CA1259006A - Heating system - Google Patents

Abstract

ABSTRACT
HEATING SYSTEM

This invention is concerned with heating systems using a plurality of boilers arranged to be switched on and off in sequence according to the load.
The invention provides not only thermostatic control for switching, but in addition a pressure sensor arranged to override the thermostat. This avoids problems due to rapid expansion which might otherwise cause unsafe or undesirable pressures in the system.

Description

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1.
HEATING SYSTEM.

This invention relates to heating systems essentially using a plurality of boilers. ~oilers are controlled by devices such as time switches and thermostats `
controlling the flow water temper~ture. While 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 predeterminea order, but such sequencers can cause problems of over pressurisation of pressurised systems on early morning cold starts unless the pressurisation uni~
is correctly designed to accept the high volumes `
of water caused by the thermal expansion of a rapid cold start.

The object of the invention is to provide an improved system to enable boilers on a cold start to be brought on line as quickly as possible without causing an excessive rise in system pressure.

In accordance with the present invention a heating system comprises a plurality of boilers connected in a common system, pressurising means for the system, a temperature sensor connected via a sequencer to bring in successive boilers on increasing load and cut out successive boilers on reducing load, and a pressure sensor arranged to over-ride the temperature sensor so as to prevent boiler op~ration or switch off boilers when pressure increa~es tow~rds or beyond a point determined by the setting of th~
sensor.

~he inven~ion enables a smaller pressurization unit to be used at the notional expense of slo~ing down system response time, but it is found experi~entally that the cost savings are sufficient to be cons~dered worthwhile.

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The system of this invention can be used with boilers of any kind, i.e~, on/off boilers, low/high fire boilers and fully modulating boilers.
Thus, if the sensed pressure approaches the predetermined value, the pressure-responsive means may modify operation of the sequencing means to reduc~ the rate of increase of any temperature rise to such an extent that it is partly or wholly negated or even reversed. For axample, despite receiving temperature information consistent with an increased demand for heat, operation of the sequencing means may be modified to reduce heat input to the system so as to allow the pressure to fall.
Preferably the pressure-response means is sensitive not only to the pressure difference between the sensed pressure and said predetermined pressure value but also the rate of change of the pressure difference whereby the corrective action taken can be related to the rate at which the system pressure approaches said predetermined value. In this way, any temperature rise being implemented by the sequencing means in response to the output of the temperature sensor may not need to be wholly negated or reversed; instead the rate of increase may be partly negated, i.e. reduced, so that the increased heat demand is met more slowly but without exceeding said predetermined pressure value.
The subject matter of the present Application is also disclosed in our co-pending Canadian Applications Serial Nos. 486,702 and 486,703.
To promote further understanding of the invention, a boiler control system incorporationg features in accordanoe with the present invention and also features which are the subject of said co-pending ~ppli~ation will now be described by way of example with reference to the 1~2~ 0~
3.
accompanying diagrammatic drawings.
The single drawing ~igure provides a schematic illustration of a heating system according to the invention.
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 10 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, ~4 and drive motor 26, 28, 30. Each boiler is also 15 equipped with its own controller 32, 3~, 36 for controlling firing of the boiler when the sequence witch 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 20 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 indentical.
The control circuits 38, 40 and 42 form part of 25 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 30 controller 46 and a sequential switching mechanism 48.
The controller 44 serves to monitor temperatura within the commom flow line 18. When pressure control is included controller ........................................ ~
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46 serves to monitor pressure from Pl. which is usually in ehe 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 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, 4'6 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 commercial1y 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 ànd 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 }ine 56 to close the gate 53 and thereby interrupt transmission of any pulses to the switching unit 48 via line 52. At the same .,"

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time, the pres~ure 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 circ~its 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 S8 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. ~hus, 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 in~erface circuit 70 includes a number of relays associated with each boiler. ~he~e relays are not shown but, for convenience, they will be referred to as relays R2-10, R3-10, R6-10 and R7-lO
in the case of boiler 10. The other boilers will have a like set of relays associated with them, 1~9~6 6.
e.g. R2-12, ~3-12 etc for boiler 12. These rel~ys ~erve 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 enerqLsed and R3 de-energised;
and the high fire~ condition corresponds to R2 energised and R3 energisæd.

