SUCTION SUPERHEAT CONROL BASED ON REFRIGERANT CONDITION AT DISCHARGE
A relationship is developed between a discharge condition of a refrigerant leaving a compressor, and the suction superheat or refrigerant quality. By relying upon measurement and control of the discharge condition, the present invention is able to achieve very low suction superheat values. Controlling the operation to very low suction superheat values results in augmented refrigerant system performance.
This application relates to a refrigerant superheat control to enhance system performance and improve compressor reliability, which relies upon a refrigerant thermodynamic condition at discharge to provide reliable suction superheat control.
In air conditioning, heat pump and refrigeration systems, a superheat of the refrigerant leaving an evaporator needs to be closely controlled. Refrigerant leaves the evaporator normally at the superheated thermodynamic state, where its actual temperature is higher than the corresponding saturation temperature (a superheat is defined as the difference between these two temperatures). A certain (positive) superheat is typically required to ensure that little or no liquid refrigerant enters the compressor and system operation is stable. If a significant amount of liquid refrigerant enters the compressor, an undesirable condition known as “flooding” will occur. Flooding could cause “liquid hammer” conditions damaging or breaking compression elements, dilute lubrication oil and wash it off the bearing surfaces, pump lubrication oil out of the compressor sump, and eventually degrade refrigerant system performance and operation.
On the other hand, it is known that in order to assure the highest performance (efficiency and capacity) of the refrigerant system, close to zero superheat values for the refrigerant leaving the evaporator are to be maintained. Further, by reducing suction superheat, the oil return to the compressor is also improved, as the oil is typically accumulated in the evaporator superheated section. Also, the oil viscosity is reduced with the superheat reduction due to the fact that more refrigerant is diluted in the oil at lower superheat values, and to a smaller degree, due to a saturation suction temperature increase. Conversely, as the superheat value is increased, refrigerant is boiled off from the oil increasing the oil viscosity and making the oil more susceptible to stagnate in the evaporator exit section or in the piping connecting the evaporator to the compressor. Of course, improving oil return is a goal of a refrigerant system designer, as it enhances compressor reliability and improves system performance by preventing oil retention in the evaporator and associated piping. Also, at some operating conditions, higher suction superheat values lead to elevated discharge temperatures, operational envelope reduction, potential oil breakdown and thermal distortion of compression elements.
While it is known to be desirable to reduce the superheat to the lowest value possible, to date, most refrigerant systems, at best, would operate with superheat values at the evaporator exit in a range of 5-10° F. The potential for a measurement error, due to temperature sensor measurement tolerances, calibration and resolution; system component manufacturing variability; ambient effects on system operation; load demand fluctuations and associated transient phenomena, concurrently occurring within the refrigerant system, have typically provided a practical bar to further reduction in the superheat setting.
It is undesirable, as mentioned above, to have significant flooding in the compressor, due to associated reliability issues. Thus, the refrigerant system designers have erred on the side of applying sufficient superheat values to eliminate any potential for such flooding at an entire spectrum of operating conditions. As mentioned above, uncontrolled flooding results in a drastic drop in compressor capacity and efficiency, and may also cause severe damage to the compressor.
SUMMARY OF THE INVENTIONThe present invention utilizes a realization that a given change in the suction superheat will result in an expected change in a discharge temperature (or superheat) of the refrigerant leaving the compressor. That is, there is an approximately linear relationship between the suction superheat and the discharge temperature (or superheat) for the refrigerant leaving the compressor. This relationship is essentially linear at any given system operating suction and discharge pressure.
Thus, by monitoring the discharge temperature (or superheat) and changing/controlling a condition of the refrigerant leaving the evaporator or entering the compressor (superheat or quality) based on discharge temperature, the system can be reliably operated at a desired low suction superheat or have a minimal controlled amount of liquid refrigerant entering the compressor suction port. The control of the suction superheat that is based on discharge temperature (or superheat) can, for example, be accomplished by varying the opening of an expansion valve or a suction modulation valve.
The relationship between the discharge temperature (or superheat) and suction superheat can be determined experimentally or can be developed analytically. Further, the relationship can be periodically tested/verified during operation to ensure that the relationship still holds, or any small adjustments need to be made to this relation based on these periodic tests.
In disclosed embodiments, the present invention allows for operation at the superheat levels that are substantially lower than the superheat levels in the 5 to 10° F. range reliably achievable in the past. With the current technique, the superheat levels leaving the evaporator can be as low as 1 to 2° F., or with appropriate control, some minimal amount of liquid refrigerant can be allowed to enter the compressor suction port.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
A basic refrigerant system 20 is illustrated in
The present invention allows achieving very low superheat values or controlled flooded conditions for the refrigerant prior to entering the compression chambers by relying upon a relationship that is illustrated in
In this example, if the compressor operation is controlled based on the discharge temperature, and at 160° F. in particular, it will correspond to a condition that falls approximately in the middle of the most efficient region of the operation, the point “E”. The point “E” is located approximately in the middle of the region defined by the points “C” and “G”. Further, the measurements error tolerance band for the discharge temperature is defined in the
On the other hand, as has been done in the prior art, if the control of suction superheat is executed by direct measurements of the suction superheat value, the suction superheat has to be set to at least 5° F., as shown in
As can be seen from
The refrigerant system can be periodically tested, to assure that the
While the graph presented in
As mentioned above, the refrigerant system can be self-learning such that, during the system operation, for a given suction and discharge pressures, the discharge temperature can be varied on an intermittent basis to establish a relation between the discharge temperature and suction superheat. Stated differently, the refrigerant system can itself develop the graph of
Furthermore, since the discharge side pressure or saturation temperature may be known, a similar relationship can be established between the suction and discharge superheat that can be used for identical purposes of the refrigerant system control.
