Refrigerant system and control method

A refrigerant system is configured to alternatingly run in an economized mode and a standard mode. A control system shifts the refrigerant system between the economized mode and standard mode responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic efficiency; condenser efficiency; evaporator efficiency; efficiency of hardware mechanically powering the compressor; and a mode-associated cycling efficiency. In a bypass mode, a bypass refrigerant flow from an intermediate port may return to the suction port. Shifting into the bypass mode may be similarly controlled based upon the determined efficiency.

BACKGROUND OF THE INVENTION

The invention relates to cooling and heating. More particularly, the invention relates to economized air conditioning, heat pump, or refrigeration systems.

U.S. Pat. No. 6,955,059 discloses an economized vapor compression system with different modes of unloading. Additionally, commonly assigned U.S. Pat. No. 4,938,666 discloses unloading one cylinder of a bank by gas bypass and unloading an entire bank by suction cutoff. Commonly assigned U.S. Pat. No. 4,938,029 discloses the unloading of an entire stage of a compressor and the use of an economizer. Commonly assigned U.S. Pat. No. 4,878,818 discloses the use of a valved common port to provide communication with suction for unloading or with discharge for volume index (Vi) control, where Viis equal to the ratio of the volume trapped gas at suction (VS) to the volume of trapped gas remaining in the compression pocket prior to release to discharge. In employing these various methods, the valve structure is normally fully open, fully closed, or the degree of valve opening is modulated so as to remain at a certain fixed position. Commonly assigned U.S. Pat. No. 6,047,556 (the '556 patent, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length) discloses the use of solenoid valve(s) rapidly cycling between fully open and fully closed positions to provide capacity control. The cycling solenoid valve(s) can be located in the compressor suction line, the compressor economizer line and/or the compressor bypass line which connects the economizer line to the suction line. The percentage of time that a valve is open determines the degree of modulation being achieved. U.S. Pat. No. 6,619,062 discloses control of scroll compressor unloading mechanisms based solely upon scroll compressor pressure ratio operation.

Nevertheless there remains room for further improvement in the art.

SUMMARY OF THE INVENTION

One aspect of the disclosure involves a refrigerant system configured to alternatingly run in an economized mode and a standard mode. A control system shifts the refrigerant system between the economized mode and standard mode responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic efficiency; condenser efficiency; evaporator efficiency; efficiency of hardware mechanically powering the compressor; and a mode-associated cycling efficiency. In a bypass mode, a bypass refrigerant flow from an intermediate port may return to the suction port. Shifting into the bypass mode may be similarly controlled based upon the determined efficiency.

DETAILED DESCRIPTION

FIG. 1shows an exemplary closed refrigeration or air conditioning system20. The system has a compressor22having suction (inlet) and discharge (outlet) ports24and26defining a compression path therebetween. The compressor further includes an intermediate port28at an intermediate location along the compression path. An exemplary compressor includes a motor29. An exemplary motor is an electric motor. Alternative motors may comprise internal combustion engines. The other variations include electric motors powered by internal combustion engine generators. An exemplary compressor configuration is a screw-type compressor (although other compressors including scroll compressors, centrifugal compressors, and reciprocating compressors may be used). The compressor may be hermetic, semi-hermetic, or open-drive (where the motor is not within the compressor housing).

A compressor discharge line30extends downstream from the discharge port26to a heat rejection heat exchanger (e.g., condenser or gas cooler)32. A trunk34of an intermediate line extends downstream from the condenser. A main branch36extends from the trunk34to a first leg38of an economizer heat exchanger (economizer)40. From the economizer40, the branch36extends to a first expansion device42. From the expansion device42, the branch36extends to a heat absorption heat exchanger (e.g., evaporator)44. From the evaporator44, the branch36extends back to the suction port24. A second branch50extends downstream from the trunk34to a first valve52. Therefrom, the branch50extends to a second expansion device54. Therefrom, the branch extends to a second leg56of the economizer40in heat exchange proximity to the first leg38. The branch50extends downstream from the economizer40to the intermediate port28. A bypass conduit60, in which a bypass valve62is located, extends between the branches (e.g., between a first location on the main branch36between the evaporator and suction port and a second location on the second branch50between the economizer and intermediate port). Optionally, a suction modulation valve (SMV)64may be located downstream of the evaporator (e.g., between the evaporator and the junction of the bypass conduit60with the suction line).

