HK1148810B - Suction superheat control based on refrigerant condition at discharge - Google Patents

Description

Suction superheat control based on discharge refrigerant condition

Background

The present invention relates to refrigerant superheat control to enhance system performance and improve compressor reliability, which relies on refrigerant thermodynamic conditions at discharge to provide reliable suction superheat control.

In air conditioning, heat pump and refrigeration systems, there is a need to closely control the superheat of the refrigerant leaving the evaporator. Normally, the refrigerant leaves the evaporator in a superheated thermodynamic state in which its actual temperature is higher than the corresponding saturation temperature (superheat is defined as the difference between these two temperatures). A certain (positive) superheat is usually required to ensure that little or no liquid refrigerant enters the compressor and the system operates stably. If a significant amount of liquid refrigerant enters the compressor, an undesirable condition known as "flooding" will occur. Flooding can result in a "liquid hammer" condition that damages or destroys the compression element, can dilute and wash the lubricant off the bearing surfaces, can pump the lubricant out of the compressor oil pan, and ultimately can degrade performance and operation of the refrigeration system.

On the other hand, it is known that in order to ensure the highest performance (efficiency and capacity) of the refrigeration system, the refrigerant leaving the evaporator should be kept close to zero superheat value. Furthermore, by reducing suction superheat, oil return to the compressor can also be improved, as oil typically accumulates in the superheat section of the evaporator. Furthermore, the viscosity of the oil decreases with decreasing superheat value, since more refrigerant is diluted in the oil at lower superheat values, and to a lesser extent, since the saturated suction temperature increases. Conversely, as the superheat increases, refrigerant vaporizes from the oil, increasing the viscosity of the oil and making the oil more susceptible to stagnation at the outlet portion of the evaporator or in the piping connecting the evaporator to the compressor. Of course, improving oil return is a goal of refrigerant system designers as it enhances compressor reliability and improves system performance by preventing oil from stagnating in the evaporator and associated piping. Moreover, under certain operating conditions, higher suction superheat values can lead to increased discharge temperatures, reduced operating range, potential oil breakdown, and thermal distortion of the compression elements.

While it is known that it is desirable to reduce superheat to the minimum possible, most refrigeration systems have heretofore been operated at best with superheat values at the evaporator exit in the range of 5 to 10 ° F. The following factors that occur simultaneously within a refrigeration system often place practical hurdles to further reducing superheat settings: measurement errors due to measurement tolerances, calibration and resolution of the temperature sensor may occur; manufacturing variability of system components; environmental impact on system operation; load demand fluctuations and associated transients.

As noted above, it is undesirable to have significant flooding in the compressor due to associated reliability issues. Thus, refrigerant system designers have made their work in "applying sufficient superheat values to eliminate any possibility of such flooding over the entire range of operating conditions". As mentioned above, uncontrolled flooding results in a substantial reduction in compressor capacity and efficiency, and may also cause severe damage to the compressor.

Disclosure of Invention

The present invention makes use of the following facts: a given change in suction superheat will result in an expected change in 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) of the refrigerant leaving the compressor. This relationship is substantially linear at any given system operating suction and discharge pressures.

Thus, by monitoring the discharge temperature (or superheat) and varying/controlling the condition (superheat or quality) of the refrigerant leaving the evaporator or entering the compressor based on the discharge temperature, the system can be made to operate reliably at the desired low suction superheat or the system can have a very small controlled amount of liquid refrigerant entering the compressor suction. For example, the control of the suction superheat based on the discharge temperature (or superheat) may be achieved by changing the opening degree of an expansion valve or a suction modulation valve.

The relationship between discharge temperature (or superheat) and suction superheat can be determined experimentally or can be obtained analytically. Furthermore, the relationship may be tested/verified periodically during operation to ensure that the relationship is still true, or any small adjustments to such relationship may be required based on these periodic tests.

In the disclosed embodiment, the present invention allows operation at significantly lower superheat levels than could be reliably achieved in the past in the range of 5 to 10 ° F. With the present technique, the superheat level upon leaving the evaporator can be as low as 1 to 2 ° F, or by suitable control, a minimal amount of liquid refrigerant can be allowed to enter the compressor suction.

