WO1988002508A1

WO1988002508A1 - Cascade-based method of controlling a fluid flow

Info

Publication number: WO1988002508A1
Application number: PCT/FI1987/000131
Authority: WO
Inventors: Erkki Johannes Niskanen
Original assignee: Erkki Johannes Niskanen
Priority date: 1986-09-26
Filing date: 1987-09-25
Publication date: 1988-04-07
Also published as: FI863906A0; FI75682B; FI75682C

Abstract

Method for controlling fluid flow in a conduit through adjusting a valve in said conduit on the basis of suitable quantities such as pressure and/or temperature making use of the method based on the principle of cascade flow. In this method a cylindrical enclosure, fitted with end walls, one of which at least is made movable, is arranged in connection with said conduit. The enclosure is connected with a first opening (d1) to the side of the inlet pressure (p1') and a second opening (d2) to the side of the outlet pressure (p3'). Regulation of the pressure (p2') of said enclosure is effected through changing, for example, the mutual area ratio of the openings (d1, d2) by means of a piston (33, 34), fixably connected to the movable wall and acting as a control means, so that the piston force, required for throttling the valve opening and control of flow, at each moment and each pressure difference is equal with the resultant of forces opposing it. To the inlet pressure (p1') side of the conduit is connected, from the side of inlet pressure (p1') a control flow conduit the control fluid flow of which is controlled by means of said method based on the principle of cascade flow. The inlet pressure (p1') and the outlet pressure (p3') are those of the pressure difference (p1'-p3'), induced over an obstacle or equivalent, located in the conduit.

Description

CASCADE-BASED METHOD OF CONTROLLING A FLUID FLOW

The present invention relates to a method, described in the introductory part of claim 1, for controlling flow of fluid in a conduit through adjusting a valve in said conduit on the basis of suitable quantities such as pressure, volume flow rate and/or, for instance, temperature or any other indicator such as the contents of CO₂. In the patent specifications FI 65500, US

4,429,709 and CA 1,172,518 are disclosed a cascadebased method for controlling the flow rate of a fluid. In the method a valve, provided in a conduit, is adjusted on the basis of suitable quantities such as the pressure and/or the temperature of fluid in said conduit, through connecting, from the environment or any other suitable space of comparison, a control flow conduit, the flow rate of fluid in which is regulated by means of the cascade-based method known as such (FI 60069), in which method an enclosure, most advantageously a cylindrical one and fitted with end walls, one of which is made movable, is arranged to said control flow conduit, and which enclosure is connected with a first opening (d₁ ) to the side of comparison - or inlet pressure (p₁ ) and a second opening (d₂) to the side of outlet - or conduit pressure (p₃) and in which regulation of pressure (p₂) of the cylindrical enclosure is effected through changing the mutual area ratio of the openings (d₁, d₂) by means of a piston or a corresponding means, fixed to. the movable wall and/or through adjusting the area ratio of the openings from outside of the cylindrical enclosure so that the piston force, required for throttling the valve opening and control of flow, at each moment and each pressure difference is equal with the resultant of opposing flow - and other forces and the displacement of the movable wall and/or piston is converted into a control move of a butterfly flap or corresponding device in the conduit. Let F_f be a force caused by the main flow in alignment with the spindle of a device in accordance with said patents, (p₂-p₁) A be a force caused by vacuum acting in the actuator cylinder, where A = the area of the actuator cylinder and F_s = a spring force. Thus, F_f and (p₂-p₁) A effect in closing and F₃ in opening direction. Thus the balance equation can be written

F_s - (p₂-P₁)A-F_f = 0 (1)

In the patents men tioned an internal pressure ratio N of the actuator has been employed so that

(2)

P

Considering that

p₁ - p₂ = (P₁ - P₃ )( = Δp (1 - N)

the balance equation (1) can be written into form

By the other hand, in the specifications of said patents, the N-value is given by means of the area ratio of the control openings in form

(4)

where k = the product of the quotients of loss coefficients and corresponding densities of the openings d₁ and d₂ and its value is taken in the following k = 1.

Each constant flow rate has its own geometry of the control cone changing the d₂-opening so that the balance equation (1) comes true. Through combining equations (3) and (4), one could see that the balance takes place at given volume flow rate V when the

orifices d₁ and d₂ are chosen so that

(5)

where

Since d₂ is a function of x only, d₂ =

(d₂(x), it follows from equation (5) that d shall be a function both of x and d₁ = d₁ (x₁ , _S). Each new

valve of is met by a new d ₁ .

