APPARATUS, SYSTEMS AND METHODS FOR LUBRICATION OF FLUID DISPLACEMENT MACHINES

Abstract

Apparatuses, systems, and methods are provided for the lubrication of fluid displacement machines, in particular positive displacement machines such as twin screw expanders utilized in organic Rankine cycle systems, comprising a working fluid mixed with a lubricant that is neither soluble nor miscible in the liquid phase of the working fluid.

Claims

1. A system for lubricating a fluid displacement machine, the system comprising: A. a fluid; B. a fluid displacement machine comprising at least one fluid input and at least one fluid output; C. at least one lubricant that is neither soluble or miscible with the fluid, said lubricant combined with the fluid to form a colloidal fluid mixture flowing through the machine from the at least one fluid input to the at least one fluid output; D. at least one mixture extraction point at which a portion of the mixture is extracted for lubrication purposes; and E. at least one desired point of lubrication in the machine in mixture receiving communication with the mixture extraction point.

2. The system of claim 1 wherein the system pressure at the at least one mixture extraction point is sufficiently higher than the system pressure at the at least one desired point of lubrication such that mixture flows from the at least one mixture extraction point to the at least one desired point of lubrication.

3. The system of claim 1 further comprising at least one lubricant filter disposed between the at least one mixture extraction point and the at least one desired point of lubrication.

4. The system of claim 1 wherein a lubrication pump is disposed between the at least one mixture extraction point and the at least one desired point of lubrication.

5. The system of claim 1 wherein the machine is a screw expander.

6. The system of claim 1 wherein the fluid comprises an HFC refrigerant and the lubricant comprises at least one of a mineral oil, an alkylbenzene oil, or a solid lubricant additive compound held in colloidal suspension.

7. The system of claim 6 wherein the HFC refrigerant comprises R-245fa refrigerant.

8. The system of claim 1 wherein agitation of the mixture is provided by the flow of said mixture through the system.

9. The system of claim 1 wherein agitation of the mixture is provided by at least one of any of fixed vanes, rotating devices, stirrers, circulators, circulation pumps, mixers, and injection jets.

10. The system of claim 1 further comprising a system pump in mixture receiving communication with the at least one fluid output and in mixture sending communication with the at least one fluid input, and wherein at least one mixture extraction point is in mixture receiving communication with the output of the system pump.

11. The system of claim 1 further comprising at least one mixture reservoir or receiver wherein the at least one mixture extraction point is in mixture receiving communication with at least one point in the at least one mixture reservoir or receiver.

12. A method of lubricating a fluid displacement machine comprising: A. providing a fluid; B. providing an apparatus comprising at least one machine which acts upon, or is acted upon by, the fluid as it passes through the machine; B. providing at least one lubricant that is neither soluble or miscible with the fluid; C. combining the fluid and the lubricant to form a colloidal fluid mixture; D. circulating the mixture through the apparatus and the at least one machine; and E. extracting a portion of the mixture and using it to lubricate at least one point in the machine.

13. The method of claim 12 wherein step (E) further comprises passing the extracted portion of the mixture through at least one lubricant filter.

14. The method of claim 12 wherein the fluid comprises an HFC refrigerant and the lubricant comprises at least one of a mineral oil, an alkylbenzene oil, or a solid lubricant additive compound held in colloidal suspension.

15. The method of claim 14 wherein the HFC refrigerant comprises R-245fa refrigerant.

16. A lubricating mixture for use with a fluid displacement machine, the mixture comprising: A. a fluid; and B. at least one lubricant that is neither soluble nor miscible with the fluid, said lubricant combined with the fluid to form a colloidal mixture (i) suitable for lubricating a fluid displacement machine when the lubricant is dispersed within the mixture and (ii) wherein said lubricant does not remain dispersed in the mixture in the absence of agitation.

17. The mixture of claim 16 wherein the machine is a screw expander.

18. The mixture of claim 16 wherein the fluid comprises an HFC refrigerant and the lubricant comprises at least one of a mineral oil, an alkylbenzene oil, or a solid lubricant additive compound held in colloidal suspension.

