US20100307177A1

US20100307177A1 - Rapid compressor cycling

Info

Publication number: US20100307177A1
Application number: US12/864,783
Authority: US
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2008-01-31
Filing date: 2008-01-31
Publication date: 2010-12-09
Also published as: WO2009096968A1

Abstract

A single speed compressor is provided with a switching device and control for turning the compressor drive motor ON and OFF in repeated succession at a selected ON time/OFF time ratio within a selected cycle time interval. The ON/OFF ratio and cycle time interval are selected to maintain desired temperature and/or humidity control within a conditioned space.

Description

TECHNICAL FIELD

This invention relates generally to rapid cycling of single speed compressors, and more particularly, to a precise control of temperature and humidity in a conditioned space by utilizing such rapidly cycled single speed compressors.

BACKGROUND OF THE INVENTION

During operation of air conditioning and refrigeration systems, it is desirable to vary the capacity of the compressor in an effort to match thermal load demands in a climate-controlled space to provide tight temperature and/or humidity control. This is frequently done by way of a variable speed drive (VSD) which controls the supply voltage and frequency by utilizing an inverter. While a variable speed drive permits continuously varying compressor speed, which in turn allows for a continuous adjustment of the capacity delivered by the variable speed compressor, thereby giving the advantage of smooth operation and better part-load performance, it is relatively expensive. The cost of variable speed drives is on the order of that of compressors. Variable speed drives also tend to be bulky and present reliability problems. Furthermore, variable speed drives themselves have associated unavoidable efficiency losses.
There are also other problems associated with the use of variable speed drives to control the capacity of compressors. For example, quite often a compressor cannot operate below a certain speed threshold, for substantial periods of time, due to problems associated with oil delivery for the lubrication of internal compressor components. Also, some compressors have internal design features which would cause the compressor to operate improperly if the speed is reduced below a certain level. For example, in the case of scroll compressors, where the fixed and orbiting scrolls have a certain radial compliance requirement, the operation below a certain speed would cause the two scrolls to separate beyond the allowable limit or oil delivery to bearings would be interrupted.
The other method of maintaining temperature and/or humidity in a climate-controlled space is turning the compressor ON and OFF for extended periods of time (normally in the order of at least several minutes, with the common practice in the industry having at least five minute intervals between subsequent compressor startups). This method has its major drawbacks as the temperature and/or humidity in the conditioned space cannot be precisely controlled. Also, it creates a discomfort as the temperature and/or humidity of the delivered air varies, especially in the cases when they fall outside the comfort zone defined by the industry standards. For example, in an air conditioning mode of operation, the temperature and humidity are lower when the compressor is ON and higher when the compressor is OFF. If the evaporator fan is also shut off for prolonged periods of time when the compressor is OFF, it creates additional discomfort as the amount of air delivered to the conditioned space is changing from no flow to full amount of flow. Further, condensate accumulated on the evaporator external heat transfer surfaces would re-evaporate during the compressor shutdowns and eventually re-enter into the conditioned environment in the form of moist air, which is obviously undesirable.
The problem would represent itself in a similar fashion if the unit is operating in the heat pump mode, when the conditioned air passes over the heat rejection heat exchanger and then delivered to the conditioned (heated in this case) environment.
There is therefore a need for a cost effective, reliable and efficient manner and apparatus for varying the capacity of a compressor.

DISCLOSURE OF THE INVENTION

A compressor is provided with a switching device and an associated control for selectively sequencing the electrical power to a single speed compressor drive motor to rapidly alternate the compressor between ON and OFF positions. The compressor is cycled in a rapid fashion such that the relatively large thermal inertia of the refrigerant system would not allow the refrigerant system to respond in a timely fashion to changes introduced by refrigerant mass flow variations as compressor alternates between ON to OFF positions. In other words, the time constant of the refrigerant system thermal inertia is higher or at least the same order of magnitude as the time period of the compressor ON/OFF cycle. In this case, the temperature of the delivered air essentially stays the same regardless whether the compressor is in ON or OFF positions as the evaporator surfaces and heat rejection heat exchanger surfaces do not have enough time to change its temperature during the compressor ON/OFF cycles. Thus, the temperature of the air being flown over these heat exchange surfaces and supplied to the conditioned environment would essentially remain constant during the period of time when the compressor is either in ON or OFF positions. Therefore, the temperature within the environment can be very tightly controlled, while the humidity can be maintained at an appropriate level as well. Consequently, the occupants of the conditioned space would be more comfortable as the temperature and humidity of the delivered conditioned air would remain the same and the flow of this air would not be interrupted. In the context of this invention, the heat rejection heat exchanger can be of a condenser type, if the transition from a single phase refrigerant vapor into a two-phase vapor/liquid mixture takes place within the heat exchanger, or it can be of a gas cooler type, where the refrigerant gas is simply cooled in the heat exchanger without any transition into a two phase region. The gas coolers are typically associated with refrigerant cycles where the refrigerant on the high side is at the pressure which exceeds the critical pressure. This operation is normally called a transcritical operation and would be typical for the refrigerants such as R744 refrigerant, commonly known as CO₂.
It should be noted that in the prior art systems instead of allowing the compressor to be rapidly cycled, the compressor cycling, even on an intermittent and infrequent basis, was strictly avoided. The thermostats, for example, were equipped with electronic boards which prevented the compressor from being rapidly cycled (so-called thermostat “jiggling”). In this case the compressor must have been shut off, for instance, for at least five minutes before starting again. In this invention, the compressor is actually expected and controlled to continuously cycle, with the cycle time normally being less than one minute. In this case, the compressor is expected to accumulate several million of cycles during its life time.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a basic vapor compression system with the present invention incorporated therein.

