US2636485A - Closed feed system for steam power plants - Google Patents

Description

April 1953 I HILLIER Q 2,636,485

CLOSED FEED SYSTEM FOR STEAM POWER PLANTS Filed June 15. 1948 :5 Sheets-Sheet 1 April .1953 H. 'HILLIER 2,636,485

' CLOSED FEED SYSTEM FOR STEAM Pom PpAN'rs Filed June' 15, 1948 s S he ets-Sfieet z CONDENSER April 1953 H. HILLIER 2,636,485

CLOSED FEED SYSTEM FOR STEAM POWER PLANTS Filed June 15, 1948 3 Sheets-Sheet 3 Patented Apr. 28, 1953 CLOSED FEED SYSTEM PLAN S Harold Hillier, Glasgow, Scotland, assignor to G. & J. Weir Limited British company FOR STEAM POWER i T Glasgow, Scotland, a

Application June 15, 1943, Serial No. 33,135

In Great Britain June 20, 1947 6 Claims. (c1. 122 45 1) This-invention relates to closed feed systems for steam power plants each including a steam generator, a turbine, a steam condenser, the associated feed water and steam connections, and more particularly to such feed systems in which a separate de-aerator is incorporated.

An object of the invention is to provide an arrangement whereby such a closed feed system can be operated entirely automatically in a stable and-satisfactory manner, and whereby two or more such closed feed systems can be operated entirely satisfactorily and stably in parallel, with a minimum of flow of feed water into and out of the system or systems.

A closed feed system, in accordance with my invention, comprises, in its simplest form, a condenser receiving steam exhausted from a turbine, a condenser extraction pump for withdrawing the condensate from the condenser, a de-aerator into which the condenser extraction pump discharges the condensate, a de-aerator extraction pump withdrawing the de-aerated feed water from the de-aerator and discharging to a feed pump which discharges the de-aerated feed water into the boiler, and the necessary interconnecting pipework between these units.

The condenser is devised to remove corrosive gases from the feed water passing through the condenser, while the condenser extraction pump withdraws such water from the condenser and discharges it into the de-aerator, wherein it is subjected to a further de-aerating action so as to ensure that all the corrosive gases in the feed water are removed before the water is delivered to the boiler.

I arrange for the make-up water admitted into the system from the feed tank to be passed into the condenser, wherein it is subjected to a deaerating action, subsequent to which it then passes through the de-aerator, wherein it is sub jected to a further de-aerating action.

To ensure the stable operation of the condenser extraction pump and the de-aerator extraction pump and provide for the automatic control of the discharge of overflow water to the external reservoir or feed tank and the admission of make-up water from the external reservoir into the feed system, float-controlled valves are provided, controlled by floats arranged in the condenser and in the de-aerator, and actuated by the rise and fall of the Water levels in the condenser and deaerator, respectively.-

The closed feed system according to the invention is designed particularly for use on board ship where rapid changesin boiler load occur, when 2 the. ship is manoeuvring, such rapid changes causing wide variations of the flow of feed water through thefeed system.

Ina practical construction, the base of the condenser is 7 provided with a float-controlled valve which regulates the flow of feed water discharged by the condenser extraction pump,

whereby a fall in water level in the condenser below a predetermined intermediate level reduces the rate or" feed flow discharged by the condenser extraction pump andvice versa. The same float actuates also an overflow feed valve whereby, on a rise of water level above said predetermined level, water is discharged from the feed system to an external feed tank by the condenser extraction pump at a relatively low temperature.

There is also provided in the base of the deaerator a float which controls the opening of an inlet valve in the discharge line of the condenser extraction pump to the de-aerator, the said valve being so arranged that it opens progressively as the water levelfalls from apred'etermined level in the base of the de-aerator.

The same float or a separate float may be used to actuate the opening of another valve which controls the-flow of make-up water from the external feed tank to the main condenser when the water level in the'de-aerator falls below a predetermined level, which may be a level corresponding to the level at which the float-controlled inlet valve is wide open.

To enable two or more such systems to operate in parallel, cross-connecting pipes with isolating valves are provided between the external reservoirs or feed tanks in the respective systems, between the discharge lines of the condenser extraction pumps, and between the discharge lines of the de-aerator extraction pumps.

