US20130255308A1

US20130255308A1 - Chiller or heat pump with a falling film evaporator and horizontal oil separator

Info

Publication number: US20130255308A1
Application number: US13/434,169
Authority: US
Inventors: Paul M. DE LARMINAT
Original assignee: Johnson Controls Technology Co
Current assignee: Johnson Controls Technology Co
Priority date: 2012-03-29
Filing date: 2012-03-29
Publication date: 2013-10-03
Also published as: JP2017075776A; TWI509207B; EP2831519A1; KR20140146598A; KR101607509B1; TW201339524A; WO2013147931A1; CN104204692A; JP2015512501A

Abstract

A refrigerant circuit using a vapor compression cycle, the circuit usable for air conditioning, refrigeration or heat pump purposes. The circuit includes a lubricated compressor connected to an oil separator vessel separate from the compressor, a falling film or hybrid falling film evaporator and a condenser. The oil separator vessel extends substantially horizontally. The oil separator vessel is separated into a primary space and a secondary space by a filter pad configured to substantially remove entrained oil droplets of about 5 μm and larger from the refrigerant entering the oil separator vessel. The primary space is in fluid connection with a discharge of the compressor. The secondary space is in fluid connection with an inlet of the condenser. The circuit has an oil entrainment flow discharge of lubricant from the compressor of at least about two percent by mass relative to refrigerant flow.

Description

BACKGROUND

This invention deals with machines used for refrigeration and air conditioning or heat pumps with medium to high cooling capacity (typically about 100 kW and higher), using vapor compression cycles, including falling film or hybrid falling film evaporators, in conjunction with lubricated compressors connected to oil separators that are separate from the compressor.
In the context of research for energy savings and reduction of the emissions of greenhouse gasses, high equipment efficiency and low refrigerant charges are being sought. To achieve these goals, improvements are being made to all the components of the systems: compressors, variable speed drives, optimized choice of refrigerant, oil separators, heat exchangers, etc. Most of the compressors require an amount of lubrication, which lubrication similarly generates carry-over of an amount of oil from the compressor into the refrigerant circuit. This oil that is entrained into the refrigerant circuit must then be returned to the compressor by and adequate oil return system, in order to avoid different adverse effects like the deterioration of the performance of the heat exchangers. It is especially so for screw compressors: these machines require a particularly large amount of lubrication in order to insure proper sealing of the gas between the rotors and to avoid the need for additional synchronization gears between the rotors. Therefore, screw compressors typically require a vessel, also commonly referred to as an oil separator, positioned between the compressor discharge and the inlet of the condenser. One challenge associated with refrigeration machines and heat pumps is the management of the oil in the refrigerant circuits. This requires a careful combination between the oil carry-over, the oil return system and the technology of the heat exchangers.
Besides separating the oil from the discharge gas, this vessel or oil separator or separator vessel usually also has the function of being the oil sump for the compressor. Separator vessels or oil separators can be based on several operating principles. The most common include:

- Impingement separation: the two-phase mixture of oil and gas is projected onto a wall or to the end of the vessel, providing a first stage of separation.
- Gravity separation: the gas mixed with oil is allowed to travel in the vessel, either horizontally or vertically upwards; this permits the larger droplets of oil in liquid phase sufficient time to be urged by gravity toward the bottom of the vessel.
- Filter pad separation: the mix is forced through a pad of closely spaced and/or finely interwoven filaments or wires that acts as a filter. In one embodiment, the filter pad may include a wire mesh. Per the instructions of such filter manufacturers, filter pads are normally installed horizontally, with gas circulation directed upwards. The filtration level of filter pads is relatively coarse; it would not stop very fine droplets entrained in the gas or mist, but still removes smaller droplets than gravity separation.
- Centrifugal separation: the two-phase oil and gas mixture is introduced tangentially in a cylindrical vessel. The whirling motion tends to project the oil droplets onto the cylindrical wall of the vessel where the droplets coalesce and fall to the bottom of the vessel. Like gravity separation, centrifugal separation allows removing the largest droplets of oil.
- Coalescing filters: the two-phase mixture of oil and gas is forced through a cartridge acting as a filter. The filter material is typically fiberglass. The filtration is much finer as compared with the filter pads (see above). For example, coalescing filters substantially prevent droplets of 1 μm (micrometers) or larger in diameter from passing through the coalescing filter during operation of a refrigerant circuit. In a typical embodiment, the coalescing filters generally permit about 1 to 10 parts per million (PPM) by mass of oil droplets entrained in refrigerant flow to discharge from the separator.

Several different principles are often implemented simultaneously in a single separator. For instance, when coalescing filters are used, they are normally installed to supplement a filter/separator that can incorporate one or more operating principles such as impingement, gravity, centrifugal, and/or filter pad separation. In the design of an oil separator, the challenge is to find the best compromise between various parameters such as price, size, ease of installation, pressure drop, reliability, and of course, efficiency of the separation.
Typical designs of oil separators include:

- A horizontal design with coalescing filters. FIG. 1 shows an example of this well-known design. The separation begins with impingement at one end of the vessel, continues with a gravity separation section that is also used as the oil sump, and is completed by coalescing filters.
- A vertical cyclone design with a coalescing filter (not shown).

