2 basic principals of refrigeration and ph charts

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 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 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


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


Cooling Tower


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


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


Pressure

Enthalpy


Liquid - Vapor

Mix

Pressure

Enthalpy


Pressure

Enthalpy


100% Liquid

Pressure

Enthalpy


Pressure

Enthalpy


100% Vapor

Pressure

Enthalpy


Pressure

Enthalpy

20% Liquid 80% Vapor


Pressure

Enthalpy


Pressure

Enthalpy

Evaporator


Pressure

Enthalpy


Evaporator

Pressure

Enthalpy


Evaporator

Refrigerant absorbs

heat from load

Pressure

Enthalpy

Evaporator


Refrigeration

Effect

Pressure

Enthalpy

Evaporator


Pressure

Enthalpy


Compressor

Pressure

Enthalpy


CompressorHead Pressure

Pressure

Enthalpy


Condenser

Pressure

Enthalpy


Condenser

Refrigerant rejects

heat to atmosphere

Pressure

Enthalpy

Metering Device


Pressure

Enthalpy


- Thermal expansion valve- Orifice

Metering Device

Pressure

Enthalpy


Evaporator

Compressor

Condenser

Metering

Device

Pressure

Enthalpy


Evaporator

Compressor

Condenser

Metering

Device

Refrigerant absorbs

heat from load

Refrigerant rejects

heat to atmosphere

Pressure

Enthalpy


Pressure

Enthalpy


95°

85°

54°

44°44°

Pressure

Enthalpy


95°

85°

44°

54°

44°

54°

Pressure

Enthalpy


95°

85°

44°

54°

Pressure

Enthalpy


85°

44°

54°

95°

Pressure

Enthalpy


85°

Cooling

Tower

44°

54°

95°

Pressure

Enthalpy


95°

85°

Cooling

Tower

44°

54°

Pressure

Enthalpy


Pressure

Enthalpy


What is this called?

Pressure

Enthalpy


Refrigeration

Effect


Refrigeration

Effect

Pressure

Enthalpy

Adding a subcooler


Refrigeration

Effect

Pressure

Enthalpy

Adding a subcooler

Increases refrigeration effect


Refrigeration

Effect

Pressure

Enthalpy

Adding a subcooler

Increases refrigeration effect

Increases energy efficiency


Refrigeration

Effect

Pressure

Enthalpy

Subcooled


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


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


SIMPLE CYCLES

Example 2




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?


SIMPLE CYCLES

Example 3





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.





Questions

2 basic principals of refrigeration and ph charts

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