GB2218827A - Fluid flow control means - Google Patents

Abstract

To stabilise the flow of fluid through a series of channels (14) supplied in parallel, individual channels or groups of channels of the series are each connected to a separate vortical flow control device. The control devices receive their fluid via tangential, oppositely disposed inlets (28, 30) from respective spaces (10, 20), the fluid in the second space (20) being received from the first space (10) via one or more apertures (21) which results in a lower pressure in the second space. Each device responds to an increase in flow in its associated channel or channels by increasing its resistance to flow, thereby tending to stabilise the flow. The apparatus can be so arranged that there is a stable relationship between pressure drop and mass flow with a considerably smaller energy loss than is dictated by conventional restrictor means employed for the same result. <IMAGE>

Description

FLUID FLOW CONTROL MEANS The invention relates to apparatus having fluid flow control means and, in particular, to means for controlling the flow of fluid through channels fed in parallel, eg. heat exchange tubes in a power generating plant.

The term "channel" as used in this specification is intended to include any conduit, pipe, tube, tubular or other hollow passage or bore or opening through which fluid flows.

When fluid flowing in a channel undergoes a phase change as a result of heat input through the channel wall, it is possible for the mass flow to vary even through the boundary conditions at the ends of the channel are kept constant. For instance, an evaporator tube of a boiler which has constant boundary conditions of inlet water pressure and enthalpy and outlet pressure may experience cyclically varying mass flow. This is usually referred to as dynamic instability.

A second kind of instability, namely static instability, may occur when a channel with constant boundary conditions is able to pass either a larger or a smaller steady flow. This is best illustrated with reference to Figure 1 of the accompanying drawings. Figure 1 is a graph showing a possible relationship (curve A) between mass flow of a fluid in a heated channel in which boiling is occurring, and the fluid pressure drop across the ends of the channel. If the pressure loss across the ends of the channel is of a magnitude indicated by the line D-E-F then it will be seen that three values of mass flow are possible. Of these, the flows at D and F, if disturbed a small amount, will return to their original values, but the flow at E will not recover from a disturbance. It is as if points D and F are points of attraction and point E is a point of repulsion.Therefore, for practical purposes, there are two possible flows for the combination of channel characteristic and driving pressure depicted by the curve A in Figure 1. Static instability may be regarded as a special case of dynamic instability in which the cycle time is infinitely long.

These types of instability are generally undesirable. In the case of a water tube boiler, for example, the resultant flow cycling leads to damage due to repeated overheating and quenching of material. Nominally identical paths through a tube bank may experience dissimilar conditions which would not have been considered in the design of the equipment. Thus, differential thermal expansion could cause unwanted stresses. Corrosion, creep and fatigue may all be accelerated. Ideally, therefore, a channel should only have one steady flow rate for each combination of boundary conditions, so that parallel channels would have flows in fixed proportions to one another throughout the operating range.

Unstable channels may be made to perform stably if additional flow resistance is provided at the inlet to the channel. The resistance typically takes the form of an orifice which has a pressure loss proportional to the square of the flow. Curve B in Figure 1 shows the resultant channel characteristic owing to the fitting of an inlet orifice to the channel. The curve B has a slope that is everywhere positive. Unlike curve A, any given pressure loss is associated with a single mass flow value.

Therefore, the channel will have a unique flow rate for any set of boundary conditions and is stable.

The channel has been stabilised because that part of the characteristic curve A which had negative slope before the orifice was added has had a positive slope of sufficient magnitude superimposed on it so that the resulting slope (curve B) is positive throughout its range.

This increased slope represents an increased pressure loss, which is undesirable because it increases both capital cost and running cost of plant.

Furthermore, it is difficult to "tune" a heat exchanger using orifices since changes in the characteristics of the channels can produce changes in the flow of hot gases over the channels thus producing feedback which can adversely affect the channel characteristics.

It has also been proposed to equalise or proportion flows in such channels by using a series of fluidic devices having venturi nozzles (eg. see GB 1342994 and GB 2007388A) with control ports connected to a common space. Flow between the control ports is used to change the resistance to flow in a channel in a sense to return the flow to the required rate. However, such venturi nozzles are similar to the inlet orifice previously mentioned, particularly in terms of pressure losses, and are undesirable.

According to one aspect of the present invention, in an apparatus comprising a plurality of flow channels for fluid supplied from a common source, there is provided plurality of fluid control devices connected to the inlets of respective channels or groups of channels, each control device comprising a chamber having first and second inlets for generating opposed vortical flows within the chamber and an outlet to the channel or channels associated with the device, the arrangement being such that, under steady conditions in said supply source, a deviation in flow in any channel causes the associated control device to change the output to the channel in the contrary sense so as to tend to restore the flow rate therein.

