CA1259005A - Heating system, sequence controller - Google Patents

Abstract

ABSTRACT
SEQUENCE CONTROLLER

The invention is concerned with the problem of time-lag arising in heating systems using a plurality of boilers 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.

Description

l~S91S
SE~UENCE 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 -5 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 10 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 15 relatively large output boilers are lit following r 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 20 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 25 sufficient to allow the first boiler to become fully operational and so on. Conventional systems caused boilers to be switched on and off excessively beca~se 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 uRe in these circumstances.

According to the invention a heating system ~. ' . .

3 ~0 5

2.
comprises a plurality of low/high fire or fully modulating boilers controlled by sequencing ~eans adapte~ 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:-Bo-ler No.l on low Boiler No.l on high Boiler No.l on low and Boiler No.2. on low Boiler No.l on high and Boiler No.2.on low Boiler No.l on high and Boiler No.2.on high Boiler No.l on high and l~os 2 and 3 on low Boilers Nos 1 and 2 on high and rlo.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.l is on high, so that boiler No.2 is to be brought on low whilst boiler ~lo.l is returned to 1OWJ the fourth step, that is bringing No.l back to high cannot take ~lace until x minutes have elapsed after the third operation ~lighting l~o.2 boiler on low and returning No.l 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 ~2S9~(~S

3.
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 boile~s 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 30 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:

. . , -~
. .
'" '' .~ , , .. , .:.- ..
:': . .:

~s~o~s , 4.
St~ge 1: ~oiler No.l firing intermittently to ~eet a load demand of 0-33~ of one boiler.

Stage 2: Boiler l~o.l modulating to meet a ioad demand of 33-90% of one boiler.
Stage 3: Boilers ~os. 1 and 2 modulating together to meet a load demand of 90-180~ of one b~iler.

Stage 4: Boiler No. 1 on high fire and boilers ~os.
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 l~o. 2 ~ e brought on at a low level with high/low 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 ~eans 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 ~emperature controller provid~d wi~h input means for regi~tering ~he te~per~ture Yalue cor~esponding to the heat demands imposed on the system and the -lZ59~ 5 5.
controller i5 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, Serial Nos. 486,702 and 486,704.
To promote further understandin~ of the invention, a boiler control system incorporaking 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.

l'~S~)V5 6.
The single drawing figure illustrates schematically a heating system including a sequence controller according to the invention.
The system illustrated serves to control operation of thres 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 o~ a modulating type or, in connection with certain aspects o~
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 15 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 ouput temperatures and from respective control circuits 38, 40, 42 when the sequence 20 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 se~uential control of the 25 boilers in such a way as to meet changes in tha heat load requirements without leading to instability in the system.
The sequence control unit comprises a modulating temperature controller 44, a modulating presure controller 46 and a sequential switching mechanism 48. The 30 controller 44 serves to monitor temperature within the common ~low line. .......................................

7.
18. When pressure control is included controller ~6 serves to monitor pressure from Pl. which i~
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 Tl and Pl deviate from the desired values so that appropriate corrections can be made.
Such control signals are dependent on both the ~agnitude and rate of change of the temperature or pressur,e 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 wherel~y 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 ~enerated 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.s. 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 35 pressure exceeds the set value, the controller 4S
provides an output on line 5S to close the gate . , , ~s~
8.
53 ar,d 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 rota~ion 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 out~ut 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 5~ and those from line 50 and 54~

The interface circuit 7U includes a number of relays associated ~ith each boiler. These relays . ..
.~

S~S
9.
are not shown but, for convenience, they will be referred ~o 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 deter~ine whether the associated boiler is to be off, on low fire or on high fire. lhe ~off~ condition corresponcs 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 fireO
stage 2. - boiler 10 high f ire.
n stage 3. - boilers 10 and 12 low fire.
" stage 4. - boiler 10 low fire, boiler 12 high fire.
n stage 5. - boilers 10 and 12 high f ire.
~ 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 on. Thus, the logical circuitry , ' ~ .,, ~'~S~0~)5 10 .

of the interface circuit 7~ is so desi~ned that an output on, for exam~,le, line 64 ener~izes rela~s ~2-1~ ana K3-l~ ana ~2-12. ~elays ~6 ana K7 are ol~eratea when a boiler is to be fired or to be switcl~ed of~ r~spectively.

