GB2225653A

GB2225653A - Optimising supply temperature of heating medium

Info

Publication number: GB2225653A
Application number: GB8925250A
Authority: GB
Inventors: Svend Joergen Kjae Christensen; Niels-Peder Middelfar Andersen; Ole Stenderup; Hans Oesterbaek
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1988-11-09
Filing date: 1989-11-08
Publication date: 1990-06-06
Anticipated expiration: 2009-11-08
Also published as: CH678887A5; DK539389D0; JPH0827028B2; FI92105C; BE1002990A3; NL8902700A; DK168247B1; GB8925250D0; SE500088C2; CA2000867A1; JPH02183734A; GB2225653B; DE3838005C2; DE3838005A1; DK539389A; NL192541C; FI92105B; CA2000867C; IT8967979A0; SE8903551L

Description

i k - 1 Method off---setting the mean value of the supply temperature of a

heating medium and means for performing the method This invention relates to a method of setting the mean value of the temperature at which a heating medium is supplied in a heating apparatus, heating the medium intermittently, of a heating system comprising at least one adjustable throttling point through which the heating medium passes, wherein a desired value for the supply temperature is determined on the basis of external factors influencing the degree of heating required and the heating medium is heated to the desired value of supply temperature with the at least, one throttling point fully open. The invention also relates to means for performing such a Method.

In such a method, the anticipated heat requiremient of the system is estimated by external factors such as outside temperature, temperaCire difference between the supply and return temperature or a predeterri-ined room temperature in a central rooin of a house. Froir, those meeasured or given values, a supply teri-,p=-ratur;-= is calculated together with a ir.an--ally adjustable heating characteristic curve. A disad-:an-.az=o--:' that- method is that ch-=n;::es in the acc-al]C'--j CCrlditL-I:CnS are not taken Lnt- W',---- 1-j=s at -1 -ott-inc L111 L 2 example, at the inlets to the radiators are used, the result is that the thermostatic valves are, in order to attain the desired temperature, always fully open when the supply temperature is too low and are for the most part closed when the supply temperature is too high. An excessively high supply temperature gives rise to a high energy loss whereas a supply temperature which is too low will not heat the rooms adequately even though the radiator valves are open.

What is desired is a supply temperature which the thermostatic valves can still exercise regulating function, that is, that they be in a a partially open or partially throttling condition.

German patent sPecification DE-OS 33 45 949 discloses an apparatus for controlling a central heating system, which apparatus seeks to determine this ideal supply temperature by measuring the changes in the "thermal resistance" with the aid of temperature and flow measuring sensors. Because of the many measuring sensors, however, that solution requires a relatively high capital outlay.

It is an object of the invention to provide an automatic regullat.Lon method for setting the mean value of the supply temperature to such a vale Ch,at a substantially optimum total through'Llow rate is attained for the 1he-azing medium.

The present inventLon Pro-s,-ides a method of the mean vai-^e cfc heating medium is supplied in a heating apparatus, heating the medium intermittently, of a heating system comprising at least one adjustable throttling point through which the heating medium passes,-wherein a desired value for the supply temperature is determined on the basis of external factors influencing the degree of heating required and the heating medium is heated to the desired value of supply temperature with the at least one throttling point fully open, characterized in that a function representing the starting characteristic with which supply temperature changes with heating is determined, at least one parameter of the starting characteristic function is changed to form a desired characteristic function, and the desired value for supply temperature is changed until the progress of the heating has conformed to the desired charaCteristic function in each heating phase.

The above-mentioned object is achieved by the features that a function representing the starting characteristic with which supply temperature changes with heating is determined, at least one parameter of the starting characteristic function is Changed to form a desired characteristic function, and the desired value for supply temperature is changed until the progress of the heating has conformed to the desired characteristic fjncton in each heating phase.

According to the invention, theref-ore, the load _h in nto cr n, y s t er..

