US2315481A - Method and apparatus for degasifying liquids - Google Patents

Description

March 30, 1943. M. K. DREWRY ET AL METHOD AND APPARATUS FOR DEGASIFYING LIQUIDS Filed Dec. 30, 1940 3 Sheets-Sheet 1 Yam. 6: 12%,

041M "FLU/Q ATTORNEYS March 30, 1943. M. K. DREWRY ET AL 2,315,481

METHOD AND APPARATUS FOR DEGASIFYING LIQUIDS Filed Dec. 30, 1940 3 Sheets-Sheet 2 C r i t 1 r c c c 1 c c c c a a c c a 5 Q s 3 5 c? @295 @Y Z 15...? 5 a 5 3 55 a IN \gNToK 6 y M; ATTORNEYS gumlw v? p 6 c r c cxc c c a c c c "m March 30, 1943. K, R WRY ETAL 2,315,481

METHOD AND APPARATUS FOR DEGASIFYING LIQUIDS Filed Dec. 30, 1940 3 Sheets-Sheet 3 f j 15 y J 6 INVENTORS 0 ATTORNEYS Patented Mar. 30, 1943 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR ,DEGASIFYING LIQUIDS Montrose K. Drewry, Milwaukee, and Maurice E.

Fitze, Hales Corners, Wis., assignors to Wisconsin Electric Power Company, Milwaukee, Wis., a corporation of Wisconsin Application December 30, 1940, Serial Nae-12,278

' (o1. lac-2.5)

2 Claims.

dioxide, and other gases in water used as the make-up feed water for steam boilers has a dele- Y terious corrosive action upon the boiler, pumps, turbines, and other machinery in the steam plant. Many methods have been resorted to to free the feed water of more or less of its dissolved gases, and the most widely used of these prior methods require the use of steam or other means for initially heating the water to or near the boiling point at atmospheric pressure (212 F.) One object of the present invention is to provide a method and apparatus for deaerating or degasifying water, such as boiler feed water which acts to extract the gases at low temperatures from the water in finely divided form and which is of decided advantage over the prior methods since it reduces the cost of the degasifying operation and the corrosive action of the gases in the water upon the equipment in which the degasiiying treatment is carried on.

The invention further consists in the method and apparatus for degasifying make-up boiler feed water as hereinafter described and more particularly defined by claims at the conclusion hereof.

Referring to the drawings:

Fig. 1 is a view, partly diagrammatic and partly in section, showing apparatus embodying the invention for carrying out our new method of degasifying boiler feed water;

Fig. 2 is a horizontal sectional view taken on the broken line 22 of Fig. 1;

Fig. 3 is a detailed vertical sectional view taken on the line 3-3 of Fig. 2;

Fig. 4 is a detailed sectional view line 4-4 of Fig. 2;

Fig. 5 is a detailed vertical sectional -view through one of the spray nozzles;

Fig. 6 is a detailed horizontal sectional view taken on the taken on the line 6-5 of Fig. 5.

According to our method 01. degasifying makeup feed water for boilers, we subject the water to the action of a reduced pressure and at the same time distribute it over a large area by spraying it in a finely dispersed condition, so that as much'of its surface area as possible is exposed while under such reduced pressure, so that a relatively high percentage of objectionable gases will be liberated from the water. For a source of reduced pressure we provide a vacuum chamber in which the vacuum is maintained with great convenience and at a minimum of cost since it may be the vacuum used in connection with the usual steam condensation equipment of the plant.

For extending the surface of the water we spray it in a series of fine sprays whereby the water is broken up into an extremely large number of very fine particles, thus increasing the surface many times, into the vacuum chamber from which the gases of the type to be removed are being continuously removed, thus producing an environment favorable to the escape of the gases "from the water being treated. In as much as the gases leave the water through the surface, the increase of exposed surface increases the rapidity with which the water is degasified in direct proportion to the increase of surface presented, and as the drops are very small, the average distance of difi'usion to the surface of the drop is very short, and consequently with efficient spraying equipment, properly arranged in the vacuum chamber we are enabled to get the water degasified in a very short space of time and at relatively little expensefor operating costs by taking advantage of the conjoint effect of the vacuum and the spray. Furthermore, in the preferred form of .the invention the temperature of the water being treated and the amount of vacuum to which it is'subjected is such as to produce flashing in one or more of the stages of the process hereinafter described, this flashing taking place in the already small drops to further subdivide them by the internal bursting of the droplet due to a flashing of some of its liquid content into vapor.

'Also it is to be noted that with increase in temperature the corrosive effect of any corrosive agent in the water including dissolved gases is increased. The general rule .of thumb is that each ten degree centigrade rise in temperature doubles the rate of reaction. There is, therefore, a definite reason for and decided advantage in keeping the temperature of the water being treated low since the corrosive tendency of the oxygen and carbonic acid content of the water is thereby minimized.

