U.S. patent application number 10/650113 was filed with the patent office on 2004-12-02 for mechanical refrigeration system with a high turndown ratio.
Invention is credited to Andrews, Craig C..
Application Number | 20040237555 10/650113 |
Document ID | / |
Family ID | 33457600 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040237555 |
Kind Code |
A1 |
Andrews, Craig C. |
December 2, 2004 |
Mechanical refrigeration system with a high turndown ratio
Abstract
Mechanical refrigeration systems, including ice machines, having
increased effective operating range. The ability to operate over a
wide range of heat transfer capacity allows the system controller
to conserve energy by closely matching the refrigeration capacity
and the heat load. In applications where the refrigeration system
is being operated from low-grade power or small power grids, the
refrigeration capacity may be adjusted to control the amount of
electrical energy taken from the grid. For the more specific
application where ice is being manufactured, to maintain product
quality the varying refrigeration capacity of the system must be
followed by a variance in the heat load that is applied to the
system.
Inventors: |
Andrews, Craig C.; (College
Station, TX) |
Correspondence
Address: |
STREETS & STEELE
13831 NORTHWEST FREEWAY
SUITE 355
HOUSTON
TX
77040
US
|
Family ID: |
33457600 |
Appl. No.: |
10/650113 |
Filed: |
August 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60474516 |
May 30, 2003 |
|
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|
Current U.S.
Class: |
62/230 ;
62/228.4 |
Current CPC
Class: |
F25B 2700/04 20130101;
F25C 2600/04 20130101; F25C 1/00 20130101; F25B 2600/2513 20130101;
F25B 2400/0409 20130101; F25B 49/02 20130101; F25B 2700/2117
20130101; F25B 2600/2511 20130101; F25B 2700/197 20130101; F25B
2600/02 20130101; F25C 2400/14 20130101 |
Class at
Publication: |
062/230 ;
062/228.4 |
International
Class: |
F25B 001/00; F25B
049/00 |
Claims
What is claimed is:
1. A method of operating a refrigeration system having a
compressor, condensor and evaporator, the method comprising:
controlling the effective surface area of the evaporator to obtain
a desired heat load on the evaporator.
2. The method of claim 1, further comprising: controlling the flow
of a refrigerant fluid into the evaporator to obtain the effective
surface area.
3. The method of claim 1, further comprising: measuring the
effective surface area of the evaporator.
4. The method of claim 1, further comprising: varying the desired
heat load to the evaporator as a function of the present capacity
of the compressor.
5. The method of claim 4, further comprising: varying the capacity
of the compressor as a function of the presently available
power.
6. The method of claim 5, further comprising: monitoring the power
available from an electrical grid; and monitoring the power
demanded by the compressor, wherein the capacity of the compressor
is varied so that the power demanded by the compressor tracks the
presently available power from the electrical grid.
7. The method of claim 5, wherein the capacity of the compressor is
varied by controlling the rate at which vapor is compressed.
8. The method of claim 7, wherein the step of controlling the rate
at which vapor is compressed comprises one or more step selected
from controlling the flow of gas into the compressor, controlling
the compression ratio of the compressor, controlling the number of
cylinders used for the compression cycle, controlling the speed of
the compressor, and combinations thereof.
9. The method of claim 7, further comprising unloading a subportion
of the compressor.
10. The method of claim 9, wherein the subportion of the compressor
is unloaded by positioning an intake valve to one or more
compressor cylinders in an open position.
11. The method of claim 5, wherein the step of controlling the
capacity of the compressor comprises one or more step selected from
hot gas bypassing, varying the amount of heat removed from the
condenser, varying the amount of heat removed from the evaporator,
and combinations thereof.
12. The method of claim 1, further comprising cycling the
compressor.
13. The method of claim 7, further comprising adjusting the
rotational speed of the compressor.
14. The method of claim 1, wherein the step of controlling the
capacity of the compressor includes varying the speed or torque of
the compressor.
15. The method of claim 1, further comprising: monitoring one or
more system variables selected from evaporator system temperature,
evaporator system pressure, condenser system temperature, condenser
system pressure, level of refrigerant in the high side liquid
receiver, refrigeration level in the evaporator, compressor
rotational speed, system power input, line voltage, line current,
and line phase factor.
16. The method of claim 15, further comprising: determining the
present capacity of the compressor.
17. The method of claim 16, further comprising: controlling the
heat load in proportion to the present capacity of the
compressor.
18. The method of claim 17, wherein the heat load is determined by
the amount of product to be processed.
19. The method of claim 18, wherein the product is ice.
20. The method of claim 1, further comprising: controlling the
power delivered to the compressor as a function of the amount of
power determined to be available; and adjusting the heat load as a
function of a condition of the evaporator, wherein the condition is
selected from physical measurements.
21. The method of claim 20, wherein the physical measurements are
selected from evaporator temperature, level of refrigerant in the
evaporator, evaporator suction pressure, evaporator discharge
pressure, and combinations thereof.
22. The method of claim 17, wherein the step of controlling the
heat load further comprises: controlling the flow of refrigerant to
maintain the evaporator in a flooded mode; and controlling the
level of refrigerant in the evaporator to effect a desired heat
load.
