1) Boyle's law states that the pressure and volume of a gas are inversely proportional at constant temperature. Charles' law states that the volume of a gas is directly proportional to its temperature at constant pressure. 2) A refrigerant is a substance used in refrigeration to absorb heat from the space being refrigerated and release it elsewhere. Common refrigerants like Freon gas are used in refrigerators. 3) A refrigerator uses a vapor-compression cycle to cool its interior. Freon gas is compressed, condenses while releasing heat outside, then evaporates in the interior, absorbing heat and cooling the refrigerator. This cycle repeats continuously.

1. 1
Module # 32
Boyle’s Law, Charles’ Law & Refrigeration
Boyle (1627-1691)
Famous physicist Boyle is well known for his Boyle's law of
gases.
Boyle’s Law
This law was formulated by Robert Boyle in 1662. This law states
that the absolute pressure of a given mass of a perfect gas varies
inversely as its volume when the temperature remains constant.
OR
Boyle's law states that “the volume of a given mass of a gas
varies inversely as its absolute pressure provided temperature
remains constant”.
If
P = absolute pressure of a gas
V = its volume
Then according to Boyle's law temperature remaining constant
V  l / P
OR

2. 2
PV = constant
In other words, the product of absolute pressure and volume of a
given mass of a gas is constant when there is no change in its
temperature.
If a gas initially at a pressure P1 and volume V1 is expanded or
contracted at a constant temperature such that its new pressure
and volume are P2 and V2 respectively then according to Boyle's
law.
P1V1=P2V2
No known gas obeys Boyle's law perfectly. There is always a
deviation which can be over looked in case of air, hydrogen and
nitrogen etc. except at very low temperature or at very high
pressure.
If we draw a graph between volume and pressure, we get a curve
called Hyperbola.
V
Fig: (1) Graph of pressure and volume

3. 3
Verification of Boyle's Law
Boyle's law can be verified by means of an experimental
arrangement shown in the figure (2). It consists of two glass tubes
A and B of uniform bore connected by a long rubber tubing. Tube
A is open and the tube B is closed at the top. They are supported
vertically by the side of a vertical scale such that tube A can be
moved upward or downward. Mercury is poured in tube A until it
partially fills both the tubes, trapping a mass of air in B. The
volume V of the air enclosed in B is noted and the difference of
heights of two mercury Columns is measured. The tube A is then
moved to another position and the readings are noted.
This process is repeated for a number of different positions of
tube A. Suppose that mercury level D in the tube A is higher than
that in tube B. The pressure P on the air would be equal to the
sum of atmospheric pressure measured by a barometer and the
difference between the levels of two mercury columns. If D were
lower than the mercury surface in B, the pressure on the air in B
would be equal to atmospheric pressure minus the difference
between the levels of two mercury columns. The product PV, for
each reading is found to be the same.

4. 4
Fig: (2) Verification of Boyle’s Law
Charles (1747-1823)
All substances expand when they are heated, but, gases expand
much more than solids or liquids. Jacques Charles observed
experimentally that when the pressure of a gas is kept constant,
then, its volume increases with temperature at a constant rate.
That is, if the absolute temperature of the gas is doubled, then,
the volume also becomes doubled and vice versa.
Charles’ Law
This law was formulated by a Frenchman Jacques A.C Charles in
about 1787.
It may be stated in two different forms
(1) The volume of a given mass of a perfect gas varies directly

5. 5
as its absolute temperature, when the absolute pressure
remaining constant.
If
V= volume of a gas
T= absolute temperature in °K
Then according to Charlie's law, pressure remaining constant,
V  T
OR
V/T = constant
V1 / T1 = V2 / T2 = V3 / T3 =.......Constant
Where suffixes 1, 2, and 3.... refer to different sets of conditions.
(2) All perfect gases change in volume by l / 273th of its original
volume at 0°C for every 1°C change in temperature, when the
pressure remains constant.
If we plot volume against temperature, we get a straight line. This
means that as T decreases, V decreases correspondingly. If this
straight line is produced towards low temperatures, it meets
temperature axis at -273° C or 0K indicating zero volume.