FIG. 2 is a graphic illustration of the compressor speed as a function of time in accordance with the present invention.

FIG. 3 is a graphic illustration of the capacity delivered by a compressor as a function of the ratio of ON-time/OFF-time for the compressor drive motor.

FIG. 3A is a graphic illustration of the conditioned space temperature as a function of time, in accordance with the present invention.

FIG. 4 is a schematic illustration of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Shown in FIG. 1 is a basic vapor compression system which includes a compressor assembly 11 a heat rejection heat exchanger (a condenser or a gas cooler) 12, an expansion device 13 and an evaporator 14. In a conventional manner, the refrigerant vapor is compressed in the compressor assembly after which it passes to the heat rejection heat exchanger 12 where the heat is transferred from the refrigerant to a secondary loop fluid such as air, water or glycol. The refrigerant is expanded in the expansion device 13, and the expanded refrigerant that is at lower pressure and temperature then passes to the evaporator 14 where it assimilates heat from an environment, with the resultant refrigerant vapor then passing back to the compressor assembly 11.
The compressor assembly 11 as shown comprises a pair of compressors 16 and 17 operating in parallel or a so-called tandem configuration. In this regard, it should be understood that the present invention is equally applicable to use in a single compressor configuration, as shown in FIG. 4. Further, the compressors 16 and 17 could be of any suitable type such as a rotary compressor, a reciprocating compressor, a scroll compressor, or a screw compressor. The compressors 16 and 17 are driven by single speed motors 18 and 19, respectively.
With a dual, parallel, compressor arrangement as shown, additional levels of the capacity for the refrigerant system 11 can be achieved as compared to a single compressor. For example, one of the compressors could be operated continuously, while the other compressor would be operated in the ON/OFF fashion. In this case, the system capacity can be adjusted between the two levels, the first low level provided by a single compressor being operated continuously, and the second level with both compressors operated continuously and simultaneously. By turning one compressor ON and OFF, the capacity of the refrigerant system 11 can be adjusted between the first and the second levels. While it is shown that the compressor 16 is the compressor that is operating in a rapid ON/OFF cycling manner, it should be understood that any of the two compressors 16 and 17, can be operated continuously while the other compressor can be operated in the ON/OFF fashion.
Electrical power to each of the compressor drive motors 18 and 19 is provided by way of an electrical power source 21 via an electrical line 22 to a control 23 and then to electrical lines 24 and 26 as shown. Electrical power to the drive motor 19 is provided by the control 23 in a conventional manner to turn the compressor on when operation of the system is desired and to turn it off when it is not desired. Electrical power to the drive motor 18, on the other hand, is provided through the electrical switching device 27 in a controlled manner, with the switching device 27 being sequentially and rapidly turned ON and OFF by the control 23 operating through an electrical line 28. The method in which this power flow is controlled will now be described.
As shown in FIG. 2, the single speed drive motor 18 may be operated either at full speed when it is turned on, or at zero speed when it is turned off. That is, by operation of the switching device 27 as controlled by the control 23 via the electric line 28, the drive motor 18 is turned on at time t₀and remains at full speed until time t₁, at which the switching device 27 is turned off and the speed of the drive motor 18 drops to zero. The drive motor 18 remains off until time t₂, when it is turned on and resumes operation at full speed until time t₃, when the drive motor 18 is turned off again and the motor speed drops to zero. This mode of operation continues as long as the capacity of the compressor 16 is desired to be at less then full capacity. In accordance with the present invention, the respective ON-time and OFF-time intervals can be varied as desired. As shown in FIG. 3, for the system configuration depicted in FIG. 4, where only one compressor is present in the system, the greater the ratio of ON-time to OFF-time, the greater the delivered capacity, Q, on an average basis. Thus, if full capacity Q₁is desired, the switching device 27 will remain ON and the drive motor 18 will continue to operate at full speed. If a lower capacity, Q₂, is desired, the switching device 27 will be operated in such a manner that the ratio of ON-time to OFF-time would be kept at a value to provide this desired capacity and could be equal, for instance, 0.5 as shown in the FIG. 3. In this manner, any capacity level between nominal capacity and full capacity can be obtained by selectively varying the ON-time/OFF-time ratio as shown in FIG. 3. It should be understood that this ratio could be anywhere between zero and infinity. It is equal zero when the compressor is continuously in the OFF position and it is equal to infinity when the compressor is continuously in the ON position. When the compressor is cycling between ON and OFF positions, this ratio could be of any value between zero and infinity. For example, if the compressor is being operated in ON and OFF positions for equal amount of time, then this ratio would be equal to unity.