Exemplary expansion devices42and54are electronic expansion devices (EEV) and are illustrated as coupled to a control/monitoring system70(e.g., a microprocessor-based controller) for receiving control inputs via control lines72and74, respectively. Alternatively, one or both expansion devices may be thermo-expansion valves (TXV). Similarly, exemplary valves52and62are solenoid valves and are illustrated as coupled to the control system via control lines76and78, respectively. Alternatively, if the expansion device54is an EEV, it may also serve as the valve52(e.g., to shut-off flow through the branch50). The control system may also control the SMV64via a control line79.

The compressor motor29may be coupled to the control system70via a control line80. The control system70may control motor speed via an appropriate mechanism. For example, the motor may be a multi-speed motor. Alternatively, the motor may be a variable speed motor driven by a variable frequency drive (VFD). Alternatively, an open drive compressor may be directly driven by an engine (motor) having variable engine speed. The exemplary control system may receive inputs such temperature inputs from one or more temperature sensors82and84. Other temperature sensors may be in the temperature-controlled environment or may be positioned to measure conditions of the heat exchangers (e.g., sensors86and88on the heat exchangers32and44, respectively). Additional or alternative sensors may include sensors indicative of the pressure at compressor suction and discharge locations and/or sensors that are indicative of pressure at the evaporator and/or condenser inlets or outlets. The control system may receive external control inputs from one or more input devices (e.g., thermostats90). Yet other sensors may be included (e.g., measuring drive voltage or frequency or compressor load).

When used for cooling, the evaporator44may be positioned within a space to be cooled or within a flowpath of an airflow to that space. The condenser may be positioned externally (e.g., outdoors) or along a flowpath to the external location. In a heating configuration, the situation may be reversed. In a heat pump system that may provide both configurations, one or more valves (e.g., a four-way reverse valve—not shown) may selectively direct the refrigerant to allow each heat exchanger structure to alternatively be utilized as condenser and evaporator.

The exemplary system has several modes of operation. For ease of reference, a first mode is a standard non-economized (standard) mode. Essentially, in this mode, both valves52and62are closed such that: refrigerant flow through the second branch50and thus the economizer second leg56is restricted (e.g., blocked); and refrigerant flow through the bypass conduit60is also restricted (e.g., blocked). Thus, refrigerant flow through the intermediate port28is minimal or non-existent. Most, if not all, refrigerant flows: from the discharge port26to the condenser32; through the condenser32; through the economizer first leg38(with no heat exchange effect as there is no flow through the second leg); through the first expansion device42; through the evaporator44; and back to the suction port24to then be recompressed along the compression path. Exemplary compressors used for heating or cooling applications normally have a peak efficiency at a system operating point corresponding to the built-in compressor volume ratio. Near this point, the pressure in the compression pocket at the end of compression is equal to or nearly equal to the discharge plenum pressure. When these pressures are equal, there are no over-compression or under-compression losses. This point occurs when the system density ratio (the density ρDof refrigerant on the system high side divided by the density ρSof refrigerant on the system low side) is equal to the compressor built-in volume ratio (compressor suction volume divided by discharge volume). Use of system density ratio may be more effective in determining optimal compressor operation than use of a system pressure ratio (pressure on the high side divided by pressure on the low side). The system pressure ratio may be less related to the compressor volume ratio. For a given compressor mode of operation, there may be multiple pressure ratios which, depending upon the suction and/or discharge temperature, would correspond to the built-in volume ratio whereas there is a single density ratio corresponding to the built-in volume ratio.

