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Predictions of fog formation due to a warm water lagoon proposed for power station cooling
663
Discussion
data, causes of persistent effects can be found. The model could indicate certain factors causing pollution effects without trying to find the physical and chemical reasons for these effects. Gnce such a causal relationship is established by the statistical analysis new research can be initiated to determine the ohvsical and chemical reason for the relationshio.__ thus a statistical model of this kind will yield further results for deeper investigations. We hope that more of such work will be published by our mathematical statisticians and that the method indicated will be practically used for the systematic processing of the data available. SELMOTAUBER
Portland State University, Portland, Oregon 97207, U.S.A.
DISCUSSION PREDICTIONS LAGOON
OF FOG FORMATION DUE TO A WARM WATER PROPOSED FOR POWER STATION COOLING*
Tms paper while reaching more or less the correct conclusions regarding the incidence of steam fog from cooling ponds contains some misconceptions whose effects have fortunately balanced out in the final analysis. First of all the restriction to situations where thermal inertia is not important is serious as sensible cooling pond design incorporates inertia as a necessary safety margin against adverse weather conditions and, more particularly, against the effects of solar heating which at certain times of the day far exceeds any art%cial heat loading. For near zero thermal inertia the resultant water temperatures could exceed statutory limits even without additional heating. The use of an “approach” based on cooling tower operation is therefore invalid on a time scale shorter than that in which the pond temperatures can respond to a change in conditions (typically 2-3 days upwards). Within the above limits there are still inaccuracies as the PARKER-KRENKELL(1970) 2°C “approach” is much too low for a cooling pond. In an unheated water body the “approach” would vary from approx. 0°C in winter to 4°C in summer in the U.K., this being the approximate differential required~ to dissipate the solar heat imput. A reasonably designed cooling pond would be expected to dissipate typically 200-300 w rn-’ of waste heat leading to a further water temperature rise of between 5 and 10°C on average, giving an overall range of “approaches” from 5 to 14”C, the actual value being a complex function of the time of year, meteorological conditions, the station load and, of course, the pond thermal inertia. Having postulated cooling pond parameters a day by day calculation is strictly necessary based on the metwrological variables. Thus in a practical case the values of AT (using the notation of the note) are much higher and more variable than specified. Furthermore, the criterion for strong steaming is based on a single observation by SAUNDERS(1964) and is not reliable. However observations on C.E.G.B. cooling ponds, to be reported, suggest that the above errors tend to cancel since, although cooling pond temperatures tend to be higher than specified, the Saunders criterion is pessimistic, higher values of AT being necessary for strong steaming. Thus the prediction of the occurrence of strong steaming are not unreasonable for a typical cooling pond within the U.K. Finally it is worth noting that the highest temperatures occur only in the immediate vicinity of the outfall owing to turbulent mixing. As steam fog is highly localized, the problem can be minimized by care in siting of a road relative to the station outfall. Scienfific Services Department, C.E.G.B., North West Region, 829, Wiimslow Road, Manchester, M20 8RU. * R. A. COX(1973) Atmospheric Environment 7,363-368.
W. MCMILLAN
664
Discussion
REFERENCES PARKER F. L. and KREKKELL P. A. (1970) Physical and Engineering Aspects of Thermal Pollution. p. 66. Butterworths, London. SAUNDERSP. M. (1964) Sea smoke and steam fog. Q. J. R. hf.ct.Sue.90. 156.
