Patent Application
20120117996
The present invention relates to a cascade refrigeration system having an upper portion that uses a modular chiller unit having ammonia as a refrigerant to provide condenser cooling for a refrigerant in a low temperature subsystem (for cooling low temperature loads) and/or for chilling a liquid that is circulated through a medium temperature subsystem (for cooling medium temperature loads). The present invention relates more particularly to a cascade refrigeration system having a critically-charged modular chiller unit that uses a sufficiently small charge of ammonia to minimize potential toxicity and flammability hazards. The present invention also relates more particularly to a modular ammonia cascade refrigeration system that uses a soluble oil mixed with the ammonia refrigerant charge. The present invention relates more particularly still to a modular ammonia cascade refrigeration system that uses intentionally-unstable superheat control to ensure positive return of any soluble oil from an evaporator of the modular ammonia chiller unit.
This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Refrigeration systems typically include a refrigerant that circulates through a series of components in a closed system to maintain a cold region (e.g., a region with a temperature below the temperature of the surroundings). One exemplary refrigeration system includes a direct-expansion vapor-compression refrigeration system including a compressor. Such a refrigeration system may be used, for example, to maintain a desired low temperature within a low temperature controlled storage device, such as a refrigerated display case, coolers, freezers, etc. in a low temperature subsystem of the refrigeration system. Another exemplary refrigeration system includes a chilled liquid coolant circulated by a pump to maintain a desired medium temperature within a medium temperature storage device in a medium temperature subsystem of the refrigeration system. The low and/or medium temperature subsystems may each receive cooling from one or more chiller units in a cascade arrangement. The chiller units circulate a refrigerant through a closed-loop refrigeration cycle that includes an evaporator which provides cooling to the low temperature subsystem (e.g. as a condenser) and/or the medium temperature subsystem (e.g. as a chiller).
Accordingly, it would be desirable to provide a cascade refrigeration system having one or more modular chiller units capable of using ammonia as a refrigerant for providing condenser cooling in a low temperature subsystem of the refrigeration system, and/or for chilling a liquid coolant for circulation through a medium temperature subsystem of the refrigeration system.
One embodiment of the invention relates to a cascade refrigeration system that includes an upper portion having at least one modular chiller unit that provides cooling to a low temperature subsystem having a plurality of low temperature loads, and/or a medium temperature subsystem having a plurality of medium temperature loads. The modular chiller unit includes a refrigerant circuit having at least a compressor, a condenser, an expansion device, and an evaporator. An ammonia refrigerant mixed with a soluble oil circulates within the refrigerant circuit. A control device may be provided that is programmed to modulate the position of the expansion device so that a superheat temperature of the ammonia refrigerant near an outlet of the evaporator fluctuates within a substantially predetermined superheat temperature range to flush an accumulation of the soluble oil from the evaporator.
Another embodiment relates to a modular ammonia chiller unit for a refrigeration system and includes a refrigerant circuit having at least a compressor, a condenser, an expansion device, an evaporator, an ammonia refrigerant, a soluble oil mixed with the ammonia refrigerant. A control device may be provided that is operated according to a control scheme configured to return an accumulation of the soluble oil from the evaporator to the compressor.
Yet another embodiment relates to a method of providing a cascade refrigeration system that is substantially HFC-free and includes the steps of providing a lower portion having a low temperature subsystem that uses carbon dioxide as a refrigerant to cool a plurality of low temperature loads, and/or a medium temperature subsystem that uses a water-glycol mixture as a liquid coolant to cool a plurality of medium temperature loads, and providing an upper portion having at least one modular chiller unit that provides cooling to the low temperature subsystem and the medium temperature subsystem, the modular chiller unit comprising a refrigerant circuit having at least a compressor, a condenser, an expansion device, and an evaporator, and charging the refrigerant circuit of the modular chiller unit with a critical charge amount of an ammonia refrigerant mixed with a soluble oil. A step may be provided for programming a control device to operate according to a control scheme configured to return an accumulation of the soluble oil from the evaporator to the compressor.
The disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements, in which:
Referring to
The terms “low temperature” and “medium temperature” are used herein for convenience to differentiate between two subsystems of refrigeration system 10. Medium temperature subsystem 80 maintains one or more loads, such as cases 82 (e.g. refrigerator cases or other cooled areas) at a temperature lower than the ambient temperature but higher than low temperature cases 62. Low temperature subsystem 60 maintains one or more loads, such as cases 62 (e.g. freezer display cases or other cooled areas) at a temperature lower than the medium temperature cases. According to one exemplary embodiment, medium temperature cases 82 may be maintained at a temperature of approximately 20° F. and low temperature cases 62 may be maintained at a temperature of approximately minus (−)20° F. Although only two subsystems are shown in the exemplary embodiments described herein, according to other exemplary embodiments, refrigeration system 10 may include more subsystems that may be selectively cooled in a cascade arrangement or other cooling arrangement.
