Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIE~D OF T~E INVENTION
This invention relates to methods and apparatus for
controlling ice rink refrigeration equipment, and more
specifically, for controlling the speed of a brine pump motor.
BACKGROUND
Refrigeration equipment for ice rinks (including
outdoor and indoor hockey, skating and curling rinks) typically
comprises a refrigeration compressor, a brine chiller, a
brine-to-ice heat exchanger usually embedded in a substrate below
the ice, brine lines and a brine pump for circulating the
refrigerated brine through the heat exchanger. The operation of
ice slab refrigeration equipment requires a significant amount of
power. For example, refrigerator compressors of about a one
hundred horsepower capacity (i.e. 77 tons), operating about 50%
of the time on the average throughout the year, are required to
maintain good ice quality in the case of a single ice rink
located in a typical community centre. The average cost for
operating refrigeration equipment for a typical ice rink is now
in the thousands of dollars monthly, and this cost is expected to
increase with ever escalating energy costs. Accordingly, there
is an increasing need to improve the energy efficiency of ice
rink equipment.
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The brine pump of a conventional ice rink refrigeration
system operates continuously at a constant speed (usually about
1800 PPM). Heretofore, no attempt had been made to vary the
speed of the brine pump, perhaps because of concerns over reduced
ice quality or perhaps because it was felt that little energy
savings could be achieved by modifying the brine pump because the
cost of running the brine pump is rather less than the cost of
running the refrigerator compressor.
SUMMARY OF THE INVENTION
The present inventors have found that it is possible to
reduce ice rink refrigeration energy costs by varying the speed
of the brine pump, without sacrificing ice quality. The present
inventors have realized that (a) the capacity of the brine pump
motor is selected based upon the maximum heat load on the ice
slab, so that the brine pump is over-designed during off-peak
load times; and (b) the flow rate of a brine pump is proportional
to the cube of the power consumption, so that a relatively small
decrease in pump flow rate can result in substantially less power
consumption (unlike the refrigerator compressor whose output is
directly proportional to its power consumption). Moreover, the
present inventors have recognized that existing systems with
constant speed brine pumps can be modified at a relatively modest
cost with relatively few changes, by apparatus which
automatically adjust the speed of the brine pump motor in
response to changes in the thermal load on the ice slab.
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Accordingly, the present invention is directed to
control apparatus for controlling the speed of an ice slab
refrigeration equipment motor, such as a brine pump motor, in
response to variations in thermal load on the ice slab. The
control apparatus comprises thermal load sensing means for
detecting the variations in ice slab thermal load and for
generating an output signal correlatable with the -thermal load
variations, and motor drive means responsive to the ther~al load
sensing means and connectable to the motor for driving the motor
at various speeds correlatable with the variations in ice slab
thermal load.
The control apparatus of the present invention is
particularly adapted for controlling the speed of an alternating
current centrifugal brine pump motor, although it could also be
used for other types of motors. The thermal load sensing means
preferably comprises brine temperature detecting means for
detecting fluctuations in the brine temperature, and drive
control means responsive to the brine temperature detector for
generating a reference signal for input into the motor drive
means, which preferably comprises a variable speed drive.
Alternatively, the thermal load sensing means could
comprise an ice slab temperature sensor placed within the ice
slab, and a set point controller for controlling the operation of
the variable speed drive.
The invention will now be described, by way of example
only, with reference to the following drawings, in which:
Figure 1 is a block diagram of a typical ice slab
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refrigeratlon system (prior art).
Figure 2 illustrates a preferred embodiment of the
control apparatus of the present invention.
Figure 3 is a detailed view of the temperature
detecting means of the preferred embodiment.
