US20090113900A1

US20090113900A1 - Methods and apparatus for operating air conditioning systems with an economizer cycle

Info

Publication number: US20090113900A1
Application number: US11/916,311
Authority: US
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2005-06-08
Filing date: 2005-06-08
Publication date: 2009-05-07
Also published as: EP1902114A1; CN101432385A; WO2006135356A1

Abstract

Methods and apparatus are provided for enhancing the performance of rooftop air conditioning systems by operating such systems with an economizer cycle and utilizing a blend incorporating R32 and R125 refrigerants as a working medium, wherein such benefits are related to at least the performance (e.g. capacity and/or the energy efficiency ratio) of the rooftop air conditioning system operating at various environments (e.g. temperatures at and above 95° F.).

Description

FIELD OF THE INVENTION

This invention relates to air conditioning systems, and, in particular, to methods and apparatus for operating packaged air conditioning systems (e.g. rooftop air conditioning systems) with an economizer cycle so as to achieve measurable performance-related benefits.

BACKGROUND OF THE INVENTION

It is known in the refrigeration art that various benefits (e.g., increased system capacity and/or efficiency) can be derived from operating a refrigeration system with a so-called “economizer cycle.” It is also understood that these benefits are magnified when there is a high pressure ratio between compressor suction and discharge, such as would occur when a high temperature differential exists during operation of a refrigeration system. For example, appreciable system benefits are achieved when operating a supermarket or transport refrigeration system with an economizer cycle in environments wherein the temperature differential (between compressor saturated suction and saturated discharge temperature) for the refrigerant circulated through the system is typically about 130° F.
In contrast, pressure ratios are much lower for air conditioning systems. This is because the temperature differential encountered during operation of air conditioning systems is markedly less than that which is typically encountered in a refrigeration context. Consequently, those in the air conditioning field have been discouraged from operating air conditioning systems with an economizer cycle, particularly since it is their belief that doing so would add non-nominal costs and complexities to the systems that would not be recouped by performance-related benefits and/or enhancements. Thus, those who manufacture, sell and/or utilize air conditioning systems have been unable, thus far, to reap the various benefits that perhaps could be realized through operation of such air conditioning systems with an economizer cycle.
While that alone is problematic, it is made more difficult by the recent introduction of various legislation and industry regulations, which have greatly affected the air conditioning industry by defining minimum efficiency standards for air conditioning systems and by instituting a gradual phase-out of (followed, over time, by a total ban on) certain thermodynamically suitable and efficient refrigerants that also happen to contain hydrochlorofluorocarbons (HCFCs), e.g., R22 refrigerant.
The phase-out/ban on R22 is particularly significant to those in the art, since there is a concern that the performance of air conditioning systems may be negatively impacted due to the resultant introduction of alternate, “environmentally friendly” refrigerants that have thermo-physical properties considerably different than those of R22. Among the most widely utilized of these alternate refrigerants is the R410A refrigerant blend, which most in the art believe exhibits performance deficiencies under certain environmental conditions (e.g., at high ambient temperatures) as compared to R22 refrigerant.
Thus, there is a need to develop air conditioning systems, methods and equipment that can be utilized within the scope of legislation and industry regulations yet that still can provide measurable performance-related benefits when operated with an economizer cycle.

SUMMARY OF THE INVENTION

These and other needs are met by the present invention, which provides methods and apparatus for operating air conditioning systems with an economizer cycle, (or so-called “vapor injection cycle”). In particular, the present invention provides for operating a rooftop air conditioning system or unit with an economizer cycle under certain conditions (e.g., in certain temperature ranges and/or using certain refrigerants) so as to achieve performance-related benefits with regard to at least the capacity and/or the energy efficiency ratio of the air conditioning system as compared to an air conditioning system that is operated utilizing a conventional (i.e., non-economized) cycle. These benefits are especially important because they are achieved while complying with all applicable legislative and industrial regulations.
In accordance with an exemplary aspect of the present invention, such benefits occur when a rooftop air conditioning unit is operated with an economizer cycle using R410A or a similar blend as a refrigerant (i.e., working medium), and/or wherein the rooftop system is operated in an outdoor setting having an ambient temperature at or above the standard ARI rating/point—for certain equipment—of 95° F.
In accordance with another exemplary aspect of the present invention in which R410A or a similar composition blend is utilized as the refrigerant for the rooftop air conditioning system, the blend can be comprised of about 47% to about 53% of R32 refrigerant and about 53% to about 47% of R125 refrigerant.
In accordance with yet another exemplary aspect of the present invention, the refrigerant blend for the rooftop air conditioning system can further include other additives such as oils (e.g., polyolester oils, polyvinylether oils, mineral oils, Alkyl Benzene oils, and combinations of one or more of these and/or other oils) and/or lubrication enhancement additives, wherein small amounts of the additive(s) are circulated within the air conditioning system along with the refrigerant blend.
Still other aspects, embodiments and advantages of the present invention are discussed in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying figures, wherein like reference characters denote corresponding parts throughout the views, and in which:

