wh | | | | | ———— ATLL a TA VENTILATION BLOWERS BSS 8100 0905:35 008s ee | optooms. | if aa — | THESIS Con nti ¥ a i Lc - ELECTRICALLY VENTILATION DRIVEN BLOWER 5. yw Le C. Brooks. on OM 19 0 8. ee 0@0@ 0 68 @6@ Ge Se O88 Ge THESIS GENERAL. In discussing the subfJect of electrically driven blowers, the idea is to included the subject only so far as it relates to ordinary ventilation purposes, as buildings and ships, the application for forced draft for boilers, foundry blast, etc., depending upon practically the same prinbiples, though specially designed wheels are required where excessive pressuresare desired. The subject matter given has been acquired partly from personal experience and partly from collected data. Thanks are extended to the General Electric 0o., for a part of the data furnished re= garding fans. A considerable amount of material has been pub-= lished during thé past few years regarding the uses of fans, but the average commercial fan does not take into account the possible high efficiencies necessary for e- conomic results, and in many cases, the customer gets something that he does not want, and which is relatively expensive to operate, because the first cost is cheap. The relative advantages of electrically driven fans over steam driven ones are as follows. (a) Small motors are more efficient than small engines thereby allowing the use of more and smaller fang. This . greatly reduces the piping and reduces the number of open ings in the water tight bulkheads. 94246 ~2Q- (b) Small ventilation sets are more easily installed ané cared for than larger ones. (e) The network of steam pipes from the boiler to the fans is avoided, thus reducing the losses from condensa~ tion and the spaces passed through are not heated by the radiation from the steam piping. (4) A well built motor will run continuously for a long period of time without repairs, while a steam engine, es- pecially of small size, requires constant overhauling, as there are a@ number of small wearing parts. Also a motor is much more clean in operation than an engine. The past prectice has been: to use both the plenum and exhaust system of ventilation, but at present, only the plenum system is used, it being more efficient and is more satisfactory, as in a hot climate better results are obtained by driving fresh air into a compartment, and let~ ting it get out the best way it can, rather than to exhaust the heated air and having it replaced by natural supply from an unknown source. The exhaust system is used and is best adapted for wash rooms and water closets. The modern battleship contains quarters for 800 to 1000 men, beside@ magazines, store rooms, engine rooms and dynamo rooms, which must be supplied with fresh air at a rate of complete change of air varying from once in 3/4 minutes in dynamo rooms to once in 8 minutes in quarters and living spaces. To accompkish this about 100,000 cu.ft., of air must be delivered per minute, the number of fans be-~- ~ Ze ing about 50, of various capacities to suit the require~ ments of the compartments ventilated. FANS. A sketch showing capacities an@ approximate sizes of fans now in use is given in platesl and 2. The fan wheels and casings are of a built up structural steel structure, the casings being made convertible, so that in case one fan is put out of commission from any cause, it may be ree Placed by one from a less important location if desired. The plates should be of sufficient thickness and the re- inforcement of sufficient strength to resist the pressure of the air and prevent vibration. All wheels and casings should be well galvanized to preveht corrosion. The wheel hubs should be brass bushed to prevent "freezing" on to the shaft. The inlet and outlet should be of practically the same area, the inlet being circular and the outlet rectan- gular in shape. The motors should be especially weal constructed as regards hard service, rigidity, commutation, and low heate ing requirements. They should be shunt wound and capable of about 20% speed reduction by varying the field strength. The fan wheel should be fastened to an extension of the motor shaft, and the construction of the motor and the set es a whole should be such as to permit the armature being ax" ~ 1 whes removed from the commtator end. Open type motors should be used wherever possible, in order to reduce weight. The motor bearings to be of sufficient sise to carry the re- quired weight with the least possible wear and to have sufficient lubrication by oil rings. Before the fans are installed on the ship, they should be given an exhausthve test(usually 2 days run) to insure that they operate satisfactorily and give the required air deliveries. In testing for air deliveries, the following procedure should be followed, the same having been found by experiment and practice to fit operating conditions. A straight tube to be fitted to the outlet of the fan, this tube being of the size of the fan outlet, and in length equal to 20 times the average length and breadth of the outlet. A double Pitot tube designed to indicate impact pressure of moving air, also static pressure to be inserted in the center of the outlet tube. A sketch of a Pitot tube that has proven satisfactory for this work, is shown on plate 3. The two outlets of the Pitot tube to be connected to accurately reading Fook gauge manome ters. The velocity and pressure of the moving air in the pipe should be such thag the pressure side of the Pitot tube shall not be less than 13.4 times the weight per cubic foot of air in pounds and the impact side of the Pittot tube should not be less than 17.4 times the weight per cubic foot of o5e air in peunds. The weight of air should be corrected for