On @run-in" condition is determined by the rate of change of the supply temperature. If a great deal of heating medium is required, that is if the supply temperature rises only slowly, then_the radiator valves are open too wide, that means that they are not working in their optimum regulating range. In that case, the mean value of the supply temperature must therefore be increased. When starting a heating system, one can make the fundamental assumption that the thermostatic valves are fully open. The load on the boiler is therefore 100% because the maximum possible amount of heating medium flows through the system. Under such load conditions, the supply temperature will rise only slowly because a ed ' large amo-int of heating medium has to be heat Lrom a constant output. The function representing the starting characteristic is determined in this operating condition. After a certain time, the rooms are warmed and the thermostatic valves start to throttle the heating medium. When such normal operation has been reached, the progression of the supply temperature at each start of the heating apparatus will be represented by a steeper curve than during the initial maximum lo-ad condition. The progress of the heating-up of the supply temiperature in this normal case in relation to the initially determined maxi-mum load curve is representative of the flow rate at which the instiallatilon operates and whether the supply -L l_ L t t= -7Z."3 tS _ a ZU r e 1 s c o r r e c t 5 4-- t -. ' Z, desired characteristic function which corresponds to the optimum progress of heating and thus the optimum flow rate and by matching the actual progress of supply temperature to this desired function, one obtains a substantaially optimum flow rate and the correct mean value for the supply temperature.

It is an advantage that no additional measuring equipment is required because, typically, temperature sensors are present for measuring the supply and return temperatures. By reason of the automatic operation of the method, frequent resetting of the curve is readily possible. The heating system can therefore be.adapted to fluctuations necessitated by the seasons of the year. The return temperature can also be determined withojt a specific return temperature sensor where a pumping phase precedes each start of the heating apparatus. If the pumping phase lasts long enough, the heating medium is fed into the supply conduit at the return temperature. The temperature sensor in the supply conduit can therefore determine the return temperature which is stored for calculating the starting and desired characteristic functions.

In a preferred embodiment of such a method, the starting and desired characteristic functions with which the supply temperature changes conform to the following auxiliary function:

p K T v (t) = ---- (1 - exp c m -c. t -------) + T R c k wherein:

T v (t) is the supply temperature, is the maximum heat output of the boiler, is the thermal capacity of the through-going medium, is time, is the thermal capacity of the boiler, and is the return temperature of the heating p K c m heating t c k T R medium.

Such an auxiliary function gives a sufficiently accuraze approximation to the actual desired progress of the supply temperature. Since heating will generally be in accordance with the exoonential or e function on starting, the Steepness and the ratio Of the gradients betwen the starting characteristic function and desired unction are easily de characteristic 1 -termined. In the given auxiliary function, the parameters are readily determinable be--a...jse it is sufficient to deterN:.n-,-= each Parameter P..,C and C /C -rl M n- k Preferably, the parameters of the starting characteristic function are determined by measuring the supply temperature at at least three instants in time. That gives an adequate number of values for forming the auxiliary function.

Advantageously, the desired characteristic function is formed from the starting characteristic function by changing the parameter C m That parameter governs the gradient of the curve representing the progress of temperature with time. In particular, the desired characteristic function may be formed by reducing the parameter C m.

By reducing the parameter C m, the curve becomes steeper. That signifies a lower quantity of throughflow. With a lower quantity of throughflow, however, the supply temperature must be higher so that an adequate amount of heat is transferred by the heating apparatus to the radiators.

A substantially optimum setting at which the thermostatic radiator valves are partially throttled is obtained if the parameter C m in the desired characteristic function is between apprcxi7,,ately 20% and approximately 40% smaller than the parameter C m in the starting charaCtE.ristic function. That means that a correspondingly smaller amount of the heating medium flows- through the h;---atLinq system., that LS, on,,. aboJt 60% to 8r_% of the largest possible quantity. the startina - 8 function is determined during each transition trom night-time operation to daytime operation. That permits daily resetting of the desired characteristic function. The heating system is thus better able to follow the switching in or out of a plurality of radiators and/or seasonally-governed fluctuations in the heat requirement.

Preferably, a predetermined dead period is provided between determination of the starting characteristic function and forming of the desired characteristic function. This dead period should amount to at least one heating-up cycle and preferably more than one. That ensures that heatingup of the rooms is not held back.

