A new design for a double-condensing chamber solar still

DESALINATION ELSEVIER

Desalination 114 (1997) 153-164

A new design for a double-condensing chamber solar still G.N. Tiwari*, A. Kupfermann, Shruti Aggarwal Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Tel. +91 (11) 666979, ext. 5040; Fax +91 (11) 686-2037

Received 23 June 1997; accepted 27 August 1997

Abstract

A design of a double-condensing chamber solar still (DCS) is described. In this design the vapour is formed in the fwst chamber after the water is heated by solar radiation. Then the vapour is allowed to move into the second chamber through a vent provided at the top of the partition wall of a solar still. The transferred vapour is condensed on the metallic condensing surface behind the partition wall. The vapour is also condensed on the inner surface of the double glass cover of a solar still. Experiments were carried out during September 1995 to August 1996. The daily yields of the DCS and a conventional solar still are compared on the basis of experimental results. It has been observed that there is significant enhancement in daily output due to a maximum vapour pressure difference between the two condensing chambers on a clear day. Keywords: Solar distillation; Double-chamber still; Efficiency

1. I n t r o d u c t i o n

A rigorous review of work done on solar distillation before 1982 was carried out by Malik et al. [1] after the review o f Howe and Tleimat [2]. In their review, they mentioned that the airinflated plastic solar still developed by Telkes [3] was used for the first time by the US Navy and Air Force during the Second World War in emergency life rafts. An air-inflated plastic solar still was designed to have a porous felt pad for *Corresponding author.

maintaining a water film, unlike conventional solar stills (CSS) for quick evaporation. Based on the concept of the water film, Sommerfeld [4] and Sodha et al. [5] studied the performance o f singleand multi-wick solar stills. These solar stills were not tried in the field due to high operating and maintenance costs and because they require skilled manpower to run them. They reported that only the CSS has been successfully tested in the field due to its long life and low operating and maintenance costs [6]. It is important to mention here that evaporating and

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condensing surfaces are in one enclosure in CSS units. However, in order to further increase the output, attempts have been made either by separating evaporative and condensing surfaces or by providing additional condensing surfaces in the same unit, or by tilting the basin to receive maximum radiation. Some of these attempts are mentioned in the following designs: • basin-type stepped solar still equipped with charging system and condenser reservoir [7] • cascade solar still [8] • solar still with internal condenser [9] ° tilted solar still [10-12]. There is a marginal improvement (1 0-15%) in the performance of the above-mentioned solar stills in comparison to CSS units. Further, the output can also be increased by using pre-heated water in the basin, generally referred to as nocturnal production. This study has been carried out in detail by Tleimat and Howe [10] and Sartori [13]. In their findings, it has been concluded that intermittent feeding of pre-heated water in the basin gives best results. However, it requires an automatic system for its operation

[14]. The output of a solar still mainly depends upon the water and glass cover temperature difference. This difference can be maximized for higher output (yield) either by increasing the water temperature or by decreasing the glass cover (condensing cover) temperature [9] or by having both conditions at the same time. Keeping the above requirements in mind, a doublecondensing chamber solar still (DCS) has been designed at the Centre for Energy Studies, Indian Institute of Technology, Delhi. In this case a double glass cover has been used to maintain higher water temperatures similar to the case of a flat plate collector [15]. The upward heat loss is also reduced to a minimum in contrast to the single glass cover solar still. Furthermore, another chamber (chamber H) behind the partition wall is created by using a metallic wall exposed to ambient air for quick release of latent heat of condensation. A small vent at the top of the

partition wall has been provided for transfer of vapour from chamber I to chamber 1/. In the DSS case the operating and maintenance cost will be lower, similar to the CSS. In this paper a comparative study on a daily as well as monthly basis between the DCS and the CSS is reported. Further, the present results have also been compared with results obtained by other investigators. It has been observed that there is a significant improvement in the performance of the DCS.