Figure 4 illustrates an alternative embodiment of the
control apparatus of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 illustrates the typical layout of a
refrigeration system for an ice rink. Brine pump 1 circulates
calcium chloride brine from brine storage tank 2 through brine
chiller 3 and rink floor piping 4 via brine lines 5a,5b. Within
chiller 3 are tubes around which circulate an ammonia or freon
refrigerant cooled by means of a refrigeration compressor 6 and
condenser 7. Associated with condenser 7 are condenser fan 8 and
water tank 9. The refigerant circulates through condenser 7 to
chiller 3 via refrigerant line lOa, and through surge drum 11,
compressor 6, and oil separater 12 via regrigerant line lOb.
Separated oil is returned to compressor 6 via oil return line 13.
The brine inside chiller 3 is cooled by heat transfer between the
brine and the refrigerant. The refrigeration compressor 6 is
typically cycled on and off as needed to keep the brine cold.
The cooled brine from chiller 3 flows through input
brine line 5a (usually about si~ inches in diameter) into rink
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floor piping 4, which is -typically small diameter thermoplastic
pipe embedded in a concrete or sand flooring onto which is poured
water for making ice. The brine passes through piping 4, which
is typically evenly spaced throughout the flooring, and heat from
the ice is transferred to the brine as it circulates through
piping 4. Warmed brine travels back to brine pump 1 via exit
brine line sb. Valves may be placed at appropriate places in the
brine lines in order to facilitate removal of the pump from the
system for repair. srine pump 1 is typically a centrifugal pump
powered by a standard "squirrel cage" induction alternating
current motor 14 (see figure 2) usually operating close to 1800
RPM.
Referring now to Figure 2, the control apparatus of the
preferred embodiment of the present invention comprises thermal
load sensing means 20 and motor drive means 22. Thermal load
sensing means 20 detects fluctuations in the thermal load on the
ice slab caused by variables such as periodic ice re-surfacing
operations, variations in ceiling lighting, the number of skaters
or curlers on the ice, the size of the audience if any, and the
rink air temperature. The thermal load sensing means 20 of the
preferred embodiment comprises temperature detecting means 30
which detects the fluctuations in the temperature of the
circulating brine, and drive control means 21, which generates a
signal correlatable with such fluctuations for input into motor
drive means 22.
It will be appreciated that the temperature of the
brine is correlatable with the fluctuations in the thermal load
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on the ice slab, for as the thermal load decreases, the frequency
of the brine temperature fluctuations also decreases. In other
words, the brine tends to stay colder longer at reduced heat
loads with the result that the brine temperature fluctuates more
slowly as a function of time (this usually results in the
refrigeration compressor operating less frequently).
Figure 3 is a detailed view of temperature detecting
means 30, which is preferably a resistance temperature detector
comprising a platinum wire encapsulated in a ceramic material
which is fixed into a stainless steel jacket 31, a coupling 32,
and three output leads 33. Temperature detecting means 30 is
preferably a one thousand ohm nominal resistance temperature
detector capable of accurately measuring temperature changes on
the order of 2F (approximately 1C) at temperatures below 32F.
Of course, other types of temperature detectors could be used.
Temperature detecting means 30 is preferably placed
directly into the output brine line 5b as shown in Figure l.
Alternatively, detecting means 30 could be placed into the input
brine line 5a, or it could be placed in the ice slab as is the
case in the alternative embodiment described below.
Drive control means 21 preferably comprises a millivott
to milliamp signal transmitter which generates a current signal
directly proportional to the output resistance of temperature
detecting means 30. The signal transmitter preferably produces a
4 mA - 20 mA or 0-10 Volt D.C. reference signal, for input into
motor drive means 22. A suitable transmitter is commercially
available from Versatile Measuring Instruments, Inc.
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Motor drive means 22 is a variable speed drive,
sometimes referred to as an inverter, which is coupled to the
drive control means and is capable of converting a fixed voltage,
fixed frequency input power signal into a variable frequency,
variable voltage, output signal proportional to the drive control
means reference signal. The output of the motor drive means 22
can be controlled so as to drive the pump motor 14 at varying
speeds, ranging for example from its usual maximum speed to a
pre-selected minimum speed. In the preferred embodiment, motor
drive means 22 comprises a commercially available transistori~ed
AC adjustable speed drive inverter such as that made by Toshiba,
which produces an output signal proportional to a 4mA-20mA input
reference signal.
