7/31/2019 sory3

1/6

a) What is the effect on the COPref as the load is increased? Why?The efficiency of a refrigerator or refrigerator performances are defined by

means of the coefficient of performance, COP denoted by COPrefwhich is

given by

W

QCOPref

1

where COP is sometimes called the performance ratio. The best COP will

be given by a circle which is a Carnot cycle operation between the given

temperature conditions. The objective of a refrigerator is to remove heat

QL from the refrigerated space. To accomplish this objective, it requires awork input of Wnet,in.

For a refrigerator the important quantity is the heat supply to the system

from the surrounding, Q1. The power input, W is important because it is

the quantity which has to be paid for and constitutes the main item of therunning cost.

b) Natural refrigerants have been gained attention over the last decade to be used asworking fluids in refrigeration application. One of the natural refrigerants is CO2

( carbon dioxide), which offers complete solution to current environmental

problems such as global warming and ozone layer destruction. CO2 has zero

ozone depleting potential and negligible global warming potential. CO2 has

excellent properties to be used as refrigerant such as; non-toxic, non-flammable,the price is only a fraction of today refrigerant, excellent thermodynamic

properties, and compact system components due to high density.The most distinction of CO2 properties compared to common refrigerants is the

low critical temperature of 31C and high critical pressure of 73.8 bar. For airconditioning

Wnet,in

QH

QL

Cold refrigerated

at TL

Warm environment

at TH>TL

Reversed

heat engine

7/31/2019 sory3

2/6

application in tropical countries, the outdoor air temperature will be

close to the critical temperature of CO2 most of the time, leading to transcritical

operation to obtain better efficiency. The efficiency of CO2 system can beincreased by lowering cooling medium temperature. Since ground water

temperature is lower than outdoor air temperature in tropical countries, the

average cooling medium temperature can be lowered in situation where there is a

simultaneous need of space cooling and hot water such as in hospitals or hotels.The present work studies the potential of a combined air-conditioning and water

heating system using CO2 as working fluid. The water heating heat exchangerrecovers part of rejected heat of the air-conditioning system to produce hot water.

When inlet water temperature to the water heating heat exchanger is lower than

cooling medium temperature of the heat rejecting heat exchanger, the average

cooling medium temperature will be lower and the efficiency of the system will

increase. This combined system offers energy saving by eliminating the need of

energy to produce hot water.

Main objectives of this study were to investigate the combined air-conditioning

and water heating system using CO2 as working fluid theoretically and

experimentally. Different operating conditions of experiments were chosen to

locate vital parameters for the combined system performance. A computer model

of the combined system was developed and verified with the experimental data.Thermophysical properties of CO2 was also written in computer code and

integrated with the model.

xiv

Combined CO2 air-conditioning and water heating system

A computer program of thermophysical properties of CO2 that can be integrated

with other program such as spreadsheet program has been developed in this work.Extended equation of state from Span and Wagner is used to calculate

thermodynamic properties and equation of state from Vesovic et al. is used to

calculate transport properties.

Some promising applications of transcritical cycle using CO2 as working fluid are

heat pump water heater and mobile air-conditioning systems. Heat pump water

heater is the most promising application compared to other refrigerants due to

better match of refrigerant temperature and water temperature. Heat rejectingprocess in transcritical cycle takes place in supercritical region where temperature

and pressure are independence properties. By regulating the discharge pressure,

the gliding temperature can be increased and a better temperature match can be

obtained. In air-conditioning application, the key point to get a higher efficiency

of transcritical cycle is to achieved a small temperature different between cooling

medium temperature and CO2 temperature leaving gas cooler. This temperature

different is called temperature approach. A lower efficiency of CO2 airconditioning

system in a higher cooling medium temperature has been reported

from several studies. This is due to a lower CO2 compressor efficiencies than

expected and improper component design leading to a higher evaporationtemperature and higher approach temperature.