FIG. 1 is a schematic view of an economizer cycle for an air conditioning system;

FIG. 2 is a schematic view of an alternative economizer cycle for an air conditioning system;

FIG. 3 is a graph depicting the absolute pressure versus specific enthalpy (i.e., a P-h diagram) for the economizer cycles of FIGS. 1 and 2;

FIG. 4 is a graph depicting the comparatively beneficial results relating to capacity that were obtained when operating a rooftop air conditioning system with an economizer cycle using an R410A refrigerant; and

FIG. 5 is a graph depicting the comparatively beneficial results relating to energy efficiency ratio that were obtained when operating a rooftop air conditioning system with an economizer cycle using an R410A refrigerant.

DETAILED DESCRIPTION

The present invention provides methods and apparatus for operating a rooftop air conditioning system with an economizer cycle. As noted above, those in the air conditioning field had largely written off the possibility of operating an air conditioning system with an economizer cycle within the confines of legislation and industrial regulations yet so as to receive performance-related benefits that would not be more than offset by the added costs and complexities of running the system. However, in accordance with the present invention it was unexpectedly discovered that operating packaged air conditioning systems (e.g., rooftop air conditioning systems) with an economizer cycle under certain operation conditions (e.g., in certain temperature ranges and/or using certain refrigerants) results in measurable performance-related benefits while complying with applicable legislative and/or industrial regulations.
An exemplary economizer vapor injection cycle 100 for an air conditioning system is depicted in FIG. 1. In accordance with the economizer vapor injection cycle (“economizer cycle”) 100, a compressor 10 delivers high pressure refrigerant to a discharge line 20 and then to a condenser 30. The refrigerant exits the condenser through a liquid line and is split between a main flow line 40 and an auxiliary flow line 50. Although the percentages of refrigerant that are routed to the main flow line 40 and to the auxiliary flow line 50 can vary, it is currently preferred that between about 8% and about 12% by weight of the total refrigerant flow be fed to the auxiliary flow line 50, wherein the balance of the refrigerant is routed to the main flow line 40.
From the main flow line 40, refrigerant is fed through an economizer heat exchanger 95 to a main expansion device 60, then to an evaporator 70, and finally back to the compressor 10. From the auxiliary flow line 50, auxiliary refrigerant flow is fed to the economizer expansion device 90 (which reduces the pressure and temperature of the auxiliary refrigerant as compared to the pressure and temperature of the refrigerant in the main flow line 40) and to the economizer heat exchanger 95 in a predetermined manner, preferably in a counter-flow configuration with respect to the main refrigerant flow. The auxiliary refrigerant flow is then fed back to the compressor 10 at an intermediate (i.e., between suction and discharge) pressure. A bypass valve 80 is present to allow a portion of partially compressed refrigerant to flow back to compressor suction (e.g., in a conventional/non-economized mode of operation) should there be a desire to unload the compressor.
The temperature difference between the main refrigerant and the auxiliary refrigerant can vary, and is dependent on system design and operating conditions; however, according to a currently preferred embodiment of the present invention, the auxiliary refrigerant will have a temperature of about 25° F. to about 40° F. less than that of the main refrigerant, wherein about 15° F. to about 35° F. of the extra temperature reduction is obtained due to heat transfer interaction between the main and auxiliary refrigerant flows in the economizer heat exchanger 95.
Thus, the economizer cycle 100 is beneficial because it causes a certain percentage (e.g., about 88% to about 92%) of refrigerant to be further subcooled (e.g., by about 15° F. to about 35° F.) to a temperature lower than the temperature that would be achieved if the air conditioning system of FIG. 1 were to be operated in a conventional (i.e., non-economized) cycle. As a result, the refrigerant will have a greater cooling potential while reaching evaporator 70.
FIG. 2 depicts an air conditioning system that operates with an alternate economizer cycle 100A. The FIG. 2 economizer cycle 100A is identical to the FIG. 1 cycle 100, with the exception that the auxiliary flow in the FIG. 2 cycle is originated downstream of the economizer heat exchanger 95, rather than upstream as it is in the FIG. 1 cycle.
FIG. 3 is a P-h diagram for the economizer cycle 100 of FIG. 1 and the economizer cycle 100A of FIG. 2, wherein the points 1, 2, 3, 4, 5, 6, 7, 7′ and 7 in the FIG. 3 diagram correspond to those same labeled points within the economizer cycles of FIGS. 1 and 2.
Still further alternate embodiments of the economizer cycles 100, 100A of FIGS. 1 and 2 are within the scope of the present invention, including but not limited to those described in the U.S. Pat. No. 6,658,867 to Taras et al., the entirety of which is incorporated by reference herein. For example, either or both of the economizer cycles 100, 100A of FIGS. 1 and 2 can be modified to incorporate a tandem compressor arrangement, wherein at least two compressors 10 are arranged and operated in parallel in which one or more of the at least two compressors can be selectively started and stopped to provide part-load operation for the refrigerant systems depicted in FIGS. 1 and 2. Additionally, multi-circuit systems (i.e., systems with multiple independent circuits, as are known in the art) can benefit from the present invention. In such systems, multiple circuits can be operated to provide similar part-load capability, such as in the case of the tandem compressors, which are known in the art.