If there is only one compressor present within the refrigerant system, such as in FIG. 4, then the nominal capacity would be equal to zero, which corresponds to the ON/OFF ratio being equal to zero with this one compressor being shut off. If there are two compressors present within the refrigerant system, such as in FIG. 1, and the other compressor 17 is operating continuously, then the nominal capacity would be equal to the capacity delivered by the compressor 17 when it is running continuously.
The ratio of ON/OFF time intervals, and therefore the capacity of the compressor 16 would be controlled such that the desired average temperature within the conditioned environment is maintained. As shown in FIG. 2, the ON/OFF cycle time interval (which is the time interval equal to the sum to of the time the compressor is ON and the time the compressor is OFF, during one cycle) is selected to be small enough to assure that that there are minimal temperature fluctuations from the average temperature value within the conditioned space. In other words, the time interval corresponding to this ON/OFF cycle should be sufficiently low such that the thermal inertia time constant of the refrigerant system would be higher than this cycle time period. As in the prior art, the average temperature within the conditioned space is controlled by the ON/OFF ratio. However, in this invention, the cycle time would control how much the temperature would fluctuate from the average value. This is illustrated in FIG. 3A, wherein the temperature fluctuations are shown by a curve A when the compressor is not allowed to cycle on a frequent basis, such as in the prior art, as explained above, as compared to a curve B wherein there are substantially reduced temperature fluctuations in the conditioned space when the compressor is rapidly cycled, as described by this invention.
The change in the cycle time would also control the humidity within the conditioned space. The humidity control may be executed simultaneously with the temperature control, as known in the art, while the refrigerant system is operated in the conventional cooling mode or any of known dehumidification modes (not shown). As stated above, similarly to the temperature fluctuations, the humidity fluctuations would be tightly controlled as well, due to a wide range and flexibility in the compressor ON/OFF cycle time ratio. For typical refrigerant systems, the cycle time would be between 10 seconds and 2 minutes. Most typically, the cycle time would be selected between 20 seconds and 1.5 minute. However, under some circumstances, when the requirements on the conditioned air temperature and humidity control can be relaxed, the ON/OFF compressor cycle time period can be extended to the range of 1.5 to 3 minutes.
Such fast cycling on a regular basis, as compared to the prior art systems, where the system control prohibited even intermittent cycling below 5 minutes, has been made technically feasible and reliable in view of recent developments such as solid state motor controls, which allow motors to be switched ON and OFF for millions of cycles with no damage, as well as control techniques to limit the starting torque on a start-up and provide for gradual shutdowns (measured in milliseconds) by gradually reducing voltage to the motor on a shutdown, and more robust mechanical designs of compressors permitting frequent cycling.
Referring now to FIG. 4, an alternative embodiment is shown, wherein a single compressor 16 and its drive motor 18 are shown to be controlled in the manner described hereinabove. However, an additional feature is also shown, wherein the control 23 is connected by an electrical line 29 to the expansion device 13, such that, during periods in which the drive motor 18 is turned off, the expansion device 13 is also to be closed. This allows for the refrigerant system to maintain pressure differential between high discharge side and low suction side during the OFF time periods of operation when the drive motor 18 is in an OFF position. This in turn will reduce cycling losses and improve refrigerant system efficiency. In addition, the control 23 can control the operation of an evaporator fan 31 and heat rejection heat exchanger fan 32 by way of electrical lines 33 and 34, respectively. The control 23 can either operate one or both of the fans continuously or turn the fans ON and OFF on an intermittent basis. Each of the fans 31 and 32 can either lead or lag the compressor ON operation. Each of the fans 31 and 32 can also be ON for the time period that is shorter or longer than the compressor ON operation. In the context of this invention, the fans 31 and 32 can be substituted by liquid pumps. For example, instead of using a fan that is blowing ambient air to cool the refrigerant in the heat rejection heat exchanger 12, a liquid pump carrying cooling fluid can be utilized.
This invention can be applied to various types of refrigerant systems, which for example include container and truck-trailer applications, supermarket refrigeration installations, and residential and commercial air conditioning and heat pump applications. It can be applied to a variety of refrigerants including, but not limited to, R410A, R134a, R22, R407c, R404A, R422D, R422A, R744 refrigerants. This invention also applies to various types of compressors including, for example, screw compressors, scroll compressors, rotary compressors, and reciprocating compressors.
Although the present invention has been particularly shown and described with reference to preferred and modified embodiments as illustrated by the drawings, it will be understood by one skilled in the art that various changes in detail may be made thereto without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. A method of controlling the temperature and/or humidity within a space being conditioned by a refrigerant system including in serial flow relationship a single speed compressor, a heat rejection heat exchanger, an expansion device and an evaporator, comprising the steps of:

