US2570247A - Condenser - Google Patents

Description

Oct. 9, 1951 Filed Jan. 5, 1945 W. KALS CONDENSER 2 Sheets-Sheet 1 INVENTOR.

W. KA LS CONDENSER Oct. 9, 1951 2 Sheets-Sheet 2 Filed Jan. 5, 1945 INVENTOR. 60% a ATT I 361K551"? patented Oct. 9, 1951 CONDENSER Walter Kals, New York, N. Y., assignor to Niagara Blower Company, New York, N. Y., a camera'- tion of New York Application January 5, 1945, Serial No 571,388

3 Claims.

This invention relates to a vapor condenser and more particularly to an evaporative type of steam or vapor condenser in which the cooling effect is obtained principally from the evaporation of Water on the exterior of the condensing coil which is arranged in an air stream passing over the coil.

One of the principal objects of the present invention is to provide a large capacity steam or vapor condenser in which the condensation of the steam is so effected by the evaporation of water on the external surface of a wetted coil, such an evaporative condensing coil having many times the cooling capacity of a dry coil of the same size. Further, such an evaporative condenser is economical in the use of cooling water, since this water is evaporated to use the latent heat of the water in providing the cooling effect instead of merely passing the water in heat exchange relation to the steam or vapor as in the usual water cooled condenser.

Another object is to provide such a steam or vapor condenser in which the removal of the air or non-condensible gases from the steam or vapor is efiected under conditions most favorable to the operation of a steam jet air ejector in ejecting the maximum amount of air with the minimum consumption of steam.

Such air is admitted in solution with the boiler feed water and a considerable quantity of air is drawn into the condenser through leakage and provision must be made for the removal of this air from the system in order to maintain the desired vacuum.

} Another object is to provide such a steam or vapor condenser in which the air can be ejected in one or two or more stages, this depending upon the conditions of vacuum under which the steam or vapor condenser operates.

Another object is to provide a steam or vapor condenser in which a separate coil or coils are used to further cool the vapor-air mixture after it has been withdrawn from the main condensing coil and in which the coils are so arranged and proportioned to provide a balanced system with the minimum of cooling surface.

' Another object of the invention is to provide such a condenser which is in the form of a compact unit housed within a single casing and in which all parts are conveniently accessible for inspection, adjustment or repair.

Another aim is to provide such an evaporative type of steam or vapor condenser which will not foul up and which will operate at uniformly high efliciency at all times. r

air therefrom. A common shaft Another object is to provide sucha steam or vapor condenser in which the cooling effect is increased or decreased in response to changes in the load on the steam condenser.

Another object is to provide such an evaporative steam or vapor condenser in which the spray water is positively prevented from freezing or falling below a predetermined minimum at all times.

Another aim is to provide such a condenser which is simple and rugged in construction and which will stand up under conditions of severe and constant use without getting out of order or requiring repairs.

Other objects and advantages will appear from the following description and drawings in which:

Fig. 1 is a vertical longitudinal section through a steam condenser embodying the present invention.

Fig. 2 is an end elevation thereof, partly broken away.

Fig. 3 is an enlarged fragmentary sectional view through the condensate outlet pipe from the main condensing coil, this section being taken on line 3-3, Fig. 2.

Fig. 4 is a fragmentary view similar to Fig. l and showing a modification of the invention in which the steam condenser has only one air ejector as compared with the two stages shown in Figs. 1 and 2.

The steam or vapor. condenser is shown as housed within a sheet metal casing ID of rectangular form, the bottom of which is closed to form a tank or sump II which contains a body I 2 of water, which water is evaporated to provide the cooling effect. A plurality of fan housings I5 are mounted in the upper part of the casing 10, the inlets of these fan housings being in communication with the interior of the casing l0 so as to exhaust 16 extends through the several fan housings l5 and is shown as driven by a pulley l8. Within each fan housing the shaft It carries a fan 20 of any suitable construction, these fans drawing the air from the interior of the casing l0 and discharging it through outlets 2| which project horizontally outwardly through one side wall of the casing Ill and toward a horizontal air discharge duct 22. This same wall of the casing I0 is provided at its lower end with an air inlet opening 23 communicating with a horizontal air intake duct 24 and through which the fans draw atmospheric air into the casing I 0.

