Selecting a working fluid for a Rankine-cycle engine

Applied Energy 21 (1985) 1-42

Selecting A Working Fluid for a Rankine-Cycle Engine O. Badr, S. D. Probert and P. W. O'Callaghan School of Mechanical Engineering, Cranfield Institute of Technology, Bedford MK43 0AL (Great Britain)

S UMMA R Y For a specified power output, the required size and obtained performance of a thermodynamic power system releasing mechanical work are very dependent upon the properties of the working fluid. Organic fluids possess several advantages over water as the working fluid in a Rankine-cycle engine utilising low-grade energy as the heat input, particularly for low power output applications. In this study, the required characteristics of the'idear workingfluid are discussed. These attributes cannot be satisfied simultaneously by any one fluid for all applications. With Rankine-cycle engines, operating between maximum and minimum temperature limits of 120°C and 40°C, respectively, the conclusions from investigating the suitabilities of sixtyeight potential working fluids are presented. The thermodynamic appropriateness, thermal stability, availability, cost and safety requirements were the primary factors to be satisfied in the screening process. Three superior fluorinated hydrocarbons--R-11, R-I I3 and R-114-commonly used as refrigerants, were eventually short listed. A final assessment, based on the performances of these selectedfluids, in the Rankine-cycle system of interest in this project, indicated that R-113 is the most suitable candidate for the application envisaged.

NOMENCLATURE AA, BA, C A ACOND,AEv

Constants of Antonie's equation (eqn (a. 1)). Heat transfer areas of the Rankine-cycle condenser and evaporator, respectively (m2). 1

Applied Energy 0306-2619/85/$03"30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

2

O. Badr, S. D. Probert, P. W. O'Callaghan

A R, BR. C R, DR As, Bs A~, B~, AR, Bp, CR,Dp, ER, Fp, Gp £l

C., D. DCOND,DEV

g P

Pc P~

QoNo, Ry S T

/;

L Tw

Vy, V1

Constants of the Reid et al. equation (eqn (b.7)). Constants of Schlessinger's equation (eqn (a.2)). Constants of the Reid et al. equation (eqn (b.9)). Constants of Martin's equation (eqn (b.5)). Specific heat of the saturated liquid of the working fluid (J/kg K). Molar specific heat of the saturated liquid of the working fluid (J/kmole K). The ideal-gas molar constant-pressure specific heat of the working fluid (J/kmole K). Constants of eqn (b.8). Outside diameters of the condenser and evaporator tubes, respectively (m). The gravitational acceleration (m/s2). Pressure of the working fluid (N/m2). Critical pressure of the working fluid (N/m2). Saturation pressure of the working fluid (N/m2). Heat transfer rates in the condenser and evaporator, respectively (W). Radius of gyration of the molecule of the working fluid (~ = 10- l°m). Specific entropy of the working fluid (J/kg K). Temperature of the working fluid (K). Normal boiling temperature of the working fluid (K). Critical temperature of the working fluid (K). Saturation temperatures of the condenser and evaporator, respectively (K). Non-dimensional temperature of the working fluid ( - T/Tc). Wall temperature of the heat exchanger surface on the working fluid side of the heat exchanger--see eqns (3), (4) and (5)~(K). Specific volumes of the saturated vapour and

Selecting a working fluid jor a Rankine-cycle engine

~b, ~c

I-[L]

r/cc

19

q~ 2~ /xt (.o

Subscripts ref T,T b 1,2

3

saturated liquid, respectively, of the working fluid (m3/kg). Nucleate boiling and film condensation heat transfer coefficients on the working fluid sides in the Rankine-cycle's evaporator and condenser, respectively (W/m 2 K). A function of the non-dimensional temperature in eqn (b.1) and defined by eqn (b.3). Latent heat of vaporisation of the working fluid (J/kg). Modified latent heat of vaporisation, defined by eqn (5) (J/kg). Carnot-cycle efficiency. Non-dimensional normal boiling point of the working fluid (_=TJTc). A function of the non-dimensional temperature in eqn (b.1) defined by eqn (b.2). An association factor of the working fluid, defined by eqn (b.8). Thermal conductivity of the saturated liquid phase of the working fluid (W/m K). Dynamic viscosity of the saturated liquid of the working fluid (kg/m s). Factor describing the working fluid, defined by eqn (b.4).

Refers to a reference state of the working fluid. At temperatures T or Tb, respectively. Refer to the state points 1 and 2, respectively-see Fig. 1.

GLOSSARY

Pay-back period The duration, in years, equal to the present capital cost of implementing a proposal divided by the financial savings (at present prices) so achieved per year.

4

O. Badr, S. D. Probert, P. W. O'Callaghan

RANKINE-CYCLE ENGINES Low-grade heat from waste streams of industrial processes or renewable sources (e.g. solar energy, geothermal heat or vertical temperature gradients in oceans), at temperatures below 200°C can be utilised by Rankine-cycle engines to produce useful shaft power or electricity. However, the financial cost of applying this technology rises rapidly as the quality of the heat source decreases. Economic studies 2.3 indicate that, although a waste-heat recovery Rankine-cycle system can n o w produce power at a real cost below that of a coal-fired steam unit with a short payback period (i.e. < 3 years), a solar-powered system will soon become economic due to the increasing cost (as well as the decreasing availability) of traditional high grade fossil fuels. The theoretical maximum effectiveness of transforming heat into mechanical work is the Carnot efficiency, namely: ~/cc = 1

TCOND TEv

(1)

where TEv and TCOND are the maximum and the minimum absolute temperatures of the cycle, corresponding to the saturation temperatures, respectively, of the evaporator and condenser. Equation (1) is an expression of the second law of thermodynamics and indicates that the maximum attainable efficiency is limited by the difference of temperature between the heat source and sink. A power cycle having a maximum temperature of 140 °C and a minimum temperature of 40 °C has a Carnot efficiency of 24.2 per cent. However, the actual efficiency achieved by a Rankine-cycle engine, in practice, is substantially lower than the Carnot efficiency due to (i) the working fluid's thermodynamic properties and (ii) the irreversibilities in the mechanical system, which are dictated by the choice of the fluid. 4'5 The working fluid used initially in Rankine-cycle engines was steam, and this is still chosen for power plants and other applications of higher temperature Rankine engines. Steam is generated readily or occurs naturally in some geographic regions (e.g. Italy, Iceland and New Zealand). It is inexpensive, chemically stable and can provide high cycle efficiencies economically for these high grade energy inputs and high power output applications. However, for low grade energy applications, particularly with low power output levels (i.e. < 10 kW), steam, as the working medium, is not the best choice technologically or the most

Selecting a working fluid Jor a Rankine-cycle engine

5

economic selection. 3'4"6- 24 For these applications, the Rankine engine's prime mover (i.e. the expander) can be either a turbine or a positivedisplacement machine (i.e. a reciprocating piston, multi-vane or screw expander).

Turbines For low power systems with moderate temperature differences across the expander, the corresponding enthalpy drops of the steam are relatively high due to its low molecular weight. If all the energy were to be extracted in a single-stage impulse turbine (i.e. by employing a single nozzle with a single wheel), the efflux velocity through the nozzle would be over 1000m/s, which requires the blades to travel at a velocity of about 500m/s. (For maximum efficiency the nozzle efflux velocity to blade travelling velocity ratio of an impulse turbine should be ~ 0"45.6' 17.19.20) This speed is over twice the practical limit, dictated by the allowable stresses for most common turbine-wheel materials and constructional techniques. 62° So single-stage impulse turbines must operate with lower than the optimal velocity ratio, resulting in poorer efficiencies. The use of multi-stage turbines may overcome this difficulty, but would lead to higher cost, complicated small turbines. Furthermore, the frictional losses in small-sized turbines are relatively high. When using steam as the working fluid for low grade energy applications, the necessary degree of superheat (required to avoid exhausting wet steam of low quality) may not be obtainable. So, erosion of the turbine blades is more likely to occur. Moreover, two-phase flow could be experienced in the turbine bearings, leading, possibly, to erratic operation: ~3.23 this complicates the bearing design.

Positive-displacement expanders If steam is chosen as the working fluid, a minimum temperature for the heat source is set, so that condensation losses are eliminated and a reasonable power output can be achieved in a positive-displacement prime mover. For steam, the desirable operational pressure is achievable only at high temperatures. This requires costly materials to be employed and increases the lubricating and sealing difficulties. ~° Also, this minimum temperature is relatively high when compared with the average available temperatures of most low grade energy sources.

6

O. Badr, S. D. Probert, P. W. O'Callaghan

? A @I~---- o-\#~'\

tm

1 '3~nlVB3dW31

o

>\

~._

ke\

__~_~__~_k

=I=----~

[~ A

',~, , . ~

~'~

< 1

'

3~nlV~3dW31

~._~ ,~, :~

~R\.

