THERMAL ENGINEERING

SYSTEMS
DIGITAL ASSIGNMENT 1

Topic : Recent developments in the

design aspect of domestic refrigerator.

Name : RAMESH KAVITHA SANJIT

Reg No : 18BME0677
Slot : C1
Faculty : Prof. Vinod Kumar Sharma
INTRODUCTION

Refrigerator design nowadays is modified according to the relevant needs of

todays world, which is higher efficiency and lesser impact to the environment.
To lower environmental impacts of refrigeration and heat pump systems,
several issues have to be addressed. In term of TEWI (Total Equivalent Warming
Impact), direct emissions of refrigerants have to be lowered and the GWP
(Global Warming Potential) of refrigerants has also to be lowered. Due to the
significant impact of energy consumption during the lifetime of any refrigeration
equipment, the essential option to lower the environmental impact is to
improve significantly the energy efficiency of the system. The boundaries of
systems have to be carefully addressed. For example, in buildings, it is not
sufficient to develop high efficiency refrigeration systems, it is also necessary to
integrate the refrigeration system in an overall efficient control system and also
to integrate the refrigeration system in a low thermal load building. Moreover,
because of the necessary strong policies for the decrease in GHG (greenhouse
gas emissions) emissions, refrigeration systems have a new role to play to
replace boilers using either oil or gas by high efficiency heat pumps in order to
limit CO2 emissions. Recovery of heat of different thermal sources allows a
significant amplification factor due to heat pumping.

LOWERING IMPACT OF DIRECT EMISSIONS

Since the beginning of the '90s, the replacement of HFCs and of HCFCs due to
the enforcement of the Montreal Protocol has led to not only the development
of new HFC refrigerants, but also to the revival of previously used refrigerants
such as CO2, and in some cases hydrocarbons, and also to revisit new possible
uses of ammonia. Moreover, as it is known, alternative refrigeration cycles
based on different physical principles other than vapor compression have also
been thoroughly revisited, such as Stirling cycle, the pulse tube, thermo-acoustic
refrigeration, magnetic refrigeration … Nevertheless, until now, vapor
compression systems at equal energy efficiency are still presenting the best
energy efficiency/cost ratios compared to alternative cycles.
To lower direct emissions the following will be addressed successively :
reduction of refrigerant charge, efficient containment and recovery policies, and
the introduction of low GWP refrigerants such as CO2 or new low GWP HFCs.
1. Reduction of refrigerant charge :

The reduction of the refrigerant charge at equal refrigerating capacity and

energy performances leads to lower refrigerant emissions due to ruptures
of the circuit, and also to limit the refrigerant emissions at the equipment
end of life.

 Compact heat exchangers : A generic way to limit refrigerant charges can

be used in any technologies when compact heat exchangers are used. For
example, going from shell-and tube heat exchangers to plate heat
exchangers leads to lower the cooling capacity to refrigerant ratio of at
least a factor 3. It is also true in air-to-air air conditioners that the tube
diameters of copper tubes are continuously decreasing. The Japanese air
conditioning industry is the technological leader for heat exchangers with
inside diameters in the range of 5 mm. New progresses are underway
specially for condensers where usual condensers with copper tubes and
aluminum fins are replaced by fully brazed aluminum heat exchangers.
Some applications are still based on huge refrigerant quantities stored in
shell-andtube evaporators (such as chillers), or also in industrial
refrigeration where large low pressure receivers feed re-circulation
pumps of refrigerant. Part of the future is based on efficient sprays on
tubes for new design of chiller evaporators, and also of possible new
design of so-called dry heat exchangers.

