JPH0654000B2 - Dryer differential pressure control device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control system for controlling the differential pressure between steam input and output lines of a web dryer, and more particularly to a control system for controlling such differential pressure between steam input and output lines of the dryer section of a papermaking machine.

2. Description of the Prior Art In papermaking machines, the formed web passes through the paper dryer section immediately after passing through the press section. Such a dryer section includes a plurality of heated rotating cylinders over which the wet paper web is passed to achieve the desired dryness of the web. More specifically, in a conventional dryer section, the wet web is passed around the outside of steam-heated super-steel dryer cylinders. The steam used to heat these dryer cylinders enters the dryer through a hollow journal by a rotating seal and condenses on the inside of the dryer shell or cylinder. As the steam condenses on the inside surface of the rotating dryer cylinder,
Such condensed water is discharged by a siphon assembly, although such rotating cylinders are not uncommon in dryer sections.5.
When operated at high web speeds of between 0.8 m/sec and 6.1 m/sec, the condensed water does not collect at the bottom of the dryer, but rather is thrown by centrifugal force around the inside surface of the dryer cylinder or shell. This behavior of condensed water within the dryer shell is known in the art as the "fringing phenomenon."
known as TAP by R. E. White.
This is fully described in an article published in PI 1958, Volume 41, No. 2. When surrounded by condensed water, the dryer shell is not exposed to "live steam" but is insulated from the live steam by a layer of condensed water that prevents conduction from the live steam to the surface of the dryer shell and the nearby paper web. Such insulation dampens the drying action, and this resistance to heat conduction can be minimized by reducing the thickness of the condensed water layer within the dryer shell.

The accumulation of non-condensable vapors within the dryer shell can exacerbate cross-machine direction non-uniformities in the drying characteristics of the dryer. This problem is discussed in TAPPI, Volume 4
6, No. 9, 1963, by R. B. Hur
Such accumulation or buildup of non-condensable vapors or gases can be minimized by continually draining some of the uncondensed vapor or water vapor from the dryer shell along with the condensed water. This uncondensed vapor or steam flow can entrain the non-condensable gases and prevent the accumulation of such gases within the dryer shell.

Moreover, such steam flow has the beneficial secondary effect of reducing the pressure differential between the input and output lines of the dryer shell;
Such a pressure difference is required to drain the condensed water.
The low-density steam stream entrains the high-density condensate and mixes with it to form a two-phase mixture of substantially lower density than the condensate. The pressure differential required to expel this relatively low-density mixture of steam and condensate against the centrifugal force generated by the rotation of the dryer shell is relatively reduced. Furthermore, this steam stream can be used in other dryer shells in the drying section that require lower-pressure steam. Alternatively, such a steam stream may be boosted or supplemented for reuse in the same dryer shell, provided, of course, that the pressure differential across the dryer shell is not too great.

A further consideration regarding condensate drainage is the need for operational stability. In practice, it has been observed that if the outer tip of the siphon tube, which is in close proximity to the condensate, becomes submerged by the condensate, the fastest condensate drainage may be discontinued. In this case, the dryer will become saturated with condensate, the drying rate will decrease, and the load on the dryer drive will increase proportionately. These issues are discussed in a Canadian Pulp & Paper Magazine article.
More specifically, in "The Art of Electron Microscopy," by T. A. Gardner in "Electron Microscopy," Volume 65, No. 14, 1964,
TAPPI Technical, published in 1983
Information Sheets, TISO 1
This is highlighted and discussed in 4-60.

From the foregoing, it is clear that several goals are pursued by the condensate drain system. First, to drain condensate at a rate at least equal to the rate of condensate formation within the dryer shell so as not to flood the dryer. Second, to maintain the condensate layer as thin as possible so as to maximize the rate of heat transfer from the "live steam" to the paper web. Third, to vent and remove noncondensable gases so that improved cross-machine direction uniformity of drying rates can be achieved. Fourth, to achieve condensate removal from the dryer shell using the minimum required pressure differential while maintaining stable operation of the system.

Many methods have been proposed to attempt to achieve the four aforementioned goals, and these proposals are often referred to as "Paper Mach
ine Steam & Condensate Sy
In a TAPPI publication entitled "Stems," H.
These basic concepts are further described in U.S. Patent No. 4,447,964 to Gardner and U.S. Patent No. 2,869,248 to Justus.
TAPPI, volume 62, No. 11,197
The article by Perrault published in Journal of Steam Engineering, Vol. 9, No. 1, 1984, also published by TAPPI, teaches the purposes of steam.
Conference Proceedings (19
Jumper on page 347 of the 1984 Engineering Conference Bulletin
The paper by also relates to the above.

