Note: Descriptions are shown in the official language in which they were submitted.
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DANFOSS A/S, DK-6430 NORDBORG
Method of setting the mean value of the supply
temperature of a heating medium and a circuit
for performing the method
The invention relates to a method of setting the mean value of the
supply temperature of a heating medium which is intermittently heated
by a heating apparatus in a heating system comprising at least one
adjustable throttling point for the heating medium, wherein a desired
supply temperature value is determined on the basis of external
influencing factors and the heating medium is heated to the desired
supply temperature value with the throttling points fully open. In
addition, the invention relates to a circuit for performing the
method.
In such a method, the anticipated heat requirement of the system is
e~timated by external influences such as the external temperature,
temperature difference between the supply and return temperature or a
predetermined room temperature in a central room of a house. From
these measured or given values, a supply temperature is calculated
together with a manually adjustable heating curve. A disadvantage of
this method is that changes in the actual load conditions are not
taken into account. When using thermostatic valves at the throttling
points, e.g. at the inlets to the radiators, this leads to the con-
dition that the thermo~tatic valves are always fully open when the
supply temperature is too low and are for the most part closed when
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the supply temperature is too high so a~ to reach the desired room
temperature. An excessively high supply temperature gives rise to a
high energy loss whereas a supply temperature which is too low will
not adequately heat the rooms even though the radiator valves are
open.
What is desired is a supply temperature at which the thermostatic
radiator valves can still exercise a regulating function, i.e. they
are in a partially open or partially throttling condition.
DE-OS 33 45 949 discloses an apparatus for controlling a central
heating system which seeks to determine this ideal supply temperature
by measuring the changes in the thermal resistance with the aid of
temperature and flow measuring sensors. However, because of the many
measuring sensors, this solution requires a relatively high investment
cost.
It is the problem Or the invention to provide an automatic regulating
method for setting the mean value of the supply temperature to such a
value that an optimum total throughflow rate is achieved for the
heating medium.
In a method of the aforementioned kind, this problem is solved in
that a starting function with which the supply temperature changes on
heating is determined, at least one parameter of this starting
function is changed to fix a desired function, and the desired supply
temperature value is changed until the course of heating has adapted
itself to the desired function in each heating phase.
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According to the invention, therefore, the load on the heating system
in the "run-in condition" is determined by the changing rate of the
supply temperature. If a lot Or heating medium is required, i.e. if
the supply temperature rises only slowly, then the radiator valves
are open too far, that is to say they are not in the optimum regulat-
ing range. In that case, the mean value of the supply temperature
must therefore be increased. When starting a heating system, one can
fundamentally assume 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. With such a load
condition, the supply temperature will rise only slowly because a
large amount of heating medium has to be heated by a constant heat
output. The starting function is determined in this operating con-
dition. After a certain time, the rooms are heated and the thermo-
static valves start to throttle. When such a normal operation has
been reached, the course of the supply temperature at each start of
the heating apparatus will be steeper than during the initial maximum
load situation. The course of heating of the supply temperature in
this normal case in relation to the initially determined maximum load
curve is an expression for the flow rate at which the installation
operates and whether the supply temperature is correctly set. By
presetting the desired function corresponding to the optimum heating
course and thus the optimum flow rate and by matching the actual
supply temperature course to this desired function, one obtains an
optimum flow rate and the correct mean value for the supply temper-
ature.
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Advantageously, no additional measuring equipment is required because
generally temperature sensors are available for measuring the supply
and return temperatures. By reason of the automatic operation of the
method, frequent resetting of the curve is possible. The heating
system can therefore be adapted to fluctuations necessitated by the
seasons of the year. The return temperature can also be determined
without its own return temperature sensor if a pumping phase precedes
each start of the heating apparatus. If the heating phase lasts long
enough, the heating medium is fed into the supply conduit at the
return temperature. The temperature sensor in the supply conduit
therefore determines the return temperature which is stored for
calculating the starting or desired function.
In a preferred embodiment of the method, the starting or desired
function with which the supply temperature changes is matched by the
following auxiliary function:
K -C . t
m k ) + TR
wherein
TV(t) is the supply temperature,
PK is the maximum heat output of the boiler,
Cm is the thermal capacity of the throughgoing water,
t is the time,
Ck is the thermal capacity of the boiler, and
TR is the return temperature
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This auxiliary function gives a sufficiently accurate approach to the
actually desired course of the supply temperature. Since heating
will generally be in accordance with the e function on starting, the
steepness and the ratio of the gradients between the starting function
and desired function are easily determined. In the stated auxiliary
function, the parameters are readily determined because it is
sufficient to determine each parameter combination PK/Cm and Cm/Ck.
Preferably, the parameters of the starting function are determined on
at least three occasions by measuring the ~upply temperature. This
gives an adequate number of values for fixing the auxiliary function.
Advantageously, the desired function is determined from the starting
function by changing, especially reducing, the parameter Cm. This
parameter governs the gradient of the curve representing the course
of the temperature.