The discharge duct and intake duct are connected by a recirculation air duct 25 so that a regulated proportion of air can be recirculated from the outlet to the inlet of the casing. For this purpose dampers 2B are provided in the air discharge duct 22, dampers 28 are provided in the air intake duct 24, and a damper 29 can be provided in the recirculation air duct 25. The recirculation air damper 29 is operated in reverse order to the exhaust air and inlet air dampers 23 and 28, that is, as the recirculation air damper 29 opens the fresh air and exhaust air dampers 26 and 28 close and vice versa. For this purpose the shafts of these dampers extend through the commonwall of their respective ducts at one end of the casing and are each provided with a lever arm 30 which lever arms are interconnected by links 3|, 32. This linkage and hence the dampers 26, 28, 29 are actuated by a damper motor 33.

Two or more banks of coils are arranged in the air stream passing through the casing Ill and water is discharged and distributed to wash the exterior of these banks ofcoils to evaporate and absorb heat therefrom, For this purpose the banks of coils are wetted from overhead spray nozzles 35. These nozzles are shown as mounted on the branches 36 of a longitudinal spray water pipe 38 which extends through one end wall of the casing 10. Thisspray water pipe connects with the outlet of a spray water pump 39 which withdraws water from the sump or tank H at the bottom of the casing l0. Makeup water can be supplied to this body l2 of water in the sump II in any usual and well known manner.

The usual eliminator plates 40 are arranged above the sprays 35 to remove entrained water from the air stream before entering the fans.

The steam or vapor to be condensed is admitted through an inlet manifold 4| which connects with a plurality of inlet headers 42 of a main condensing coil 43. arranged within the casing ID. This main condensing coil 4.3 has a series of pitched tubes 44 which connect with a plurality of outlet headers 45 arranged directly on the opposite side of the casing. The tubes 44 are pitched so as to drain into. the headers 45. Each of these outlet headers has a horizontal outlet pipe 43 of relatively large diameter which extends through the adjacent casing end wall and discharges into. one or more vertical pipes 48 also of relatively large diameter and hereafter referred to as an air removal header. The. lower end of this vertical air removal header 4.8 can connect with a hot well (not shown) and the upper end thereof extends above the main condensing coil 43 and connects. with a horizontal pipe 49 extending through the adjacent end wall of the casing I and forming the air-vapor mixture inlet and condensate outlet for the lower lower header 50 of an air-vapor mixture cooling coil hereinafter referred to as an air devaporizing, coil. The tubes 52, of this air. devaporizing coil 5| pitch upwardly to an upper header 53 of this coil, this upper header 53' being shown as arranged adjacent the opposite end wall of the casing ID.

The air-vapor mixture outlet 54 from the header 53 extends through the adjacent end wall of the casing I0 and connects with the inlet of a firststage steam jet air ejector 55 which is supplied with steam from a steam line 56, The steam jet air ejector 5.5- discharge's, at an intermediate vacuum, the air-vapor mixture withdrawn from the coil '5.| into-the adjacent upper header 58 of an intercondensing coil 59.. The tubes 60 of this intercondensing coil 59pitch downwardly to a lower header 5| arranged ad,- jacent the first end wall of the casing I0 and ill having a horizontal outlet pipe 62 extending through this end wall of the casing. This outlet pipe 62 connects with a vertical air removal header 53, the lower end of which can connect with any suitable form of vacuum drain (not shown) to remove the condensate from the intercondensing coil 59, or this condensate can be returned to the hot well in any conventional manner.

The upper end of the vertical air removal header 63 connects with a horizontal pipe 64 which forms the air-vapor mixture inlet and condensate outlet for the lower header 65 of a second stage air devaporizing coil 66. As with the first stage air devaporizing coil 5|, the tubes 68 of this second stage air devaporizing coil 66 pitch upwardly to an upper header 69, this upper header being shown as arranged adjacent the opposite end wall of the casing Ill.