When properly implemented, these designs normally provide a highly efficient oil separation thanks to the coalescing elements. Yet, coalescing filters have some drawbacks. They tend to be relatively expensive. If the coalescing filters are not mounted properly, some separators in a series may not meet operating specifications. If the possibility of inspection and filter removal is desired, costly additional flanges or access-providing man holes are required that also increase the risk of refrigerant leaks. In addition, hydraulic pressure safety testing of vessels having internal coalescing filters raises risk of damaging these filters, as well as associated difficulties in emptying and drying the vessel properly after completing the testing.
In case of accidental high fluid mass flow, coalescing filters can also suffer loss of performance, and are susceptible to clogging and/or destruction under the effect of elevated fluid forces. Increased fluid mass flow is especially a problem for heat pump applications with high pressure halogenated refrigerants such as HFC's. Even when used in air conditioning applications, coalescers or coalescing filters need to be oversized when high pressure HFC's, such as R-410A or R-507 are used. Use of high temperature heat pumps increases problems associated with such applications. In such high temperature heat pumps, the evaporating temperature is substantially higher than when using the same machines and refrigerant in corresponding air conditioning applications, due to higher temperature of the water or other medium being cooled at the evaporator. In high temperature heat pumps, the leaving water from the evaporator is typically above 20° C., and can reach up to 60° C. or even higher. The resulting higher evaporation temperatures substantially increase density and hence the mass flow of refrigerant, even when using a relatively low pressure refrigerant such as R-134a or even lower pressure refrigerants like R-245fa for instance.
For heat pumps or chillers using compressors, such as a screw compressor, what is desirable is an oil separator having a radically simplified design that does not include coalescing filters, while providing a sufficient oil separation during operation.

SUMMARY

For heat pumps or chillers using a screw compressor, the present disclosure is directed to the use of falling film evaporators or hybrid falling film evaporators, such as described, for instance, in U.S. Pat. No. 7,849,710, which is incorporated by reference in its entirety. These evaporators offer the best state of the art compromise between optimized performance and reduced refrigerant charge. In addition, falling film and hybrid falling film evaporator performance is less sensitive to oil carry-over than traditional flooded evaporators, permitting an oil separator, such as the filter pad, to be used without sacrificing evaporator performance.
For purposes of the present disclosure, the term filter pad generally incorporates the following performance characteristics: substantially prevents droplets of about 5 μm (micrometers) in diameter or larger from passing through the filter pad during operation of a refrigerant circuit. In one embodiment, the filter pad operates between about 50 to about 100 parts per million (PPM) of oil entrained in refrigerant flow from the separator. In another embodiment of the filter pad, the void percentage of the filter pad is between about 97 and about 99 percent. In a further embodiment of the filter pad, the diameter of filaments and/or wires generally ranges from between about 0.15 mm to about 0.35 mm (millimeters) in diameter.
The present invention is directed to a refrigerant circuit using a vapor compression cycle, the circuit usable for air conditioning, refrigeration or heat pump purposes. The circuit includes a lubricated compressor connected to an oil separator vessel separate from the compressor, a falling film or hybrid falling film evaporator and a condenser. The oil separator vessel extends substantially horizontally. The oil separator vessel is separated into a primary space and a secondary space by a filter pad configured to substantially remove entrained oil droplets of about 5 μm and larger from the refrigerant entering the oil separator vessel. The primary space is in fluid connection with a discharge of the compressor. The secondary space is in fluid connection with an inlet of the condenser. The circuit has an oil entrainment flow discharge of lubricant from the compressor of at least about two percent by mass relative to refrigerant flow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 and shows a prior art oil separator.

FIG. 2 shows an exemplary embodiment of an oil separator of the present disclosure.

FIG. 3 shows an exemplary embodiment of an oil separator of the present disclosure.

FIG. 4 shows an exemplary embodiment of an oil separator of the present disclosure.

FIG. 5 schematically shows an exemplary embodiment of a vapor compression system of the present disclosure.

FIG. 6 schematically shows an exemplary embodiment of a vapor compression system of the present disclosure.

FIG. 7 shows an exemplary embodiment of an oil separator of the present disclosure.