 Indirect systems : Water chillers are typical indirect systems that have
won significant market shares for cooling large buildings. This concept can
also be applied to lower cooling capacities due to the interest of low
refrigerant charges, specially when using HFCs. Indirect systems can also
be used in mobile air conditioning (MAC) systems, specially in buses and
trains, where the usual circuit is long and the refrigerating capacity /
refrigerant charge ratio is high. For refrigeration, since the mid '90s a
number of tests have been performed in Europe and also in the U.S. to
study the interest of indirect systems in the commercial sector in order to
limit refrigerant charges of centralized systems installed in supermarkets.
Different concepts have been tested. One of them seems to present high
energy performances at acceptable costs: indirect systems using CO2 at
the low temperature level. In those systems the CO2 is partially
evaporated in display cases or evaporators of cold chambers, and is then
condensed in evaporator condenser where HFC or Ammonia evaporates
for cooling CO2. For medium temperature applications, the use of heat
transfer fluids seems still to be the preferred option, but new
developments are underway for the use of CO2 even at the medium level
temperature. Indirect systems in commercial refrigeration may lead to
reduce the refrigerant charge by 50% to 75% compared to the reference
line. Indirect systems imply an additional difference of temperatures
between evaporating temperature and air to be cooled. Moreover the
heat transfer fluid needs to be pumped and so the pumping system adds
and additional energy consumption. Comparisons of direct expansion
systems with indirect systems in commercial refrigeration show
interesting results where the energy efficiencies of both systems are
exactly in the same range because of the inherent inefficiencies of direct
expansion where superheat hampers the efficiency of evaporators.
Detailed measurements on heat exchangers show a better heat exchange
efficiency of heat transfer fluid compared to refrigerant.
 2. Containment and recovery policies :

Containment and recovery policies have been promoted by association of

engineers, a number of OEMs and regulatory bodies of different countries. The
European regulations 2037/2000 for HCFCs and 842/2006 for HFCs imply regular
leak tightness control of refrigeration systems (for refrigerant charges higher
than 3 kg), systematic refrigerant recovery at end of life of equipment, and
systematic follow up of refrigerant quantities sold on the market. A number of
standards have addressed improvement of leak tightness of components,
among them EN 378, ASHRAE 147, and EN 14624. A special mention has to be
made for the system developed in The Netherlands, called STEK, where the leak
prone components have to be analyzed and the origin of the emission detailed.
It is still a significant part of future developments to make new design of leak
prone components such as safety valves, service valves, and different types of
fittings. The methods used in many companies in order to reach higher level of
quality along the lifetime of the equipment has to be generalized for leak
tightness of components. New methods of tests are needed in order to
guarantee initial leak tightness.

3. Efficient applications of CO2 :

Significant developments have been made in the last 10 years for new CO2
systems taking advantage of the low GWP of this fluid and its classification as a
safe refrigerant (A1 under EN 378 and Ashrae 34). The drawbacks of CO2 are
well known and are mainly related to its low critical temperature of 31°C. This
low critical temperature leads to transcritical cycle when delivering heat at
higher temperature. Efficient developments of CO2 are summarized here after.

 Low temperature cascading system : When condensing CO2 at

temperature lower than 31°C, and possibly around –10 to 0°C, CO2
systems show high energy performances due to efficient thermophysical
properties (low viscosity and high thermal conductivity) and the
development of cascading systems. Those cascading systems use CO2 at
evaporating temperatures varying between –50 and –35°C associated
with a high temperature refrigerating system (of the cascade) using either
ammonia or HFCs have led to very efficient systems in the food industry,
and more scarcely in the commercial sector.

 CO2 water heat pumps : Japan has developed high efficiency water
heaters using CO2 as working fluid. The domestic hot water consumption
of Japanese families are significantly higher than those of European
standards and so the energy bill for hot water is significant. Heat pump
water heaters are appealing in the Japanese context when COPs range
between 3 and 4. Due to the very large temperature glide for water
heating, typically from 15°C up to 60, and more generally in Japan 90°C,
the continuous temperature glide of CO2 in the supercritical region leads
to minimization of entropy losses of the heat exchange. The Japanese
government has created an incentive of about 1/3 of the price (the typical
price is about 6,000 US $) in order to develop rapidly the market. The
market in 2006 has been superior to 250,000 units and more than 15
different companies are competing on this market.