~ )eration of the system will now be explain~-by re~erence to typical situations that arise in practice. ~`or convenience assu,ne that the systeln lS o~eratin~ at a ~)oint in the se~uence w~lere t~le first tiv~ switclles S~ have been clos~c.
lhis corresLIon~s to sta-Je 5 a~ove. If now tne heatin; re~uirements are increased by a~pro~riate chanye in tlle value set into the controller 44, the temperature reyistere~ by sensor Tl will aift r fr~ the new set value and as a conse~uence the controller will produce ~ulses on line 52 to aavc ~ce the motor/can arive towaras stage 6. A certain number of pulses have to be accumulated before the transfer from stal~e 5 to staye 6 occurs a~
when sufficient pulses have been accumulatea to ~roouce an output si~nal on line 6~, a ti~liny ~evice (not shown) of the interface circuit 7l) is operated to su~ply a siynal via line 74 to the yate 53 for a preaeterminea interval of ti~e so as to allow suf~icient time ~or the o~eratiny conôitions o~ the boilers to be chanyed an~ sta~lizea before any further chan~es can be brouyht about by the controller 44. This time delay occurs at each ste~) in tne se~uence.

The valves 26, 2~ will at this time alrea~y - be open but valve 3U will be closed. ln respons~
to production o~ the si~nal on line ~, t~e relay, ~-14 ana K6-14 are eneryised to initiate o~erat~n of the boiler 14 at the low fire level. Relay ~6-14 closes contact5 K6/1 in control circuit ~,,.- '' , :

l'~S90(~5 11 .
3~ to provine a si~nal which is fed to control unit 3~ via line ~ an(i is utilised to effect oL~enincj of the valve 3U. Althou~h oi~ening of th~ valve 3U will leac to the admission of cold water into the system Which~ in turn, will cause a re~uction in temperature, this will not upset tne intende~ functioniny of the system even thouyh the controller 44 may respon~ to the lower tem~erature transient.lhis is because the ~ulses on line are at this stage sul~,ressea Dy the si~nal on line 74. ~nen valve ~U is fully o~en, a microswitch is o~erated to si~nify tnis to control unit 3 and the condition of relay ~1 is investiyated by al,plyiny a si~nal alon~ line 76. ~epenaing on the con~ition of relay ~1, this is routed back to the boiler control unit 36 either via contacts ~ when relay Kl is ener~isea~ and line 7~
or via contacts ~1/1, K2/1 and line 80. The fon~er route si~nifies that tlle se~uence control unit is to be overriden and the boiler 14 is to be o~,erated by means of its own control unit 36 and thermostat T7. The latter route si~nifies normal control of the ~oiler 14 via the se~uence control unit and in this case timin~ relay RT4 is ener~ised for a L)redetermined time interval sufficient for ~ro~er firin~ of the boiler 14 to take place.
If correct firin~ occurs in time, the control unit ~ feeds a si~nal on line ~2 to ener~ise relay K4 which, in turn, opens contacts ~4/1 to ae-ener~ise relay Kl~.

If however, correct firin~ does not occur within the ~redtermine~ time interval, relay KT4 times out, closes contacts KT4/1 ana ener~izes relay ~1 with conse~uent override of the se~uence control unit. An audible and/or visual warnin~

~ ~ .