An upper portion (e.g., the upper cascade portion 12) of the refrigeration system 10 includes one or more (shown by way of example as four) modular ammonia chiller units 20, that receive cooling from a cooling loop 14 having a pump 15, and one or more heat exchangers 16, such as an outdoor fluid cooler or outdoor cooling tower for dissipating heat to the exterior or outside environment. Outdoor fluid cooler 16 cools a coolant (e.g., water, etc.) that is circulated by pump 15 through cooling loop 17 to remove heat from the modular ammonia chiller units 20.
One exemplary modular ammonia chiller unit 20 is shown in more detail in
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According to one exemplary embodiment, the modular ammonia chiller units 20 are compact modular chiller units that are critically charged with approximately 10 pounds of ammonia. System 10 may include a multitude of the compact modular ammonia chiller units 20 arranged in parallel as low temperature refrigerant condensing units and/or as medium temperature liquid chillers. The number of compact modular ammonia chiller units 20 may be varied to accommodate various cooling loads associated with a particular commercial refrigeration system. Likewise, the number of medium temperature cases 82 and low temperature cases 62 may be varied.
Referring to
Notably, in order to provide a chiller unit 20 that is less complex, less expensive, and more easily operated, serviced and maintained by technicians that may otherwise be unfamiliar with ammonia refrigerant systems, the chiller unit does not include an oil management system (e.g. piping, valves, controls, oil reservoir, filters, coolers, separators, float-switches, etc.) for providing lubrication to the compressor. Rather, the modular ammonia chiller unit 20 of the illustrated embodiment uses a soluble oil that is mixed with the ammonia refrigerant to provide lubrication to the compressor. According to one embodiment, the soluble oil is a PolyAlkylene Glycol (PAG) oil, such as a Zerol SHR 1202 ammonia refrigeration oil that is commercially available from Shrieve Chemical Products, Inc. of The Woodlands, Tex. Unlike conventional systems that may use a mineral oil (which is generally insoluble and tends to accumulate in the evaporator and degrade system performance), the PAG oil is soluble within the ammonia refrigerant and thus circulates through the closed loop circuit 30 with the ammonia refrigerant to provide compressor lubrication. Further, although PAG oil is hygroscopic by nature and has an affinity for absorbing water (which is detrimental to the performance of refrigeration systems), the relatively small, modular and “tight” nature of the ammonia chiller units (e.g. with no piping connections associated with a conventional oil system, and that use piping connections that are as leak-tight as possible, etc.), permits the unique usage of PAG oil as a soluble lubricant in an ammonia chiller unit 20.
In order to provide further improved performance of the compact modular ammonia chiller unit 20 of the present disclosure, control device 34 provides a unique intentionally-unstable control scheme for operation of the expansion device 28 to modulate the superheat temperature of the ammonia refrigerant at the exit of the evaporator 22 between a range of approximately 0-10 degrees F. (although other superheat temperature ranges may be used according to other embodiments). The “superheat temperature” as used in the present disclosure is understood to be the temperature of the superheated ammonia vapor refrigerant (in degrees F.) that is above the saturation temperature of the ammonia refrigerant for a particular operating pressure. For example, a superheat temperature of 10 degrees F. is intended to mean the ammonia is superheated to a temperature that is 10 degrees F. above its saturation temperature at the operating pressure. According to one embodiment, the control device 34 provides a signal to the expansion device 28 to operate the chiller unit 20 with a preferred superheat temperature within a range of approximately 6-8 degrees F. to provide for effective performance of the evaporator 22. However, the control device 34 is also programmed to operate the expansion device 28 in an “intentionally-unstable” manner such that the expansion device 28 modulates (e.g. periodically, cyclically, oscillatory, etc.) to provide a superheat temperature within the range of approximately 0-10 degrees F. over a desired time range, such as approximately 1-2 minutes. Referring to
According to one embodiment, the control device 34 is (or comprises) a closed-loop proportional-integral-derivative (PID) controller of a type commercially available from Carel USA of Manheim, Pa., and such an intentionally-unstable control scheme may be programmed using appropriate proportional, integral, and/or derivative settings on the controller that may be preprogrammed, or established empirically during an initial system testing and startup operation to be slightly “over-reactive” such that the controller directs the expansion device 28 to reposition in a manner that raises and lowers the superheat setpoint within the desired temperature range and time period. The control settings for the control device 34 may also be set to provide a lower limit for the superheat temperature range, such as a superheat temperature of approximately 1 degree F., according to one embodiment. The applicants believe that by permitting the superheat temperature to occasionally decrease such that the ammonia refrigerant in the evaporator 22 generally remains in a saturated state (i.e. does not become a saturated vapor), any of the soluble oil that may have accumulated within the evaporator 22 can be reabsorbed (due to its solubility in the ammonia refrigerant) and carried-through (e.g. flushed from, etc.) the evaporator and back to (i.e. returned to) the compressor via the ammonia accumulator to ensure positive oil return. The time range setting for the control device 34 is established with the intent to permit a decrease from the optimum superheat temperature only as often as needed to return any accumulating soluble oil from the evaporator 22. Accordingly, the intentionally-unstable operating scheme for the control device 34 is intended to “provide the best of both operating modes” by permitting occasional flushing or returning any accumulating soluble oil from the evaporator 22, while maintaining the superheat temperature within a higher range that is associated with optimum evaporator thermal performance for a majority of the time so that the overall performance of the chiller unit 20 is maintained.
