Dinc Ibrahim. Refrigeration systems and applications 2th edition

Подождите немного. Документ загружается.

Advanced Refrigeration Cycles and Systems 261

spoilage and maintain freshness and quality. Food freezing systems are required for longer term

storage at −18 to −35

◦

C. Food storage and transport take place in chambers covering a wide

range of sizes from cold stores to household refrigerators. Solar cooling is of great interest

especially in developing countries, where food preservation is often as difﬁcult a problem as food

production.

From an energy-saving view, a solar cooling system has the capability of saving electrical energy

in the range of 25–40% when compared to an equivalent cooling capacity of a conventional water-

cooled refrigeration system. Therefore, the use of solar cooling systems will save energy, especially

during the summer season. The contribution of these systems to the food processing sector and

consequently to the economy will be high (Dincer, 1997).

Solar-powered mechanical cooling, of whatever type, is presently in the developmental phase.

The technology is ready, but cost factors stand in the way of vigorous marketing programs. At

present, active solar cooling is not in a reasonably competitive position with respect to conventional

cooling systems (energized by electricity or fossil fuel). During the last decade, the situation has

changed quickly because of increasing interest in renewable energy sources, especially solar energy,

for reducing the use of fossil fuels and electricity.

Solar energy can be used in different systems available for cooling applications. These systems

are (Dincer and Dost, 1996) as follows:

• Rankine cycle vapor-compression system,

• absorption cycle system,

• adsorption system,

• jet ejector system,

• Rankine cycle-inverse Brayton cycle system,

• nocturnal radiation system.

Among these systems, the solar-powered absorption cooling cycle is the most popular system

for solar cooling applications because of the following advantages (Dincer and Dost, 1996):

• quiet operation,

• high reliability,

• long service life,

• effective and economic use of low-grade energy sources (e.g., solar energy, waste energy, geother-

mal energy, natural gas),

• easy implementation and capacity control,

• no cycling losses during on–off operations, and

• meeting the variable cooling load easily and efﬁciently.

5.9.2 Solar-Powered Absorption Refrigeration Systems (ARSs)

Solar energy is a renewable and ozone-friendly energy source. Solar cooling is the most attractive

subject for many engineers and scientists who work on solar energy applications. Most of the

research and development efforts have been carried out using an absorption cooling system. This

system is usually a preferable alternative, since it uses thermal energy collected from the sun without

the need to convert this energy into mechanical energy as required by the vapor-compression system.

Besides, the absorption cooling system utilizes thermal energy at a lower temperature (i.e., in the

range 80–110

◦

C) than that used by the vapor-compression system.

Research and development studies on solar absorption refrigeration systems (ARSs) using dif-

ferent combinations of refrigerants and absorbents as working ﬂuids have been done. These ARSs

have good potential where solar energy is available as low-grade thermal energy at a temperature

level of 100

◦

C and above.

262 Refrigeration Systems and Applications

The principle of operation of a solar-powered absorption cooling system is the same as that of

the absorption cooling system shown in Figure 4.33, except for the heat source to the generator. In

Figure 4.33, we presented a solar absorption cooling system using an R-22 (refrigerant)-DMETEG

(absorbent) combination as a working ﬂuid (Dincer et al., 1996). Its operation can be brieﬂy

explained as follows. In the absorber, the DMETEG absorbs the R22 at the low pressure and

absorber temperature supplied by circulating water, and hence a strong solution occurs (2). This

strong solution from the absorber enters a solution pump, which raises its pressure and delivers

the solution into the generator through the heat exchanger (3–6). The generator, which is heated

by a solar hot water system, raises the temperature of the strong solution, causing the R-22 to

separate from it. The remaining weak solution ﬂows down to the expansion valve through the heat

exchanger and is throttled into the absorber for further cooling as it picks up a new charge of

the R22 vapor, becoming a strong solution (6−2) again. The hot R-22 vapor from the generator

passes to the condenser and is released to the liquid phase (8–9). The liquid R-22 enters the second

heat exchanger and loses some heat to the cool R-22 vapor. The pressure of the liquid R-22 drops

signiﬁcantly in the throttling valve before it enters the evaporator. The cycle is completed when

the desired cooling load is achieved in the evaporator (10–12). Cool R-22 vapor obtained from the

evaporator enters the absorber while the weak solution comes to the absorber continuously. The

R-22 vapor is absorbed here (12−1). This absorption activity lowers the pressure in the absorber,

causing the vapor to be taken off from the evaporator. When the vapor goes into liquid solution,

it releases both its latent heat and a heat of dilution. This energy release has to be continuously

dissipated by the cooling water.

