Patent Application
20130086937
The present invention relates to refrigeration chillers, particularly to the circulation of compressor lubricant trapped in the evaporator. Oil migration from the chiller compressor to the evaporator is a very old problem and occurs in virtually all compressor-driven chiller systems. Without proper lubrication, the compressor must be shut down to avoid excessive wear. This is especially true for chillers using screw compressors which require large amounts of oil to lubricate, seal, and cool the compressor.
Numerous lubricant circulation techniques exist for recovering oil entrained in the chiller's refrigerant charge. Oil separators placed in the refrigeration circuit remove all but a tiny fraction of the oil in the refrigerant but cannot capture all of it. The uncaptured compressor lubricant migrates into the evaporator where it rests in the refrigerant charge as an oily liquid that must be circulated back to the compressor by other means. Eductors are commonly used for this purpose and are often part of a factory installed oil return system. In this application of an eductor, a high pressure stream of refrigerant gas flows through the eductor nozzle drawing the oily liquid from the evaporator back into the compressor. In this way, an eductor can recover oil effectively provided the pressure differential between the compressor inlet and compressor discharge is sufficiently high. This pressure differential (compressor “lift”) is proportional to the cooling load on the chiller and is highest when the chiller is under heavy load.
However, relying on compressor lift to recover compressor lubricant often causes increased oil migration with decreased circulation and recovery. Chiller compressors often operate at low lift for extended periods resulting in further pressure reductions in the eductor and reduced oil circulation from the evaporator back to the compressor. These reductions in compressor lift can also cause oil to slide down out of the compressor inlet into the evaporator. As a result, with the exception of the most recent chiller designs, rotary screw compressors cannot operate below 25 to 30% of their rated capacity for an extended period of time without experiencing excessive oil loss. When compressor lift is too low to properly circulate oil, oil migration continues until either the load on the chiller increases thereby increasing compressor lift, or the chiller shuts down due to low oil volume to avoid harming the compressor. When the chiller shuts down for lack of oil, it is then very difficult to recover oil from the refrigerant charge in the evaporator.
Adding extra oil to the compressor oil reservoir is also not always possible either. First of all, as lubricant builds up in the evaporator, the evaporator's performance efficiency is degraded. Secondly, excessive foaming can occur when the chiller is started which causes liquid refrigerant to be carried into the compressor, reducing the compressor discharge temperature. As the compressor discharge temperature approaches the condenser saturation temperature, the affinity of the refrigerant gas to absorb oil can increase to a point where oil can no longer be separated from the refrigerant.
There remains, then, a need for a reliable cost efficient system for continuously circulating compressor lubricant from the chiller refrigerant under all operational loads that can be added relatively quickly to a wide range of existing chillers without expensive modifications.
The present invention addresses the concerns mentioned above as well as others by providing a system for circulating lubricant from the evaporator to the compressor of a refrigeration chiller that operates regardless of compressor lift and can be added to an existing chiller with minimal modifications.
The invention adds a second pump which operates in tandem with the chiller's existing oil circulation system to introduce a discharge of oily liquid from the evaporator to the compressor. Both pumps take the same oily liquid from the evaporator and pump it to a discharge near the compressor suction inlet. This discharge can be a discharge common to both pumps, or a separate discharge for each. In one embodiment of the invention, the existing oil circulation system relies on a factory installed eductor pump while the second pump is a magnetically coupled gear pump coupled to an atomizing nozzle positioned near the compressor suction inlet. The second pump sprays a fine mist of oily liquid from the nozzle near the compressor suction inlet that is sucked into the compressor and circulated back through the compressor regardless of compressor load. The invention also includes automatic controls to operate the second pump as a backup only when the primary oil circulation system fails or only when a low lift situation develops.
The present invention also provides a relatively easy and inexpensive method to retrofit the invention to existing refrigeration chillers. By obtaining the second pump and coupling it to the evaporator and a discharge near the compressor suction inlet, the chiller is inexpensively readied to take advantage of the benefits of the invention.
Various forms, objects, features, additional aspects, advantages, and embodiments of the present invention will become apparent to those of ordinary skill in the art from the following detailed description when read in light of the accompanying drawing.
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. One embodiment of the invention is shown in great detail, although it will be apparent to those skilled in the relevant art that some features that are not relevant to the present invention may not be shown for the sake of clarity.
The refrigeration chiller at 10 includes a compressor 12, an oil separator 16, a condenser 19, an expansion valve 24, and an evaporator 26 coupled together to form a complete refrigeration circuit. The refrigeration cycle begins when compressor 12 compresses a refrigerant gas which is delivered to condenser 19 through the oil separator 16. Inside condenser 19, the refrigerant gas cools as it condenses on the exterior surfaces of tube bundle 21 and falls to the bottom of condenser 19 as a liquid. A cooling medium flowing through tube bundle 21 carries away heat from the refrigerant gas which is cooled by other means to allow this condensation to occur continuously inside condenser 19.
