IL78656A - Fluid pressure control system - Google Patents

Description

'7TU vn nipj ii iyn FLUID PRESSURE CONTROL SYSTEM The present invention relates to a fluid pressure control system for actuating external devices. The invention is particularly applicable for controlling the cleaning operations of a plurality of filters each constructed as described in US Patent No.4, 614, 581 , and the invention is therefore described below with respect to this particular application, but it will be appreciated that the invention could advantageously be used in many other applications as well .

US Patent No.4, 614, 581 describes a filter including a control unit effective to automatically initiate a cleaning operation when the filter becomes overly clogged as manifested by a large pressure differential between the upstream and downstream sides of the filter body. That specification further describes a control unit for actuating an external device, particularly the cleaning means of the filter, which control unit comprises: a housing having a partition wall dividing its interior into at least two compartments; the partition wall being formed with an opening therein establishing communication between the two compartments; a displaceable member displaceable in one of the compartments and defining a first and a second chamber on opposite sides thereof; a stem fixed to the displaceable member; a valve member carried at one end of the stem and movable thereby to open and close the valve opening; a spring biassing the displaceable member to the position wherein its valve member closes the valve opening; a first inlet port in the housing communicating with the chamber on the side of the displaceable member opposite to the partition wall, for inletting a first control pressure; a second inlet port in the housing communicating with the chamber between the displaceable member and the partition wall for inletting a second control pressure into the chamber effective, when the second control pressure is lower than the first control pressure by a predetermined magnitude, to cause the first control pressure to displace the displaceable member against the bias of the spring to move the valve member to open the valve opening, and thereby to transfer the second control pressure via the valve opening to the second compartment; and means communicating with the second compartment for actuating the external device in response to the transfer of the control pressure from the inlet port to the second compartment.

According to the present invention, the control unit of that patent is modified such that the housing is formed with a second valve opening to a second external device and is also formed with a vent to the atmosphere; the stem including a second valve member normally located to disconnect the second valve opening from the vent, but movable, upon the movement of the stem by the actuation of the displaceable member, to connect the second valve opening to the vent and thereby to actuate the second external device.

In the preferred embodiments of the present invention described below, the second external device is a second control unit of like construction as the first-mentioned control unit; the second valve opening of the first-mentioned control unit being connected to the second inlet port of the second control unit to vent the second chamber of the second control unit to the atmosphere when the displaceable member of the first-mentioned control unit is actuated.

According to a further feature in the described preferred embodiments, in each of the two control units, the first valve member is integrally formed with the one end of the stem and includes a first section having a cross-sectional area equal to that of its first valve opening so as to close it when the difference in pressure at its two inlet ports is below the predetermined magnitude and a second section adjacent to the first section and having a smaller cross-sectional area so as to open its first valve opening when the sensed difference in pressure rises to the predetermined magnitude. The one end of each stem includes a third section adjacent to the second section and on the side thereof opposite to the first section, the third section having a cross-sectional area at least equal to that of its first valve opening so as to reclose it when the sensed difference in pressure rises to a second predetermined magnitude above the first-mentioned predetermined magnitude. The arrangement is such that when the stem of the first-mentioned control unit moves from its first position closing its first valve opening, to its second position opening its first valve opening, it vents the second chamber of the second control unit to the atmosphere, thereby causing its stem to move from its first position closing its second valve opening, through its second position opening its second valve opening, to its third position reclosing the second valve opening; and when the stem of the first-mentioned control unit moves back to its first position closing its first valve opening, it automatically causes the stem of the second control unit to move from its third position closing its second valve opening to its second position opening its second valve opening to the atmosphere, to thereby actuate a third control device.

In most applications of the present invention, there would be more than two control units, each of like construction as the first-mentioned control unit; the control units being connected in series, with each control unit connected to its immediately preceding one in the same manner as the second control unit is connected to the first-mentioned control unit so that the control units are operated sequentially upon the completion of the operation of its immediately preceding control unit.

The invention is particularly useful in applications wherein each of the control units is connected to a filter to initiate, when actuated, a cleaning operation in the respective filter.

Further features and advantages of the invention will be apparent from the description below .