Although the above-mentioned patents and other disclosures discuss the above objectives and propose systems for achieving those objectives, these prior methods and apparatus all suffer from certain inherent control problems. While each of the above systems can be adjusted to operate acceptably under particular conditions, they are unable to accommodate, both in direction and magnitude, changes necessitated by occasional system malfunctions or changes in machine operating conditions.

As an example showing the ineffectiveness of these previous proposals,
per Machine Steam And Con
Conventional differential pressure controls, such as those outlined in "Densate Systems" (Figure 1), accept an input with only a single set point. However, the required set point changes as the machine's operating speed, steam pressure, and condensate flow rate change. Because set point changes are a complex function of the above variables, as shown in Figure 2, machine operators will often set the differential pressure set point to the highest value required to satisfy a wide range of operating conditions. Setting the differential pressure set point to the highest value results in inefficient operation. Furthermore, such systems also suffer from a tendency to fill with water. In addition, if one of the dryer's siphons floods, having a fixed differential pressure set point will cause the steam flow control valve to close slightly, even though the appropriate control action would be to open the valve slightly in an attempt to drain the water from the dryer.

U.S. Patent No. 2,888,892 to Justus, shown in FIG.
The flow control concept of '69,248 avoids the latter of the aforementioned problems by measuring and controlling the amount of steam flow that is discharged with the condensate. If one of the dryers then begins to fill with water, the control valve will open slightly.
However, this system operates at only one fixed set point which may not be appropriate for all operating conditions.

In the aforementioned paper by Jumper, the system depicted in Figure 4 uses a microprocessor to adjust the set point based on the condensate flow rate from the separator tank. The controller establishes the set point by continually decreasing the set point until the condensate flow rate decreases. However, such approach results in the dryer being near or below a stable operating point during operation. In many high-speed dryers, the condensate flow rate will not decrease until the differential pressure is low enough that the dryer fills with water. Once this occurs, the dryer will not be able to recover from the flooded condition, even if the differential pressure is subsequently increased.

The present invention overcomes the aforementioned deficiencies of various prior art proposals by recognizing the importance of parameters that indicate what the optimum differential pressure is for stable and efficient operation of the dryer section. This method requires inputs of at least the machine's operating speed and condensation rate. However, this method also generally requires a steam pressure input, and may also utilize a signal from a sheet break sensor as an input to adjust the set point during a sheet break condition.

In addition to using the aforementioned parameters that characterize the system's operation as inputs, the proposed system also provides a setpoint signal for the steam flow momentum. This parameter, as described below, is important for ensuring stable and efficient operation of the exhaust system. The steam flow momentum is proportional to the product of the steam flow specific gravity and the square of the steam flow velocity. This parameter is provided as an output parameter instead of differential pressure, which is a mass or volumetric flow rate.
The optimum differential pressure for normal operation as recognized by the present invention is required to be set somewhat higher than the minimum differential pressure to accommodate occasional operating irregularities.
These occasional annoyances include increased condensate flow, small differential pressure fluctuations, and increased operating speed.
An applied pressure difference of 4 bar has proven to be adequate.

The above method does not require frequent set point adjustments and consequential response, as in the system described in the aforementioned Jumper paper. As noted in prior proposals, such control action often leads to an unstable region near the minimum differential pressure shown in the curve of Figure 2. Rather, the present system utilizes experimentally measured relationships, such as those shown in Figure 2, to adjust the siphon system to its most stable and efficient operating point. This system, in accordance with the present invention, is further enhanced by the use of a small radial siphon pump with a steam outlet and a low-loss vortex flowmeter. Regarding such enhancements, it is recognized that the low pressure loss requirement can be achieved by increasing the radial pipe size or by using a smaller steam flow. Previous implementations have typically utilized increasing the radial pipe size.
However, as shown by the top curve in Figure 2, the sensitivity of steam flow to differential pressure increases, and the steam flow rate when the dryer is operated at a stable differential pressure is generally excessively high. That is, the minimum differential pressure is about 0.14 b
The present invention takes advantage of the fact that when the radial pipe size is reduced, the minimum differential pressure increase is relatively small, while the reduction in steam flow sensitivity is very significant.
By controlling the differential pressure to a higher value of 1 bar and using small radial pipes, the steam flow does not change significantly during machine upsets. Therefore, the valves, condensers, and connecting pipes are small enough that the system continues to operate in a stable condition even at low differential pressures.