In order that the invention may be more clearly understood, it is hereafter described with reference to the accompanying drawings in which:

Fig. 1 shows diagrammatically the boiler feed system provided with means for supplying deaerated waterin accordance with the invention.

Fig. 2 shows diagrammatically the boiler feed system provided with means for supplying deaerated water in accordance with the invention, but slightly modified relatively to Fig. 1.

Fig. 3 shows diagrammatically two or more boiler feed systems provided with means for supplying de-aerated water in accordance with the invention connected so as to enable them to operate satisfactorily in parallel and automatically at all conditions of loading.

Referring to Fig. 1, the steam condenser l is oi! the regenerative type, with a well 2 in the base of the condenser. The condensate extraction pump 3 draws water from the well 2 through a conduit to and discharges it through a conduit 31) provided with a non-return valve 30 and a floatcontrolled discharge valve 4 which will be fully open with the float 5 at; a predetermined inter-. mediate water level in the condenser, closing progressively as the water level in the condenser well 2 falls until the valve 4 is completely closed at a predetermined low level. The float 5 on rising above the predetermined intermediate level, opens an overflow valve 6 whereby feed water is discharged from the surplus feed system by the condensate extraction pump 3, through pipe 31), a connecting pipe 6a and the overflow valve 5. by way of a pipe I to the external feed tanl; 8. The size of the overflow valve 6 is proportioned to pass a maximum of say 30% of the maximum feed flow required by the boiler 9 at full power.

The discharge from the, condensate extraction pump 3. through the valve l ledby aconduit 3d through an air ejector i0, where it condensesthe ope in steam omih r e ec or l 1- he opcra in t a is su p ed to. t e i ej c or H, by way o a v v d pipe 12, and the r e ect r W thdraws. air and non-condensable gases. from the condenser i by way of a pipe L3, The condensed air elector steam can be drained to the main condenser l by way oi a pipe, 114, while the air and gases are discharged to the atmosphere through a trap, in the. usualmanner, not shown While a single stage ejector is, Shown for simp ity t will. be u de tood; ha e electo ma be o h wo-s ge or h e-stage ty e, with the corresponding app opr at pipin connece me The condensate passed; through the air ejector i condenses the operating steam, from the air ejector H and conserves the heat in the feed water. The feed water may also pass through other heatexchangers for the cooling of drainage water, condensation of evaporator vapour or con densat-ion of the auxiliary exhaust steam or bleed steam, such heat exehangers being omitted; from the diagram in the interest of simplification. After passing through such heat exchansers the feed water is discharged through a de-aerator float-controlled inlet valve l which is actuated by a float l6, After passing through the valve 15, the feed water passes in a continuation on d t r h. a hea x ha er 1. to s ray n e ce r anged t he we o he tie- aerator 5,8.

The tie-aerator I8 is provided with an air ejector l9 supplied with operating; steam through a valved pipe 20, the air being drawn through a pipe 2% while the discharge from the air ejector i9 is. passed through a pipe 200 and the steam condensed in the heat exchanger I611, the resultant condensate being drained through a pipe 20d. into the drain trap 21m from which it flows into the de-aerator 18. The air in the trap 29a is discharged to the atmosphere in the usual manner. The heat in the operating steam of the air ejector i9 is thereby conserved in the feed water passing throughthe heat ex h nge fiao its way tothenspraying devices H in the top of the de-aerator 18. Heating steam for the deaerator is supplied through the pipe 2|, means being provided within the de-aerator to ensure intimate contact between the heating steam supplied and the fee d wa spr ng, he. deaerator.

A afi i l fid aist e .2. forms base of the de-aerator 18, the float I6 being arranged in the feed tank 22, so that the valve 15 is closed at a predetermined high level in the feed tank 22 and is progressively opened as the water level in the feed tank 22 falls to a predetermined intermediate water level, at wh ch point the float it. has caused the inlet valve 5 to be wide open.

The feed tank 22 is so proportioned that from the predetermined intermediate water level there is contained a suffici'ent volume of water to supply the weight of water required by the boiler 9 when ity is changed in load from full-load to noload, Another float 23 is arranged in the feed tank 22, so that, as the water level falls below the predetermined intermediate level, the float 23 pregressivel-y opens a make-up valve 2a which permits the flow of cold un-de-aerated water from the external feed tank 3 by way of the pipe 25, the make-up valve 24 and the pipe 26, to the main condenser i. The make-up valve 24 is so pro- QOlTtiQRBd that the maximum quantity of water which can pass through the make-up connection it controls is about 30% of the feed flow required by the boiler at full power.