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 illustrates a control unit of the type described in US Patent No.4, 614, 581 but modified in accordance with the present invention; Fig. 2 illustrates a system including two control units each according to the structure of Fig. 1 ; and Fig. 3 illustrates a system of two control units of a slightly different construction.

The control unit illustrated in Fig. 1 is basically of the same structure, and operates in substantially the same manner, as the control unit described in US Patent No.4, 614, 581. To facilitate understanding its structure and operation, Fig. 1 includes the same reference numerals with respect to the parts that are common to that control unit, the parts which are new or modified being identified by reference numerals beginning with "300".

Thus, the control unit illustrated in Fig. 1 and therein generally designated 20, includes a housing 202 whose interior is divided into two compartment by means of a partition or wall plate 204 formed with a central opening 206 permitting communication between the two compartments. To facilitate assembly and disassembly, the housing is formed of two sections 202a, 202b secured together by a nut 202c with partition or wall plate 204 clamped in between.

Disposed within the compartment on one (left) side of partition wall 204 is a displaceable member 210 dividing that compartment into two chambers 211 and 212. The other compartment on the opposite side of partition wall 204 is also divided into two chambers 213, 214 by a second displaceable member 215. The amount of displacement of member 210 is relatively small, and therefore it includes a diaphragm 216, clamped at its outer periphery between the two housing sections 202a, 202b, and at its inner periphery between the two displaceable member sections 210a, 210b.

Displaceable member 210 acts as a differential-pressure sensor for sensing the pressure drop across the filter body, i.e., the difference in pressure between its upstream and downstream sides, and moves a control valve in response thereto to control the flow through opening 206 of the partition plate 204. Displaceable member 215 controls the displacement of a main valve member 46 in the filter, and is in the form of a piston movable within a cylindrical portion of housing 202. The construction of the filter, and the manner in which piston 215 controls a backwash operation therein, are disclosed in the above-cited patent.

With respect to displaceable member 210, it will be seen that its main section 210a is formed with a central stem 220 aligned with central opening 206 through partition wall 204. The inner end of stem 220 is of reduced cross-sectional area, shown at 222, so that its cross-sectional area is less than that of opening 206. The inner tip of stem 220 receives a ring 224 to increase the cross-sectional area of this inner tip to equal that of opening 206 such that the stem includes an inner section (that defined by ring 224) of the same cross-sectional area as opening 206, an adjacent section 222 of smaller cross-sectional area than opening 206 (which section is of longer axial length than the axial length of the opening), and a third section 225 (that defined by the next adjacent section of the stem) of the same (or greater) cross-sectional area as opening 206.

The outer end of stem 220 projects through an opening 226 formed in the control unit housing 202 and is circumscribed by a cylindrical collar 228. This end of stem 220 projecting externally of the sensor housing is provided with a cap 230 carrying a plurality of differently-marked (e.g., differently colored) rings 232, for a purpose to be described below.

Section 210a of displaceable member 210 is formed with a socket 234 for receiving a coiled spring 236 between it and an abutment ring 238 carried by the partition wall 204. Spring 236 thus biases stem 220 further through the housing opening 226, the position of the stem being limited by another abutment member 240 engageable with the opposite side of socket section 234 of the displaceable member 210.

Housing 202 includes an inlet port 138 connected to the upstream side of the filter body; therefore the pressure within chamber 211 will be that at the upstream side of the filter body. Since port 138 receives the unfiltered water at the upstream side of the filter, it includes a screen 139. The pressure in chamber 212 at the opposite side of displaceable member 210 is that of the downstream side of the filter body, this pressure being transmitted to chamber 212 via a port 136, an annular groove 242 through the control device housing section 202b, and a pair of bores 244 through the partition wall 204.

