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Fridges

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by

Saleh Abdalla

on 7 January 2013

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Transcript of Fridges

Liquification Stage Evaporation Stage How it works The refregiration cycle Refrigeration cycle (cont'd) Refrigeration cycle (cont'd) Refrigerants and alternatives Alternative 1: HCFC
(Hydrochlorofluorocarbon) Chemical trends (C,N,O,S,H,F,Cl) How it works (cont'd) History 1856 Chemicals used in refregiration Done by:

Saleh Juan Abdalla 100036816
Ali Al Nuwais 100036582
Hamdan Al Shehhi 100035220
Arman Al Awadhi 100036489 Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman Chem 115 - Fall 2012 - Saleh,Ali,Hamdan,Arman The originally scottish James Harrison invented the very first and premitive refrigerator in Australia using the principle of vapor compression 1927 General Electric produced 1,000,00 cooling units that used compressors. They used Sulphur dioxide as a refregirant 1931 Dupont contrinuted widely to the refregiration industry by producing commercial quantities of refregirators that use the nontoxic and non-inflammable dichlorodifluoromethane 20th century Refregirators that use CFC's appeared and the quest for finding alternative refregirants started due to public demands *Most refregirators use a phase change heat pump.

*The pump makes use of a low boiling point liquid typically called a "refregirant".

*The refregirant absorbs energy as heat from the surroundings when its evaporated and releases the energy as heat to the surroundings when condensed.

*The pumps keeps this process going in a cycle called the refregiration cycle. *The refrigerator's electric grid at the back
drives an electric motor to run a mechanical compressor.

*Since there is a cycle: there is movement of substances back and forth: energy is needed to move them: it comes from mechanical motors: they are fed by electric components.

Remember: energy is conserved! The compressor compresses the low-pressure and low-temperature vapor to a high-pressure and high-tempreature vapor then passes it to a condenser to turn it to liquid.
The work of compression heats up the vapor. Remember PV=mRT! The warm high-pressure liquid comes out of the condenser and gets pumped by the compressor into an evaporator.
The compressor pumping action drops the pressure and temperature of the liquid well below its boiling point.
The liquid evaporates to a low-pressure, low-temperature vapor and repeats the cycle. Condenser Evaporator *The preceding reactions are examples of physical reactions. The refregirant changes from the liquid phase to the vapor phase back and forth.

*An exothermic reaction takes place at the condenser.
(ΔH = -ve)

*An endothermic reaction takes place at the evaporator.
(ΔH = +ve)

*Heat of vaporization of the refregirant is considered here. *The refregiration cycle allows a small amount of refrigerant to be used and for durable amounts of time.

*A good refregirant has to have the following properties:

- non-inflammable
- noncorrosive
- safe
- boiling point lower than desired refrigerator's temperature
- high heat of vaporization *The most common refrigerant used is Chlorofluorocarbon (CFC).

*When CFCs break down after the refrigeration process, they release atomic chlorine which breaks down ozone.

*Advantages: Non-flammable and non-toxic refrigerant gas.

*Disadvantages: causes depletion of ozone layer allowing harmful ultraviolet rays to enter the Earth. image from: http://undsci.berkeley.edu/images/ozone/ccl3f.gif *By adding hydrogen to the chemical formulation chemists made CFCs less stable in the lower atmosphere enabling them to break down before reaching the ozone layer.

*Advantages: Lower atmospheric lifetimes (2 - 25 years) than CFCs (100 years or more)

*Disadvantages: Can contribute to global warming (it’s a greenhouse gas)
More expensive to produce than CFCs image from: http://0.tqn.com/d/chemistry/1/G/J/D/1/Chlorodifluoromethane.jpg Alternative 2: HFC
(Hydrofluorocarbons) *Hydrofluorocarbons like CFC molecules, except hydrogen atoms have replaced the chlorine atoms.

*Advantages: Do not contain chlorine means no ozone depletion potential.
Lower stability than CFCs
Lower atmospheric lifetimes (2 - 25 years) than CFCs (100 years or more)

*Disadvantages: Can contribute to global warming (it’s a greenhouse gas)
More expensive to produce than CFCs *Increasing carbon generally increases the molecular weight and the boiling point.
*Increasing nitrogen generally makes the compound more reactive. This can lead to toxicity and instability.
*Increasing oxygen generally reduces atmospheric stability, but may lead to toxicity, flammability and reactivity.
*Increasing sulfur generally increases toxicity and decreases stability.
*Increasing hydrogen generally reduces atmospheric lifetime,but increases flammability.
*Increasing fluorine attached to carbon increases global warming potential.
*Increasing chlorine improves ozone depletion potential and toxicity. *A device called the thermocouple (basically a thermometer) works so your refrigerator stays in any temperature you want.

*The thermocouple will shut off the power supply to the compressor as soon as it reaches the assigned temperature.

*Since the refrigerator is not sealed of completely (around the doors and where the pipes go through) some heat will leak.

*The thermocouple senses the change in the temperature and turns the compressor back on to cool the refrigerator again. How does the inside temperature stays the same? 10 11
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