A de-aerator extraction pump 2i withdraws water from the feed tank 22 by way of the pipe 23 and discharges the de-aerated waterthrough the pipe as, provided with a non-return valve 29a to the boiler feed pump 38-, the boiler feed pump 39 discharging the water by way of pipe 31, provided with a non-return valve 31a, through the feed regulating valve 32 into the boiler 9. The operation of the feed regulating valve 32- is controlled by a float 38 which opens the valve 32' as the boiler water level falls and closes the valve 32 as the boiler water level rises.

The pumps 3, 2i and 3d are preferably of the centrifugal type. The float 5 in the condenser well 2 ensures that there is an adequate head of water on the suction side of the condenser extraction pump 3, greater than required for the satisfactory operation of the pump 3, and at the same time the pump is constrained at all times to operate on its natural discharge pressure characteristic, thereby ensuring that the pump 3 operates in a stable and satisfactory manner under all conditions of loading.

In the same way the float 23 in the ale-aerated feed tank 22, which is positioned relative to the ole-aerator extraction pump 21, together with the proportions of the tank 22 are all arranged as necessary to ensure that there is an adequate head of water on the suction side ofthedeaerator extraction pump 21 greater than that required for the satisfactory operation of" that pump.

The action of the boiler feed regulator 32, located between the de-aerated feed tank 22 and the boiler 53 ensures that both the tie-aerator extraction pump El and the boiler feed pump 3! are constrained to operate on their natural discharge pressure characteristics, thereby ensuring that those pumps operate in a satisfactory and stable manner under all conditions of loading.

When the boiler load is increased, it is necessary to discharge water from the feed system to the external feed tank 8. The water withdrawn from the boiler- 8 as steam is condensed in the maincondenser l and the surplus water accumulates in the well 2 at the base of the condenser; As the water level rises, the float 5 reaches the point at which it opens the overflow valve 5, and the surplus water is discharged by the condenser extractionpump 3 by way of pipe 6a, the valve 6 and pipe I to the external feed tank 8.

When the boiler load is reduced, additional water must b supplied to the boiler 9. The additional water supplied to the boiler 9 is taken from the de-aerated feed tank 22, thereby causing the float 23 to open the make-up valve 24, so that water flows from the external feed tank 8, through pipe 25, the float-controlled make-up valve 24 and pipe 26, into the main condenser I, in sufficient quantity to furnish the extra water required by the boiler 9. v

For a steady boiler load the float 5 in the well 2 of the condenser will settle at a level at which the opening of the extraction pump discharge valve 4 permits the passage of the steady feed flow required by the boiler. In the same way, the float I6 in the de-aerated feed tank 22 will settle at a level which will permit the feed discharged by the condenser extraction pump 3 to pass into the de-aerator I8, while the boiler feed regulator 32 will settle at a water level which permits the required feed flow to pass into the boiler 9.

At full power the water level in the well 2 of the condenser I will be at the predetermined intermediate level while the water level in the deaerated water feed tank 22 will also be at its predetermined intermediate level. In the event of the boiler load being suddenly reduced to zero, the water required to maintain the level in the boiler 9 will be drawn from the de-aerated feed tank 22 and the level of water in the deaerated feed tank 22 will fall rapidly until it reaches the predetermined low level, the proportions of th tank being such that this level is reached without uncovering the suction to the de-aerator extraction pump 21 and at the same time maintaining the head of water on the de-aerator extraction pump 21 as necessary to ensure its satisfactory operation.

The make-up valve 24 under the fall of the float 23 will have opened wide and make-up water will be passed into the condenser I from the external feed tank 8 in sufiicient quantity to restore the de-aerated feed tank level to the predetermined intermediate level.

As the float-controlled inlet valve I5 on the de-aerated feed tank is wide open, the condenser extraction pump 3 will be delivering water to the de-aerator I8, but, as the water level in the well 2 falls, th quantity of water discharged to the de-aerator I8 by the condenser extraction pump 3 will be gradually reduced until it reaches a feed flow corresponding to about 30% of the full power feed flow. It will be seen that the quantity of water discharged into the de-aerator I8 is reduced automatically as the boiler load is reduced, thereby minimising fluctuations in the de-aerator temperature and pressure which would be liable to derange the operation of the deaerato extraction pump 21 At the same time the arrangement provides an immediate and adequate supply of hot de-aerated feed water for discharging to the boiler, thereby maintaining stable conditions for the boiler feed delivery s stem, and thus avoiding operating diliiculties which might arise if fluctuating temperatures were permitted.

