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Basic Principals of Refrigeration
DefinitionRefrigeration is the transfer of heat from one place to another by a change in state of a liquid.
Alcohol on ones handevaporates and cools
Sensible Heat
Heat which causes a change in temperature of a substance
Sensible HeatHeat which causes a change in temperature of a
substance is called sensible heat.
If a substance is heated and the temperature rises as the heat is added, the increase in heat is called
sensible heat.Likewise, heat may be removed from a substance. If
the temperature falls, the heat removed is, again, sensible heat.
Sensible heat can be measured in degree of temperature.
Latent HeatHeat energy absorbed in the process of changing form of a substance without a change in temperature or pressure. Heat energy that cannot be measured with a thermometer.
32o
Latent HeatHeat which brings a change in state with no
change in temperature.
When we think about ‘change of state’ conditions we are considering Solids to
Liquids and Liquids to Vapours.
Latent or Sensible?
• What changed from previous slide
• What type of heat was involved
32o
Latent Heat
32o 32o
What is Refrigeration?The process of reducing heat energy.
One example is by the removal of sensible heat, chilled water from 54 F to 44 F.
Another example is removing latent heat from water at 32 F to change it to ice at 32 F. A change
of state without a change of temperature
What is Refrigeration?
What is required to make a refrigeration unit?
1) Compressor
2) Condenser
3) Expansion device
4) Evaporator
5) Refrigerant
What is Refrigeration?What does the compressor do?
The compressor has two main functions.
1) To circulate the refrigerant around the system.2) To increase the pressure of the refrigerant vapour to a pressure at which it can be condensed back into a liquid.
What is Refrigeration?What happens in the condenser?
The condenser provides the surface area and storage space to:-
1) Remove sensible and latent heat from the high pressure refrigerant vapour to change it to a liquid.
2) To store sufficient liquid to create a vapour seal between the condenser and expansion device.
What is Refrigeration?
What does the expansion device do?
The expansion device is a throttling device or orifice. It causes the high
pressure liquid from the condenser to convert to a mixture of low pressure
vapour and liquid.
What is Refrigeration?
What happens in the evaporator?
The evaporator provides the surface area to change the low pressure liquid
refrigerant into vapour. To change the liquid to vapour, latent heat is required.
This heat is taken from the medium being refrigerated.
What is Refrigeration?What are Refrigerants?
A substance that can change state from a liquid to a vapour at temperatures to suit the required application, normally between 70 to 150 F and
evaporate from liquid to vapour at temperatures from-40 to 80 F. These processes should take
place at reasonable pressures.
Different refrigerants will be selected for different applications depending on operating
temperatures.
Saturated Vapor
Vapor and liquid are in contact with one another.
SubcoolingCooling the refrigerant below its condensing
temperature. A refrigerant must be in a complete liquid state before it can be subcooled.
SuperheatTo raise the temperature of a vapor beyond
its boiling point. No liquid can remain when a refrigerant is superheated.
Pressure / Temp. Charts
Boiling points at atmospheric pressure (14.7PSIA)
R-11 = + 75o
R-123 = + 82o
R-12 = - 21o
R-134a = - 15o
R-22 = - 41o
Vapor Pressure 80°F(Saturated state)
RR--1111 RR--2222RR--134a134a
1.5 PSIG 86.4 PSIG 143.6 PSIG
TerminologySaturation Point: - The boiling temperature of a substance at a given pressure.
Saturation: - A mixture of liquid and vapour at its saturation temperature and pressure.
Superheat: - Vapour at a temperature above its saturation temperature, measured in degrees of superheat.
Subcooled: - Liquid below its saturation temperature, measured in degrees of subcooling.
What is Refrigeration?How does the system work?
Follow the diagram of a basic system starting at the compressor discharge.
Vapor Compression
Evaporator
Condenser
Expansion ValveCompressorMotor
Cooling Tower
What is Refrigeration?Refrigerant in vapour state, at high pressure and
high temperature, passes into the condenser. When the high temperature vapour contacts the
cool tubes, it firstly gives up sensible heat (superheat) and becomes (saturated vapour). Latent heat is then transferred to the cooling
water in the tubes and the vapour changes state to liquid.
The pressure of the refrigerant remains the same.
Vapor Compression
Evaporator
Condenser
Expansion ValveCompressorMotor
Cooling Tower
What is Refrigeration?The high pressure, medium temperature liquid now passes through the expansion
device (orifice). As the liquid passes through the expansion device some of the liquid will
‘flash off’ creating a low temperature liquid/vapour mixture, normally about 10 F colder than the medium being cooled. The sensible heat removed from the medium
being cooled is transferred to the refrigerant liquid where latent heat is absorbed which changes the state of the liquid to a vapour.
Vapor Compression
Evaporator
Condenser
Expansion ValveCompressorMotor
Cooling Tower
What is Refrigeration?
The low pressure, low temperature vapour is then sucked into the
compressor where it is compressed into high pressure, high temperature vapour.
Vapor Compression
Evaporator
Condenser
Expansion ValveCompressorMotor
Cooling Tower
More Terminology
Evaporator split: - The temperature difference betweenthe evaporator saturation temperature and the leavingprocess temperature.
Evaporator range: - The temperature difference betweenthe entering and leaving process temperature.
Condenser split: - The temperature difference between thecondenser saturation temperature and the leavingcondenser water temperature.
Condenser range: - The temperature difference betweenthe entering and leaving condenser temperature.