According to an alternative embodiment, the control device 34 may be programmed to return oil from the evaporator 22 to the compressor 24 using a different control scheme. For example, the control device 34 may be programmed to periodically (e.g. on a predetermined frequency) turn-off and then restart the compressor 24 as a method for periodically ensuring positive return of any soluble oil that may have accumulated in the evaporator 22 back to the compressor 24. The frequency of the shutdown-restart operation for each unit 20 may also be based upon a designation of which of the chillers is the “lead” chiller (i.e. the chiller with the most run time, as other of the chillers may be started or shutdown as needed to maintain the desired cooling capacity for the lower portion of the commercial refrigeration system). For commercial refrigeration systems that use multiple modular ammonia chiller units, the shutdown-restart operation and frequency may be established (e.g. sequenced, etc.) so that only one modular ammonia chiller unit is shutdown at any one time. Accordingly, such alternative embodiments are intended to be within the scope of this disclosure.
Referring further to
According to any preferred embodiment, a commercial cascade refrigeration system 10 is provided having an upper cascade portion 12 that includes one or more compact modular ammonia chiller units 20 that provide cooling to a lower portion 18 having a low temperature CO2 subsystem 60 and/or a medium temperature chilled liquid coolant subsystem 80, where the ammonia chiller units 20 use a soluble oil for lubrication of a compressor, and in some embodiments an intentionally-unstable superheat temperature control to provide positive return of any accumulated soluble oil from the evaporator 22 back to the compressor 24.
According to the illustrated embodiment of the present disclosure, the use of critically-charged compact modular ammonia chiller units 20 to provide cascade cooling to a low temperature CO2 refrigeration subsystem 60 and a medium temperature chilled liquid coolant (e.g. glycol-water, etc.) subsystem 80 results in an all-natural refrigerant solution for use in commercial refrigeration systems, such as supermarkets and other wholesale or retail food stores or the like, that entirely avoids the use of HFC refrigerants and provides an effective and easily maintainable “green” solution to the use of HFC's in the commercial refrigeration industry. The use of relatively small, critically-charged chiller units 20 permits a series of such modular low-charge devices to be combined as necessary in an upper cascade arrangement 12 in order to cool the load from a large lower refrigeration system 18 using a naturally occurring refrigerant. In addition to being HFC-free, the system as shown and described is intended to have near-zero direct carbon emissions, one of the lowest “total equivalent warming impact” (TEWI) possible, and is intended to be “future-proof” in the sense that it would not be subject to future rules or climate change legislation related to HFCs or carbon emissions.
Referring generally to
As utilized herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.
It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples).
The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.
It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure.
It is important to note that the construction and arrangement of the elements of the refrigeration system provided herein are illustrative only. Although only a few exemplary embodiments of the present invention(s) have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible in these embodiments (such as variations in features such as connecting structure, components, materials, sequences, capacities, shapes, dimensions, proportions and configurations of the modular elements of the system, without materially departing from the novel teachings and advantages of the invention(s). For example, any number of compact modular ammonia chiller units may be provided in parallel to cool the low temperature and/or medium temperature cases, or more subsystems may be included in the refrigeration system (e.g., a very cold subsystem or additional cold or medium subsystems). Further, it is readily apparent that variations and modifications of the refrigeration system and its components and elements may be provided in a wide variety of materials, types, shapes, sizes and performance characteristics. Accordingly, all such variations and modifications are intended to be within the scope of the invention(s).