The release of fixed gas in solution is dependent upon the operation of Henry's law. That is to say, the relationship commonly expressed as (wherein C1 is the concentration of solute gas in the contacting vapor phase, C2 is the concentration of the solute gas in the solvent liquid and K is a constant dependent upon temperature) determines the condition being approached in any such system wherein a fixed gas is being released.

Most gases, and all those of consequence in the technical utilization of water, exhibit an inverse temperature coeflicient of solubility, and this fact has misled to the utilization of temperatures at or near the atmospheric boiling point in practically all prior water-degasiiying methods. At such temperatures the vapor environment is at approximately one atmosphere pressure and is relatively dense. The mass of this environmena sufliciehtly low concentration (C1) so that the release will effectively proceed is substantial in amount and heat must be supplied to create this vapor.

Heretofore unrecognized and unavailed of is the fact that reduction in the pressure of the environmental vapor reduces the mass thereof for an equivalent diluent effect much more rapidly than the adverse shift in (K), i. e., more rapidly than solubility of the fixed gases increases due to the lowering of the treating temperature. Therefore, at 0.03 atmosphere only about three per cent of the water required at one atmosphere need be vaporized to produce an environment in which C1 is the same, although K will have shifted to a very much smaller degree. By reason of this wide divergence and because it is not feasible in atmospheric pressure treatments to volatilize the large amount of liquid required, we are able by our method and at great economy to produce a markedly lower concentration (C1) of the fixed gas in the environmental vapor. By direct application of Henrys law, it therefore becomes apparent that by our method a more complete removal of the fixed gases may be procured than has been heretofore procurable.

Not only is a more favorable equilibrium procurable, as a practical matter, by our method, but a more rapid carrying forward of the release of the gas is also produced. One reason for this is that the equilibrium interface concentrations of gases being, removed are lower, due to the lower (C1), as indicated above. Hence, the concentration differential in the liquid film, which is the driving force for the controlling diflusion mechanism, is greater with our process than with prior atmospheric pressure processes. This is further enhanced in our method by forcibly spraying the water treated and preferably at the same time volatilizing a portion thereof. Heretofore exten sion of the surface has been obtained by gently boiling, or by blowing steam through the water undergoing treatment or the running of the water over a number of trays so as to break it up and expose it for a considerable length of time to the surrounding deaerating medium. We have discovered that where the liquid itself is split up by forcibly spraying the same, the water can be sufficiently subdivided to render the film thickness a matter of secondary importance. In addition, by our process the liquid presents a continually eroding surface, which further enhances the efiective transport coeihcients for the several gases being removed.

We, therefore, are able, through this invention, to eifectively treat a larger quantity'of liquid in a relatively smaller and less expensive unit of equipment within a given space of time. This, coupled with the greater degree of gas removal obtained, the more favorable energy requirement, both from the standpoint of level and quantity, the reduction in Waste of water treated, the lowtal vapon at one atmosphere required to produce titions 8, 9, and 10 into three compartments ll,

- l2, and I3 and a collecting chamber H. The

external pressure and should be tight and free of leaks.

Each compartment ll, 12, and I3 is provided with a series of headers i5, and each of the headers is provided with a plurality of nozzles l6 arranged in three rows, the middle row positioned vertically, the other two rows on opposite sides of said middle row and in spaced relation thereto. Each of the nozzles is of the centrifugal type, having a spirally grooved whirl plug I! mounted therein adjacent the discharge orifice l8 so as to impart a whirl to the spray as it leaves the orifice l8 and thus increase the angle of divergence and the atomization of the spray over a plain nozzle type. With the exception of the central header, the other headers are connected in pairs by piping l9, and the supply of water to these pairs of headers and the central header are each controlled by a suitable on and off valve 2!),

it being noted that the valves are preferably not regulating or throttling valves, but when open allow for the full volumetric capacity of the nozzles it, so that they will be used at their greatest spraying efiiciency. mechanism whose specific structure forms no part of the present invention is provided for controlling each of the valves 2t, so that one or more of these valves may be operated at a time to turn the supply of make-up water to be treated "on or ofi."

The partitions 8 and 9 each have outlet vent openings 2i protected from the direct passage of the spray by hoods or covers 22, and deflector plates 23 are mounted beneath each opening and above the spray headers of the compartments below. The lower partition M) is provided with a single drain passage 23 associated with an outlet cred rate of corrosion in the treating equipment I nates a treating tower, divided by apertured parcontainer mixer 25 for receiving suitable water treating reagents from a pipe 25' if desired.

At the top of the tower or tank there is an air or gas outlet pipe 26 that connects with the vacuum producing steam jet pump on the main condenser for the main steam power unit by which pressures substantially below atmospheric pressure, for example 15 inches of mercury vacuum or preferably from 24.3 to 29.5 inches of mercury vacuum, are produced and which pump acts to discharge the fixedgases at atmospheric pressure. At the bottom of the tower there is an outlet pipe 2? for the degasified water by which it is conducted to boiler-feed or other pumps (not shown).