O" \ i J C

o v

h'< I '3U~IVU3dH31

i ,e

"~. = [--,

N~

e~

Selecting a working fluid Jor a Rankine-cyele engine

7

Using fluids with greater molecular weights than that of water can result in greater cycle efficiencies with less complex and less costly singlestage expanders. 6 - 8 , 1 0 - 1 2 , 1 6 . 1 7 . 2 2 , 2 3 I n a turbine, if a heavy organic-fluid vapour is used instead of steam, the velocity of the efflux is reduced approximately as the ratio of the inverse square roots of the molecular weights of the fluids. Thus, a vapour which is 5-6 times as dense as steam would make a single-stage turbine, with the proper velocity ratio, a feasible prospect. There is a further advantage. Because the specific enthalpy drop is less for heavy vapours than for steam (i.e. approximately in the inverse proportion of their molecular weights), more vapour must flow through the turbine for the same power output. This is a great advantage in small turbines because it allows the blades to be larger and makes satisfying the full-admission condition of the turbine possible~ even for small power outputs. Consideration of these factors leads to higher nozzle and blade efficiencies. Using a high molecular weight working fluid, with a sufficiently low saturation pressure in the condenser will also lead to less disc friction losses. With regard to the problem of excessive moisture content of the vapour after the expansion through the nozzle (as occurs with steam having an insufficient degree of superheat), it is interesting to note that many common organic fluids exhibit a vapour saturation curve on the temperature-entropy diagram with an approximately zero or positive slope ds/d T--see Fig. l(b) and (c). Steam has a negative slope--see Fig. l(a). As a consequence, isentropic expansions of saturated organic vapours result in saturated or superheated vapours, so that erosion of the blades and erratic operation of the bearings are avoided. On the other hand, the use of an organic working fluid, in a positive-displacement expander offers the following main advantages over steam for systems in the low power output range: (i) Higher overall efficiency for low maximum cycle temperature. (ii) Lower costs of the materials for production of the expander. (iii) Less internal leakage losses in the expander due to the better sealing of the clearances between its moving and stationary elements. (iv) No condensation losses. Although fluids, with zero or positive values of the slope, ds/dT, of the saturated vapour line, are more desirable than water for the operation of the expander, media with high positive slopes result in low Rankine-cycle efficiencies. A fluid of type (c) in Fig. 1 yields a less useful enthalpy drop in

8

O. Badr, S. D. Probert, P. W. O'Callaghan

the expander and results in a higher average temperature at which the heat rejection occurs than for a fluid of type (b). However, this drawback may be overcome by employing the superheated vapour at state point 2 to raise the temperature of the liquid entering the boiler. This 'regeneratioff, or 'feed heating', process requires an additional heat exchanger in parallel with the boiler and condenser. Practically, the final performance depends upon the heat transfer characteristics of the fluid--and hence upon the cost of the additional heat exchanger equipment. Consequently, fluids are sought which have nearly zero values of the slope ds/d T for the saturated vapour lines on their T-s diagrams. Using Trouton's empirical rule, Garay 17 concluded that fluids having two or more atoms in their structures and a molecular weight of more than about 160 have positive slopes of their saturated vapour lines. However, a theoretical study by Goldstein (see Tabor and Bronicki 6) showed that the slope of the saturated vapour line is, to a first approximation, a function only of the number of atoms in the molecule and not of their weight or character. Therefore, molecules with small numbers of atoms would have T-s diagrams similar in shape to that given in Fig. l(a), whereas, for molecules with large numbers of atoms, the charts would resemble that shown in Fig. l(c). The shape shown in Fig. l(b) would require the number of atoms to be between five and ten. 625

W O R K I N G FLUIDS FOR LOW GRADE ENERGY STIMULATED RANKINE-CYCLE ENGINE As suggested by many authors, 4'6'8'16. x7,19,20,23 34 a fluid for a Rankinecycle engine should satisfy the following thermodynamic and physical criteria. (1) Its critical temperature should lie well above the highest temperature of the proposed cycle. Evaporation of the working fluid-and thus the significant addition of heat---can then ensue at the maximum temperature of the cycle. This results in a relatively high cycle efficiency. (2) The saturation pressure at the maximum temperature of the cycle should not be excessive. Very high pressures lead to mechanical stress problems and, therefore, unnecessarily expensive components may be required. (3) The saturation pressure at the minimum temperature of the cycle -

Selecting a working fluid Jor a Rankine-cycle engine

9

(i.e. the condensing pressure) should not be so low as to lead to problems of sealing against infiltration of the atmospheric air into the system. (4) The triple point should lie below the expected minimum ambient temperature. This ensures that the fluid does not solidify at any point during the cycle nor whilst being handled outside the system. (5) A low value for the specific heat of the liquid or, alternatively, a low ratio of number of atoms per molecule divided by the molecular weight 16.~7 (i.e. leading to ds/d T ~ 0 for the saturated liquid line in Fig. 1) and a high ratio of the latent heat of vaporisation to the liquid's specific heat should appertain. This reduces the amount of the heat required to raise the temperature of the subcooled liquid of the working fluid to the saturation temperature corresponding to the pressure in the Rankinecycle's evaporator. So most of the heat is added at the maximum cycle temperature, and the Rankine cycle can approach more closely the Carnot cycle. (6) The working fluid should possess a low value of the liquid viscosity, a high latent heat of vaporisation, a high liquid thermal conductivity and a good wetting capability. These ensure that the working fluid pressure drops in passing through the heat exchangers and the auxiliary piping are low and that the heat transfer rates in the exchangers are high. (7) The working fluid should have low vapour and liquid specific volumes. These properties affect the rates of heat transfer in the heat exchangers. The vapour specific volume relates directly to the size and cost of the cycle components. Moreover, a high vapour specific volume leads to larger volumetric flows requiring a multiplicity of exhaust ends of the expander and resulting in significant pressure losses. The specific volume of the liquid at the condenser pressure should be as small as possible in order to minimise the required feed-pump work. (8) The slope ds/dT of the saturated vapour line in the T-s diagram--see Fig. 1 should be nearly zero. This prevents excessive moisture production or excessive superheat occurring during the expansion. It also ensures that all the heat rejection in the condenser occurs at the minimum cycle temperature. (9) Non-corrosivity and compatability with common system materials are important selection criteria. (10) The fluid should be chemically stable over the whole temperature range employed. The thermal decomposition resistance of the working fluid in the presence of lubricants and container materials is a highly important criterion. In addition to making the replacement of the

10

O. Badr, S. D. Probert, P. W. O'Callaghan

working fluid necessary, chemical decomposition of the fluid can produce non-condensable gases which lower the heat transfer rate in the condenser, as well as compounds, which have corrosive effects on the materials of the system. (11) Non-toxicity, non-flammability, non-explosiveness, non-radioactiveness and current industrial acceptability are also desirable attributes. (12) The fluid should possess good lubrication properties. (13) The substance should be of low cost and readily available in large quantities. SCREENING PROSPECTIVE F L U I D S FOR USE IN RANKINECYCLE ENGINES

No one fluid has been identified (and it is extremely unlikely that one will ever be developed or discovered) which will possess, simultaneously, all of the mentioned desirable attributes. So, for a particular application, a compromise has to be adopted. Various criteria have been proposed to facilitate selecting the best fluid for a given application. 4-1°'13'16"17'19"2°'23-25'27-4° These depend upon the particular application envisaged and are based upon the following considerations for the fluid. (i) Thermodynamic and thermophysical suitability over the evaporation and condensation temperature ranges of interest. (ii) Thermal stability and compatability with the constructional materials of the system and lubricants (if necessary) to ensure the long-life operation of the engine with minimal servicing and maintenance requirements. (iii) Accepted as 'safe', provided adherence to current industrial practices is ensured. (iv) Low cost for the amount of fluid required. Also, ideally, the choice of this fluid should result in the following properties of the Rankine-cycle engine. (i) Maximum overall efficiency. (ii) Minimum capital cost and simple design of the system hardware, including the heat exchangers, expander and feed pump. (iii) Least weight and/or volume of the system. (iv) Lowest noise level and rate of atmospheric pollution.

Selecting a working fluid Jor a Rankine-cycle engine

11

Table 1 lists sixty-eight of the working fluids which, in this survey, have been considered either as possible candidates or have actually been used in operational Rankine engines4- 20.22-49 (see also Tables 2 and 3). The Table also shows the collated relevant data for the mentioned fluids at the evaporator and condenser temperatures of 120 °C and 40 °C, respectively, of interest in this project. Specifying an upper cycle temperature of 120 °C eliminates R-12, R-22, R-152a, R-500, R-502, R-C318 and R-290 from the list of prospective working fluids because of their low critical temperatures. Moreover, if the corresponding maximum cycle pressure is limited to 25 x l0 s N/m 2 in order to minimise the weight and cost of the evaporator and the high pressure piping, then R-142b, R-717 (ammonia), R-600a, R-764 (sulphur dioxide), R-40 and R-630 also have to be omitted. On the other hand, for the specified minimum temperature of the cycle, i.e. 40°C, the corresponding lowest condenser saturation pressure should be limited to reduce the likelihood of the infiltration of atmospheric air into the condenser and the low pressure piping, and to provide a sufficient suction head to ensure the reliable and smooth operation of the feed pump without the occurrence of cavitation. By limiting the minimum condenser pressure to 5 x 104 N/m 2, fluids like CP-9, CP-17, CP-25, CP-27, CP-28, CP-32, CP-34, FC-75, Allied P-1D, iso-octane, CP-O, o-xylene, Dowtherm E, iodobenzene, butylacetate, acetonitrile, monobromobenzene, n-propanol, R-1120, trichlorobenzene, tetrachloroethylene, methanol, ethanol, ethylene dichloride, iso-propanol, PP2, PP3, PP5, PP9 and Dowtherm A, all have to be eliminated from our consideration. It is apparent that the specification of the temperature (and the corresponding pressure) limits of the cycle is a convenient starting point for the preliminary screening of the available fluids. A short list of twentyfive candidates can then be drawn up from the sixty-eight prospective candidate fluids considered. A vital factor in deciding which working fluid to adopt (for a Rankinecycle engine) is its thermal stability. The chosen fluid must be resistant to chemical decomposition under repeated evaporation-condensation cycles up to the selected maximum operating temperature and while in contact with conventional constructional materials, lubricants and probable contaminants (e.g. air and water vapour). There is, unfortunately, a dearth of published information concerning the thermal stabilities of most organic fluids at temperatures of interest in Rankinecycle applications. Moreover, the thermal stability of a chemical