An accurate solution for balance can by obtained through measuring x together with where upon d₁ can

by calculated from equation (5). Naturally, instead of position x, also the pressure difference Δ p can be measured. The result of this, together with known function Δ p(x₁ ,V_S) provides a balance position x which is different of new position x.

An approximate solution can be found through presentation of equation (5) in product form:

g(x, ) =

g₁(x) g₂

(7)

whereupon d₁ is dependent on the volume flow rate V_S only; d₁ = d₁ (V_S) and the equatation (5) comes true by tak ing

d₁(

) = g₂

(8) d₂(x) = 1/g₁(x) (9) The result is improved by a proper choice of the spring and the geometry of the valve itself. The best approximate result can be obtained through numerical optimization. In a device according to the patents mentioned above, the balance equation (1) is transformed to correspond a new volume flow rate through alteration of the values of factors F_f and F_s through adjusting simultaneously both the position of the main flow adjusting device, like butterfly flap, and the rate of compression of the spring by means of the adjusting means located on the spindle.

It is apparent from the above mentioned that for effecting an accurate volume flow with said device, a sensor, measuring the values x or p, a computing unit and an actuator controlled by it for adjusting the opening d₁ , is required. Thus, since the shape of the cone adjusting the openinig d₂ is fixed, only an approximate value is obtainable with a device according to said patents. In addition, adjusting means of the flap and the spring are needed in the device raising the costs.

The object of the invention is to avoid the disadvantages mentioned above and to realize such a cascade-based method, by means of which the range of the volume flow, to be held constant, can by chosen accurately through control of the opening d₁ without the values of x or Δp. This is achieved by the aid of the characteristic features of the invention presented in the patent claims enclosed.

A method according to the invention is not bound to any particular system, application or medium and it can be varied in various ways for different purposes. It will be described in the following in detail by means of an adjustable constant flow valve, which is one of its possible applications in the field of air conditioning technique. The specification is associated with drawings, in which figure 1 is a cross-sectional top view of a valve with an actuator. The valve is adjusted with a manually controllable thermostat; figure 2 illustrates the operation of the valve by means of curves.

A flow obstacle or equivalent causing pressure difference is located in a conduit 1. In this case the flow obstacle is a metering flange 3 located in a metering housing 2 over which flange the flow induces a pressure difference p₁ ' - p₃ '. Since p₃ ' is metered immediately behind the flange, p₃' is considerably lower than p₃, which is the pressure of a valve housing 10 upstream the valve after the turbulant flow has became steady.

A gate valve 11, which forms a lock face having such as cylindrical or spherical surface and which can be cut to a shape which results in a proper valve characteristic, has its bearings fitted in the valve housing 10. The valve turns centrally in relation to a gate 12, but can also have its bearings fitted eccentrically so that it gets loose slightly off the gate when turning. Additonally it can be balanced when required by the position of the assembly. Behind the gate valve pressure p₄ is prevailing.

The springs 13 and 14 are linked to the gate valve 11 in such angles of installation that the sum curve of the torque curves obtained creates to the spindle 33, by means of a connection rod 16, a constant directional force, which-tends to open the valve. The direction can also by changed to opposite through changing the working direction of the actuator and the spring force can be replaced with a gravity force like a weight. A control housing 20, whereto pressure p₃ ' is introduced through a connection pipe 21, is fixably engaged with said valve housing 10. Into a cascade actuator 30 located in said control housing 20 is introduced a control pressure p₂', which Is determined by the ratio of the openings d₁ and d₂ in accordance with the formula 4. The actuator 30 comprises a fixably mounted cylinder bottom 31, a linear bearing 32 and a piston 34, which is supported by said spindle 33 for axial displacement and joined hermetically by means of bellows or a roller membrane with said cylinder bottom 31. When required, the spindle 33 is fitted with a control cone 35 in order to compensate, for instance an error of spring forces.

The actuator 30 is connected with a connecting 1 pipe to a controller 22, where the opening d₁, is controlled by a thermostat 23. The control air of pressure 1 p ₁ is introduced, through a connecting pipe 24, to the controller 22, wherefrom it continues to the actuator 31 through a connecting pipe 25. The diameters of the connecting pipes shall be multiples of those of the control openings d ₁ , d₂. Since the flow in the gate valve does not cause any torque in it as far as it is of proper form, the flow force F_f = 0, and, since the sum of spring forces is a constant, the balance equation is simply

(p₂'- p₃')A = F_S. (10)

where A is the cross area of the actuator 31 and F_S is the sum force of the spring forces a constant.

