REFRIGERATION &

LIQUEFACTION
Lilik Suprianti, ST, MSc
Refrigeration
Refrigeration is best known for its use in the air conditioning of buildings and in the
treatment, transportation, and preservation of foods and beverages. It also finds large-
scale industrial application, for example, in the manufacture of ice and the dehydration
of gases. Applications in the petroleum industry include lubricating-oil purification, low-
temperature reactions, and separation of volatile hydrocarbons.
■ The word refrigeration implies the maintenance of a temperature below that of the
surroundings.

■ This requires continuous absorption of heat at a low temperature level, usually

accomplished by evaporation of a liquid in a steady-state flow process.

■ The vapor formed may be returned to its original liquid state for re evaporation in
either of two ways.

■ Most commonly, it is simply compressed and then condensed. Alternatively, it may

be absorbed by a liquid of low volatility, from which it is subsequently evaporated at
higher pressure.
THE CARNOT REFRIGERATOR
■ In a continuous refrigeration process, the heat absorbed at a low temperature is continuously
rejected to the surroundings at a higher temperature.

■ Basically, a refrigeration cycle is a reversed heat-engine cycle. Heat is transferred from a low
temperature level to a higher one; according to the second law, this requires an external source of
energy.

■ The ideal refrigerator, like the ideal heat engine (Sec. 5.2), operates on a Carnot cycle, consisting in
this case of two isothermal steps in which heat Qc is absorbed at the lower temperature Tc and
heat QH is rejected at the higher temperature TH, and two adiabatic steps

■ The cycle requires the addition of net work W to the system. Since AU of the working fluid is zero for
the cycle, the first law is written:

𝑊 = 𝑄𝐻 − 𝑄𝑐 (9.1)
The measure of the effectiveness of a refrigerator is its coefficient of performance 𝜔
𝐡𝐞𝐚𝐭 𝐚𝐛𝐬𝐨𝐫𝐛𝐞𝐝 𝐚𝐭 𝐭𝐡𝐞 𝐥𝐨𝐰𝐞𝐫 𝐭𝐞𝐦𝐩𝐞𝐫𝐚𝐭𝐮𝐫𝐞 𝑸𝒄
𝝎= = (9.2)
𝐧𝐞𝐭 𝐰𝐨𝐫𝐤 𝑾
Equation (9.1) may be divided by 𝑄𝑐
𝑊 𝑄𝐻
= −1
𝑄𝑐 𝑄𝑐
Combination with eh 5.7
𝑊 𝑇𝐻 𝑇𝐻 − 𝑇𝑐
= −1=
𝑄𝑐 𝑇𝑐 𝑇𝑐
Eq. 9.2 become
𝑇𝑐
𝜔= (9.3)
𝑇𝐻 −𝑇𝑐
This equation applies only to a refrigerator operating on a Carnot cycle, and it gives the
maximum possible value of w for any refrigerator operating between given values of TH andTc
THE IDEAL VAPOR-COMPRESSION REFRIGERATION
CYCLE
■ The vapor-compression refrigeration cycle is the ideal model for refrigeration systems.
Unlike the reversed Carnot cycle, the refrigerant is vaporized completely before it is
compressed and the turbine is replaced with a throttling device.

This is the most widely

used cycle for
refrigerators, A-C
systems, and heat
pumps.
■ The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process
to make it a more realistic model for the actual systems.
■ Replacing the expansion valve by a turbine is not practical since the added benefits
cannot justify the added cost and complexity.
On the basis of a unit mass of fluid,
the equations for the heat absorbed
in the evaporator and the heat
rejected in the condenser are:

𝑄𝑐 = 𝐻2 − 𝐻1
𝑄𝐻 = 𝐻3 − 𝐻4

These equations follow from Eq. (2.32) when the small changes in potential and kinetic energy
are neglected. The work of compression is simply:

𝑊 = 𝐻3 − 𝐻2
■ and by Eq. (9.2), the coefficient of performance is:
𝐻2 −𝐻1
𝜔= (9.4)
𝐻3 −𝐻2

■ To design the evaporator, compressor, condenser, and auxiliary equipment one must know the
rate of circulation of refrigerant m. This is determined from the rate of heat absorption in the
evaporator by the equation:
𝑄𝑐
𝑚ሶ = (9.5)
𝐻2 −𝐻1

The vapor-compression cycle of Fig. 9.1 is shown on a P H

diagram in Fig. 9.2. Such diagrams are more commonly
used in the description of refrigeration processes than TS
diagrams because they show directly the required
enthalpies.
Although the evaporation and condensation processes
are represented by constant-pressure paths, small
pressure drops do occur because of fluid friction.
ABSORPTION REFRIGERATION SYSTEMS
■ When there is a source of
inexpensive thermal energy at
a temperature of 100 to
200°C is absorption
refrigeration.
■ Some examples include
geothermal energy, solar
energy, and waste heat from
cogeneration or process
steam plants, and even
natural gas when it is at a
relatively low price.
Ammonia absorption refrigeration cycle.
• Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by a transport
medium.
• The most widely used system is the ammonia–water system, where ammonia (NH3) serves as
the refrigerant and water (H2O) as the transport medium.
• Other systems include water–lithium bromide and water–lithium chloride systems, where
water serves as the refrigerant. These systems are limited to applications such as A-C where
the minimum temperature is above the freezing point of water.
• Compared with vapor-compression systems, ARS have one major advantage: A liquid is
compressed instead of a vapor and as a result the work input is very small (on the order of one
percent of the heat supplied to the generator) and often neglected in the cycle analysis.
• ARS are often classified as heat-driven systems.
• ARS are much more expensive than the vapor-compression refrigeration systems. They are
more complex and occupy more space, they are much less efficient thus requiring much larger
cooling towers to reject the waste heat, and they are more difficult to service since they are
less common.
• Therefore, ARS should be considered only when the unit cost of thermal energy is low and is
projected to remain low relative to electricity.
• ARS are primarily used in large commercial and industrial installations.
■ The work required by a Carnot refrigerator absorbing heat at temperature Tc and
rejecting heat at the temperature of the surroundings, here designated Ts, follows from
Eqs. (9.2) and (9.3):
𝑇𝑆 − 𝑇𝐶
𝑊= 𝑄𝑐
𝑇𝐶
■ where IQcI is the heat absorbed. If a source of heat is available at a temperature above
that of the surroundings, say at TH, then work can be obtained from a Carnot engine
operating between this temperature and the surroundings temperature Ts. The heat
requireed I QHI for the production of work I W I is found from Eq. (5.8):
Liquefaction
■ Liquefied gases are in common use for a variety of purposes: liquid propane in cylinders
serves as a domestic fuel, liquid oxygen is carried in rockets, natural gas is liquefied for ocean
transport, and liquid nitrogen is used for low-temperature refrigeration. In addition, gas
mixtures (e.g., air) are liquefied for separation into their component species by fractionation.

■ Liquefaction results when a gas is cooled to a temperature in the two-phase region. This may
be accomplished in several ways:

1. By heat exchange at constant pressure.

2. By an expansion process from which work is obtained.

3. By a throttling process.
Many important scientific and engineering processes at cryogenic temperatures (below about 100°C) depend
on liquefied gases including the separation of oxygen and nitrogen from air, preparation of liquid propellants
for rockets, the study of material properties at low temperatures, and the study of superconductivity.

The storage (i.e., hydrogen) and

transportation of some gases (i.e., natural
gas) are done after they are liquefied at very
low temperatures. Several innovative cycles
are used for the liquefaction of gases.