With advantage, the difference between the changed desired supply temperature and the actual supply temperature value is used to form an input value for an integrator which switches the heating apparatus ON and OFF by ipeans of a switch with a hysteresis characteristic. Such a switch with a hysteresis characteristic is known from, for exam-ple, German patent specification DE-PS 34 26 937. That method permits simple regulation.

Preferably, the threshold values for the hysteresis characteristic of the switch are determined from the Parameters of the desired characzeriss-zic function. That reprezsen-s an ad%.-antaaec-_,s addi-:-ional use czE --Bra:,r--_=-,ers off th-_- a_xiliarv -=icr - 9 mean value of the supply temperature can easily be adapted to the desired set values by varying the threshold values of the switch with the hysteresis characteristic.

The invention also provides means, in particular a circuit, for setting the mean value of the temperature at which a heating medium is supplied in a heating apparatus, heating the medium intermittently, of a heating system comprising at least one adjustable throttling point through which the heating medium passes and in which the heating medium is heated to the desired value of supply temperature with the at least one throttling point fully open, the means comprising:

mearr.s arranced to determine a desired value for the supply temperature on the basis of external factors influencing the degree of heating required, means arranged to determine a function representing the starting characteristic with which supply temperature changes with heating, r,-,eans arranged to chance at least. one parameter of the starting characteristic function to form a desired characteristic function, and means to change the desired value for supply teff.perat-cre until the progress of the heating has conformed to the desired characteristic function in eacin heatinc pi-ase.

The -Ln-:enl--ion also provides -cr carrying c--- a - 10 invention, the means comprising pre-setting means arranged to produce, as a result of external factors influencing the need for heating, a desired temperature value signal, an integrator arranged to be supplied with the difference between a modified desired supply temperature signal and an actual supply temperature value signal, a switch with a hysteresis characteristic arranged to produce a boiler control signal for switching the state of the heating apparatus when the integrator output signal exceeds a first predetemined value or falls below a second predetermined value, parameter identification means arranged to determine the parameters of the starting characteristic function, computing means arranged to calculate the desired characteristic function and form the difference between the desired characteristic function and the measured progress of heating of the heating medium, an error signal prod-Jcing means to produce, in dependence on the desired charateristic function formed in the computing means, an error signal and, depending on the error signal, to produce at least two values of temperature signal of which at least one is a positive change and one is a negative chanae, and summing means arranged to add the values of the temperature signal every time the boiler is switched OFF, the output of the summing mea.ns being arranged to be added to the OUtpUt of the pre-settina me-ans to modify the desired 1 Advantageously, the error signal Producing means is arranged to produce three values of temperature signal corresponding to a temperature change _of -0.20C., OOC. and +0.20C. The r ate of change of the changed desired supply temperature value is therefore relatively small. The heating system can readily follow the change.

A method and means in accordance with the invention will now be described, by way of example only, with reference to the single figure of the accompanying drawing which is a diagrammatic representation of a heating system.

Referring to the accompanying drawing, the -hree radiators heating system comprises, for example, t 13, 14 and 15 which are supplied by a supply conduit 11 with hot water from a boiler 5. After flowing through the radiators 13, 14 and 15 the water returns to the boiler 5 through a return conduit 12. The amount of Water flowing through each radiator 13, 14 and 15 is defined by a respective one of valves 16, 17 and 18. These valves 16, 17 and 18 are in the fcrrr.. of conventional thermostatic valves, that is, their decree of opening depends on the te7., perature Of the room which the radiator heats. If the temperature in the room iS below the set desired temperature, the thermostatic radia±c,- val--e opens and, if it is hIcher, the -a'L-,-e throttles the supply of hot water into t,-,;-= rad.atcr.

As usual, boier 5 a - 12 apparatus, for example, an oil, gas or like burner or an electric heating apparatus and a storage vessel for water.

The supply temperature T v and the return temperature T R are measured at the supply conduit 11 and return conduit 12 or in the boiler 5, for example, by means of a thermometer 25 connected to a measurement transducer which converts a temperature value into electrical signals taken via lines 19, 20 and 23 for further processing. Although two separate temperature sensors will give more accurate measurements of the supply and return temperatures, it is sufficient to have a single temperature sensor (not shown) for the supply temperature.