In accordance with the above-described aspects of the present invention, the operation of the exhaust system is further stabilized by the use of a steam vent as described in the aforementioned Justus patent. While the present invention controls the operation of the dryer away from points of instability, the use of a steam vent ensures that the dryer can recover from even major system upsets. For example, if the differential pressure were reduced to zero even for a short period of time, the tip of the siphon would be submerged in condensed water. With conventional differential pressure control, the dryer would remain full of water because the differential pressure set point would be inadequate to counteract centrifugal force and remove the condensed water. The aforementioned Jumpeter patent
In the system of U.S. Pat. No. 6,869,292, the differential pressure will be increased by the controller only until the controller determines that the increase will no longer result in an increased condensate flow rate.
No. 48 attempts to increase the differential pressure to meet the steam flow setpoint only. However, in high speed machines,
The required flow rate may be achieved by only a few unflooded dryers in the dryer section, even though the differential pressure is insufficient to rescue the remaining dryers from flooding. In a system according to the present invention, the steam flow momentum set point will similarly increase the differential pressure to achieve the set point flow rate. At the same time, the differential pressure required to drain a flooded dryer is reduced by the reduction in the specific gravity of the discharged condensate due to the additional steam flow entering through the steam outlet located above the condensate level. Furthermore, the system will automatically increase the set point due to the reduction in condensate flow rate. The combined effect of these three actions should provide a previously unattainable range of operational stability.

A third feature associated with the system of the present invention is the use of low-loss meters. These low-loss meters may include simple orifice meters with small restrictions or vortex meters. The former are used in the art and are
The orifice flowmeter provides a pressure drop that is directly proportional to the momentum of the steam flow. This pressure drop can be measured and used as an input to a control system. Such orifice flowmeters are commercially available, but the signal they provide is often processed to provide a volumetric or weight flow rate. In accordance with the present invention, it is instead proposed that the vortex shedding frequency be used as a direct input to the control system. This frequency is also related to the momentum of the steam flow. Such a device is advantageous because it can be used as part of a control system without providing a pressure drop.

Another feature of the present invention is the method for selecting the set point for the steam flow rate. By careful testing, a set of curves similar to those shown in Figure 2 can be established. The set point for the desired operation can be determined by first locating the minimum differential pressure for the given conditions of operating speed, dryer pressure, condensation rate, and siphon size.

An increment of approximately 0.14 bar is added to this value to allow for minor upsets during operation, as previously described. The steam flow rate corresponding to this differential pressure is then used in the steam flow momentum calculation to be used as the set point.

This series of calculations can be performed for any siphon configuration, and the momentum set point value is then plotted as a function of condensation load for each operating speed. The controller then uses the measured condensation rate and operating speed as inputs to calculate the desired set point using the curves of Figure 2. Some typical curves of this type are shown in Figure 5.
It has been found that, from time to time, the set point determined by this procedure provides a steam volumetric flow rate less than that required for adequate exhaust of noncondensable gases. Therefore, it may be advisable to establish a certain volumetric flow rate minimum and to use a control system to check and ensure that this minimum is always met.

The primary object of the present invention is to overcome the aforementioned deficiencies of prior art proposals and to make a significant contribution to web drying technology.
A method and apparatus for extracting condensed water from the rotating cylinder of a paper dryer is provided.

Another object of the present invention is to provide a method for indirectly controlling the pressure differential across a heated dryer as a function of the dryer's operating speed and condensate flow rate by directly controlling the kinetic flow rate of uncondensed steam.

Another object of the present invention is to provide a control system for controlling the differential pressure between steam input and output lines of a web dryer such that control signals generated by an operating speed sensor and a condensation rate sensor, respectively, are compared by the control system to determine the optimum ratio setting for the output valve so that the differential pressure between the input and output lines is kept as low as possible while simultaneously preventing the dryer from filling with condensate.

Other objects of the present invention will become readily apparent to those skilled in the art from the drawings, description and appended claims.