O. Badr, S. D. Probert, P. W. O'Callaghan

12

TABLE 1: Flutd

Chemical Jormula

Number oJ atoms per molecule

Molecular weight (kg/kmole)

Critical temperature

Freezing point

(°C)

(°C)

Saturated-liquid properties at 40 °C (b)

Saturation pressure (a) At 40~C At 120°C (lO s N/m')

vt (10 -4 m a/kg )

cl (J/kgK)

,ut (10 -4 kg/m s )

,~j (W/mK)

R.II (Trichlorofluoromethane)

CCIaF

5

137'38

198"0

- 111"1

1"735

12"293

6"95

898"5

3"67

0'083

R-12 (Dichloro(difluoromethane)

CCI2F2

5

120.93

112'0

- 157"8

9.606

[41'239]

7.98

1014"0

1"94

0"064

R-21 (Dichlorofluoromethane)

CHCI ~F

5

102-92

178"5

- 135-0

2"962

19-640

7-52

615'5

2"66

0"097

R-22 (Chlorodifluoromethane)

CHCIF 2

5

86"48

96-0

- 160"0

15"335

[49"751]

8.84

I 326'4

1"84

0"080

R-30

CH2CI 2

5

84'93

237'0

-96.7

1.058's~

9.446 oo

7.97tGI

I 210.0 ~ (20)

3.56 ~R)

0"148e (20)

R-II3 ITrichlorotrifluoroethane)

CCI2FCCIF2

8

187"39

214"1

- 35.0

0.783

6.892

6.55

973 8

5.63

0.072

R-II4 (Dichloro tetrafluoroethane)

CCIF2CCIF 2

8

170.94

145.7

-93-9

3'372

20'603

7.10

1047.5

2.91

0'061

R-133a (Chlorotrifluoroethane)

CH2CICF3

8

118.0

152.0

- 106.0

3.100

22'761 •

R-142b (Chlorodifluoroethane)

CH3CCIF2

8

100.5

137.1

131.1

5"251

30-902

R-152a (Difluoroethane) R-500

CH3CHF,~

8

9' 126

CCI2Fz/CH3CHF2

-

66.05

113.5

- 117"0

99-3I

105.5

158.9

111.63

82.2

.

.

.

.

9.35

I 328.1

--

0-078

{44'9881

11.64

1843.0

2.0Y R'

0.097

11-362

[44-25]

9.08

1299"6

1.73

0"069

16.769

[40.71

8-77

938.7

1-53

0"058

[28.766]

6.96 IM~

I 163.9"*

3.18"*

0.060**

~ 0.027*

10'21 z ~ (25)

1970"0z~' (100)

45"00Az' (38)

0'121 ~ ' (94)

0"247°

2'990°

12"43tcj~

1 695"1(L)

4"92e

0' 141oo

0"070

1307 `>

12"23~

1 738'9lu

4"71 t

0 "133~

0.033 Isl

0.733 o

6.39 e

0"128

173"8/26"2per cent by weight) R-502

CHCIF jCCIF2CF 3 (48"8/51"2 per cent by weight) C4Fa

12

200.04

115.3

-41.4

(C6H~XCbH,)-

31

196.3

522.0

-55-0

C6H6

12

78.11"

289"0"

5"0

CP-25 (Toluene)

C~Hs

15

92"13"

320-6"

-95"0

CP-27 (Monochlorobenzene)

C,H~CI

12

R-C318 (Octafluorocyclobutane) CP-9 IMonoisopropylbiphenyl) CP-17 (Benzene)

112.56

359"2

45"2

4.800 °

9-60~1

997.P L~

Selecting a working fluidfor a Rankine-cycle engine

13

Possible Working Fluids for a Rankine-cycle Engine Latent heat o] l:aporisation at 120°C (c) (kJ/kg)

135"748

Slope oJ the .saturated t~pour line ds/d r

~0

<0

157'791

Thermal stability Approximate maximum operating temperature (d) I°C)

Survival time at 382°C (hours)

120" (110 v)

SaJety considerations

Cost (h) (£:kg)

Flammability

Toxwity (e) Flash point (°C)

lgnttion temp (°C)

Explosire lim~t.~ in atmospheric air (I3 Loner I~pper Per cent b) colume

Suggested hazards (g)

Non-flammable'

0.43~a (BOC) -0.84**

6 (i ooo')

Non-flammable'

0.90a~ (BOC/

4-5

Practically non-flammable'

5a

(] 000 ~)

(120 v) 20411 3008

<0

8.40a~ (BOC)

(lOOO')

<0

5a

200* (150 v)

Source' o: mJormanon (re/ereme nos m square bratket~)

Practically non-flammable'

1.15aa (8OC)

[501 A A [ t 6 I 4t-[241 • • [251

v[511 I

, [521 [501

t [521

~71511 A A [ 1 6 ] 11137] ~[ 3 6 ]

15fl] * [521 AAIt61 [501 AA[16I "k[241

v[st] t[521 270.246 ~w~

<01,

110.178

>0

175" d i 0 v)

>0

Non-flammable

0.67,~ (BDH) 0.438 ##

4-5

Practically non-flammable*

0.7a.~ (BOC)

(I 0003

204II 71.989

4a (250) 1500")

175" (120 v)

6 (1 000')

Practically non-flammable* (BOC)

1501 0 0 [531 AA[16[ • 1541 [5Ol ~[241 V[5q 111371 [50]

t * [4] • • [251

t 1521 z~& U61

A ~ [161

[241 v lSq + 1521

~0

200

[241 A[3t]

92.970

<0

[5o[

<0

[5Ol

<0

<0

5a

(150 v)

Non-flammable

5a

1-04 ~A (BOC)

Non-flammable

1.66z~ (BOC)

>0 A

257.592 tw~

>0

363.345 ~w)

>0

19,00 ~c~ (BOC)

370 28%711

> 336 vv

353.849 ~

>0

480 482.2 s

> 336 vv

330.967 ~w,

>0

320 426.711

48 vv

(25 aa)

(10'0'~a)

(75*)

- 11

560 z~a

4 5 a'~

480 ~'a

28.9 *zx

640 z~a

1.40'

1.27'

7"10'

6.75 t

3 ~a

3~.

[501

~.~ [161

15o1 V1511 AAIt61

[501 O [55l ~r~r [56J A[311 [24] • [291 AA[16] • [37] 0,55c, z~ [311 (BDH) ~ t [4] 0 1551 • [54] 0 . 5 ~. [241 (BDH) t t [41 O [551 t [521 0.95~.~ [571 (BDH) • [54] • [371

A&[16)

O 0 [571 V'V[381 AA[16I t [521 • [371 v v 1381

AAil6] V V [381 & & [161

O. Badr, S. D. Probert, P. W. O'Callaghan

14

TABLE 1--Contd. Flu ~d

Chemk~l Jormu~

Number Molecular of atoms weight per (kg/kmole) molecule

12

CP-28 (Perfluorobenzene)

C6F6

CP-32 (Pyridine)

CrH~N

I1

CP-34 (Thiophene)

C.,H~S

FC-75

186.0

Critical temperature (°C)

Freezing point (°C)

Saturation pressure (a) At 40°C At 120°C (10 ~ N/m z)

243.3

-55'0

0'227 °

3"066 °

Saturated-liquid properties at 40 ° C (b) rl

ct

~0

),l

(J/kg K)

(10 -~ kg/ms)

(W/m K)

6.18 ~'~ (20)

I 190.8 tu

9.00 ~c~ (25)

0.098 '~a

~(10 -~ m~/kg)

7.14 ~'

(20)

79'1

347"0

-41.7 °

0-060 ~A'

1.173 ¢

10.83 IGI

I 769.8 ° 0

0,169 ° °

9

84.14

307"0

-40.0

0.207 °

2.666 °

10-29 ~6~

1430-0 z ~ (38)

5.1Y ~

0.137 ~ a (93)

CsF~oO

25

416.08

227.0

-62'0

0"091 v v

1"7737v

5"81 v ~

1 062'7 v v

11'02 ~7v

0'063 v v

FC-88 (Perfluoropentene)

C~F~

17

288'0

150"0

- 115'0

Allied P - I D

C~0F~O ~

34

570.0

243.0

-85'0

(Perfluoro2-butyl-tetrahydrofuran) I '466 °

0.02

11'866 ~

--

~0.638 't

(perfluoroether) R-717 (Ammonia)

NH3

4

17-03

133.0

77-7

15.541

R-290

C~Hs

I1

44.10

96-8

- 187.7

13"671

58.13

135.0

-75.5

5"266

91.700 ~

17.25

4872.6

1.12

0.446

[42.455]