It will thus be seen that when the differential-pressure between the upstream and downstream sides of the main filter body are less than a predetermined magnitude (indicating a relatively clean filter), displaceable member 210 will be located, by virtue of spring 236, in the illustrated position wherein ring 224, at the end of stem 220, is disposed within opening 206 through the partition wall 204, thereby preventing any flow from chamber 212 on one side of the partition wall to chamber 213 on the opposite side of the partition wall. However, when the pressure at the downstream side drops such that the differential-pressure rises to a predetermined magnitude, member 210 will be displaced, against the action of spring 236, to move the reduced cross-sectional area 222 of its stem 220 within opening 206 (rightly) through the partition wall 204, thereby J establishing communication between the two chambers 212 and 213. However, should the differential-pressure further increase in magnitude, displaceable member 210 will be moved further (rightly) against the action of spring 236 to bring section 225 of the stem into opening 206 thereby again terminating the communication between the two chambers 212, 213 on the opposite sides of the partition wall 204.

Piston 215, movable in the compartment on the opposite side of partition wall 204, is biased towards the partition wall by a coil spring 250 interposed between the piston and the end wall 251 of the control device housing 202. Thus, spring 250 tends to contract chamber 213 defined by the piston 215 and the partition wall 204. Piston 215 is formed with a cylindrical recess 252 receivable within a cylindrical embossment 254 formed in partition wall 204, and of smaller diameter than recess 252, when the piston is urged against partition wall 204 by spring 250.

Piston 215 is further formed with an axial bore extending therethrough and slidably receiving a cylindrical sleeve 260. This sleeve is open at one end but is closed at the opposite end by a plug 262. Sleeve 260 is of greater length than the thickness of the piston at this location, and is adapted to engage an annular abutment 264 formed in the inner face of end wall 251 when the piston is moved to its extreme extended position wherein chamber 213 is of maximum volume and chamber 214 at the opposite side of the piston is of minimum volume. When sleeve 260 engages abutment 264, the end of the sleeve carrying plug 262 projects into chamber 213. This projecting end of the sleeve is formed with a radially-extending bore 266 which establishes communication between chambers 213 and 214 on the opposite sides of the sleeve.

Piston 215 is further formed with a small bore 267 to provide a slow-acting vent between the two chambers on the opposite sides of the piston.

Control device housing 202 includes a further port 124 which leads to the compartment on the side of the partition wall 204 within which piston 215 is movable. In the illustrated extended position of piston 215, port 124 communicates with chamber 213, but in the retracted position of the piston, wherein the piston is moved towards or into contact with partition wall 204, port 124 communicates with chamber 21 on the opposite sides of the piston. Port 124 is connected via a fitting to a pressure sustaining valve in the filter for sustaining the outlet pressure during a backwash operation, and also to a timer for terminating the backwash operation, both as described in the above-cited patent application.

Control device housing 202 further includes an additional port 268 leading to the atmosphere directly, or via the timer. Port 268 always communicates with chamber 214.

Piston 215 includes a stem 270 carrying the main valve member 46 which initiates or terminates the backwash operation. Thus, in the retracted position of piston 215, wherein it is in contact with partition wall 204 and chamber 213 is at its minimum volume, the main valve member 46 is- seated within a valve opening so that no backwash operation is performed; but when the displaceable member 215 is moved to its extended position, away from partition wall 204, valve member 46 is unseated from its valve opening to initiate a backwash operation.

The illustrated control device, when used with a backwash filter as described in the above-cited patent application, operates as follows: When the filter is initially connected to the fluid supply line, and the line has not yet been filled, spring 236 in the control unit 20 urges the differential-pressure sensing member 210 to its normal position wherein ring 224 at the end of its stem 220 blocks communication between the two chambers 212, 213 on opposite sides of the partition wall 204; and spring 250 urges piston 252 to its fully retracted position wherein it is against the partition wall 204 and its chamber 213 is at minimum volume. In this initial position of piston 252, its control sleeve 260 will have been moved by embossment 254 such that the plugged end 262 of the sleeve blocks communication between chambers 213 and 214. Also, in this initial position of piston 215, its valve member 46 is in the position shown wherein it closes a valve within the backwash filter, thereby preventing a backwash operation.

When the line pressure is turned on, the water flows to the filter body of the filter, and since the filter body is still clean, the pressure on its downstream side as applied to port 136 is substantially equal, to the pressure at its upstream side as applied to port 138, so that displaceable member 210 is located within valve opening 206, thereby blocking communication between chambers 212 and 213.