It has been unexpectedly discovered in accordance with the present invention that utilizing either of the economizer cycles 100, 100A of FIGS. 1 and 2 or any of those described in the '867 patent to Taras et al. in certain contexts can provide highly beneficial results with regard to air conditioning system performance, especially with regard to the capacity and the energy efficiency ratio (EER) of a rooftop air conditioning system. For purposes of the present invention a “rooftop air conditioning system” refers to a packaged air conditioning system (in contrast to what is referred to in the art as a “split air conditioning system”) that is sited above ground, e.g., on top of a building or structure. Also, for purposes of the present invention, an “economizer cycle” refers to the economizer cycle 100 of FIG. 1, the economizer cycle 100A of FIG. 2, one of the economizer cycles depicted and/or described in the '867 patent to Taras et al., or any other known economizer cycles.
For example, as shown in the data reflected in FIGS. 4 and 5, it was discovered through experimentation and modeling that utilizing R410A as a refrigerant with an economizer cycle for a rooftop air conditioning system provides certain performance-related benefits as compared to usage of R410A in a rooftop system employing a conventional (i.e., non-economized) cycle. FIG. 4 depicts a graph of the relative capacity (with respect to the R22 conventional cycle) versus ambient temperature results of modeling and experimental validation for a rooftop air conditioning system that was operated (a) with an economizer cycle using R410A as a refrigerant in accordance with the present invention (described on the graph as “R410A Economized”), and (b) with a conventional cycle using R410A as a refrigerant (described on the graph as “R410A Conventional”). Similarly, FIG. 5 depicts a graph of energy efficiency ratio (EER) versus ambient temperature results of modeling and experimental validation for a rooftop air conditioning system that was operated (a) with an economizer cycle using R410A as a refrigerant in accordance with the present invention (described on the graph as “R410A Economized”), and (b) with a conventional (i.e., non-economized) cycle using R410A as a refrigerant (described on the graph as “R410A Conventional”).
It should be noted that for the experiments reflected in both FIGS. 4 and 5, the equipment utilized to perform the “R410A Economized” and “R410 Conventional” testing typically was not identical, since the conventional system would include larger heat exchangers than the economizer system, in order to obtain performance parity at the ARI conditions of 80° F./67° F. indoor dry bulb/wet bulb temperatures and 95° F. ambient temperature. Upsizing heat exchangers is a typical measure taken by those in the art in an attempt to improve the performance (i.e., capacity and/or energy efficiency ratio) of an air conditioning system.
As shown in FIGS. 4 and 5, the capacity and the energy efficiency ratio of both the conventional system and the economizer system are substantially equal at the standard ARI rating/design point. These results indicate the benefits of the economizer system of the present invention, since it was able to achieve capacity and an energy efficiency ratio values comparable to those observed with respect to the conventional system at the standard ARI rating/design point of 80° F./67° F. indoor and 95° F. ambient temperatures despite typically not being equipped with larger heat exchangers. As used herein, the phrase “ambient temperature” refers to the outdoor temperature where a rooftop air conditioning system is sited, wherein such temperature can be (and typically is) effectively higher than the temperature reading that would be registered on a thermometer, e.g., due to a direct sunlight. Also, saturated discharge temperatures corresponding to specific ambient temperatures could be higher than expected due to the natural aging of the air conditioning equipment.
The results in FIGS. 4 and 5 also indicate that the benefits of the economizer system of the present invention become even more pronounced (versus the conventional system) as the systems are operated at ambient temperatures above the ARI rating/design point of 95° F. For example, the data reflected in FIG. 4 indicates that the capacity of a rooftop air conditioning system operated with a conventional cycle using R410A as a refrigerant begins to show a noticeable degradation (i.e., decrease) at temperatures above 95° F. despite being equipped with larger heat exchangers. Specifically, as shown in FIG. 4, the capacity degradation of the conventional system is already 9% (in comparison to an R22 system not equipped with enlarged heat exchangers) at 125° F. ambient temperature. In contrast, and as also shown in FIG. 4, when the rooftop air conditioning system was operated in accordance with the present invention with an economizer cycle using the same R410A blend as the refrigerant, the system exhibited a much less precipitous capacity degradation, e.g., only 5% at 125° F. ambient temperature.