Solar-operated ARSs have so far achieved limited commercial viability because of their high

cost–beneﬁt ratios. The main factor which is responsible for this drawback is the low COP associated

with these systems, which generally operate on conventional thermodynamic cycles with common

working ﬂuids. It is essential to investigate the possibility of using alternative working ﬂuids

operating in new thermodynamic cycles. Also, development of more efﬁcient, less expensive solar

collectors will be a continuing need for solar energy to reach its full potential.

5.10 Magnetic Refrigeration

Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique

can be used to attain extremely low temperatures (well below 1 K) as well as the ranges used

in common refrigerators, depending on the design of the system. The magnetocaloric effect is a

magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable

material is caused by exposing the material to a changing magnetic ﬁeld. One of the most notable

examples of the magnetocaloric effect is in the chemical element gadolinium and some of its alloys.

Gadolinium’s temperature is observed to increase when it enters certain magnetic ﬁelds. When it

leaves the magnetic ﬁeld, the temperature returns to normal.

In the magnetic refrigeration cycle, depicted in Figure 5.33, initially randomly oriented magnetic

moments are aligned by a magnetic ﬁeld, resulting in heating of the magnetic material. This heat is

removed from the material to the ambient temperature by heat transfer. On removing the ﬁeld, the

magnetic moments randomize, which leads to cooling of the material below ambient temperature.

Heat from the system to be cooled can then be extracted using a heat-transfer medium. Depending

on the operating temperature, the heat-transfer medium may be water (with antifreeze) or air, and

for very low temperatures, helium.

Magnetic refrigeration is an environmentally friendly cooling technology. It does not use ozone-

depleting chemicals (CFCs), hazardous chemicals (NH3), or greenhouse gases (hydrochloroﬂuoro-

carbons [HCFCs] and hydroﬂuorocarbons [HFCs]). Another key difference between vapor cycle

refrigerators and magnetic refrigerators is the amount of energy loss incurred during the refrig-

eration cycle. The cooling efﬁciency in magnetic refrigerators working with gadolinium has been

Advanced Refrigeration Cycles and Systems 263

Expelled heat

Heat

load

Figure 5.33 Schematic representation of a magnetic refrigeration cycle that transports heat from the heat

load to the ambient. Left and right depict material in low and high magnetic ﬁeld, respectively (Adapted from

Bruck, 2005).

shown (Zimm et al., 1988) to reach 60% of the theoretical limit, compared with only about 40% in

the best gas compression refrigerators. This higher energy efﬁciency will also result in a reduced

release (Bruck, 2005).

Magnetic refrigeration has three prominent advantages compared with compressor-based refrig-

eration. First, there are no harmful gases involved; second, it may be built more compactly as the

working material is a solid; and third, magnetic refrigerators generate much less noise. Recently,

a new class of magnetic refrigerant materials for room temperature applications was discovered

(Bruck, 2005).

This technology is still at the research stage for mainstream HVAC&R applications. Target future

applications include residential central air conditioners and heat pumps, ductless unitary applica-

tions, and small chillers. In the ﬁrst two of these applications, the small-scale, variable capacity

chilled water system of the magnetic refrigeration cycle would replace the direct expansion refriger-

ant system. Ultimately, to achieve commercial success in target applications, magnetic refrigeration

will need to achieve similar or superior efﬁciency at similar or lower cost than conventional vapor

compression equipment. Consequently, much recent and ongoing research efforts have focused on

enhancing the performance of prototype systems using the magnetocaloric materials and permanent

magnet materials currently available, and developing magnetocaloric materials that yield greater

temperature changes at lower magnetic ﬁeld strengths (Dieckmann et al., 2007).