21.28

2855.0

0.89

0.090

28"424

18'81

2645.2

1.42

0-101

4.00 °°

0"093 O°

(Propane) R-600a (Iso-Butane)

(CH~)~CH

Iso-Octane

Cs H ~

26

114-22

271.1

- L07"4 ° °

O' 124 ° e

1'724 ° °

14.81oo

2 129"7 ° e

.-Pentane

CH~(CH~),CH3

17

7215

196"6

- 129"75 °

1"158I^/

8"897 ~sl

17"20 IG~

2 382'8 ~

1'92 I~

0"105 e °

CPO

(C6H~) 2

22

154.2

498'0

69.0

0.015 ~'

7,56 'cJ

1290.0 ' ~ (30)

22.79 ~R'

0.137 z~a (80)

O-Xylene

(CH3)~C~H,~

18

106,17

357.05 e

-27

0.021 ~

0.0500 °

11.87 ~GI

I 848.9 IL'

6.25 °

0.133 ° o

Dowtherm E (O-Dichlorobenzene)

C6H~C12

12

147"0

424'15 °

- 17'0

0-003 °

0"173 <>

8"09 I~1

I 150"0~

10"78~p

0121 z ~

Iodobenzene

C6H51

12

204'01

448"0

Butylacetate

C6H~O 2

20

116"16

305'9

Acetone

(CH,)zCO

10

58"05

Acetonit rile

CH3CN

6

Acetaldehyde

CHjCHO

7

(Biphenyl)

(20)

31"4

(20)

0'003 '^1

0'141 '^~

6"38 I~'

792'0 'u

12'55 ''1

-73'5

0"035 °

0853 °

12"89~°~

1956"0 ° o

5"63*

0'131 ° 0

235"0

-95'0*

0"543 °

6"000 °

13"34~cl

2211'8 cu

2"70 ° 0

0"151e

41.05

274.7

-43.9 °

0"227 °

3"019 ~A~

13'34 IG)

2 489'9 eL'

2'99 ~R~

0"169 ° °

44.05

187.8

13.71 t6~

1401.5 ° o

1.84 tR~

0139 ° o

-123.0

2.068 ° o

14.893 ° o

0120'

Selecting a working fluidfor a Rankine-cycle engine

Latent heat oJ vaporisaoon at 120~C (c) (kJ/kg)

Slope oj Thermal stability the ~aturated Approximate Surrit,al tz~pour maximum time at line operating 382°C ds/dT temperature (hours) (d)

Safety eonsideratmns . . Toxlclt.) ~e(

.

Ignition temp. ( C)

(~(7) 153"364 tw~

~0

454.4 II

Cost (h) I£ kg)

. Flammabiht)

Flash point ("C)

E~plosae limtt~ m atmospheric air (j) l.o.er 8pper Per cent bt tolume

Suggested ha:ard~ (g)

240 ~

> 336 v • 450 ~

440.895 ~w'

>0

370

336 v v

349765 ~w'

< 0 for T < 66 °C, > 0 for T > 66°C

290

72 ~'*

15

(Emannuel)

(5+)

20 a"':-

482 :~±

l 81'

[2"4'

3 ''~'-

2 3 =z~ (BDH)

Sourte ol inJormaoon (rejeremv nos In vquare hratketsl

[31] V V [381 <> [55] [] [36] ~ . A B6] • 1371 [241 V V [38] • [54] t [521 O[55] A & B 6 1

© 0 [57]

77'389 v v

> 0 '~

320 371A

>0

200

> 144 v v

I I m'~

3'~

(3M) (~4000 v )

20 s ~ (BDH)

9,75 ' ~ (3M)

[24] ~ [55] &~.[(6] WV[381 [24] . ~ A {16] ~ , , ~ [58] &Dtl '~'V [381 [241

¢, [55) >0 A

~ 444.266 ~

~75

<0

[24] A[3t) V W [381

>336 "v

2 (50+) 5b

<0

155"

270 )

-

5b

~0

<

66*

4661'

.1. +

935 ~

4 ~-~

<

6.6 +

5433 +

18 +

844'

4 z,~:,

0"15 'z'~ (BOC)

4"0~':~ (BOC)

257.316 o ~

f501

[t2] *1152]

( I 000 z z ) 142.026

-

>0~

[50] & A [161 -t 1521

[50] t [52] & A [161 141

0 0 [531 &.A [61 (500 ~ )

267.776~*

< --40 ~aa

260 ±~

14 +

7.8"

4 ~,~

3'33 ~ (BDH)

>0

36(,44T ~'

370 426.7 s

(0"2 +)

112'8 n ~

540 *¢~

315.51

(100~

32 2 ~

465 a a

1+

6+

I '~z'

2.54 ~ (BDH)

3"':'

0.53 z ~ iBDH)

[41 • [54] OO[53] ~ . A [16]

[24] ¢ [52] ~ [55] &,~[16] •1371 [61 • [37] •[54] ~ [16]

o [551 O 0 1571 277"0 L~ (180.2)

>0

220

(50')

661 : ~

648 z~:~

2 '~'~

115 L ; ' (BDH)

t [52]

t [52]

[6] t [52 I 0[541 ,
~2,[161 193'6 (188.4)

12"0 ± (Emannuel)

313.684 'w'

( 150 *)

I I+

36 AA

{54] t [52] [54] & & [ 6 ]

o [551 0 0 [57] + [52] 423"995 ~w'

-0

(1000~

2.55 +

128'

[4]

t [521

• [54] 0 [551 00[57] 702'736 (~'

<0

(40 +)

[41

~ [521

@[541

334720 o'

(10 ~)

38 ~zx

175 ~ ;

397 +

570'

4'~

1'54 s s (BDH) -0"75"*

¢~ ]551 00[571 [54} A A [16] 0 0 [57] • • [25 I * [521

O. Badr, S. D. Probert, P. W. O'Callaghan

16

TABLE 1--Contd. Chemical [ormula

FluM

Number o] atom~ per molecule

Molecular weight (kgikmole)

Critical temperature (°C)

Monobromobenzene

C~HsBr

12

15701

396"9

Ethylene oxide

CzH~O

7

44 05

1950

n- Propanol

CH 3CH ~CH ~O H

12

60.10

2636

R-610 1Ethyl ether)

CzHsOCzH s

15

74.12

Methyl mercaptan

CH3SH

6

R-61 I (Methylformate)

HCOOCH 3

3-MethylI-butene

(CHj)zCHCH:CH ~

Methylbromide

Freezing point (°C)

-30'9

Saturation pressure (a) ............. At 40°C At 120°C (10 ~ N/m ~)

Saturated-liquid properties at 40°C (b) t~ (10 -a ma/kg)

c~ (J/kgK)

#t (10 -a kg/m s)

2t (W/mK)

0'013 ~

0"360 °

7'11 ~

994"8 ~Lp

8"76 tar

0'111 °©

2'827 ~ °

21347 ° #

13"82~

2117"7 Ct~

2"15 tal

0"142 ° 0

- 1263 °

0'068 (>

2.266 °

12-OY~'

2559'2 't~

194"0

-116'3

1'12 ~

9466 <,

15"0P ~'

2246'5 ~L~

2"02 O

48"0

182.8

- 123.15

3'133 °

21332 e>

12.70 ~G~

I 851-T t~

293 L~ (20)

8

60"05

214.0

-98-9

1'369 ts~

11.782 ~s~

I l O 0 ''~p

I 9 3 2 T L~

2'8Y a~

15

70.14

176'9

168.5

17.44 ~

2 200.0~a (16)

2.62~ (0)

CH~Br

5

94.94

190.9

830'0 a a

347 IR~

0"140 a ~ (20)

Dibromodifluoromethane

CBHF:

5

210.0

197'8

6280.0 (24)

4.70 (25)

0.114 (21)

Boron trichloride

BCIz

4

117-17

178.9

R-764 ISulfur dioxide)

SO,

3

64.07

157.5

1448.5 ~t~

230 'R~

R-40 (Methyl chloride)

CH3CI

5

50'49

R-630 (Methyl amine)

CH3NH z

7

R-600 (n-Butane)

CH~CH2CH2CH 3

R-160 IEthyl chloride)

R-63 I

112"15

-118.55

1933 °

3465 t'~

20"599 ~sp

72916r

14-00 °

(2.4)

141.6

-107.25

1933 °

4.39 (21)

-

0"137' (30)

0190 '~A (30)

2324 ~sr

1744P sr

8"70 tG~

75.5

6.266 ~>

41.330 °

~752'

143-1

978

8.563

45.330 °

1135

1653'8

239

0.145

31.06

156'9

92.5

5.280 ~s~

43.77P s~

195Y ~

37070 °0

1'69 ~R~

0'182 ° °

14

58.13

152'0

138-5

3.782

22.200

2528.8

1.43

0. II1

CH3CH2CI

8

64.52

187.2

-138.3

2.46T ~''

16.134 g o

1221 ~G~

16495 "~

2"20 m~

0'111 o °

C2HsNH2

10

45"08

183"3

80"6

2"266 °

17"999°

15"71 ~

2850"8 ~LI

2"08 ~a~

0 "147°c

C sCIzFo

11

220'93

180"0

-125"4

0"808 A

2855-0 "~'*

0"89**

0"090 't~v*

9.13

(Ethyl amine) R-216 (Dichlorohexafluoropropane)

9-423 A

Selecting a working,fluid for a Rankine-cycle engine

Latent heat oj raporisation at 120¢C (c) (kJ/kg)

Slope oJ the saturated rapour line ds/dT

Thermal stability Approximate maximum operating temperature (d) (¢C)

Sum'ival time at 382°C (hours)

Toxicit) (e)

251.04--')

-

413781 ~

(50')

.