19. The mixture of claim 18 wherein the HFC refrigerant comprises R-245fa refrigerant.

20. The mixture of claim 16 wherein the mixture flowing through the machine comprises between 1% and 3% lubricant by mass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Without limiting the invention to the features and embodiments depicted, certain aspects this disclosure, including the preferred embodiment, are described in association with the appended figures in which;

[0043] FIG. 1 is a block diagram of a prior art ORC system used to convert heat energy into electric power;

[0044] FIG. 2 is a block diagram of an ORC system used with this invention depicting a lubrication feed system from the outlet of the system pump to the positive displacement machine;

[0045] FIG. 3 is a graph that depicts the concentration of non-soluble immiscible lubricant present at the outlet of a system pump as a function of time after startup; and

[0046] FIG. 4 is cross sectional side view of a receiver tank in an ORC system depicting the stratification of the mixture of working fluid and non-soluble immiscible lubricant.

DETAILED DESCRIPTION

[0047] FIG. 2 depicts an ORC system configuration suitable for use with the present invention. Here, lubrication line 108 is operatively connected between the output of system pump 105 and one or more points requiring lubrication in positive displacement machine 102. In some embodiments, these points are bearing housings within which one or more ball, roller, sleeve, or other configuration of bearings are housed. The flow of WF/NSIL mixture under positive pressure from the system pump 105, which may be controlled by a microprocessor-directed variable frequency drive (VFD) system, provides a stream of lubricating mixture to the bearings and/or other lubrication points. While the WF/NSIL mixture may be extracted from any convenient or desired point in the system, the output of system pump 105 is a particularly advantageous point of extraction for several reasons. It is the point of greatest positive pressure of any WF/NSIL mixture location in the ORC system, as system pump 105 is the sole source of such motive pressure for the WF/NSIL mixture in the particular system depicted. No additional pressure-inducing components are required if a small portion of the positive pressure generated by system pump 105 is used to supply a stream of WF/NSIL mixture for lubrication purposes.

[0048] Another significant advantage of obtaining WF/NSIL mixture for lubrication purposes at the output of system pump 105 is that this point also presents the lowest temperature WF/NSIL mixture anywhere in the ORC system. The WF/NSIL mixture at this point has been fully condensed and will provide the maximum heat dissipation when applied to the bearings or other lubrication points at the machine. While the use of warmer mixture may be acceptable or even desired in some embodiments, the cooler lubrication source is often preferred.

[0049] Regardless of the preferred source of WF/NSIL mixture used for lubrication, the flow rate may be controlled by one or more valves or other flow control devices so as to achieve the desired flow rate. This is particularly useful when the WF/NSIL mixture is obtained at the output of system pump 105, as the speed of the VFD-controlled pump is dictated by the larger operational requirements of the ORC system and cannot be varied to accommodate lubrication concerns. In cases where a dedicated supplemental lubrication pump is employed for provide adequate pressure for the lubrication feed, the flow of WF/NSIL mixture to the bearings or other points of lubrication may be controlled in whole or in part by controlling the operation of said dedicated supplemental pump in place of, or in combination with, suitable valves or other flow control devices.

[0050] The choice of lubricant to be mixed with the chosen working fluid is critical. There are a wide variety of working fluids suitable for use in the many applications to which this disclosure applies. The essential characteristic of the WF/NSIL mixture of this invention is that the working fluid and the non-soluble immiscible lubricant form a colloidal mixture rather than a homogenous, uniform solution. By way of illustration and not limitation, examples will be provided using preferred ORC systems. The same principles apply to other applications when appropriately adjusted for their specific requirements. Some ORC systems utilize water, vaporized into steam by the input heat, as a working fluid. For those systems, a wide variety of oils and other lubricants not soluble in water may be appropriate for use, potentially including petroleum-based lubricants. Many ORC systems utilize refrigerants, including but not limited to organic refrigerants, in lieu of water as a working fluid. The complex chemical composition of refrigerants is an area of active development driven in large measure by concerns surrounding the potential effect of legacy refrigerants on the environment. As another non-limiting example, the refrigerant discussed above (R-245fa) is classified as a hydrofluorocarbon (HFC) compound and lacks the chlorine component of the earlier generation of chlorofluorocarbons (CFCs), such as R-12, as well as the later generation of hydrochlorofluorocarbon (HCFC) refrigerants, such as R-22, both now deprecated since being deemed environmentally undesirable. Due to their different compositions, certain lubricants soluble or miscible in chlorinated refrigerants are not similarly soluble or miscible in non-chlorinated refrigerants, including but not limited to HFCs such as R-245fa.