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.

Drawings

Figure 1 is a view of a refrigeration system incorporating the present invention.

FIG. 2 is a graph showing the variation of discharge temperature as a function of suction superheat.

Fig. 3 shows the system efficiency with respect to suction superheat and discharge temperature.

Detailed Description

A basic refrigeration system 20 is shown in fig. 1, the refrigeration system 20 including a compressor 22 for delivering compressed refrigerant downstream to a heat rejecting heat exchanger 24 (a condenser for subcritical applications and a gas cooler for transcritical applications). The expansion device 26 is preferably an electronic expansion device and is generally known in the industry. The refrigerant having passed through the expansion device 26 flows in sequence through the evaporator 28, optional suction modulation valve 30, and suction line 38 back to the compressor 22. As shown in this figure, a temperature sensor 46 is provided on or in the discharge line exiting the compressor. Temperature sensor 46 may also be positioned to measure a discharge temperature on or within the compressor housing. The temperature sensor 46 communicates with the electronic controller 32, and the electronic controller 32 in turn controls the electronic expansion device 26 and/or the optional suction modulation valve 30 to regulate and control suction superheat. A sensor 56 for measuring the suction superheat value is also provided between the expansion device 26 and the compressor 22. More preferably, the sensor 56 is disposed on the suction line 38 between the outlet of the evaporator 28 and the inlet of the compressor 22. The sensor 56 may be a sensor that directly measures superheat (the difference between the actual refrigerant temperature and the saturated refrigerant temperature at approximately the same location). Alternatively, the sensor 56 may be a temperature sensor. For example, the temperature sensor 56 may be of the thermocouple type or thermistor type. In the latter case of the temperature sensor 56, an additional temperature sensor 58 may be provided within the evaporator 28 (in the refrigerant flow or outside the evaporator surface) in the two-phase region to determine the saturated refrigerant temperature in the evaporator. The difference between these two temperature measurements provided by the temperature sensors 56 and 58 will determine the suction superheat value. Instead of utilizing a temperature-type sensor 58 within the evaporator, a pressure-type sensor 60 may be used that will determine the refrigerant pressure at or near the location where the superheat value is to be obtained (e.g., the suction side of the refrigeration system 20). After the pressure value of the refrigerant is measured by the pressure sensor 60, the pressure value may be converted to a corresponding saturation temperature value, as is well known. The suction superheat value is then easily calculated by subtracting the obtained saturated refrigerant temperature from the measured actual refrigerant temperature. As described above, the temperature sensors 56 and 58 may be mounted on an exterior surface (e.g., air side) of the ductwork, compressor, heat exchanger, or the like. These sensors may also be mounted internally or in the refrigerant flow (so-called "in-flow sensors"). In the latter case, the "in-flow sensor" would directly measure the temperature of the refrigerant. If the temperature sensors are mounted externally, they are preferably isolated or shielded from the ambient environment to reduce measurement errors.

The present invention allows for a very low superheat value or controlled flooding condition for the refrigerant before it enters the compression chambers by relying on the relationship shown in fig. 2. As shown in fig. 2, the discharge temperature is plotted as a function of suction superheat for a particular operating condition. In addition, the data shown in FIG. 3 illustrates the thermodynamic efficiency of the system with respect to suction superheat and discharge temperature. As can be seen in fig. 2, the change in suction superheat causes a substantially linear change in discharge temperature until the compressor begins to experience a flooded condition. The overflow condition is represented by a vertical line starting at point "O" and extending downward. As the amount of flooding increases, the discharge temperature continues to decrease while the suction superheat value remains zero (when superheat is zero, the actual temperature of the refrigerant equals the saturation temperature). As shown in fig. 2 and 3, the most efficient operation of the compressor, evaporator and overall refrigeration system is achieved in the region between points "C" and "G" shown in the figures. Point "C" corresponds to a suction superheat of about 4F, and point "G" corresponds to a slight compressor flooding condition.

