US3251138A - Drying system - Google Patents

Description

y 1966 L. WHITTAKER 3,251,138

DRYING SYSTEM Filed July 15, 1963 3 Sheets-Sheet 1 TO ATMOSPHERE I: I 46 I 3'0 4! Inventor L.' WHITTAKER A TTORNEYS Maym17, 1966 L. WHITTAKER 3,

DRYING SYSTEM Filed July 15, 1963 2 Sheets-Sheet 2 Inventor L. WHITTAKER ima 7%? A T TOR NE YS United States Patent 3,251,138 DRYING SYSTEM Leslie Whittaker, Long Sault, Ontario, Canada, assignor to Dominion Tar & Chemical Company, Limited, Montreal, Canada Filed July 15, 1963, Ser. No. 295,112 Claims priority, applicstgtiozrzfanada, July 20, 1962, 3 9 Claims. (Cl. 34-48) This invention relates to drying systems and particularly to drying systems which include large drying surfaces such as the drying cylinders of paper making machines and other like machines, and includes the control of the introduction of steam and the removal of condensate from the various sections of the machine for the purpose of providing a predetermined constant heat diflerential in the supply of steam to various sections of the machine.

In most drying systems and particularly in the drying sections of a paper machine, it is essential that the heat in the various sections of the machine be controlled and that a constant pressure differential be maintained in the supply of steam to the sections and in order that the constant differential in steam supply be maintained it is essential that a controlled withdrawal of condensate from the sections be ensured.

The present invention consists essentially in feeding steam through a differential control valve to a first drying section of the machine and evacuating the condensate, then passing the evacuated condensate through a restricted passage to a flash tank in which the level of the condensate is maintained relatively constant by means of a level control. Steam is also fed to a second drying section through a differential control valve whereby the pressure of the steam supplied to the second drying section is at a predetermined value below the pressure supplied to the first drying section. Excess heat in the flash tank in the form of flashed condensate is fed to a second drying section of the machine and after the flashed condensate has given up heat it is passed through a valved steam trap and by-pass system and heat exchanger and thence to a cooled condensate tank. A part of the cooled condensate from the cooled condensate tank is returned as a spray into a condensate header 10- cated between the valved steam trap system and the heat exchanger. If, during the drying operation, suflicient heat to maintain the desired temperature in the second drying section is not available in the condensate from the flash tank to provide the required drying, then extra steam is admitted to the section through a differential control valve in the main header supplying the second section of the machine. It is desirable in selecting the drying load in the first and second drying sections that the balance be such that for most of the time the differential control valve is admitting a small amount of steam to the second section of the machine. Since the diiferential valve is on differential control between the pressure of the steam fed to the first drying section and the flash heat supply to the second drying section, it follows that when the differential control valve admitting steam to the first section is adjusted to some new steam flow, the diflerential control valve for the second section will hold the differential between the two sections so that the second section will follow the first section, holding a constant differential through its control valve.

The level of the condensate in the cooled condensate tank is controlled by withdrawing condensate from the tank through a pump and passing a portion of it through a level control valve which is opened when the level in the tank is above a predetermined level and closed when the condensate reaches the predetermined level or is below that level. The condensate passing through the and noncondensables from the dryer is ensured.

level control valve is returned to the boiler room for reuse. A steady flow of condensate from the pump is passed through a pressure control valve and is delivered in the form of a spray into the header receiving condensate from the second section of the machine in order to kill off any tendency for the condensate to flash into steam. This spray of condensate is recycled back to the cooled condensate tank.

A vacuum pump is connected to the top of the cooled condensate tank in order to maintain the differential between the inlet and outlet pressures on the second sectionof the machine.

The object of the invention is to provide a constant differential in heat at different sections in a drying system and the utilization to the best advantage of the condensate from the sections in maintaining the desired differential.

A further object of the invention is to provide pressure control means for both steam and condensate at different sections of a dryer system in order to provide a controlled pressure differential between various sections of a drying system.

A further object of the invention is to provide a large degree of control of the flow of steam and condensate from a drying cylinder by utilizing a long orifice in the condensate line.

A further object of the invention is to provide means whereby the controlled flow of condensate from a first drying section is utilized in a second drying section, and means are also provided to permit a free flow of condensate from the second drying section without flash.