The order in which the boi}ers are called on line is as follows :-Heating stage 1. - boiler 10 low fire.
" stage 2. - boiler 10 high fire.
n stage 3. - boilers 10 and 12 low fire~
- ~ staye 4. - boiler 10 low fire, boiler 12 high fire.
n stage 5. - boilers 10 and 12 high fire.
n stage 6. ~ boiler 10 high fire, boilers 12 and 14 low fire.
n stage 7. - boilers 10 and 12 high fire, boiler 14 low fire.
~ $~age 8. - boilers 10, 12 and 14 high fire.

Sta~e ~ 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 of the interface circuit 70 is so designed that an output on, for example, line 64 energizes relays R2-10 30 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 æystem will n~w be explained by reference to typical ~itu~tion~ that arise in practice. For convenience as~ume th~t the ~yste~

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is operating at a point in the sequence where the first five switches 5~ have been closed. Thi~
corresuonds to stage 5 above. If now the heating re~uirements are increased by appropriate chanye in the value set into the controller 4~, the tem2erature registere~ by sensor Tl will differ from the ne~
set value and as a conse~uence the controller will produce pulses on line 5~ to advance the motor/cam 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 tim~ny device (not shown) of the interface circuit 7~ is operated to supply a slgnal 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 cha~es can be brought about by the controller 44. This

2~ time delay occurs at each step in the sequence.

The valves 2~, 2~ 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 ener~ised to initiate operation of the boiler 14 at the low fire level. ~elay R~-14 closes contacts ~6/1 in control circuit 32 to provide a signal which is fed to control unit 36 via line 8~ and is utilised to effect opening of the valve 3U. Althouyh opening of the valve 30 3~ 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 ~. ~

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line 74. ~ihen valve 30 is fully o~en, a microswitch is o~)eratea to si,~nify this to control unit 36 and the conaition of relay ~1 is investi~atea by al,~lyin(J a si~Jnal alon~ line 76. ~e~en~in~
on the condition of relay ~1, this is routed Dack to the boiler control unit 36 either via contacts ~1/1 (when relay ~1 is ener~ise~ and line 7~
or via contacts ~1/1, ~2/1 and line ~. The former route si~nifies tnat the se~uence control urlit 1~ is to be overri~en and the boiler 14 is to be operatec by means of its own control unit 36 an,a themlostat T7. The latter route si~nifies nonnal control of the boiler 14 via the se~;uence control unit an~ in tnis case timiny relay ~r4 is ener~;isea for a pre~etermine~ time interval sufficient ~or ,ro~er ririn~ of the boiler 14 to take ~lace.
If correct firin~ occurs in time, the control unit 3~ feeds a si~nal on line ~2 to ener~ise relay K4 which, in turn, o~ens contacts K4/1 to dc-erler~ise relay K'1'4 .

lf ~owever, correct firiny aoes not occur within the ~reatermined tin~e interval, relay ~14 times out, closes contacts ~14/l an~ eneryizes relay ~1 with conse~uent overrlde of the se~uence control unit. An audible and/or visual warniny si~nal may be ~enerated in this event. ~ner~ization of relay Kl in any of the control circuits 3~, 4~ ana 4~ automatically causes the se~uence control unit to be overriden for all boilers in the system.

3~ h similar situation may arise if, at any time, the nonnal safety functions monitored by the boiler control unit indicate incorrect operation. In this case, the boiler control units 32, 34, 3 ' ' . :

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will produce a boiler lock-out signal via line ~4 to energize relay ~5 and thereby close contacts R5/1 to energize relay Rl.

Assuminy correct firing of boiler 14 occurs, the si~nal on line 82 is also used to check the condition of relay R3 to determine whether low or high fire is re~uired. In the latter case, relay K3 Will be eneryized and its contacts R3-1 will close to route this signal back to the control unit 36 via line ~6 to signify hiyh fire opera~ion.
~owever, in the example under consideration, relay ~3-14 will not be eneryise~ and the boiler 14 will therefore be ouerated at low fire.

As well as brin~iny boiler 14 on line, the switch into stage 6 of the se~uence re~uires the boiler 1~ to be switched from hiyh fire to low fire. This will be im~lemented in response to de-ener~i7ation of relay R3-12 since the control unit 34 will no longer receive any feedback via ~0 line ~6. After the timing device of the interface circuit 17 has timed out and rernoves the sup~ressing signal from line 74, normal o~eration of the controller 44 is restored. If, at this time, the new boiler operating conditions are not adequate to meet the heat re~uirements, further ~ulses on line 5~ are transmitted via gate 53 to increment the switching unit 4~ towards stage 7.
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When the heating demand reduces an~ the sequence is reversed, it will be seen that the proyression 3V from one stage to the next (e.g. stage 6 to stage 5) may involve taking a boiler off line. In these circumstances, the associated relay R7, e.g. relay ~ ,....