5.11 Supermarket Refrigeration

An important application of refrigeration is supermarket refrigeration. Nearly all supermarkets

today use ozone-depleting HCFC refrigerant, usually R-22, or a blend consisting entirely or pri-

marily of HFCs. HCFCs and the HFCs are also greenhouse gases. Most supermarkets use direct

expansion refrigeration systems. Two of the most common advanced refrigeration technologies for

264 Refrigeration Systems and Applications

supermarkets are distributed system and secondary loop system. Below, we present the operation

of each system brieﬂy.

5.11.1 Direct Expansion System

Supermarket refrigeration systems have traditionally been direct expansion systems (used in about

70% of the supermarket refrigeration market). These systems typically use refrigerants R-22, R-502

(a blend of R-22 and CFC-115), R-404A (a blend of HFCs), or R-507A (a blend of HFCs).

The average emission rate of direct expansion systems is believed to be between 15 and 30%.

Most of the emissions are due to leaks in the system, including leaks in the valves and compres-

sors. In a direct expansion system, the compressors are mounted together and share suction and

discharge refrigeration lines that run throughout the store, feeding refrigerant to the cases and cool-

ers (Figure 5.34). The compressors are located in a separate machine room, either at the back of the

store or on its roof, to reduce noise and prevent customer access, while the condensers are usually

air-cooled and hence are placed outside to reject heat. These multiple compressor racks operate at

various suction pressures to support display cases operating at different temperatures (IEA, 2003).

As shown in Figure 5.34, the hot refrigerant gas from the compressors is cooled and condensed

as it ﬂows into the condenser. The liquid refrigerant is collected in the receiver and distributed to the

cases and coolers by the liquid manifold. The refrigerant is expanded turning a fraction of liquid into

vapor before ﬂowing into the evaporator. After cycling through the cases, the refrigerant returns

Rooftop

Remote condenser

Discharge manifold

Multiple

parallel

compressors

Suction manifold

Skid-mounted

components

Receiver

Sales area

Evaporator

Display case line-ups

Liquid manifold

Refrigerant

piping

Machine room

Figure 5.34 The schematic of direct expansion system (Adapted from IEA, 2003).

Advanced Refrigeration Cycles and Systems 265

to the suction manifold and the compressors. Supermarkets tend to have one direct expansion

system for “low-temperature” refrigeration (e.g., ice cream, frozen foods, etc.) and one or two

direct expansion systems for “medium-temperature” refrigeration (e.g., meat, prepared foods, dairy,

refrigerated drinks, etc.).

5.11.2 Distributed System

Unlike traditional direct expansion refrigeration systems, which have a central refrigeration room

containing multiple compressor racks, distributed systems use multiple smaller rooftop units that

connect to cases and coolers, using considerably less piping (Figure 5.35). The compressors in a

distributed system are located near the display cases they serve, for instance, on the roof above the

cases, behind a nearby wall, or even on top of or next to the case in the sales area. Thus, distributed

systems typically use a smaller refrigerant charge than direct expansion systems and hence have

decreased total emissions (IEA, 2003).

Rooftop

Multiple

parallel

compressors

Fluid pump

Evaporator

Display case line-

ups

Sales area

Refrigerant piping

fluid loop for rejection

Evaporative fluid

cooler

Water-cooled

condenser

Compressor

cabinet

Figure 5.35 The schematic of distributed system (Adapted from IEA, 2003).

266 Refrigeration Systems and Applications

As shown in Figure 5.35, the refrigerant is compressed in multiple parallel compressors and the

superheated refrigerant gas is cooled and condensed in a water-cooled condenser. The refrigerant

is then expanded before entering the evaporator. It absorbs heat from the cooled products before

returning to the compressors as a vapor. The water that is heated by the condensing refrigerant in

the condenser is sent to an evaporative cooler. It is cooled and pumped back to the condenser to

repeat the cycle.

5.11.3 Secondary Loop System

Secondary loop systems have recently seen increased introduction into retail food equipment, and

now make up about 4% of the market (Figure 5.36). These systems generally use R-404A or

R-507A, although some earlier systems used R-22. Their average leak rate is between 2 and 15%.