After condensing to a liquid, refrigerant flows from condenser 19 to expansion valve 24. The pressure on the condenser side of expansion valve 24 is much higher than on the evaporator side. Because of this, refrigerant passing through expansion valve 24 expands and cools changing from a relatively warm liquid to a much cooler two phase liquid and gas mixture as it flows into evaporator 26. The cooler liquid refrigerant phase flowing into evaporator 26 then contacts tube bundle 28 through which a second external cooling medium flows. As the cool liquid refrigerant phase contacts the exterior walls of the tubes in tube bundle 28, heat from the cooling medium flowing inside the tubes warms the liquid refrigerant causing it to vaporize. The cooling medium then leaves tube bundle 28 at a lower temperature to cool an external heat load while the now vaporized refrigerant gas rises to the upper interior portions of evaporator 26. Evaporator 26 in
Various types of compressors are used to compress refrigerant gas in a refrigeration chiller. One embodiment is the very common screw-type compressor which operates by rotating screw rotors within a working chamber. In this embodiment of compressor 12, relatively large quantities of oil are required to lubricate, cool, and seal the compressor. This means compressor lubricant is in direct contact with hot refrigerant gas under high pressure which results in significant quantities of oil being dissolved in the refrigerant, or refrigerant dissolved in the oil, or both. Furthermore, some oil is physically picked up with the high velocity refrigerant exiting the compressor discharge as well. Other embodiments of compressor 12 are envisioned which may operate differently but suffer the same loss of significant quantities of compressor lubricant into the refrigerant charge during compression.
To recapture lubricant lost from compressor 12, oil separator 16 is placed between compressor 12 and condenser 19. Most embodiments of oil separator 16 used in refrigerant chillers are very efficient and will capture all but a very small portion of the oil combined with the refrigerant moving from compressor 12 through oil separator 16. However, it is not uncommon for a small amount of oil to be carried through oil separator 16 with the refrigerant gas and into condenser 19.
Because of the relatively high pressures and temperatures, lubricant does not rest long in the condenser, but migrates into evaporator 26 where it resides as an oily liquid having a higher concentration of oil than the liquid refrigerant phase in evaporator 26. As previously noted, this oily liquid is lubricant in various forms that the oil separator 16 failed to capture and includes oil, refrigerant dissolved in oil, oil dissolved in refrigerant, or combinations thereof. Depending on the properties of the liquid refrigerant phase in evaporator 26, the oily liquid may separate to the top of the liquid refrigerant phase, or it may collect at the bottom of it. In some cases and at various times, the oily liquid may develop into a foam on the surface of the liquid refrigerant phase, or be more dispersed in some areas of evaporator 26 than others depending on the unique situation present. Regardless of precisely how it is dispersed, unless the oily liquid in evaporator 26 is timely returned to compressor 12, the entire supply of compressor lubricant will eventually migrate to evaporator 26 resulting in negative outcomes such as reduced efficiency, damage to compressor 12, or premature shutdown of refrigeration chiller 10.
Various methods have been devised for capturing and circulating the oily liquid back to the compressor. Many oil circulation systems installed as original equipment on chillers rely on eductors to perform this task. Eductors, also known as “jet pumps” or “venturi pumps,” move the oily liquid using a high rate of flow of refrigerant vapors past an opening that is coupled to the evaporator. The high rate of flow creates a vacuum that draws the oily liquid from the evaporator.
However, in some situations, eductor pump 31 will not pump enough oily liquid from evaporator 26 to avoid excessive oil loss. If high pressure line 36 does not carry enough pressure to move vapors fast enough past the opening in eductor pump 31 to create enough vacuum to draw oily liquid up suction line 43, insufficient lubricant will circulate back to compressor 12. This can occur in various situations, most notably, when the chiller is operating at low load. As the heat load begins to drop, refrigeration chiller 10 responds by reducing the discharge pressure of compressor 12. Reduced load results in reduced “lift,” or a narrowing of the difference between the compressor suction and discharge pressures. As compressor 12 begins to reduce its discharge pressure and lift is reduced, the flow of vapors motivating eductor pump 31 to move oily liquid out of evaporator 26 is also reduced. If lift is reduced too far, eductor pump 31 will cease to adequately circulate lubricant, and oil will collect in evaporator 26 with inadequate quantities returning to compressor 12 until lift is increased.
The present invention addresses the problem of excessive oil loss due to low compressor lift by adding a second pump, shown in
The addition of a second pump as envisioned by the invention offers numerous opportunities for enhanced lubricant circulation. In one embodiment of the invention, mechanical pump 50 is added to an existing chiller installation and connected directly to the same power source (not shown) as refrigeration chiller 10 and operates whenever the chiller is operating. In this embodiment, both eductor pump 31 and mechanical pump 50 operate simultaneously at all times to achieve a constant flow of oily liquid back to the compressor regardless of chiller load and corresponding compressor lift. Even if eductor pump 31 ceases to be effective for any reason, mechanical pump 50 continues to return oily liquid to compressor 12 as the mist continuously spraying from nozzle 55 near the compressor suction inlet 29 is constantly drawn into compressor 12.
It is important to note several alternate embodiments from the depictions in
Other configurations of mechanical pump 50 are envisioned as well. In one such embodiment of the invention, mechanical pump 50 is added to an existing chiller installation and operated as a backup when compressor lift drops below a minimum preset threshold to avoid excessive oil loss. In this embodiment, mechanical pump 50 is wired into the chiller's existing control system 69 via pump control cable 54 such that when compressor lift drops too low, mechanical pump 50 is activated. When the low lift condition ceases, mechanical pump 50 is deactivated. In another embodiment of the invention, mechanical pump 50 is installed as original equipment to operate simultaneously along with eductor pump 31. These and other embodiments are meant as merely examples of numerous ways in which a mechanical pump system could be used to augment or replace an eductor pump system as a simple and efficient system for circulating oil to prevent excessive oil loss. Although the operational requirements will dictate the optimum arrangement of pumps, suction lines, nozzles, and other elements of the invention, the required circulation of compressor lubricant from the evaporator to the compressor is preferably achieved by pumping the oily liquid from evaporator 26 to a discharge near compressor suction inlet 29.
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only one embodiment has been shown and described and that all changes, equivalents, and modifications that come within the spirit of the inventions defined by following claims are desired to be protected.