To determine the return temperature, prior to each start of the heating apparatus, the heating medium in the boiler is then pumped around the heating circuit for a certain time so that the supply temperature is equal to the return temperature. This supply terruperature is then stored and then utilized for the next heating period as a constant value of return temperature.

To control the boiler, that is, to set the mean value of the supply temperature T v a presetting means 1 is r)rovided in which a desired s--, pply temperature value T S is produced froin, several external Ide relevant factors such as -,_he outsL T and a value H re.. 9 1 presentin the graa--eni c: a eXLernal k - 1 1 - be formed according to a known formula in which:

TS = W22 - T external) + 22 + 2/H In that formula, H is the gradient of the characteristic curve, a comparatively low mean supply temperature being reached with a low H value, whereas a higher means supply temperature value is reached with a higher H value. This desired value is changed into a modified desired value T F at a summating point 2 by a correcting value that will be described hereafter. By way of a signal line 20, the actual value of the supply temperature T v is derived at a differential-forming point 29 from this modified desired value T F This difference is fed to the input of an integrator 3. The integrator 3 integrates this signal with respect to time. The output of the integrator 3 is fed to a switch 4 with a hysteresis characteristic which switches the heating apparatus of the boiler 5 OFF when - output Lhe value of the integrator 3 exceeds a predetermined first value and switches the heating apparatus of the boiler 5 ON again when the output. value of the integrator falls below a predeteri-r..ined second value.

k In the heating-up phase, that is, when the heating apparatus heats the water, the progress with time of the raising of the supply temperature TV can be expressed by the following auxiliary function:

p K -c. t m T v (t) = ---- (1 - exp -------) + T R c m c k wherein:

T v (t) is the supply temperature, p K is the maximum heat output of the boiler, c 1r, is the thermal capacity of the through-going heating medium, t c k T R medium.

is time, is the thermal capacity of the boiler, and is the return temperature of the heating Parameter identification means 7 determine the supp'y temperature T v at several different instants in time, preferably at three instants, and from these determinations ascertain the parameters P,, C m and C k In order to work out the auxiliary function precisely, it IS ceneral ly suf f icient to dee.-rt-!ine only the q-.,otients P C and C /C, TIL-1- pararreter -i',--nt-if icattion m m X r-eans 7 r-= e S a s i n p u. t s a t i r,-- e s i an;-= th e 11 temperature T v by way of connected to the signal temperature T R by way of a signal line 26 which is line 19, and the return a signal line 24 which is connected to the signal line 23. The parameter identification means 7 operates only when the heating liquid is heated for an initial time, for example, upon transition from night-time to daytime operation. The parameters that are determined in the parameter identification means 7 therefore define a starting characteristic function.

The parameters are transmitted to computing means 6 where they can be modified to form a desired characteristic function. During a subsequent heating-up cycle, a desired characteristic function is for-med with -s the the aid of modified parameters and represent desired progress with time of the raising of the supply temperature T V This computed progress with time T v is fed by way of a signal line 28 to a differential-forminq point 8 to which is fed the value of the supply temperature T by way of a signal line 27 v connected to the signal line 19. At the differential- form.ing point 8, therefore, the difference is formed between the calculated value o- T v and the mr easured value of T V -.his difference is fed to an error signal producing unit 9. This error signal producing unit 9 prod-ices an error signal _From the calenlated at the di'Li-eren4Iial--.:or.-inj - 16 supplied by way of a signal line 30. At its output, the error signal producing unit 9 puts out one of three temperature signal values A depending o.r the determined error, namely, according to the following rule:.

If the error lies between -2% and +2%, then A 0 IT if the amount of error is larger than 2%, then A = 0.2'C., and the sign of A depends on the sign of the error.

The output of the error signal producing unit 9 is added in a summating unit-10 during each stop phase of the heating apparatus of the boiler 5. The output of the summating unit 10 is added at the summating point 2 to the output T S of the presetting means 1. At the summating point 2, a changed or modified desired supply temperature value T F is therefore formed. During normal operation, this modified desired supply temperature value T. is used in the above describedd manner tocether with the actual supply temperature T v to form a difference value that is then fed to the integrator 3.