23. The method of claim 17, wherein the refrigeration unit includes
an icemaker, and further comprising adjusting the feed water
distribution to change the contact between the water and the
evaporator.
24. The method of claim 23, wherein the icemaker is selected from a
block ice machine, a tube chunk machine, a spinning disk flake
machine, a drum and scraper flake machine, and slush ice
machine.
25. The method of claim 1, further comprising: delivering the
refrigerant to the evaporator continuously or periodically based on
system operating parameters.
26. The method of claim 1, further comprising: evacuating, during
periods of surplus power, a low pressure reservoir; and providing
communication, during periods of limited power availability, of the
low pressure reservoir with the suction side of the compressor.
27. The method of claim 1, further comprising: controlling the
position or number of water introduction conduits across the
evaporator surface.
28. The method of claim 1, further comprising: (d) controlling the
pressure of the evaporator to control the evaporator
temperature.
29. A refrigeration unit, comprising: means for monitoring the
electrical supply; and a controller for varying the heat load so
that the electrical demand tracks the electrical supply.
30. The refrigeration unit of claim 29, further comprising an ice
maker.
31. The refrigeration unit of claim 29, wherein the compressor is a
non-reciprocating compressor.
32. The refrigeration unit of claim 29, wherein the controller is a
power distribution controller.
33. The refrigeration unit of claim 29, wherein the means for
monitoring the electrical supply comprises one or more sensors for
measuring an electrical supply characteristic selected from
voltage, current, power factor, an independent external signal and
combinations thereof.
34. The refrigeration unit of claim 29, further comprising means
for varying the compressor speed or torque.
35. The refrigeration unit of claim 34, wherein the means for
varying the compressor speed or torque is selected from a
transmission converter, a torque converter, a direct current motor,
an alternating current motor with variable speed controller, and an
alternating current motor with variable torque controller.
36. The refrigeration unit of claim 34, wherein the means for
varying the compressor speed provides turndown ratios of 10:1 or
greater.
37. The refrigeration unit of claim 29, further comprising: a valve
for selectively communicating the low pressure reservoir with the
low pressure side of the compressor.
38. The refrigeration unit of claim 29, wherein the refrigeration
unit is an air cooler.
39. The refrigeration unit of claim 30, further comprising a water
introduction conduit defining a water introduction point, and
wherein the water introduction point is selectively positionable at
various distances above the base of the evaporator surface.
40. The refrigeration unit of claim 29, further comprising: means
for removing oil from the refrigerant.
41. The refrigeration unit of claim 29, further comprising: a
baffle in the evaporator to encourage natural circulation due to
rising refrigerant bubbles.
42. The refrigeration unit of claim 41, wherein the baffle is
perforated or louvered.
43. A refrigeration unit, comprising: an absorption system; and a
controller for varying the heat load to the absorption system.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/474,516 filed May 30, 2003.
FIELD OF THE INVENTION
[0002] This invention relates to the design and operation of
mechanical refrigeration systems to increase the effective
operating range of the system.
BACKGROUND OF THE INVENTION
[0003] Electricity will play the major role in the growth of many
areas of the world that are now considered underdeveloped, poor, or
remote. Because many of these locations will never be serviced by
large area electrical distribution systems, electricity is, or will
be, provided on a local scale by various means. When supplying
power to these small grids, the balancing of electrical supply and
demand is critical to the distribution of high quality power. Quite
often the source of electricity, such as diesel generators and
renewable energy sources, has its own variability. Wind turbines,
small hydroelectric devices, solar arrays, etc., all have
electrical power outputs that vary with the available power input.
For these electrical distribution systems, the ability to utilize
the electrical energy at the same rate it is being generated is a
useful property of any electrical load. When the electrical demand
is larger than the supply, the grid suffers a brownout where the
line voltage drops below its normal range. In the extreme, this
condition may lead to damage or destruction of the generating
system as it is overloaded beyond its design capacity. More likely,
the generating system will detect the fault or overload and take
steps to protect itself by tripping a circuit overload protection
device. This, of course, leads to a blackout where electricity
delivery to the grid has completely stopped.
[0004] Therefore, it is desirable for the electrical loads on the
distribution system to be able to operate efficiently and perform
useful work over a wide range of operation. As the overall
electrical demand approaches that of the supply, it may be
desirable to target individual loads for capacity reduction,
specifically those loads where the average output, rather than the
instantaneous output, is of interest. These loads may be shut down
completely or, in some cases, merely turned-down so that their work
output decreases but more importantly their power consumption
decreases. Given the ability to control the demand of the
electrical grid, a power generation facility may increase the
overall quality of the electrical energy being supplied to the end
user.
[0005] The voltage and frequency variances inherent to many small
grid systems complicate the suitability of individual loads to be
turned down. Most motor loads are unable to operate over a wide
range of operating voltages since they overheat if the voltage
increases beyond nameplate rating and their power output decreases
if operated below nameplate. Likewise, the ratio of the voltage to
frequency must be within a very narrow window for the motor to
operate efficiently and to provide useful work. Even while the
operating voltage and frequency fall within acceptable limits, the
process being driven may have limitations to the range of
operation
[0006] Quite often, conditions are such that there is the capacity
to generate electricity at a rate higher than it is being consumed.