 CO2 transcritical cycle (experiment) :

Assessment of alumina nanofluid as a coolant in double pipe gascooler for
trans-critical CO2 refrigeration cycle :-
The fundamental trans-basic CO2 refrigeration cycle is appeared in Fig. 1
(a). Procedure 1-2 is adiabatic pressure in blower, trailed by warmth
dismissal in gas cooler (process 2-3) at that point isenthalpic extension for
example process 3-4 out of a development valve lastly vanishing of
refrigerant in evaporator as given by procedure 4-1. The counter stream
type twofold pipe gas cooler measurements accepted pursues: tube
length of 14 m, internal distance across of external pipe as 10 mm and
external and inward breadth of internal pipe as 6.35 mm and 4.72 mm
individually.

Coolant (water or Nanofluid) is bolstered into the cylinder side while

supercritical refrigerant streams in the annulus side. There are two
fundamental troubles in demonstrating: first is settling huge thermo-
physical property varieties of CO2 in both hub and outspread bearings in
pseudocritical area and second reenactment of the steady suspension of
base liquid and Nanoparticle. The previous is taken consideration by
utilizing connection created by Pitla et al., while for the last single stage
approach is received. One dimensional warmth move and liquid stream
for both coolant (water and Nanofluid) and refrigerant is demonstrated
independently. The basic preservation conditions are discretised utilizing
a limited distinction plot as appeared in Fig. 1 (b). The model is first settled
for one component at that point created for complete geometry
component by component.
Refrigerant side modelling : The mass, force and vitality preservation
conditions for twofold pipe gas cooler in discretized structure for a
solitary component as appeared in Fig. 1 (b) is given in conditions (1) to
(3). For every component, there is a warmth balance among coolant and
the refrigerant as appeared in condition (4).

Pitla et al. proposed a relationship for evaluating mean Nusselt number

for CO2 as given by condition (5) and the equivalent is embraced here. In
condition (5), Nubt and Nuwt are the Nusselt number determined at mass
and divider temperature based thermo-physical properties individually.
Mass and divider temperatures are settled for each component utilizing
suitable iterative strategy by at first expecting them as the refrigerant and
coolant temperature separately. Nusselt number for each case is
determined utilizing relationship given by Gneilinski given in condition (6)
in which f is the rubbing element evaluated as given by condition (7).
Further, conditions (8) and (9) are used to gauge warmth move coefficient
and all out weight drop separately for refrigerant side.
Coolant side modelling : For water, temperature dependent thermo-
physical properties are considered in this analysis as given in equations
(10) to (13). Further, Nusselt number, heat transfer coefficient and
pressure drop for water are calculated using correlations given previously
in equations (6) to (9). For modelling nanofluid, single phase approach is
employed. In this approach, mixture of Nanoparticles and base fluid
(water in our case) are treated as homogenous solution assuming no
relative motion between Nanoparticles and basefluid particles.
Buongiorno investigated possible seven slip mechanisms for two phase
behavior of NanoFluids and reported Brownian motion and
thermophoresis to be the most dominating mechanisms. Similar
conclusion was reported by Sadeghi et al.. However, Ahmed et al.
reported that for NanoFluids flow with Re higher than 100, both Brownian
motion and thermophoresis may be safely neglected. This implies that
adopting single phase approach for our range of investigation (i.e.
turbulent flow conditions) is justified. However, two phase modelling
approach is more accurate but complex and time consuming. Haghighi et
al. claimed that heat transfer and pressure drop behavior of NanoFluids
can also be predicted within 10 % error using classical correlations
developed for base fluid provided that appropriate thermo-physical
properties of NanoFluids are utilized. Following this, Nusselt number, heat
transfer coefficient and pressure drop of NanoFluids are also calculated
using equations (6) to (9).