The air-vapor mixture outlet 10 from the header 69 extends through the adjacent end wall of the casing Ill and connects with the inlet of a second stage air ejector 1| which is supplied with steam from the steam line 56. The air ejector discharges, at atmospheric pressure, the air-vapor mixture withdrawn from the coil 56 into the adjacent upper header 12 of an atmos pheric or above pressure aftercondensing coil' 13. Since the temperature of the. vapor in this at.- mospheric aftercondensing coil is at 212 F., corresponding to atmospheric pressure, it is unnecessary that this aftercondensing coil be sprayed with water to provide an evaporative cooling effect and this aftercondensing coil can be located above the eliminator plates 40, as shown. The tubes 14 of this coil 13 are prefer-.- ab-ly finned hairpin tubes which pitch downwardly to anoutlet header 15 which isshownas arranged under the header [2. and as having an atmospheric drain l6. 7

To insure complete separation of thev streams of condensate from the coils 43 and 5| and the air-vapor mixture from. the main condensing coil 43, the horizontal pipes 46 and- 49 and the vertical air removal header 48 connecting these pipes with the hot well are sufiiciently lar e to provide an open channel flow for the condensate. That is, the condensate flows, through these pipes and air removal header in an open stream of considerably smaller cross sectional area than the pipes and header so as not, to impede. the free flow of the air-vapor mixture. in any direc tion through thispiping, as illustrated in Fig. 3. Further, as illustrated in Fig. 3, the flow of the air-vapor mixture countercurrent to the flow of condensate can be facilitated by aperforated tube 83- in the vertical air removal header 4.8, which tube is open at its bottom and has. lateral hori-L- zontal branches 8i. and 82, the branches. 8| ex.-

tending into. the upper parts of the horizontal pipes 43, of the outlet headers 4.5 and the. branch 82 extending into the upper part of the pipe 49 of the outlet header 50,

The damper motor 33 controlling the. amount of air recirculated through the casing ||1 is shown as under controlv of a. pressure actuated controller or pressurestat 85 arranged in the steam inlet 4| and responsive to changes in the steam pressure therein and hence responsive. to changes in load upon the condenser. Further, to insure a minimum temperature. of this spray water under all operating conditions, particularly to avoid freezing in the wintertime, steam from the steam line 56; can be injected into, the body l2 of spray waterin the tank [1- through a steam injector 86, the injection of such steam 5. being shown-as under control of a thermostat 88 in the body 2 of spray water.

In the condenser as above described, the airvapor mixture from the main condensing coil 43 is ejected by the air ejectors 55 and H in two stages, intercondensing and air .devaporizing being provided between the two stages. For a vacuum less than 26.5 inches two stage separation may become unnecessary, and a single stage of air ejection can then be provided as shown in Fig. 4. As shown in this figure, the outlet from the first air ejector 55 is connected directly with the inlet header (2 of the atmospheric aftercondensing coil '|3, the second stage devaporizing and intercondensing coils and air ejector being omitted. Since in other respects the single stage air ejection system shown in Fig. 4 is the same asthe double stage air ejection system shown in the preceding figures, the same reference numerals have been applied.

Operation In the operation of the steam condenser, atmospheric air is drawn into the casing ||l through the inlet 23 from the inlet duct 24 by the fan wheels 20 and discharged through the outlet duct 22. from the fan outlets 2|. The stream of air so drawn upwardly through the casing I successively passes the tubes of the main condensing coil 43, first stage air devaporizing coil 53, intercondensing coil 59, and second stage air devaporizing coil 66, following which the air stream passes the water spray nozzles 35 and eliminator plates 40 and then passes the tubes of the atmospheric aftercondensing coil 13.

At the same time water from the body l2 of water maintained in the sump or tank II at the bottom of the casing II] is withdrawn by the spray water pump 39 and forced through the spray water pipe 38 to the nozzles 35 which spray this water over the coils 66, 59, and 43, the excess spray water collecting in the sump or tank These coils are therefore maintained in a wetted condition so that the withdrawal of heat from the steam or air-vapor mixture passing through these coils is principally effected by the evaporation oi the water on the surface of these coils into the passing air stream.

The steam enters the inlet headers 42 of the main condensing coil 43 through the steam inlet manifold 4|, condensation of this steam taking place in the tubes 44 of this main steam condensing. coil 43 and this condensate flowing as an open channel stream along the bottom of the horizontal pipes 46 from the outlet headers 45 into the vertical air removal header 4B which leads to the hot well (not shown).

The steam entering the main steam condensing coil 43, however, contains a very considerable quantity of air, this air being admitted insolution with the boiler feed water and also entering through leakage in the section of the system maintained under vacuum by the steam condenser. This air mixes with the steam or vapor in any proportion if the steam is at the saturation temperature corresponding to the prevailing pressure.

