Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Refrigeration Cycles and Systems 181

(b) The exergy of the heat transferred from the low-temperature medium is

=−



1 −



=−(13.91 kW)



1 −

280

233



= 2.81 kW

This is the minimum power input:

min

= 2.81 kW

The second-law efﬁciency of the cycle is

net

2.81

37.62

= 0.075 = 7.5%

The total exergy destruction in the cycle can be determined from

dest,total

net

−

= 37.62 − 2.81 = 34.8kW

=−15

◦

C = 258 K

=−



1 −



=−(13.91 kW)



1 −

280

258



= 1.19 kW

min

= 1.19 kW

net

1.19

37.62

= 0.032 = 3.2%

dest,total

net

−

= 37.62 − 1.19 = 36.4kW

(d) The simple gas refrigeration cycle analysis is as follows (Figure 4.18):

= T





(k−1)/k

(

253 K

)(

)

0.4/1.4

= 376.0K

− h

− T

−−−→ 0.82 =

376.0 − 253

− 253

−−−→ T

= 402.9K

= T





(k+1)/k

= (289 K)





0.4/1.4

= 194.5K

− T

−−−→ 0.84 =

289 − T

289 − 194.5

−−−→ T

= 209.6K

=˙mc

− T

) = (0.45 kg/s)(1.005 kJ/kg · K)(253 − 209.6) kJ/kg = 19.63 kW

net,in

=˙mc

− T

) −˙mc

− T

)

= (0.45 kg/s)(1.005 kJ/kg · K)

[

(402.9 − 253) − (289 − 209.6) K

]

= 31.91 kW

COP =

net

19.63

31.91

= 0.615

182 Refrigeration Systems and Applications

16 °C

−20 °C

Refrig

Figure 4.18 Temperature–entropy diagram of simple gas refrigeration cycle considered in Example 4.5,

part (d).

4.7 Absorption–Refrigeration Systems (ARSs)

Although the principle of the absorption–refrigeration cycle has been known since the early 1800s,

the ﬁrst one was invented by French engineer Ferdinand P.E. Carre in 1860, an intermittent crude

ammonia absorption apparatus based on the chemical afﬁnity of ammonia for water, and produced

ice on a limited scale. The ﬁrst ﬁve Absorption–Refrigeration System (ARS) units Carre produced

were used to make ice, up to 100 kg/hour. In the 1890s, many large ARS units were manufactured

for chemical and petroleum industries. The development of ARSs slowed to a standstill by 1911 as

vapor-compression refrigeration systems came to the forefront. After 1950, large ARSs gained in

popularity. In 1970s, the market share of ARSs dropped rapidly because of the oil crisis and hence

the government regulations. Because of the increasing energy prices and environmental impact of

refrigerants, during the past decade ARSs have received increasing attention. So, many companies

have concentrated on ARSs and now do research and development on these while the market

demand increases dramatically.

ARSs have experienced many ups and downs. The system was the predecessor of the vapor-

compression refrigeration system in the nineteenth century, and water–ammonia systems enjoyed

a variety of applications in domestic refrigerators and large industrial installations in the chemical

and process industries. They were energized by steam or hot water generated from natural gas,

oil-ﬁred boilers, and electrical heaters. In the 1970s, the shift from direct burning of oil and natural

gas struck a blow at the application of the ARSs but at the same time opened up other opportunities,

such as the use of heat derived from solar collectors to energize these systems.

The concept of absorption refrigeration developed well before the advent of electrically driven

refrigerators. In the last decades, the availability of cheap electricity has made absorption systems

less popular. Today, improvements in absorption technology, the rising cost, and the environmental

impact of generating electricity are contributing to the increasing popularity of absorption systems.

ARSs for industrial and domestic applications have been attracting increasing interest throughout

the world because of the following advantages over other refrigeration systems:

• quiet operation,

• high reliability,

• long service life,

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

mal energy),

• easy capacity control,

• no cycling losses during on-off operations,

Refrigeration Cycles and Systems 183

• simpler implementation, and

• meeting the variable load easily and efﬁciently.