Normally this may be corrected by reducing the generator output or
in rare instances by increasing a dump load whose purpose is to
simply dissipate electrical energy while providing no useful work.
If the electrical energy is being provided by a fuel powered
device, such as diesel generator, then turning down the generating
rate provides a decrease in fuel consumption and there is little
wasted energy since the fuel may be stored indefinitely for later
use. However, when the electrical energy is being provided by an
alternate energy source having no fuel requirements, reducing the
energy generation capacity is a wasted opportunity to provide
surplus energy to either accomplish useful work or to be converted
to a form suitable for storage and later use.
[0007] The concept of load leveling includes storage of surplus
energy for a short period of time and reconversion of the stored
energy to useable work at a later time when there is less energy
available. An example of loading leveling includes hydro storage
where water behind a hydroelectric dam is pumped to a lake above
the dam in the spring when there is a surplus of water, and that
water is allowed to flow through the dam later in the year to turn
the turbines. Another example is the regenerative fuel cells where
electrical energy is used to electrolyze water into stored hydrogen
and oxygen that is later used to operate a fuel cell. A final
example is a flywheel storage system where the electrical energy is
converted to mechanical energy and vice-versa.
[0008] Another form of load leveling is to simply increase the
production of a useful commodity while the energy is available so
that the commodity does not have to be produced when the electrical
energy is in limited supply or unavailable. An example would be the
production of ice for those locations, such as fishing villages,
where ice is required for the local economy. In that application
load leveling is accomplished by making as much ice as possible
while there is surplus energy so that the icemaker may be turned
down or turned off when there is a shortage of energy. To continue
the example, if the electrical energy is being provided by a
renewable energy source, such as a wind turbine, any surplus ice
that can be generated may be stored for later use when there is
less energy available from the renewable energy source. However, a
problem arises when the refrigeration capacity of a mechanical
refrigeration system is varied beyond a narrow window, because the
refrigeration capacity of a typical refrigeration system is
adjusted for the purpose of controlling the temperature of the
process, not for the purpose of limiting the amount of power being
consumed.
[0009] There are four basic methods of capacity control employed in
the industry. In a first method, if the refrigeration capacity is
only slightly too large, then a portion of the hot gas exiting the
compressor is shunted across the condenser and injected into the
evaporator or liquid refrigerant line. Known as hot gas bypassing,
this spoils the efficiency of the refrigeration cycle since the
heat of compression is dumped into the cold reservoir
(process/evaporator) rather than the hot reservoir
(ambient/condenser). The second method of capacity control is very
similar in that the amount of heat removed from the condenser is
varied, e.g., in an air cooled condenser the fans are slowed to
reduce the airflow through the condenser. As with hot gas bypassing
the efficiency is spoiled since a portion of the heat of
compression is left in the refrigerant when it enters the liquid
evaporator. While these first two methods of capacity control
decrease the effectiveness of the refrigeration cycle, they do not
significantly change the power being consumed by the refrigeration
system. Therefore, they are equivalent to operating a dump load
where the surplus energy is simply wasted.
[0010] A third method of capacity control is available on systems
having multi-cylinder compressors. In these systems, the intake
valve of the compressor may be held open during both the intake and
compression phases of the compression cycle, commonly referred to
as unloading the compressor. When held open, the intake valve
allows the refrigerant in the suction side of the compressor to
enter the cylinder during the intake stroke and then exit the
cylinder back into the suction piping system. Since the pressure of
the refrigerant is unchanged, the compressor performs no work.
Therefore, on compressors having an unloaded cylinder the work
performed by the motor is reduced since the only work done by the
unloaded cylinder is due to friction. As an example, a six cylinder
compressor may have programmed stages where a specific system
parameter is used to determine how many cylinders are compressing
refrigerant and how many cylinders are unloaded.
[0011] Yet another method of capacity control is the cycling of the
compressor. Generally reserved for when the capacity is much higher
than the heat load, the compressor system will turn off when the
target temperature is achieved and then return to operation when
the upper limit is reached.
[0012] With the exception of cylinder unloading, these methods of
capacity control work equally well with non-reciprocating forms of
compressors, such as rotary lobe compressors, scroll compressors,
etc., and have complementary capacity reduction techniques with
absorption systems.
[0013] There are also less commonly used forms of capacity control
such as rotational speed adjustment. However, returning to the
example of ice making, reducing the capacity of the icemaker simply
by slowing down the compressor and reducing the amount of
refrigerant pumped through the cycle will result in a sharp
decrease in the quality of ice.
[0014] Therefore, there is a need to control the refrigeration
capacity of a vapor phase compression system in order to reduce the
power demands of the refrigeration unit. It would be desirable to
control or vary the refrigeration unit capacity without effecting
the quality of the refrigeration.
SUMMARY OF THE INVENTION
[0015] The present invention provides a means of controlling the
refrigeration capacity of a vapor phase compression machine, for
the purpose of reducing the power demands of the refrigeration
unit. The quantity of input power consumed by the refrigeration
system may be controlled by an external means, such as a power
distribution controller that adjusts the refrigeration system as a
part of load balancing procedure. The operation and capacity of the
refrigeration system may be adjusted by internal means, such as
sensors on the compressor, motor, evaporator, condenser, target
temperature, target capacity, etc.