Density and specific heat of Al2O3/water NanoFluids can be estimated

accurately by the weighted average of the densities and specific heats of
the nanoparticles and the base fluid respectively as shown in equation
(14) and (15). Buongiorno et al. normalized experimental data of
Nanofluid’s thermal conductivity from thirty organizations worldwide and
reported that predictions from Maxwell model, as given in equation (16),
to be in good agreement with and the same is utilized in this study.
Correlation proposed by Maiga et al. for effective viscosity for alumina
NanoFluids based on scarce experimental data collected from literature is
adopted here as well, given in equation (17). The absolute values of
thermo-physical properties of alumina nanoparticles are taken from
Purohit et al. The overall heat transfer coefficient for gas cooler is
calculated as given by equation (18). The pump work required for coolant
as shown in equation (19), is calculated as product of total pressure drop
and discharge. Finally, cooling capacity, compressor work and COP of
refrigeration system are calculated as given by equation (20), (21) and
(22) respectively.

The simulation is developed in MATLAB environment and REFPROP is

utilized for estimating refrigerant properties. The double pipe gas cooler
is divided into number of segments in order to resolve large varying CO2
thermal and transport properties. Fig. 2 shows the grid independence test
and it can be observed that beyond 20 number of divisions, the COP and
the gas cooler capacity values does not vary more than 1%. The
evaporator temperature of 0o C and compressor efficiency of 0.65 is
adopted throughout investigation. The mass flow of refrigerant for entire
range of investigation is taken as 0.02 kg. s-1. Gas cooler pressure and
coolant inlet temperatures are taken as 10 MPa and 30o C respectively,
unless specified other way. The heat loss from the gas cooler into the
surroundings is neglected.
In this study, performance of alumina NF cooled double pipe gas cooler in
trans-critical CO2 refrigeration cycle is theoretically compared to that of
water cooled gas cooler. A more practical equal pumping power
comparison criterion is adopted besides conventional equal Re
comparison base. NFs are loaded with 0.5%, 1.5% and 2.5% of particle
volume fraction under turbulent flow conditions. Both refrigerant side
and coolant side modelling is made with extra care of CO2 thermo-
physical property variations in vicinity of pseudo critical temperature
zone. The prominent conclusions drawn are as follows:

-> COP of the NF cooled refrigeration system is found higher only at equal
Re comparison criterion while for equal pumping power the water cooled
system dominates. Gas cooler overall conductance, effectiveness and
capacity shows similar trend under both comparison criteria. As expected,
gas cooler performance is improved only under same Re comparison base

->With the increase in gas cooler pressure, COP is found to increase

irrespective of comparison criteria due to imposed high gas cooler outlet
temperature. Gas cooler effectiveness shows a distinct behavior with
respect to increase in gas cooler pressure: first it decreases till 10 MPa
and then it increases. This is due to the imposed mass flow rates of both
 refrigerant and coolant. Overall conductance of gas cooler decreases with
increase ingas cooler pressure, while reverse is true for gas cooler
capacity. Performance of NF cooled gas cooler with respect to gas cooler
pressure is found better only at equal Re comparison base.

->The pump work for coolant is found negligible as compared to

compressor work and gas cooler capacity. Even at equal mass flow rate
comparison base, NF cooled gas cooler shows no potential improvement
in performance. This implies that by increasing only mass flow rate of
water, the performance of water cooled system can be improved as much
as that of NF cooled systems.

4. New low GWP HFCs :

Refrigerants are used in a wide variety of heating, ventilation, air conditioning,

and refrigeration (HVAC&R) equipment. The current generation of refrigerants,
hydrofluorocarbons (HFCs), have zero ozone depletion potential; however,
when released to the atmosphere, they have significant global warming
potential (GWP). The growing international emphasis on global warming
mitigation has stimulated interest in a new generation of low-GWP refrigerants.
 Low GWP HFCs : Although the goal of an eventual HFC phase-down is to
replace current HFC refrigerants with low-GWP alternatives, two HFCs in
particular warrant consideration as viable replacement options: HFC-32
and HFC-152a. HFC-32 is classified as A2L and has a GWP of 677. HFC152a
is classified as A2 and has a GWP of 138. While these GWP values are
higher than other single-digit-GWP alternatives, they represent a
significant improvement over most current HFC refrigerants that have
GWP values between 2,000 and 4,000. HFC-32 is a versatile refrigerant
that is particularly suitable for air conditioning and heat pump
 applications. The use of HFC-32 has accelerated in the past two years,
with at least one manufacturer having announced a switch to using HFC-
32 in all successive models of residential air conditioners launched in
Japan beginning in late 2012. HFC-152a has been investigated as an option
for replacing HFC-134a in mobile vehicle air conditioning applications, but
its A2 flammability classification poses a major barrier to widespread
adoption. HFC-152a may also be a viable replacement in commercial
refrigeration applications, chillers, and industrial refrigeration.