The heating system operates as follows. When the SYS-LeM is switched from nicht-time reduced operati to normal daytime opera-lion, one can assor-,, e tlhat all rad-:'a-c-- 16, 17, 18 are ful, 1%- open er-5 --"j= 1 k - 17 maximum amount of water flows through the radiators 13, 14, 15. The boiler 5 is started. The supply temperature T v thereupon rises and is measured. With the aid of the measured characteristic curve, the constants P k' Cm' Ck of the auxiliary function of the heating system can be calculated in the parameter identification unit 7, for example, by making use of a microprocessor. Since these constants are calculated upon starting the heating system, one therefore obtains a starting characteristic function, that is, the equation which applies to the heating system at 100% flow.

With the aid of this starting characteristic function, a desired characteristic function can now be calculated by inserting, for example, a new value for c m. The new value can, for example, be 20% to 40%, particularly 30%, smaller than in the starting characteristic function. The object of the regulator formed by the integrator 3, the switch 4 with a hysteresis characteristic, the boiler 5, the return conduit 20 and the differentialforming point 26 is, now, to set a mean value for the supply temperature such that the supply temperature T v keens within the modified desired supply temperature value T F given by the desired characteristic curve. If the suiD.n'j7 temperature T v has the desired progress with one obtains a thro--,an'flcw amounting to about 6C1% to 7,' t preferably 70%, of the rr,-mximum through ilew. er i- c c. c va- e s thro f ow, e th - - a t 1 1 - 18 the radiators are in a partially throttling state, that is, they can react to temperature changes in the room by opening further or throttling more and thereby fulfil their regulating function.

After each stopping of the boiler, that is, after each switching off of the heating apparatus, the measured progress of temperature with time of the supply temperature T v is compared with the calculated desired characteristic function at several points. Depending on the result of this comparison, the mean value of the supply temperature is held constant, raised by 0.2' C., or reduced by 0.2' C. (that is changed by -0. 2' C.). Such a change is so small that the system has adequate time to become set to the new boundary conditions. This adaptation of the mean value to the load is carried out as long as the daytime operation is set.

An additional advantage of the system is that, from the calculated constants C 1 C k and P K' one can determine a socalled alpha value which can be fed to the switch 4 with the hysteresis characteristic by way 1 of a signal line 31. This alpha value serves to fix or change the two predetermined threshold values in respect of which, when exceeded or fallen below, a boiler L.

conLrol signal is produced for switchi ng the state of the heating apparatus. That avoids a somewhat uncertain manual settiDg Of this value.

Loading of the system is therefore not- -Trie,. ell y the ---se' hc-a- -4 1 1 19 ascertained. The supply temperature T v is so controlled that the radiator thermostats can always remain within their regulating range despite changing external conditions.

The external temperature T external and the curve gradient H fed to the pre-setting means 1 are also further used during daytime operation to modify the desired value T S depending on external conditions. That input of the summating point 2 does not therefore necessarily have to be constant throughout the daS,.

Claims

C L A I M S:

1. A method of setting the mean value of the temperature at which a heating medium is supplied in a heating apparatus, heating the medium intermittently, of a heating system comprising at least one adjustable throttling point through which the heating medium passes, wherein a desired value for the supply temperature is determined on the basis of external factors influencing the degree of heating required and the heating medium is heated to the desired value o-' supply temperature with the at least one throttling point fully open, characterized in that a function representing the starting characteristic with which supply temperature changes with heating is determined, at least one parameter of the starting characteristic function is changed to form a desired characteristic function, and the desired value for supply temperature is changed until the progress of the heating has conformed to the desired characteristic function in each heating phase.

2. A method as claimed in claim 1, wherein the starting or desired characteristic function with which the supply temperature changes conforms to the following auxiliary function:

p K -C. t T -) -- T R (t) = ---- (1 - ex;D ------- X Ir.

