GB2161293A - Sequence controller - Google Patents

Abstract

The invention is concerned with the problem of time-lag arising in heating systems using a plurality of boilers (10, 12, 14) which are successively switched in or out according to load requirements, and provides that during times of increasing load when an additional boiler is required, that boiler is initially switched in at a low setting and at the same time a previously switched in boiler is reduced to low setting. This may reduce unnecessary switching in and out particularly when the structure of the previously switched in boiler is not at normal operating temperature at the time when the next boiler is called in. <IMAGE>

Description

SPECIFICATION Sequence controller This invention relates to heating systems essentially using a plurality of boilers. Such boilers control by devices such as time switches and thermostats controlling the flow water temperature. When control thermostats are fitted to the flow pipe of individual boilers, it is known to use a sequencer to arrange for the boilers to switch in and out in a predetermined order. Such sequence systems can lead to instability with boilers switching in and out excessively causing inefficient operation and fuel wastage.

One of the problems with multi boiler installation occurs at the commencement of load or at times of increasing load and is due to the time taken to bring a boiler into operation. This is because relatively large output boilers are lit following a predetermined cycle of purging, pressurization and the like, and may require a further time interval before the boiler structure itself rises in temperature and any appreciable effect is felt in the temperature of the output water. It is conventional to provide a fixed delay to allow for this so that when the system calls for heat and the first boiler is lit, the second boiler is only lit if the heat demand is not satisfied after a predetermined interval sufficient to allow the first boiler to become fully operational and so on.Conventional systems caused boilers to be switched on and off excessively because if the load is just above that for one boiler, second unit will cut in and out regularly.

The object of the present invention is to provide an improved control system for use in these circumstances.

According to the invention a heating system comprises a plurality of low;high fire or fully modulating boilers controlled by sequencing means adapted and arranged to bring the boilers on line in a predetermined sequence such that, when a new boiler is brought on line, the new boiler and at least one of any boilers previously brought on line and operating at a high fire level are both initially operated at a low fire level.

Hence, an operating system for three on/low/high boilers will have the following possibilities arranged in order of increasing load: Boiler No.1 on low Boiler No.1 on high Boiler No.1 on low and Boiler No.2. on low Boiler No.1 on high and Boiler No.2. on low Boiler No.1 on high and Boiler No.2. on high Boiler No.1 on high and Nos 2 and 3 on low Boilers Nos 1 and 2 on high and No.3 on low All boilers on high The sequencing means is preferably arranged so that there is a delay when the new boiler is brought on line before any further changes in boiler operation can be implemented.Thus, for example, when insufficient heat is being provided and boiler No.1 is on high, so that boiler No.2 is to be brought on low whilst boiler No.1 is returned to low, the fourth step, that is bringing No.1 back to high cannot take place until x minutes have elapsed after the third operation (lighting No.2 boiler on low and returning No.1 to low) so as to allow time for the system to stabilize between the operations.

The invention may be contrasted with a conventional system which would for example successively bring boiler 1 between low and high positions and would then bring on the second boiler in a low state while keeping the first boiler in the high state. If the load is marginally between that required for boiler 1 on high and boiler 1 on high plus boiler 2 on low, the result is likely to be that boiler 2 goes on, off repeatedly or in other words system instability with consequent waste and inefficiency. By having boilers 1 and 2 on low and switching boiler 1 to high or back to low to cope with the fluctuating load in the same marginal area, both boilers are kept running continuously and the start up loads and wastage are avoided.

In addition to controlling the start up sequence, means may be provided to prevent instability in the system in shut down sequences. According to a feature of the invention all the boilers are connected between a common return main and a common flow main and a separate control valve is connected between each boiler and the return main and a separate temperature sensor connected between each boiler and the flow main, each temperature sensor being arranged to operate the corresponding control valve.

Hence by these means, the flow through an individual boiler can be terminated on a temperature basis rather than a time basis, and the risk of unnecessary boiler start-ups for this reason can be avoided.

The invention is applicable in all its aspects also to fully modulating boilers. Thus, in terms of fully modulating boilers, references hereinbefore to "high" and "low" are to be construed as referring to higher and lower heat outputs which are not necessarily discrete values as in the case of low/high fire boilers. In this context, a typical sequence employing three modulating boilers is as follows: Stage 1: Boiler No.1 firing intermittently to meet a load demand of 0-33% of one boiler.

Stage 2: Boiler No.1 modulating to meet a load demand of 33-90 /O of one boiler.

Stage 3: Boilers Nos. 1 and 2 modulating together to meet a load demand of 90-180% of one boiler.