 Hydrocarbons : The three most suitable hydrocarbon refrigerants

incorporate propane, isobutane, and propylene. These hydrocarbons
have GWP estimations of 3 or less, and they are delegated A3 refrigerants
due to their high combustibility. Hydrocarbons are possible substitutions
for some HFC410A frameworks, regardless of having somewhat lower
volumetric limit and execution. Hydrocarbon refrigerants have
fundamentally lower cost contrasted with other manufactured options.
Hydrocarbons are in fact feasible for little and medium-sized refrigeration
and cooling applications, just as chillers. In any case, because of their high
combustibility, they would be viewed as hazardous in most direct-
extension (DX) HVAC&R applications, aside from applications with low
charges. Charge cutoff points forced by Underwriters Laboratories (UL).
Benchmarks and the EPA Significant New Alternatives Program (SNAP)
limit the capacity to utilize hydrocarbons in applications requiring bigger
volumes of refrigerant. In 2011, the EPA issued a last principle permitting
the utilization of isobutane and propane in family unit size fridges and
coolers and little independent refrigeration units, gave they consent
charge limit confinements forced by security codes. Various makers are
presently selling private fridges utilizing isobutane in the United States
and internationally. Reasonability in bigger refrigeration applications
would require broad hazard evaluations to help changes to current charge
limits.
Hydrocarbons are actually practical for private and business cooling
applications, however ASHRAE Standard 1520 charge cutoff points and
limitations right now counteract usage of hydrocarbons in these
applications. Propane, in any case, demonstrates huge guarantee for
auxiliary extension frameworks in general stores. Propane could likewise
be suitable in some chiller applications. Hydrocarbons have equivalent
efficiencies to the present HFC refrigerants, so actualizing them doesn't
fundamentally decrease framework proficiency.
 Ammonia : Ammonia is classified as B2 and has a GWP value of 0. 21
Industrial refrigeration systems often use ammonia as a refrigerant. Due
to its Class B toxicity rating, ammonia is not a likely candidate for comfort
conditioning applications or indoor commercial refrigeration applications.
However, ammonia could be viable for chillers and secondary expansion
systems, particularly for supermarkets. Like other naturally occurring
refrigerants, ammonia has a much lower cost than other synthetic
alternatives. Ammonia has comparable efficiency to the current HFC
refrigerants, so implementing ammonia as an alternative would not
significantly reduce system efficiency.

 CO2 : Carbon dioxide (CO2) is classified as A1 (non-flammable, non-toxic)

and has a GWP of 1, by definition. CO2 has been demonstrated as a viable
alternative for several applications including heat pump water heaters,
commercial refrigerated vending machines, supermarket refrigeration,
secondary expansion systems, and industrial and transport refrigeration
systems. Carbon dioxide is also a technically viable option in mobile
vehicle air-conditioning (MVAC) systems. The higher design pressure
required for CO2 systems presents some safety concerns. The higher
pressures also add to the overall component costs of the system. EPA
SNAP has cited concern about the potential lethality of carbon dioxide at
high concentrations, which is especially relevant to passenger car volumes
or small room volumes. Implementing CO2 as an alternative to HFCs often
requires a complete system redesign due to the high pressure and
supercritical behavior. This poses a major barrier to widespread adoption.
The theoretical cycle efficiency of CO2 is significantly lower than that of
HFCs, which can result in a reduction of overall system efficiency.
Advances in transcritical CO2 systems have enabled the use of CO2 in
some refrigeration applications such as supermarkets and vending
machines. However, CO2 is unlikely to be viable for air conditioning
applications due to the inherent thermodynamic disadvantages
compared to other candidate fluids. In Europe, CO2 supermarket systems
have approached the efficiency of traditional systems when used in areas
with mild climates such as Denmark. This approach has also been proved
successful in certain parts of the U.S.; however, this approach is not viable
for hotter climates.