SUMMARY OF THE INVENTION The present invention relates to a method and control system for controlling the differential pressure between the steam input and output lines of a web dryer. The system includes a selectively controllable output valve disposed in the output line of the dryer for selectively controlling the flow of steam, condensate, and non-condensable gases out of the dryer. Output valve actuation means is disposed adjacent the output valve for selectively controlling operation of the output valve between its fully open and fully closed settings. Operating speed sensing means is disposed adjacent the dryer for sensing the rotational speed of the dryer and generating a first control signal proportional to the sensed rotational speed of the dryer. Condensation rate sensing means is provided for sensing the rate at which a layer of condensate accumulates inside the dryer and generating a second control signal proportional to the sensed rate of accumulation. Control means is connected to the output valve actuation means for selectively energizing the output valve actuation means in response to the control signals generated by the operating speed sensing means and the condensation rate sensing means, respectively. The control means is arranged to compare signals from the operating speed sensing means and the condensation rate sensing means to determine a maximum ratio setting for the output valve so that the differential pressure between the input and output lines is kept as low as possible while simultaneously preventing the dryer from filling with condensate.

In a more particular embodiment of the present invention, the control system includes input steam pressure sensing means located adjacent the steam input line for sensing the pressure of steam entering the dryer and generating a third control signal proportional to the sensed input line pressure, the third control signal from the input steam pressure sensing means being further compared by the control means to determine an optimum ratio setting for the output valve.

Additionally, the control system includes sheet break sensing means disposed adjacent the web for sensing a break in the web and generating a fourth control signal indicative of such web break.
The fourth control signal from the break sensor is compared by a controller to further determine an optimum ratio setting for the output valve to prevent excessive waste of steam flow when such a web break occurs.

Additionally, the control system includes steam flow sensing means disposed in the output line for sensing the momentum of the steam flow exiting the dryer, the steam flow sensing means generating a fifth control signal proportional to the momentum of the steam flow, the fifth signal being compared by the control means to further determine an optimum ratio setting for the output valve to ensure stable and efficient operation of the system for draining condensate from within the dryer.

The control system includes an orifice flow meter means disposed in the output line for measuring the momentum of the steam flow, the orifice flow meter having a restricted flow passage for providing a pressure drop directly proportional to the momentum of the steam flow.
A vapor flow sensing means is connected across the passageway for sensing the momentum of the vapor flow.

In a particular embodiment of the present invention, the control means is a microprocessor and the dryer includes radial siphon means disposed within the dryer, the siphon pipe having an inside diameter of 2.29 cm or less.

As set forth below, although particular embodiments of the present invention are described in the accompanying drawings and detailed description, it will be appreciated by those skilled in the art that such illustrated embodiments of the present invention are provided merely as examples of how the apparatus and method according to the present invention may be practiced, and that numerous variations of this basic concept may be employed without departing from the spirit and scope of the present invention as defined in the appended claims.

Moreover, although the present invention has been described with particular application to the dryer section of a papermaking machine, it will be appreciated that the present invention, as defined by the appended claims, is intended to apply to control systems for drying webs of any suitable material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a "Paper Mach"
ine Steam & Condensate Sy
1 is a block diagram of a prior proposal for conventional differential pressure control as outlined by H. P. Fishwick in "Systems for Controlling Differential Pressure Flows."

FIG. 2 is a graph showing the differential pressure of the dryer versus steam flow rate.

FIG. 3 illustrates the flow control concept outlined in FIG. 3 of US Pat. No. 2,869,248 to Justus, mentioned above.

FIG. 4 shows the Jumper method taught by Jumper's paper in the aforementioned TAPPI 1984, p. 347.
1 shows the prior disclosure by ter.

FIG. 5 is a graph illustrating a typical curve using a given siphon configuration and plotting condensation rate versus momentum of vapor flow.

FIG. 6 is a schematic diagram of a control system according to the present invention.

FIG. 7 shows a conventional differential pressure and/or pressure regulator similar to that shown in FIG. 6, but with a backup manual operation.
1 is a schematic diagram of a device to which the flow control system is coupled;

Like reference numerals are used to denote like parts throughout the various views of these drawings.

Below are the abbreviations used throughout the various figures of each drawing:
This is a list with their meanings.

