Multi-Compressor Refrigeration System and Method for Operating It

Abstract

A refrigeration system (20) has a first compressor (24) and a second compressor (26). The second compressor has at least a first condition t least partially in parallel with the first compressor along a refrigerant flowpath. A heat rejection heat exchanger (50) is downstream of the first and second compressors along the refrigerant flowpath. An expansion device (54) is downstream of the heat rejection heat exchanger along the refrigerant flowpath. A heat absorption heat exchanger (56) is downstream of the expansion device along the refrigerant flowpath. The first compressor is a variable speed compressor coupled to a variable speed drive (32). The second compressor is a fixed speed compressor.

Description

CROSS-REFERENCE TO RELATED APPLICATION

Benefit is claimed of U.S. patent application Ser. No. 61/486,496, filed May 16, 2011, and entitled “Multi-Compressor Refrigeration System”, the disclosure of which is incorporated by reference herein in its entirety as if set forth at length.

BACKGROUND

The disclosure relates to refrigeration. More particularly, the disclosure relates to refrigerated transport containers using CO₂-based refrigerant.

CO₂-based refrigerant such as R744 has drawn increasing attention for use in refrigerated transport containers. Exemplary refrigerated transport containers include shipping containers and containers integral with trucks, trailers, or rail cars. Such containers, especially shipping containers, may be subject to a wide variety of operating conditions. The operating conditions reflect both the external/environmental temperature and the interior temperature. Interior temperature varies based upon the nature of the goods being transported, with low temperatures being required for frozen goods and higher temperatures being required for non-frozen refrigerated perishable goods. Exemplary systems include an electrically-powered compressor for driving refrigerant along a circuit/flowpath through an exterior heat rejection heat exchanger and an interior heat absorption heat exchanger.

SUMMARY

One aspect of the disclosure involves a refrigeration system having a first compressor and a second compressor. The second compressor has at least a first condition at least partially in parallel with the first compressor along a refrigerant flowpath. A heat rejection heat exchanger is downstream of the first and second compressors along the refrigerant flowpath. An expansion device is downstream of the heat rejection heat exchanger along the refrigerant flowpath. A heat absorption heat exchanger is downstream of the expansion device along the refrigerant flowpath. The first compressor is a variable speed compressor coupled to a variable speed drive. The second compressor is a fixed speed compressor.

In various implementations, the fixed speed compressor may have a larger displacement than the variable speed compressor. The compressors may be reciprocating compressors. This may be in an operational condition with the fixed speed compressor connected directly to a line voltage and the variable speed compressor connected to the line voltage via its variable speed drive.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a refrigeration system.

FIG. 2 is a view of a refrigerated container.

FIG. 3 is a control flowchart for the system of FIG. 1.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a vapor compression system 20 having a compressor subsystem 22. The exemplary compressor subsystem 22 includes a first compressor 24 and a second compressor 26 at least partially in parallel with the first compressor along a refrigerant flowpath 500. The exemplary first and second compressors are both reciprocating compressors. The exemplary compressors have respective electric motors 28 and 30. The exemplary motor 28 is powered by a variable frequency drive (variable speed drive (VSD)) 32 in turn powered by line power 34. The exemplary motor 30 is directly powered by the line power 34. Exemplary line power is 50 Hz or 60 Hz providing three-phase power to the motor 30 and VSD 32. For example, in the case of a cargo container drawing line power from a generator on the ship carrying the container, the exemplary voltage is 460V.

The exemplary motors 28 and 30 are hermetic induction motors. An alternate motor 28 is a permanent magnet motor. The exemplary motor 28 is rated for adjustable speed duty (ASD) under applicable industry standards. An ASD-rated motor will likely have more robust winding insulation than a non-ASD-rated motor. The line voltage may represent a maximum of the VSD output voltage, but the VSD may be configured to provide a higher than line frequency at such maximum voltage. The exemplary VSD is capable of running the motor 28 across a frequency range spanning the line frequency. An exemplary low end of the range is 15-20 Hz. An exemplary high end of the range is at least 110 Hz or 120 HZ. For example, with an exemplary line power of 460 V and 60 Hz, at a VSD output frequency of 120 Hz, the VSD output voltage may be that maximum 460 V. With a linear V/f curve, the VSD output voltage is 230 V@ 60 Hz and 77V@20 Hz (an exemplary lowest operating frequency of the motor 28 noted above and further discussed below).