 Hydrofluoroolefins (HFOs) : HFOs as a replacement for HFCs requires a

tradeoff between GWP and system efficiency HFOs are some of the most
viable emerging alternative refrigerants. Refrigerant manufacturers have
 developed numerous HFO blends tailored to specific applications. HFO-
1234yf and HFO1234ze are furthest along in development. HFO-1234yf
and HFO-1234ze are both classified as A2L and have GWP values less than
1. Cost represents a major concern with HFOs and HFO blends. While
actual costs under full scale production conditions are unknown, current
HFO-based refrigerants will almost certainly have a much higher cost than
the refrigerants they would replace. Additionally, with HFO systems, the
efficiency tends to decrease as the GWP of the refrigerant decreases.
Therefore, implementing.

IMPROVING ENERGY EFFICIENCY

Energy efficiency is one of the most important aspects when it comes to

developing any machine and it also a key factor in attaining the goal of
sustainable development. Efficient energy conversion means that there
will be lesser wastage of fuel and thus there will be lesser emission of
harmful gases into the atmosphere. For refrigeration and heat pump
systems, it requires not only to develop efficient components and
equipment, but also to integrate efficiently those equipment in larger
systems: for buildings it is essential to think of efficient buildings as a
whole and not only using efficient equipment.

1. New designs with lower heating or cooling loads :

The development of passive houses or near zero energy houses leads to

the development of smaller heat pumps and also reverse heat pumps
(heating and cooling) as well as integrated heat pumps. The larger
thermal need becomes to be the generation of domestic hot water in
low energy houses. In Switzerland, new design of heat pumps with heat
recovery of the exhaust air producing hot water and heating air instead
of a hydronic circuit correspond to those new stakes.
Many studies and research have been carried out since 15 years on thermal
comfort. One of the most important results is to show that the human body can
adapt widely to thermal conditions. The comfort temperature in summer can be
set around 26°C in rooms, and leads to better comfort than a lower
temperature. Taking that into account Japanese companies have promoted a
new dressing code for employees in summer: no jacket and no tie.

2. System design and overall control :

The integration of refrigeration systems and heat pumps requires to

integrate the variation of source and sink temperatures in order to adapt
the thermodynamic system and to reach highest possible COPs. A typical
example is to lower the hot water temperature for air-to-water heat
pumps according to the outdoor temperature. Another interesting
example is to disconnect dehumidification and cooling when the relative
humidity is at acceptable level to control the evaporating temperature
above the dew point and so high COPs for cooling can be reached,
specially for air-to-air systems.
 New specifications for residential heat pumps require that the heating
capacity is fully generated by the heat pump at the lowest outdoor
temperature (no complementary heat can be produced by a
complementary electric resistance on the hydronic circuit). The span of
outdoor air temperatures for the heating season varies typically from –
15°C to 14°C and the water for the hydronic circuit has to be produced at
temperatures varying from 65 to 35°C, the largest difference of
temperatures (-15/+65°C) corresponding to the highest thermal needs.
Such constraints require reverse systems capable to switch from 2-stage
to 1-stage operation according to the outdoor temperature and to the
water temperature.