[0051] An essential element of this invention is the non-soluble immiscible character of the WF/NSIL mixture. It is not sufficient to identify a fluid and a lubricant independently of this requirement. Due to the differences in composition of both components, each must be carefully selected in full consideration of the characteristics of the other. In the embodiment described above, one such combination experimentally and operationally verified to produce the desired non-soluble immiscible WF/NSIL mixture consistent with this disclosure is the refrigerant R-245fa and mineral oil or its closely-related synthetic alternatives such as alkylbenzene oil. This example is illustrative of one preferred embodiment and is not limiting upon the scope of this invention in any way, as it is believed that numerous other combinations of fluids (refrigerants and non-refrigerants) and lubricants may be used to comprise an appropriate colloidal mixture for a wide variety of applications consistent with this disclosure.

[0052] Because the WF/NSIL mixture is colloidal in nature, it is by definition non-uniform at the microscopic level and for a certain sample range above that. Unlike soluble or miscible compositions where the components in a homogenous mixture may be difficult or even impossible to separate without elaborate processing, the colloidal WF/NSIL mixture is self-separating. Even with extreme agitation, visual inspection of the WF/NSIL mixture reveals the presence of NSIL droplets (as the discontinuous phase) distributed throughout the working fluid (as the continuous phase). The NSIL droplets constantly seek to combine with each other, forming larger droplets that collect on the upper layers of any accumulation of WF/NSIL mixture at rest as they are displaced in the mixture by the working fluid of greater specific gravity settling to the lower layers due to gravitational force.

[0053] With regard to any assessment of the composition of the colloidal WF/NSIL mixture, it must be understood that determination of the proportional composition of the colloidal WF/NSIL mixture requires a sample of appropriate size for the purpose at hand. By way of example and not limitation, a sample size of 5 mL or less may be optimal for the purpose of characterizing a WF/NSIL mixture at rest that has essentially separated into strata when the task at hand is to determine the boundaries of such strata as precisely as possible. When assessing the overall composition of a colloidal WF/NSIL mixture that is only slightly more agitated than in its fully separated state, a 5 mL sample taken at a particular location may be highly misleading due to the lack of uniformity in the WF/NSIL mixture. Instead, a sample between 100 and 500 mL, or greater, may be advisable. In circumstances involving a highly agitated and well-dispersed colloidal WF/NSIL mixture, a sample size of between 10 and 50 mL may suffice to accurately determine its proportional composition. All discussions herein regarding the proportional composition of a WF/NSIL mixture are predicated on the basis that such composition is based a suitable sample size for the state of dispersion of NSIL within the WF/NSIL mixture, as such state will vary greatly throughout the system as discussed below.

[0054] The time-dependent variation in the relative concentration of NSIL in the WF/NSIL mixture should understood to be a function of many characteristics of the materials and the system within which the WF/NSIL mixture circulates in a closed loop. Factors which affect the time-dependent concentration of NSIL in the WF/NSIL mixture include, but are not limited to, a) the time-dependent propensity for the WF/NSIL mixture to separate while at rest, b) the amount of time that has lapsed since the ORC system's last shutdown and/or the state of the WF/NSIL mixture at commencement of operation, c) the physical operating constants of the system, such as mass flow rate of the WF/NSIL mixture, the capacity of any WF/NSIL mixture receiver or storage tanks, temperature and pressure of the WF/NSIL mixture at any point, and the like, 4) the absence or presence of any mechanical or other agitation that would affect the time required for the WF/NSIL mixture to reach its optimum state of lubrication equilibrium, 5) sheer randomness in location and/or other factors under which the WF/NSIL mixture separates, and 6) any other factors that would enhance or retard the process of attaining an optimal WF/NSIL mixture. The tendency of the unstable colloidal WF/NSIL mixture to naturally separate on its own when the mixture is at rest and not subject to agitation (listed as factor (a) above) is a characteristic of the properties of the working fluid component(s) and non-soluble immiscible lubricant component(s) of the WF/NSIL mixture and is largely independent of the system in which the WF/NSIL mixture is utilized.