With the above construction the water to be treated at ordinary room temperatures, 33 to F. and not exceeding F., is conducted to the first spraying mechanism from-the supply pipe 28. There it is sprayed into the first compartment ii while the same is under a reduced pressure, and the spray, coupled with some fiashing of water into vapor depending upon the temperature of the water and the amount of the vacuum, causes the water to expose itself in a greatly extended surface in an environment where the concentration of the disssolved gases is very low since the dissolved. gases and the vapor evolved are drawn ofi continuously through the outlet 2 5, as noted above in connection with the general explanation of the process. This vacuum and spraying treatment liberates the greater proportion of free oxygen, carbon dioxide, and other gases that may be in the water. If necessary or Suitable control desired in order to remove a further percentage of the gases, the water may be subjected to several of these spraying under vacuum treatments, and each time a certain portion of the gases that may remain in the water is removed. The

the upper compartment, and taken from this 'well by a centrifugal pump 32 which delivers it to the temperature level restoring apparatus 33 which raises the temperature of the water 3 to 8 F. (not over 10 F.) to provide for flashing in' the nozzles of the second stage, thence the water is carried by thepiping 3,4 to the valve controlled spray headers I for the second stage which spray the water into the evacuated space of compartment l2 so as to act on the remainder of the free gas content of the water and remove substantially the same percentage of this remainder as was done in the first stage. Thereafter the treated water may be subjected to a third spraying treatment, and to compensate for the temperature drop in the second stage it is again brought back to its initial temperature by conducting it from the bottom of compartment,

I2 through a sump, similar to the sump 28, and

piping 35 to a well 36 vented by a connection 31 with the vent connection 3| and taken from this well by a centrifugal pump 38 which passes it through a temperature level restoring apparatus 39 similar to the apparatus 33 which raises the temperture of the water 3 to 5 F. (not over F.) to provide for flashing in the nozzles of the third stage. The water is then conducted by the piping 40 to the valve controlled spray headers l5 for the third stage which spray the water into the evacuated space of compartment 13 so as to act on the relatively small free gas content then in the water and remove substantially the same percentage of these remaining gases as was done in the second stage, and then the degasified water passes down through the passage 24 and is treated by suitable chemical reagents if desired which may ebe mixed with it in the mixer 25. The treated water then passes into the chamber l4 and is ready for introduction into the remainder of the water used as feed water for the boiler or boilers of the power plant. The released gases and vapor pass upwardly under the vacuum suction from one compartment to another through the vented openings 2| and thence through the outlet 26. It has been found that the oxygen and carbon dioxide gas removalffrom the water with this process affords a large margin of safety against the deleterious effects of free gas content of such water when used as a make-up supply for boiler feed water. Tests show that about ninety per cent of inlet oxygen is removed in each stage. For a three stage degasifler outlet oxygen is, therefore, luix /iox /m or /iooo of the inlet oxygen (.008 cc./liter out, 8.0 cc./liter in are typical values).

As noted above temperatures from 33 to 140 F. with vacuum pressures of 24.3 to 29.5 inches of mercury corresponding, respectively, to these pressures so as to produce flashing in the first stage and addition of 3 to 10 F. of heat in subsequent stages to permit flashing in these stages is preferred and will give the best results but vacuum pressures below-24.3 inches of mercury and or which at the temperature of the water being treated are not high enough to cause flashing may also be used with good results and with less depreciation of equipment thanthe usual methods.

From the foregoing it will be noted that the water is treated at a low temperature in a very finely subdivided form and in a space of reduced pressure. The low pressure and fine degree of subdivision are the things which make feasible the use of relatively cool water. The low temperatures used result in two major advantages;- flrst, the corrosion rate in the treating equipment itself is markedly slowed down, and second, due to the elimination of high temperature heating of the water and the elimination of the necessity for volatilization of a large proportion of the water to the liquid between successive sprayings, maintaining in said chambers a continuously changing atmosphere of mixed vapor and gas having a low concentration of the gas to be removed from the liquid and at a pressure which will cause a portion of the liquid to vaporize in each stage as it is being sprayed, and removing the changing atmosphere of mixed vapor and gas from a later stage through the chamber of an earlier stage to assist in sweeping out mixed vapor and gas in this earlier stage.

2. In anapparatus for degasifying liquids, the combination of a chamber capable of being evacuated and having an outlet, means for withdrawins gas and vapor from said chamber under a pressure below atmospheric pressure, means for spraying the liquid to be degasifled into said chamber at a temperature above the flash point thereof at the vacuum prevailing in said chamber, a second chamber capable of being evacuated, a vapor and gas passage joining said second chamber with said'flrst chamber to permit gas and vapor to be evacuated from said second chamber under vacuum by passage through said first chamber to said outlet, means for collecting the liquid sprayed into said first chamber, means for heating said collected liquid to a. temperature in excess of the flash point thereof at the reduced pressure prevailing in said second chamber.

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