A further object of the invention is to provide a recirculation of cool condensate, sprayed into the condensate as it leaves the dryers to prevent flashing of the condensate.

A further object of the invention is to provide a modified steam trap and by-pass in the condensate line from a dryer section whereby the evacuation of condensate These and other objects of the invention will be apparent from the following detailed description of one concept of the invention and from the accompanying drawings, in which:

FIG. 1 is a schematic layout of one concept of the invention particularly applied to the drying section of a paper machine, in which separate drying sections are represented by a .singledrying cylinder.

FIG. 2 is an enlarged detail of the steam trap and bypass system shown in FIG. 1.

FIG. 3 is an enlarged perspective view of the condenshown representing each individual section of the dryers.

Also only a first and a second section of the dryers is shown. It is to be understood that there may be any number of sections of dryers and that each individual section may include more than one cylinder.

The first or main section of the dryers is represented by the letter A, while the second section and subsequent sections are represented by the letter B in FIG. 1 of the drawings.

The dryer cylinder 5 of the first section A is provided in well known manner with a steam inlet 6 and a condensate outlet 7, the condensate outlet being connected to an internal syphon pipe 8. Similarly the dryer cylinder 9 of the second section B is provided in well known manner with a steam inlet 10 and a condensate outlet 11 also connected to an internal syphon pipe 12.

The main steam supply for the drying system is obtained from any suitable source and is fed into a main supply header 13. A branch line 14 connects the header 13 with the header 15 and is provided with two pressure control valves 16 and 17. A pipe line 18 connects the inlet 6 of the dryer cylinder 5 with the branch line 14 intermediate the valves 16 and 17.

The valve 16 is controlled by the pressure control 19 while the valve 17 is controlled by the pressure difierential control 20.

The condensate in the dryer cylinder 5 is forced up by steam pressure through the syphon pipe 8 and passed through the pipe 21 to the long restricted orifice 22, shown in detailin FIG. 3 of the drawings, and thence to-the header 23. The condensate is taken from the header 23 to a point about midway of the height of the flash tank 24. A pump draws the condensate from the bottom of the flash tank 24 and returns it to the boiler. This pump 25 is operated under the control of level control 26 in order to maintain a predetermined level of condensate in the tank 24. Excess heat in the flash tank 24 in the form of flashed condensate, is fed from the top of the flash tank through the pipe 27 to the header 15 and thence to the inlet 10 of the dryer cylinder 9.

The steam in the dryer cylinder 9 after giving up heat is drawn ofi as condensate by the syphon pipe 12 through the pipe 28 from which it can be directed through the valve 29 to the condensate header 30 or through the valve 31, trap 32 and valve 33 to the header 30. It is preferable that the header 30 be of oversize diameter, iii. if a 2 pipe would normally be used to carry otf the condensate, a 4" pipe would be used in this case in order to make the condensate sprays efiective in cooling the condensate and eliminating any chance of it flashing into steam. The gate 34 of the valve 29 is shown in detail in FIG. 4 and is provided with an aperture 34a to ensure a minimum constant condensate by-pass of the trap 32.

V The trap 32 is preferably of the inverted bucket type and is provided with an inlet 35, an outlet 35a and a relief valve 35b. Freely mounted on the top of the inverted bucket 36 is a floating valve 36a which is positioncd to control flow through orifice 36b. An enlarged vent 37 is positioned in the top of inverted bucket 36. The valves 31 and 33 are isolating valves which are closed when maintenance of the trap 32 is necessary.

The header 30 is provided with one or more spray nozzles 38 whereby cooled condensate can be sprayed into the header, for a purpose which will be explained later.

A pipe connection 39 leads from the header 30 to one side of the heat exchanger 40 through which cooling water is passed from the pipe 41 to the pipe 42. The other side of the heat exchanger 40 is connected through the pipe 43 to the cooled condensate tank 44 at approximately midway of the height of the tank.

A constantly operated pump 45 draws off the cooled.

condensate from the bottom of the tank 44 through the line and delivers it through the valve 47 and thence through the line 48 to the spray nozzles 38 in the condensate header 30. The valve 47 is controlled by the pressure control 49. Excess condensate in the tank 44 is drawn ofi also by the pump 45 through the valve 50 which is controlled by thelevel control 51 and opens the valve when the level of the condensate in the tank 44 is above a predetermined level and closes the valve when the level of the condensate is at the predetermined level or below. Condensate passing through the valve 50 is returned to the boiler house through the line 52 for reuse and normally would be connected to a hot well for this purpose.

reducing the amount of steam that can pass.