By reducing the parameter C , the curve becomes steeper. This means
a lower throughflow quantity. However, with a lower throughflow
quantity, the supply temperature must be higher so that an adequate
amount of heat is transferred by the heating apparatus to the
radiators.
An optimum setting at which the thermostatic radiator valves are
partially throttled is obtained when the parameter C in the desired
function is about 20% to 40% smaller than the parameter Cm of the
starting function. This means that a correspondingly smaller amount
CA 02000867 1998-02-11
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of the heating medium flows through the heating system, i.e. only
about 60% to 80% of the largest possible amount.
Advantageously, the starting function is determined during each
transition from night-time to daytime operation. This permits daily
resetting of the desired function. The heating system can thus
better follow the inclusion or switching off of a plurality of
radiators and/or seasonally governed fluctuations in the heat require-
ment.
Preferably, a predetermined dead period is provided between determin-
ing the starting function and fixing the desired function. This dead
period amounts to at least one heating-up cycle and preferably more
than one. This ensures that heating up of the rooms is not delayed.
With advantage, the difference between the changed desired supply
temperature value and the existing supply temperature value is used
to form an input quantity for an integrator which switches the heating
apparatus on and off by way of a hysteresis switch. Such a hysteresis
switch is for example known from DE-PS 34 26 937. This method
facilitates simple regulation.
Preferably, the threshold values for the hysteresis switch are deter-
mined from the parameters of the desired function. This represents an
advantageous additional use of the parameters of the auxiliary
function. The mean value of the supply temperature can be easily
adapted to the desired set values by varying the threshold values of
the hysteresis switch.
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According to the invention, provision i~ also made for a clrcuit for
performing the method, comprising pre-setting means which produce a
desired supply temperature value signal as a result of external
influencing factors, an integrator which is supplied with a difference
between a modified desired supply temperature value signal and an
existing supply temperature value signal, a hysteresis switch which
produces a boiler control signal for switching the heating apparatus
when the integrator output signal exceeds a first predetermined value
or falls below a second predetermined value, parameter identification
means which determine the parameters of the starting function, a
computer which calculates the desired function and forms the difference
between the desired function and the measured heating course of the
heating medium, an error signal producing unit which, depending on the
desired function formed in the computer and the determined difference,
forms an error and, depending on this error, produces three temperature
signal values of which at least one is positive and one is negative,
and a summating unit which adds the temperature signal values each
time the boiler is switched off, the output of the summating unit
being added to the output of the presetting means.
Advantageously, the three temperature signal values correspond to a
temperature change of -0.2~, 0~ and +0.2~ C. The rate of change of
the changed desired supply temperature value is therefore relatively
small. The heating system can readily follow the change.
A preferred example of the invention will now be described in conjunc-
tion with the drawing in which the single figure is a diagrammatic
representation of the heating system.
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The heating system comprises for example three radiators 13, 14, 15
which are supplied by a supply conduit 11 with hot water from a boller
5. After flowing through the radiators 13, 14, 15, the water
returns to the boiler 5 through a return conduit 12. The amount of
water flowing through each radiator 13, 14, 15 is determined by a
respective valve 16, 17, 18. These valves 16, 17, 18 are in the form
of conventional thermostatic valves, i.e. their degree of opening
depends on the temperature of the room which the radiator heats. If
the temperature in this room is below the set desired temperature, the
thermostatic radiator valve opens and, if it is higher, the valve
throttles the supply Or hot water in the radiator.
As usual, the boiler 5 comprises a heating apparatus, for example an
oil, gas or like burner or an electric heating apparatus and a storage
vessel for water.
The supply temperature TV and the return temperature TR are measured
at the supply conduit 11 and return conduit 12 or in the boiler 5, for
example with the aid of a thermometer 25 with a connected measurement
convertor which converts a temperature value into electric signals
supplied via conduits 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 starting of the heating
apparatus, the heating medium in the boiler is then pumped around in
the heating circuit for a certain time so that the supply temperature
is equal to the return temperature. This supply temperature is then
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stored and then utilised for the next heating period as a constant
return temperature.
To control the boiler, i.e. to set the mean value of the supply temper-
ature Tv, provision is made for pre-setting means t in which a desired
supply temperature valueTs is formed from several external influencing
factors such as the external temperature TeXternal and a curve gradient
H. The quantity TS can, for example, be formed according to a known
formula in which
S ( external) 22 2/H
In this formula, H is the gradient of the curve, a comparatively low
mean supply temperature being reached with a low H value whereas a
higher mean supply temperature value is reached with a higher H value.
This desired value i~ changed into a modified desired value TF in a
summating point 2 by a correcting quantity that will be described
hereinafter. By wayofasignal conduit 20, the existing value of the
supply temperature TV is derived at a differential formation point 29
from this modified desired value TF. This difference is fed to the
input of an integrator 3. The integrator 3 integrates this signal
over the time. The output of the integrator 3 is fed to a hysteresis
switch 4 which switches the heating apparatus of the boiler 5 off when
the output 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 predetermined second
value.