Since in this particular case the pressure difference p₁' -p₃' = Δp' depends on the volume flow only,

P₁'-P₃' = Δ_P

). (11)

Thus, the balance equation (10) gives

(12)

The pressure difference p, controlling the rate of control flow, is now independent of the position x of the spindle 33. Thus, both the orifice d₂ and the spring force F_s can be chosen as constants and independent of x. Therefore, the equilibrium state of the equation 12 maintains a volume flow rate if d₁ is determined according to the equation

(13)

Operation of the valve can be described by means of Fig. 2, where the x-axis shows displacement of the spindle to the right so, that the valve is closed in origo. Downwards on the vertical axis is given the rate of main flow as a function of the pressure differen ces p₁, p₂, p₃ , p₄ and p₅, measured o ver the valve and upwards the axis the control forces Fm₁ , Fm₂ and Fm₃, which are piston forces, obtained by means of the pressure differences, caused by the flow rates

and in the measurement flange 3. The resultant spring

force F_j (=F_S) is presented also in the same axis system.

If, for example, the pressure difference over the valve, with the position x₁, of the spindle, increases from pressure difference p₄ to pressure difference p₅, the volume flow rate increases from point b of level V₂ to the level of point e, where upon the pressure difference Δp increases increasing the control force Fm₂ from point f to point g. Since the N-value, wh ich = F_j/F_m2 is too high now, the spindle moves in closing direction to position x₂, where the N-value is the same with pressure difference p₅.

If the valve is moved from the position x₄ of the spindle to the position x₃ or opened, the volume flow rate increases, from level V₂ to the level of point i, whereupon p' increases and the control force Increases from point j to point k. For very same reason as above the spindle moves in closing direction back to point x₄, where the N-value is valid with pressure difference p₂. If the N-value is increased at pressure difference p₂ and the position x₅ of the spindle, for example, through increasing the opening d₁ with manual control or by means of a thermostat or any other actuator, the control force Fm₃ increases from point n to point o, whereupon the valve is closing up to the position x₆, where the N-value is the same again but now at lower level of point q on the pressure difference curve p₂.

As it becomes evident in Fig. 2, the N-values are constants over the pressure difference range p₁- p₅ at all levels of so that control of the volume

flow rate can be made continuously by means of the opening d₁.

It is to be noted that the flow obstacle 3 can be located in another conduit where the proper valve Is fixed. For example, the val ve in ventilation systems can be located in an inlet conduit while the flow obstacle 3 can be located in an outlet conduit. Thus the flow in the inlet conduit Is controlled by means of the flow in the outlet conduit.

Claims

CLAIMS:

1. A method for controlling the volume flow of a fluid in a conduit (1), provided with a valve, in whic method the valve is adjusted on the basis of suitable quantities such as the pressure prevailing in the

and/or the temperature of the flowing fluid, through making use of the method of cascade flow, in which an enclosure, preferably a cylidrical one, is provided in connection with said conduit and fitted with end walls, one of which at least is made movable and which enclosure is connected with a first opening (d₁) to the side of inlet pressure (p₁ ') and with another opening (d₂) to the side of outlet pressure (p₃') and in which the adjustment of the pressure (p₂') of the cylindrical enclosure is effected through changing the mutual area ratio of the openings (d₁, d₂) by means of a piston (33, 34 ) or a corresponding means operating as a control means fastened to said movable wall, and/or through adjusting the area ratio of the openings from outside so that the piston force, throttling the flow opening and required for control of the flow at each moment and each pressure difference is equal with the resultant of opposing forces resisting it, and so that a control flow conduit is connected to said conduit from the side of inlet pressure (p₁ '), in which conduit the control volume flow of fluid is adjusted by means of said cascadebased method, in which said cylindrical enclosure or a corresponding one, is provided into connection with said control flow conduit, and in which enclosure the displacement of said movable wall and, at the same, the displacement of the piston, is converted into control movement of the valve or a corresponding one in said conduit, c h a r a c t e r i z e d in that the inlet pressure (p₁ ') and the outlet pressure (p₃') are the pressures of the pressure difference (p₁ ' - p₃'), induced over an obstacle or a corresponding one, provided in a conduit.

2. A method, according to claim 1, c h a r a c t e r i s e d in that said obstacle is a plate provided with an orifice, such as a metering flange (3).

3. A method, according to claim 1 or 2, c h a r a c t e r i z e d in that the valve used is a gate valve (11) fitted with a lock face having such as a spherical or cylindrical surface.

4. A method according to claim 3, c h a r a c t e r i z e d in that the lock face of the valve is provided with two springs (13, 14), which are, in relation to each others, in such an angle that the resultant spring force is constant.