As mentioned above, previous attempts to use suction and discharge pressures and discharge temperature to reliably control suction superheat to extremely low values have failed, since they relied on refrigerant properties and compression process polytropic exponent both of which highly depend on operating conditions and compressor design characteristics. This becomes particularly difficult for the compressors with a built-in volume ratio that are subjected to over-compression or under-compression conditions. Therefore, the prior art methods could be used as the first order approximations only and could not be relied upon to control suction superheat to near zero values.
It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, or reciprocating compressors can be employed.
The refrigerant systems that utilize this invention can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems.
Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
Claims
1. A refrigerant system comprising:
- a compressor, said compressor having a suction inlet line and a discharge outlet line;
- a compressed refrigerant passing from said compressor downstream to a heat rejection heat exchanger and then downstream to an expansion device;
- an evaporator positioned downstream of said expansion device; and
- a sensor for sensing a condition of a discharge refrigerant leaving the compressor and
- a control utilized to control a refrigerant thermodynamic state at a location between the expansion device and the compressor based upon the sensed condition of the discharge refrigerant.
2. The refrigerant system as set forth in claim 1, wherein said sensed condition is the discharge temperature of the refrigerant.
3. The refrigerant system as set forth in claim 2, wherein said refrigerant thermodynamic state is at least one of the suction superheat or refrigerant quality.
4. The refrigerant system as set forth in claim 1, wherein said sensed condition is the discharge superheat of the refrigerant.
5. The refrigerant system as set forth in claim 1, wherein said location is between the evaporator and the compressor.
6. The refrigerant system as set forth in claim 5, wherein a relationship is found between at least one of the discharge temperature and discharge superheat, and at least one of the suction superheat and refrigerant quality at a location between the evaporator exit and the compressor inlet.
7. The refrigerant system as set forth in claim 6, wherein said relationship is essentially linear.
8. The refrigerant system as set forth in claim 6, wherein said relationship is periodically checked by varying said suction superheat value during refrigerant system operation.
9. The refrigerant system as set forth in claim 6, wherein said relationship for at least one of suction pressure and discharge pressure is stored in the system controller memory.
10. The refrigerant system as set forth in claim 9, wherein a control retrieves said relationship from said memory.
11. The refrigerant system as set forth in claim 9 wherein said relationship is a lookup table.
12. The refrigerant system as set forth in claim 9, wherein said relationship is determined experimentally.
13. The refrigerant system as set forth in claim 12 wherein said relationship is determined during unit operation.
14. The refrigerant system as set forth in claim 9, wherein the relationship is determined analytically.
15. The refrigerant system as set forth in claim 1, wherein the sensed condition is used to achieve suction superheat values equal to or less than 2° F.
16. The refrigerant system as set forth in claim 1, wherein the sensed condition is used to achieve suction superheat values equal to or less than 4° F.
17. The refrigerant system as set forth in claim 5, wherein said suction superheat is calculated based on the difference between the saturated temperature and measured temperature sensed between the evaporator and compressor.
18. The refrigerant system as set forth in claim 17, wherein said suction temperature is measured by a temperature sensor.
19. The refrigerant system as set forth in claim 18 wherein said sensor is one of the thermistor or thermocouple type.
20. The refrigerant system as set forth in claim 17, wherein said saturated temperature is measured by a temperature sensor located within the evaporator.
21. The refrigerant system as set forth in claim 17, wherein said saturated temperature is calculated based on the pressure measurement.
22. A method of operating a refrigerant system including the steps of:
- providing a compressor, said compressor having a suction inlet line and a discharge outlet line;
- passing a compressed refrigerant from said compressor downstream to a heat rejection heat exchanger and then downstream to an expansion device;
- passing the refrigerant to an evaporator downstream of said expansion device;
- sensing a condition of a discharge refrigerant leaving the compressor; and
- controlling a refrigerant thermodynamic state at a location between the expansion device and the compressor based upon the sensed condition of the discharge refrigerant.
23. The method as set forth in claim 22, wherein a relationship between said refrigerant thermodynamic state at a location between the expansion device and the compressor and said sensed condition of the discharge refrigerant used for the control is determined experimentally.
24. The method as set forth in claim 22, wherein a relationship between said refrigerant thermodynamic state at a location between the expansion device and the compressor and said sensed condition of the discharge refrigerant used for the control is determined analytically.
25. The method as set forth in claim 22, wherein the sensed condition is used to achieve suction superheat values equal to or less than 2° F.
Type: Application
Filed: Oct 10, 2007
Publication Date: Sep 1, 2011
Inventors: Alexander Lifson (Manlius, NY), Michael F. Taras (Fayetteville, NY)
Application Number: 12/671,970
International Classification: F25B 1/00 (20060101);