Recently, there has been increasing interest in the industrial (Figure 4.19) and domestic use of

the ARSs for meeting cooling and air conditioning demands as alternatives, because of a trend

in the world for rational utilization of energy sources, protection of the natural environment, and

prevention of ozone depletion, as well as reduction of pollution. There are a number of applications

in various industries where ARSs are employed, including the following:

• food industry (meat, dairy, vegetables, and food freezing and storage, ﬁsh industry, freeze

drying),

• chemical and petrochemical industry (liquefying if gases, separation processes),

• cogeneration units in combination with production of heat and cold (trigeneration plants),

• leisure sector (skating rinks),

• HVAC,

• refrigeration, and

• cold storage.

The absorption cycle is a process by which the refrigeration effect is produced through the use of

two ﬂuids and some quantity of heat input, rather than electrical input as in the more familiar vapor-

compression cycle. In ARSs, a secondary ﬂuid (i.e., absorbent) is used to circulate and absorb the

primary ﬂuid (i.e., refrigerant), which is vaporized in the evaporator. The success of the absorption

process depends on the selection of an appropriate combination of refrigerant and absorbent. The

most widely used refrigerant and absorbent combinations in ARSs have been ammonia–water and

lithium bromide-water. The lithium bromide-water pair is available for air-conditioning and chilling

applications (over 4

◦

C, because of the crystallization of water). Ammonia-water is used for cooling

and low-temperature freezing applications (below 0

◦

C).

The absorption cycle uses a heat-driven concentration difference to move refrigerant vapors

(usually water) from the evaporator to the condenser. The high concentration side of the cycle

absorbs refrigerant vapors (which, of course, dilutes that material). Heat is then used to drive off

these refrigerant vapors thereby increasing the concentration again.

(a) (b) (c)

Figure 4.19 (a) An ARS of 2500 kW at −15

◦

C installed in a meat factory in Spain. (b) An ARS of 2700 kW

at −30

◦

C installed in a reﬁnery in Germany. (c) An ARS of 1400 kW at −28

◦

C installed in a margarine

factory in The Netherlands (Courtesy of Colibri b.v.-Stork Thermeq b.v.).

184 Refrigeration Systems and Applications

Both vapor-compression and absorption-refrigeration cycles accomplish the removal of heat

through the evaporation of a refrigerant at a low pressure and the rejection of heat through the

condensation of the refrigerant at a higher pressure.

Extensive studies to ﬁnd suitable chemicals for ARSs were conducted using solubility measure-

ments for given binary systems. Although this information is useful as a rough screening technique

for suitable binary systems, more elaborate investigations now seem necessary to learn more of the

fundamentals of the absorption phenomena.

During the last decade, numerous experimental and theoretical studies on ARSs have been

undertaken to develop alternative working ﬂuids, such as R22-dimethyl ether tetraethylene gly-

col (DMETEG), R21-DMETEG, R22-dimethylformamide (DMF), R12-dimethylacetamide, R22-

dimethylacetamide, and R21-dimethyl ester. Previous studies indicated that ammonia, R21, R22,

and methylamine hold promise as refrigerants, whereas the organic glycols, some amides, esters,

and so on fulﬁll the conditions for good absorbents. Recently, environmental concerns have brought

some alternative working ﬂuids to the forefront, for example, R123a-ethyl tetrahydrofurfuryl ether

(ETFE), R123a-DMETEG, R123a-DMF, and R123a-triﬂuoroethanol, because of the CFCs’ ozone

depletion effects.

The cycle efﬁciency and the operating characteristics of an ARS depend on the thermophysical

properties of the refrigerant, the absorbent, and their combinations. The most important properties

for the selection of the working ﬂuids are vapor pressure, solubility, density, viscosity, and thermal

stability. Knowledge of these properties is required to determine the other physical and chemical

properties, as well as the parameters affecting performance, size, and cost.

Note that ammonia will quickly corrode copper, aluminum, zinc, and all alloys of these metals,

therefore these metals cannot be used where ammonia is present. From common materials only

steel, cast iron, and stainless steel can be used in ammonia ARSs. Most plastics are also resistant

to chemical attack by ammonia, hence plastics are suitable for valve seats, pump parts, and other

minor parts of the system.