barometric pressure and wet and dry bulb thermometer tene- peratures. Standard conditions of air being 70°F, 70% hume idity and barometric height of 30 inches, the weight being 0.07465 lbs. per cu. ft. In connection with pressures, it has been found that @ pressure of 5.2 pounds per aq.ft.,(one inch of water) is the most satisfactory for ordinary ventilation purposes, the size of pipe being such as to give 2000 to 2200 ft. velocity per min. Higher velocities than this giving ex cessive noises in the piping due to eddies and producing circulation in the living quarters that is uncomfortable. In measuring pressures, the Pitot tube has been found to give most reliable results as all forms of anemometers have*been proven entirely unreliable, care must be exere cised in making the Pitot tube to prevent the aspiration effect of the current of air across the small round hole, thereby producing eratic results. In case extreme accur- ecy is desired, a number of Pitot tubes may be inserted in the outlet. pipe, all of them being connected to one pres- sure recording manometer. However for practical purposes the one tube is sufficient. An increase of 10% above the minimum pressures noted above may be allowed. In calculating the air delivered the following de- veloped formulae are applied. ~6e W 3 weight of air in lbs. per cu.ft. Vv Volume in cu.ft. per min. V VU= Velocity in ft. per min. hl.= impact pressure in inches of water. h2 <= Static "* _ * 8 " , h3 = hl - h2 = velocity head in " w A. = area of outlet in square ft. H.P.= horse power in air delivered by fan from which Vv . 997 Ths We Ve Av HePeo 8.2 xhixvV 33000 This will give the horse power in moving air. The average fan gives an efficiency of about 40 to 45 %, from which the required horse power of the motor may be determined.By making the ouklet and inlet of the fan slightly bell mougked with special wheels, the efficiency is increased to 50 or 55 %. In fact, fans have been constructed with an effic-~ iency of 70 %, but these are not yet a commercial possi-~ bility. In case higher pressures were allowed, the eff- iciency would be increased. ~Tu PIPING. After the question of the fan has been settled, the distributing piping should receive careful attention, for & satisfactory ventilation system may not be obtained af the piping is improperly done. It is not the intention to discuss this subject in minute detail, but rather to state a few thoughts which have been found by experience to exist. Of course, the great obstacle to be overcome in a piping system, is friction. This may exist in the form of rough interior pipe, sharp turns, or improperly designed outlets and branches. It is generally understood that the frictional ree sistance of air in pipes varies as the square of the vee locity of flow, and the work of forcing the air through the pipe increases as the @ube of the increase in quantity, but that the coefficient of friction does not change with the size of the pipe or velocity of air. That is if we double the quantity of air passed in the same pipe, the work lost by friction is inoreased 8 times, and if we double the line ear dimensions of the pipe, the work lost is decreased to one fourth. The coefficient of friction for air in well constructed pipes has been found by manufacturers to be about 0.0001. As stated above the best results are obtained with air in the mains at a pressure of about 5.2 pounds per ~8~ sqeft., plus about 11/2 lbs. for velocity head and a ve- locity of 2000 to 2200 ft. per min. The angle at which the branches leave the mains may be vary from 30° to 45° with equally satisfactory results. But the air leaving the branches should not exceed 1000 ft. per min. velocity. This of course requires that the branch pipe be enlarged, This enlargement should be in the form of a cone (or taper) to prevent eddies, expanding at the rate of 11/2 inches per ft. In small pipes this may be increased to 3 inches per ft. The accepted formila for loss in head due to friction of the piping is :; H 49.5. ta V.? H = loss in head. f = Coefficient of friction. 1 = length and @ & diameter of pipe in inches or ft. V, # = velocity of air in ft. per sec. from which the size of piping required at any part of the system may be determined. In designing the mains where branches are taken off along the mains, the sise need not be changed until the ve- locity of the air reamining in the mains is reduced to 1500 ft. per min., then contract the main with a taper, as exe plained above. Where elbows are necessary in the piping, they should be made very smooth, the radius of the centre of the pipe being not less than 11/2 diameters. In figuring lengths, 1 = 90° elbow is equal to about S feet of pipe. ee? Je THEORY of FANS. A ventilation blower may be considered to be a special type of centifugal pump. As it is sometimes erron~ ously supposed that the pressure developed by a blower or pump is due to the peripheral velocity of the motor as it leaves the wheel, that is Pressure head, H & P » & brief discussion may g be in order, as a great portion of this peripheral velocity is spent in eddies, unless proper Ai Pfusion vanes are used. ¥2 Ww A aoe ~ ws : \ / | \ ' Oo _ ji _ SE] : \ ; / ~ + _7 N\ _ L -” Fig-2. In fig. 1 let w represent a body revolving about centre 0. Its mass is w and centrifugal force s wr" s uy” & gr r If a tube is filled with fluid, fig. 2, and open at the inner end, each particle exerts a centrifugal force dependent upon its mass, and distance from the centre. The ou’ sw? z Z¢g where M = mass of unit volume, assured in air calculations to be 1 cubic foot. total centrifugal force or pressure per sqeft. P ~10= 2) 2 a» ” Vv If the tube does not reach the centre, P = ¥ (Vp S é 2 | Ae KH ¥