. Flash point t °(')

Salet ~ tonsideration~ --Flammabilitl . . Ignition Evptosire limit~ in temp atmo~pheri( air (C) ~) Louer Upper Per cent b ) totume

--

<

18 ~

429 ~x-~

657"051 'w~

260"454 ° o

(400 t)

45 ~

1604-~'-

361-294 (w~

(05*)

18 ~

-

365'176 cw~

3 I100 ~)

19 ~

465 ~:~

233"750 'v'a

Not t o x i c * *

174.072 ,w,

4t (Skin 20'1

<-7

~

3*

80"

2"15'

135'

185'

365 ~

4.5 ~

20.0'

365 =:~

537 ' ~

(o~t (h) I£ k g )

145 t

Soum e ol a{Iormatum { rt!/erem e no~ m wuare hra~ketv~

Suggested hazard.~ (g)

2:'"

5.25 ,sz~ (BDH)

[541 ~ Q [55] OO[571

{161

4 ~4'

13:. tgoc) -547**

[25]

4 ~z-

1"66 =~" (BDHI -149"*

4 ``,<

13.0 's~ (BOC)

4 ~'~

3 5 :~:~ (BDH) ~1 2 1 " *

[541 00[53] ¢ {521 AA[161 [4] 0[54] o [55} * [52] [50] ©[55] @[541 + [521 [54] © [551 AA[16] [50] A A [16] f [52] • • [251 [54} O [551 ~A[161 • * [25i [541 &&[16] * [52]

4 ~:"

13'5'

17

0 ~'s

5 5 0 ~= (BOC) -84.4** I 1 L= (BOC) ~ 8 0 2 ~*

••

O C ' [57] ~.z3, [16] ••[251 * [521

• • [25] 4 (100*%

76 (Fluorochem)

[161 O [55] [251 [54] • , [251 • •

- 1053"* ~ 211.666 ,s

<0 A

Non-flammable'

I

(5")

2 0 0 6 T w'

<0

475250 ~

4 ( 100 *)

3333*

(10")

6322*

107 +

I 1.4'

-

495'

2075 t

405 U '

186'

8.41'

4 ~;~

128 :~':~ (BOC)

[50] 0 I551 A[121 &[311 [50] <3 [551 + [521 AA[16] [50] 00157] +

214597

>0

5

266.043 ~w~

4-5 (I 000')

429,58T w~

(10 t)

>0 A

-

-

- 50'0 ~

- 18 c'~

-

# 1521

4 ~"

29.5 ~L (BOC)

519 ~

40 ~

14'8'

4 ±z:

~0.27"*

403 ~'±

355'

13-95 ~

4 "~"

6"3 ~ (BOC) -6-91"*

152l

[50} [161 + [52] [50] 0 0 [25] @ @ [53l t [52} AA[161 [50] & g k [16] 0 [55] @ , • [25] 00[57[ * [~2] A~

{5ol &[311 - k - k [561

O. Badr, S. D. Probert, P. W. O'Callaghan

18

TABLE 1--Contd. Fluid

Chemical ]ormula

Number Molecular of atoms weight per (kg/kmole) molecule

Critical temperature (°C)

Freezing point (°C)

Saturation pressure (a) At 40°C

At 120°C

[10 ~ N/m 2)

Saturated-liquid properties at 40 °C (b) t't (10 -4 m3/kg)

ct (J,'kgK)

Itt (10 ~ kg/m s)

/.z (W/mK)

R-I 130 (Dichloroethylene)

CHCI==CHCI

6

9695

2433

-50'0

0758 ee

7515 ° °

821 e °

9205 e°

3"60 ° °

0"097 ° °

R-1120 ITrichloroethylene)

CHCI==CCI 2

6

13139

2711

-72-8

0.183 °

2482 ee

6"99 e °

983"2 ° °

5.3@ R~

0"110 O°

Trichlorobenzene

C6H3CL3

12

181 45

0.001

0053

Tetrachloroethylene

C2CI,t

6

165"83

3469

- 122

0051 o

0.947 o

6 59,{~

0360 °

6"399 ~

1251 '(~

3088"P c)

456*

7.2Y~ 0"205 e e

0.173 °

4-000 °

1229 ~

2 7 9 4 7 (u

8.26

0.152

Methanol

CHsOH

6

32'04

240'0

-97.5

Ethanol

C2HsOH

9

64.07

243.1

114-1

Ethylene dichloride

CICHzCH2CI

8

98 96

287"9

35.6

0-193 ~s~

2598 ~s~

8'64 (~t

I 306Y u

Iso-propanol

CH3CH(OH)CHs

12

60"10

235'2

-88"5

0.125 ~st

3.374 ~s~

12.26 c~j

2 708"9 (u

PPI (Perfluoron-hexane)

C~Ft4

20

338

177.9

-90

0"547

5785

6'11

I 140

5,32

0"064

PP2 (Perfluoromethyl cyclobexane)

C,F~4

21

350

212.8

30

0.273

3.366

5,72

1 010

I 1.50

0.059

PP3 [Perfluoro( 1,3-dimethyl cyclobexane)]

CsF14

22

400

2415

- 70

0"101

1.706

558

I 010

14.30

0"060

PP5 I Perfluorodecalin)

CioFts

28

462

292"0

0'021

0"538

5'31

I 085

33"0

0"056

PP9 ( Perfluoro1-methyldecalin)

CttF2o

31

512

313"4

- 70

0"008

0"281

5"15

1 125

44'0

0.057

PP50 (Perfluoron-pentane)

CsFtz

17

228

148"7

- 120

1'477

11690

6.44

1095

Dowtberm A

BiphenylBiphenyl oxide eutectic (26.5/73.5 per cent by weight}

165

497

~0.022 g

9"43 (25)

1600"0 (20)

6 4 Y R~

0.126'

(20)

11"2~18'0

-

12

~ 14.37

0-141'

3"82

0.063

35.00 (25)

0140 (25)

Selecting a working fluidfor a Rank ine-cycle engine

Latent heat oJ taporlsation at 120°C (c) (kJ kg)

Slope oJ the Saturated vapour fine ds/dT

Thermal stability . Approximate maximum operating temperature (d)

Survival time at 382°C (hours)

. . Toxwit) (e)

.

.

Flash point (°C)

Sajety considerations . . . . . Flammabtlio Ignition temp. (:C)

(°C) 246.856 e l

4 {200')

Cost (h) {£/kg)

.

Explosive limits m atmospheric air (/) Lo~er Upper Per cent by t'olume 5.6'

233.975 ~ "

.

19

Source inJormation (reJerenee no~ in square bracket~)

Suggested hazards (g)

1501 eO153] * 1521 IS01 o1551

l I 4*

Non-flammable at normal temperature'

DO[53J t 1521

155] 154]

20981# ~

o1551 952049 ~'

-

(200~

672*

365'

1541

o155] t [521 OO[53] 54495

(1000')

300.050 ~ '

(200*)

595098 ~

14.00~

3.28'

18.95'

6.2 t

159'

2.02'

11.8'

[541 O1551

t [521 [541 t

--

[521 [541

t [521 63.56

%0

440

Non-flammable

500 a ~ (Emannuel)

.~ 73.60

~<0

450

~ ._=

Non-flammable

7792

<~0

450

~ ~

Non-flammable

-

82.98

<0

400

~ '~ ~

Non-flammable

320 ~z~ (Emannuel)

82.90

~<0

400

_.~ ta

5320

<0

440

> 0A

426"7 m

[591 A A [161 [59]

[59]

159] A A [16]

=

298"0 (257'7)

124

617

Non-flammable

[59]

Non-flammable

[59]

-

[16] A[31] 111371

e~

~o I

Vl v Vl v

r~

+

r

I

Vl

+

~

~"

o

~6

'T ~

÷~-~

i

+

o _o

u

I

r~"

I

c~ II o

~

,t

II

?

;:

~

0

~ m

e~

~

~

+

~

~~ .m

.g

.~

0

~.~

g 0

=

8

0

0 0

e-

0 o

c~

8. S

oo

03

0

0

0

~r

e~ ," .1=

~

'z

"a

¢.

>,

0

0

~d

~o

oo

I ®

~b

a.~

7 + --I@

I --

--

+ ~

I o

I

0

.,9 r-I

I

II

II

~.-~

'~,

"-

o=

~=~¢'~

N~,

~.~

,-,

~l . -

.~,

0

A

g

-

~,

0 ..=

~.co

~,

0

~

II

,,~

0

o ~ .0." ~

~

"~

e~o..) w

°

°

0

,7

"o

~

oo

~:~

E~ ~

o

...o

o

~

u':,

~

"~

.=

._=

~:~

~=

. . . - ~,

~.~

~ ~

~=-~ ~)

o

>'~

£

~ . ~

_~ ~=

~.--,.-~,.,-,

,,

~

..