[0016] To provide suitable turndown ratios, i.e., the ratio of
maximum capacity to minimum capacity, the compressor speed and
torque may each be varied to adjust and maintain the desired power
delivery draw of the refrigeration system. This may be accomplished
through any method of speed control such as variable speed
controller and an inverter grade alternating current (AC) motor.
With these systems infinite speed turndown ratios of the compressor
are possible with turndown ratios of 1000:1 easily achieved. To
further increase the turndown ratio, individual cylinders of the
compressor may be partially or fully unloaded as the result of
commands from the control system. Unloading cylinders allows the
work being performed by the compressor to be reduced in an
efficient manner, and therefore reducing the power requirements to
maintain the compressor within a safe window of operation (e.g.,
minimum speed).
[0017] Optionally, the control system may allow the refrigeration
system to control the speed of the process that is utilizing the
refrigeration. As the real time capacity of the refrigeration
system is determined, the amount of product that can be processed
(e.g., food frozen, ice manufactured, etc.) can then be controlled
so that the quality of the refrigeration process is maintained even
as the capacity of the refrigeration process is reduced.
[0018] One method of controlling the heat input to the system
includes maintaining the refrigeration evaporator in a flooded mode
of operation and adjusting the refrigerant level within the
evaporator to control the heat load delivered to the system. The
effective surface area of the evaporator may be reduced by lowering
the refrigerant level and thereby reducing the cold surface area in
contact with the product or process.
[0019] In another aspect of the invention, the heat load is
controlled through modification of the amount, location, or
distribution of the heat load within or upon the system evaporator.
For the specific example of the icemaker, the feed water
distribution may be lowered or raised to adjust the portion of the
evaporator in contact with the water.
[0020] Optionally, a low capacity method for operating the
refrigeration system includes maintaining the velocity of
refrigerant within the system piping and the turbulence inside the
evaporator within acceptable ranges by continuously operating the
compressor but delivering the refrigerant to the evaporator in
small batches. Likewise, a means of increasing the flow rate of the
low side of the refrigeration system is provided by providing
periodic surges of gas flow from the evaporator to the
compressor.
[0021] In another embodiment of the invention, the compressor is
coupled to a wind turbine, either directly, through a transmission,
or through a torque converter. In this system, the cylinder
unloading method of capacity reduction may be used to reduce the
power input requirements of the compressor for periods of light
wind and to more closely match the low speed torque capabilities of
the energy source.
[0022] In another embodiment of the invention, an absorption system
may be utilized in place of the vapor phase compression system. In
these systems the capacity control is the direct result of
controlling the heat applied to the absorption system. Typically,
absorption systems have an efficiency advantage over vapor phase
compression when the energy input is in the form of heat rather
than fuel or electricity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] So that the above recited features and advantages of the
present invention can be understood in detail, a more particular
description of the invention, briefly summarized above, may be had
by reference to the embodiments thereof which are illustrated in
the appended drawings. It is to be noted, however, that the
appended drawings illustrate only typical embodiments of this
invention and are therefore not to be considered limiting of its
scope, for the invention may admit to other equally effective
embodiments.
[0024] FIG. 1 shows refrigerant flow control of the effective
surface area.
[0025] FIG. 2 shows physical control of the effective surface
area.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The present invention provides a means of controlling the
refrigeration capacity of a vapor phase compression machine, for
the purpose of reducing the power demands of the refrigeration
unit. The quantity of input power consumed by the refrigeration
system may be controlled by an external means, such as a power
distribution controller that adjusts the refrigeration system as a
part of a load balancing procedure. The operation and capacity of
the refrigeration system may be adjusted by internal means, such as
sensors on the compressor, motor, evaporator, condenser, target
temperature, target capacity, etc. In another mode of operation,
the power demands of the refrigeration system may be matched to the
available power by monitoring the voltage, current, power factor
and similar grid parameters. For example, the refrigeration system
may include a controller to monitor the grid power at an
appropriate place and then reduce the capacity of the refrigeration
system to maintain the quality of the electrical grid, e.g.,
preventing brownouts, large power factors, etc.
[0027] Another aspect of the invention is the compressor speed and
torque may each be varied to adjust and maintain the desired power
delivery draw of the refrigeration system. This may be accomplished
through any method of speed control including the use of
conventional electrical power source coupled to a transmission or
torque converter, direct current (DC) motor or alternating current
(AC) motor and variable speed controller. The DC motor controller
may be any type that is compatible with the grid and does not place
an extreme amount of harmonics or other noise onto the power
source. The variable speed AC controller may include open or closed
loop vector control, constant volts/hertz, or any number of other
methods for speed or torque control of AC motors. Infinite speed
turndown ratios of the compressor are possible with turndown ratios
of 1000:1 easily achieved.
[0028] To further increase the turndown ratio of the refrigeration
system, the compressor may be fitted with a means of reducing the
capacity by reducing the performance of the compressor system. One
method of reducing the capacity of the compressor system is know as
hot gas bypassing, where a controlled amount of hot refrigerant
exiting the compressor is injected into the low pressure side of
the refrigeration system, e.g., the evaporator. As a result of
bypassing the condenser, where the heat of compression is removed
from the refrigerant, this hot gas spoils the efficiency of the
system by boiling off a portion of the liquid refrigerant in the
evaporator. The disadvantage of hot gas bypass, and other systems
which merely spoil the efficiency of the refrigeration process, is
that the amount of power consumed by the refrigeration system
remains fairly constant, with only the refrigeration capacity
reduced.