[0055] The degree of dispersion of NSIL in the WF/NSIL mixture is of interest only at certain points in the system. One such point is the location in the system where a portion of the mixture is extracted for application at desired points of lubrication. It important that the mixture obtained for direct injection lubrication contain the desired quantity of NSIL lubricant. Extracting lubricant-depleted mixture for lubrication purposes, particularly when done unintentionally, would jeopardize the operation of the machine. As described above, extracting a portion of the WF/NSIL mixture at the output of the system pump would be preferred in some embodiments. At this point in the system, having just been churned by the pump's impellers, the mixture would be relatively homogeneous and well-dispersed, and if the input flow to the system pump contained an appropriate concentration of lubricant, the portion extracted for lubrication purposes would likewise contain an appropriate concentration of NSIL evenly dispersed within the output flow of the system pump. In another embodiment, the mixture extracted for lubrication injection may be taken from a reservoir or receiver where the mixture has been allowed to rest relatively undisturbed for a period of time. Due to the self-separating nature of the colloidal mixture, the location of the extraction point within the reservoir or receiver tank will largely determine the concentration of lubricant in the extracted mixture. As described elsewhere herein, extracting fluid from the upper strata of separated mixture will yield a much higher concentration of lubricant than if the sample is extracted near the bottom of the tank. At certain points in the system, the relative concentration of lubricant in the WF/NSIL mixture is not critical to the operation of the system, although due to the closed-loop circulating nature of the system, the relative proportion of working fluid and lubricant(s) will generally be constant on the whole for a similar and appropriate sample size obtained between the source point and the exit point if a similar degree of agitation is maintained for the mixture.

[0056] In FIG. 3, empirical test data related to the variation in NSIL concentration as a function of time after startup of an ORC system is depicted. In this series of measurements, the total WF/NSIL mixture contained in the closed-loop of an ORC system was 5.8% NSIL by mass (depicted by curve 301). For each trial, WF/NSIL mixture samples of sufficient quantity were collected at the output of system pump 105 in an ORC system configuration similar to that depicted in FIG. 2. The machine was started and the proportional composition of the WF/NSIL mixture was measured at the start, at 10 minute increments for the first 30 minutes of operation, and again after 60 minutes of operation. Following collection of the data, the ORC system was stopped and the WF/NSIL mixture in the closed-loop system was allowed to rest without movement or agitation until it was believed to have reached its naturally quiescent state.

[0057] Curve 302 represents the data associated with the iteration with the maximum observed concentration of NSIL at the start, curve 304 represents the same data for the iteration with the lowest observed concentration at the start, and curve 303 represents the average (mean) data for all test iterations performed. It can be seen that the starting values varied widely over a range of almost 3:1. This variation is attributable to the fact that the WF/NSIL mixture readily separates when the ORC system is stopped and the data provides insight that the separation of the WF/NSIL mixture within the closed-loop circuit has at some degree of randomness and therefore is not a highly repeatable or predictable phenomenon.

[0058] A particularly valuable conclusion that may be drawn from the data is that regardless of the starting concentration of NSIL in the WF/NSIL mixture, the measured concentration of NSIL in the WF/NSIL mixture was seen to converge on a highly repeatable value of approximately 2%. It is also important to observe the difference between this value and the overall NSIL concentration of 5.8% based on known and carefully measured quantities installed at the test commissioning of this particular system. It is also important to note that this 2% concentration of lubricant flowing within the active portion of the system is substantially less than the 5% taught by Smith in that prior art system.