Downstream from the compressor discharge ports, the refrigerant flowpath sequentially passes through a first heat exchanger 50, a receiver 52, an expansion device 54, and a second heat exchanger 56. In the first mode, the heat exchanger 50 is a heat rejection heat exchanger (e.g., a gas cooler or condenser) and the second heat exchanger 56 is a heat absorption heat exchanger (e.g., an evaporator). The exemplary heat exchangers 50 and 56 are refrigerant-air heat exchangers wherein electric-powered fans 60 and 62, respectively, drive air flows 64 and 66 across the transfer elements (e.g., coils).

A controller 70 may be coupled to various controllable system components (e.g., the compressor motors, the fans, the expansion device or any other control valves, and the like). The controller may be coupled to receive inputs from sensors (e.g., pressure and/or temperature sensors). Exemplary sensors include a supply air sensor 72 positioned to measure the temperature of the flow 66 exiting the heat exchanger 56, a return air temperature sensor 74 positioned to measure the temperature of the air 66 entering the heat exchanger 56, and an ambient/external temperature sensor 76 (e.g., positioned to measure ambient/external air temperature of the flow 64 entering the heat exchanger 50).

The exemplary system 20 is used in a refrigerated transport system 200 (FIG. 2). An exemplary system is shown as a shipping container 201 having a refrigerated compartment 202. An equipment compartment 204 is located at one end of the container and contains the components of the system 20. The evaporator 56 in the refrigerated compartment 202 (or in air flow communication with the refrigerated compartment 202 via the recirculating air flow 66). Other similar refrigerated transport systems include trucks and trailers as described above. The exemplary shipping container draws power from an external source (e.g., the generator of a ship). However, truck and trailer systems are more likely to include electrical generators (e.g., diesel engine-powered electrical generators). Such electrical generators may be in housings external to the main box of the truck or trailer (e.g., along with the compressor and heat rejection heat exchanger).

In configuring the system, total cost of ownership (TCO) is an important consideration. Common industry practices have arisen involving measuring cost at specific operating conditions (the TCO points) which may be associated with specific users (e.g., at which such users spend majority of time). Each TCO point is characterized by an ambient temperature and a refrigerated compartment temperature. If a single compressor were used and sized to meet the full load pulldown capacity requirement, it would have substantial extra capacity at partial load conditions which provide the majority of TCO points. Such excess capacity would involve inefficient operation. Accordingly, the presence of multiple compressors may allow sizing to provide better efficiency at the lower load TCO points and meet the full load pulldown capacity requirement (even if use of multiple compressors provides lower efficiency during pulldown).

In an exemplary implementation, the variable speed compressor is run alone over a load range from minimal load to an intermediate load. The variable speed compressor is thus sized to provide maximum efficiency over this range which includes the majority of TCO points. Once the variable speed compressor has been sized, the fixed speed compressor may be sized to make up for the difference between the maximum capacity of the variable speed compressor and the maximum required system capacity. The maximum required system capacity is usually defined by an extreme of anticipated need to provide cool down (pulldown) in an initial operating condition. Steady state operating condition is typically at a substantially lower capacity. The exemplary fixed speed compressor is larger than the exemplary variable speed compressor. An exemplary size measurement is displacement per revolution. The exemplary fixed speed compressor is 110 to 350% the size of the variable speed compressor, more narrowly, 125-350%. Yet, more narrowly, 125-250%. Even though the variable speed compressor has a smaller displacement, the ability of the VSD to output frequencies greater than line frequency allows the smaller displacement variable speed compressor to be run at capacities which may exceed the capacity of the fixed speed compressor run at line power. This allows the variable speed compressor to be operated alone at low loads without having a gap in load handling. More particularly, the peak capacity of the variable speed compressor may meet or exceed the combination of the capacity of the fixed speed compressor and the minimum capacity of the variable speed compressor. This allows a smooth handover between operations in: a mode where only the variable speed compressor runs; and a mode where both compressors run (without a gap between available capacities of those two modes).