4.7.1 Basic ARSs

It is considered that the ARS is similar to the vapor-compression refrigeration cycle (using the

evaporator, condenser, and throttling valve as in a basic vapor-compression refrigeration cycle),

except that the compressor of the vapor-compression system is replaced by three main elements – an

absorber, a solution pump, and a generator. Three steps, absorption, solution pumping, and vapor

release, take place in an ARS.

In Figure 4.20, a basic ARS, which consists of an evaporator, a condenser, a generator, an

absorber, a solution pump, and two throttling valves, is schematically shown. The strong solution

(a mixture strong in refrigerant), which consists of the refrigerant and absorbent, is heated in the

high-pressure portion of the system (the generator ). This drives refrigerant vapor off the solution.

The hot refrigerant vapor is cooled in the condenser until it condenses. Then the refrigerant liq-

uid passes through a throttling valve into the low-pressure portion of the system, the evaporator.

The reduction in pressure through this valve facilitates the vaporization of the refrigerant, which

ultimately effects the heat removal from the medium. The desired refrigeration effect is then pro-

vided accordingly. The weak solution (weak in refrigerant) ﬂows down through a throttling valve

to the absorber. After the evaporator, the cold refrigerant comes to the absorber and is absorbed

by this weak solution (i.e., absorbent), because of the strong chemical afﬁnity for each other. The

strong solution is then obtained and is pumped by a solution pump to the generator, where it is

again heated, and the cycle continues. It is signiﬁcant to note that the system operates at high

vacuum at an evaporator pressure of about 1.0 kPa; the generator and the condenser operate at

about 10.0 kPa.

Refrigeration Cycles and Systems 185

Figure 4.20 AbasicARS.

4.7.2 Ammonia–Water (NH

–H

O) ARSs

In practical ARSs, the utilization of one or two heat exchangers is very common. Figure 4.21

represents a practical ARS using a working ﬂuid of ammonia as the refrigerant and water as the

absorbent, with two exchangers. As can be seen from the ﬁgure, in addition to two heat exchangers,

this system employs an analyzer and a rectiﬁer. These devices are used to remove the water vapor

that may have formed in the generator, so that only ammonia vapor goes to the condenser.

The system shown in Figure 4.21 utilizes the inherent ability of water to absorb and release

ammonia as the refrigerant. The amount of ammonia vapor which can be absorbed and held in a

water solution increases with rising pressure and decreases with rising temperature. Its operation is

same as the system given in Figure 4.20, except for the analyzer, rectiﬁer, and heat exchangers. In

the absorber, the water absorbs the ammonia at the condenser temperature supplied by circulating

water or air, and hence a strong solution (about 38% ammonia concentration) occurs.

Because of physical limitations, sometimes complete equilibrium saturation may not be reached in

the absorber, and the strong solution leaving the absorber may not be as fully saturated with water as

its pressure and temperature would require. This strong solution from the absorber enters the solution

pump (the only moving part of the system), which raises its pressure and delivers the solution into

the generator through the heat exchanger. Pumped strong solution passes into generator via heat

exchanger where strong solution is preheated before being discharged into ammonia generator.

Note that the pumping energy required is only a few percent of the entire refrigeration energy

requirement. The generator, which is heated by an energy source (saturated steam or other heat

source via heating coils or tube bundles), raises the temperature of the strong solution causing the

ammonia to separate from it. The remaining weak solution (about 24% ammonia concentration)

absorbs some of the water vapor coming from the analyzer/rectiﬁer combination and ﬂows down

to the expansion valve through the heat exchanger. It is then throttled into the absorber for further

cooling as it picks up a new charge of the ammonia vapor, thus becoming a strong solution. The

hot ammonia in the vapor phase from the generator is driven out of solution and rises through

the rectiﬁer for possible separation of the remaining water vapor. Then it enters the condenser

and is released to the liquid phase. Liquid ammonia enters the second heat exchanger and loses

some heat to the cool ammonia vapor. The pressure of liquid ammonia signiﬁcantly drops in the

throttling valve before it enters the evaporator. The cycle is completed when the desired cooling

load is achieved in the evaporator. Cool ammonia vapor obtained from the evaporator passes into

186 Refrigeration Systems and Applications

Analyzer

Rectifier

Generator

First heat

exchanger

Throttling valve

Evaporator

Absorber

Condenser

Second hea

exchanger

Solution pump

Figure 4.21 A practical ammonia–water ARS.

the absorber and is absorbed there. This absorption activity lowers the pressure in the absorber and

causes 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 or air.