In this example, if the operation of the compressor is controlled based on the discharge temperature, and particularly at 160 ° F, it will correspond to a condition that falls approximately in the middle of the most efficient operating region, point "E". Point "E" is located approximately at the middle of the area defined by point "C" and point "G". Further, a measurement error tolerance band of the discharge temperature is defined between the point "D" and the point "F" in fig. 2 and 3. Since the measurement error tolerance band falls within the most efficient operating region, it can be concluded that: by controlling the suction superheat based on the discharge temperature, the refrigeration system 20 can always operate at or near the most efficient point. Further, in this particular example, the refrigeration system 20 may be comfortably operated with discharge temperature control at point "E", where point "E" corresponds to a suction superheat of only 1 ° F.

On the other hand, if the control of the suction superheat is made by directly measuring the suction superheat value, as is done in the prior art, the suction superheat must be set to at least 5 ° F, as indicated by point "B" in fig. 2, to ensure that an uncontrolled flooding condition does not occur ("uncontrolled flooding" representing a large amount of liquid refrigerant reaching the compressor suction). For example, if the superheat value is set below 5 ° F, the actual operating point may be well below point "H" due to potential measurement error, which corresponds to a severe flooding condition of the compressor and its potential damage. Thus, for this example, the minimum acceptable set point for suction superheat using the prior art is a value of 5 ° F.

As can be seen in fig. 3, operation at point "B" is not as efficient as operation at point "E". For example, for typical freeze mode operating conditions in a heavy duty trailer or shipping container application, as the suction superheat value increases from 1 ° F to 5 ° F, this translates into (and is reflected in) a 2% reduction in system performance (capacity and efficiency). It should be noted that if the prior art technique is employed, for example if the actual superheat value is set at point "a", the performance of the refrigeration system may actually be reduced even more due to measurement errors and reliability issues. In other words, the present invention allows the suction superheat to be accurately controlled using the discharge temperature sensor by using such a relationship of "defining the suction superheat value based on the measured value of the discharge temperature". For example, when using this relationship, if the discharge temperature is set to 162 ° F, the refrigeration system can operate at a suction superheat of 1 ° F without any risk of reaching point "H" which corresponds to the starting point of a region of severe flooding conditions that could damage the compressor. The controller 32 of the refrigeration system 20 may utilize the sensed discharge temperature to predict the amount that must be changed to achieve the desired suction superheat value discharge temperature. In the disclosed embodiment, the controller 32 will then control the expansion device 26 and/or the suction modulation valve 30 to vary the discharge temperature to adjust the suction superheat of the refrigerant to the compressor. In the disclosed embodiment, the suction superheat may be reduced to a value below 2 ° F, and may be maintained at about 1 ° F.

The refrigerant system can be periodically tested to ensure that the relationship in fig. 2 is still true by raising the suction superheat from a lower level to a higher level and then measuring the correlation between the discharge temperature change and the suction superheat to ensure that the expected change will still occur. This may occur at a sufficiently large suction superheat value so that the suction temperature can be reliably measured. As an example, the suction superheat may be varied from 1 ° F to 16 ° F, and the corresponding discharge temperature variation may be measured at the superheat region of 16 ° F. Thus, the refrigerant system may operate in a self-learning or adaptive manner to achieve the highest possible operating performance.

While the graph shown in fig. 2 shows the relationship between discharge temperature and suction superheat for particular suction and discharge pressures, similar graphs may be obtained for other operating conditions. These graphs can then be used to control suction superheat at any desired operating conditions. Instead of forming multiple plots, the results may be aggregated into a look-up table and then interpolated as needed for actual values of suction and discharge pressures. Further, instead of utilizing a look-up table or graph, an equation relating suction superheat to discharge temperature may be developed for a given suction and discharge pressure.

As described above, the refrigeration system may be self-learning such that during system operation, for a given suction and discharge pressure, the discharge temperature may be intermittently varied to establish a relationship between the discharge temperature and the suction superheat. In other words, the refrigeration system may itself derive the graph of fig. 2 during operation. The corresponding map values as a function of suction and discharge pressures may then be stored in memory of the refrigerant system controller and later retrieved from memory by the controller as needed.