FIG. 3 shows a basic control algorithm 300. There is a start-up 302. The relationship between a measured or otherwise determined system temperature and a target temperature is then determined. For example, the measured temperature may be the temperature T_C(S) of the supply air measured by the sensor 72 or T_C(R) of the return air measured by the sensor 74. The desired temperature may be a user-entered temperature set point. The relationship may involve determining 304 whether the measured temperature exceeds the set point by at least a given threshold (DT). Exemplary DT may be preset based upon the nature of the use (e.g., refrigerating frozen goods versus refrigerating non-frozen perishable goods) and the particular measured temperature may be determined by such use. For example, in a frozen goods scenario, a main focus of operation may be to avoid temperature high enough to melt the goods. The return temperature T_C(R) may be measured to ensure that it does not exceed a user-entered temperature setpoint. An exemplary target return temperature may be relatively low (e.g., less than 14.4 F). For non-frozen perishable goods, it may be more important to measure the supply temperature to ensure that the supply temperature is not so low as to freeze the goods that the air initially comes in contact with. Exemplary supply temperature is thus in excess of 14.4 F. For frozen goods, there may be more flexibility in DT than for non-frozen perishable goods. Thus, an exemplary DT for frozen goods is approximately 4 F whereas a DT for non-frozen perishable goods is approximately 0.5 F. This effectively determines whether the system is in a high capacity situation or a low capacity situation. If yes (high capacity situation), then the compressors are run simultaneously 310 with the variable speed compressor at a temperature-dependent speed up to its maximum speed. This provides the fastest possible cool down/pull down.

If not in the high capacity situation, then it is determined 312 whether any cooling is required (e.g., the measured temperature is greater than the set temperature by less than the DT) (low capacity situation). If no, then both compressors are shut put in their off conditions 314 and the cycle can repeat. If yes, then it is determined 316 whether the fixed speed compressor has been off or it has run for its minimum time. This determination 316 helps avoid short cycling of the fixed speed compressor. If yes at 316, then only the variable speed compressor is run 318. It is run at a speed appropriate for the needed capacity. If no, then there is simultaneous operation 310 to avoid short cycling of the fixed speed compressor. If the variable speed compressor only is run at 318, then it is determined 320 whether it has been at its maximum speed for a predetermined time. Yes indicates that the variable speed compressor alone is not effective to bring down the temperature quickly enough. Thus, if yes, then simultaneous operation is resumed at 310. If no, however, there is a minimum capacity which the variable speed compressor may provide in continuous operation. It is therefore determined 322 whether required speed has reduced to this minimum speed for a predetermined time. If no, the control cycle merely repeats at step 304. If yes, then the variable speed compressor is shut off at 314.

Once in simultaneous operation at 310, there is then a determination 330 of whether the variable speed compressor is being operated within a certain proximity of its minimum speed. if no, then the cycle repeats at 304. If yes, then the system shifts to variable speed compressor only operation at 318.

In the foregoing example, the controller is configured to in no part of a normal operational range operate the fixed speed compressor alone. However, this may be done in abnormal situations such as a failure of the variable speed compressor or associated components, a service mode, or a manual override mode where the user commands shut down of the variable speed compressor.

Although an embodiment is described above in detail, such description is not intended for limiting the scope of the present disclosure. It will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, when applied to the reengineering of the configuration of an existing system, details of the existing system may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. A refrigeration system (20) comprising: wherein:

a first compressor (24);

a second compressor (26) having at least a first condition at least partially in parallel with the first compressor along a refrigerant flowpath;

a heat rejection heat exchanger (50) downstream of the first compressor and second compressor along the refrigerant flowpath;

an expansion device (54) downstream of the heat rejection heat exchanger along the refrigerant flowpath; and

a heat absorption heat exchanger (56) downstream of the expansion device along the refrigerant flowpath,

the first compressor is a variable speed compressor coupled to a variable speed drive (32) and the second compressor is a fixed speed compressor; and

the second compressor is larger than the first compressor.

2. (canceled)

3. The system of claim 1 wherein:

the second compressor has a larger displacement per revolution than a displacement per revolution of the first compressor.