The optimal compressor volume ratio may vary depending upon the compressor mode of operation. If the compressor is operated in an unloaded mode wherein part of the refrigerant from an intermediate location along the compression path is bypassed back to suction conditions, an optimal volume ratio may be reduced relative to a standard mode of operation. Similarly, if additional refrigerant is returned to the compressor at the intermediate location, the optimal value of volume ratio would be generally higher relative to the standard mode.FIG. 2shows a plot200of compressor isentropic efficiency ηISENTROPIC—COMPRESSOR(%) against density ratio for standard mode operation.

A second mode of operation is an economized mode. Generally, in the economized mode, the first valve52is open and the second valve62is closed. Flow from the compressor is split, with a main portion flowing through the main branch36as in the standard mode. An economizer portion, however, flows through the second branch50, passing through the valve52and economizer second leg56wherein it exchanges heat with the refrigerant in the first leg38. In this mode, the economizer40provides additional subcooling to the refrigerant along the first leg38. The additional subcooling increases the system capacity and thus provides more system cooling (e.g., of the space being cooled) in the cooling mode and heating in the heating mode. Therefrom, the economizer flow returns to the intermediate port28to be injected (as vapor) into and recompressed along the downstream portion of the compression path.FIG. 2further shows a plot202of economized mode compressor isentropic efficiency against density ratio. Above an approximate density ratio504, the economized mode offers higher compressor efficiency than the standard mode.

A third mode is a bypass mode. Generally, in the bypass mode, the valve52is closed and the valve62is open. Additionally, an intermediate pressure relief bypass flow will, in the illustrated embodiment, exit the intermediate port28and pass through the bypass conduit60to return to the suction port24.FIG. 2further shows a plot204of compressor isentropic efficiency against density ratio for the bypass mode. Below a ratio506, the bypass mode offers a higher compressor isentropic efficiency than the standard and economized modes. In the exemplary embodiment,506is less than504and, therefore, intermediate these density ratios the standard mode offers higher compressor efficiency than the bypass and economized modes.

To determine the most efficient mode of operation for a given system operating condition, other factor or factors beyond compressor isentropic efficiency are taken into account.FIG. 3shows ideal cycle efficiency (e.g., with no losses in the compressor, motor, or other associated components, and with infinitely large heat exchanger coils) as a function of density ratio at a constant discharge pressure. Plots210,211, and212respectively identify standard, economized, and bypass modes. The ideal system efficiency is expressed in terms of EER (ideal system capacity divided by compressor power for a compressor operating at 100% efficiency). The economized mode has the highest cycle efficiency in a high density ratio domain above a ratio510. The bypass mode has the highest efficiency in a lower density ratio domain (e.g., below the ratio510). In the example, the standard mode efficiency is never above the higher of the bypass and economized mode efficiencies. However, other variations may differ.

Additionally, the mass flow rate {dot over (m)} of refrigerant in through the heat exchangers may be considered in determining the most efficient mode.FIGS. 4 and 5respectively relate to temperature differential ΔT across the condenser and evaporator for a fixed ambient temperature and fixed temperature of the conditioned environment.

Where ΔT is the absolute temperature difference between the saturated temperature of the refrigerant in a heat exchanger and the air temperature downstream of the heat exchanger.FIG. 4shows the temperature differential as a function of refrigerant mass flow rate {dot over (m)} through the condenser. A plot220shows ΔT for the standard mode, a plot221shows the economized mode, and a plot222shows the bypass mode. The mass flow rate {dot over (m)} can be varied, for example, by driving the compressor at various operating speeds.

FIG. 5shows temperature differential as a function of mass flow rate through the evaporator. A plot225shows the evaporator ΔT for the standard mode, a plot226shows the economized mode, and a plot227shows the bypass mode. The temperature differential is illustrated for a specific compressor operating speed. As shown, for example,FIG. 4, for the chosen operating speed the mass flow rate through the condenser at by-pass mode is ˜60% of the standard mode, and the mass flow rate at economized mode is ˜140% of the standard mode (the difference between the mass flow rates at different modes is shown for illustration purpose, only, as the exact percentages would vary with a specific compressor type and system operating condition). Similarly, forFIG. 5, the mass flow rate for the same operating speed through the evaporator at by-pass mode is ˜60% of the economized mode, and mass flow rate at standard mode is ˜105% of the economized mode.