TABLE 2 Underwriters' Laboratories Group's Classification of Comparative Life Hazards of Gases and Vapours s2 (This classification is used in Table 1 of the prospective working fluids)

Group

Definition

1

Exposures to 0.5 to 1 per cent concentration for about 5 minutes' duration are lethal or produce serious injury

3

Exposures to 0.5 to 1 per cent concentration for about 0.5 hours' duration are lethal or produce serious injury

3

Exposures to 2 to 2.5 per cent concentration for about 1 hour's duration are lethal or produce serious injury

4

Exposures to 2 to 2.5 per cent concentration for about 2 hours' duration are lethal or produce serious injury

Between 4 and 5 5a

Less toxic than group 4, but more toxic than group 5 Much less toxic than group 4 but more toxic than group 6

5b

Ambiguous available data, some of which satisfy group 5a whereas other data for nominally similar situations satisfy group 6

6

Exposures to concentrations up to at least 20 per cent by volume for 2 hours' duration do not produce injury

TABLE 3 National Fire Protection Association's Classification of Comparative Hazards of Materials and Recommendations for Action in the Case of an Emergency 16 (This classification is used in Table l of the prospective working fluids)

Group

Definition Highly flammable gases or very volatile liquids. Shut off flow and keep cooling-water streams on exposed tanks or containers Such materials can ignite under almost all normal ambient environmental temperature conditions. Water may be ineffective to deal with a hazard involving this material because of the material's low flash point These materials must be heated moderately before ignition will occur. A water spray may be used to extinguish any resulting fire,because:the material, by this means, can be cooled below its flash point This group embraces materials that must be preheated before ignition will occur. The application of water in the event of such a fire may cause frothing if it gets below the surface of the liquid and there turns to steam. However, a water mist gently applied to the surface will cause frothing, which will extinguish the fire Such materials will not burn

Selecting a working fluid Jor a Rankine-cycle engine

23

compound cannot be defined precisely by a single temperature, which would be relevant for all possible uses. Unfortunately, the reported results for thermal stability tests of a given fluid show considerable variations, depending upon the test equipment used, the test procedure, as well as the duration and the nature of the material-fluid contact. The approximate thermal stability limit values given in Table 1 are representative of temperatures at which a tolerable rate of chemical degradation (but not mentioned specifically in any of the literature surveyed) would occur. Considerable scatter in the thermal stability limit data are noticeable for R-12, R-113, CP-9 and CP-27. Angelino and Moroni x4 reported, after comparing the thermal stabilities stated in different publications, that a discrepancy of several hundred °C in the decomposition temperature and a factor of 102 to 104 for the decomposition rate were often encountered. However, the thermal stability is closely related to the molecular structure of the fluid. 17'38'61'62 Wigmore et al. 63 identified three classes of chemicals: this ensued from an examination of the atom-to-atom bond strengths of molecular structures with good thermal stabilities--namely, the fluorinated hydrocarbons, the fluorinated silicons and the aromatic hydrocarbons. These considerations enable us to restrict the search for suitable fluids, particularly because of the absence of fluorinated silicons from the short list and with our previous decision to eliminate the aromatic hydrocarbons because of their inconvenient very low saturation pressures at the lowest cycle temperature. Only the remaining fluorinated compounds in the short list will be considered further: these are R-I 1, R-21, R-II3, R-114, R-133a, FC-88, dibromodifluoromethane, R-216, PPI and PP50. Another important criterion when selecting working fluids is their availability and cost. The 3M's fluorinated fluid FC-88 was withdrawn from the market because of its high cost and was replaced by relatively cheaper products. 48 It did not even appear in the 1979 product manual from 3M. s8 Similarly, the du Pont products R-133a and R-216 were not included in the company's product information bulletin of 1982, 51 and they are of doubtful availability in the market at reasonable prices. A comparison between the costs of the other seven fluids, based on O w e n ' s 16 data, is given in Table 4. The figures shown there serve to eliminate fluids PP1 and PP50 from any further consideration in this investigation. Besides fluids R-21 and dibromodifluoromethane being expensive, the former has a negative slope ds/d T for the saturated vapour line, whereas

O. Badr, S. D. Probert, P. W. O'Callaghan

24

TABLE 4 Relative Prices of the Candidate Working Fluids for Rankine-Cycle Engines a

Fluid

R-I1

R-21

R-113

R-114

Dibromodifluoromethane

PPI

PP50

Relative prices

1.00

19.53

1.63

2.56

176.74

1 162.79

1 162.79b

a The prices (all based on 1975 data) were evaluated relative to the cheapest price ofR-I 1 (0-43 £/kg). b Estimated assuming the same prices apply for I.S.C. Chemicals Limited's flutec products, PP1 and PP50.

the latter is relatively toxic due to its bromine content. Accordingly, these fluids are also eliminated from the short list of candidate fluids. The screening criteria imposed in this section have yielded three remaining fluids--R-11, R-113 and R-114~-out of a starting list of sixtyeight, as potentially the most suitable prospective candidates for the envisaged Rankine-cycle application. This selection agrees with the operating experience data of Curran. 24 According to his collated data on about 2150 operational Rankine-cycle engines, which used sixteen different organic working fluids, these three selected candidates represent the most usable fluids for low-grade energy applications as intended in the present investigation.

SELECTION OF THE MOST SUITABLE W O R K I N G F L U I D The performances of the working fluids in a Rankine-cycle engine depend not only on their thermodynamic and thermophysical properties, but also, to a significant extent, on the design configuration of the engine. The stated required characteristics of the ideal working fluid can serve as guidelines, but the final selection of the most appropriate working fluid should be based on detailed analyses of the main processes occurring, as well as on the overall behaviour of the particular assembly in which the working fluid has to perform. In this section, a comparison between (i) the heat transfer characteristics in both the Rankine-cycle evaporator and condenser, (ii) feedpump power requirements and (iii) the overall system efficiencies, for the final three selected working fluids, is carried out. In particular, the

Selecting a workingfluid Jor a Rankine-cycle engine

25

configuration of the organic Rankine-cycle test rig at Cranfield Institute of Technology (CIT) is considered. A more detailed survey of the available thermal-stability data of the fluids is also included. Heat transfer characteristics

In the considered Rankine-cycle engine both the working fluid evaporator and condenser are shell-and-tube heat exchangers with the working fluid on the shell side. The mechanism of heat transfer on the shell side of the evaporator is pool boiling on a bank of horizontal plain tubes, through which condensing steam--simulating the low grade energy source--is flowing. In the condenser, the mechanism of heat transfer on the shell side is via film condensation on a plain horizontal tube bundle, through which cooling water is pumped. The effectivenesses of the heat transfer processes in the evaporator and condenser depend essentially upon (i) the mean temperatures at which the heat is added to, or rejected from, the working fluid and (ii) the heat transfer coefficients on the working fluid sides of the heat exchangers. Perjormances o] the working fluids in the evaporator The heat exchange between the condensing steam and the working fluid in the Rankine-cycle evaporator can be considered to occur in two stages-see Fig. 2. (i) Raising the temperature of the sub-cooled liquid entering the evaporator, at approximately the saturation temperature of the condenser, to the saturation temperature, TEv, corresponding to the pressure of the evaporator. (ii) Supplying the latent heat of vaporisation to transform the saturated liquid into the delivered saturated vapour: this process occurs at the maximum cycle temperature (i.e. the saturation temperature of the evaporating liquid). The higher the mean temperature at which heat is added (or, alternatively, the larger the percentage of the heat transferred to the working fluid at the maximum cycle temperature), the higher the efficiency of the Rankine-cycle engine will be. Figure 3(a) presents the calculated percentages of the heat added at the evaporator's saturation temperature, compared with the total amount of heat exchanged in the evaporator, as functions of T for the fluids under consideration. The

26

O. Badr, S. D. Probert, P. W. O'Callaghan

i

!

EONDENSING STEAM

I

Safurated

TEV

_

.

I

[.!quid. _.'~.,

/

==

I

WORKING FLUID

Saturated

Jl~ Vapour

I

!

f HEAT EXCHANGED

Fig. 2.

( tinear scaLe )

Temperature variations of the working fluid and the heating steam in the evaporator.

results indicate that the performance of R- 11 is superior to that of R- 113 which, in turn, is better than the behaviour of R-114. This is a consequence of the steeper saturated-liquid line of the R-11 in the temperature-entropy diagram (i.e. a lower liquid specific heat), its higher latent heat of vaporisation and its higher ratio of the latent heat of vaporisation to the liquid phase's specific heat--see Figs 3(b) and (c). The principal heat transfer process between the submerged steamheated surfaces of the tube bundle (with 10°C superheat) and the large volume of the approximately stagnant liquid of the working fluid surrounding it, is nucleate pool-boiling. 63-67 Such a phenomenon is complicated because of its dependence, not only upon the fluid properties, but also on the characteristics of the heating surface and the fluid-surface interactions. Many authors, 68- 73 starting from simplified models of the boiling process, proposed different correlations for nucleate boiling heat transfer expressed in terms of a number of dimensionless groups. Others v4.75developed simpler dimensional correlations based on the law of corresponding states. More recently, Chongrungreong and Sauer 76 suggested a correlation for predicting the nucleate boiling performance of refrigerants. The proposed equation agrees well with the experimental data for R-11, R-113 and R-114, and has errors always

Selecting a working fluid jor a Rankine-cycle engine

27

~I00ud c~ ~-BO uJ ac u.J ..j oc

~u Z~-

0

I

I

I

80

60

I

q

100

I

I

120

I

I

14.0

I

160

I

m

180

EVAPORATOR'S SATURATION TEMPERATURE, TEV , ['E (a)

~a 200

160 z 180

160

R~113

oc 120

.14.0 ~i

o~ 160

1.2

'100 o BO

0.~

.60

BO"

",,

\

60" "z 200 4.00 600 800 SATURATED LIQUID SPECIFIC ENTROPY,[ J l ~ K ]

Fig. 3.