As shown in Figure 2, the output leads of temperature
detecting means 30 are connected to the input terminals 23 of
drive control means 21, and the output terminal 25 of drive
control means 21 is connected to the input reference signal
terminal 26 of motor drive means 22. The input power line is
connected to the input power terminals 27 of motor drive means
22, and the output terminals 28 of motor drive means 22 are
connected to the power input terminals of the pump motor 14.
Power terminal 24 of drive control means 21 may be connected to a
115 volt, 60 cycle power line from motor drive means 22 or to a
separate power supply.
Preferably, the reference signal of drive control means
21 is selected to be directly proportional to the temperature
fluctuations detected by temperature detecting means 30.
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Likewise, it is preferable that the output signal of the motor
drive means 22 be directly proportional to the reference signal
of drive control means 21, so that the control apparatus of the
present invention drives the pump motor at a speed which is
directly proportional to the fluctuations in brine temperature.
The motor drive means 22 may be calibrated to produce a
minimum output signal for driving the pump motor at a
pre-selected reduced speed, which may be field adjustable. This
minimum speed can be empirically determined for each
refrigeration system, and should be high enough:
(a) to prevent freeze-up in the compressor
chiller;
(b) to avoid excessive cycling of the refrigerant
compressor; and
(c) to avoid cavitation caused by the inlet pressure of
the pump falling below the net positive suction
head.
In the case of a refrigerant plant which is set up such
that the compressor turns on when the brine temperature reaches a
maximum temperature (e.g. 19F) and shuts off when it reaches a
i minimum temperature (e.g. 17F) it is preferable to calibrate the
control apparatus so as to drive the brine pump at maximum power
output as the brine temperature reaches its maximum value, so
that the brine circulates at maximum speed when the refrigeration
compressor is operating. As the compressor operates and as the
brine becomes colder, the resistance of the temperature detecting
means drops, which results in reducing the reference signal of
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control means 21, causing drive motor means 22 to reduce its
power output, thus dropping the speed of pump motor 14.
Preferably, the control apparatus reduces the pump motor speed to
its minimum value, when the brine reaches the temperature at
which the refrigera-tion compressor shuts off. As the brine
temperature begins to warm, the control apparatus of the present
invention causes the pump flow to increase, and once again to
reach a maximum value about the same point in time as the
refrigeration compressor is once again activated.
The control apparatus of the present invention will
conserve energy during periods of less than maximum heat load.
During such period (e.g. the winter months), the refrigeration
system is over-designed; therefore, the flow rate of brine, which
is directly proportional to the pump speed, can be reduced
without sacrificing ice quality. Running the pump motor at a
reduced speed greatly reduces the power consumption by the pump
motor, since the power consumption is proportional to the cube of
the pump speed. Moreover, heat generated by the friction caused
by the flowing brine (for example, a 25HP brine pump operating
continually at maximum speed is equivalent to a 20KW electric
heater in the ice slab) is decreased as the brine flow is
decreased, thus reducing the cost of operating the chiller
compressor.
Figure 4 illustrates an alternative embodiment of the
control apparatus of the present invention, comprising thermal
load sensing means 120 and motor drive means 122. Thermal load
sensing means 120 comprises ice slab temperature sensor 130, and
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set point controller 121. Motor drive means 122 is preferably an
adjustable speed drive inverter like inverter 22 of the preferred
embodiment. Ice slab temperature sensor 130 is preferably a
resistance temperature detector, generally like temperature
detecting means 30 of the preferred embodiment, which is adapted
for insertion into the ice slab 100, preferably in a relatively
warm area of the ice slab, so as to monitor the fluctuations in
the ice slab, and preferably to generate an ohmic signal directly
proportional to the such temperature fluctuations. Pump 101,
pump motor 114, brine chiller 103, and brine lines 105a, 105b are
similar to their counterparts described with reference to Figure
2.