The heat introduced into the absorption system in the generator (from steam heat) and the evapo-

rator (from actual refrigeration operation) has to be rejected to the outside. One heat ejection occurs

in the ammonia condenser and other heat ejection occurs in the ammonia absorber. Reabsorption of

ammonia into weak solution generates heat and unfortunately this heat has to be rejected so that the

absorption process can function. Aqua ammonia consists of water and ammonia. Water can easily

absorb ammonia and stay in solution under normal temperature; hence the absorber has to be cooled

with cooling water or air. Evaporated ammonia in the generator is passed through the distilling

column where the ammonia is concentrated into nearly pure ammonia vapor before going into the

condenser. Once ammonia is turned into liquid it is let down into the evaporator, low-pressure side,

where ammonia is again turned into vapor, by evaporation, while picking up heat from the conﬁned

refrigerated space. Ammonia vapor is then absorbed in the absorber to complete the cycle.

For ammonia–water ARSs, the most suitable absorber is the ﬁlm-type absorber for the following

reasons (Keizer, 1982):

• high heat and mass transfer rates,

• good overall performance, and

• large concentration rates.

Refrigeration Cycles and Systems 187

Further information and detailed discussion of energy and mass balances and limiting conditions

in the analyzer and rectiﬁer, along with some examples, can be found in Gosney (1982).

4.7.3 Energy Analysis of an ARS

As mentioned earlier, energy analysis of an ARS refers to the ﬁrst law of thermodynamic analysis

of an open (control volume) system. Therefore, each component in the ARS is considered a steady-

state steady-ﬂow process, and we will write energy balance equations, equating that input energies

(including work) to output energies. Note that in vapor-compression refrigeration systems, the mass

ﬂow rate of the refrigerant was constant throughout the cycle. However, here in ARS we have two

ﬂuids (making a working ﬂuid) as refrigerant and absorbent and their composition at different points

is different, particularly in the absorber and generator. Therefore, we also include mass balance

equations for those two components in addition to energy balance equations. We refer to Figure 4.21

for the state points in the following equations.

• Absorber:

Energy balance: ˙m

+˙m

=˙m

(4.42)

Mass balance equation: ˙m

+˙m

=˙m

(4.43)

where

is the absorber head load in kW; X is the concentration; ˙m

=˙m

is the mass ﬂow

rate of the weak solution in kg/s; ˙m

=˙m

is the mass ﬂow rate of the strong solution in kg/s;

and ˙m

is the mass ﬂow rate of the refrigerant in kg/s. Here, state 1 is a saturated liquid at the

lowest temperature in the absorber and is determined by the temperature of the available cooling

water ﬂow or air ﬂow.

• Solution pump:

˙m

=˙m

(4.44)

The compression is almost isothermal.

• First heat exchanger:

˙m

+˙m

=˙m

+˙m

(4.45)

• Generator:

Energy balance: ˙m

gen

=˙m

+˙m

(4.46)

Mass balance: ˙m

+˙m

=˙m

(4.47)

where

is the heat input to generator in kW; ˙m

=˙m

and ˙m

=˙m

• Condenser:

˙m

=˙m

(4.48)

• Second heat exchanger:

˙m

+˙m

=˙m

+˙m

(4.49)

• Expansion (throttling) valves:

˙m

=˙m

⇒ h

= h

(4.50)

˙m

=˙m

⇒ h

= h

(4.51)

The process is isenthalpic pressure reduction.

188 Refrigeration Systems and Applications

• Evaporator:

˙m

=˙m

(4.52)

For the entire system, the overall energy balance of the complete system can be written as

follows, by considering that there is negligible heat loss to the environment:

W +

gen

(4.53)

The COP of the system then becomes

COP =

gen

(4.54)

where

is the pumping power requirement, and it is usually neglected in the COP calculation.