Secondary loop systems use a much smaller refrigerant charge than traditional direct expansion

refrigeration systems, and hence have signiﬁcantly decreased total refrigerant emissions. In sec-

ondary loop systems, two liquids are used. The ﬁrst is a cold ﬂuid, often a brine solution, which is

pumped throughout the store to remove heat from the display equipment. The second is a refrigerant

Rooftop

Remote condenser

Skid-mounted

components

Receiver

Machine room

Brine pump

Parallel

compressors

Brine chiller H-X

Brine piping

Brine coil

Display case line-ups

Sales area

Refrigerant piping

Brine piping

Figure 5.36 The schematic of secondary loop system (SCEFM, 2004).

Advanced Refrigeration Cycles and Systems 267

used to cool the cold ﬂuid that travels around the equipment. Secondary loop systems can operate

with two to four separate loops and chiller systems depending on the temperatures needed for the

display cases (SCEFM, 2004).

As shown in Figure 5.36, the refrigerant is compressed in parallel compressors and the super-

heated refrigerant gas is cooled and condensed in a remote condenser. The liquid refrigerant is then

collected in the receiver, expanded in a throttling device, and evaporated by absorbing heat from a

cold ﬂuid (i.e., brine). The cooled brine is distributed in the sales area (refrigerated area) absorbing

heat from the products before returning to the evaporator to repeat the process.

5.12 Concluding Remarks

This chapter has dealt with advanced refrigeration cycles and systems and their energy and exergy

analyses. New refrigeration systems and their technical and operational details and applications are

provided. Illustrative examples and some practical cases are also provided to better understand the

technical details of the advanced refrigeration systems.

Nomenclature

COP coefﬁcient of performance

constant-pressure speciﬁc heat, kJ/kg·K

ex speciﬁc exergy, kJ/kg

Ex exergy rate, kW

h enthalpy, kJ/kg

˙m mass ﬂow rate, kg/s

P pressure, kPa

q speciﬁc heat, kJ/kg

Q heat load; power, kW

s entropy, kJ/kg

gen

entropy generation rate, kW/K

T temperature,

◦

CorK

v speciﬁc volume, m

/kg

V volumetric ﬂow rate, m

w speciﬁc work, kJ/kg

W work input to compressor or pump, kW

X concentration of refrigerant in solution, kg/kg

Greek Letters

η efﬁciency

Study Problems

Multistage and Cascade Refrigeration Cycles

5.1 Consider a two-stage cascade refrigeration system operating between the pressure limits of

1.4 MPa and 120 kPa with refrigerant-134a as the working ﬂuid. Heat rejection from the

lower cycle to the upper cycle takes place in an adiabatic counter-ﬂow heat exchanger where

the pressure is maintained at 0.4 MPa. Both cycles operate on the ideal vapor-compression

268 Refrigeration Systems and Applications

refrigeration cycle. If the mass ﬂow rate of the refrigerant through the upper cycle is

0.12 kg/s, determine (a) the mass ﬂow rate of the refrigerant through the lower cycle, (b) the

rate of heat removal from the refrigerated space, and (c) the COP of this refrigerator.

Condenser

Evaporator

Compressor

Expansion

valve

Condenser

Evaporator

Compressor

Expansion

valve

5.2 A two-stage cascade refrigeration system operates between the pressure limits of 1.6 and

0.18 MPa with refrigerant-134a as the working ﬂuid. Heat rejection from the lower cycle to

the upper cycle takes place in an adiabatic counter-ﬂow heat exchanger where the pressure

is maintained at 0.6 MPa. Both cycles operate on the ideal vapor-compression refrigeration

cycle. If the refrigeration load is 11 tons, determine (a) the power input to the cycle, (b)

the COP, and (c) the power input and the COP if this refrigerator operated on a single

ideal vapor-compression cycle between the same pressure limits and the same refrigera-

tion load.

5.3 Consider a two-stage cascade refrigeration system operating between the pressure limits

of 1.2 MPa and 200 kPa with refrigerant-134a as the working ﬂuid. Heat rejection from

the lower cycle to the upper cycle takes place in an adiabatic counter-ﬂow heat exchanger

where the pressure in the upper and lower cycles are 0.4 and 0.5 MPa, respectively. In both

cycles, the refrigerant is a saturated liquid at the condenser exit and a saturated vapor at the

compressor inlet, and the isentropic efﬁciency of the compressor is 82%. If the mass ﬂow

rate of the refrigerant through the lower cycle is 0.06 kg/s, determine (a) the mass ﬂow rate

of the refrigerant through the upper cycle, (b) the rate of heat removal from the refrigerated

space, and (c) the COP of this refrigerator.