Design of a reverse 2-stage/1-stage system

3. Motor and compressor improvements :

Exergy analyses indicate always the paramount impact of motor-

compressor for refrigeration and heat pump systems. For electric motors,
the breakthrough has been made several years ago with permanent
magnet motors, which have been introduced in large scale by Japanese
OEMs. The electrical efficiency even for electric motors in the range of 1
to 2 kW is superior to 92%. A complementary breakthrough has occurred
progressively, also coming from Japanese OEMs, with electronic variable
speed control. All efficient reverse heat pumps sold in Japan for cooling
capacities ranging from 2.4 to 5 kW are equipped with variable speed
motors. The variable speed control allows fine tuning of the refrigerant
 mass flow rate for the large span of heating/cooling capacities. Typically
those revere air-to-air heat pumps are capable to provide 3 times more
heat than the maximum cooling capacity. COPs superior to 6 referring to
ISO 5151 summer conditions T1 have been measured on such systems.
Even if reciprocating compressors are still widely used for small
refrigerating capacities (domestic) and also in many medium capacity
systems (up to 30 kW input power), for capacities ranging from 1 to 25
kW input power, scroll compressors and in a lesser extent rotary
compressors are replacing reciprocating ones. The precision of
manufacturing machines allows such a change because the number of
rotating pieces is smaller, friction losses can also be significantly lower,
the discharge valve can be avoided, and more generally, the pressure
losses for valves can be significantly lowered with scroll and rotary
compressors. By design those compressors can reach higher efficiency
compared to reciprocating ones.
One of the significant on-going improvements is the development of oil
less compressors and high efficient integrated oil separator. When going
to very efficient heat exchangers, specially when reducing dramatically
the diameter or the equivalent diameter of heat exchanger tubes, the
presence of oil hampers significantly the heat exchange. Moreover, the
oil return creates an accumulation at the end of the evaporator, and oil
slugging is always possible. The drastic limitation of oil circulation has
been also significantly improved for CO2 in MAC systems. What has been
gained for CO2 can also be gained for any refrigerants. In parallel, the
development of magnetic bearings allows no lubrication for centrifugal
machines and new relatively small centrifugal compressors have been
appearing on the market since five years. This trend is also associated with
significant increase in RPMs of those centrifugal compressors. Velocities
higher than 25,000 rpm are foreseen. The development of new software
enables rapid design of wheels and possible new developments of high
speed non-lubricated centrifugal compressors seem a new opportunity.

4. Heat exchanger improvements :

The development of heat exchangers is driven by high efficiency heat

exchange and the decrease of materials quantities (for mass production
components, the mass of the material is a key parameter for the price).
The trends are obvious: the thickness of copper tubes has been reduced
to 0.3 mm, the thickness of aluminum fins is in the range of 0.1 mm, the
diameters have been constantly reduced, and high efficiency fins, groove
tubes, have led to improve of a factor 2 to 3 the heat exchange coefficient
of air-to-refrigerant heat exchangers in the last 20 years. In parallel with
those developments of copper tubes / aluminum fins, fully brazed all
aluminum heat exchangers have won the competition for radiators,
condensers, and evaporators in the automotive industry.
Condensers have been designed with extruded micro-channel aluminum
tubes. Those tubes are brazed with accordion louvered fins, leading to a
significantly higher surface ratio of fins compared to stationary
application. For equivalent air velocity, the heat exchange coefficients are
higher of nearly a factor 2 compared to stationary condensers. For
evaporators, design of plate fin heat exchangers has been the leading
technology for the last twenty years, and due to the constraints of
pressure associated with CO2, new designs for evaporators has been
realized with the same technology as for condensers using micro-channel
tubes organized in different circuitry. The high compacity the lower cost
of aluminum compared to copper has led to the beginning of the transfer
from the automotive industry to the stationary industry.
CONCLUSION

 The decision as to which refrigerant should be used in a

refrigerating or air-conditioning system is based on the major
criteria of safety, costs and environment protection. But against the
background of constantly increasing energy prices, the energy
consumption of a system also plays an increasingly important role.
Ideally, the chosen refrigerant should have excellent
thermodynamic properties, high chemical stability and good
physical characteristics. Furthermore, it should have no or only a
negligible impact on the environment, while also being inexpensive
and available worldwide.