The air ejectors 55 and H perform best when handling an air-vapor mixture containing the least amount of vapor. This is because the air ejectors handle a certain weight of gaseous fluid and hence the portion of vapor in this air vapor mixture greatly reduces the air removal capacity of the air ejectors.

Accordingly, the present inventionproposes to use a separate air devaporizing coil 5| arranged above the" main steam condensing coil 43 and through which the air-vapor mixture from the main condensing coil 43 passes before reaching th air ejector. Thus, the steam enters the in let 4| of the main steam condensing coil 43 and in passing through the tubes 44 the vapor is condensed, the condensate flowing from the lower outlet headers 45 in an open channel stream, as indicated in Fig. 3 to the large vertical air removal header 48 where it flows along the walls of this air removal header, still in an open channel stream, to the hot well (not shown). Assuming that the main condenser 43 temperature is in the order of 126 F. at an absolute pressure of 2 pounds, the air-vapor mixture leaving the outlet pipes 46 of this main condenser 43 will have this same, or. a slightly lower temperature.

The greater part of this air-vapor mixture from the main steam condensing coil 43 will flow into the horizontal branches 8| of the vertical tube and flow upwardly through this tube 80 and through its upper horizontal branch 82 into the lower header 50 of the air devaporizing coil 5|. That part of the air-vapor mixture which does not enter the horizontal tubes 8| flows into the horizontal inlet 50 of the air devaporizing coil 5| either directly, through the vertical air removal header 48, or through the perforations of the vertical tube 88. The purpose of the tubes 80, 8| and 82 is to secure complete segregation of the upwardly moving stream of air-vapor mixture from the downwardly flowing stream of condensate and may be omitted.

In flowing through the tubes 52 of the air devaporizing coil 51 the temperature of the airvapor mixture will be reduced to, say, 118 F. and a corresponding part of the vapor of this airvapor mixture will be condensed and flow from the lower header 50 and horizontal pipe49 into the vertical air removal header 48 to the hot well. Since the surface of the air devaporizing coil 5| is quite large for the relatively small load, the airvapor mixture in this coil will be cooled very effectively, this in turn resulting in a low vapor content of the air-vapor mixture drawn into the air ejector 55 and hence provide the optimum operating condition for this air ejector as previously described. The air-vapor mixture leaving the air devaporizing coil 5| will contain only so much vapor as will saturate this mixture at the prevailing temperature and pressure.

This first stage air ejector 55 compresses the saturated air-vapor mixture from the air devaporizing coil 5| to an intermediate vacuum at which steam from this air ejector will condense. Under the assumed condition of operation the temperature in the intercondensing coil 59 would be in the order of 153 F. at a pressure in the order of 4 pounds absolute. Condensation therefore takes place in this intercondensing coil 59, the condensate leaving the bottom connection 62 of its lower header 6| into the vertical air removal header 53 which is connected to a vacuum drain (not shown).

The air-vapor mixture from the intercondens-e ing coil 59 is drawn upwardly through this vertical air removal header 53 and through the horizontal pipe 64 into the second stage air devaporizing coil 66. As with the first stage air devaporizing coil 5|, the surface of this second stage air devaporizing coil 66 is quite large for its small load and hence the air-vapor mixture passing therethrough is cooled very effectively to aitemperature in the order of F. The com.

paratively' small amount of condensate condensed in this second stage air devaporizing coil 66 discharges into. the vertical air removal header 63 to mix with the condensate from the intercqndensing coil 59 flowing to the vacuum drain. In the vertical air removal header 63 the condensate flows downwardly, while the air-vapor mixture flows upwardly. As with the pipes 46 and 49 and air removal header 48, the pipes 62, and 64 and air removal header 63 can be equipped with the perforated tube 80 and branch tubes 8! and 82 to segregate the flow of condensate from the flow of the air-vapor mixture.

The mixture in the second stage air devaporizing coil 66, of air saturated with vapor at the prevailing temperature and pressure is drawn into the second. stage air ejector I I. The second stage air ejector H discharges into the atmospheric aftercondensing coil 73, the temperature in this aftercondensing coil being, of course, in the order of 212 F. At this temperature the atmospheric aftercondensing coil need not be wetted to provide evaporative cooling and can be located above the eliminator plates 46, as shown. The air reaching this coil is sufficiently low in temperature to effect the necessary cooling. The condensate leaving the lower header 75 of this aftercondensing coil 13 through the outlet 16 can be conducted'to an atmospheric drain.