2. Working principle 2.1. Conventional solar stills (CSS)

The solar radiation, I(t), incident on a glass cover is transmitted inside the distiller unit after partial reflection and absorption. The transmitted radiation is further reflected and absorbed by the water mass and finally it reaches the blackened surface of the basin where it is mostly absorbed. After absorption of solar radiation from the blackened surface, most of the thermal energy is convected to the water mass, and the rest, which is very small, is lost to the atmosphere through bottom insulation. Thus, the water gets heated. The heat transfer, namely radiation, convection and evaporation, occurs between the water and glass cover. The evaporated water gets condensed on the inner surface of the glass cover after releasing the latent heat of condensation. This condensed water trickles into the distillate output under the effect of gravity. The cross-sectional view of the CSS is shown in Fig. la. 2.2. Double-condensing (DCS)

chamber

solar still

The solar radiation, after reflection and absorption from the glass cover I and II, respectively, partially reaches the water surface directly, and the rest is received by the water surface after the reflection from the mirror. The total radiation available on the water surface is

G.N. Tiwari et al. /Desalination

I

155

114 (1997) 153-164

Fig. la, Cross-sectional view rofa conventional solar still.

I

Blackened basin liner

Qlfscovcr2 --Ghs

cover-1

~xttaptlt-1

outlet

Fig. 1b. Cross-sectional view of a double-condensing chamber solar still.

further partially reflected from the mirror. The total radiation available on the water surface is further partially reflected and absorbed by the water mass, and the rest is absorbed by the blackened surface. Afier absorption most of the radiation is convected to the water mass which gets heated. There is a heat transfer due to radiation, convection and evaporation. Since double-glass cover is used in a DCS, in contrast

to CSS, the temperature of the inner glass cover is increased significantly due to a reduced heat loss coefficient from the top of the inner glass cover. Hence, the overall temperature difference between the water and inner glass cover is reduced, and the pressure in this chamber (chamber I) is increased significantly due to a higher operating temperature. At the same time the pressure in chamber II is relatively very low

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G.N Tiwari et at. /Desalination 114 (1997) 153-164

due to its direct exposure to environment conditions through a metallic sheet, generally known as condensing cover II. Thus a pressure difference is created between the two chambers, and the vapour is dragged into the second chamber. This vapour is condensed on cover II as well as at the back of the partition wall, after releasing its latent condensed heat. The percentage of condensed water at the back of the partition wall is very small and is negligible. The condensed water is collected through a channel provided at the bottom of the vertical walls. In addition, there is also distillate output from the inner glass cover because its temperature is lower than that of the water mass. The cross-sectional view of the system is shown in Fig. lb.

Fig. 2a. Conventionalsolar still.

....... ~iiiiiiiiiiiiiiiill

3. Design of solar stills 3.1. Conventional solar still (CSS) The conventional solar still with an effective basin area of I m × I m has been fabricated by using fibre-reinforced plastic (FRP) material. The inner surface of the FRP body is blackened. A 5 mm thick glass cover was used as a transparent material. This glass cover with an inclination of 15 o is fixed to the vertical wall of the still. At the lower end of the glass cover, a provision to collect the condensed water was made with the help of an adhesive (m-ceal trademark) to ensure that no vapour would be lost. The body of the solar still is 3 mm thick to provide minimum heat loss from the bottom as well as from the sides of the still. The vertical wall (inside distiller unit) facing the sun is polished to reflect incident radiation into the basin. A provision for an inlet and outlet was made at the top of the vertical wall and at the bottom of the basin. The whole unit is placed on an angled iron stand. The photograph of a CSS is shown in Fig. 2a. The solar still faces due south to receive maximum solar radiation throughout the year.

Fig. 2b. Double-condensingchamber solar still. 3.2. Double-condensing (DCS)

chamber

solar still

The double-condensing chamber solar still also has an effective area of 1 mx 1 m. The vertical heights of the still are 455 mm (metallic surface) and 213 mm, respectively. The enclosure of a DCS was divided into two chambers--I and II, which are separated by a partial wall 70 mm away from the condensing cover II as shown in Fig. 2c. The partition wall, 280ram high, consists of wood (15 mm) and polyurethane (30 ram). The condensing cover II, measuring l 1 4 5 m m ×

157

G.h: Tiwari et al. / Desalination 114 (1997) 153-164

Fig. 2c. Double-condensing chamber solar still without glass cover.