Stage 4: Boiler No. 1 on high fire and boilers Nos. 2 and 3 modulating to meet a load demand of 180 300% of one boiler.

Thus, for example, at the changeover from Stage 2 to Stage 3 which involves the introduction of the second boiler, boiler No. 1 will be returned to a low firing level and boiler No. 2 will be brought on at a low level with hightlow operation or a level between high and low with a modulating burner and, on the expiry of the previously mentioned time delay, both boilers will commence to modulate until the load demand is met.

The sequencing means preferably comprises a series of switches corresponding to respective stages in the sequence and means for operating the switches in the desired sequence, the switch-operating means being responsive to a temperature sensor associated with the flow main of the system (and, optionally, a pressure sensor for sensing the pressure within the system) so that the point reached in the switching sequence is dependent upon the sensed temprature and sensed pressure (if employed).

The temperature sensor is conveniently associated with a temperature controller provided with input means for registering the temperature value corresponding to the heat demands imposed on the system and the controller is advantageously arranged to provide output signals which represent the differences between the sensed temperatures and the set temperature values and also whether the sensed temperature is above or below the set value. Such output signals are preferably also indicative of the rate of change of the temperature difference.

In one convenient embodiment, the controller provides output signals in pulse form which are effectively accumulated by the sequencing means with appropriate regard to whether the pulses are "up" pulses (corresponding to a set value higher than the sensed temperature) and "down" pulses (corresponding to a set value lower than the sensed temperature). For example, the sequencing means may comprise some form up/down counting device for registering the pulses such that each step in the sequence corresponds to a predetermined count value, the arrangement being such that the "up" pulses negate the "down" pulses and vice versa. The counting device may be electronic or it may comprise a reversible stepping motor having an output shaft whose angular position is dependent upon the number of pulses "accumulated".The shaft may carry a number of cams co-operable with the sequence switches so that the position reached in the sequence is determined by the angular position of the shaft and hence the difference in the number of up and down pulses received by the motor.

The subject matter of the present Application is also disclosed in our co-pending Applications of even date and the disclosures contained in said co-pending Applications are incorporated herein by reference.

To promote further understanding of the invention, a boiler control system incorporating features in accordance with the present invention and also features which are the subject of said co-pending Application will now be described by way of example with reference to the accompanying diagrammatic drawings.

The system illustrated serves to control operation of three low/high boilers 10, 12, 14 but it will be understood that the system is applicable to other numbers of boilers and that the boilers may be of the modulating type or, in connection with certain aspect of the invention, the boilers may be of the on/off variety.

Water circulation through the boilers takes place via return and flow lines 16, 18. Associated with each boiler, there is a motorised return flow valve 20, 22, 24 and drive motor 26, 28, 30. Each boiler is also equipped with its own controller 32, 34, 36 for controlling firing of the boiler when the sequence switch is out of circuit. Each controller 32, 34, 36 receives inputs from temperature sensors T2-T4 and T5- T7, which respond to the boiler output temperatures and from respective control circuits 38, 40 and 42 when the sequence control is in circuit, of which only control circuit 42 is shown in detail since they are all substantially identical.

The control circuits 38, 40 and 42 form part of a sequence control unit for sequential control of the boilers in such a way as to meet changes in the heat load requirements without leading to instability in the system. The sequence control unit comprises a modulating temperature controller 44, a modulating pressure controller 46 and a sequential switching mechanism 48. The controller 44 serves to monitor temperature within the common flow line 18. When pressure control is included controller 46 serves to monitor pressure from PI. which is usually in the return.

Each controller 44, 46 is pre-settable with the desired system temperature and pressure values (which can be adjusted according to requirements) and serve to provide control signals whenever the input from sensors TI and PI deviate from the desired values so that appropriate corrections can be made. Such control signals are dependent on both the magnitude and rate of change of the temperature or pressure differences between the set values and the sensed values. In this way, the controllers 44, 46 can, in effect, predict when the set value will be achieved whereby corrective action can be taken in advance to prevent overshoot or hunting. Such controllers are commercially available and a suitable controller for present purposes is the Phillips LD30 controller.

The control signals generated by the controllers 44, 46 are in pulse form and the frequency of the pulse train produced varies according to the magnitude and rate of change of the temperature or pressure difference, e.g. as the sensed temperature approaches the set value the pulse frequency decreases. In the case of the temperature controller 44, the pulses are fed to the switching unit 48 via one of two output lines 50, 52 depending upon whether the sense temperature is above or below the set temperature. If it is above, the pulses are fed via line 50 and if it is below they are fed via line 52 and a normally open gate 53. In the case of the pressure controller, when the pressure is below but within a certain range of a set value, pulses are fed via line 54 to the switching unit 48.If however the sensed pressure exceeds the set value, the controller 46 provides an output on line 56 to close the gate 53 and thereby interrupt transmission of any pulses to the switching unit 48 via line 52. At the same time, the pressure controller continues to provide output pulses on line 54 at an increased frequency.