FIG. 1 dpc...differential pressure controller pc...pressure controller Δp...(steam) differential pressure dpt...differential pressure transducer pcv...pressure control valve dpcv...differential pressure control valve vb...vacuum bleed port vc...vacuum condenser vcv...vacuum control valve lc...liquid level controller vr...vacuum vessel vac...vacuum pump s...separator lcv...liquid level control valve cp...condensate pump vp...vacuum pump FIG. 3 frc...flow recorder and controller prc...pressure recorder and controller dpt...differential pressure transmitter pt...pressure transmitter FIG. 4 pc...pressure controller dp...differential pressure FIG. 7 p...(steam) pressure μp...microprocessor (computer) pc...pressure controller dp...(steam) differential pressure r/c...recorder and controller sht bk...sheet breakage DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 3 and 4 show various prior art control devices for controlling the discharge of condensate outside the dryer shell.

FIG. 2 shows a graph used to adjust the siphon system to its most stable and efficient operating point.

FIG. 5 is a graph used to adjust the controller by using the measured condensation rate and run speed as inputs to calculate the desired set point.

6 illustrates one specific embodiment of the present invention, showing a control system, generally indicated at 10, for controlling the differential pressure between a steam input or steam supply line 12 and an output line of a web dryer 16, generally indicated at 14. System 10 includes an input control valve 18 disposed in steam input line 12 for selectively controlling the flow of steam from a supply header 20 into dryer 16. A selectively controllable output valve 22 is disposed in output line 1 of dryer 16 for selectively controlling the flow of steam, condensate, and incompressible gases out of dryer 16.
4. An input valve actuation means 24 is disposed adjacent the input valve 18 for selectively controlling operation of the input valve 18 between its fully open and fully closed settings in accordance with a pressure controller 26. An output valve actuation means 28 is disposed adjacent the output valve 22 for selectively controlling operation of the output valve 22 between its fully open and fully closed settings. A first control signal is generated proportional to the sensed rotational speed of the dryer 16.
An operating speed sensing means 30 is located adjacent to the dryer 16. A condensation rate sensing means 32 is connected to the condensate pump 3 for sensing the rate of accumulation of the condensed water layer within the dryer 16 and generating a second control signal proportional to the sensed rate of accumulation.
4 and the condensate return line 36. A control device, indicated generally at 38, is connected to the output valve actuating means 28 such that the control means 38 compares signals received from the speed sensing means 30 and the condensation rate sensing means 32 to determine an optimum ratio setting for said output valve, and selectively actuates the actuating means 28 in response to control signals generated by the speed sensing means 30 and the condensation rate sensing means 32 so that the pressure differential between the input and output lines is maintained as low as possible while preventing flooding of the dryer 16 with condensate.

As shown in FIG. 6, the control system 10 additionally senses the steam pressure between the input valve 18 and the dryer 16 and controls the input valve 18.
input steam pressure sensing means 4 for generating a third control signal proportional to the pressure sensed between the steam pressure sensing means 4 and the dryer 16;
The third control signal from the input steam pressure sensing means 40 is compared by the control means 38 to determine an even more optimum ratio setting for the output valve 22.

In addition to the aforementioned sensing means, the control system 10 also includes sheet break sensing means 42 located adjacent the web for sensing a break in the web and generating a fourth control signal indicative of such a break. The fourth control signal from the break sensor 42 is compared by the control means 38 to determine a further optimum ratio setting for the output valve 22 to prevent excessive waste of steam flow in the event of such a web break.

The controller 10 additionally includes a separator tank 46 and an output valve 22 for sensing the momentum of the vapor flow exiting the dryer 16.
and a vapor flow sensing means 44 disposed between the
Steam flow sensing means 44 generates a fifth control signal proportional to the momentum of the steam flow, which is compared by control means 38 to determine an even more optimum ratio setting for output valve 22 to ensure stable and efficient operation of the system for removing condensate from within dryer 16.

7 illustrates one alternative embodiment in which the control system 10A includes an orifice flow meter means, generally designated 43A, located in the output line 14A for measuring the momentum of the steam flow. The orifice flow meter 43A includes a flow restriction passage 45A for creating a pressure drop directly proportional to momentum. The steam flow sensing means 44A is connected to the passage 45A for sensing the momentum of the steam flow.
are connected across the network.

In the preferred embodiment of the present invention, the control means 38 is a microprocessor and the dryer 16 includes a radial siphon means 48, shown diagrammatically in Figure 6, located within the dryer 16 for removing condensed water therefrom. The siphon means 48 includes a siphon tube having an inside diameter of less than 2.29 cm.