Set point controller 121 receives the signal of the ice
slab temperature sensor 130, and compares that signal with a
pre-determined set point, which is selected depending upon a
number of factors. For example, for an ice slab having a
thickness of one inch, 22F might be satisfactory for hockey,
24F might be satisfactory for curling, 26F might be
satisfactory for figure skating, etc. Set point controller 121
periodically compares the ice slab temperature with the set point
temperature. If the ice slab temperature is greater than the set
point temperature, set point controller 121 sends a reference
signal to motor drive means 122 directing it to increase the
speed of pump motor 114. Likewise, if the signal received from
temperature sensor 130 corresponds to an ice slab temperature
less than the set point temperature, set point controller 121
will generate an output reference signal directing motor drive
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means 122 to decrease -the speed of pump motor 114. The reference
signals generated by the set point controller could comprise, for
example, a 4mA signal in respone to an ice slab temperature below
the set point, and a 20mA signal in response to an ice slab
temperature above the set point, which would cause motor drive
means 122 to drive the pump motor alternately at high and low
speeds. Alternatively, the reference signal could be
proportional to the difference between the set point temperature
and the ice slab temperature. A further alternative is to
increase/decrease the reference signal after a given interval of
time, as long as the ice slab temperature remains above/below the
set point temperature, until the maximum/minimum reference signal
value is achieved.
It may be necessary or desirable, when utili~ing this
alternative embodiment of the invention, to coordinate the
operation of the brine pump and refrigeration compressor, to
ensure, for example, that the refrigeration compressor operates
when the pump motor is running at high speed. This can be
achieved, for example, by also controlling the operation of the
compressor by set point controller 121. Moreover, it may be
desirable to drive the compressor motor at variable speeds by
means of a variable speed drive like motor drive means 122.
It will be appreciated that a motor drive means other
than an alternating current inverter can be used as part of the
control apparatus of the present invention. For example, a
direct current converter could be used to power a brine pump
having a D.C. motor. Moreover, motor drive means other than a
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variable speed drive could be used; for example, an eddy current
clutch or mechanical means such as continuously variable
transmission (e.g. a variable pitch belt), could be used to drive
the pump motor at variable speeds.
It will also be apparent that the thermal load sensing
means is not limited to the temperature detecting means of the
preferred and alternative embodiments. One alternative is a
non-contact thermometer such as a spot radiometer mounted above
and directed at the ice surface.
Another alternative thermal sensing means is compressor
suction pressure detecting means, which measures the suction
pressure on the refrigeration compressor, for in some
refrigeration plants, the compressor operation is controlled
according to the refrigerant pressure in the suction line from
the brine chiller. A change in brine temperature causes a change
in refrigerant vaporization which in turn changes the suction
line pressure. Thus, suction like pressure is correlatable with
the brine temperature which, as noted above, is correlatable with
the ice slab heat load. Compressor suction pressure detecting
means could comprise a pressure transducer, which sends a current
signal proportional to the pressure to the motor drive means.
Furthermore, while the control apparatus has been
described specifically with reference to controlling a brine pump
motor, it will be apparent that it could also be utilized to
control the operation of other ice slab refrigeration equipment
motors, such as a refrigeration compressor motor.
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The method of the present invention may comprise the
steps of: (a) detecting variations in the ice slab temperature;
(b) generating an output signal correlatable with the variations
in the ice slab temperature; (c) making a comparison between the
output signal and a predetermined ice slab temperature value;
(d) generating a reference signal correlatable with the said
comparison; and (e) adjusting the speed of the motor in response
to the reference signal.
Alternatively, if comparing means such as a set point
controller is not used, the method may comprise the steps of:
(a) detecting variations in the ice slab thermal load; (b)
generating an output signal correlatable with the thermal load
variations; and (c) adjusting the speed of the motor in response
to the output signal.
While the present invention has been described and
illustrated with respect to the preferred and alternative
embodiments, it should be understood that numerous variations of
these embodiments may be made without departing from the scope of
the invention, which is defined in the appended claims.
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