455mm, is made of stainless steel. The vertical mirror (277× 1110 mm) is also fitted on the partition wall, facing chamber I, to reflect back most of the radiation falling on it to the water in the basin (Fig. 2c). A GI sheet metallic tray

(1004 x 1000 x40 mm), which is blackened from the inside, is used as a basin and is placed over the horizontal platform. The outer structure of the DCS was made up of wood 15 mm thick, which is painted from the outside and inside with waterproof paint. Water-proof form, 23 mm thick and 7 2 m m high, is fixed at the top along the peripheral of the still to ensure it is vapourleakage proof. A double glass cover (5 mm) fitted in an aluminium frame (1142 x 1289 x 18mm) is placed over the structure. In order to have uniform water flow over the back wall (condensing cover II), a stainless steel pipe 12 mm in diameter with a number of holes at equidistance was fitted to the top of the back wall from the outside along with a provision for a channel at the bottom to drain out water. Provision for filling water into the tray was also made along with a drainage system. Views of the DCS and the glass cover are shown in Figs. 2b and 2d, respectively. The volume of chamber I of a DCS is the same as that o f a CSS.

Double

Plywood

-Stoinl=ss

i

Thcr rnocot¢

f

gloss covet ste'~l c o n c l ~ n s i n g

i--" At~JmiNlum

surfoce

frorrl¢

i

Mirl

L

~._y____:_- - ~ , --.__-~_~ /

~.Orainoge for woter

-L-o,o,,.g,., (2)

/

~- D r a i n a g e ;

Wotgr

, Chonnel

f

(1)

- lfl

fray

/

often

|of Oroinoge-

in~uloted

II

woII

Orainoge - I /

/

Support for oluminium frome

/

~--Woodcn

.~Jppor( tot wQ.ter Ircly

Fig. 2d. Pictorial view of double-condensing solar still with glass cover. (1) and (2) Fresh water outputs (compare Fig. 1b).

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G.N. Tiwari et al. / Desalination 114 (1997) 153-164

4. Experimental procedure In order to see the improvement in the performance o f a DCS, it was decided to compare the performance of both the DCS and a conventional fibre reinforced plastic (FRP) CSS with the same effective area for a given water depth. Both solar stills faced south to receive the maximum and equal amount of solar radiation. The experiments were conducted for the following cases with the DCS: 1. without dye and without water flow over condensing cover II 2. without dye and with water flow over condensing cover II 3. with dye and without water flow over condensing cover II 4. with dye and with water flow over condensing cover II. It is important to mention here that the provision of water flow over the back of the CSS has not been considered. This is due to the fact that the back wall acts as a reflecting surface, not as a condensing surface, similar to the DCS. The effect of dye has been considered uniformly in both stills. The water mass in both solar stills was kept at about 20 kg to avoid the storage effect during offsunshine hours and for a faster evaporation rate. The water was filled once a week. The experiment was conducted from September 1995 to August 1996. At least weekly observations (on a daily basis) were carried out for each case. Later, the experiment was continued in Case 3 by measuring the yield in the morning at 10 am and in the evening at 5 pm. This decision was taken to avoid high operating and maintenance costs due to water flow over condensing cover II.

in chamber I are transferred the chamber II for the case of water flow over condensing cover II. This is due to the fact that the partial vapour pressure difference is maximized in this case. When there is no water flow over the second condensing cover, the amount of water collected from both chambers is approximately the same in the case of DCS. It is further noted in Table 2 that the DCS gave more than 50% daily yield in comparison to the CSS with clear sunny weather conditions during September 1995 to August 1996. The average monthly performances are given in Table 3. It is inferred that the improvement in the performance of the DCS is between 35-77%. It is also important to mention here that the yield for a number of typical days in the months of May, June, July, etc. were about 4-4.5 l/d. The hourly variation of solar intensity, ambient air temperature, and water temperature for the CSS and DCS for a typical day (23/12/95) is shown in Fig. 3. It is clear that the water temperature in the DCS is significantly higher than in the CSS due to a minimum top heat loss from the water to ambient in the former. This happens due to the double glass cover in the DCS, unlike the CSS.

TI +

The results obtained during September 1995 to August 1996 are given in Tables 1-3. Table 1 gives the results of the four cases mentioned above. It is clear that most of the vapours formed

I(t) 9(, Tw(DCS)

-~Tw(CSS)

- 500

5O

Y

~4o 21111 20

10

5. Results and discussion

] 1500 |

6O

10

11

12

13 Time

14

15

16

17

(Hours)

Fig. 3. Hourly variation of intensity [I(t)], ambient (Ta) and water (Tw) temperatures of double-condensing chamber and conventional solar stills on December 23, 1995.