As the dirt removed by the filter accumulates on its upstream surface, the pressure drop across the filter body increases, thereby reducing the pressure at the downstream side. This downstream pressure is transmitted, via port 136, to chamber 212 of the control unit 20 so that as the pressure differential increases, the upstream pressure in chamber 211 tends to move stem 220 of displaceable member 210 through opening 206 in the partition wall 204, against the action of spring 236. When this differential-pressure reaches a predetermined magnitude, indicating an undue amount of dirt accumulating on the upstream face of the filter body, the reduced cross-sectional areas section 222 of stem 220 becomes located within opening 206, which thereby establishes communication between chambers 212 and 213 on opposite sides of the partition wall 204. When this occurs, the downstream filter pressure in chamber 212 is communicated to chamber 213, thereby causing piston 215 to move away from the partition walls 204 against the action of spring 250. This movement of piston 215 causes it to move its valve member leftwardly to initiate a backwash operation, as described in the above-cited patent application.

Early during the displacement of piston 215 which started the backwash operation, the piston passes port 124, thereby connecting the latter port to chamber 213 of a sustainer valve assembly in the filter to sustain the pressure therein during the backwash operation, and also to actuate a timer which terminates the backwash operation, if not terminated by a preset time interval, both as described in the above-cited patent application .

When piston 215 reaches its fully actuated position, wherein chamber 213 is of maximum volume and chamber 214 is of minimum volume, sleeve 260 movable within the piston engages the annular abutment 264 of end wall 251 , thereby shifting the sleeve to the position illustrated in the drawing, wherein its plugged end 262 and also its radial bore 266, are disposed within chamber 213, thereby establishing communication between this chamber and the interior of the sleeve. However, the opposite side of sleeve 260 is still in engagement with abutment 264, and therefore there is still no effective communication between chambers 213 and 214 on the opposite sides of the piston.

This is the condition of the control device so long as stem 220 of the differential-pressure sensing member 210 is located with its section 222 disposed within valve opening 206, thereby transmitting the downstream-filter pressure from chamber 212 to chamber 213. However, as soon as the filter body has been sufficiently cleaned by the backwash operation so as to reduce the pressure drop across it the desired amount, differential-pressure sensor 210 moves its stem 220 back to its normal position with the inner section of the stem defined by ring 224 located within opening 206, thereby terminating the communication between chambers 212 and 213. As soon as this occurs, the slow-acting vent 267 formed through piston 215 starts to vent the pressure within chamber 213 to the atmosphere via chamber 214 and port 268. After the pressure within chamber 213 has thus dropped sufficiently, spring 250 moves piston 215 away from abutment 264. As soon as the end of sleeve 260 separates from abutment 264 the pressure within chamber 213 is immediately vented to the atmosphere via the interior of sleeve 260, chamber 214, and port 268', whereupon spring 250 quickly returns piston 215 back to its normal, retracted position, against partition wall 204. As this occurs, sleeve 260 is again moved by embossment 254 to bring its plugged end within the bore in piston 215, thereby effectively interrupting the communication between chambers 213 and 214. When, piston 215 is thus returned to its fully retracted position against partition wall 204, its valve member 46 is moved back to close the valve opening and thereby to terminate the backwash operation.

After a backwash operation has started, it may occur that the pressure at the downstream side of the filter still does not rise, but rather further drops. This might indicate some malfunction in the device which could be caused, for example, by a jam or by the presence of such a large amount of dirt within the supply line that the filter is incapable of cleaning the filter body faster than the dirt accumulates thereon. In either event, the differential-pressure sensing member 210 is further displaced to bring the enlarged cross-sectional section 225 into opening 206 through the partition wall 204, thereby terminating the communication between chambers 212 and 213. This will also terminate the backwash operation and thereby prevent the wastage of water.