Further, since the discharge side pressure or saturation temperature may be known, a similar relationship may be established between the suction and discharge superheat, which may be used for the same purpose of refrigeration system control.

As noted above, previous attempts to reliably control the suction superheat to extremely low values using suction and discharge pressures and discharge temperatures have failed because they are dependent on refrigerant properties and compression process polytropic indices, both of which are highly dependent on operating conditions and compressor design characteristics. This becomes particularly difficult for compressors with inherent volume ratios that are subject to "over-compression" or "under-compression" conditions. Thus, the prior art method can only be used as a first order approximation and cannot be relied upon to control suction superheat to be close to zero.

It should be noted that many different compressor types may be used in the present invention. For example, scroll, screw, rotary, or reciprocating compressors may be employed.

Refrigeration systems employing the present invention may be used in a number of different applications including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigerated load 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 (13)

1. A refrigeration system comprising:

a compressor having a suction inlet line and a discharge outlet line;

a compressed refrigerant flowing downstream from the compressor to a heat rejecting heat exchanger and then downstream to an expansion device;

an evaporator positioned downstream of the expansion device; and

a sensor for sensing a condition of a discharge refrigerant exiting the compressor; and

a controller for controlling a refrigerant thermodynamic state at a location between the expansion device and the compressor based on a sensed condition of a discharged refrigerant, the controller utilizing a functional relationship between the sensed condition and a suction superheat and controlling the refrigerant thermodynamic state to ensure a low suction superheat value,

wherein the functional relationship exists between at least one of a refrigerant quality and a suction superheat at a location between the evaporator outlet and the compressor inlet and at least one of a discharge temperature and a discharge superheat,

wherein the functional relationship is periodically verified by changing the value of the suction superheat during operation of the refrigeration system.

2. The refrigerant system as set forth in claim 1, wherein said low suction superheat value is a positive value.

3. The refrigerant system as set forth in claim 1, wherein said low suction superheat value is zero and said compressor is operated under conditions of slight flooding.

4. The refrigerant system as set forth in claim 1, wherein said functional relationship is substantially linear.

5. The refrigerant system as set forth in claim 1, wherein said functional relationship for at least one of suction pressure and discharge pressure is stored in a system controller memory.

6. The refrigerant system as set forth in claim 5, wherein said functional relationship is experimentally determined.

7. The refrigerant system as set forth in claim 5, wherein said functional relationship is determined during operation of the refrigerant system.

8. The refrigerant system as set forth in claim 5, wherein said functional relationship is analytically determined.

9. The refrigeration system of claim 1, wherein the sensed condition is used to achieve a suction superheat value equal to or less than 2 ° F.

10. The refrigerant system as set forth in claim 1, wherein said suction superheat is calculated based on the difference between a saturation temperature and a measured temperature sensed between said evaporator and said compressor.

11. A method of operating a refrigeration system comprising the steps of:

providing a compressor having a suction inlet line and a discharge outlet line;

flowing compressed refrigerant from the compressor downstream to a heat rejecting heat exchanger and then downstream to an expansion device;

flowing the refrigerant to an evaporator downstream of the expansion device;

sensing a condition of a discharge refrigerant exiting the compressor;

controlling a refrigerant thermodynamic state at a location between the expansion device and the compressor based on a sensed condition of a discharge refrigerant, the controlling utilizing a functional relationship between the sensed condition and a suction superheat, and the controlling the refrigerant thermodynamic state to ensure a low suction superheat value, wherein the functional relationship exists between at least one of a discharge temperature and a discharge superheat and at least one of a refrigerant quality and a suction superheat at a location between the evaporator outlet and the compressor inlet; and

the functional relationship is periodically checked by changing the value of the suction superheat during operation of the refrigeration system.

12. The method of claim 11 wherein the low suction superheat value is maintained at a positive value.

13. The method of claim 11 wherein the low suction superheat value is maintained at zero and the compressor is operated under conditions of slight flooding.