The Refrigeration Cycle

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27 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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The Refrigeration Cycle

A Thermodynamic Approach




Introduction


The refrigerator is an appliance that seems to disobey the laws
of thermodynamics, which
dictate how heat travels from a high
temperature source to a low temperature source and generally
not in the reverse direction. So then how does a refrigerator
remove heat from a cold source and where does that heat go?


The answer comes from

Kelvin and
Pl
anck
’s

statement of the
second law of thermodynamics. They explain how, within a
cyclic heat engine, work
-
energy can be produced with a supply
of heat (Q
h
) from a high
-
temperature reservoir (T
H.
) However
,

a
rejection of excess heat (Q
l
) into a low
-
temper
ature reservoir
(T
L
) is also required (Figure 1a).


It is i
deally a reversible process, where with the help from a
source of work
-
energy, heat can be removed from a low
-
temperature reservoir and rejected into a high
-
temperature
reservoir (Figure 1b). The
reversed heat engine is the basi
s of
the refrigeration process. The refrigeration cycle is 4 stage cyclic
process that relies on thermodynamic principles to extract heat
from a cold space.



The cycle uses a refrigerant as
the working fluid to travel

within
the system. The refrigerant is a
substance that has a low boiling
point, generally around
-
30


F at
atmospheric pressure.
Figure 2
outlines the basic cycle that the
refrigerant will follow, which
includes the following stages:




Stage 1: Compression


Stage 2: Condensation


Stage 3: Expansion



Stage 4: Evapora
tion




Figure 2

The refrigeration cycle


Figure 1a

Kelvin
-
Planck heat engine





Figure 1b

Reversed heat engine
(Refrigerator)


So u r c e:
St e p h e n R.
T u r n s,

T h e r mo d y n a mi c s C o n c e p t s
a n d Ap p l i c a t i o n s
, ( N e w Y o r k:
C a mb r i d g e U n i v e r s i t y P r e s s,
2 0 0 6 ).

So u r c e:

Ge o 4 VA,
h t t p://w w w.g e o 4 v a.v t.e d u/A3/A3.h t m
.





Stage 1: Compression

To start the process, we look at the
compressor, which is the source of work for the
system in Figure 1b. At this stage, the
refrigerant enters the compressor as a
saturated vapor, w
hich can be seen as
Point 1

on the refrigerant phase diagram in Figure 3. At
this point, the refrigerant pressure and
temperature is lower than the pressure and
temperature within the cold space of the
refrigerator. By going through the compressor,
the ref
rigerant undergoes adiabatic
compression, which is to say it gains pressure
without the loss or gain of heat energy. This
process continues until the refrigerant is 4 or 5
times atmospheric pressure.

Increasing

the pressure of a vapor adiabatically
results in the internal temperature of the vapor
rising dramatically as well. Now the vapor is
superheated well above room temperature as
shown by
Point 2.


Stage 2: Condensation

The superheated vapor travels from t
he compressor to the condenser, which is the series of coils
that are exposed to the surrounding air in the kitchen. This ambient air acts as the high
-
temperature reservoir seen in Figure 1a. However, while the air is at a high
-
temperature relative
to the
overall system, the vapor at this point is even hotter. In an isobaric (at constant pressure)
environment, the vapor continues through the coils, releasing heat to the surrounding air.

Once it reaches
Point 3

on the phase graph, it begins a phase change f
rom vapor to liquid. As
the substance undergoes an isobaric phase change, the temperature does not change, even
though it is continually losing energy. This stability at a higher temperature prolongs the
temperature differential between coils and the air.
This allows for a more efficient heat transfer.
Upon reaching the end of the condensing coils, the refrigerant is now a saturated liquid.

















1
-
2: Compression of vapor

2
-
3: Superheated vapor cooled in condenser

3
-
4: Vapor converted to liquid in condenser

4
-
5: Liquid becomes liquid + vapor mix in
expansion valve

5
-
1: Liquid + Vapor mix converted to vapor in evaporator

Figure 3

Refrigerant phase diagram & refrigeration cycle path

So u r c e:
Wi k i p e d i a, h t t p://e n.w i k i p e d i a.o r g/w i k i/Va p o
r
-
c o mp r e s s i o n _ r e f r i g e r a t i o n.


Stage 3: Expansion

From the condenser, the liquid refrigerant enters an expansion
valve. This valve
depressurizes the fluid almost
instantaneously until it is below the atmospheric pressure.
This process is also adiabatic
,

causing the temperature to
drop alongside the pressure. The fluid, now a mixture of both
liquid and vapor, is resting at a temperatu
re much colder
than the freezer compartment of the refrigerator.


Stage 4: Evaporation

The liquid
-
vapor mixture now enters a
second set of coils
placed inside the refrigerator. These coils serve as the heat
exchanger with the cold air within the refrigerator. In this
case, the refrigerant in the coils is even colder than the
chilled air in of the refrigerator compartments, mean
ing that
the refrigerant draws out and absorbs heat from air within
the cold space (Figure 4). The absorption of heat does not
raise the temperature of the fluid because
--
just like in the
condenser
--

the fluid is going through an isobaric phase
change.
Thu
s

all the heat being absorbed is going into
converting the remaining liquid into vapor. Upon the
conclusion of this stage, the refrigerant is ready to enter stage
1 again.


Final Remarks

It is important to note that this cycle does not run continuously.
The cycle is powered by the pressure differenc
e created by the
compressor, which forces the movement of the refrigerant.
However the compressor itself is only powered on when
necessary, for example when the cold space of the refrigerator
warms up. This happens due to users opening and closing the
door

as well as the inevitable leakage of heat entering the
space. The heat absorbed through the evaporating coils is
simply the excess heat that entered the refrigerator's cold space
from the outside.

At first glance, the refrigerator seemingly violates the
laws of
thermodynamics, with heat flowing from cold to hot. With a
more detailed look, however, it is shown that the spontaneous
flow of heat from hot to cold is what allows the refrigeration
cycle to work.


Note:

Modern refrigerators
have better insulation than
the old mo
dels. In the long
run it is usually more
economical to replace old
refrigerators because older
models are running the
compressor more often.

Figure 4

Heat transfer from
condensing coils to ambient
air


Heat transfer from
refrigerator cold space to
evaporator coils