[0029] A preferred method of reducing the capacity of the
compressor is to limit the rate at which gas is being compressed.
This may be achieved by limiting the flow of gas into the
compressor, limiting the compression ratio of the compressor, or by
limiting the number of cylinders used for the compression cycle. A
common method of reducing the number of cylinders actively
compressing the refrigerant is to hold the intake valve open
allowing refrigerant to be drawn into the cylinder from the low
pressure suction line and then rejected back into the low pressure
suction line. This effectively reduces or eliminates the gas
throughput of that cylinder and is called "unloading" of the
cylinder. Since there is little or no work being done on the
refrigerant, the capacity of the compressor resulting from that
cylinder is eliminated. Furthermore, since there is relatively
little amount of power required to move the unloaded cylinder, the
power consumption of the compressor is reduced along with the
capacity. Since the amount of energy required to operate the
compressor decreases with the capacity, this method of capacity
reduction can be used to accurately control the power usage of the
compressor. The present invention provides the ability to adjust
the power demand of the refrigeration system by reducing the
refrigeration capacity of the compressor, such as by unloading
cylinders. Individual cylinders of the compressor may be partially
or fully unloaded as the result of commands from the control
system. Unloading cylinders allows the work being performed by the
compressor to be reduced in an efficient manner, and therefore
reducing the power requirements to maintain the compressor within a
safe window of operation (e.g., minimum speed). Likewise, the
individual cylinders may be returned to operation as energy becomes
available, allowing the compressor to remain within desired
operating parameters.
[0030] In some heat loads, such as comfort air conditioning or
refrigerated storage, it is acceptable to have any rate of heat
transferred between the cold and hot reservoirs since there are no
short term demands for continuous operation. However, for those
heat loads where the refrigeration is being used to cool or freeze
on demand, such as a food process line or ice maker, it is critical
that the heat load does not exceed the capacity of the
refrigeration equipment. Therefore, on systems where the capacity
of the refrigeration equipment may possibly be varied on a
continuous basis, the refrigeration control system may be given the
ability to control the speed of the process that is utilizing the
refrigeration. In this control system, a controller determines the
present capacity of the refrigeration equipment by measuring the
temperature (and/or pressure) of the evaporator system and the
condenser system. Additional data, such as the refrigerant level in
the high side liquid receiver, refrigerant level in the evaporator,
compressor RPM, system power input, line voltage, line current,
line phase factor, etc., may also be used to determine the real
time capacity of the system. After the real time capacity of the
system is determined, the amount of product that can be processed
(e.g., food frozen, ice manufactured, etc.) can then be
controlled.
[0031] A preferred refrigeration output control is as follows: The
controller, or central control station, sets the desired amount of
power to be consumed by the refrigeration system. The controller
then limits the power (voltage and/or current) delivered to the
motor and therefore the amount of power delivered to the
compressor. The temperature of the cold reservoir (the evaporator
making the ice) is then maintained even with reduced power input by
reducing the flow rate of the water being delivered to the
icemaker. This reduces the heat load of the evaporator allowing the
evaporator temperature to be maintained. Alternately, the level of
refrigerant in the evaporator, suction pressure, or discharge
pressure may be used to maintain control of the water flow.
[0032] The evaporator may be operated in a flooded mode to allow
the efficiency of the evaporator to be maintained across the entire
range of refrigeration output. The liquid level of the refrigerant
is then used to control the flow of refrigerant and to control the
heat load (water to the icemaker).
[0033] In another optional aspect of the invention, a fixed- or
variable-speed, supplemental oil pump, that is operated by a source
other than the motor driving the compressor, is included in the
system to ensure proper lubrication. The supplemental oil pump may
be cycled off and on in response to one or more parameters such as
oil pressure or compressor RPM. Yet another optional aspect of the
invention is the use of a low-pressure reservoir in communication
with the suction side of the compressor. This reservoir may simply
increase the volume of the low side reservoir so that short-term
changes in the pumping capacity are damped. Alternatively, the
reservoir may be valved into and out of the low-pressure side of
the system to provide short-term load leveling, i.e., allowing the
capacity of the compressor to be changed while allowing the short
term pumping capacity upon the evaporator to remain stable. As the
pumping capacity of the compressor is increased, due to an increase
in the available power, a portion of the excess pumping capacity is
used to evacuate the reservoir in preparation for the next
short-term reduction in pumping capacity. This may be accomplished
by an additional compressor, or more preferably, by alternately
pumping on the reservoir and the evaporator such that the
temperature of the evaporator is maintained while the pressure of
the reservoir is taken significantly below that of the evaporator
and more preferably to a partial vacuum.
[0034] An optional aspect of the control system is the ability of
the refrigeration system to control the speed of the process that
is utilizing the refrigeration. To determine the real time capacity
for heat transfer, probes are provided so that the controller is
able to measure the temperatures (and/or pressures) of the
evaporator system and of the condenser system. Additional data,
such as the level of refrigerant in the high side liquid receiver,
refrigeration level in the evaporator, compressor RPM, system power
input, line voltage, line current, line phase factor, etc., may
also be used to determine the real time capacity of the system.
