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A Refrigerator Heater

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A Refrigerator Heater


A refrigerator is a cooling device meant to keep substances at
temperatures lower than the outside temperature, as low as 2 to 3 °C.
A refrigerator works on the basic principle of heat transfer like a
heat engine. While a heat engine takes in heat from a higher
temperature, expends some of it for doing work and rejects the
remaining as exhaust at a much lower temperature, a refrigerator does
quite the converse. It takes in heat from a body making it cold, has
work done upon it and rejects the heat at a much higher temperature
(Fig.1; Appendix). A heat pump as the name suggests, is a device which
can pump heat in or out from an enclosure like a room. It is very
similar to that of a refrigerator, the only difference being that it
takes in heat from a cold reservoir like the outside air and passes it
to a room at a higher, suitable temperature. It can also operate vice
versa by introducing a valve where the process of cooling or heating
can be adjusted. Theoretically, a refrigerator should be able to work
like a heat pump. However, it has its own possibilities and
limitations which shall be discussed in this essay.

A schematic diagram of a refrigerator is shown in the Appendix
(Fig.2). The working of a refrigerator comprises of four major steps
namely – evaporation, compression, condensation and expansion. A
chemical called a refrigerant is used in this process to remove the
heat from the refrigerator. Refrigerants must have certain specific
properties – they must be easily liquefiable, they must have low
specific heat capacities, they must be non-toxic, non-flammable and
non reactive with foodstuff etc. The first refrigerants used were
ammonia, ether and chemogene (a mixture of petrol ether and naphtha).
However, nearly all were toxic, flammable and reactive. Thomas Midgley
made a new species of compounds from fluorine called Chloro-Fluoro
Carbons (CFC)[1]. They were non-toxic, non flammable and removed large
quantities of heat from the refrigerator and were ideal refrigerants.

The refrigerant is contained in the refrigerant tank in a cold liquid
state. First, it goes to the evaporator. The evaporator is a snake
like pipe which is in close contact with the refrigerator cabin. The
refrigerant absorbs this heat cooling the cabin. It uses it as latent
heat and changes from cold liquid to cold vapour without any change in
temperature. Thus, the evaporation is isothermal.

The cold vapour then goes to the compressor. The compressor is of a
big volume with a piston attached to it. Electrical energy is used to
move the piston. The cold vapour is compressed suddenly increasing the
pressure and decreasing the volume rapidly. Because of the sudden
compression, there is no time for the heat generated in the vapour to
flow out and temperature increases. Thus, this process is adiabatic.
Since work is done on the gas, W <0. The cold vapour now changes into
very hot vapour.

The hot vapour then flows into the condenser. The condenser is a
vertical grating of coils located at the back of the refrigerator.
This is quite the opposite of what happens in the evaporator. The hot
vapour condenses and gives away its thermal energy in the form of
latent heat. It changes from hot vapour to hot liquid at the same
temperature, making the process isothermal. The heat is given out from
the back to the surroundings.

Hot liquid from the condenser passes through an expansion valve. This
is a chamber of large volume but with a series of narrow, twisted
tubes before it. The hot liquid passes through the loops and turns and
becomes hotter and an increase in pressure. It then rapidly expands
into the large volume. Pressure decreases rapidly, volume increases
rapidly and temperature drops. The hot liquid turns into cold liquid.
Because of all three changes, this process is adiabatic. Work is done
by the gas so W> 0.

A heat pump, as the name implies, moves heat from one place to
another, usually used in homes. In winter, it moves in heat from
outside to inside the house and in summer, moves heat from inside to
outside. This dual nature of a heat pump can be done with a valve
which can reverse the direction of the transfer of heat. Thus, heat
pumps can act as heaters as well as air conditioners.2

Most heat pumps are air-source pumps; they use air to get heat into
the house. Water-source heat pumps also exist which get heat from an
outside source of water, usually well water. Heat pumps can also use
refrigeration techniques to cool or warm a house. A diagram of how a
heat pump can use refrigeration techniques to warm or cool a house is
shown in the Appendix (Fig. 3 and 4). As it can be seen, the process
is fairly similar to that of a standard refrigerator, the only
difference being that the interior is the house and the exterior is
the air.3

Analysing the heat transfer in a refrigerator, a refrigerator could
also, theoretically, be made to function as a heat pump. The heat that
is given out in the third phase after condensation could be captured
and delivered to the interior of a house through a pump. Similarly,
introducing a valve, the vice versa should also be possible. However,
the second process would seem rather difficult since in condensation,
heat is given out of the refrigerator by the vapour. When cooling the
house, the heat would have to be given to the condenser. This would
change the liquid refrigerant back to vapour upsetting the
refrigeration cycle. However, this process could theoretically be
prevented if while cooling the house, the heat is supplied not to the
condenser but to a place which needs heat to be given in. the most
suitable place is when the hot vapour is moving from the compressor to
the condenser. Giving in heat will raise its temperature further. It
should not upset the refrigeration process.

It is necessary to see that the above mentioned process is theoretical
to a large extent. The first objective of warming the house is not
very difficult since a pump can be attached to the condenser and the
heat rejected can be supplied to the house, suitable regulating the
flow of heat. However, the second objective is far more difficult to
achieve because it requires a modification of the framework of a
refrigerator. It requires an additional, alternate way from the pump
supplying the heat to the house to the vapour when it is going from
the compressor to the condenser only when the house is being cooled.
This requires a fairly large amount of money and modification in the
building of a refrigerator. Present refrigerators do not have this
modification because the aim of a refrigerator is to cool stuff kept
in it; making it function like a heat pump is an additional theory
that can make the refrigerator serve for a double purpose. It is,
though, not impossible to make the refrigerator function like a heat
pump to warm the house; it should rather be called a one way heat pump
since heat transfer in only one direction will take place.

