaves

Methods used for detecting refrigerant leaks include:

GENERAL REFRIGERANT GENERAL REFRIGERANT NOTESNOTES

Refrigerants should boil at low temperatures at Refrigerants should boil at low temperatures at atmospheric pressure so that low temperatures atmospheric pressure so that low temperatures can be obtained without going into a vacuumcan be obtained without going into a vacuum

It is illegal to intentionally vent refrigerant to the It is illegal to intentionally vent refrigerant to the atmosphere (Stiff fines for violations)atmosphere (Stiff fines for violations)

Mandatory certification for techniciansMandatory certification for technicians The EPA set refrigerant phase-out schedules The EPA set refrigerant phase-out schedules Refrigerant cylinders and drums are color-codedRefrigerant cylinders and drums are color-coded

RECOVERY, RECYCLING AND RECOVERY, RECYCLING AND RECLAIMING OF REFRIGERANTSRECLAIMING OF REFRIGERANTS

Refrigerant recovery is mandatory Refrigerant recovery is mandatory during service and installation during service and installation operationsoperations

Intended to reduce the emissions of Intended to reduce the emissions of CFC, HCFC and HFC refrigerantsCFC, HCFC and HFC refrigerants

Recovery equipment must be used Recovery equipment must be used according to manufacturer’s according to manufacturer’s instructionsinstructions

PLOTTING THE PLOTTING THE REFRIGERATION CYCLEREFRIGERATION CYCLE

Pressure-enthalpy chartPressure-enthalpy chart Used to create a graphical representation of a Used to create a graphical representation of a

systemsystem Pressure scales on the vertical axis (psia)Pressure scales on the vertical axis (psia) Enthalpy scale along the bottom of the chartEnthalpy scale along the bottom of the chart Horseshoe curve represents the saturation curveHorseshoe curve represents the saturation curve Refrigerant is saturated on and under the curveRefrigerant is saturated on and under the curve

Enthalpy is defined as heat contentEnthalpy is defined as heat content

THE PRESSURE ENTHALPY CHART

Saturation curve

Saturated refrigerant

Superheated vapor region

Subcooled liquid region

Enthalpy scale (btu/lb)

Pressure (psia)

THE PRESSURE ENTHALPY CHART: A SAMPLE PLOT

Enthalpy scale (btu/lb)

Pressure (psia)

Compressor

Condenser

Metering device

Evaporator

THE PRESSURE ENTHALPY CHART: AN R-22 EXAMPLE

Cond. sat. temp. 130°F (296.8 psig, 311.5 psia)

311.5 psia


Evap. sat. temp. 40°F (68.5 psig, 83.2 psia)

311.5 psia

83.2 psia


0°F of subcooling, so refrigerant enters the metering device at 130°F

311.5 psia

83.2 psia

130°F

40 btu/lb


311.5 psia

83.2 psia

130°F

40 btu/lb

Evaporator superheat is 10°F, so refrigerant leaves evaporator at 50°F

50°F

110 btu/lb


311.5 psia

83.2 psia

130°F

40 btu/lb

Suction line superheat is 10°F, so refrigerant enters compressor at 60°F

110 btu/lb

60°F

112 btu/lb


311.5 psia

83.2 psia

130°F

40 btu/lb 110 btu/lb 112 btu/lb

190°F

Refrigerant leaves compressor at 190°F

127 btu/lb


311.5 psia

83.2 psia


127 btu/lb



127 btu/lb

A

B C

D

EMetering device: “A to B”

Evaporator: “B to C”

Suction line: “C to D”

Compressor: “D to E”

Condenser “E to A”

PRESSURE ENTHALPY PRESSURE ENTHALPY CALCULATIONSCALCULATIONS Net Refrigeration Effect (NRE) = C – B Net Refrigeration Effect (NRE) = C – B

= 110 btu/lb – 40 btu/lb = 70 btu/lb= 110 btu/lb – 40 btu/lb = 70 btu/lb Heat of Compression (HOC) = E – C = Heat of Compression (HOC) = E – C =

127 btu/lb – 110 btu/lb = 17 btu/lb127 btu/lb – 110 btu/lb = 17 btu/lb Total Heat of Rejection (THOR) = E – A Total Heat of Rejection (THOR) = E – A

= 127 btu/lb – 40 btu/lb = 87 btu/lb= 127 btu/lb – 40 btu/lb = 87 btu/lb Heat of Work = E – D = Heat of Work = E – D =

127 btu/lb – 112 btu/lb = 15 btu/lb127 btu/lb – 112 btu/lb = 15 btu/lb

UNIT SUMMARYUNIT SUMMARY Common refrigeration temperature ranges are Common refrigeration temperature ranges are

high, medium and lowhigh, medium and low High temperature refrigeration is also referred High temperature refrigeration is also referred

to as air conditioning or comfort coolingto as air conditioning or comfort cooling Refrigeration is the process of transferring heat Refrigeration is the process of transferring heat

from a place where it is objectionable to a place from a place where it is objectionable to a place where it makes little or no differencewhere it makes little or no difference

Heat flows naturally from warm to cool Heat flows naturally from warm to cool substancessubstances

UNIT SUMMARY - 2UNIT SUMMARY - 2 Saturated refrigerants follow a P/T relationshipSaturated refrigerants follow a P/T relationship One ton of refrigeration is equal to 12,000 btu/hourOne ton of refrigeration is equal to 12,000 btu/hour The evaporator is the component that absorbs heatThe evaporator is the component that absorbs heat The condenser is the component that rejects heatThe condenser is the component that rejects heat The metering device is the component that The metering device is the component that

regulates refrigerant flow to the evaporatorregulates refrigerant flow to the evaporator The compressor creates the pressure difference in The compressor creates the pressure difference in

the system that allows the refrigerant to flowthe system that allows the refrigerant to flow

UNIT SUMMARY - 3UNIT SUMMARY - 3 Superheat is equal to evaporator outlet Superheat is equal to evaporator outlet

temperature minus evaporator saturation temperature minus evaporator saturation temperaturetemperature

Subcooling is equal to the condenser saturation Subcooling is equal to the condenser saturation temperature minus the condenser outlet temperature minus the condenser outlet temperaturetemperature

Superheated and subcooled refrigerants do not Superheated and subcooled refrigerants do not follow a pressure/temperature relationshipfollow a pressure/temperature relationship

Modern refrigerants must be safe and detectableModern refrigerants must be safe and detectable The pressure enthalpy chart provides a graphical The pressure enthalpy chart provides a graphical

representation of a refrigeration systemrepresentation of a refrigeration system

Documents

Section 1: Theory of Heat Unit 3: Refrigeration and Refrigerants