4. The system of claim 1 wherein:

the first compressor and the second compressor are reciprocating compressors.

5. The system of claim 1 in operational condition with the second compressor connected directly to a line voltage (34) and the first compressor connected to the line voltage via its variable speed drive.

6. A transport system (200) comprising:

the refrigeration system (20) of claim 1; and

a refrigerated container (201) having an interior (202) containing or in air flow communication with the heat absorption heat exchanger.

7. (canceled)

8. The system of claim 1 wherein:

a displacement per revolution of the second compressor is 110-350% of a displacement per revolution of the first compressor.

9. The system of claim 1 wherein:

the first compressor has an induction motor or a permanent magnet motor; and

the second compressor has an induction motor.

10. The system of claim 1 further comprising a controller configured to:

at high required capacity (upper range), operate (310) both the first compressor and the second compressor, the second compressor being operated at a fixed speed; and

in low required capacity (lower range), operate (318) only the first compressor, over at least a portion of said lower capacity range the operating being with variable speed.

11. The system of claim 10 wherein the controller is configured to in no part of a normal operational range operate the second compressor alone.

12. A method for operating the system of claim 1, the method comprising:

at high required capacity (upper range), operating (310) both the first compressor and the second compressor, the second compressor being operated at a fixed speed; and

in low required capacity (lower range), operating (318) only the first compressor, over at least a portion of said lower capacity range the operating being with variable speed.

13. The method of claim 12 wherein:

the lower range meets the upper range.

14. (canceled)

15. The method of claim 12 wherein:

the operation of the first compressor in an uppermost portion of the lower capacity range is at a power frequency in excess of a line power frequency.

16. The method of claim 12 wherein:

a cooldown phase comprises the operation in the high capacity range; and

a post-cooldown phase comprises the operation in the lower capacity range.

17. The method of claim 12 wherein:

the control is responsive to a sensed air temperature of a controlled space.

18. A method for operating a refrigeration system, the refrigeration system comprising: wherein:

a first compressor (24);

a second compressor (26) having at least a first condition at least partially in parallel with the first compressor along a refrigerant flowpath;

a heat rejection heat exchanger (50) downstream of the first compressor and second compressor along the refrigerant flowpath;

an expansion device (54) downstream of the heat rejection heat exchanger along the refrigerant flowpath; and

a heat absorption heat exchanger (56) downstream of the expansion device along the refrigerant flowpath,

the first compressor is a variable speed compressor coupled to a variable speed drive (32) and the second compressor is a fixed speed compressor,

the method comprising: controlling operation of the compressor responsive to a sensed air temperature of the controlled space.

19. The method of claim 18 wherein:

the sensed air temperature is used to control transitions between operation of only one of the compressors and both of the compressors.

20. A method for operating a refrigeration system, the refrigeration system comprising: wherein: wherein:

a first compressor (24);

a second compressor (26) having at least a first condition at least partially in parallel with the first compressor along a refrigerant flowpath;

a heat rejection heat exchanger (50) downstream of the first compressor and second compressor along the refrigerant flowpath;

an expansion device (54) downstream of the heat rejection heat exchanger along the refrigerant flowpath; and

a heat absorption heat exchanger (56) downstream of the expansion device along the refrigerant flowpath,

the first compressor is a variable speed compressor coupled to a variable speed drive (32) and the second compressor is a fixed speed compressor,

the method comprising: at high required capacity (upper range), operating (310) both the first compressor and the second compressor, the second compressor being operated at a fixed speed; and in low required capacity (lower range), operating (318) only the first compressor, over at least a portion of said lower capacity range the operating being with variable speed,

the lower range comprises a lower sub-range wherein the first compressor is operated in a cyclic mode with essentially fixed speed when operating and, an upper sub-range operated continuously with speed increasing with required capacity.

21. The method of claim 20 wherein:

the lower range meets the upper range.

22. The method of claim 20 wherein:

the operation of the first compressor in an uppermost portion of the lower capacity range is at a power frequency in excess of a line power frequency.

23. The method of claim 20 wherein:

a cooldown phase comprises the operation in the high capacity range; and

a post-cooldown phase comprises the operation in the lower capacity range.