6. 6
However, volume of gas can never be zero. We cannot draw this
line anymore, because it will indicate negative volume which is a
meaningless idea.
Thus -273°C is the lowest possible temperature that is possible to
attain. This temperature is called the absolute zero.
For this reason, -273°C is taken as the zero of the absolute scale
of temperature measured in Kelvin.
V
Fig: (1) Temperature Volume Graph
Refrigerant
The working substance used in a refrigerator is called refrigerant.
Generally speaking, a refrigerant is anybody or substance which
acts as a cooling agent by absorbing heat from another body or

7. 7
substance. The refrigerant must be circulated in a cyclic process
during a refrigeration process and must be used again and again.
Hence, vapor compression cycle and vapor absorption cycle are
the two most important cycles of refrigeration.
A heat carrier must be used to move heat from the interior of a
cabinet or room to the outside. Thus in refrigerating systems fluids
which absorb heat inside the cabinet and release it out side are
called refrigerants.
The ideal refrigerant would be one that could liberate all the heat
that it is capable of absorbing.
The refrigerant changes from liquid to vapor during the process of
absorbing heat and condenses again to liquid while liberating
heat, in most of the refrigerating systems.
The refrigerant may be defined as a carrier of heat from a low
temperature region to a high temperature region.
Refrigeration
In general, refrigeration is defined as any process of heat
removal. More specifically, refrigeration is defined as that branch
of science which deals with the process of reducing and
maintaining the temperature of a space or material below the
temperature of the surroundings.

8. 8
Since the heat removed from the refrigerated body is transferred
to another body, it is evident that refrigerating and heating are
actually opposite ends of the same process. Often only the
described result distinguishes one from the other.
Refrigeration may be defined as the process by which the
temperature of a given space or a substance is lowered below
that of the atmosphere or surroundings. The system maintained at
the lower temperature is known as refrigerated system and the
equipment which is used to maintain this lower temperature is
known as the refrigerating system. In order to maintain a
temperature below the surrounding atmosphere, it is essential
that whatever amount of heat gets into refrigerated space, must
be extracted out to maintain the desired temperature difference.
Ton of Refrigeration
Due to the fact that in early stages refrigeration was produced by
ice, the rate of removal of heat in a cooling operation was
expressed in terms of tons of ice required per unit time, usually
per day (24 hours).
In F.P.S. system, the latent heat of fusion of ice is 144 B.T.U. /lb.,
which means that one Ib. of ice absorbs 144 BTU of heat when it
melts. One short ton (2000 Ibs.) of ice, therefore, absorbs 2000 x
144 or 288,000 BTU of heat. In other words, when one ton of ice

9. 9
melts in 24 hour it will produce cooling effect at the rate of
288,000/ 24 = 12,000 BTU per hour or 12,000/60 = 200 BTU per
minute. This rate of cooling has been designated as a ton of
refrigeration. So, a ton of refrigeration is not a unit of mass but it is
a measurement of the rate of heat transfer.
In M.K.S. system of units 72, 000 kcal per 24 hours or 3000 kcal
per hour or 50 kcal per minute rate of cooling means one-ton
refrigeration.
Hence, a machine having its capacity to produce cooling effect of
200 BTU/min or 50kcal/min is classed as one-ton machine.
Refrigerator
The machine which is used to keep the things cool is commonly
known as refrigerator.
OR
It is a device which makes the heat flow from a body at lower
temperature to a body at higher temperature.
Principle of Refrigerator
Gas is liquefied under pressure and is then let through a valve
into the evaporator. But under normal pressure this liquid
evaporates and absorbs a lot of heat from the area surrounding

10. 10
the evaporator and again converts into gas. This process
continues and things in it always remain cool.
Construction
Fig: The Refrigerator
A refrigerator consists of three parts.
1. Evaporator
2. Compressor
3. Condenser
Working of Refrigerator
In the three parts of the refrigerator, the following processes
occur.

11. 11
A refrigerator uses such gases which can easily liquefy under
pressure at normal temperature. Usually Freon gas is used for
this purpose.
Compressor
In this part, the gas is first compressed and then it is fed into the
condenser.
Condenser
In it, gas is condensed into liquid under pressure. Here, heat of
condensation is evolved. Now, the liquid gas moves into the
evaporator through a valve.
Evaporator
In the evaporator, the liquid Freon evaporates under normal
pressure absorbing a lot of heat from the area surrounding the
evaporator, and, thus cooling it down. The gas is then again
brought to the condenser with the help of compressor. In the
condenser, it loses a large amount of heat on being liquefied.
This cycle is then continuously repeated. The condenser is
placed outside the body of the refrigerator so that it can lose its
heat in the atmosphere. The evaporator is placed inside the body.