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K7-14 is ener~ised to close contacts ~7/1 ana provi~e via line ~ a si~nal which is utilise~
~y the associate~ control unit ~ to initiate closiny o~ the valve ~ owever, valve closure is not e~ecte~ instantaneously. Insteaa, a ~artial closin~ siynal is pro~uce~ ~y the boiler control unit to ef~ect ~artial closiny o~ the valve, e.~.
to Wit~ % o~ its ~ully closed ~osition. A
~icroswitch is operated when the valve reaches lU the partially closea position an~ the valve ncw r~ ains in tnat ~osition until the flow telnper~ture as sensed ~y the associate~ sensor l'~-T4 ~alls to a ~,re~eterrnined value in~icatin~ that all of tile heat fr~m t~lat boiler has been ~issi~ate~.
~t this l~oint, the valve is closed canpletely to cut off water circulation to the associate~
boiler. 'I'he fore~oin~ procedure ap~lies to shut-off of all the boilers except tne last line. In this case, the valve is not closea otherwise this woul~
stop all water recirculation in the system.

The pressure controller 46 an~ sensor ~1 are employea to l,lonitor and control pressure within the system. lt is conventional practice to operate a water heatin~ systeln under pressure, by means of a suitable ~um~inLJ arran~ement, so as to raise the boilin(J point o~ the water. l`he controller 46 serves to ~revent excessive ~ressure buil~-up within the systeln an~ does so by overridin~y the tem"erature controller 44 at least insofar as the latter ~ay be causin~ the heat out~ut to increase.
Thus, if the sensed L-ressure ai~roaches the set value without exceedin~ the salne, the ~ressure controller be~ins to ~ee~ ~ulses alon~ line 54 at a fre~uency de~endin~ u~on how close the sense value is to the set value. Conse~uently this 59(~

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at least ~artly ne~ates any ~ulses fed by the em~erature controller alon~ line 52. If the ~ressure increases to a certain extent that the set value is exceeded, the controller 46 ~roduces a si~nal on line 56 to close t~e yate S~ and also su~lies ~ulses on line 5~ so as to reduce the heat out~ut of the boiler. ~nce the ~)ressure has fallen to a safe level, the overriae is re~nove~ to allow control of t~le system to ~e resunlea by the controller lU 44-

Claims (11)

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 boilers connected in a common system, pressurising means for the system, a temperature sensor connected via a sequencer to bring in successive boilers on increasing load and cut out successive boilers on reducing load, and a pressure sensor arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise when the pressure increases towards a point determined by the setting of the sensor.

2. A heating system comprising a plurality of boilers connected in a common system, pressurising means for the system, a temperature sensor connected via a sequencer to bring in successive boilers on increasing load and cut out successive boilers on reducing load, and a pressure sensor arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise when the pressure increases beyond a point determined by the setting of the sensor.

3. A heating system according to claim 1 or claim 2, wherein the pressure sensor is sensitive to the rate of change of pressure.

4. A heating system according to claim 1, wherein said pressure sensor is arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise to such an extent that it is partly negated when the pressure increases towards said point determined by the setting of the sensor.

5. A heating system according to claim 1, wherein said pressure sensor is arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise to such an extent that it is wholly negated when the pressure increases towards said point determined by the setting of the sensor.

6. A heating system according to claim 1, wherein said pressure sensor is arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise to such an extent that it is reversed when the pressure increases towards a point determined by the setting of the sensor.

7. A heating system according to claim 4, claim 5 or claim 6, wherein the pressure sensor is sensitive to the rate of change of pressure.

8. A heating system according to claim 2, wherein said pressure sensor is arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise to such an extent that it is partly negated when the pressure increases beyond said point determined by the setting of the sensor.

9. A heating system according to claim 2, wherein said pressure sensor is arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise to such an extent that it is wholly negated when the pressure increases beyond said point determined by the setting of the sensor.

10. A heating system according to claim 2, wherein said pressure sensor is arranged to modify operation of the sequencing means to reduce the rate of increase of any temperature rise to such an extent that it is reversed when the pressure increases beyond said point determined by the setting of the sensor.

11. A heating system according to claim 8, claim 9 or claim 10, wherein the pressure sensor is sensitive to the rate of change of pressure.