2199

2249

1752.33

Third week of September, 1995 (5)

Case 2: without dye and with water flow

Case 3: with dye Fourth week and without of September, water flow 1995 (4)

Case 4: with dye Second week and with water of October, flow 1995 (3)

Average yield (ml) for conventional solar still

2082

Period (no. of days)

Case 1: without Second week dye and without of September, water flow 1995 (5)

Mode of experiment

640.3

1588.25

626

1400

Average yield from first condensing chamber (ml)

1947.33

140

2592

1490

Average yield from second condensing chamber (ml)

Double-condensing chamber solar still

2587.6

2992.2

3218

2890

Total average yield (ml)

47.6

33

46.3

38.8

(%)

Average improvement

Table 1 Comparative study of conventional and double-condensing chamber solar stills with clear skies for different cases

36.8

36.9

36.1

24

25

24

24

(°c)

(°c) 34

Ta, min

Ta, max

Climatic conditions

Total daily yield (ml) for conventional solar still

2000 2495 1515 1140 930 970 910 1810 1050 900 1800 1800 2540 2360 3089

Date

13.9.95 23.9.95 2.11.95 17.11.95 5.12.95 7.12.95 4.1.96 22.1.96 8.2.96 15.2.96 11.3.96 20.3.96 9.4.96 26.4.96 17.6.96

1515 870 1240 970 770 860 700 1655 880 850 1240 1420 1530 1720 1899

Yield from first chamber (ml) 1500 2970 1100 865 911 785 750 1820 920 930 1930 1665 2160 1790 2116

Yield from second chamber (ml)

Double-condensing chamber solar still

3015 3840 2340 1835 1681 1713 1450 3475 1800 1780 3170 3085 3690 3510 4015

Total average yield (ml) 51 35.6 54.5 60.9 80.7 76.6 59.3 91.9 71.4 97.7 76.1 71.4 45.3 48.7 29.9

Improvement (%)

34.1 35.6 29.1 28.7 33.6 33.8 21.8 22.6 25.6 25.9 30.2 35 36 38.5 43

Ta, max (°C)

25.4 25.2 15.5 12.8 9 8.8 8.9 11 10.1 14 15,6 21.5 21 27 31

Ta, min (°C)

Climatic conditions

Table 2 Comparative study of conventional and double-condensing chamber solar stills without water flow for typical days of the month with clear skies

i

4~

t~

2.

2273.3 1955.5 1220.33 867.76 966.57 1300 1796.56 2382 2084 2700 2517.7 2090

Sept., 1995 (15) Oct., 1995 (9) Nov., 1995 (15) Dec., 1995 (16) Jan., 1996 (14) Feb., 1996 (19) Mar., 1996 (16) Apr., 1996 (10) May, 1996 (10) June, 1996 (8) July, 1996 (8) Aug., 1996 (6)

aWater flow over condensing cover II.

Total daily yield (ml) for conventional solar still

Month (no. of clear days)

1494.3 1337.5 904.33 639.19 733.6 1043.4 1354.4 1551.5 1442 1462.57 743.86 1223.25

1712.7 1314 860.67 746.5 831.73 1260 1667.5 1772 1450 1840.86 2332.86 a 1327

Yield from Yield from first chamber second chamber (ml) (ml) 3207 2651.5 1765 1387.5 1565.3 2304.5 3021.9 3323.5 2892 3303.43 3076.7 2550.25

Total average yield (ml)

Double-condensing chamber solar still

41 35.6 44.6 59.7 65.4 77.2 68.2 39.4 38.77 22.35 22.2 22.02

Average improvement (%)

33.8 29 29 20 22 23.3 30.5 36.8 40.5 39.9 35.3 33.7

Ta, max (°C) 25.2 15 13.2 7.4 6.0 8.2 15.8 22 26.6 28.7 27.2 26.1

Ta, min (°C)

Climatic conditions

Table 3 Comparative study of conventional and double-condensing chamber solar stills on a monthly basis without water flow

15.78 13.7 12.04 11.46 12.04 13.7 15.8 17.28 16.92 17.64 16.92 17.28

Average daily intensity (MJ/m2)

t.*a

q~

3.