'120

~=1.o~.~,~-.-.'~ '-.--~~.-JS.-~.--.-.~"~ 1.0

BO

(b)

1.~.

60

"

u~

4.0 o

~0 I

100 %0 180 EVAPORATOR'S SATURATION TEMPER~URE,TEv, ['C ] {c)

Behaviours of the short-listed working fluids in the Rankine-cycle evaporator: condenser saturation temperature = 40°C.

28

O. Badr, S. D. Probert, P. W. O'Callaghan

smaller than 16 per cent. This correlation: r(Q/A) D "~0"569F#C "]0"395 ~b=5"3708X 10-2[ EV EVl / t 11

L-

J

1.695r DEV "] - 0"4"44"FU9]' "579

LO.O- S8j

Lv,J

(2)

will be used for predicting the boiling heat transfer coefficients for R-11, R-113 and R-114. For comparison purposes, only a single plain copper tube with an outside diameter of 9.5 mm will be considered. Noting that the heat transfer rate per unit surface area (Q/A)Ev can be expressed as: (Q/A)E v = c%(Tw - TEv)

then eqn (2) can be rewritten as: TEv)DEv-]1"32F[.llCll °916 x

-

L 3'933[- DEV-] xps

L0.0-~88J

J

LWJ

1'°3~v~'] 3"664 LV,J

(3)

Using eqn (3) and considering 10 °C superheat of the heating surface over the saturation temperature of the working fluid in the evaporator, the predicted boiling heat transfer coefficients are presented in Fig. 4. Again, R-11 has higher heat transfer coefficients than those exhibited by R-113 and above 80°C by R-114, and therefore an evaporator with less heat transfer area needs to be employed--see Fig. 5. Performances of the working fluids in the condenser The heat transfer process between the working fluid and the cooling water in the condenser can be considered to occur in the following two stages-see Fig. 6: (i) The first is to decrease the temperature of the superheated vapour (exhausted from the expander) to the saturation temperature, TcoNo, corresponding to the condenser's pressure. (ii) The second is the rejection of the latent heat of vaporisation to transform the saturated vapour into a saturated liquid at the minimum cycle temperature, TcoNo. Figure 7(a) shows the calculated percentages of the heat rejected at the condenser's saturation temperature compared with the total amount of

//t

~,3ooo % 2Boo.

/

/

/

~""~,~

f

\

\

X

.c= 260~ ~24.00-

/

u.J

/

/

/

/

/

2200"

~ 2000" ~ 1Boo-

/ ......"

'x

/ /"; . . . . . . . . . .

..-.'/ ~lSOO- / ' /

% %.

*J o 14001200-

/

1000"

Tw- TEV

:

DEV

= 9.5 mm

10"C

BOO600

I

60

I0

I

I

B

I

100

i

I

120

I

I

I

160

I

160

I

IBO

EVAPORATOR'S SATURATION TEMPERATURE, TEV , ['[]

Fig. 4.

Boiling heat transfer coefficients for the short-listed working fluids. I,

condenser's saturation temperature= 40"C expander'sefficiency : 70 %

-x 28 .~

feed-pump's efficiency net power oufpuf temperature difference

- 26 "~

Tw-TEv

: 63 % = 1 kW : 10"C

\

]

4"O~"E 3.6

~ 22-

3.2

_~ 20 ~}~',

2.6 ~ •'.,, \

16 14.

\k'\

2.o 'XX\ \ \

II 10~

,,',.,. /

R-11

0

"1.6

.

8 -0.4

8

60

810

I

(~ 1 0

h-

i --4~--~,---,-~ 120 14.0

EVAPORATOR'S SATURATION

Fig. 5.

~ 160

m

0 180

TEMPERATURE. TEV, [ ' [ ]

Heat loading and required heat transfer area of the Rankine-cycle evaporator for 1 kW net power output.

O. Badr, S. D. Probert, P. W. O'Callaghan

30

~"

5

Saturated Vapour

t--.

TCO~

=~,~.,

~ /

WORKING FLUID ~,

Saturated q,,1 Liquid i

~OLINr,

,

,,

I HEAT EXCHANGEO ( linear scale )

Fig. 6. Temperature variations of the working fluid and the cooling water in the

condenser. heat exchanged in the condenser, for the selected fluids. According to the Carnot principle, the lower the mean temperature at which the heat is rejected, or, alternatively, the larger the percentage of the heat transferred from the working fluid at the minimum temperature of the cycle, the higher the Rankine-cycle efficiency will be. The results indicate that the performance of R-11 is superior to those of R-113 and R-114 because of the approximately zero value of ds/d Tfor the saturated vapour line of the former--see Fig. 7(b). In the Rankine-cycle condenser, the slow moving vapour of the working fluid condenses as a film on the outer surfaces of the water-cooled horizontal tube bundle. This phenomenon has been studied by many investigators.77-80 For our comparative study, the Nusselt relation: 77

F

i/ g g# Ah',o, 1 o:5

c¢c = 0' 725 LvlD---CONDm'~" ~ COND~ ~ ) ]

(4)

,

(5)

where: 3

Ahlat = Ahl, , + ~ct(TcoNo -- Tw)

as suggested by Ganic and Wu, 34 McAdams, 64 Kreith 65 and Butterworth, 66'81 was selected for predicting the values of the condensation heat-transfer coefficients. For simplicity, only one plain

Selecting a working fluidJor a Rankine-cycle engine

31

E J

96

R-11

1-

94

o_

92

~-

8g

c:? N

g6

evaporator's saturation temperature = 120 "C expander's efficiency =70 %

....,. I

R-113

~-~..._

z~ 8~

~ 82 80

I

30

35

I

---

410 45 50 [0NDENSER'S SATURATION TEMPERATURE, TEOND, ['C ] (a)

200 o Critical

point

o------~......

\k

160

~~2O8o

•/R'-114 R-113 i/// /

40 0

.

I 600

///I

~ 650

I

I

|

I

.

I

700

750

.

R-~

.

.

I

.

I

I

800

B50

I

'

---

900

SPECIFIC ENTROPY OF SATURATED VAPOUR, [ J l k g K ] (b)

Fig. 7.

Behaviours of the short-listed working fluids in the Rankine-cycle condenser.

% -- 2000 R-11

1800

16oo

R-113 R-114

i~oo

T[OND- Tw = 10 "C DCOND = 12,7 mm

z 1200

1000

Fig. 8.

'

35 40 45 50 CONDENSER'SSATURATIONTEMPERATURE, TCOND, ['[] Condensation heat transfer coefficients of the short-listed working fluids.

-

30

evaporafo~ saturation fe~erature = 120 ~ expander's efficiency = 70 %

-

" •c~o~. z~ 11 o

feed- pump's efficiency = 63 % nef power oufpuf =1 kW femperature dif ference TCOND-Tw= 1 0 ~

| | | 8 ~--

a_

i I

9:

7

G ~:

i CONDENSER'S

Fig. 9.

SATURATION

TEMPERATURE, TCOND , ['C ]

Heat loading and required heat transfer area of the Rankine-cycle condenser/or a 1 k W net power output.

Selecting a working fluid Jor a Rankine-cycle engine

33

horizontal tube of 12.7 mm outside diameter is considered, and a mean temperature difference (TcoND -- Tw) of 10 °C is assumed. The calculated values of ac (see Fig. 8) indicate that R-11 exhibits higher values of ~c compared with those for R-113 and R-114 and, accordingly, requires a condenser with less heat transfer area--see Fig. 9.

Pumping requirements The predicted values for the percentage of the expander's power output required to drive the feed pump of the Rankine-cycle engine--see Fig. 10--indicate that R-113 is the preferred choice. For a specified set of saturation temperatures of the evaporator and condenser, the relatively low powers required to drive the feed pump in the case of R-113, compared with R-11 and R-114, is a direct result of its lower pressure differential across the pump--see Fig. 11--and its lower liquid specific volume. In calculating the values shown in Fig. 10 for the three fluids, a value of 63 per cent for the pump efficiency was assumed. For a multi-vane feed pump, which is the type installed in the Rankine-cycle engine at CIT, the results of the laboratory tests carried out by Bala 82 using R- 11 and R- 113 indicated that the pump efficiency is about 50 per cent higher with R-113. The considerable difference is mainly because of the higher molecular weight and liquid viscosity of R- 113 and, therefore, less internal leakage and better lubrication of the rubbing surfaces in the pump.