[0059] The difference between the overall concentration of NSIL and the observed concentration at the output of system pump 105, which also represents the concentration at the output of positive displacement machine 102 due to the closed-loop circuit between those two points, is attributable to several factors. First, NSIL has extremely strong affinity to bond with metal surfaces in the ORC system, including but not limited to the surfaces and bearings of the positive displacement machine, the metallic inner surfaces of heat exchanger 101, and metallic inner surfaces of condenser subsystem 104, all of which are directly in contact with the WF/NSIL mixture flow. This affinity causes a thin film of NSIL to be deposited on these surfaces, providing lubrication on the case of the surfaces, bearings, and other lubrication points of the positive displacement machine. While no lubrication is specifically required for the inner metallic surfaces of the heat exchanger 101 and condenser subsystem 104, the deposition of NSIL on these surfaces was observed to have a negligible effect on their thermal properties and performance. At the overall ORC system concentration of 5.8% NSIL by mass, the comprehensive performance of the ORC system was only de-rated by approximately 2%, which includes both the effect of the oil deposition within the thermal subsystems and the addition of non-refrigerant NSIL to the refrigerant working fluid required for proper operation of the ORC system. This 2% degradation in system performance is notable in that it is far less than reported in the prior art for similar systems utilizing a mixture of working fluid and soluble or miscible lubricants.

[0060] Additionally, the difference between the overall ORC system concentration of 5.8% NSIL by weight and the observed 2% concentration at the point of lubrication equilibrium is partially attributable to the accumulation of NSIL in the receiver tank associated with the condenser subsystem. FIG. 4 presents a representative depiction of the stratification of the components in the receiver tank 401 measured during ORC system operation at the point of lubrication equilibrium. While the boundaries between adjacent stratum are not clearly defined, the regions have distinct characteristics that provide valuable insight into the nature of this invention.

[0061] Stratum 402 is a faintly milky colloidal mixture comprising primarily organic refrigerant working fluid with a small quantity of suspended NSIL. This stratum extends upward approximately 9.5 inches from the bottom of the tank. Stratum 403 is a transition zone approximately 1 inch in depth and, although similarly milky in appearance, further comprises droplets of NSIL of increasing size and number toward its upper edge. Stratum 404, approximately 1.5 inches thick, is largely comprised of NSIL with random droplets of working fluid refrigerant. Stratum 405, approximately 0.5 inches high, is a region comprised of agitated working fluid and NSIL. Due to the agitation, the upper surface is irregular and subject to variation. Partially vaporized working fluid occupies the remaining volume between the upper surface of stratum 405 and the upper inside surface of receiver tank 401.

[0062] The demonstrated and observed affinity of NSIL for the surfaces, bearings, and other lubrication points in the machine represent a noticeable and significant improvement over the present use of lubricants that are soluble or miscible in the working fluid. Experimental observations reveal a much higher concentration of NSIL at the critical points in the system despite the absence of sufficient bearing temperatures necessary for proper lubrication in the prior art. Further, experimental testing has revealed that the use of NSIL in lieu of soluble or miscible lubricants as taught in the prior art results in decreased bearing wear over significant periods of use. In the case of NSIL, bearing temperature under operating conditions is irrelevant as it is no longer necessary to vaporize working fluid to provide adequate lubrication as taught in the prior art. The use of lubricants that are inherently insoluble and immiscible in the working fluid represents a clear departure from prior teaching in this field. It is believed that the present art relied upon a presumption that a mixture of working fluid and lubricant was best achieved through the use of lubricants that were either soluble or miscible in the liquid phase of the working fluid that would yield a stable, homogenous mixture of lubricant and working fluid. However, the use of NSIL as taught herein provides superior performance despite the fact that the WF/NSIL mixture can, by definition, never be completely homogenous and its instantaneous composition inherently stable in colloidal form.

[0063] The description of this invention is intended to be enabling and not limiting. It will be evident to those skilled in the art that numerous combinations of the embodiments described above may be implemented together as well as separately, and all such combinations constitute embodiments effectively described herein.