4 4 Vacuum is maintained at the top of the tank 44 by the vacuum pump 53 connected on its other side to atmosphere.

Having now described a typical installation in detail, the operation will now be described.

When steam at pressures normally well above atmospheric is admitted to a group of dryer cylinders here represented by the cylinder 5, good evacuation is ob-.

tained through the condensate syphon 8 of the cylinder 5, particularly as the condensate portion of this system from the restrictor orifice 22 down through the group of dryer cylinders here represented by the cylinder 9 is under controlled environment as dictated by the dilferential control valve 17 and the vacuum pump 53.

Similarly, the steam entering the dryer cyl nder 9, even though it may be at a much lower pressure, has sufficient differential pressure applied to it through the condensate side of the system starting at the steam syphon 12 in the cylinder 9, that it is easily evacuated in its condensate form through the condensate system starting at the trap 32 and valve 29 and running down to the pump 45 and these two condensate systems, with their associated controls, allow a wide range of pressures to .be applied to either cylinder 5 or cylinder 9 provided that the pressure in cylinder 5 is 3 to 4 lbs., for most drying conditions, above the pressure in cylinder 9, this condition being maintained by the valve 17. The actual differential to be maintained by valve 17 is dependent on the diameter and speed of revolution of the drying cylinders.

Steam admitted to the dryer cylinders contains heat and it is this heat which it is desirable to transfer to the dryer surface and hence to the medium to be dried. It is understandable if there is no restriction on the discharge side of the dryer cylinder 5, that much of the steam would blow straight through the dryer, out through the condensate line and pass through so quickly that it would not give up very much of its heat to the dryer.

Heat not used in the main section or drying cylinder 5 is reused or expanded in the second stage or drying cylinder 9, hence, it is not necessary to extract of the heat from the steam as it passes through the dryer 5. However, if the steam is allowed to pass freely through the dryer 5, too much would be available to properly be utilized by the succeeding flash or drying cylinder 9. Restriction of the flow of steam from the dryer 5 is offered by the restrictor orifice 22. The resistance to entry or exit .from an orifice is vastly different for steam and condensate because of the great difference in their viscosities. Also, steam always introduces a small amount of non-condensables in the form of gases which must be removed, or in time they will fill the heating or drying area and hence prevent drying from taking place. It is therefore, necessary that a small amount of steam which is mixed with the noncondensables be passed through the discharge side of the dryer at all times to make sure the non-condensables are removed. The orifice 22 in the condensate discharge line 21 should be sized so that it passes all the condensate that will be produced, plus a small amount of steam along with the non-condensable gases.

At a low drying rate very little condensate is produced as compared to a high drying rate when a great deal of condensate will be produced. It is the ideal situation that all condensate be removed at any drying rate, and

at the same time a very small percentage of steam..

However, it is easy to see that when the drying rate is low, very little condensate is passing through the condensate restrictor 22, that a good deal of steam, therefore, can pass through. On the other hand, when the drying rate is high, and a good deal of condensate is produced, it fills the restrictor, or tends to fill it, thus Since the production of non-condensables is in direct proportion to the amount of steam used, then at high drying rate it is likely, if the condensate orifice 22 is not correctly sized to adequately pass all the condensate plus a little steam, that condensate will be left in the dryer along with non-condensable gases. If a fiat plate orifice were used where the only restrictions present were the entry and exit losses, then at low drying rates the size of the orifice would be much too great because the steam would easily pass through with this type of restriction and a good deal of steam would be wasted or be in excess of the requirements in the succeeding flash section or dryer 9. By using a long orifice such as that shown in FIG. 3 at 22, an additional restriction to the passage of steam and condensate is introduced in the form of a coefiicient of friction on the surface of the orifice 22a. By the use of this restriction in the long orifice 22a it is possible to more nearly equate the rate of flow of steam and condensate so that at even low drying rates when little condensate is passing through the restriction, the steam itself when it tries to pass through, will be restricted by surface friction and hence the loss of steam will be reduced at low drying rates. This has a tendency to make the conditions for which the orifice is designed at high drying rates more nearly correct for low drying rates although, of course, a perfect condition can never be achieved from a single orifice. The long orifice 22a has the additional advantage that the erosion from hot condensate and steam must take place over the full length of the orifice giving a long orifice an infinite life compared to a flat orifice which soon becomes significantly larger. Thus, the long orifice gives a reasonable control of the proportion of steam and condensate that will be passed over a wide range of drying loads with a minimum of maintenance and replacement.