The switching unit 48 may take various forms but will be described herein with reference to a motorised cam- operated switching unit. The motor is reversible and has a series of cams attached to its output shaft, each cam being cooperable with a respective one of the eight switches 58 which are closed and opened in sequence as the motor shaft rotates and, when closed, provides signals along the respective output lines 61 to 68 which are routed to the control circuits 38, 40, 42 via an interface circuit 70.

Electrical power for the switching unit is derived from supply circuit 72. Thus, as the motor rotates in one direction from a start position initially switch 61 closes and as the motor continues to rotate in the same direction switches 62 to 68 are closed in succession. Reverse rotation of the motor reverses the sequence with consequent opening of the switches. Operation of the motor and the direction of rotation is governed by the pulsed output from the controllers 44, 46. Thus, pulses fed via line 52 produce the closure sequence 61 through to 68 whereas pulses fed via lines 50 and 54 produce the opening sequence 68 through to 61.It will be noted that the motor and shaft effectively constitute a mechanical counting unit and the angular position of the shaft depends upon the difference between the number of pulses accumulated from line 52 and those from line 50 and 54.

The interface circuit 70 includes a number of relays associated with each boiler. These relays are not shown but, for convenience, they will be referred to as relays R2-10, R3-10, R6-10 and R7-10 in the case of boiler 10. The other boilers will have a like set of relays associated with them, e.g. R2-12, R3-12 etc for boiler 12. These relays serve to control the operating state of the respective boiler and its associated valves 26, 28, 30. Relays R6 and R7 control opening and closing of the respective valves 26, 28, 30 whilst relays R2 and R3 determine whether the associated boiler is to be off, on low fire or on high fire. The "off" condition corresponds to R2 and R3 both de-energised; the "low fire" condition corresponds to R2 energised and R3 de-energised; and the high fire" condition corresponds to R2 energised and R3 energised.

The order in which the boilers are called on line is as follows: Heating stage 1. -boiler 10 low fire.

stage 2. -boiler 10 high fire.

stage 3. -boilers 10 and 12 low fire.

stage 4. -boiler 10 low fire, boiler 12 high fire.

stage 5. -boilers 10 and 12 high fire.

stage 6. -boiler 10 high fire, boilers 12 and 14 low fire.

stage 7. boilers 10 and 12 high fire, boiler 14 low fire.

stage 8. -boilers 10, 12 and 14 high fire.

Stage 1 corresponds to an output on line 61, stage 2 corresponds to an output on lines 61 and 62 and so one. Thus, the logical circuitry of the interface circuit 70 is so designed that an output on, for example, line 64 energizes relays R2-10 and R3- 10 and R2-12. Relays R6 and R7 are operated when a boiler is to be fired or to be switched off respectively.

Operation of the system will now be explained by reference to typical situations that arise in practice.

For convenience assume that the system is operating at a point in the sequence where the first five switches 58 have been closed. This corresponds to stage 5 above. If now the heating requirements are increased by appropriate change in the valve set into the controller 44, the temperature registered by sensor T1 will differ from the new set value and as a consequence the controller will produce pulses on line 52 to advance the motor/can drive towards stage 6.A certain number of pulses have to be accumulated before the transfer from stage 5 to stage 6 occurs and when sufficient pulses have been accumulated to produce an output signal on line 66, a timing device (not shown) of the interface circuit 70 is operated to supply a signal via line 74 to the gate 53 for a predetermined interval of time so as to allow sufficient time for the operating conditions of the boilers to be changed and stabilized before any further changes can be brought about by the controller 44. This time delay occurs at each step in the sequence.