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In the heating-up phase, i.e. when the heating apparatus heats the
water, the time heating course of the supply temperature TV can be
expressed by the following auxiliary function
TV(t) = - ( 1 - exp m ) + TR
m k
wherein
TV(t) is the supply temperature,
PK is the maximum heat output of the boiler,
C is the thermal capacity of the throughgoing water,
t is the time,
Ck is the thermal capacity of the boiler, and
TR is the return temperature.
Parameter identification means 7 determine the supply temperature TV
at several different instances, preferably on three occasions, and
from these determine the parameters PK, C and Ck. In order to fix
the auxiliary function precisely, it is generally sufficent to deter-
mine only the quotients PK/C and C ICk. As input quantities, the
parameter identification means 7 are fed with a time signal, the
supply temperature TV by way of a signal conduit 26 which communicates
with the signal conduit 19, and the return temperature TR by way of a
signal conduit 24 which communicates with the signal conduit 23. The
parameter identification means 7 operate only when the heating liquid
is heated for the first time, for example upon transition from night-
time to daytime operation. The parameters that are determined in theparameter identification means 7 therefore define a starting function.
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The parameters are transmitted to a computer 6 where they can be
modified to form a desired function. During a subsequent heating-up
cycle, a desired function is formed with the aid of the modified
parameters and represents the desired time course of heating the
supply temperature Tv. This computed course of TV is fed by way of a
signal conduit 28 to a differential formation point 8 to which is fed
the value of the supply temperature TV by way of a signal conduit 27
communicating with the signal conduit 19. At the differential form-
ation point 8, therefore, the difference is formed between th~ calcul-
ated value of TV and the measured value of Tv. This difference is fed
to an error signal producing unit 9. This error signal producing unit
9 determines an error from the difference calculated at the differen-
tial forma~ion point 8 and the values of the desired function supplied
by way of a signal conduit 30. At its output, the error signal pro-
duction unit 9 emits three temperature signal values A depending on
the determined error, namely according to the following rule. If the
error lies between -2% and +2%, A = 0. If the amount of the error is
larger than 2%, A = 0.2 C. The sign of A depends on the sign of the
error.
The output of the error production unit 9 is added in a summating unit
10 during each stop 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 TS of the presetting means 1. At the summating point 2, a
change or modified desired supply temperature value TF is therefore
formed. During normal operation, this modified desired supply temper-
ature value TF is used in the above described manner to form a differ-
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ence together with the existing supply temperature TV that is then fed
to the integrator 3.
The heating system operates as follows. When the system is switched
from night-time reduced operation to normal daytime conditions, one
can assume that all radiator thermostats 16, 17, 18 are fully open and
the maximum amount of water flows through the radiators 13, 14, 15.
The boiler 5 i~ started. The supply temperature TV thereupon rises
and is measured. With the aid of the measured curve, the constants
PK, Cm, Ck of the auxiliary function of the heating system can be
calculated in the parameter identification unit 7, for example with
the aid of a microprocessor. Since these constants are calculated
upon starting the heating system, one therefore obtains a starting
function, i.e. an equation, which applies for the heating system at
100% flow.
With the aid of this starting function, a desired function can now be
calculated by inserting for example a new value for C . The new value
can for example be 20% to 40%, particularly 30%, smaller than in the
starting function. The object Or the regulator formed by the integrat-
or 3, the hysteresis switch 4, the boiler 5, the return conduit 20 and
the differential formation point 26 is, now, to set a mean value for
the supply temperature such that the supply temperature TV keeps
within the modified desired supply temperature value TF given by the
desired curve. If the supply temperature TV has the desired course,
one obtains a throughflow amounting to about 60% to 80% preferably
70%, of the maximum throughflow. At this throughflow, the thermostatic
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valves 16, 17, 18 of the radiators are in a partially throttling
condition, i.e. they can react to temperature changes in the room by
opening further or more throttling and thereby fulfil their regulating
function.
After each stopping of the boiler, i.e. after each switching off of
the heating apparatus, the measured temperature course of the supply
temperature TV is compared with the calculated desired function at
several points. Depending on the result of this comparison,-the mean
value of the supply temperature is held con~tant raised by 0.2 C or
reduced by -0.2 C. This change is so small that the system has
adequate time to become set to the new marginal conditions. ThiA
adaptation of the mean value to the lQad is carried out as long as the
day-time operation is set.
An additional advantage of the system is that, from the calculated
constants C , Ck and PK, one can determine a so called alpha value
which can be fed to the hysteresis switch 4 by way of a signal conduit
31. This alpha value ~erves to fix or change the two predetermined
threshold values for which, when exceeded or fallen below, a boiler
control signal ls produced for switching the heatlng apparatus. This
avoids a somewhat uncertain manual setting of this value.
Loading of the system is therefore not only estimated but the actual
consumed heat consumption is determined. The supply temperature TV is
so controlled that the radiator thermostats can always remain in their
regulating range despite changing external conditions.
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The external temperature TeXter al and the curve gradient H fed to the
presetting means 1 are also continued to be used during daytime oper-
ation to modify the desired value TS depending on the external con-
ditions. This input of the summating point 2 does therefore not have
to be necessarily constant throughout the day.