From the foregoing, it will be seen that the differential-pressure sensor 210 may assume any one of three positions, namely: (1) a normal position during the normal filtering mode of the filter wherein its ring 224 is disposed within the partition wall opening 206 to terminate communication between chambers 212 and 213; (2) a first actuated position reached when the differential-pressure between the upstream and downstream sides of the filter body rises to a predetermined magnitude to initiate a backwash operation, in which case the reduced-area section 222 of stem 220 is located within opening 206; and (3) a second actuated position reached when the differential-pressure increase to a second predetermined magnitude, higher than the first, whereupon the larger cross-sectional section 225 of the stem 220 becomes located within the partition wall opening 206 to again terminate the communication between chambers 212 and 213, resulting in the termination of the backwash operation. All three of these conditions of the filter are immediately indicated to the observer by the extent of projection of the stem 220 through collar 228 of the control device housing 202, and particularly by the colored rings 232 carried a the end of the stem.

It will also be appreciated that this projection of stem 220 externally of the control device not only indicates the state of the filter (i.e. under which condition it is then in, and/or how close it is to initiating a backwash operation), but also may be used for manually controlling the filter to initiate a backwash operation by pressing it to the above-described first actuated position of the stem, or to terminate the backwash operation by further pressing it to the second actuated position, or returning it to its home position.

The above-described construction and operation of the control unit illustrated in Fig. 1 is basically the same as described in US Patent No.4, 614, 581. Following is a description of the modifications in the construction of the control unit in order to perform additional functions, as will be described more particularly below with respect to Figs. 2 and 3. The modifications in the structure of the control unit are indicated by reference numerals starting with "300" in Fig. 1.

Thus, housing section 202a adjacent to the projecting end of stem 220 is formed with a port 300 normally vented to the atmosphere, and a second port 302 connected to another external device to be controlled by control unit 20. In the system illustrated in Fig. 2, the second external device is another control unit of like construction as control unit 20, and also controls the initiation of a cleaning operation in its respective filter. The arrangement is such that when a cleaning operation is initiated in the filter controlled by control unit 20 of Fig. 1, as soon as that cleaning operation has been completed control unit 20 automatically controls the second control unit connected to its outlet port 302 to initiate a cleaning operation in its respective filter.

In this manner, a plurality of control units may be connected in series such that when a cleaning operation is initiated in the filter of the first control unit, each of the subsequent control units automatically initiates a cleaning operation in its respective filter as soon as the cleaning operation of the preceding filter in the sequence has been completed.

For this purpose, port 302 is connected to a valve opening 304 controlled by a valve member 306 carried by stem 220. As described above, stem 220 may be moved, under the influence of diaphragm 210 displaceable by the difference in pressure in the two chambers 211 and 212, to one of the following three positions : 1. Stem 220 is normally in its first position, as illustrated in Fig. 1 , wherein its end ring 224 closes opening 206 in partition wall 204; this is the position of the stem when the pressure differential in the two chambers 211, 212 acting on displaceable member 210 is below a predermined magnitude, so that the filter controlled by control unit 20 merely performs its normal filtering operation; 2. Stem 220 is moved to its second position when the pressure differential in chambers 211, 212 rises to the predermined magnitude, indicating an unduly clogged filter; in this case, displaceable member 210 moves stem 220 to its second position aligning the annular recess 222 with opening 206, thereby establishing communication between chambers 212 and 213 to initiate a filter cleaning operation, as described in the above-cited US Patent No.4, 614, 581 3. Stem 220 is moved to its third position, wherein its shoulder 225 again closes opening 206, terminating the cleaning operation whenever, because of a malfunction or other reason, the dirt in the filter accumulates faster than it can be cleaned.

It will thus be seen that, when stem 220 is in its first position, it blocks communication between valve opening 304 and vent 300; but as soon as it moves to its second position, wherein it initiates a cleaning operation in its respective filter, it automatically vents port 302 to the atmosphere via valve opening 304, valve member 306 and vent 300. As soon as port 302 is thus vented to the atmosphere, it automatically actuates the control device connected to this port 302.

Fig. 2 illustrates a control system wherein the external control device connected to port 302, for actuation by the movement of stem 220 to its second position, is another like control unit as control unit 20, and controls the initiation of a cleaning operation in another filter. To facilitate understanding the structure and operation of the second control unit, its parts are identified by the same reference numerals as control unit 20 but followed with a " 1 " mark; thus, the second control unit, corresponding to control unit 20, is designated in Fig. 2 as 20 ' .