Similarly beneficial results with respect to the energy efficiency ratio (EER) are demonstrated with reference to FIG. 5—that is, there was a measurable benefit achieved though use of refrigerant blends such as R410A with an economizer cycle for a rooftop air conditioning system above 95° F. in accordance with the present invention. Specifically, a rooftop air conditioning system operated with a conventional cycle using R410A as a refrigerant exhibits an energy efficiency ratio degradation (i.e., decrease) of 12% (once again, in comparison to an R22 system not equipped with enlarged heat exchangers) at 125° F. ambient outdoor temperature, despite of being equipped with larger heat exchangers, whereas a rooftop air conditioning system exhibits only a 5% energy efficiency ratio degradation at 125° F. outdoor ambient temperature when operated with an economizer cycle and utilizing R410A as the refrigerant in accordance with the present invention.
The comparative capacity and energy efficiency benefits shown in FIGS. 4 and 5 for the present invention are particularly significant because they occurred for ambient temperatures at and especially above 95°, which are routinely encountered at the top of a structure or building (e.g., a roof) during the daytime in certain hot, dry, populated climates (e.g., Nevada, Arizona, The Middle East), and which is where such benefits are most needed because an air conditioning systems are relied upon at such high temperatures to deliver as much cooling as possible.
Moreover, the fact that these beneficial results occurred with respect to a rooftop air conditioning system is also very important. In standard (i.e., “split” or residential) air conditioning systems, certain portions of the system (e.g., the condensing unit) are typically installed on the side of a structure, not atop the structure as they are for a rooftop system. Thus, rooftop air conditioning systems are exposed to significant additional heat loads not present in other (e.g., “split” or residential) air conditioning applications because the hot ambient air that blows over the evaporator and condenser coils is additionally preheated by hot rooftop surfaces exposed to direct sunlight, which also acts to provide extra radiant heat load by directly or indirectly (e.g., through conduction and convection) heating various components of the refrigerant system. Further, due to the location and conditions under which they are operated, rooftop air conditioning systems are vulnerable to edging and/or infrequent maintenance, both of which can cause the effective operation temperature of the system to be higher than usual.
Thus, it is very significant that air conditioning systems of the present invention can be operated with an economizer cycle under the demanding conditions of rooftop air conditioning systems and with environmentally friendly refrigerant blends such as R410A yet still exhibit comparatively less capacity and energy efficiency degradation than air conditioning systems that are operated with a conventional cycle and that are equipped with a larger heat exchanger.
In summary, significant capacity benefits (e.g., a lower capacity degradation) and energy efficiency benefits (e.g., a lower energy efficiency ratio degradation) are achieved when a rooftop air conditioning system is operated with an economizer cycle using R410A refrigerant as compared to the same system being operated with a conventional cycle and/or for standard (i.e., “split” or residential) air conditioning applications. Moreover, these benefits are especially pronounced when the system is operated under conditions wherein the ambient outdoor temperature is above 95° F., in the range of about 95° F. to 125° F., or above 125° F., and when extrapolated over the usable lifetime of the rooftop air conditioning system.
It should be noted that although R410A is generally a blend of 50% by weight of R32 refrigerant and 50% by weight of R125 refrigerant, any references to “R410A” herein should be interpreted to refer to a blend of between about 47% and about 53% (both inclusive) by weight of R32 and between about 53% and about 47% (both inclusive) by weight percentage of R125. Moreover, these ranges can be adjusted in accordance with the present invention, and/or some amounts of other refrigerants (e.g., R134a) can be added to the blend. In an embodiment wherein RI 34 a is added, it is currently preferred to add no more than 5% by weight thereof.
Further, one or more additives can be included in the R410A-like refrigerant blend in accordance with the present invention. Exemplary such additives include, but are not limited to oils (e.g., polyolester (POE) oils polyvinylether (PVE) oils, Alkyl Benzene oils, mineral oils, or a mixture or combination of one or more of these oils, wherein the viscosity grades of such oils can vary but are generally in the range of about 20 to about 70 centistokes and when the oil viscosity is measured without the refrigerant at temperature of 100 F.) and/or one or more lubrication enhancement additives known in the art. The additives can be added to the refrigerant blend as is generally known in the art, e.g., by being circulated within the air conditioning system along with the refrigerant.
Although the present invention has been described herein with reference to details of currently preferred embodiments, it is not intended that such details be regarded as limiting the scope of the invention, except as and to the extent that they are included in the following claims—that is, the foregoing description of the present invention is merely illustrative, and it should be understood that variations and modifications can be effected without departing from the scope or spirit of the invention as set forth in the following claims. Moreover, any document(s) mentioned herein are incorporated by reference in their entirety, as are any other documents that are referenced within the document(s) mentioned herein.