Referring now to the withdrawal of condensate from the drying cylinder 9 and the operation of the trap and by-pass system illustrated. Normally, a trap is designed to pass condensate and restrict entirely, or at least as much as possible, the passage of steam. Such traps are inflexible devices designed for specific pressures of steam and specific rates of condensation or depletion of heat from the steam so that the trap will get, over a specified time, a definite amount of condensate passed to it which it is, in turn, designed to pass on through the condensate system. When all condensate available in the system has passed through the trap and steam attempts to follow, the cup of the trap will not float on the loW density steam following the condensate and therefore the cup will drop and close the trap and prevent the exit of steam into the condensate header. This is a cycling action and as such, over a wide range of loads. such as are required in a flexible drying system, the cycle can be very quick on heavy loads to the extent that the trap wears out quickly, or on light loads can be so slow that the trap will remain closed for long periods. During this time the condensate accumulates in the dryer together with non-condensables and there is no steam flow through the dryer. The result is a very definite drop in the temperature of the dryer during this time and possible localized heating in the dryer because of the inadequacy of currents through the dryer.

In the trap and by-pass arrangement shown in FIGS.1 and 2 these disadvantages are overcome. By hooking the trap 32 up with the gate valve 29 in the bypass line 29a it is possible by cracking this valve 29 open to allow a constant bleed of steam and non-condensables to pass through the valve and therefore lay-pass the trap 32. To ensure that this takes place, a small hole 34:: is drilled in the gate 34 of the valve 29 so that even although the gate 34 is closed, there is a small amount of bleed through the valve. This hole 34a is sized for minimum bleed requirements of the drying system. This by-pass arrangernent results in a more constant cycle over a wide range of loads.

When condensate is passing through trap 32, inverted bucket 36 remains down and a through flow is maintained. Enlarged vent 37 is sized to permit an increased Volume of non-condensable gases to escape and also to enable the inverted bucket 36 to vent steam at an increased rate.

When steam enters trap 32 in a quantity sufficient to float inverted bucket 36, floating valve 3611 also rises and closes orifice 3612. As more condensate enters inlet 35 bucket 36 drops, orifice 36b is opened and the flow of condensate through trap 32 is again resumed.

By carefully sizing the trap several times larger than would normally be required by the condensate rate at which it would run on the average, it is possible to have a smooth, soft and regular action of the trap under a wide range of drying loads. This arrangement of metered by-pass and softened trap action gives a steady drying rate on the machine dryer with a slight loss of efficiency on the use of steam. However, this loss of efficiency is more than overcome by the increased efliciency within the dryer itself where all non-condensable gases have been removed and the condensate removal is constant over a wide range of drying loads.

In order to maintain apressure differential suficient in the dryer section Bdrying cylinder 9, to drive the steam and condensate up out of the syphon pipe 12 and down through the trap 32, it is desirable to obtain low pressures in the condensate header 30. Ofteh to obtain suflicient differential to evacuate the dryer cylinder 9 the pressure in the condensate header 30 must be close to or below atmospheric pressure. In order to accomplish this the vacuum pump 53 produces a vacuum at the top of the tank 44 and hence up to the condensate header 30 to the dryer 9. This vacuum likely and often does cause the condensate to flash into steam in the header 30, because of residual heat content thus cancelling out any vacuum produced. To overcome this the cool condensate from the tank 44 is pumped by the pump 45 through the pressure control valve 47. This pressure control valve 47 permits a steady flow of the cooled condensate to go up through the line 48 and hence through the spray nozzles 38 into the condensate header 30. The pressure on the line 48 is controlled by the action of the valve 47 and holds a constant pressure on the spray nozzles 38. This cool condensate sprayed into the condensate in the header 30 kills off any flash and hence allows the vacuum pump 53 to maintain a vacuum back through the header 30. The condensate sprayed through the nozzles 38 into the header 30, of course, then recycles down through the heat exchanger 40, tank 44 and through the pump 45.