AUTHOR’S
REPLY
THE RESTRICTION to cases where the effective thermal inertia is low was made to facilitate an analysis in which it was not necessary to involve the temperature history of the air. This was considered justified because the volume of water taking part in evaporative cooling at any one time is small and so the rni~rn~ surface temperature of the lake should follow the wet bulb temperature quite closely, even though the main body of the lake has not had time to respond. The intake is submerged, however, and convective mixing in the cooler parts of the lake will mean that the cool surface layers will be well mixed before reaching the intake. Thus, as Mr. McMillan points out, thermal inertiawill be significant in cases where the lagoon is deep, especially in the region of the intake. With regard to the “approach”, it is difficult to see how the additional heat input from the power station of ZOO-300w m-Z quoted by Mr. McMilian could raise the temperature at the station intake by 5-10°C because this is about the differential required to dissipate the load if it were uniformly distributed across the lake. This is not the situation in power station cooling, where most of the cooling takes place near the outfall with correspondingly large horizontal temperature gradients there. The intake temperatures would therefore be relatively insensitive to station load since the intake is as far from the outfall as possible. Thus the “approach” would be close to its natural value and my assumption of 2°C seems to be in close agreement with Mr. McMillan’s range of O-&C for this quantity. Finally, it may be of interest to point out that in cases where the thermal inertia is considered important, the minimum surface water tem~rature should depend on the meteorologi~l history and the inertia in a fairly straightforward way and the incidence of steaming could be determined in a way similar to that described in the paper. This would, however, involve a considerably greater effort in anaiysis of the meteorological data. Department of Mathematics, imperial College of Science and Techrwiogy, London S Wl.
R. A. Cox
DISCUSSION ETUDE DE LA TRANSFORMATION DE L’ANHYDRIDE SULFWREUX EN ACfDE SULFURIQUE EN RELATION AVEC LES DONNEES CLIMATOLOCIQUES DANS UN ENSEMBLE URBAIN* I READ it with interest, primarily because there is reason to feel that the health effects of sulfur oxide pollution are a consequence, to a very substantial extent,of the conversion of sulfur dioxide to sulfuric acid mist and to acid sulfates. Unfortunately, our present monitoring techniques do not report the latter in a way which is satisfactory. This paper suggests that the need for such monitoring is great and the methods are available. The major contribution of this paper has to do with the importance of black suspended matter as an agent which would, presumably catalytically, oxidize sulfur dioxide into sulfuric acid. The mechanisms for doing this are apparently related to the metal content contained in the particulate. It is important to use such studies as these under a variety of conditions, to attempt to determine what the mechanism might be. The f&ding that there is a bigher fraction of sulfur dioxide converted into sulfuric acid during low winds and low temperatures is not surprising. However, the extent of the apparent conversion at low temperatures (between 1 and 4°C) represents a new insight into this probkm. A 23 per cent * M. BENARIE,T, MENARD and A. NONAT (1973) Atmospheric Erwironment 7, 403-421.
Discussion
data, causes of persistent effects can be found. The model could indicate certain factors causing pollution effects without trying to find the physical and chemical reasons for these effects. Gnce such a causal relationship is established by the statistical analysis new research can be initiated to determine the ohvsical and chemical reason for the relationshio.__ thus a statistical model of this kind will yield further results for deeper investigations. We hope that more of such work will be published by our mathematical statisticians and that the method indicated will be practically used for the systematic processing of the data available. SELMOTAUBER
Portland State University, Portland, Oregon 97207, U.S.A.
DISCUSSION PREDICTIONS LAGOON
OF FOG FORMATION DUE TO A WARM WATER PROPOSED FOR POWER STATION COOLING*
Tms paper while reaching more or less the correct conclusions regarding the incidence of steam fog from cooling ponds contains some misconceptions whose effects have fortunately balanced out in the final analysis. First of all the restriction to situations where thermal inertia is not important is serious as sensible cooling pond design incorporates inertia as a necessary safety margin against adverse weather conditions and, more particularly, against the effects of solar heating which at certain times of the day far exceeds any art%cial heat loading. For near zero thermal inertia the resultant water temperatures could exceed statutory limits even without additional heating. The use of an “approach” based on cooling tower operation is therefore invalid on a time scale shorter than that in which the pond temperatures can respond to a change in conditions (typically 2-3 days upwards). Within the above limits there are still inaccuracies as the PARKER-KRENKELL(1970) 2°C “approach” is much too low for a cooling pond. In an unheated water body the “approach” would vary from approx. 0°C in winter to 4°C in summer in the U.K., this being the approximate differential required~ to dissipate the solar heat imput. A reasonably designed cooling pond would be expected to dissipate typically 200-300 w rn-’ of waste heat leading to a further water temperature rise of between 5 and 10°C on average, giving an overall range of “approaches” from 5 to 14”C, the actual value being a complex function of the time of year, meteorological conditions, the station load and, of course, the pond thermal inertia. Having postulated cooling pond parameters a day by day calculation is strictly necessary based on the metwrological variables. Thus in a practical case the values of AT (using the notation of the note) are much higher and more variable than specified. Furthermore, the criterion for strong steaming is based on a single observation by SAUNDERS(1964) and is not reliable. However observations on C.E.G.B. cooling ponds, to be reported, suggest that the above errors tend to cancel since, although cooling pond temperatures tend to be higher than specified, the Saunders criterion is pessimistic, higher values of AT being necessary for strong steaming. Thus the prediction of the occurrence of strong steaming are not unreasonable for a typical cooling pond within the U.K. Finally it is worth noting that the highest temperatures occur only in the immediate vicinity of the outfall owing to turbulent mixing. As steam fog is highly localized, the problem can be minimized by care in siting of a road relative to the station outfall. Scienfific Services Department, C.E.G.B., North West Region, 829, Wiimslow Road, Manchester, M20 8RU. * R. A. COX(1973) Atmospheric Environment 7,363-368.