Rankine-cycle efficiency The predictions presented in Fig. 12 assume a 70 per cent expander efficiency and a 63 per cent pump efficiency: R-11 exhibits slightly higher efficiencies than those obtained with R-113 whereas the resulting efficiencies, using R-114, are noticeably lower. The efficiencies shown in Fig. 12 were estimated assuming the same expander and pump efficiencies for the three considered fluids. The expander's isentropic efficiency in a Rankine-cycle engine is the most influential factor dictating its overall thermal efficiency. 5 The performance of the expander depends upon the irreversibilities exhibited mainly by the working fluid during the expansion process. For multi-vane expanders of the type used in the considered engine, internal leakage of the working fluid within the machine represents the major cause of

O. Badr, S. D. Probert, P. W. O'Callaghan

34

\

\

\

o

l l l

\

eu

l l l l l

\

II

11

~E

l

O

~

Z

~

\ \

\

S ~£

\ o

(%]'03033N

~

~

1NdlNO ~3/,~Od ~30NVdX3 -10 :~gVIN33~3d

~

I°/01 'O303:1N 117d1170 ~t~Od ~30N~'dX3 :I0 ~3VIN~3~3d

a

.,.~

~e

Selecting a working fluidJor a Rankine-cycle engine

s°t

/

~0

///// //////

30-I - -

35

20-

z

//

~0-

// ,/II

//

6s. a_ C-

3-

t,o

10.8. 06.

/ / / /

11

/ '/ //// / /

o Eriticat point

05 04 03 0.2 -40

0

t~O

BO

120

160

200

2~0

TEMPERATURE,[E]

Fig. il. Saturationpressure temperaturerelationships for the short-listed fluids. efficiency loss. 83 - 87 The higher the molecular weight of the working fluid the smaller the internal leakage rate in the expander, as and therefore the greater is its expected isentropic efficiency. Accordingly, higher overall cycle efficiencies are expected to be achievable practically when using R-113 instead of R-I 1. The few available experimental measurements, although obtained using turbine expanders, corroborated this assertfon. 23 Thermal stability Despite the very large discrepancies between reported data concerning the thermal stabilities of organic working fluids for Rankine-cycle engine applications, the following useful comparative results, for the three selected fluids, can be deduced. 88-91 (i) Under dynamic testing conditions, in which the working fluid is

O'Callaghan

O. Badr, S. D. Probert, P. W.

36

32¸ .f

28

...'

.,..-

....

..j.,J" .......'"

24 ¸

.~/.."

>_" ~. 2o

?~,/ .."

16

...J

R-113

u

8

, ......... ~ ~

d

e

r

'

s saturation temperature = "C efficiency =7.0 %

s

feed-pump's efficiency

:"

b

80

Fig. 12.

i

I

I

I

~

h

~

= 63 %

i

'

-----

100 120 %0 160 180 EVAPORATORtS SATURATION TEMPERATURE,TEV, ['£]

Variations of the predicted thermal efficiency of the Rankine-cycle engine with the evaporator's saturation temperature for the short-listed fluids.

alternatively vaporised and condensed, 9° at an evaporator temperature of 537.8 °C, R-11 is the least stable, followed in sequence by R-113 and R- 114, respectively. (ii) Under both dynamic testing conditions 9° and for sealed-tube static tests, 89.91 in the presence of metals, the order of increasing stability is as follows: R-11, R-113 and then R-114, which is the most stable of the three considered. (iii) When a lubricating oil is present, and with metals in contact with the considered working fluids, the order of stability appears to be: R-114 is the most stable followed by R-113 and then R-11. 88'89'91 As a result of the previous analyses and deliberations, it is apparent that R-113 is the most appropriate fluid for the Rankine-cycle application envisaged. R-I 13 exhibits (i) satisfactory heat transfer characteristics in the heat exchangers; (ii) low feed pump power requirements and (iii) high Rankine-cycle efficiencies. It has an approximate thermal stability limit exceeding 149 °C when in contact with lubricating oils and constructional metals. 23'24 On the other hand, R-11, which possesses satisfactory thermodynamic and heat transfer properties, has the relatively low

Selecting a working fluid Jor a Rankine-cycle engine

37

thermal stability limit of 1 2 0 ° C . 23'24 Although R-114 has a superior thermal stability, it exhibits poor thermodynamic and heat transfer behaviours. CONCLUSIONS AND RECOMMENDATIONS There is no unique working fluid which satisfies all the stated attributes simultaneously for every application. However, for a specified application, some fluids are more suitable than others. It is up to the designer or the investigator to set up his own selection criteria in order to identify the most appropriate fluid amongst those available. In doing so, the design configurations of the actual system, in which the working fluid has to perform, must be taken into account. For the Rankine-cycle engine at CIT, R-113 is the best choice. The thermal stability data reported in the literature are of little quantitative use. They can only provide a crude guide in screening prospective working fluids. Once the most appropriate working fluid is selected, thermal stability tests of the fluid when in extended contact for many hours with the used lubricant and the constructional materials of the proposed system, up to the highest expected operating temperature of the cycle, should be carried out to justify the selection that has been made.

REFERENCES 1. B. Sternlicht, Waste-energy recovery: An excellent investment opportunity, Energy Convers. Mgmt, 22 (1982), pp. 361-73. 2. R. E. Barber, Potential of Rankine engines to produce power from wasteheat streams, Proc. 1974 IECEC, Aug., 1974. No. 749018, pp. 508-14. 3. R. E. Barber, Current costs of solar-powered organic Rankine-cycle engines, Solar Energy, 20 (1978), pp. 1-6. 4. D. A. Palmer and B. E. Sirovich, Selection of a Rankine-cycle fluid for recovery of work from heat at moderate temperature, Proe. 1978 IECEC, Aug., 1978. No. 789606, pp. 1500 6. 5. R. Barber and D. Prigmore, Solar-powered heat engines. In: Solar-Energy Handbook, Chapter 22 (J. F. Kreider and F. Kreith (Eds)), McGraw-Hill, USA, 1981. 6. H. Tabor and L. Bronicki, Establishing criteria for fluids for small vapourturbines. SAE Paper 931C, presented at the National Transportation, Powerplant, Fuels and Lubricants Meeting, Baltimore, USA, Oct. 19-23, 1964.

38

O. Badr, S. D. Probert, P. W. O'Callaghan

7. S. K. Ray and G. Moss, Fluorochemicals as working fluids for small Rankine-cycle power units, Advanced Energy Conversion, 6 (1966), pp. 89-102. 8. S. S. Wilson, New working fluids for power units, New Scientist, 36 (571) (Nov., 1967), pp. 412-13. 9. S. Luchter, A quantitative method of screening working fluids for Rankinecycle power plants. ASME paper 67-GT-12, 1967. 10. D. E. Stoddart, Waste-heat recovery by the use of low boiling-point liquids, Process Engineering (Nov., 1968), pp. 5-8. 11. D.T. Morgan, E. F. Doyle and S. S. Kitrilakis, Organic Rankine-cycle with reciprocating engine, Proc. 1969 IECEC, Sept. 1969. No. 699001, pp. 1-10. 12. B. Wood, Alternative fluids for power generation, Proc. 1. Mech. E., 184 (Pt 1) (No. 40) (1969/70), pp. 713-40. 13. J. E. Boretz, Low peak-temperatures and hydrodynamic bearings: Key to long-life organic Rankine-cycle systems, Proc. 1972 1ECEC, Aug. 1972. No. 729054, pp. 296-302. 14. G. Angelino and V. Moroni, Prospective for waste-heat recovery by means of organic fluid cycles. Trans. ASME, Journal oJ Engineering jor Power, 95(2) (April, 1973), pp. 75-83. 15. R. E. Barber, Rankine-cycle systems for waste-heat recovery, Chemical Engineering (Nov. 25, 1974), pp. 101-6. 16. J.R. Owen, The organic Rankine-cycle: A Review oJapplications andjactors aJJecting working-fluid selection, Research Memorandum M I 76, The City University, London, June, 1975. 17. P. N. Garay, Application of chemical fluids in Rankine-cycle plant, Proc. 1975 IECEC, Aug. 1975. No. 759209, pp. 1435 8. 18. R. E. Barber, Solar-powered organic Rankine-cycle engines: Characteristics and costs, Proc. 1976 IECEC, Sept., 1976. No. 769200, pp. 1151-6. 19. R. E. Niggemann, W. J. Greenlee and P. D. Lacey, Fluid selection and optimisation of an organic Rankine-cycle waste-heat power-conversion system. ASME Paper 78-WA/Ener-6. Presented at the Winter Annual Meeting, San Francisco, Calif., USA, Dec. 10-15, 1978. 20. M.N. Bahadori, Solar water-pumping, Solar Energy, 21 (1978), pp. 307-16. 21. R. K. Rout, B. Fortunato and S. M. Divakruni, Simulation of solarpowered Rankine-cycle systems, Proc. 1978 1ECEC, Aug. 1978. No. 789502, pp. 1641-9. 22. A. A. Samuel, U. S. P. Shet and M. C. Gupta, Solar thermal-power plant: Thermodynamic analysis. Proceedings oj' the International Symposium." Workshop on Solar Energy, June, 1978, Cairo, Egypt, Vol. 3, pp. 1279-92. 23. E. Wali, Optimal working fluids for solar-powered Rankine-cycle cooling of buildings, Solar Energy, 25 (1980), pp. 235-41. 24. H. M. Curran, Use of organic working fluids in Rankine-engines, J. Energy, 5(4) (July/Aug., 1981), pp. 218-23. 25. G. Horn and T. D. Norris, The selection of working fluids other than steam for future power generation cycles, The Chemical Engineer (Nov., 1966), pp. 298-305.

Selecting a workingfluid Jor a Rankine-cyele engine

39

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