It will be seen that the second control unit 20' in Fig. 2 is of identical construction as control unit 20 described above with respect to Fig. 1. In the second control unit 20', however, port 136' connected to chamber 212', at one side of the displaceable diaphragm 210', is connected to port 302 of control unit 20 such that when port 302 is vented to the atmosphere, it will cause chamber 212' of the second control unit 20' also to be vented to the atmosphere.

The second control unit 20' is automatically controlled by the first control unit 20 in the following manner: In the normal operation of the two control units 20 and 20' , port 138 of control unit 20 is connected to the upstream side of its respective filter, and port 136 of control unit 20 is connected to the downstream side of its respective filter.

Similarly, port 138' of control unit 20' is connected to the upstream side of its filter, and port 136' is connected to the downstream side of its respective filter; in addition, port 136' is connected to port 302 of control unit 20.

Thus, when stem 220 of control unit 20 is in its first position, as illustrated in Fig. 2, it controls its respective filter to produce a normal filtering operation.

However, as soon as stem 220 of control unit 20 moves to its second position, to initiate the cleaning operation in its respective filter, it immediately connects chamber 212' of the second control unit 20' to the atmosphere via port 136' of control unit 20', port 302, valve opening 304, valve 306 and port 300 of control unit 20.

This venting of chamber 212' to the atmosphere immediately causes its stem 220' to move from its first position closing its opening 206', quickly through its second position opening its opening 206', and then to its third position reclosing opening 206'. When stem 220' is in its third position, its filter performs a regular filtering operation, and no cleaning operation is started.

Now, as soon as the cleaning operation in the first filter, controlled by control unit 20, has been completed such that the pressure differential applied to displaceable diaphragm 210 of control unit 20 drops again below the predermined magnitude, stem 220 moves back to its first position reclosing opening 206 and terminating the cleaning operation. This movement of stem 220 back to its first position also disconnects port 302 from the atmosphere, so that the pressure within chamber 212' again builds up by the downstream pressure in its respective filter.

Thus, the pressure within chamber 212' of the second control unit 20' again builds up until its stem 220' is moved from its third position back to its second position, and as soon as this occurs, the second control unit 20' initiates a cleaning operation in its respective filter. When the latter filter completes its cleaning operation, the pressure within its chamber 212' builds up so that the difference between it and that in its chamber 211' is below a predermined magnitude, whereby stem 220' in the second control unit 20' returns to its first position terminating its cleaning operation.

It will be appreciated that a control system could include a larger number of control units, each constructed and operated as described above with respect to control units 20 and 20', all connected in series, with chamber 212 of each control unit connected to the outlet port 302 of the immediately preceding control unit so as to be controlled in the same manner as described above with respect to control units 20 and 20'. Thus, the initiation of a cleaning operation by one control unit will automatically prime the immediately succeeding control unit (by moving its stem from its first position to its third position) for initiating a cleaning operation in its respective filter, which cleaning operation will be automatically initiated as soon as the cleaning operation in the filter controlled by the preceding control unit has been completed.

Many advantages are provided in such an arrangement. Thus, the described arrangement permits a plurality of filters and control units to be connected in the form of a battery, with the initiation of a cleaning operation in one filter automatically priming the next succeeding filter for a cleaning operation to be initiated immediately upon the completion of the cleaning operation performed by the preceding filter. In the event there is a malfunction in one filter of the series, the next filter in the series will, immediately upon sensing a clogging condition, initiate a cleaning operation in its filter and in the immediately succeeding filters.

While the control system described above and illustrated in Fig. 2 is particularly applicable with respect to filters and control units as described in US Patent No.4 , 61 4 , 581 mentioned above, the invention could advantageously be used with other contructions of control units and in other applications .

For example, Fig. 3 illustrates two control units, therein designated 320 and 320 ' , of a slightly different construction from that of Figs. 1 and 2 , and connected in series for controlling other types of filters or other external devices. The construction and operation of control units 320 and 320 ' are basically the same as described above with respect to control units 20 and 20 ' , except that instead of using the build-up of pressure within chamber 21 3 (Fig. 1 ) for moving piston 21 5 in order to initiate a cleaning operation, the build-up of pressure within chamber 31 3 of unit 320 , and within chamber 31 3 ' of unit 320 ' , may be used for controlling any other external control device, generally designated as 400 and 400' in Fig. 3. The latter devices are connected to the outlet port 324 and 324' communicating with chambers 313 and 313' , respectively. The external control devices 400, 400' may also be filters having other forms of cleaning operations, which operations are initiated as soon as the pressure within chambers 313, 313' has been built up to a predermined magnitude.