Higher temperature differential is associated with a less efficient heat exchanger or less efficient operation. For example, an ideal heat exchanger would be 100% efficient and have infinitely large heat exchange surfaces, zero temperature differential and no pressure drop losses.FIGS. 6 and 7show heat exchanger efficiency for the condenser and evaporator, respectively for a fixed ambient temperature and fixed temperature of the conditioned environment where the efficiency is plotted against refrigerant mass flow. InFIG. 6, plots230,231, and232are respectively associated with the standard, economized, and bypass modes. Similarly, inFIG. 7, plots235,236, and237identify evaporator efficiencies the standard, economized, and bypass modes. The efficiency of each mode is also illustrated for a chosen specific compressor operating speed. Each combination of ambient temperature and temperature of the conditioned environment will have unique graphs similar to those illustrated inFIGS. 4-7. In setting the controller programming (e.g., one or more of hardware, software or entered setting), the system designer may analyze these graphs for each ambient temperature and temperature of the conditioned environment to select the most efficient mode of operation, considering the constraints of required system capacity. The controller may be so programmed or configured to operate the system in to shift the system between the modes responsive to a determined efficiency reflecting a combination of efficiency components including those above and those discussed below.

As shown in the above examples, the heat exchangers operate less efficiently as the mass flow rate through the heat exchangers is increased (the heat exchangers become more “loaded” as well as additional pressure drop loss being introduced as refrigerant mass flow rate is increased.

Other factors may include losses associated with the motor (e.g., with an electric motor and its variable frequency drive).FIG. 8shows a plot250motor efficiency ηMOTORas a function of load (% of rated load). Each of the three exemplary modes: standard; economized; and bypass will load the motor differently. Points251,252, and253respectively identify the loads associated with the standard, economized, and bypass modes.

FIG. 9shows a plot260of variable frequency drive efficiency ηVFDas a function of VFD load (e.g., % of rated VFD load). The rated VFD load may or may not correspond to the rated motor load. The correspondence will depend on how and how well the VFD and motor load characteristics are matched. Points261,262, and263respectively identify the loads associated with the standard, economized, and bypass modes. If the compressor is driven by an engine (either directly or indirectly) then the engine efficiency may be considered in lieu of or along with the motor efficiency for the various modes of operation. Additionally, the effective cycling losses can also be considered. For example, the identified modes of operation may be subject to different degrees of cycling and cycling may have different effects upon each mode. For example, in the economized mode, the system would be expected to cycle more frequently than in the bypass mode. This is because in the economized mode of operation more cooling capacity is generated than in the standard mode or bypass mode. Therefore, to match the generated capacity to the required capacity, the system would need to cycle on and off more frequently in the economized mode than in the bypass mode. Accordingly, a cycling efficiency factor ηCYCLINGmay be considered. For example, if the system operates continuously, the cycling efficiency is 100%. Accordingly, an overall EER value may be calculated based upon an ideal EER value modified by the various efficiencies discussed above:
EEROVERALL=EERCYCLE—IDEAL·ηISENTROPIC—COMPRESSOR·ηEVPORATOR·ηCONDENSER·ηMOTOR·ηVFD·ηCYCLING

Some of these factors or their associated components may be unknown to a system designer. For example, at the time of designing/selecting the compressor, the particular variable frequency drive efficiency may be unknown. Such unknown factors may be ignored or merely estimated. In a basic example, only the compressor isentropic efficiency is considered and the other efficiencies are neglected. This basic example yields an exemplary method of operation involving operating the system: in the bypass mode below the density ratio506; in the standard mode between the density ratios506and504; and in the economized mode above the density ratio504. Rough exemplary values for one implementation involve density ratios506and504of about 2.9 and about 3.25, respectively.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a modification or a reengineering of an existing system, details of the existing system may heavily influence details of the implementation. Accordingly, other embodiments are within the scope of the following claims.