mv (hc hfg )

C.O.P.
ms (hA hfg )

2.28 Advantages and Disadvantages of Steam Jet Refrigeration System

Following are the advantages and disadvantages of a steam jet refrigeration system:

Advantages:

1. It is simple in construction and rigidly designed.

2. It is a vibration- free system as pumps are the only moving parts.
3. It has low maintenance cost, low production cost and high reliability.
4. It has relatively less plant mass (kg/TR). Hence, there are now a number of
air- conditioning application ranging up to 300 TR in capacity as well as many
industrial applications of even larger size.
5. It uses water as a refrigerant. Water is very safe to use as it is non-poisonous
and non-inflammable.
6. This system has an ability to adjust quickly to load variations.
7. The running cost of this system is quite low.

Disadvantages:

1. The system is not suitable for water temperature below4ºC.

2. For proper functioning of this system, maintenance of high vacuum in the
evaporator is necessary. This is done by direct vaporization to produce
chilled water which is usually limited as tremendous volume of vapor is to
be handled.

2.3 THERMO ELECTRIC REFRIGERATION SYSTEM

2.30 Introduction

Thermoelectric refrigeration owes its origin to the discovery of Seebeck and Peltier effects in
t 1821. Seebeck found that if two dissimilar metals are joined at two junctions, one at high
temperature and other at low temperature, current was produced. Peltier in 1834 observed
that if current was passed through two dissimilar metals joined at two junctions, one was
cooled and the other was heated. In 1838, Lenz used both effects to freeze water and
thermoelectric refrigeration was born. Further development occurred in 1930 when
semiconductors were discovered.

Figure 2.8 Thermoelectric Refrigeration Set-up

2.31 Applications of this refrigeration system are:

1) Portable refrigerator

2) Water coolers

3) Space apparatus

4) Blood analyzers

5) CCD cameras

6) Laser diodes

The typical thermoelectric system contains thin ceramic wafers with a series of P and-N
doped bismuth-telluride semiconductor materials sandwiched between them. The ceramic
material adds rigidity and necessary insulation. N type material has excess electrons while P
type material has deficiency of electrons. The P and N materials are joined as shown in fig. 5.
The thermoelectric couples are electrically in series and thermally parallel. As electrons move
from P type to N type material through an electrical connector, electrons jump to a higher
energy state, absorbing energy from the cold side. The electrons then flow from N type to P
type through electrical connector, thereby releasing energy to the hot side. Thus, one junction
is hot while the other is cool. Thermoelectric system can thus be used to heat or cool
depending on current direction.

2.32 Analysis of Thermoelectric System

Three parameters are important. They are

i) Hot surface temperature (Th)

ii) Cold surface temperature (Tc)

iii) Heat load to be absorbed at cold side (Qc)

Let Ta = ambient temperature, Qh = heat released to hot side

Th = Ta + KQh, where,

K = thermal resistance of path

Qh = Qc + P

Where P = electrical power required for producing thermoelectric effect = V x I.

Temperature difference across the thermoelectric system is T = Th - Tc

Figure shows variation in COP of system with current. The system COP can be calculated as
follows:
Where a = thermoelectric power or See back coefficient

U = overall conductance

R = overall resistance Current I

2.33 Advantages of Thermoelectric Refrigeration

1) Absence of moving parts, so no vibration problems

2) Long life and no wear and tear

3) Load can be controlled by adjusting current.

4) Very compact and portable

5) Light weight

6) No leakage problems; no refrigerant required

7) Can be used for year-round air-conditioning

8) Design and manufacture are simple.

9) It can be operated in any position, vertical or horizontal.

10) Suitable for production of cooling suit

2.34 Disadvantages of Thermoelectric Refrigeration

1) Unavailability of suitable material with high figure of merit 'Z'

2) Costly as initial expenditure high

3) Running cost is high.

4) Lower COP of the order of 0.1 to 0.2

2.4 AIR REFRIGERATION SYSTEM
2.40 Introduction

In an air refrigeration cycle, the air is used as a refrigerant. In olden days, air was
widely used in commercial applications because of its availability at free of cost. Since air
does not change its phase i.e. remains gaseous throughout the cycle, therefore the heat
carrying capacity per kg of air is very small as compared to vapour absorbing systems. The
air-cycle refrigeration systems, as originally designed and installed, are now practically
obsolete because of their low coefficient of performance and high power requirements.
However, this system continues to be favored for air refrigeration because of the low weight
and volume of the equipment. The basic elements of an air cycle refrigeration system are the
compressor, the cooler or heat exchanger, the expander and the refrigerator.