The invention also provides, in another aspect, a fluid control device suitable that can provide the form of flow control required, comprising a chamber in the form of a body of revolution and axially spaced first and second inlets opening tangentially into the chamber, and, at an end of the chamber remote from said inlets, an axial outlet having a restricted throat portion.

Preferably, said common source provides a flow at a first pressure at which the first inlets of the devices are supplied and from which a flow is directed through restriction means to provide a lower pressure supply to the second inlets, and the sizes of said inlets are selected such that flow through said second inlets determines the direction of said resultant vortical flow.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a graph showing three different characteristic curves of pressure drop in a channel against mass flow of fluid in that channel, including the curves A and B already described; Figure 2 is a schematic cross-section through part of a heat exchanger incorporating control devices in accordance with the invention; and Figure 3 is an enlargement of the encircled area in Figure 2 showing the device in cross-section.

The heat exchanger of Figures 2 and 3 typically has a water feed 10, eg. a large inlet pipe or header which terminates in a tube plate 12. Fluid flow channels 14 extend between the tube plate 12 and a second tube plate (not shown) of a steam header, the channels 14 being contacted by hot gases in the space (indicated by the legend "GAS SIDE") between the headers to heat the fluid flowing through the channels between the headers.

A fluid flow control device 16 is connected to the inlet of each channel and extends upstream thereof to pass through a tube plate 18 spaced from the tube plate 12 to define a compartment 20. The water feed 10 and the compartment 20 respectively constitute first and second common sources of water for the devices 16. In this example, the compartment 20 is fed from the water feed 10 through one or more holes 21 in the tube plate 18 which create a resistance to flow and, consequently, a pressure drop between the water feed 10 and the space 20.

Referring to Figure 3, each device 16 has a body 22 defining a chamber 24 which is symmetrical about an axis 26. The chamber 24 has two tangential inlets 28,30 oppositely disposed for generating opposed vortical flows therein, and an axial outlet 32 which is registered with the inlet to the associated channel 14. The inlet 28 communicates with the water feed 10 and is slightly smaller than the inlet 30 which communicates with the compartment 20. At the outlet 32 the body has a restricted throat portion 34.

In a typical design of the devices 16 as described above, if a hundred units of pressure drop are available between the water feed 10 and the inlets to the channels 14, then if the channel inlet pressure is taken as datum the pressure distribution will be roughly as indicated by the numbers in the rectangular boxes in Figure 3.

As previously mentioned, the inlet 30 is slightly larger than the inlet 28, which fact coupled with the pressure distribution shown in Figure 3, causes the resultant vortical flow in the chamber 24 to be in the sense dictated by the inlet 30. It is the presence of a vortex in the chamber 24 which accounts for the pressure loss across the chamber 24, ie. from 54 units at the greatest radius to 18 units at the axis 26.

Because the inlets 28,30 of the devices 16 are all open to their respective common sources of water, ie.

the water feed 10 and the compartment 20, a flow fluctuation in a single channel 14 has a negligible effect on the pressures there. Thus, if the mass flow in one channel changes, because the pressures externally of the device 16 associated with that channel 14 remain substantially constant the internal pressures in the device have to change. The effect is that each device responds to an increase in flow in its associated channel by increasing its flow resistance.

As an example of the operation of a device 16, if the internal pressure at the greatest radius changes from 54 to 53 units, then the pressure differences across the inlets 28 and 30 change by 1 part in 46 and by 1 part in 18, respectively. Therefore, the fractional momentum change of flow through inlet 30 is greater than the fractional momentum change of flow through the inlet 28.

As the strength of the resultant vortex in the chamber 24 is determined by the difference in the momentum streams flowing through the inlets 28,30 and as the streams have momentum values of similar magnitude, those fractional momentum changes result in relatively large changes in the strength of the resultant vortex and, consequently, in the pressure drop across the chamber.

The group characteristic of the set of channels in the heat exchanger of Figs. 2 and 3 might appear as the curve C in Fig. 1, ie. with a considerably smaller pressure loss than the orifice - stabilised example of curve B which represents the characteristic that might be obtained if designing for about half the pressure drop.

(Alternatively, for the same pressure drop the characteristic might have about double the slope of curve B). It will be noted that the curve C still retains a reflex portion of the kind described in curve A. However, with the heat exchanger operating at the point X, for example, on the curve C, the deviation occurring in a single channel as just described, follows instead the positive characteristic Y shown in broken lines. This result is a consequence of the relatively large changes in the vortex strength and pressure drop in response to relatively small changes in the mass flow through a channel 14 shown in Figure 1 and so stabilise the flow through the channel 1.