Claims

1. An air conditioning system, comprising:

a rooftop air conditioning unit operated with an economizer cycle, wherein the rooftop air conditioning unit utilizes at least one predetermined refrigerant as a working medium.

2. The air conditioning system of claim 1, wherein the at least one predetermined refrigerant is a blend of R32 refrigerant and R125 refrigerant.

3. The air conditioning system of claim 2, wherein the at least one predetermined refrigerant is a blend of about 47% to about 53% by weight of R32 refrigerant and about 53% to about 47% by weight of R125 refrigerant.

4. The air conditioning system of claim 1, wherein the at least one predetermined refrigerant includes at least one additive.

5. The air conditioning system of claim 4, wherein each of the at least one additive is an oil.

6. The air conditioning system of claim 5, wherein each of the at least one oil has a viscosity grade between about 20 and about 70 centistokes.

7. The air conditioning system of claim 5, wherein each of the at least one oil is selected from the group consisting of polyolester oil, polyvinylether oil, Alkyl Benzene oil, mineral oil, and a mixture of two or more thereof.

8. The air conditioning system of claim 4, wherein the at least one additive is an additional refrigerant.

9. The air conditioning system of claim 8, wherein the additional refrigerant is R134a.

10. The air conditioning system of claim 9, wherein the amount of R134a added to the system is up to and including 5% by weight.

11. The air conditioning system of claim 4, wherein the at least one additive is a lubrication enhancement additive.

12. A rooftop air conditioning system which is operated with an economizer cycle and which uses a working medium comprised of a mixed refrigerant of R32 and R125.

13. The rooftop air conditioning system of claim 12, wherein the working medium is comprised of 47% to about 53% by weight of R32 refrigerant and about 53% to about 47% by weight of R125 refrigerant.

14. The rooftop air conditioning system of claim 12, wherein the working medium includes at least one additive.

15. The rooftop air conditioning system of claim 14, wherein each of the at least one additive is an oil.

16. The rooftop air conditioning system of claim 15, wherein each of the at least one oil has a viscosity grade between about 20 and about 70 centistokes.