~.

.~

162

G.N. Tiwari et al. / Desalination 114 (1997) 153-164

60

Inner glass(DCS)

~- Outer glass(DCS)

-~ Metallic sheet(DCS) -e- GIsIs(CSS)

Z,.G

50

3.5 13

E ao

%

2c

n

3-(3

.

5 11

12

13

14

15

16

17

~ 1.5 / pl

6. E x p e r i m e n t a l u n c e r t a i n t i e s

The measured values are not accurate. In any experiment the exactness of the measured value is affected by the deviations caused by various errors, namely systematic and random ones. Systematic errors are caused by instrument and environmental errors. Random errors are caused by the random variations in the parameters or the system of measurement. An estimate of internal and external uncertainty has been carried out for both stills. To estimate these uncertainties, data of the output for both of the solar stills are taken. The values of

I

0 ° . "1(~

Time (Hours)

Fig. 4 shows the hourly variation of condensing cover temperatures of the CSS and the DCS for Case 3. This indicates that the temperature of condensing cover II, which is exposed to shaded ambient air, is lower than the temperature of condensing cover I in the forenoon; in the afternoon, its temperature rises. The results obtained for the DCS were compared with others and are given in Fig. 5. The average yields of DCS and CSS are plotted in the same figure. It has been observed that the DCS a performs better in these tests when compared with the results obtained by EI-Bassuoni [8] and Wibulswas [16].

o.%/.J OjO

Fig. 4. Hourlyvariation of condensing cover temperatures of double-condensing chamber solar stills and conventional solar stills for Case 3 (23 December 1995).

,

Oo _ . , : ' /

"~ 2.5

10

eget,

°

I

9

0

II

O

I

O

I •

.

I

.

.

I0

.

.

I

.

[.

.

_

15

I

.

.

20

.

.

i

.

25

Total horizontal radiation (MJ/rn 2 d)

Fig. 5. Comparison of DCS with other solar still. • reflectingback wall [16], x regenerative back wall [16], o absorbing back wall [16], - still with n improvement, [] double-condensingchamber solar still, A conventional solar still, • solar still equipped with condenser reservoir [7], o modified cascade still [8]. available data for all clear days in a month are taken. The uncertainty is calculated by using the data for 1 year.

6.1. Estimate o f internal uncertainty

u,--

2 2 2 2 01 + 0 2 + 0 " 3 . . . +0"12 _ )"]~1~-~'~ 2

12

No

from Nakra and Choudhary [17], where o is the standard deviation, X-37 the deviations of the observations from the mean, and N O the number of observations taken to find the mean.

% uncertainty = Average o f total number of observations

G.N. Tiwari et al. / Desalination 114 (1997) 153-164

163

Table 4 Sample calculations of experimental uncertainties of DCS and CSS Months

X

September, 1995 October, 1995 November 1995 December, 1995 January, 1996 February, 1996 March, 1996 April, 1996 May, 1996 June, 1996 July, 1996 August, 1996

Y,

G __

CSS

DCS

CSS

DCS

CSS

3207 2651.3 1765 1385.7 1565.3 2304.5 3021.9 3323.5 2892 3303.4 3076.7 2550.2

2273.3 1955.7 1220.3 867.76 966.6 1300.3 1796.6 2382 2084 2700 2517.7 2090

352,450 134,284.8 1,869,058 960,995.44 5,329,018.8 9,506,604.9 5,138,844.5 812,802.5 748,680 794,041.72 559,687.43 352,310.75

170,837.5 137,014.62 8,336,079.4 179,300.97 963,264 879,460.99 2,201,857 332,160 1,176,920 425,358.87 359,949.43 463,526

148.2 91.61 91.14 61.26 164.89 162.27 141.68 90.15 173.05 127.29 106.87 148.38

103.3 92.53 192.48 26.46 70.1 49.36 92.74 57.63 216.97 93.17 85.7 170.20

2 +O12

12

410.27 12 = 34.3

% uncertainty -

34.2 x 100

- 1.83%

1871.2

For DCS: 451.55 U~- - - 37.62 12 % uncertainty -

3 7.62 x 100 2591.8

6.2. External uncertainty

External uncertainty is evaluated by taking into account the errors caused in taking the readings o f yield, temperatures, intensity, etc. It has been carried out by taking the least count o f the measuring instruments. % error % error % error Total % Total %

For CSS,

UI =

(l

DCS

Sample calculations o f experimental uncertainties for the D C S and CSS are given in Table 4. Using these values, we obtain 2 2 2 O1 + 0 2 + 0 3 . . .