.

iZS9005 12.
siynal may be yenerate~ in this event. ~neryization of relay ~1 in any of the control circuits 3~,

4() ana 42 automatically causes the se~uence control unit to be overrider~ for all boilers in t~le system.
S A similar situation may arise if, at any tilne, tJIe nonrlal safety functions monitorea by the boiler control unit indicate incorrect o~eration. In this case, the boiler control units 32, 34, 36 will produce a boiler lock-out si~nal via line ~4 to eneryize relay ~5 ana ther~by close contacts ~5/1 to en~ryize relay ~

Assumin~ correct firiny of t~oiler 14 occurs, t~le si~,nal on lioe ~ is also uset~ to check the conaition of relay K3 to determine whether low or hiyl~ fire is re~uirea. ln tlle latter case, relay K3 will ~e eneryizea an~ its contacts K3-1 will close to route this signal ~ack to the control unit 36 via line ~ to siynify hiyh fire operation.
~owever, in the exami~le under consideration, relay k8-14 will not be ener~ised and the boiler 14 will therefore be o~erated at low fire.

As well as ~rinyiny boiler 14 on line, the switch into staye 6 of the se~uence re~uires the boiler 1~ to be switched from high fire to low fire~ This will be im~)lemented in resl~onse to ae-eneryization of relay ~3~12 since the control unit ~4 will no lonyer receive any feedback via line ~6. After the timing aevice of the interface circuit 17 has timea out and removes the su~pressiny signal froln line 74, normal o~eration of the controller 44 is restore~. lf, at this time, the new boiler operatiny conditions are not ade~uate to meet the heat re~uirements, further ~ulses on line

5~ are transmitted via ~ate 58 to increment ~he switchiny unit 48 towards stage 7.

, ' ' `' ' .

~2~9005 13.
~ ihen the heating dernand reduces and the se~uence is reversea; it will ~e seen that the ~royression from one sta~e to the next (e.g. sta~e 6 to stage 5) may involve takin~ a boiler off line. In these circumstances, the associated relay R7, e.g. relay R7-14 is eneryised to close contacts R7/1 and provide via line ~ a siynal which is utilised by the associated control unit ~6 to initiate closing of the valve 24. ~owever, valve closure is not effecte~ instantaneously. Instead, a partial closing si~nal is produced by the boiler control unit to effect ~artial closing of the valve, e.g.
to within ~U% of its ~ully closed ~osition. A
microswitch is operated when the valve reaches the ~artially closed position an~ the valve now remains in that position until the flow tem~erature as sensed by the associated sensor 12-T4 falls to a predetermined value indicatiny that all of the heat from that boiler has been dissi~ated.
At this i~oint, the valve is closed completely to cut off water circulation to the associate~
boiler. The foregoing proce~ure 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 Pl are employed to monitor an~ control pressure within the system. It is conventional practice to operate a water heatin~ system under pressure, by means of a suitable ~uml)ing arrangement, so as to raise the boiliny point of the water. The controller 4~ serves to prevent excessive pressure build-uu within the system and does so by overriding the 1,Z590~)5 14.
temt,erature controller 44 at least insofar as the latter may be causiny the heat output to increase.
'rhus, if the sensed pressure approaches the set value without exceediny the same, the pressure controller beyins to feed pulses alony line 54 at a fre~uency del~endiny u~on how close the sense value is to the set value. Conse~uently this at least l~artly negates any pulses fed by the emL~erature controller along line 52. If the ~ressure increases to a certain extent that the set value is excee~ed, the controller 46 pro~uces a si~na~
on line 5~ to close t~e gate 53 and also supplies ~ulses on line 54 so as to reauce the heat output of the boiler. ~nce the pressure has fallen to lS a safe level, the overri~e is removed to allow control of the system to be resumed by the controller 44.

Claims (4)

15.

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heating system comprising a plurality of low/high fire 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 low 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.

3. A heating system comprising a plurality of 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 the boilers previously brought on line and operating at a higher heat output are both initially operated at lower heat outputs.

4. A heating system as claimed in claim 3 wherein a predetermined delay occurs between successive switching-in operations, irrespective of demand.