The other promising application of CO2 transcritical cycle is combined airconditioning

and water heating system. There will be at least two gas coolers inthis system, one for rejecting heat and the other for recovering heat to produce hot

water. These gas coolers can be arranged in series or parallel. In series

configuration, gas cooler as water heating heat exchanger is placed in front of gascooler as heat rejecting heat exchanger. The approach temperature will become

lower and will tend to zero. This arrangement is similar to a heat recovery using

desuperheater in subcritical cycle. In parallel configuration, gas cooler as water

heating heat exchanger is placed in parallel with gas cooler as heat rejecting heat

exchanger. Hot gas CO2 discharged from compressor is split into two streams,

one stream enters heat rejecting heat exchanger and the other stream enters waterheating heat exchanger. The split ratio of hot gas CO2 depends on load ratio of

hot water load to total rejected heat of air-conditioning system. The load ratio will

determine the performance of the air-conditioning system.

7/31/2019 sory3

3/6

c) What do you understand by the term load? Give example of actual loads inrefrigeration practice in a domestic fridge, in a room, and in a factory.

Domestic Fridge

The present invention concerns domestic refrigerators of the type having a refrigerator body

defining a first internal refrigerated cavity, and, in this first internal cavity, at least one compartment

in which at least a part is manually movable.

Known domestic refrigerators generally have two compartments of the type mentioned above, namely

the compartment for making ice, commonly called "freezer", containing a refrigeration coil and closed

by means of a pivoting door, and a compartment in the form of a vat, generally disposed opposite the

"freezer" and serving as a removable vegetable tray, in an environment at a low temperature between

2 and 5 C.

There is a need for the consumers to be able to rely on refrigerators which are provided with

means enabling to substantially extend the period of preservation of fruits and vegetables, experience

showing that in known vegetable trays, the vegetables have a tendency to fade or rot relatively

rapidly.

Processes for extending the length of preservations of plant food products, in particular fruits and

vegetables, are known, and these processes consist in placing these products under an oxygen starved

controlled atmosphere and to keep them at low temperature, typically between 0 and 15 C. These

processes are found either in storage silos of substantial size, where the composition of the controlled

atmosphere is permanently supervised and adjusted, namely for their conditioning in wrappings

intended for sale and having selective properties of gas diffusion.

It is an object of the present invention to propose a domestic refrigerator enabling to establish and

maintain in a portion of its internal cavity an atmosphere adapted for the extended preservation of

fruits and vegetables, in an autonomous arrangement, at low cost, with reliable operation and which

does not modify the overall size of the refrigerator.

For this purpose, according to a characteristic of the invention, the compartment of the refrigerator

defines a second internal cavity which is substantially water tight with respect to the first internal

cavity and communicates with a gas source containing less oxygen than air.

According to a more particular characteristic of the invention, the gas source comprises a separation

module periodically supplied with air under pressure, advantageously by means of a moto-compressorunit controlled by a control module comprising a timer, and preferably, coupled to a detector which is

responsive to the movement of the movable part of the compartment.

It is another object of the present invention to propose a process for feeding an oxygen starved gas

into a compartment of a refrigerator of the above type, enabling, at lower cost, to keep, in the

compartment, an atmosphere which is adapted for the preservation of fruits and vegetables.

Room

In a room, the specification that needs to know is the chilling or freezing times now

worked out, the size of the room can be determined. To achieve this, the operation of the

7/31/2019 sory3

4/6

whole abattoir may have to be changed and also the flow of carcasses to and from chiller

or freezer, the position of doors and so on.

If it size and position of the room has been rigidly fixed before this stage, the cooling

times determined above will not be met. When loading a chiller the doors are invariably

left open for long periods allowing a fully established air flow to take place to and fromthe room either from gravity through a single door or by a through flow of air if more

than one door is open.

