effect of cross draughts on the exhaust air volume required for hot knock-out . the obstruction offered by the side of a mould does not shield the depressed velocity zone above the mould from disturbance by the horizontal motion of cross draughts . consequently , cross draughts can enhance the rate of diffusion of rising thermal currents and blow them sideways into exhaust air streams at a point nearer to the grid , where the exhaust air velocities are higher . it follows that the performance of down-draught systems can be improved by the influence of cross draughts only if the thermal currents are blown into exhaust air streams moving at higher velocities than the cross draughts , so that the resultant direction of all dust-bearing air streams is towards the grid . if the grid is unduly blocked on the down-wind side of the cross draughts , the thermal currents will be blown into a zone of reduced exhaust air velocities , and control of the dust-bearing air streams can be impaired , particularly if the speed of the cross draughts is high in relation to the exhaust air movement . the important conclusion is that the performance of correctly designed and operated down-draught systems for the knock-out of hot moulds is not unduly affected by cross draughts of the order usually present in foundries . obviously , high velocity cross draughts , such as may be found when the knock-out is situated near large open doors , will seriously impair their performance . nevertheless , cross draughts are so variable and unreliable that the assistance they may provide should not be considered when designing a system . effect of cross draughts on the exhaust air volume required for cold knock-out . the effect of the cross draughts is to increase the strength of the exhaust air velocities on the windward side of the grid and to reduce those on the down-wind side . since cross draughts not only diminish the exhaust air velocities on the down-wind side of the grid , but also blow the dust and fumes into this zone , it follows that the exhaust air volume must be increased by an amount that will counteract the fall in exhaust air velocities . the main distinction between the effects of cross draughts of normal velocity on thermal currents and cold air streams is that the former are deflected into exhaust air streams of unchanged or even higher velocities , while the latter are blown into weaker air streams , and therefore additional exhaust air volume is required . relationship of grid size , box height and exhaust air volume . examination of the results shown in figs 6.9 and 10 shows that the minimum exhaust air volume does not increase in direct proportion to the increase in the size of the grid . the proportional increase in air volume is , however , never greater than the corresponding increase in grid area . when considering these results it is important to remember that engineering methods of air flow measurement are not precise , and errors of 10 per cent and even more , in some cases , may occur . nevertheless , by considering a large number of test results , it is possible to distinguish two marked trends in the amount of exhaust air volume required by the 6-ft x 4-ft grid in relation to the 4-ft 6-in x 3-ft 6-in grid . ( 1 ) . increase in exhaust air volume . the exhaust air volume required by the 6-ft x 4-ft grid with the 8-in deep hot and cold moulds and the 16-in deep cold moulds tested in the absence of appreciable cross draughts exceeded the volumes required by the 4-ft 6-in x 3-ft 6-in grid by between 25 and 40 per cent . ( 2 ) . constant exhaust air volume . the exhaust air volume required by the 6-ft x 4-ft grid , with 16-in deep hot and cold moulds tested in cross draughts of 75-100 f.p.m was approximately equal to ( and in some cases even less than ) the volumes required by the 4-ft 6-in x 3-ft 6-in grid . insufficient experimental data are available to provide a complete explanation of the conditions responsible for the similarity of exhaust air volumes measured between the two grids with the 16-in deep boxes in 75-100 f.p.m cross draughts . the many variable factors present during the tests produced complex air flow conditions which do not facilitate comparison , but the resultant effect of the following two factors emerges as a predominant influence : ( a ) . the effect of cross draughts on the sideways entrainment of dust-bearing air currents from the depressed velocity zone into relatively higher exhaust air velocities near to the down-wind top edge of the moulding box . ( b ) . the effect of the grid area and , therefore , grid velocity diminishes with increasing distance from the grid until the exhaust air velocities are almost identical , regardless of the size of the grid , as explained earlier . in practice , however , the number of possible variations in the factors controlling the distance from the grid at which air velocities become constant for a given exhaust air volume is so large that the distance must be calculated afresh for each individual case . in addition to the variation in the area of the vertical gaps at the sides of the grids and in the horizontal unblocked grid area , the pattern of grid blockage may be such that the zone above the grid is divided into separate regions so far apart that the exhaust streams found in them only lose their identity at a considerable height above the top of the moulding box . the important conclusion is that the effectiveness of down-draught systems of knock-out ventilation will not necessarily be improved by changes in the size and design of knock-out grids - regardless of exhaust air volume - if the depth of the box is too great . field observations indicate that for the conditions described above , 11-in or 12-in is about the maximum permissible depth when knocking out hot , and that the blockage due to the box and sand should be less than 50 per cent of the grid area . selection and performance of down-draught systems . importance of the down-draught system - the ease with which a