Basics of refrigeration engineering section b

BASICS OF MECHANICAL ENGINEERING
(ME-101 F)
Lectures on
REFRIGERATION AND AIR-CONDITIONING
By
Priyanka Singh
M.Tech (Applied Mechanics)
Motilal Nehru National Institute of Technology, Allahabad
•Introduction to RAC
•Rating of refrigeration machines
•Coefficient of performance
•Simple refrigeration cycle
•Vapour compression cycle
•Psychrometric charts and its use.
•Human comforts

Process of producing and maintaining temperature of a
region below the surrounding temperature by transfer
of heat from lower temperature regions to higher
temperature ones is called refrigeration. Devices that
produce refrigeration are called refrigerators, and the
cycles on which they operate are called refrigeration
cycles. The working fluids used in refrigerators are
called refrigerants. Refrigerators used for the purpose
of heating a space by transferring heat from a cooler
medium are called heat pumps.
Introduction

The performance of refrigerators and heat pumps is
expressed in terms of coefficient of performance
(COP), defined as
The standard of comparison for refrigeration cycles
is the reversed Carnot cycle. A refrigerator or heat
pump that operates on the reversed Carnot cycle is
called a Carnot refrigerator or a Carnot heat pump,
and their COPs are

Refrigerator and Heat Pump Objectives
(fig. 10-1)
The objective of a refrigerator is to remove heat (QL) from the cold medium;
the objective of a heat pump is to supply heat (QH) to a warm medium

The power cycles can be used as refrigeration cycles
by simply reversing them. Of these, the reversed
Brayton cycle, which is also known as the gas
refrigeration cycle, is used to cool aircraft and to
obtain very low (cryogenic) temperatures after it is
modified with regeneration. The work output of the
expanding device can be used to reduce the work
input requirements to the compressor. Thus the COP
of a gas refrigeration cycle is
Simple Gas (Air) Refrigeration Cycle

Simple Gas (Air) Refrigeration Cycle
Schematic and T-s Diagram

Many of the impracticalities associated with the reversed Carnot
cycle can be eliminated by vaporizing the refrigerant completely
before it is compressed and by replacing the turbine with a throttling
device, such as an expansion valve or capillary tube. The cycle that
results is called the ideal vapor-compression refrigeration cycle, and
it is shown schematically and on a T-s diagram in Figure. The vapor-
compression refrigeration cycle is the most widely used cycle for
refrigerators, air-conditioning systems, and heat pumps.
It consists of four processes:
1-2 : Isentropic compression in a compressor
2-3 : Constant-pressure heat rejection in a condenser
3-4 : Throttling in an expansion device
4-1 : Constant-pressure heat absorption in an evaporator
Vapor-Compression Refrigeration Cycle

(Fig. 10-3)
Schematic and T-s Diagram

In an ideal vapor-compression refrigeration cycle,
the refrigerant enters the compressor as a
saturated vapor and is cooled to the saturated
liquid state in the condenser. It is then throttled in
Expansion valve to the evaporator pressure and
vaporizes as it absorbs heat from the refrigerated
space.
Very low temperatures can be achieved by
operating two or more vapor-compression
Systems in series, called cascading. The COP of a
refrigeration system also increases as a result of
cascading.

P-h Diagram of an Ideal Vapor-
Compression Refrigeration Cycle

Schematic and T-s Diagram for Actual Vapor-
Compression Refrigeration Cycle
(Fig. 10-7)

Another form of refrigeration that becomes
economically attractive when there is a source of
inexpensive heat energy at a temperature of 100 to
2000º
C is absorption refrigeration, where the
refrigerant is absorbed by a transport medium and
compressed in liquid form. The most widely used
absorption refrigeration system is the ammonia-
water system, where ammonia serves as the
refrigerant and water as the transport medium. The
work input to the pump is usually very small, and
the COP of absorption refrigeration systems is
defined as
Vapour Absorption Refrigeration Cycle

Vapor Absorption Refrigeration
Cycle
(Fig. 10-21)

The maximum COP an absorption refrigeration
system can have is determined by assuming totally
reversible conditions, which yields
where T0, TL, and Ts are the absolute temperatures
of the environment, refrigerated space, and heat
source, respectively.

Ordinary Household Refrigerator

Heat Pump Heats a House in Winter and Cools it in Summer
(Fig. 10-9)

Air conditioning is the process of maintain the quality of air. Quality
of air includes temperature, velocity, humidity and purity (oxygen
content) of air maintained at the perfect level.
Properties of air
Air is the sole working substance of an air conditioning rig. It is
actually the mixture of gases in which water vapour exists in
superheated or saturated state, hence air is considered as binary
mixture and hence requires three independent variables to fix its state.
These properties of moist air is known as psychrometric properties.
These are as follows
1.Specific humidity or humidity ratio
Specific or absolute humidity or humidity ratio or moisture content(ω)
is defined as the ratio of the mass of water vapour present to the mass
of dry air in a given volume of mixture.
Introduction to air-conditioning

All data essential for
the complete
thermodynamic and
psychrometric
analysis of air-
conditioning process
when summarised in
a graph format is
known as
psychrometric chart.
Plotting of
psychrometric chart
is done as dry bulb
temperature that is
normal temperature
to specific humidity
and water vapour
pressure.
Psychrometric Chart

Human comfort is the condition of supplied air which in turns proved
suitable for people experiencing it, that is to maintain the quality and
velocity of supply so that person should feel comfortable within the
atmosphere created.
Governing parameters: These are air temperature, humidity and air-
velocity along with air purity. It is too hard to define or fix human
comfort with four different variables, hence, effective temperature is
used as an index, that is temperature at 100% relative humidity in which
subject will feel the same experience as in actual environment.
Because human comfort varies person to person and by age group and
gender, and many such factors affect it, vast study is needed. Such
studies have shown correlation between comfort level, temperature,
humidity, gender, length of exposure, age group and other causes. These
are tabulated in the form of comfort charts.
Human Comfort

Human comfort usually depends upon certain causes as follows:
1.Metabolic rate: The rate at which body produces heat.
2.Mechanism of body heat loss.
3.Heat exchange between man and environment.
END

Basics of refrigeration engineering section b

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Basics of refrigeration engineering section b