providing a switching device between a power source and a drive motor for said single speed compressor;

providing a control for selectively operating said switching device between ON and OFF positions; and

using said control to turn said switching device ON and OFF in repeated succession at a selected ON time/OFF time ratio within a selected cycle time interval.

2. A method as set forth in claim 1 wherein said selected cycle time interval is in the range of 20 seconds to 1.5 minutes

3. A method as set forth in claim 2 wherein said selected time interval is in the range of 10 seconds to 20 seconds.

4. A method as set forth in claim 1 wherein said selected cycle time is in the range of 1.5 minutes to 4 minutes.

5. A method as set forth in claim 1 wherein said refrigerant system includes another single speed compressor operably connected in parallel with said single speed compressor, with respect to refrigerant flow.

6. A method as set forth in claim 1 wherein said heat rejection heat exchanger and said evaporator have associated fans, and including the further step of turning at least one of said fans ON and OFF in coordination with the turning ON and OFF said switching device.

7. A method as set forth in claim 6 wherein the turning ON and OFF said at least one fan is selectively timed so as to either lead or lag the turning ON and OFF said switching device.

8. A method as set forth in claim 6 wherein the selected cycle time interval of said at least one fan is larger or smaller than the selected cycle time interval of said switching device.

9. A method as set forth in claim 6 wherein at least one of said fans is operated on a continuous basis.

10. A method as set forth in claim 1 and including the further step of closing said expansion device when said switching device is turned OFF.

11. A method as set forth in claim 10 wherein the closing of said expansion device is selectively timed to either lead or lag the time of turning said switching device OFF.

12. A method as set forth in claim 10 wherein the cycle time interval of said expansion device is larger or smaller than the selected cycle time interval of said switching device.

13. Control apparatus for a drive motor of a single speed compressor in a refrigerant system which includes in serial flow relationship, the single speed compressor, a heat rejection heat exchanger, an expansion device and an evaporator, comprising:

a drive motor for said single speed compressor; a switching device connected between a power source and said drive motor; and a control for selectively operating said switching device been ON and OFF positions in repeated succession at a selected ON time/OFF time ratio within a selected cycle time interval to control the temperature and/or humidity within a conditioned space.

14. A control apparatus as set forth in claim 13 wherein said selected cycle time interval is in the range of 20 seconds to 1.5 minutes.

15. A control apparatus as set forth in claim 13 wherein said selected cycle time interval is in the range of 10 seconds to 20 seconds.

16. A control apparatus as set forth in claim 13 wherein said selected cycle time interval is in the range of 1.5 minutes to 4 minutes.

17. A control apparatus as set forth in claim 13 wherein said refrigerant system includes another single speed compressor operably connected in parallel with said single speed compressor, with respect to refrigerant flow.

18. A control apparatus as set forth n claim 13 wherein said heat rejection heat exchanger and said evaporator have associated fans, and further wherein said control is adapted to turn at least one of said fans ON and OFF in coordination with the turning ON and OFF said switching device.

19. A control apparatus as set forth in claim 18 wherein the turning ON and OFF said at least one fan is selectively timed so as to either lead or lag the turning ON and OFF said switching device.

20. A control apparatus as set forth in claim 18 wherein the selected cycle time interval of said at least one fan is larger or smaller than the selected cycle time interval of said switching device.

21. A control apparatus as set forth in claim 18 wherein at least one of said fans is adapted to be operated on a continuous basis.

22. A control apparatus as set forth in claim 13 wherein said control is operative to close said expansion device when said switching device is in said OFF position.

23. A control apparatus as set forth in claim 22 wherein the closing of said expansion device is selectively timed to either lead or lag the time of turning said switching device OFF.

24. A control apparatus as set forth in claim 22 wherein the cycle time interval of said expansion device is larger or smaller than the selected cycle time interval of said switching device.