As shown in Figure 6, the control means 38, which may be a microprocessor, has a number of inputs including a machine speed input 50, a condensate flow rate input 52, an input line pressure input 54, a break input 56, and a steam flow input 58.
The output of the controller 38 has at least one set point for controlling the steam flow rate, which is then sensed again for feedback control. The control means 38 receives a condensate flow rate input 52 and a machine speed input 50. In addition, the controller may have a steam pressure input 54. Moreover, the steam flow control set point is proportional to the momentum of the steam flow. The set point is preferably 0.14 bar, which corresponds to 0.07 to 0.21 bar above the minimum differential pressure. The system 10 uses a steam outlet 60 on the dryer siphon and a radial siphon tube 48 having an internal diameter of less than 2.29 cm.

In the preferred embodiment of the present invention, the flow sensing meter 44 is a vortex meter, and the system may be used for condensable vapors other than water vapor. The control means output 62 may provide set points for both the circulation valve and the heat compressor valve in the conventional heat compressor system of FIG.
The control means may be set to maintain a particular volumetric flow rate as a minimum that can ensure the discharge of an adequate volume of non-condensable gas.

The set point value for steam flow momentum will decrease with increasing condensate flow rate and increase with increasing machine operating speed.

As shown in FIG. 7, this system may be coupled with a conventional differential pressure control system and/or flow control system for backup manual operation.

To operate the system normally with an adequate differential pressure, it must be set somewhat higher than the minimum differential pressure to accommodate occasional upsets during operation. Experience has shown that an applied differential pressure of approximately 0.14 bar is adequate. The operation of the system is further enhanced by the use of a small radial siphon, steam outlet, and low-loss vortex flowmeter, as described above. Low pressure losses can be achieved by increasing the radial pipe size or by reducing the steam flow. The present invention takes advantage of the fact that when the radial pipe size is reduced, the increase in minimum differential pressure is relatively small, while the reduction in sensitivity to steam flow is quite significant. By controlling the momentum to a value that can lift the differential pressure approximately 0.14 bar above its minimum value, and by using a small radial pipe, steam flow does not change significantly during upsets in machine operation. As a result, valves, condensers, and connecting pipes are custom-sized to be smaller, and the system continues to operate in a stable condition even with small differential pressures. The use of a steam outlet ensures that the dryer can recover from even major system upsets. Increasing the setpoint flow rate will also increase the differential pressure, automatically increasing the setpoint due to a decrease in system condensate flow. In addition, the differential pressure required to drain a flooded dryer is simultaneously reduced by the increased susceptibility of the condensate to being drained due to the additional steam flow entering from the steam outlet located above the condensate layer. The combined effect of these three actions should provide a level of operational stability previously unattainable.

By providing a simple orifice flow meter with a small restriction in the vortex meter, the pressure drop can be measured and used as an input to the controller.

The desired operating set point can be determined by first locating the point of minimum differential pressure for the given conditions of operating speed, dryer pressure, condensation rate, and siphon size. A small increment, typically 0.14 bar, is added to this value to allow for minor upsets during operation. The steam flow rate corresponding to this differential pressure is then used to calculate the steam flow momentum, which is used as the set point.

The present invention uses the aforementioned parameters as inputs to a controller which in turn calculates the appropriate set point; the system does not require frequent adjustment of the set point and monitoring of the resulting response as described in prior art proposals.

──────────────────────────────────────────────────── Continued from the front page (72) Inventor: Garvin, Stanley P. Jr. Janesville, North Washington, Wisconsin, USA 485

Claims (1)

[Claims] 1. A dryer comprising: a selectively controllable output valve disposed in an output line of said dryer for selectively controlling the flow of steam, condensate, and non-condensable gases out of said dryer; output valve actuation means disposed adjacent said output valve for selectively controlling operation of said output valve between its fully open and fully closed settings; operating speed sensing means disposed adjacent said dryer for sensing the rotational speed of said dryer and generating a first control signal proportional to said sensed rotational speed of said dryer; and condensation rate sensing means for sensing the rate at which a layer of condensate accumulates within said dryer and generating a second control signal proportional to said sensed rate of accumulation.
and control means operatively connected to said output actuating means for selectively actuating said actuating means in response to said control signals generated by said operating speed sensing means and said condensation rate sensing means, respectively, so that the differential pressure between the input line and the output line is kept as low as possible while preventing the dryer from being filled with condensed water.