down-draught system of ventilation can be applied to a knock-out without interfering with other foundry operations frequently commends it to the planning engineer . the practical advantage of the absence of ventilating equipment above floor level is that all four sides of grids are available for the accommodation of foundry equipment , the movement of operators , boxes and castings , and no limitations are imposed upon the travel of cranes and hoists . the comfort of knock-out operators is greatly affected by radiant heat . the quantity of heat energy radiated from a surface depends upon its area , temperature , and radiation coefficient . since no hood and baffles are fitted and the net area of the hot grid bars is small , the source of heat radiated to operators is effectively limited to the hot casting and the mould . consequently , a down-draught system can give not only control of dust , but also less discomfort to the operators when dealing with a large number of very hot castings . limitations in the application of down-draught systems - down-draught systems can , as indicated by the experiments illustrated in figs 6.10 and 10a , and do , as shown by test 1 in table 2.2 , provide effective protection from the dust and fumes produced by relatively small castings in fairly shallow boxes . this system , therefore , finds the greatest application in highly mechanized foundries producing large quantities of light repetition castings . the down-draught system has , however , certain limitations and various factors must be considered before installing such a system . depth of boxes - thermal currents can not be reversed with economical exhaust air if the distance between the grid and the top of the boxes exceeds 12-in , unless special provision is made . boxes must always be knocked out at grid level and never turned over on rails above the grid . size of grid - the larger the grid , the greater the area of boxes that can be knocked out and , consequently , the greater the distance between the side and centre of the boxes . the size of grids for hot moulds should not exceed 4-ft 6-in x 3-ft 6-in , or 6-ft x 4-ft in special cases . shape of grid - the ratio of the grid length to width should be similar for both boxes and grid , so that exhaust air streams are concentrated around the sides of the box . height of grid above the floor - the floor restricts the direction from which replacement air can approach a grid and acts as an air baffle , so that exhaust air velocities are highest when the grid is mounted level with the floor . raised grids should not exceed 18-in in height . grid design - green sand clogs between the bars of fixed grids and restricts the flow of exhaust air . a knock-out point should not be ventilated by a down-draught system unless sand is shaken through a vibrating grid at about the same rate as it is spilt from the box . blockage of the grid - the blocked section of a grid should not greatly exceed the area of the box if the vibrating grid is efficient . the area of the box and spilt sand together should not exceed 50 per cent of the grid if the exhaust air volumes given in figs 6.9 , 10 and 10a are to be used as the design basis . experiments have shown that if the blockage is increased from 50 to 75 per cent , the minimum exhaust air volumes required to control dust and fumes are increased by amounts up to 50 per cent , or even more in some cases . air seals - it is essential for knock-out units to be provided with effective air seals . the air seals at the sand transfer point between the hopper and belt must remain effective regardless of the rate at which sand spills from the hopper . extraction of sand and fines . in the down-draught system , air is exhausted through the sand falling into the hopper . should this sand , or a large proportion of it , be completely dry , a considerable amount of the fines will be exhausted . with very high velocities the fines may be accompanied by fairly coarse grains . in consequence , the composition of the sand will be radically changed . the amount of material to be collected will be large and there may be abrasion of the ducting . the extraction of sand and fines can be reduced by consideration of the three following factors in design . usually a combination of all three is necessary : ( 1 ) . the frequency of knocking-out in relation to the size of the hopper , rate of sand removal , and location of air ducts should be determined , so that the sand inside the hopper can never rise unduly close to the air inlets . the external angle of the base of the hopper should not be less than 60 &amp;deg; . ( 2 ) . the velocity of the exhaust air close to the falling sand inside the hopper should be reduced by enlarged inlets . ( 3 ) . the air ducts in the hopper should be located and arranged so that sand does not fall directly into the exhaust inlet , and the openings should be protected by shields . in addition , the sand-to-metal ratio and the time between pouring and knock-out should be such that only part of the mould is completely dry by the time the knock-out is reached ( see chapter 3 ) . if this condition can not be fulfilled a down-draught system should not be used . sludging of sand in the exhaust air ducts . steam is released from hot moist sand moulds as they disintegrate and fall through the grid into the hopper . should this steam exceed the amount which can be retained by the exhaust air , it will condense on the exhaust ducts . sand and dust in the air stream will deposit on the moist surfaces or on any water at the bottom of the duct , forming a sludge which may eventually choke the duct to such an extent that efficient ventilation becomes impossible . the amount of water that can be retained by the air depends on the air volume and temperature . if the saturation level is exceeded , the moisture condenses to form droplets which are sufficiently small to remain in suspension as visible steam , but are readily deposited on objects with which they come into contact . 