Many other modifications, variations and applications of the invention may be made, respect to several embodiments, it will be appreciated that many other variations, modifications and applications of the invention may be made.

Claims (5)

WHAT IS CLAIMED IS;

1. A fluid pressure control system including a control unit for actuating a first and a second external device, comprising: a housing having a partition wall dividing its interior into at least two compartments; said partition wall being formed with a first valve opening therein establishing communication between said two compartments; a displaceable member displaceable in one of said compartments and defining a first chamber on the op it side thereof appointO' to said partition wall, and a second chamber on the side thereof facing said partition wall. a stem fixed to said displaceable member; a first valve member carried at one end of said stem and movable thereby to open and close said first valve opening; a spring biassing said displaceable member to the position wherein said first valve member closes said first valve opening; a first inlet port in said housing communicating with said first chamber for inletting a first control pressure; a second inlet port in said housing communicating with said second chamber for inletting a second control pressure into said chamber effective, when said second control pressure is lower than said first control pressure by a predetermined magnitude, to cause said first control pressure to actuate said displaceable member against the bias of said spring to move said first valve member to open said first valve ) opening, and thereby to transfer said second control pressure via said valve opening to said second compartment; means communicating with the second compartment for actuating said first external device in response to the transfer of said control pressure from said second inlet port to said second compartment; said housing being formed with a second valve opening communicating with said second external device and also being formed with a vent to the atmosphere; said stem including a second valve member normally located to disconnect said second valve opening from said vent, but movable, upon the movement of said stem by the actuation of said displaceable member, to connect said second valve opening to said vent, and thereby to actuate said second external device.

2. The control system according to Claim 1, wherein said second external device is a second control unit of like construction as said first-mentioned control unit; said second valve opening of the first-mentioned control unit being connected to said second inlet port of said second control unit to vent said second chamber of the second control unit to the atmosphere when said displaceable member of the first-mentioned control unit is actuated.

3. The control system according to Claim 2, wherein, in each of said two control units, said first valve member is integrally formed with said one end of the stem and includes a first section having a cross-sectional area equal to that of its first valve opening so as to close it when the difference in pressure at its two inlet ports is below said predetermined magnitude, and a second section adjacent to said first section and having a smaller cross-sectional area so as to open its first valve opening when the sensed difference in pressure rises to said predetermined magnitude; said one end of each stem including a third section adjacent to said second section and on the side thereof opposite to said first section, said third section having a cross-sectional area at least equal to that of its first valve opening so as to reclose it when the sensed difference in pressure rises to a second predetermined magnitude above said first-mentioned predetermined magnitude; whereby when the stem of said first-mentioned control unit moves from its first position closing its first valve opening, to its second position opening its first valve opening, it vents said second chamber of the second control unit to the atmosphere, thereby causing its stem to move from its first position closing its second valve opening, through its second position opening its second valve opening, to its third position reclosing said second valve opening; and when the stem of said first-mentioned control unit moves back to its first position closing its first valve opening, it automatically causes the stem of said second control unit to move from its third position closing its second valve opening to its second position venting its second valve opening to the atmosphere, to thereby actuate a third control device.

4. The control system according to Claim 3, wherein there are more than two control units, each of like construction as said first-mentioned control unit; said control units being connected in series, 78656/2 - 32 - with each control unit connected to its immediately preceding one in the same manner as said second control unit is connected to said first-mentioned control unit, so that all said control units are operated sequentially upon the completion of the operation of its immediately preceding control unit.

5. The control system according to any one of Claims 1-4, wherein each of said control units connected to a filter to initiate, when actuated, cleaning operation in the respective filter. Jona an Pa n n Benjamin J. Barish & Co. P.O. Box 23008 Tel-Aviv 61 230