The valves 26, 28 will at this time already be open but valve 30 will be closed. In response to production of the signal on line 66, the relays R2-14 and R6-14 are energised to initiate operation of the boiler 14 at the low fire level. Relay R6- 14 closes contacts R6/1 in control circuit 32 to provide a signal which is fed to control unit 36 via line 88 and is utilised to effect opening of the valve 30.Although opening of the valve 30 will lead to the admission of cold water into the system which, in turn, will cause a reduction in temperature, this will not upset the intended functioning of the system even though the controller 44 may respond to the lower temperature transient.This is because the pulses on line 52 are at this stage suppressed by the signal on line 74.When valve 30 is fully open, a microswitch is operated to signify this to control unit 36 and the condition of relay R1 is investigated by applying a signal along line 76.Depending on the condition of relay R1, this is routed back to the boiler control unit 36 either via contacts R1/1 (when relay R1 is energised) and line 78 or via contacts R1/1, R2/1 and line 80. The former route signifies that the sequence control unit is to be overriden and the boiler 14 is to be operated by means of its own control unit 36 and thermostat T7. The latter route siqnifies normal control of the boiler 14 via the sequence control unit and in this case timing relay RT4 is energised for a predetermined time interval sufficient for proper firing of the boiler 14 to take place. If correct firing occurs in time, the con trol unit 36 feeds a signal on line 82 to energise relay R4 which, in turn, opens contacts R4/1 to de energise relay RT4.

If however, correct firing does not occur within the predetermined time interval, relay RT4 times out, closes contacts RT4/1 and energizes relay R1 with consequent override of the sequence control unit.An audible andior visual warning signal may be generated in this event. Energization of relay R1 in any of the control circuits 38, 40 and 42 automatically causes the sequence control unit to be overriden for all boilers in the system. A similar situation may arise if, at any time, the normal safety functions monitored by the boiler control unit indicate incorrect operation. In this case, the boiler control units 32, 34, 36 will produce a boiler lock-out signal via line 84 to energize relay and thereby close contacts R5/1 to energize relay R1.

Assuming correct firing of boiler 14 occurs, the signal on line 82 is also used to check the condition of relay R3 to determine whether low or high fire is required. In the latter case, relay R3 will be energized and its contacts R3-1 will close to route this signal back to the control unit 36 via line 86 to signifty high 'fire operation. However, in the example under consideration, relay R3-14 will not be energised and the boiler 14 will therefore be operated at low fire.

As well as bringing boiler 14 on line, the switch into stage 6 of the sequence requires the boiler 12 to be switched from high fire to low fire. This will be implemented in response to de- energization of relay R3-12 since the control unit 34 will no longer receive any feedback via line 86. After the timing device of the interface circuit 17 has timed out and removes the suppressing signal from line 74, normal operation of the controller 44 is restored. If, at this time, the new boiler operating conditions are not adequate to meet the heat requirements, further pulses on line 52 are transmitted via gate 53 to increment the switching unit 48 towards stage 7.

When the heating demand reduces and the sequence is reversed, it will be seen that the progression from one stage to the next (e.g. stage 6 to stage 5) may involve taking a boiler off line. In these circum stances, the associated relay R7, e.g. relay R7- 14 is energised to close contacts R7/1 and provide via line 90 a signal which is utilised by the associated control unit 36 to initiate closing of the valve 24. However, valve closure is not effected instantaneously. Instead, a partial closing signal is produced by the boiler control unit to effect partial closing of the valve, e.g. to within 90% of its fully closed position.A micros witch is operated when the valve reaches the partially closed position and the valve now remains in that position until the flow temperature as sensed by the associated sensor T2-T4 falls to a predetermined value indicating that all of the heat from that boiler has been dissipated. At this point, the valve is closed completely to cut off water circulation to the associated boiler. The foregoing procedure applies to shut off of all the boilers except the last line. In this case, the valve is not closed otherwise this would stop all water recirculation in the system.

The pressure controller 46 and sensor p1 are employed to monitor and control pressure within the sytem. It is conventional practice to operate a water heating system under pressure, by means of a suita ble pumping arrangement, so as to raise the boiling point of the water. The controller 46 serves to pre vent excessive pressure build-up within the system and does so by overriding the temperature controller 44 at least insofar as the latter may be causing the heat output to increase. Thus, if the sensed pressure approaches the set value without exceeding the same, the pressure controller begin to feed pulses along line 54 at a frequency depending upon how close the sense value is to the set value. Consequently this at least partly negates any pulses fed by the emperature controller along line 52. If the pressure increases to a certain extent that the set value is exceeded, the controller 46 produces a signal on line 56 to close the gate 53 and also supplies pulses on line 54 so as to reduce the heat output of the boiler. Once the pressure has fallen to a safe level, the override is removed to allow control of the system to be resumed by the controller 44.

Claims (2)

1. A heating system comprising a plurality of low/high fire or fully modulating boilers controlled by sequencing means adapted and arranged to bring the boilers on line in a predetermined sequence such that, when a new boiler is brought on line, the new boiler and at least one of any boilers previously brought on line and operating at a high fire level are both initially operated at a low fire level.

2. A heating system as claimed in Claim 1 wherein a predetermined delay occurs between successive switching-in operations, irrespective of demand.