After the real time capacity of the system is determined, the
amount of product that can be processed (e.g., food frozen, ice
manufactured, etc.) can then be controlled so that the quality of
the refrigeration process is maintained even as the capacity of the
refrigeration process is reduced. Likewise, as additional energy
becomes available the capacity of the refrigeration process may be
increased followed by an increase in the rate of product handling
or generation. The system controller may be set for the maximum
amount of ice produced, the maximum amount of power consumed, the
minimum incoming line voltage to be maintained, or may be used to
even out the loads in the individual phases of the incoming power
lines or to remove harmonics.
[0035] A specific example of the above feature of the control
system is using the refrigeration system to produce ice, where the
final temperature of the ice must be maintained to produce high
quality ice. In typical icemaker systems, water is placed in
contact with a cold surface to freeze the water into ice. In each
icemaker system the placement or quantity of water (load) may be
modified to more closely match the load to the refrigeration
capacity. In the simplest case of a block ice machine, the rate at
which the ice cans are cycled defines the overall heat load. In the
case of the tube chunk machines and cube machines, the number of
active surfaces (tubes, pans, etc.) and the flow rate of feed water
may be adjusted to maintain the evaporator temperature(s) of the
system. In spinning disk flake machines the water level in the
reservoir containing the disk may be raised or lowered so that
varying amounts of the disk are coated with water. Finally, in the
example of the drum and scraper (or auger) flake machines,
modifying the feed water flow rate as well as the surface area of
the drum being utilized will allow the production to be closely
matched to the capacity.
[0036] A similar example is using the refrigeration system to
refrigerate, dehumidify, or otherwise process air and the flow rate
of air across the evaporator may be adjusted to maintain a fixed
air temperature on the discharge side of the evaporator.
[0037] In another aspect of the system, the refrigeration
evaporator is maintained in a flooded mode of operation and
adjustments to the refrigerant level within the evaporator are used
to control the heat load delivered to the system. The heat load is
modified since the substantial portion of the heat is removed from
the area of the evaporator located below the top liquid level of
the refrigerant. Lowering the refrigerant level and thereby
reducing the cold surface area in contact with the product or
process reduces the effective size (i.e., height) of the
evaporator. In the specific example of a tube or barrel type
icemaker, the feed water in contact with the evaporator surface
located above the level of the liquid refrigerant will only
marginally be cooled. Only when the water approaches the height
corresponding to the top surface of the refrigerant within the
evaporator will heat be removed from the feed water. If the
refrigeration capacity is reduced to some fraction of full output,
lowering the refrigerant liquid level to approximately the same
fraction of the full level effectively eliminates the upper portion
of the icemaker that is above the refrigerant level liquid.
[0038] In another aspect of the invention, the placement and number
of water sources or nozzles to the evaporator for the production of
ice may be varied. The positioning of the initial point of
introduction of the water on the surface of the evaporator may be
varied on a real time basis or any other suitable basis to increase
or decrease the heat load upon the evaporator. As examples, the
height of the water source or nozzle above the base of the
evaporator surface may be increased to increase the heat load and
the height of the water source lowered to reduce the heat load.
Likewise, the utilized surface area of the evaporator may be
adjusted in terms of its width or angular portion of a circular
evaporator by adjusting the placement or number of water sources
across the top surface. Furthermore, it is possible to reduce the
heat load of the evaporator more rapidly than simply adjusting the
internal liquid-level of the refrigerant, by reducing, redirecting
or stopping the water flow to one or more portions of the
evaporator.
[0039] In another embodiment of the controlled level mode of
operation, the temperature of the liquid refrigerant within the
evaporator is maintained at a preset value to control the quality
of the process (e.g., ice manufacturing). As the temperature of the
evaporator changes, the amount of refrigerant entering the
evaporator is adjusted such that an increasing evaporator
temperature leads to a reduction of incoming refrigerant. Continued
pumping on the evaporator (withdrawing vaporized refrigerant) with
a reduced refrigerant liquid flow into the evaporator will result
in the lowering of the refrigerant level and therefore reductions
in the active area of the evaporator and the heat load. Since the
temperature of the refrigerant in a two-phase evaporator is well
characterized in terms of the pressure of the evaporator, the
pressure of the evaporator may be used as the controlling
parameter. Furthermore, a pressure-operated valve controlling the
introduction of liquid refrigerant to the evaporator may be used to
eliminate the need for an electronic or other control system.
[0040] Another benefit of adjusting the liquid refrigerant level in
the evaporator is the capability provided to the system to maintain
the turbulence within the evaporator to keep the refrigeration oil
entrained in the refrigerant. Under full heat load conditions, the
refrigerant is continuously undergoing vigorous boiling as the
incoming heat vaporizes the liquid refrigerant over the entire
surface of the evaporator. However, during high turndown modes of
operation, the boiling per unit area of evaporator reduces as the
heat load reduces and the boil becomes less vigorous, creating the
potential for the refrigerant oil to separate and remain near the
surface of the liquid. A benefit of adjusting the level of the
liquid refrigerant is to maintain the heat transfer per unit area
of the evaporator (effective area) at a value that results in a
sufficiently rapid boil of the refrigerant.