5.4 Consider a two-stage cascade refrigeration system operating between the pressure limits of

1.2 MPa and 200 kPa with refrigerant-134a as the working ﬂuid. The refrigerant leaves the

Advanced Refrigeration Cycles and Systems 269

condenser as a saturated liquid and is throttled to a ﬂash chamber operating at 0.45 MPa.

Part of the refrigerant evaporates during this ﬂashing process and this vapor is mixed with

the refrigerant leaving the low-pressure compressor. The mixture is then compressed to

the condenser pressure by the high-pressure compressor. The liquid in the ﬂash chamber

is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in

the evaporator. The mass ﬂow rate of the refrigerant through the low-pressure compressor

is 0.06 kg/s. Assuming the refrigerant leaves the evaporator as a saturated vapor and the

isentropic efﬁciency is 82% for both compressors, determine (a) the mass ﬂow rate of the

refrigerant through the high-pressure compressor, (b) the rate of heat removal from the

refrigerated space, and (c) the COP of this refrigerator. Also, determine (d) the rate of heat

removal and the COP if this refrigerator operated on a single-stage cycle between the same

pressure limits with the same compressor efﬁciency and the same ﬂow rate as in part (a).

Condenser

High-pressure

Compressor

Expansion

valve

Evaporator

Low-pressure

Compressor

Expansion

valve

Flash

chamber

5.5 Consider a two-stage cascade refrigeration system operating between the pressure limits of

1.6 MPa and 100 kPa with refrigerant-134a as the working ﬂuid. The refrigerant absorbs

heat from a space at 0

◦

C and rejects heat to ambient air at 25

◦

C. Heat rejection from

the lower cycle to the upper cycle takes place in an adiabatic counter-ﬂow heat exchanger

where the pressures in the lower and upper parts are 0.40 and 0.32 MPa, respectively. Both

cycles operate on the ideal vapor-compression refrigeration cycle. If the mass ﬂow rate of

the refrigerant through the upper cycle is 0.15 kg/s, determine (a) the mass ﬂow rate of the

refrigerant through the lower cycle, (b) the rate of heat removal from the refrigerated space

and the COP of this refrigerator, (c) the exergy destruction in the heat exchanger, and (d)

the second-law efﬁciency of the cycle and the total exergy destruction in the cycle. Take

= 25

◦

270 Refrigeration Systems and Applications

Liquefaction of Gases

5.6 Consider the simple Linde–Hampson cycle shown in the ﬁgure. The gas is nitrogen and

it is at 25

◦

C and 1 atm (101.325 kPa) at the compressor inlet, and the pressure of the gas

is 20 MPa at the compressor outlet. The compressor is reversible and isothermal, the heat

exchanger has an effectiveness of 100% (i.e., the gas leaving the liquid reservoir is heated

in the heat exchanger to the temperature of the gas leaving the compressor), the expansion

valve is isenthalpic, and there is no heat leak to the cycle. Determine (a) the refrigeration

load per unit mass of the gas ﬂowing through the compressor, (b) the COP, (c) the minimum

work input per unit mass of liquefaction, and (d) the exergy efﬁciency. Various properties

of nitrogen in the cycle are given as follows:

=−0.036 kJ/kg s

=−0.000059 kJ/kg · K

=−32.67 kJ/kg s

=−1.6806 kJ/kg · K

= h

=−232.70 kJ/kg s

= s

=−4.0025 kJ/kg · K

= h

=−431.54 kJ/kg

Compressor

Expansion

valve

Liquid

removed

Heat

exchanger

Makeup gas

5.7 Repeat Problem 5.6 for argon gas. Various properties of argon in the cycle are given as

follows:

=−0.1933 kJ/kg s

=−0.000516 kJ/kg · K

=−33.34 kJ/kg s

=−1.1930 kJ/kg · K

= h

=−111.51 kJ/kg s

= s

=−2.4985 kJ/kg · K

= h

=−272.65 kJ/kg