(X-X-) 2

- 1.45%

in measuring intensity = 2% in measuring temperatures = 0.1% in measuring yield = 1 % error for D C S = 2+4×0.1 +3 × 1 error for CSS = 5.4% = 2+2×0.1+1 = 3.2%

7. C o s t c o m p a r i s o n The cost o f CSS and D C S for an effective area o f 1 m 2 are Rs 10,800 ($300) and Rs 12,500 ($347), respectively. The costs o f CSS include wooden dye and the FRP mould. On large-scale production o f D C S and CSS, there will be a marginal difference in costs due to the additional expense for glass covers and metallic condensing covers. This marginal higher cost for D C S can be recovered within 1 or 2 years due to a higher yield in the DCS.

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8. Conclusions and recommendations T h e performance o f DCS gives a higher daily output o f about 3 5 - 7 7 % over the CSS. It was observed that the experiment should be continued for several years to generate more data on an hourly and daily basis. It is further suggested that a theoretical model by developed for experimental validation and optimization o f design parameters.

Acknowledgment The authors are grateful to German Technical Cooperation (GTZ), N e w Delhi, for partial financial support.

References [1] M.A.S. Malik, G.N. Tiwari, A. Kumar and R.C. Tyagi, Solar Distillation, Pergamon Press, UK, 1982. [2] E.D. Howe and B.W. Tleimat, in: Solar Energy Engineering, A.A.M. Sayigh, ed., Academic Press, New York, 1977, pp. 431464. [3] M. Telkes, Solar distiller for life rafts. U.S. Office of Science, Report No. 5225, PB 21120, 1945. [4] J.V. Sommerfeld, Informe sobre trabajos de investigacion en destiladores solares que se realizan en la USM. Scientia, 139, Valparaiso, Chile, 1970.

[5] M.S. Sodha, A. Kumar, G.N. Tiwari and R.C. Tyagi, Solar Energy, 26 (1981) 127. [6] A. Delyannis and E. Delyannis, Proc., 4th International Symposium on Fresh Water from the Sea, 4 (1973) 487. [7] S.M.A. Moustafa and G.M. Brusewitz, Solar Energy, 22 (1979) 141. [8] A.M.A. El-Bassuoni, Int. J. Solar and Wind Tech., 3 (1986) 189. [9] S.T. Ahmed, Int. J. Solar and Wind Tech., 56(6) (1988) 637. [10] B.W. Tleimat and E.D. Howe, Solar Energy, 10(2) (1966) 61. [11] B.M. Achilov, T.D. Zhurauv and R. Akhtamov, Appl. Solar Energy (Geliotekhnika), 10(4) (1974) 104. [12] O.St.C. Headley and J.B. Morris, Int. J. Ambient Energy, 1(4) (1980) 209. [13] E. Sartori, in: Advances in Solar Energy Technology, Vol. 2, Pergamon Press, 1987, p. 1427. [14] G.N. Tiwari, in: Reviews and Contemporary Physics Solar Energy and Energy Conservation, R. Kamal, K.P. Maheshwari and R.L. Sawhney, eds., Wiley Eastern, India, 1992, pp. 32-149. [15] J.A. Duffle and W.A. Beckman, Solar Engineering of Thermal Processes, 2nd ed., Wiley Interscience, New York, 1991. [16] P. Wibulswas and S. Tadtiam, Improvement of a basin-type solar still by means of a vertical back wall, Int. Symposium Workshop on Renewable Energy Sources, Lahore, Elsevier, Amsterdam, 1984. [17] B.C. Nakra and K.K. Choudhary, Instrumentation Measurement and Analysis, 1st ed., Tata McGrawHill, New Delhi, 1985, pp. 33-36.