[0041] In a related optional aspect of the invention, during a low
capacity mode of operation the velocity of refrigerant within the
system piping and turbulence within the evaporator may be
maintained within acceptable values by operating the compressor
continuously but delivering the refrigerant to the evaporator in
small batches. The addition or increase in capacity of a low side
accumulator will assist in providing a larger volume for the
low-pressure gas. In this mode of operation the refrigerant flow to
the evaporator is controlled or periodically stopped to allow an
increased pressure difference between the high side liquid
reservoir (receiver) and the low side reservoir (evaporator and
accumulator). When the liquid refrigerant valve is reopened, the
flow rate of refrigerant will reach higher peaks than its average
value. In a similar manner, a valve placed between the evaporator
and low side accumulator will allow a pressure differential to be
generated between those two low-pressure reservoirs with the
accumulator acting as a vacuum source. As with the high side
liquid, periodically closing and reopening this valve will
temporarily increase the velocity of the gas in the low side of the
system, flushing trapped refrigerant and oils from the piping.
[0042] As another optional aspect of the invention related to
adjusting the level of liquid refrigerant in the evaporator, the
evaporator may be provided with a floating oil skimmer to provide
removal of refrigeration oil that forms on the surface of many
refrigerants. Alternatively, the evaporator may be fitted with
multiple taps at different heights to remove the oil film. Each tap
may be provided with a valve so that only the tap level with the
top surface of the liquid refrigerant is utilized. Another
embodiment would consist of a tube that may be adjusted by an
external means to track the top surface of the refrigerant.
Finally, the system may be fitted with a mechanical or refrigerant
driven mixer, or designed and operated in such a manner that there
is sufficient turbulence to maintain the oil in solution, allowing
oil to be more easily removed with less concern for the location of
the liquid level. In the latter embodiment, the oil would remain
entrained in the refrigerant and would be carried away with a
controlled amount of liquid refrigerant from a fixed port that is
not necessarily near the top surface. In any of these embodiments
where a portion of the liquid is being drawn from the evaporator,
this liquid refrigerant/oil mixture may be expanded in a heat
exchanger and the energy utilized in another portion of the
refrigeration cycle before returning the vaporized refrigerant and
oil to the compressor. Conversely, for operation with refrigerants
that are lighter than oil (such as ammonia) the oil may simply be
withdrawn from the bottom of the evaporator sub-system.
[0043] A common method of maintaining circulation within flooded
evaporator systems, such as a barrel type ice maker, is to place a
baffle within the evaporator with the first side of the baffle
facing the heat source and the second side of the baffle facing
away from the heat source. An open area above and below the liquid
level of the refrigerant allows natural circulation due to the
rising refrigerant bubbles near the heat source to keep the
refrigeration oil entrained in the refrigerant. However, this
method of creating natural circulation cannot be utilized when the
level of the refrigerant is varied and more specifically when the
top level of the refrigerant is lowered below the top of the
baffle. Therefore, another optional aspect of the invention is a
baffle arrangement that provides natural circulation regardless of
the level of the refrigerant in the evaporator. Replacing the solid
baffle with a baffle that is perforated, louvered, or is otherwise
substantially open-will allow circulation upward with the bubbles
near the heat source and liquid flow downward on the second side of
the baffle, away from the heat source. In another embodiment,
replacing the baffle with chevrons, or another means of redirecting
the bubbles to columns, will provide `chimneys` where the rising
bubbles draw the refrigerant upwards and areas with fewer bubbles
where the liquid refrigerant will more easily downflow. Spacing of
these chimneys around the circumference of the evaporator will
provide circulation of the refrigerant and oil independent from the
liquid level of the refrigerant.
[0044] In another aspect of the invention, following a significant
reduction in the available power to the compressor, a portion of
the liquid refrigerant in the evaporator may be rapidly removed
from the evaporator and evaporated in a separate heat exchanger
such that the heat taken up by the evaporation is provided by the
high pressure liquid in a high-side liquid storage reservoir.
Therefore, the liquid level in the primary evaporator, (the
evaporator connected to the process) may be quickly lowered to
reduce the heat load to the primary evaporator. The heat uptake
capacity of the excess refrigerant will then be utilized to
sub-cool the liquid refrigerant feeding the evaporator, and
therefore reduce the enthalpy (heat content) of the high-pressure
liquid refrigerant. In effect, this method of load leveling
utilizes a short-term surplus of liquid refrigerant in the process
evaporator to reduce the temperature of the refrigerant available
to enter the process evaporator (and therefore making that
refrigerant more effective in cooling the evaporator when the
refrigerant is later released into the evaporator). Further, the
liquid refrigerant may be evaporated into a low-pressure reservoir
so that the load on the compressor is not increased. Vapor
refrigerant may then be removed from the low-pressure reservoir
during periods of higher pumping capacity. Conversely, during short
term periods of excess refrigeration capacity, a portion of the
liquid refrigerant in the high-pressure reservoir or liquid
refrigerant from the process evaporator may be used (i.e.,
evaporated in a heat exchanger and the vapor returned to the
compressor) to sub-cool the remaining liquid refrigerant in the
high-pressure reservoir.