The pipe connection 34 and control valve 35 are provided between the de-aerated feed tank 22 and the condenser I to permit the circulation of water under light loads to ensure an adequate flow ofwater through the heat exchangers Ill an I511 as necess y to obtain the eflicient condensation of the operating steam of the ejectors II and I9 and the condensation of steam supplied to any heat exchangers which may be arranged in the feed system between the condenser extraction pump 3 and the de-aerator I8. I also arrange for a pipe connection 36 with control valve 31, between the pipes 'I and 26, whereby feed water may be drawn from the external feed tank 8 into the condenser I under manual control independently of the operation of the float controls in the system.

The valves I5 and 24 may be so arranged that they are controlled by a single float. Make-up into the system may be controlled by means of the valve 31 manually or the position of the floats I 8 and 23 may be so arranged that the predetermined intermediate level in the de-aerator is such that the make-up valve 24 is open as necessary to pass the desired quantity of makeup water into the system from the external feed tank 8, in such quantity as is necessary to offset the various inevitable losses from the system.

Referring to Fig. 2, the same reference numerals used in Fig. 1 are used to denote like parts.

'In addition to the elements of the system shown in Fig. 1, Fig. 2 includes an evaporator 38 supplied with sea or fresh water through the float-controlled feed regulator 39, the vapour produced in the evaporator passing by way of pipe 38 to the heat exchanger 4|, from which the condensed vapour passes into the main condenser I by way of pipe '32. The heat exchanger II is arranged in the feed system so that the condensate discharged by the condenser extraction pump 3 through conduit 3d passes through the heat exchanger II on its way to the de-aerator I8, whereby the heat in the evaporator vapour is conserved in the feed water. I may arrange for the heat exchanger 4| to be cooled by circulating sea water. The condensed vapour from the evaporator 38 may, alternatively, be led into the de-aerator I8 or the external feed tank 8, but it is always subjected to de-aerating action before it is charged to the boiler 9.

Heating steam from any convenient source is supplied to the evaporator coils 43 by way of a valved pipe 44, and the resulting condensate is drained through a pipe 44a into the heat exchanger M or other convenient place. The density of the water in the evaporator 38 is maintained at a proper value by opening the blowdown connection 45.

In the de-aerated water feed tank 22 shown in Fig. 2, the volume of de-aerated water available between the intermediate water level at which the float 23 opens the make-up valve 24 and the bottom of the de-aerated water feed tank is sufficient tosupply the boiler 8 in a sudden shut-down from full power to no-load, as shown in relation to Fig. 1. Between the intermediate water level corresponding to the closed position of the float 23 and the operating position of the float it, I may allow suflicient volume to contain the water which is discharged from the boiler when the boiler is working up from no-load to full-load, so that the de-aerated feed water tank 22 provides sufficient volume to deal with working up from no-load to full power or a crash stop from full power to no-load, without drawing water from the external feed tank or discharging any water to that tank. I

To enable two or. more such systems as described with reference to Figs. 1 and 2 to operate in parallel, I arrange suitable cross-connecting pipes and valves as shown diagrammatically in 3, where the same numerals used in Figs. 1 and 2 refer to similar parts. While the. two systerns shown in Fig. 3 are shown one above the other inthe drawing, it is to be understood that they are not superposed above one another but these systems are at substantially the same level as they would be on board a ship. For this reasong the pipes connecting the two systems are referred to; as crosseconnecting pipes.

When two or more closed feed systems such as have been described in relation to. Figs. 1' and 2 are required to operate in parallel, there are me evils-able inequalities in the quantities of steam evaporated. by the several boilers 3-, and in the quantities of feed water delivered by the respective feed. pumps To, look after such inequalities in flow in the parallel systems, the cross-com nection pips ts, having valves etc, is, provided between the externalieed tanks so that any differences in the weight ofwater discharged to the external feed tanks 8; through the overflow connections 1 or other sources or supply can balance out through the cross-connection dd.