A r C wherein:

T v (t) is the supply temperature, is the maximum heat output of the boiler, is the thermal capacity of the through-going medium, is time, is the thermal capacity of the boiler, and is the return temperature of the heating c m heating t c k T R medium.

3. A method as claimed in claim 1 or claim 2, wherein the parameters of the starting characteristic function are determined by measuring the supply temperature at at least three instants in time.

4. A method as claimed in claim 2 or-claim 3 when dependent on claim 2, wherein the desired characteristic function is formed from the starting characteristic function by changing the parameter C m

5. A method as claimed in claim 4, wherein the desired characteristic function is formed by redicing the parameter C m.

6. A method as claimed in claiin 5, wherein the parainter C JP. in the desired characteristic function is -ely 40% smaller between approximately 20% and approximat than the parameter C m in the starting characteristic funct-ion.

7. A method according to any preceding clai.-,p, s-:ar-l-inw iS 1 1 C determined during each transition from night-time operation to daytime operation.

8. A method as claimed in any preceding claim, wherein a predetermined dead period is provided between determination of the starting characteristic function and forming of the desired characteristic function.

9. A method as claimed in any preceding claim, wherein the difference between the changed desired supply temperature and the actual supply temperature value is used to form an input value for an integrator which switches the heating apparatus ON and OFF by means of a switch with a hysteresis characteristic.

10. A method as claimed in claim 9, wherein the threshold values for the hysteresis characteristic of the switch are determined from the parameters of the desired characteristic function.

11. A method of setting the mean value of the temperature at which a heating medium is supplied substantially as herein described with reference to, and as illustrated by, the single figure of the accompanying drawing. the

12. Means for setting the mean value o4L temperature at which a heating medium is supplied in a -ing the medium intermittently, heating apparatus, heat of a heating system comprising at least one adjustable throttling point through which the heating medium passes and in which the heating medium is heated tO Lhe desired of supply tem-,)erat,,..re with the at 1 J 11 k ' 1 - 23 one throttling point fully open, the means comprising: meams arranged to determine a desired value for the supply temperature on the basis of external factors influencing the degree of heating required, means arranged to determine a function representing the starting characteristic with which supply temperature changes with heating, means arranged to change at least one parameter of the starting characteristic function to form a desired characteristic function, and means to change the desired value for supply temperature until the progress of the heating has conformed to the desired characteristic function in each heating phase.

13. Means for carrying out a method as claimed in any one of claims 1 to 11, the means comprising pre-setting means arranged to produce, as a result of external factors influencing the need for heating, a desired temperature value signal, an integrator arranged to be supplied with the difference between a modified desired supply temperature signal and an actual supply temperature value signal, a switch with a hysteresis characteristic arranged to produce a boiler control signal for switching the state of the heating apparatus when the integrator output signal exceeds a first prede-Lemined value or falls below a second predetermined value, parameter identification rineans arra nwed to the para:-etp--rs of ---.art:' ng characteristic function, computing means arranged to calculate the desired characteristic function and form the difference between the desired characteristic function and the measured progress of heating of the heating medium, an error signal producing means to produce, in dependence on the desired charateristic function formed in the computing means, an error signal and, depending on the error signal, to produce at least two values of temperature signal of which at least one is a positive change and one is a negative change, and summing means arranged to add the values of the temperature signal every time the boiler is switched OFF, the output of the summing means being arranged to be added to the output of the pre-setting means to modify the desired temperature value signal.

14. Means as claimed in claim 13, wherein the error signal producing means is arranged to produce three values of temperature signal corresponding to a temperature change of -0.2'C., O'C. and +0.2'C.

15. Means for carrying out a method as claimed in any one of claims 1 to 11, the means being substantially as herein described with reference to, and as illustrated by, the single figure of the accompanying drawing.

16. A heating system including means as claimed in any one of claims 12 to 15.

Published 1990 atThe Patent Office. State House, 66,71 High Holborn. London WCIR 4TP. Further copies maybe obtainedfrom. The Patent Office Sales Branch, St Mary Cray, Orpington. Kent BR5 3RD Printed by Multiplex techniques ltd. St Mary Cray. Kent. Con 1'87 X j A