Example 4.6

Consider a basic ARS using ammonia–water solution as shown in Figure 4.22. Pure ammonia enters

the condenser at 2.5 MPa and 60

◦

C at a rate of 0.022 kg/s. Ammonia leaves the condenser as a

saturated liquid and is throttled to a pressure of 0.15 MPa. Ammonia leaves the evaporator as a

saturated vapor. Heat is supplied to the generator by geothermal liquid water that enters at 135

◦

at a rate of 0.35 kg/s and leaves at 120

◦

C. Determine (a) the rate of cooling provided by the system

and (b) the COP of the system. (c) Also, determine the second-law efﬁciency of the system if the

ambient temperature is 25

◦

C and the temperature of the refrigerated space is 2

◦

C. The enthalpies

of ammonia at various states of the system are given as h

= 1497.4 kJ/kg, h

= 482.5 kJ/kg,

= 1430.0 kJ/kg. Also, take the speciﬁc heat of water to be 4.2 kJ/kg·

◦

Solution

(a) The rate of cooling provided by the system is

=˙m

− h

) = (0.022 kg/s)(1430.0 − 482.5) kJ/kg = 20.9kW

Figure 4.22 The basic ARS considered in Example 4.6.

Refrigeration Cycles and Systems 189

(b) The rate of heat input to the generator is

gen

=˙m

geo

geo,in

− T

geo,out

) = (0.35 kg/s)(4.2kJ/kg·

◦

C)(135 − 120)

◦

C = 22.1 kW

Then the COP becomes

COP =

gen

20.9kW

22.1kW

= 0.946

reversible heat engine and a reversible refrigerator as shown in Figure 4.23. Heat is absorbed

from a source at T

by a reversible heat engine and the waste heat is rejected to an environment

. Work output from the heat engine is used as the work input in the reversible refrigerator,

which keeps a refrigerated space at T

while rejecting heat to the environment at T

.Usingthe

deﬁnition of COP for an ARS, thermal efﬁciency of a reversible heat engine and the COP of

a reversible refrigerator, we obtain

COP

abs,rev

gen

= η

th,rev

COP

R,rev



1 −



− T



Substituting,

COP

abs,rev



1 −



− T





1 −

(25 + 273) K

(127.5 + 273) K



(2 + 273) K

(25 − 2) K



= 3.06

Reversible

heat engine

Reversible

refrigerator

gen

Figure 4.23 The system used to develop reversible COP of an absorption-refrigeration system.

The temperature of the heat source is taken as the average temperature of geothermal water.

Then the second-law efﬁciency of this absorption system is determined to be

COP

abs,rev

0.946

3.06

= 0.309 = 30.9%

190 Refrigeration Systems and Applications

4.7.4 Three-Fluid (Gas Diffusion) ARSs

The two-ﬂuid ARS succeeded in replacing a compressor which requires a large amount of shaft

work by a liquid pump with a negligible energy requirement compared to the refrigeration effect.

By addition of a third ﬂuid, the pump is removed, completely eliminating all moving parts. This

system is also called the von Platen–Munters system after its Swedish inventors. This type of system

is shown in Figure 4.24. The most commonly used ﬂuids are ammonia (as refrigerant), water (as

absorbent), and hydrogen, a neutral gas used to support a portion of the total pressure in part of

the system. Hydrogen is called the carrier gas. The unit consists of four main parts: the boiler,

condenser, evaporator, and absorber. In gas units, heat is supplied by a burner, and when the

unit operates on electricity the heat is supplied by a heating element. The unit charge consists of a

quantity of ammonia, water, and hydrogen at a sufﬁcient pressure to condense ammonia at the room

temperature for which the unit is designed. This method of absorption refrigeration is presently

used in domestic systems where the COP is less important than quiet trouble-free operation. In

the system shown in Figure 4.24, the cold ammonia vapor with hydrogen is circulated by natural

convection through a gas–gas heat exchanger to the absorber, where the ammonia vapor comes

in contact with the weak solution from the separator. At the low temperature of the ammonia

and hydrogen, absorption of the ammonia occurs and hence hydrogen alone rises through the heat

exchanger to the evaporator, while the strong solution ﬂows down by gravity to the generator.

4.7.5 Water–Lithium Bromide (H

O–LiBr) ARSs

These ARSs utilize a combination of water (as the refrigerant) and lithium bromide (as the

absorbent), as the working ﬂuid. These systems are also called absorption chillers and have a wide

range of application in air conditioning and chilling or precooling operations and are manufactured

Figure 4.24 A three-ﬂuid ARS.