Another point to notice is that the load on the room, when used as a store, even when

the outside temperatures are very high, is very small compared to both the peak and

average load and is for the most part due to the evaporator fans running continuously. The

load then increase when the doors are opened and the room is washed out or possibly

unloaded. Warm carcasses are then loaded into the room and the load rapidly reaches the

peak product load that occurs at the end of the loading period. Thereafter, the doors are

closed and the load rapidly declines. At the end of the chilling cycle, the doors are again

opened to remove the carcasses and the infiltration load so caused increase.

Factory

An example of actual loads is storage of specific food. A refrigerator is design to

maintain the freezer section at -18C and the refrigerator section at 3C. Lower freezer

temperatures increase energy consumption without improving the storage life of frozen

food significantly. Different temperatures for the storage of specific foods can be

maintained in the refrigerator section by using special-purpose compartment.

Generally, all full size refrigerators have the a large air-tight drawer for leafty

vegetable and fresh foods to seal moisture and protect from drying effect. Some have a

temperature controlled meat compartment maintained at -0.5C, which keeps meat at

lowest safe temperature without freezing it.

For specified external dimensions, a refrigerator is desired to have maximum food

storage volume, minimum energy consumption, and the lowest possible cost to the

consumer.

The size of compressor and another components of a refrigeration system are

determined on the basis of the anticipated heat load (or refrigeration load), which is heat

flow into the refrigerator. The heat load consists of the predictable part, fan motor; defrostheaters and the unpredictable part.

7/31/2019 sory3

5/6

The cross section of a refrigerator showing the relative magnitudes of various effects that

constitutes the predictable heat load.

d) What is the effect on the condenser temperature as the load is increased? Why?The effect of the condenser temperature is when the load is increase, the temperature also

increased. When the more load is added, the more heat is rejected from the system. When

more heat is rejected, the temperature is more cooled do to the increases of the load. Cooling

effect of the refrigeration system, which is the rate of heating being removed from low

temperature region with specified evaporator and condensation temperatures. The unit duty

measurement is in the watts (for 1 ton of refrigeration = 351 W)

The function of the refrigeration system is to transport the heat from the lower

temperature in the conditioned space to the higher temperature outside. It is thus a heat pump.

The required energy input of a heat pump is a function not only of the amount of cooling

it has to do but also of the temperature elevation against which it is pumping. The reduction

of this temperature difference is the goal of optimization.

The isothermal processes in this cycle are also isobaric (constant pressure). Theefficiency of a refrigerator is defining as the ratio between the heat removed from the processand the work required to achieve this heat removal.

CONCLUSION

As a conclusion, it is proved that the variation in Coefficient of Performance (COP ref) of a

vapor compression refrigeration system has been obtained by using the method of refrigerant

7/31/2019 sory3

6/6

enthalpy change rate with assuming the system is adiabatic which no heat transfer occurs

during the process. The function of the refrigeration system is to transport the heat from the

lower temperature in the conditioned space to the higher temperature outside. It is thus a heat

pump. The required energy input of a heat pump is a function not only of the amount of

cooling it has to do but also of the temperature elevation against which it is pumping. The

reduction of this temperature difference is the goal of optimization for refrigerator the

important quantity is the heat supplied to the system from the surroundings, QL.The power

inputw 1-2,is important because is the quantity which has to be paid for and constitute the

main item of the running cost. There are two type of refrigeration system viz vapor

compression and vapor absorption system. Out of these two vapors, compression system is

commonly use in the buildings. Vapor Absorption System is also becoming economically

attractive where waste heat is available. The Commercial Refrigeration (based on vapor

compression system) and its auxiliaries are the major consumers of electricity in a building.

Therefore, it is necessary to understand some of the terms, concepts, and equipment involved.

Thus, the P-h diagram is a widely used and very useful tool for doing an approximate

evaluation of a refrigerator system. It also useful I order to find out the coefficient

performance.Eventually this experiment is essential because it is fundamental experiment for

the advanced technology in the future.