 Ammonia refrigeration convinces with top energy efficiency:

Ammonia is the refrigerant with the demonstrably best
thermodynamic properties. It is the only natural refrigerant that
industry never wanted to dispense with on account of its high
efficiency. Ammonia is also unbeatable in ecological terms: it has
no ozone depletion potential and no global warming potential (ODP
and GWP = 0), with a favourable TEWI balance thanks to the high
COP of ammonia systems. In industrial systems with capacities
exceeding 500 kW, ammonia is simply unsurpassed in terms of
energy and cost efficiency. And it is also finding increasing use on a
smaller scale, for example in systems with a capacity of less than
500 kW where the quantity of ammonia can be reduced when
choosing a suitable secondary refrigerant. At present, intensive
research is in progress here in particular in the range of small-
capacity systems, with the objective among others of developing
small, semi-hermetic and hermetic compressors with output below
100 kW. Reduced quantity heat exchangers are also being
developed along the same lines. Furthermore, various research
projects are also looking at simplified oil management with soluble
oils to facilitate DX systems as well. Moreover, today ammonia is
also being used increasingly in areas that used to be dominated by
synthetic refrigerants. For example, all large exhibition buildings in
Germany have been equipped with ammonia liquid chillers for air-
conditioning. Banks, insurance companies and office buildings also
increasingly use ammonia liquid chillers for energy-saving air-
conditioning. Even modern airports make increasing use of
ammonia systems, in the light of risk analysis results indicating no
 greater hazard potential for the general public or airport employees
than systems using synthetic refrigerants.

 Carbon dioxide can be used to save energy and money: The

efficiency of a CO2 system is clearly superior to a plant operating
with synthetic refrigerants when used in the subcritical range. But
in the supercritical range too, success is also being achieved in
optimising system efficiency.

 Hydrocarbons : Hydrocarbons have excellent thermodynamic

properties, which is why refrigerating and air-conditioning systems
operating with these substances are particularly energy-efficient.
They are well miscible with conventional refrigerating oils and have
a relatively high critical temperature. While the flammability of
hydrocarbons requires hermetically sealed systems with explosion
protection for electrical components, all components are easily
available and current technology copes well with the demands of
safe operation. Given the high energy saving potential of systems
with hydrocarbons, a number of companies have announced their
intentions of operating new refrigerating systems with
hydrocarbons.

 Apart from the choice of refrigerants the various advancements

and improvements in refrigeration devices are also very important
in determining the COP of the refrigerator.
REFERENCES

 http://www.centrogalileo.it/nuovapa/Articoli%20tecnici/INGLESE
%20CONVEGNO/XII%20Convegno%20English/I%20SESSIONE/Clodi
c.pdf
 https://www.energy.gov/sites/prod/files/2014/12/f19/Refrigeran
ts%20Roadmap%20Final%20Report%202014.pdf
 Protocol K. United Nations framework convention on climate
change. Kyoto Protoc Kyoto 1997;19.
 Kim MH, Pettersen J, Bullard CW. Fundamental process and system
design issues in CO2 vapor compression systems. Prog Energy
Combust Sci 2004;30:119–174.
 Gullo P, Elmegaard B, Cortella G. Energy and environmental
performance assessment of R744 booster supermarket
refrigeration systems operating in warm climates. Int J Refrig
2016;64:61–79.
 Purohit N, Gupta DK, Dasgupta MS. Thermodynamic analysis of CO2
trans-critical booster system for supermarket refrigeration in warm
climatic conditions. 4th IIR Conf. Sustain. Cold Chain. held Massey
Univ. Auckl., IIR; 2016.
 Gupta DK, Dasgupta MS. Simulation and performance optimization
of finned tube gas cooler for trans-critical CO2 refrigeration system
in Indian context. Int J Refrig 2014;38:153–167.
 Sarkar J. Performance improvement of double-tube gas cooler in
CO2 refrigeration system using nanofluids. Therm Sci 2015;19:109–
118.
 Pitla SS, Groll EA, Ramadhyani S. New correlation to predict the
heat transfer coefficient during in-tube cooling of turbulent
supercritical CO2. Int J Refrig 2002;25:887–895.
 Choi SUS, Eastman JA. Enhancing thermal conductivity of fluids
with nanoparticles. ASME-Publications-Fed 1995;231:99–106.
 Haghighi EB, Saleemi M, Nikkam N, Khodabandeh R, Toprak MS,
Muhammed M, Palm B. Accurate basis of comparison for
convective heat transfer in nanofluids. Int Commun Heat Mass
Transf 2014;52:1–7.