To permit two or more closed feed systems to be operated in parallel with only one vie-aerator IS in operation, a cross-connection pipe ll having valves lla, is provided between the conduits 3d of the discharges of the condenser extraction pumps 3-, so that they have a common discharge line to the de-aerator i8 which is in operation.

Isolating valves 38 are provided so that, if one tie-aerator i8 is shut down, the valve id associated with that de-aerator is closed and the corn densate discharged by the condensate pump 3 associated with that (lo-aerator is delivered through the cross-connection pipe 41 to the other deaeratcr which is in operation with the other condenser extraction pump. Likewise a cross-connection line 49 having valves 4%, is, provided between the discharge lines 29 or the tie-aerator extraction pumps 2'5, so that the (fie-aerator extraction pump 21 on the de-aerator i3 which is in operation can discharge through such connection 49 to any of the several boiler feed pumps 38 which may be working. There may also be several de-aerators H3 in operation with only one boiler feed pump 38. in service, and the CI'OSSe connection 49 will enable the several ole-aerator extraction pumps 2'1 to deliver in common to the single boiler feed pump 33' or more feed which may be in service.

Inequalities in the flow through the several condenser extraction pumps 33 and/or ale-aerator extraction pumps 2? will cause the waterlevels in the condenser wells 2 and the ole-aerated water feed tanks 22 to rise or fall, thereby causing the make-up valves or overflow valves'to open and pass water into or out of the several systems, the differences between the systems being levelled out through the cross-connection 43 between the external feed tanks 3a If there are morethan two systems, cross-connections are provided between all the similar pumps and the similar feed tanks.

Arrangements may be made to have one common feed tank for several closed feed systems and where several feed tanks are used the efiect of using cross-connections 46 is to operate the several feed tanks as one common feed tank.

Standby feed pumps 58 are respectively provided for emergency use in each system and so arranged that they can draw water direct from the external feed tank 8 through external pipes 55, with a. non-return valve 52 and deliver it through valved pipes 53 into the pipes 3i, res selectively.

The float-controlled valves in the condenser i, the de-aerated water feed tank 22, and the boiler 8' may be operated by floats acting directly on the control valves, or the floats may be arranged to actuate pilot or relay mechanisms to effect the desired regulation of the several control valves.

Where: relay valves are used, 1 preferably make use ofthe teed water to control hydraulically the operation of the several control valves by means of the relay mechanism which is actuated in accordance with the movement of the floats.

The de-aerator inlet valve 15 and the make-up valve 24 may. be controlled from thesame float which operates at a predetermined high level to close the inlet valve it, operates at a predetere mined intermediate level to open the inlet valve [5 wide open, and which, as the level falls below the intermediate level, opens the make-up valve 2-4 to permit make-up water to. enter the feed 5Y5". term by way of the main condenser i. The open-. ing of the make-up valve 24 may be arranged to overlap the full opening of the inlet valve l5.

The space available on board ship is frequently very restricted, and when necessary the deaerated water feed tank 22 in the base of the deaerator Ill-may be restricted to a small capacity such as is requisite to ensure the satisfactory op.- eration of the. float-controlled inlet valve i5, and the float-controlled make-up valve 26. In such circumstances, the float-controlled make-up valve 2 3 is of suiiicient size to pass water into th feed system at a rate of flow equivalent to the maximum rate of feed flow of which the boiler feed pump 38 is capable. In the same way, it may be in some circumstances advantageous to make the overflow valve 6 of larger capacity than described above.

It will-be clear that the arrangement of the re+ generative type condenser i with a closed feed system embodying a, ole-aerator l8 ensure that all water passing into the boiler is subjected to both the d e-aerating action of the condenser I and thede aeratingaction of the lie-aerator 18, so that extremely efiicient Clo-aeration is obtained of all feed water charged to the boilers 9.

The action of the several float-controlled valves is such that the system iscompletely automatic in action from no-load to full-load and vice versa and the operation of the several float controls ensures that the several pumps in the system have a satisfactory suction head at all times, are constrained to operate at all times along their natural discharge pressure characteristics, and that the changes in flow called for by variations in load on the boilers are reduced to the absolute minimum in flow through the (lo-aerator so that pressure and temperature conditions in'the deaerator are the-optimum for efiiicient Clo-aeration and satisfactory operation of the pumping units.