System analysis
Look at the following exercises and see if you can analyze the system operation
R22 System operating logs analysis
Look at the following operating logs of a R22 water chiller and evaluate the system
operation based on the actual readings compared to design conditions.
Questions
Pressure Enthalpy Charts
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Heat Content
(BTU / lb.)
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Liquid - Vapor
Mix
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
100% Liquid
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
100% Vapor
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
20% Liquid 80% Vapor
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Evaporator
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Evaporator
Pressure
Enthalpy
Pressure - Enthalpy Chart
Evaporator
Refrigerant absorbs
heat from load
Pressure
Enthalpy
Pressure - Enthalpy Chart
Evaporator
Refrigerant absorbs
heat from load
Pressure
Enthalpy
Evaporator
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Evaporator
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
Compressor
Pressure
Enthalpy
Pressure - Enthalpy Chart
Compressor
Pressure
Enthalpy
Pressure - Enthalpy Chart
CompressorHead Pressure
Pressure
Enthalpy
Pressure - Enthalpy Chart
Condenser
Pressure
Enthalpy
Pressure - Enthalpy Chart
Condenser
Refrigerant rejects
heat to atmosphere
Pressure
Enthalpy
Pressure - Enthalpy Chart
Condenser
Refrigerant rejects
heat to atmosphere
Pressure
Enthalpy
Metering Device
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
- Thermal expansion valve- Orifice
Metering Device
Pressure
Enthalpy
Pressure - Enthalpy Chart
Evaporator
Compressor
Condenser
Metering
Device
Pressure
Enthalpy
Pressure - Enthalpy Chart
Evaporator
Compressor
Condenser
Metering
Device
Refrigerant absorbs
heat from load
Refrigerant rejects
heat to atmosphere
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
95°
85°
54°
44°44°
Pressure
Enthalpy
Pressure - Enthalpy Chart
95°
85°
44°
54°
44°
54°
Pressure
Enthalpy
Pressure - Enthalpy Chart
95°
85°
44°
54°
Pressure
Enthalpy
Pressure - Enthalpy Chart
85°
44°
54°
95°
Pressure
Enthalpy
Pressure - Enthalpy Chart
85°
Cooling
Tower
44°
54°
95°
Pressure
Enthalpy
Pressure - Enthalpy Chart
95°
85°
Cooling
Tower
44°
54°
Pressure
Enthalpy
Pressure - Enthalpy Chart
Pressure
Enthalpy
Pressure - Enthalpy Chart
What is this called?
Pressure
Enthalpy
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Adding a subcooler
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Adding a subcooler
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Adding a subcooler
Increases refrigeration effect
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Adding a subcooler
Increases refrigeration effect
Increases energy efficiency
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Subcooled
Pressure - Enthalpy Chart
Refrigeration
Effect
Pressure
Enthalpy
Subcooled Superheat
System Calculations
PRESSURE ENTHALPY CHARTSSIMPLE CYCLES
Lets now plot some simple cycleson a PH chart.
For this exercise we will use a basic water chiller operatingon R22
Pressure - Enthalpy Chart
SIMPLE CYCLES
Example 1
Suction Pressure 75psia
Suction Temperature 50 F
Liquid Line Temperature 112 F
Discharge Pressure 250 psia
Plot the above points on the PH chart and calculate thefollowing.
1) Heat of Rejection2) Refrigeration Affect3) C.O.P
Pressure - Enthalpy Chart
SIMPLE CYCLES
Example 2
Suction Pressure 75psia
Suction Temperature 50 F
Liquid Line Temperature 90 F
Discharge Pressure 250psia
Plot these new points and answer the following questions.
1) What is the total Subcooling?2) What difference does this have on the system?3) If the discharge pressure increases to 350psia and there
is no subcooling, what affect does this have on thesystem?
Pressure - Enthalpy Chart
SIMPLE CYCLES
Example 3
Suction Pressure 40psia
Suction Temperature 15 F
Liquid Line Temperature 90 F
Discharge Pressure 250psia
Plot these new points and answer the following questions.
1) What happens to the R.E when the suction pressure isreduced?
2) What happens to the H.O.C when the suction pressure isreduced?
SYSTEM CAPACITY
We can now calculate the system capacity if we knowthe following information.
Enthalpy btu’s/lbs
Mass flow lbs/min
We can calculate the mass flow if we know thefollowing.
Actual pumped volume of the compressor cu.ft/min
Specific volume of the refrigerant at the suction ofthe compressor cu.ft/lb
The actual pumped volume of the compressor can beobtained from the manufacturer. The rest of theinformation we can read off our PH chart.
Example 4
If we use the system parameters plotted in theprevious example 3 and we have a compressor thatpumps 500cu.ft/min. What would our system capacitybe?
Enthalpy=67btu/lbSpecific volume=1.3cu.ft/lbActual volume=500cu.ft/lb
Mass flow lbs/min= actual volume/specific volume
Capacity btu/min=lbs/min x btu/lbs
Btu’s/min / 200 = Tons refrigeration
500/1.3 = 384.6lb/min
384.6 x 67 = 25,769 btu/min
25,769/200 = 128.8 tons
Example 5
Plot the following parameters on a PH chart and calculatethe system capacity if the compressor has a actual volumeof 8000cu.ft/min.
Suction Pressure 20psia
Suction Temperature 0 F
Liquid Line Temperature 80 F
Discharge Pressure 200psia
Questions
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