Another fact to be noted is that despite the heat given out from the
condenser, the temperature of the heat is grossly insufficient to warm
a house of moderate size. This can be seen by placing our hands behind
a refrigerator. The air coming out is warm, no doubt, but if thought
practically to use this heat to warm a room, the idea would seem
rather absurd because the heat, is in no way, sufficient. Despite
this, to use this heat, one would further have to heat the air using
another source of energy and then convey this heat to the house.
Instead of following this complicated process, we can buy an external
heater itself. Thus, it can be seen that the functioning of a
refrigerator as a heat pump is very, very theoretical.

The fact that the use of a refrigerator as heat pump is not suitable
practically leads us to the question why? Why isn’t it possible? This
leads us to the concept of efficiency. Efficiency, as the name
implies, can best be described as how “good” a machine is, how well it
does its work. The scientific definition is “The ratio of the useful
energy delivered by a dynamic system (such as machine, engine, or
motor) to the energy supplied to it over the same period or cycle of
operation.”4 If an amount Q of energy is expended and an amount W of
work is obtained, then the efficiency is W/Q. Efficiency is often
expressed as a percentage; it gives a clearer idea of how good and
efficient the machine is. A good and efficient machine should waste as
little heat as possible and do maximum work. 100% efficiency is not
possible because there is always some way, where the machine will
waste some heat. There are reasons for this – “perfect” insulation of
the system is not possible, machine parts will absorb some energy in
the form of heat etc.

In the case of a refrigerator, the concept of efficiency is slightly
modified because of its nature of working. As mentioned before, it
takes heat from a colder body and releases it to a hotter body. Thus,
the work done will be the heat it removes from the refrigerator cabin.
For a refrigerator, efficiency is called Coefficient of Performance
(COP). It is “the ratio of the amount of heat removed at the lower
temperature to the work put into the system”.5

Let heat removed be or = Qlow

work done

Heat given out = Qhigh

at high temperature

Heat supplied = Qhigh - Qlow

COP = W

Q

= Qlow

Qhigh - Qlow

For a heat pump as well, the efficiency is called the COP. In this
case, the work done by the heat pump will be the heat given or removed
from the house and the energy supplied will be the heat supplied or
removed from the house minus the heat taken or given to the
surrounding.

Let heat removed/given = Qhouse

house

Heat given/removed = Qoutside

surrounding

Heat supplied = Qhouse - Qoutside

COP = W

Q

= Qhouse

Qhouse - Qoutside

In conclusion, a refrigerator will be a bad heat pump because its aim,
objective is to cool stuff kept inside. To behave like a heat pump
will require certain modifications mentioned before. In the present
world, such technology is not available and is also not worth it. It
is highly possible in the years to come, refrigerators be used for
double purposes of cooling and heating. They would no doubt be
different in structure to the refrigerators we use today. Restricting
ourselves to the present situation, it is highly absurd to think about
using refrigerators as heat pumps.

Bibliography

1. Examples of Heat Pump and Refrigerator (2005).
http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ThermLaw2/HPandRefer.html
(updated 4th April 2005, accessed 4th April 2005)

2. Hypertextbook (2005), Refrigerators.
http://hypertextbook.com/physics/thermal/refrigerators/ (updated
4th April 2005, accessed 4th April 2005)

3. Physics202 (2005), Carnot Engines, Heat Pumps and Refrigerators.
http://oak.cats.ohiou.edu/~piccard/phys202/carnot/carnot.html
(updated 4th April 2005, accessed 4th April 2005)

Appendix

[IMAGE](Fig. 1)6

[IMAGE] (Fig. 2)7

[IMAGE](Fig.3)8

Heating with a heat pump

[IMAGE](Fig.4)9

Cooling with a heat pump


---------------------------------------------------------------------

[1] Trane (2005) Refrigerant History.
http://www.trane.com/commercial/issues/environmental/cfc6b.asp
(updated 4th April 2005, accessed 4th April 2005)

2 The Energy Outlet (2005), Heat Pumps – How does a heat pump work.
http://energyoutlet.com/res/heatpump/how.html (updates 4th April 2005,
accessed 4th April 2005)

3 The Energy Outlet (2005), Heat Pumps – How does a heat pump work.
http://energyoutlet.com/res/heatpump/how.html (updates 4th April 2005,
accessed 4th April 2005)

4 US DOE Energy Efficiency and Renewable Energy (EERE), Efficiency.
www.eere.energy.gov/ (updated 4th April 2005, accessed 4th April 2005)

5 Heat Engines (2005).
http://electron9.phys.utk.edu/phys136d/modules/m3/heatpump.htm
(updated 4th April 2005, accessed 4th April 2005)

6 The open door website, Thermal Physics – Heat Pumps.
http://www.saburchill.com/physics/chapters/0126.html (updated 4th
April 2005, accessed 4th April 2005)

7 UNIFIED ENGINEERING September 1996, Refrigeration cycles.
http://web.mit.edu/16.unified/www/FALL/thermodynamics/chapter_5.htm
(updated 4th April 2005, accessed 4th April 2005)

8 The Energy Outlet, Heat Pumps – Pumping heat uphill.
http://energyoutlet.com/res/heatpump/pumping.html (updated 4th April
2005, accessed 4th April 2005)

9 The Energy Outlet, Heat Pumps – Pumping heat uphill.
http://energyoutlet.com/res/heatpump/pumping.html (updated 4th April
2005, accessed 4th April 2005)

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