I preferably arrange the de-aerated water feed tank under the tie-aerator so that it is capable of accommodating the whole of the variation in water volume covering the extreme fluctuations of boiler load from zero to full load and from full load to zero, in particular it being desirable to provide in the ale-aerator sufficient tie-aerated water to supply the boiler requirements on a sudden shut-down from full power to no-load, such withdrawal of water from the de-aerator being replaced comparatively slowly from the external feed tank by way of the main condenser.

It will be understood that, while I have described one arrangement embodying my invention, other combinations of make-up and overflow valves may be arranged in conjunction with the condensers and de-aerator in accordance with my invention.

What is claimed is:

1. In an automatically operable closed feed system for a boiler installation including a boiler, a condenser for condensing the steam generated in the boiler, a tie-aerator for de-aerating the feed water for the boiler, a conduit and a condensate pump therein for passing condensate from said condenser into said de-aerator, a tank connected into the de-aerator for receiving deaerated water therefrom, means including a boiler supply pump for drawing water from said tank and forcing it into said boiler, a first valve in said conduit, means responsive to variations in the level of the water in said condenser for operating said valve at full open position at an intermediate condensate level in the condenser and for closing said valve as the condensate level in the condenser falls, a second valve in said conduit downstream of said first valve therein, means responsive to variations in the level of the water in the de-aerated water tank for operating said second valve at full open position at a predetermined water level in the de-aerated water tank and for moving the said second valve toward closed position as the water level rises from said predetermined level, a feed water supply tank, a pipe connected into the portion of said conduit leading from the outlet of the condensate pump for passing condensate to said supply tank, a valve in said pipe operable to open position by said means responsive to variations in the level of the water in the condenser as the condensate level in the condenser rises above said intermediate level in the condenser to permit delivery of condensate through said pipe to said supply tank, a water supply pipe for conducting water from the supply tank to the condenser wherein the water is subjected to a de-aerating action. a valve in said water supply pipe, and means responsive to variations in the level of the water in the de-aerated water tank for closing the valve in said water supply pipe at an intermediate water level in the de-aerated water tank and for opening the valve in said water supply pipe as the level in the de-aerated water tank falls, to permit flow of water from said supply tank through said water supply pipe to the condenser.

2. A closed system as claimed in claim 1, in which the de-aerated water tank has a capacity for holding sufiicient water below an intermediate level therein to supply the requirements of the boiler when the boiler is shut down from full power to no-load.

3. A closed system as claimed in claim 1, in which the de-aerated water tank has a capacity 10 for holding a volume of water between an intermediate level therein and a predetermined high level corresponding to the volume of Water discharged from the boiler between no-load and full load operating conditions.

4. The combination of a plurality of automatically operable closed feed systems arranged in parallel each having the features and elements defined in claim 1, characterized by including a valved pipe line interconnecting the feed water supply tanks of the plurality of systems for conducting water from the supply tank of one systern to the supply tank of another system, whereby any inequalities in the demand for water in the different systems may be adjusted.

5. The combination of a plurality of automatically operable closed feed systems arranged in parallel each having the features and elements defined in claim 1, characterized by including a valved pipeline intercom'iecting the said conduits of the plurality of systems for conducting condensate from the condenser of one of the systems into the de-aerator of another of the systems, a valved pipeline interconnecting the inlets to the boiler supply pumps of the plurality of systems for conducting de-aerated water from the deaerator of one of the systems to the boiler of another system of the plurality of systems when the de-aerator of said last-mentioned system is out of operation.

6. An automatically operable closed feed system for a boiler installation as claimed in claim 1, including a heat exchanger for heating the water passed through said conduit to the deaerator, a pipe connection between the lower portion of the de-aerated water tank and the upper portion of the condenser for conducting water from the de-aerated water tank back to the condenser for providing an adequate flow of water through the heat exchanger, and a control valve in said pipe connection.

HAROLD HILLIER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,596,423 Gibson Aug. 17, 1926 1,599,334 Lang Sept. 7, 1926 1,629,028 Fothergill et a1. May 17, 1927 1,636,361 Gibson July 19, 1927 1,793,119 Moore Feb. 17, 1931 1,991,929 Hillier Feb. 19, 1935 2,078,288 Sherman Apr. 27, 1937 2,371,443 Hillier Mar. 13, 1945 FOREIGN PATENTS Number Country Date 551,063 Great Britain Feb. 5, 1943

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