Before discussing the air refrigeration cycle, we should first know about the unit of
refrigeration, coefficient of performance of a refrigerator and the difference between the heat
engine, a refrigerator and a heat pump.

2.41 Difference Between a Heat Engine, refrigerator and Heat Pump

In a heat engine, as shown in Fig.2.9 (a), the heat supplied to the engine, is converted
into useful work. If Q2 is the heat supplied to the engine and Q1 is the heat rejected from the
engine, then the net work done by the engine is given by

Figure 2.9 Difference between a heat engine, refrigerator and heat pump.

The performance of a heat engine is expressed by its efficiency. We know that the
efficiency or coefficient of performance of an engine.

Workdone WE Q2 Q1
E or (C.O.P.) E
Heat sup plied Q2 Q2
 A refrigerator as shown in Fig. 2.9 (b), is reversed heat engine which either cool or
maintain the temperature of a body (t1) lower than the atmospheric temperature (ta). This is
done by extracting the heat (Q1) from a cold body and delivering it to a hot body (Q2). In
doing so, work WR is required to be done on the system. According to First Law if
Thermodynamics,

WR Q2 Q1

The performance of a refrigerator is expressed by the ratio of amount of heat taken

from the cold body (Q1) to the amount of work required to be done on the system (W R). This
ratio is called coefficient of performance. Mathematically, coefficient of performance of a
refrigerator,

Q1 Q1
(C.O.P.) R
WR Q2 Q1

Any refrigerating system is a heat pump as shown in Fig.2.9 (c), which extracts heat
(Q1) from a cold body and delivers it, to a hot body. Thus there is no difference between the
cycle of operations of a heat pump and a refrigerator. The main difference between the two is
in their operating temperatures. A refrigerator works between the cold body temperature (T1)
and the atmospheric temperature (Ta). A refrigerator used for cooling in summer can be used
as heat pump for heating in winter.

In the similar way, as discussed for refrigerator, we have

Wp = Q2-Q1

The performance of a heat pump is expressed by the ratio of the amount of heat
delivered to the hot body (Q2) to the amount of work required to be done on the system( W p).
This ratio is called coefficient of performance of energy performance ratio (E.P.R.) if a heat
pump. Mathematically, coefficient of performance if energy performance ratio of a heat
pumps.

Q2 Q2
(C.O.P.) P orE.P.R.
WP Q2 Q1
 Q1
(C.O.P.) R 1
Q2 Q1

From above we see that the C.O.P. may be less than one or greater than one
depending on the type of refrigeration system used. But the C.O.P. of a heat pump is always
greater than one.

2.42 Open Air Refrigeration Cycle

In an open air refrigeration cycle, the air is directly led to the space to be cooled (i.e. a
refrigerator), allowed to circulate through the cooler and then returned to the compressor to
start another cycle. Since the air is supplied to the refrigerator at atmospheric pressure,
therefore, volume of air handled by the compressor and expander is large. Thus the size of
compressor and expender should be large. Another disadvantage of the open cycle system is
that the moisture is regularly carried away by the air circulated through the cooled space. This
leads to the formation of frost at the end of expansion process and clog the line. Thus in an
open cycle system, a drier should be used.

2.43 Closed or Dense Air Refrigeration Cycle

In a closed or dense air refrigeration cycle, the air is passed through the pipes and
component parts of the system at all times. The air, in this system, is used for absorbing heat
from the other fluid (say brine) and this cooled brine is circulated into the space to be cooled.
The air in the closed system does not come in contact directly with the space to be cooled.

The closed air refrigeration cycle has the following thermodynamic advantages:

1. Since it can work at a suction pressure higher than that of atmospheric pressure,
therefore the volume of air handled by the compressor and expender are smaller as
compared to an open air refrigeration cycle system.
2. The operating pressure ratio can be reduced, which results in higher coefficient of
performance.
2.44 Air Refrigerator Working on Reversed Carnot Cycle

In refrigerating systems, the Carnot cycle considered is the reversed Carnot cycle.
We know that a heat engine working on Carnot cycle has the highest possible efficiency.
Similarly, a refrigerating system working on the reversed Carnot cycle will have the
maximum possible coefficient of performance. We also know that is not possible to make an