While the invention has been described in relation to a single drying cylinder in two sections of a drying installation it will be apparent that the invention can be applied to any number of sections in which a heat differential must be maintained to ensure uniform drying of the material. It will also be apparent that there can be any desired number of drying cylinders in each section and that the control system shown can be applied to any grouping of dryers in order to achieve the desired results set forth above.

What I claim is:

1. In a steam drying system employing drying sections between which a relatively constant heat differential must be maintained, a first drying section and a second drying section, a source of steam supply feeding steam to each of said drying sections, individual means to maintain a pressure differential between the steam supply to the said first and second drying sections, a steam and condensate exhaust line from said first drying section, means to restrict the flow of condensate through a predetermined length'of said exhaust line, a flash tank in said condensate exhaust line, a supply line to deliver flashed steam from said flash tank to said second drying sections,

means to exhaust the condensate from said second drying section, said latter means including a steam trap and a oy-pass around said trap, means to cool the condensate from said second drying section, and means to return the cooled condensate as a spray to the condensate after it passes through said trap and trap by-pass.

2. In a steam drying system as set forth in claim 1 in which the means to restrict the flow of condensate in the exhaust line from said first drying section comprises a length of pipe of smaller diameter than that of the condensate exhaust line.

3. In a steam drying system as set forth in claim 1 in which the said flash tank includes means to control the level of the condensate therein.

4. In a steam drying system as set forth in claim 1 in which the feed of steam from the source of supply and flashed steam from said flash tank to the said second drying section are :fed to a common header.

5. In a steam drying system as set forth in claim 1 in which the feed of steam to the drying sections includes a main feed line and a branch feed line to each drying section and the means to maintain a pressure differential between the first and second drying sections include a first pressure control valve in the main feed line controlled by the pressure in the branch line to the first drying section, and a second pressure control valve in the branch line to the second drying section, the said second pressure control valve being controlled by the pressure in the main feed line and in the supply line from the said flash tank to the said second drying section.

6. In a steam drying stystem as set forth in claim 1 in which the said steam trap is provided with a bleed aperture permitting a continuous flow of condensate through the trap from the said second drying section.

7. In a steam drying system as set forth in claim 1 in which the means to cool the condensate includes a heat exchanger and a condensate tank.

8. In a steam drying system as set :forth in claim 1 in which the means to return the cooled condensate as a spray includes a pressure control valve and a series of nozzles injecting the cooled condensate into the condensa-te after it passes from the said trap and trap by-pass.

9. In a steam drying system as set forth in claim 7 in which a vacuum pump maintains a vacuum at the top of the condensate tank to provide a differential between inlet and outlet pressure at the said second drying section.

References Cited by the Examiner UNITED STATES PATENTS 2,142,037 12/1938 Stam-m 34-119 X 2,696,679 12/1954 Cram 34119 2,811,787 11/1957 Clements 34-119 X 2,885,790 5/1959 Cram 34-48 3,190,867 6/1965 Oldweiler et a1. 3410X WILLIAM F. ODEA, Primary Examiner.

D. A. TAMB URRO, Assistant Examiner.

Claims (1)

1. IN A STEAM DRYING SYSTEM EMPLOYING DRYING SECTIONS BETWEEN WHICH A RELATIVELY CONSTANT HEAT DIFFERENTIAL MUST BE MAINTAINED, A FIRST DRYING SECTION AND A SECOND DRYING SECTION, A SOURCE OF STAEM SUPPLY FEEDING STEAM TO EACH OF SA ID DRYING SECTIONS, INDIVIDUAL MEANS TO MAINTAIN A PRESSURE DIFFERENTIAL BETWEEN THE STEAM SUPPLY TO THE SAID FIRST AND SECOND DRYING SECTIONS, A STEAM AND CONDENSATE EXHAUST LINE FROM SAID FIRST DRYING SECTION, EMANS TO RESTRICT THE FLOW OF CONDENSATE THROUGH A PREDETERMINED LENGTH OF SAID EXHAUST LINE, A FLASH TANK IN SAID CONDENSATE EXHAUST LINE, A SUPPLY LINE TO DELIVER FLASHED STEAM FROM SAID FLASH TANK TO SAID SECOND DRYING SECTIONS, MEANSZ TO EXHAUST THE CONDENSATE FROM SAID SECOND DRYING SECTION, SAID LATTER MEANS INCLUDING A STEAM TRAP AND

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