On the local dissipation of natural fog

Abstract

It is submitted that the hazards of landing aircraft in fog could be greatly reduced by providing relatively small clearings into which approaching planes could be safely guided by radio or other navigational aids which are now available. The same general method might also be used to facilitate the entrance of ships into fog-bound harbors or docks. In the first section of this paper the properties of fog which are of importance in the discussion of methods of fog dissipation are summarized on the basis of measurements made at Round Hill. The minimum dimensions of a cleared space of useful size are taken as 500 to 1000 meters long, 30 to 50 meters wide and 10 to 20 meters high. From extensive investigations at Round Hill it is known that in order to maintain a clearing of this size under typical wind conditions, fog must be cleared at a minimum rate of about 2000 cubic meters per second. This figure is used for all subsequent computations. It is pointed out that the known methods of fog dissipation can be divided into two general classifications: (1) those in which the fog particles, are physically removed from the air, and (2) those in which the particles are evaporated in the air. Numerous specific methods are then described and critically examined with respect to their ability to provide cleared air at a rate of 2000 cubic meters per second in a reasonably practical manner. The more important methods considered involve the use of intense sound fields, charged or uncharged falling particles, electrical precipitation, mechanical precipitation, evaporation by heating and evaporation induced by the condensation of atmospheric water vapor on hygroscopic particles. It is concluded that the evaporation methods as a class are superior to the physical removal methods because they lower the relative humidity of the cleared air and thereby greatly reduce the limiting effects of atmospheric turbulence which act to "fill in" the cleared space. The method involving the condensation of water vapor by means of calcium chloride is chosen as being probably the most practical of the fog dissipation methods considered. The second section of the paper presents a detailed examination of one application of the calcium chloride method of fog dissipation. In this method drops of a saturated solution of calcium chloride are released above the volume of fog which is to be cleared. These hygroscopic drops, which are large enough to fall fairly rapidly, condense a suffcient quantity of water vapor from the air through which they descend to effect the evaporation of the fog particles. The investigation of this method of fog dissipation is divided into three parts. The first part deals with the determination of the relative humidity required to cause the evaporation of the fog drops as a function of the time of evaporation and the size of the fog drops. The second part is concerned with the rate of condensation of water vapor on drops of calcium chloride solution as a function of the drop size and concentration. In the third part the criteria for the selection of the optimum size of the solution drops are presented and the development of spray nozzles capable of forming drops of approximately the desired size is described briefly. Finally, the quantity of calcium chloride solution required for the dissipation of fog under typical conditions is computed. It is found that, with the best available spray nozzles, fog can be dissipated at a rate of 2000 cubic meters per second (suffcient to maintain a cleared space of useful size under typical conditions) by spraying from 4 to 5 liters of saturated solution per second. It is concluded that the method is practicable on the scale proposed. The third section of the paper is an account of the design and successful operation of a fullsized experimental fog dissipator operating according to the method described in the second section. The major considerations which influenced the determination of the size and spraying capacity of the apparatus are summarized and the essential features of the actual installation are described. The test procedure is outlined and the average results of eight successful tests conducted during a period of two and one-half years are indicated. The tests were made at air temperatures ranging from 4° to 20°C and at wind velocities up to 7 meters per second. The clearings formed were usually from 500 to 700 meters long, 30 to 50 meters wide and 15 to 20 meters high. After the installation of an improved type of spray nozzle clearings of the same size were maintained by spraying only 5 liters of saturated calcium chloride solution per second. The data obtained in two typical fog dissipation tests are presented in detaiL. It is found that the experimental results are in excellent agreement with the computations presented in the second section of the paper. It is concluded that the local dissipation of natural fog by means of sprayed calcium chloride is entirely feasible. Certain practical disadvantages of the experimental installation are discussed and a new type of apparatus which has recently been constructed to overcome some of these limitations is briefly described. Methods for practically eliminating the corrosive action of the calcium chloride solution are also noted. The fourth and final section of the paper describes a new type of apparatus in which the general method of fog dissipation by means of hygroscopic particles is applied in a different manner. By substituting finely-divided calcium chloride powder for the relatively coarse spray it is possible to confine the hygroscopic material entirely within the dissipating apparatus which is constructed in the form of a short tunnel. The spent hygroscopic material is removed from suspension by means of a special eliminator and only cleared and dehumidified air is discharged. A powerful engine-driven blower facilitates proper distribution of the cleared air under all wind conditions. The advantages of this type of apparatus in comparison with that described in the third section are: the absence of an external spray of calcium chloride, its independence of wind velocity over a considerable range, and its smaller size which reduces the obstruction hazard and permits it to be made mobile. From the results of tests with the spray-type fog dissipator it was known that the new apparatus should be capable of reducing the relative humidity to 90% in 2000 cubic meters of fog per second in order to maintain a clearing of useful dimensions under typical conditions. A unit of excessive size would be required to handle this quantity of air. However, it is possible to remove a suffcient quantity of water vapor from a fraction of the air so that the required relative humidity of 90% can be produced in the total volume of air by proper admixture of the dried portion. A commercial calcium chloride powder was selected as the most suitable hygroscopic material and computations were made to determine the quantity of powder required and the time necessary for it to act. In order to check these computations, which involved several simplifying assumptions, and also to develop the essential features of the proposed apparatus, a working model was set up outdoors. Results from the tests with the model are in fairly good agreement with the computations. It was determined that the lowest practical exit relative humidity is about 50%. Since a relative humidity of 90% suffces for the dissipation of fog only one-fifth of the total volume of air to be cleared need be handled by the apparatus on this basis. The eliminator which mechanically removes the spent calcium chloride particles from the dehumidified air is an important part of the apparatus. After numerous tests on typical eliminators it was found necessary to develop a new type which would be effective at the required high flow velocities. The important problem of properly distributing the dried air was studied with the aid of a large mobile blower unit. It was concluded that, although it would be preferable to employ a number of appropriately spaced discharge ports when-possible, a satisfactory distribution could be effected from a single large opening by using a discharge velocity of from 20 to 30 meters per second. Preliminary designs for two units of the new type are presented to show that the size, weight, blower power requirements and quantity of calcium chloride required for apparatus capable of maintaining cleared spaces of useful size are not unreasonable.