[0045] Optionally, the overall efficiency of the unit may be
established, displayed, and/or transferred to a control station, or
used within the software control strategy to modify the operating
parameters. For example, as less energy becomes available and the
refrigeration capacity is reduced, (the overall efficiency of the
system will decrease due to ancillary loads, e.g., control system,
pumps, fans, etc.) it may be more desirable to stop the
refrigeration process and utilize the energy somewhere else on the
grid.
[0046] In another embodiment of the invention, the compressor is
coupled to an alternate energy source that produces rotational
motion (e.g., wind turbine or water turbine), either directly,
through a transmission, or through a torque converter. In this type
of system, the cylinder unloading method of capacity reduction may
be used to reduce the power input requirements of the compressor
for periods of light wind. Since many alternate and renewable
energy sources, such as wind turbines, have poor starting torque
characteristics, cylinders may be unloaded, to minimize the
starting torque requirements of the compressor so that they more
closely match the low speed torque capabilities of the energy
source.
[0047] In another embodiment of the invention, an absorption system
may be utilized in place of, or in conjunction with the vapor phase
compression system. In these systems the capacity control is the
direct result of controlling the heat applied to the absorption
system. Typically, absorption systems have an efficiency advantage
over vapor phase compression when the energy input is in the form
of heat rather than fuel or electricity.
EXAMPLE
[0048] An apparatus having a mechanical vapor phase compressor,
condenser, flooded evaporator, and control system was designed
having a maximum capacity of 5 tons of ice per day delivered at
-10.degree. F. with a condenser temperature of 105.degree. F. and
freshwater input at a temperature of 65.degree. F. The system
included a variable speed motor controller capable of operating a
15 horsepower inverter grade motor across a speed range from zero
to 1800 RPM. This motor was directly coupled to a four cylinder,
open frame compressor (Carrier Model 5F40), fitted with individual
cylinder unloaders, auxiliary oil cooler, water cooled heads, and
suction and discharge service valves. The system was operated on
R-22. Hot refrigerant gas exited the compressor and was condensed
in a water cooled shell and tube condenser having a capacity of
approximately 8-tons. The condensed refrigerant was then held in a
liquid receiver having a capacity of 80 pounds of refrigerant.
Using a solenoid valve refrigerant was delivered to the evaporator
system. The refrigerant liquid level was controlled by an
evaporator pressure controlled expansion valve. Liquid refrigerant
and refrigeration oil was recovered from the evaporator and
vaporized in a low temperature heat exchanger with heat being
provided by the liquid refrigerant being delivered to the
evaporator. This liquid refrigerant/oil recovery line was provided
with a solenoid valve that was closed when the temperature of the
refrigerant exiting the low temperature heat exchanger was below a
predetermined value. This may occur when the compressor is
operating at low capacity, when there is not sufficient heat load
to draw liquid refrigerant into the evaporator, or when the control
system has adjusted other parameters of the system so that the
capacity of the system is reduced. This refrigerant/oil recovery
valve also closed when the compressor was cycled off to prevent
liquid refrigerant from entering the low-pressure accumulator.
[0049] A barrel type evaporator served as the primary evaporator
and cold reservoir heat exchanger for the system. The level of
refrigerant within the evaporator was raised or lowered to
effectively change the size (height) of the evaporator and
therefore adjust the amount of chilled surface area available for
generating ice. The system parameters were adjusted such that the
temperature of the liquid within the evaporator was maintained at
approximately -15.degree. F.
[0050] Ice was generated on the surface of the evaporator and
removed by a mechanical scrapper. The thickness of ice is
controlled by adjusting the rate at which the scraper harvests the
ice. The rate of ice generation is controlled by both the liquid
refrigerant within the evaporator as well as the fraction of the
drum over which water is applied. The drum may be divided into four
portions. The first portion is the fraction of the circumference of
the drum over which water is delivered or poured over the drum. The
water delivery pumping rate and solenoid valve are controlled so
that water is only delivered to a well defined portion of the drum.
The second portion of the drum is where the water that has become
ice is allowed to sub-cool to the desired temperature, related to
both the rate of ice making and the temperature of the liquid
refrigerant within the drum. The third portion of the drum is where
the harvesting scraper is removing the ice from the surface and
delivering the ice to the output hopper. The fourth portion of the
drum is the fraction of the drum that is left unused to match the
heat load to the capacity of the compressor. The speed of rotation
of the harvester is monitored and adjusted by the control system.
The water delivery nozzle preferably rotates with the harvester
with the angle between the nozzle and harvester adjustable by the
user.
[0051] Vapor phase refrigerant is removed from the top of the
evaporator and travels through a low side accumulator before being
returned to the compressor. A solenoid valve between the evaporator
and accumulator allows a pressure differential to be generated and
dumped, causing surges of vapor refrigerant in the piping system.
This surge mode is utilized during extremely low capacity operation
to prevent oil accumulation in the piping.
[0052] While the foregoing is directed to the preferred embodiment
of the present invention, other and further embodiments of the
invention may be devised without departing from the basic scope
thereof, and the scope thereof is determined by the claims which
follow.
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