W. MCMILLAN
664
Discussion
REFERENCES PARKER F. L. and KREKKELL P. A. (1970) Physical and Engineering Aspects of Thermal Pollution. p. 66. Butterworths, London. SAUNDERSP. M. (1964) Sea smoke and steam fog. Q. J. R. hf.ct.Sue.90. 156.
AUTHOR’S
REPLY
THE RESTRICTION to cases where the effective thermal inertia is low was made to facilitate an analysis in which it was not necessary to involve the temperature history of the air. This was considered justified because the volume of water taking part in evaporative cooling at any one time is small and so the rni~rn~ surface temperature of the lake should follow the wet bulb temperature quite closely, even though the main body of the lake has not had time to respond. The intake is submerged, however, and convective mixing in the cooler parts of the lake will mean that the cool surface layers will be well mixed before reaching the intake. Thus, as Mr. McMillan points out, thermal inertiawill be significant in cases where the lagoon is deep, especially in the region of the intake. With regard to the “approach”, it is difficult to see how the additional heat input from the power station of ZOO-300w m-Z quoted by Mr. McMilian could raise the temperature at the station intake by 5-10°C because this is about the differential required to dissipate the load if it were uniformly distributed across the lake. This is not the situation in power station cooling, where most of the cooling takes place near the outfall with correspondingly large horizontal temperature gradients there. The intake temperatures would therefore be relatively insensitive to station load since the intake is as far from the outfall as possible. Thus the “approach” would be close to its natural value and my assumption of 2°C seems to be in close agreement with Mr. McMillan’s range of O-&C for this quantity. Finally, it may be of interest to point out that in cases where the thermal inertia is considered important, the minimum surface water tem~rature should depend on the meteorologi~l history and the inertia in a fairly straightforward way and the incidence of steaming could be determined in a way similar to that described in the paper. This would, however, involve a considerably greater effort in anaiysis of the meteorological data. Department of Mathematics, imperial College of Science and Techrwiogy, London S Wl.
R. A. Cox
DISCUSSION ETUDE DE LA TRANSFORMATION DE L’ANHYDRIDE SULFWREUX EN ACfDE SULFURIQUE EN RELATION AVEC LES DONNEES CLIMATOLOCIQUES DANS UN ENSEMBLE URBAIN* I READ it with interest, primarily because there is reason to feel that the health effects of sulfur oxide pollution are a consequence, to a very substantial extent,of the conversion of sulfur dioxide to sulfuric acid mist and to acid sulfates. Unfortunately, our present monitoring techniques do not report the latter in a way which is satisfactory. This paper suggests that the need for such monitoring is great and the methods are available. The major contribution of this paper has to do with the importance of black suspended matter as an agent which would, presumably catalytically, oxidize sulfur dioxide into sulfuric acid. The mechanisms for doing this are apparently related to the metal content contained in the particulate. It is important to use such studies as these under a variety of conditions, to attempt to determine what the mechanism might be. The f&ding that there is a bigher fraction of sulfur dioxide converted into sulfuric acid during low winds and low temperatures is not surprising. However, the extent of the apparent conversion at low temperatures (between 1 and 4°C) represents a new insight into this probkm. A 23 per cent * M. BENARIE,T, MENARD and A. NONAT (1973) Atmospheric Erwironment 7, 403-421.