Since the condensation of the steam is effected through evaporative cooling, it will be seen that the amount of cooling done by the steam condenser embodying the present invention depends upon the wet bulb temperature of the air entering thecasing l6 and drawn upwardly past the several banks of coils. It will further be seen that the amount of cooling done can be regulated by adjusting the wet bulb temperature of theair entering the casing 16 and that thiscan be regulated by varying the proportions of recirculated air returned from the air discharge.

duct 22 to the air intake duct 24 through the recirculation air duct 25. Thus, the air leaving the casing through the air discharge duct 25 is warm and humid due to its having passed the .1

wetted coils heated by the steam being condensed. When a regulated amount of this warm humid leaving air is added to the fresh air entering the fresh air inlet duct Zfithewet bulb temperature of the air passing through the casing i6 is raised and hence less evaporative cooling takes place on the wetted coils. The regulation of the cooling done bythe condenser is made responsive to the load, greater cooling being effected under heavy load' conditions, and less cooling being effected under light load conditions. While various forms of controls can be provided for so regulating the cooling done by the condenser, this is shown as efiected by the pressurestat 85 in the steam inlet and which actuates the damper motor 33 to control the dampers 26-; 28 and 29. As the pressure in the steam inlet 4*I' rises under a heavy load condition, the pres surestat 85- actuates the damper motor 35;" to open the dampers 26 and 28 and-close the recirculated airdamper'zfl; Hencea greater proportion ofslow wet bulb outside air'is admittedfrom the air intake duct 26 to the casing andhence' more evaporation of the spray water-takes place on-the'wetted'coil's and greater cooling is effected; Conversely; with. a lowering load; the pressure actuated instrument 85' in the steam inlet 41': actuates the damper motor'33 to 'close'th'edampe. ers= 2B: and 2B in the air intake and dischargeducts 214". and'IZF-andopen the damper 29: the

8. recirculation air duct 25.- Consequently, the air admitted through the inlet 23 of the casing 16 is more humid and less evaporative cooling is efiected thereby to balance the operation of the steam condenser to this lighter load.

To maintain a minimum temperature of the spray water I2 under all conditions of operation; particularly to prevent freezing, when the temperature of this spray' water drops to the set ting. of the thermostat 88, this thermostat ac tuates the steam injector 86 to inject steam from the steam line 56 into the body l2 of spray wa' ter in the bottom of the casing II) and thereby prevent its temperature from falling. below the desired minimum.

The operation of the form of the invention shown in Figs. l'-3, as above described, is ofa two stage air ejection system, the two air ejec tors 55 and H being provided for this purpose and the intercondensing coil 59 and second stage air devaporizing coil 66 being interposed between these two air ejectors. Such two stage air ejection is customary where a vacuum greater than 26.5 inches is being maintained. Where so high a vacuum is not being maintained, the single stage air ejector system shown in Fig. 4 is cu's: tomary. It will be noted that in this single stage air ejection system the single ai'r ejector 55 charges directly into the atmospheric pressure aftercondenser 13 thereby to eliminate the inter condensing and sec-0nd stage air devaporizing coil 59 and 66, together with their vacuum drains}. the form of the invention otherwise being iden tical to that shown in Figs. 1-3.

From the foregoing it will be seen that the present invention provides for an open channel flow of the condensate to permit the ready take-' off of the air-vapor mixture remaining a'fter condensation takes place. Further, by the useoran air devaporizing coil in the same casing as the condensing coil and also evaporatively'cooled'; this residual air-vapor mixture is continuously removed andits water vapor content reduced be fore being admitted to the air ejector. Also by placing the air devaporizing coil above the condensing coil, the condensate from both coils candrain into a common channel. It will further be seen that where the vacuum'maintained requires two stages of air ejection, the intercondensing and second stage air devaporizing coils between the" two air ejectors can function in the same manner. and obtain the sameadvantagesas" the main condensing and first stage air devapor izing coils; The regulation of the amount of recirculated air admitted to the inlet of the cas ing also balances the cooling effect of the condenser to the load and under all conditionsa? minimum temperature of the spray water" is maintained.