In other words, although flow through the channels would appear to be unstable because a part of the curve C still has a negative slope, if the flow through an individual channel attempts to deviate from the required flow (say point X on curve C), then the control device operates to follow a positively sloping characteristic as shown by the dotted line Y allowing stable correction of the deviation. Therefore, despite the fact that the flows in all the channels acting together follow curve C, if any single channel tends to depart from the group characteristics, its control device functions to correct the flow through that channel back to the point X.

In the embodiment described above, the devices 16 are substantially identical and the proportion of flow passed by adjacent channels is 1:1. Although there are applications of the invention in which a proportionality of flow of 1:1 is desirable, in other applications it may be necessary to have other proportionalities of flow. For example, in many heat exchangers, owing to the geometry of the enclosure of the gas space and the arrangement of channels therein, the channels will not experience equal heat exchange conditions. Thus, across a tube bank it may be necessary to vary the proportion of flow passed by the channels between say 0.9 and 1.1. Such flow proportioning is achieved by suitable sizing of the inlets 28,30 and outlet 34 of the devices 16 and/or, when flow proportioning between groups of channels is required, control of the pressures of fluid in the common sources.

Although the device 16 in accordance with the invention has been described with particular reference to its use with heat exchange channels, it will be appreciated that it is applicable to other situations in which the flow of fluid through parallel fed channels has to be controlled, eg. the proportioning of flow to oil burners in power generating plant.

Furthermore, the device 16, by controlling the pressure of fluid applied to either or both of the inlets 28,30 thereof, can be used to control flow through either a single channel or a group of channels. In the latter instance, either a single device 16 can control the flow to a group of channels, or, as described with reference to the accompanying drawings, each channel has its own device 16 which communicate with the common sources of fluid, the pressures of which are controlled.

Claims (14)

1. Apparatus comprising a plurality of flow channels for fluid supplied from a common source and a plurality of fluid control devices connected to the inlets of respective channels or groups of channels, each control device comprising a chamber having first and second inlets for generating opposed vortical flows within the chamber and an outlet to the channel or channels associated with the device, the arrangement being such that, under steady conditions in said supply source, a deviation in flow in any channel causes the associated control device to change the output to the channel in the contrary sense so as to tend to restore the flow rate therein.

2. Apparatus according to claim 1 wherein said common source provides a flow at a first pressure at which the first inlets of the devices are supplied and from which a flow is directed through restriction means to provide a lower pressure supply to the second inlets.

3. Apparatus according to claim 2 wherein the second inlet of each device is larger than its first inlet, whereby the flow direction through said second inlet determines the direction of the resultant vortical flow.

4. Apparatus according to any one of claims 1 to 3 in which the channels are heat exchange channels extending between inlet and outlet header sections.

5. Apparatus according to claim 2 or claim 3 together with claim 4 wherein the flow control devices extend through a partition in the inlet header that divides an intermediate space from an entry space of the header, the respective inlets of each control device opening one from the entry space and the other from the intermediate space, the intermediate space receiving fluid from said entry space through flow restriction means whereby to lower the fluid pressure in the intermediate space relative to the entry space.

6. Apparatus according to any one of the preceding claims wherein said first and second inlets enter their respective chambers tangentially.

7. Apparatus according to any one of the preceding claims wherein each control device chamber has the form of a body of revolution.

8. Apparatus according to any one of the preceding claims wherein each control device chamber has an outlet with a restricted through portion.

9. Apparatus according to any one of the preceding claims wherein the sizes of the inlets and outlets of the control devices are so selected that, for the required flow conditions, at least some of the channels carry different proportions of the total flow.

10. Apparatus comprising a plurality of flow channels for fluid supplied from a common source and a plurality of fluid control devices connected to the inlets of respective channels or groups of channels, each control device comprising a vortex chamber with first and second inlet means, and an outlet means to the channel or channels associed with the device, first and second spaces supplying the flows to the first and second inlet means respectively, said second space receiving its fluid through restriction means from said first space whereby the fluid to said second inlet means is at a lower pressure than that to the first inlet, the flow through said two inlet means being dependent upon variations in the supply pressures thereto in relation to that in the outlet means, the arrangement being such that, under steady conditions in said supply source, a deviation in flow in any channel causes the associated control device to change the output to the channel in the contrary sense so as to tend to restore the flow rate therein.

11. A fluid control device comprising a chamber in the form of a body of revolution and axially spaced first and second inlets opening tangentially into the chamber, and, at an end of the chamber remote from said inlets, an axial outlet having a restricted throat portion.

12. A control device according to claim 11 wherein, over the axial extent of the chamber containing the first and second inlets, the chamber has an annular form.

13. Apparatus for controlling the supply of fluid to a plurality of flow channels connected in parallel, constructed and arranged for use substantially as described herein with reference to the accompanying drawings.

14. A fluid control device constructed and arranged for use substantially as described herein with reference to the accompanying drawings.