17. The air conditioning system of claim 15, wherein each of the at least one oil is selected from the group consisting of polyolester oil, polyvinylether oil, Alkyl Benzene oil, mineral oil, and a mixture of two or more thereof.

18. The air conditioning system of claim 14, wherein the at least one additive is an additional refrigerant.

19. The air conditioning system of claim 18, wherein the additional refrigerant is R134a.

20. The air conditioning system of claim 19, wherein the amount of R134a added to the system is up to and including 5% by weight.

21. The air conditioning system of claim 14, wherein the at least one additive is a lubrication enhancement additive.

22. A method of improving the performance of a rooftop air conditioning system, comprising the steps of:

providing a rooftop air conditioning system; and

operating the rooftop air conditioning system with an economizer cycle.

23. The method of claim 22, further comprising the step of:

utilizing a mixture of R32 and R125 refrigerants as a working medium for the system.

24. The method of claim 23, wherein the working medium includes at least one additive.

25. The method of claim 24, wherein each of the at least one additive is an oil.

26. The method of claim 25, wherein each of the at least one oil has a viscosity grade between about 20 and about 70 centistokes.

27. The method of claim 25, wherein each of the at least one oil is selected from the group consisting of polyolester oil, polyvinylether oil, Alkyl Benzene oil, mineral oil, and a mixture of two or more thereof.

28. The air conditioning system of claim 24, wherein the at least one additive is an additional refrigerant.

29. The air conditioning system of claim 28, wherein the additional refrigerant is R134a.

30. The air conditioning system of claim 29, wherein the amount of R134a added to the system is up to and including 5% by weight.

31. The air conditioning system of claim 24, wherein the at least one additive is a lubrication enhancement additive.

32. The method of claim 22, wherein the step of operating the rooftop air conditioning system is performed in an outdoor ambient temperature above 95° F.

33. The method of claim 22, wherein the step of operating the rooftop air conditioning system is performed in an outdoor ambient temperature between about 95° F. and about 125°.

34. A rooftop air conditioning system, comprising:

a compressor;

a condenser in communication with the compressor via at least a first refrigerant line;

a first expansion device in communication with the condenser via at least a second refrigerant line;

an evaporator in communication with the first expansion device via at least a third refrigerant line and in communication with the compressor via at least a fourth refrigerant line,

a second expansion device in communication with the condenser via at least a fifth refrigerant line; and

a heat exchanger in communication with the second expansion device via at least a sixth refrigerant line and in communication with the compressor via at least a seventh refrigerant line;

wherein the rooftop air conditioning system is operated with an economizer cycle and utilizes at least one predetermined refrigerant as a working medium.

35. The rooftop air conditioning system of claim 34, wherein the system comprises at least two compressors.

36. The rooftop air conditioning system of claim 35, wherein at least two compressors are tandem compressors.

37. The rooftop air conditioning system of claim 34, wherein the system is a multi-circuit system.

38. The air conditioning system of claim 34, wherein the at least one predetermined refrigerant is a blend of R32 refrigerant and R125 refrigerant.

39. The air conditioning system of claim 38, wherein the at least one predetermined refrigerant is a blend of about 47% to about 53% by weight of R32 refrigerant and about 53% to about 47% by weight of R125 refrigerant.

40. The air conditioning system of claim 34, wherein the at least one predetermined refrigerant includes at least one additive.

41. The air conditioning system of claim 40, wherein each of the at least one additive is an oil.

42. The air conditioning system of claim 41, wherein each of the at least one oil has a viscosity grade between about 20 and about 70 centistokes.

43. The air conditioning system of claim 41, wherein each of the at least one oil is selected from the group consisting of polyolester oil, polyvinylether oil, Alkyl Benzene oil, mineral oil, and a mixture of two or more thereof.

44. The air conditioning system of claim 40, wherein the at least one additive is an additional refrigerant.

45. The air conditioning system of claim 44, wherein the additional refrigerant is R134a.

46. The air conditioning system of claim 45, wherein the amount of R134a added to the system is up to and including 5% by weight.

47. The air conditioning system of claim 40, wherein the at least one additive is a lubrication enhancement additive.