I claim as my invention:

1. In a condenser for vapor containingnoncondensible gases, means forming a casiii' means passing a stream of atmospheric a throughsaid casing, a main condensing coil said casing in the path of the stream of airpass ing therethrough and'having an inlet header and an outlet header, means for admitting the mix ture of vapor and non-condensible gases into said inlet header, a gas'devaporizing coil in said casingabovet said: main condensing coil and the path of the. stream of air passing therethrough' and having an inlet header arranged above said'outleti header of said main condensing coil and an outlet header, a vertical condensate drain pipeconnectin'gtheoutlet of said: outlet header of said main condensing coil with the inlet of said inlet header of said gas devaporizing coil, a vertical tube in said vertical condensate drain pipe and having horizontal branches extending into said inlet and outlet to conduct the mixture of vapor and non-condensible gases from said main condensing coil to said gas devaporizing coil, and a gas ejector removing the non-condensible gases together with any remaining vapor from said outlet header of said gas devaporizing coil.

2. In a condenser for vapor containing noncondensible gases, means forming a casing, means passing a stream of atmospheric air through said casing, a main condensing coil in said casing in the path of the stream of air passing therethrough and having an inlet header and an outlet header, means for admitting the mixture of vapor and non-condensible gases into said inlet header, a gas devaporizing coil in said casing above said main condensing coil and in the path of the stream of air passing therethrough and having an inlet header arranged above said outlet header of said main condensing coil and an outlet header, a vertical condensate drain pipe connecting the outlet of said outlet header of said main condensing coil with the inlet of said inlet header of said gas devaporizing coil, a vertical tube in said vertical condensate drain pipe and having horizontal branches extending into said inlet and outlet to conduct the mixture of vapor and non-condensible gases from said main condensing coil to said gas devaporizing coil, said vertical tube being perforated to receive vapor and non-condensible gases from said vertical condensate drain pipe, and a gas ejector removing the non-condensible gases together with any remaining vapor from said outlet header of said gas devaporizing coil.

3. In a condenser for vapor containing noncondensible gases, means forming a casing, means passing a stream of air through said casing, a main condensing coil in said casing in the path of the stream of air passing therethrough and having a sufiiciently large area of heat exchange surface to effect substantially complete condensation of said steam and also having an inlet header and an outlet header, means for admitting the mixture of vapor and non-condensible gases -into said inlet header, a gas devaporizing coil arranged in said casing above said main condensing coil and in the path of the stream of air passing therethrough and having a substantially smaller area of heat exchange surface than said main condensing coil and also having an inlet header arranged above said outlet header of said main condensing coil,

a separate vertical condensate drain pipe arranged in spaced relation to said outlet header of said main condensing coil and to said inlet header of said gas devaporizing coil and extending a considerable distance above said outlet header of said main condensing coil to provide a disengagement space for the saturated non-oondensible gases, a horizontal pipe connecting an intermediate part of said separate vertical condensate drain pipe with said outlet header of said main condensing coil and through which the condensate and the released, saturated non-condensible gases flow from said main condensing coil into said separate vertical condensate drain pipe, another horizontal pipe connecting the extreme upper end portion of said separate vertical condensate drain pipe with said inlet header of said gas devaporizing coil and through which said released, saturated non-condensible gases from said separate vertical drain pipe flow into said inlet header of said gas devaporizing coil to be cooled below the dew point thereof and reduced in weight, and through which the condensate from said gas devaporizing coil flows from said gas devaporizing coil into said separate vertical condensate drain pipe, said separate vertical condensate drain pipe and horizontal pipes being sufiiciently large to provide an open channel flow of the condensate therethrough and in said separate vertical drain pipe and said another horizontal pipe directly countercurrent to the flow of said released, saturated non-condensible gases and providing effective separation of these two media, and a gas ejector at the outlet end of said gas devaporizing coil and removing the non-condensible gases of reduced weight thererom.

WALTER KALS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 21,761 Ashley Apr. 8, 1941 Re. 22,533 Olstad et al. Aug. 22, 1944 1,046,303 Josse et al Dec. 3, 1912 1,651,900 Pagel Dec. 6, 1927 1,730,350 Bell Oct. 8, 1929 2,170,802 Bowman Aug. 29, 1939 FOREIGN PATENTS Number Country Date 237,893 Great Britain Nov. 26, 1925 226,321 Germany Apr. 30, 1921

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