FIG. 8 shows an exemplary embodiment of an oil separator of the present disclosure.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 2 shows a horizontal vessel 1 with a filter pad 2 providing a longitudinal separation of the vessel into two spaces: a primary space 3 having an inlet 4 to receive discharge from the compressor, and a secondary space 5 having an outlet 6 in communication with an inlet of a condenser (not shown). In an exemplary embodiment, inlet 4 receives compressor discharge in which a gas and oil mixture 15 entering separator vessel 1 can be arranged to provide impingement separation at an end of vessel 1 in primary space 3. In addition, a second filter pad 7 can be disposed on or near end 17 of vessel 1 where this impingement occurs, to limit the re-entrainment of liquid with gas discharged from the compressor after the gas discharge collides with the end of the vessel.
In this arrangement, as further shown in FIG. 2, the oil that is separated from the gas is allowed to move freely from primary space 3 to secondary space 5. In one embodiment, filter pad 2 is installed across the complete cross section of vessel 1, transverse to the longitudinal direction of vessel 1. The lower part of vessel 1 is collecting liquid oil 19 and performing the function of an oil sump. As filter pad 2 offers very little resistance to the circulation of liquid oil 19 between primary space 3 and secondary space 5, the level of liquid oil 19 is essentially the same for both spaces. It is generally better to collect the liquid oil 19 from secondary space 5 through an oil pipe 8 to be returned to the compressor for lubrication, because the oil has an opportunity to become separated from foam and bubbles by filter pad 2 while migrating from primary space 3 to secondary space 5. With this arrangement, evaporators 12 (FIG. 5), such as falling film evaporators or hybrid falling film evaporators, a circuit having an oil entrainment flow discharge of lubricant from the compressor of about two percent or more by mass relative to refrigerant flow, i.e., percentage of mass flow total of refrigerant plus lubricant, such as associated with screw compressor operation, can be accommodated by the oil separator. In one embodiment, the oil separator is separate from the compressor.
In one arrangement, a filter pad 2 separating respective primary and secondary spaces 3, 5 is substantially planar and is installed vertically, i.e., perpendicular to a longitudinal axis of vessel 1, and about at mid-length of the separator vessel. In alternative arrangements, this filter pad 2 can be positioned non-perpendicular with respect to the vessel axis, such as shown in FIG. 3. This arrangement has an advantage of reducing the velocity of the gas flowing through filter pad 2. As a result, a vessel of the present disclosure can have a smaller vessel diameter in comparison to the diameter of a vessel of conventional construction, with the smaller diameter vessel of the present disclosure operating with a gas flow velocity through the filter pad 2 that may be similar to the operating gas flow velocity through the filter pad 2 of the larger, conventional vessel of FIG. 2. In one embodiment, the smaller diameter vessel of the present disclosure may operate with a gas flow velocity through the filter pad 2 that may be less than the gas flow velocity of the larger, conventional vessel. In another embodiment, filter pad 2 may be composed of two or more portions 2 a, 2 b arranged at an angle to each other, e.g., in the shape of a “V” as shown in FIG. 4, which is a plan view of the vessel. In yet another embodiment, portions 2 a, 2 b may be of unequal length.
In still a further arrangement, two oil separation sections or spaces utilizing the same principle can be integrated in a single vessel 1, each section or space or secondary space 5 receiving approximately one half of the volume of discharge gas and oil from primary space 3. In this arrangement, there is one vessel 1 with two filter pads 2, and two possible options about the direction of the flow. In one embodiment as shown in FIG. 7, there is one primary space 3 positioned substantially in the middle of vessel 1 and between the two filter pads 2, and two secondary spaces 5, with one secondary space 5 arranged at each end of the vessel. As shown, there is one common gas inlet 4 substantially positioned in the middle of primary space 3, and one outlet 6 positioned in each of opposed secondary spaces 5, which secondary spaces 5 are positioned at each end of primary space 3. These two outlets 6 are connected to the condenser inlet (not shown). The interconnection piping between both outlets can be internal or external to vessel 1. In another embodiment (FIG. 8) the flows are reversed: there is one primary space 3 positioned at each end of vessel 1 with an end of gas inlet 4 extending into each respective primary space 3, and one common secondary space 5 positioned between opposed primary spaces 3 with a common gas inlet 4 entering secondary space 5. In this embodiment, sections of inlet 4 connected to the compressor discharge are divided into two pipes or portions extending to each of the two primary spaces, one at each end of vessel 1. The interconnected piping or portions of inlet 4 can be arranged internal or external of the vessel. For instance, in FIG. 8, there is one common inlet 4 extending to a bifurcated internal pipe 13 distributing the gas to each end of vessel 1.
In an alternate embodiment of FIG. 7, both outlets 6 can be connected to form a single pipe that extends to one condenser inlet. In an alternate arrangement, the condenser (not shown) can have two inlets, one inlet at each end; with each of the two separator outlets 6 being connected to one of the condenser inlets.
The arrangement with two sections or spaces in a common vessel offers several advantages. As the flow to each section or space is reduced, such as being reduced by a factor of two, so too is a reduction of the required cross section of the vessel. Therefore, in spite of the additional length, the reduction in diameter will result in a less expensive vessel. A further advantage is that a vessel of smaller diameter will typically radiate less noise, because there is less potential for wall resonance in a shell of smaller diameter. Finally, the added length to the vessel does not raise packaging problems with other system components as long as the length of the separator or vessel does not substantially exceed that of the heat exchangers, such as the condenser and/or evaporator.
As the oil separator vessel is horizontal, the arrangement lends itself to easy packaging with horizontal shell and tube heat exchangers, and with a horizontal screw compressor driveline. In a possible arrangement as shown in FIG. 5, the discharge of compressor 9 is directed downwards to separator vessel 1. In another embodiment as shown in FIG. 6, the discharge of compressor 9 may be directed from one side of compressor 9 to oil separator vessel 1. In yet another embodiment, the compressor discharge may be directed at an orientation that is between the downward direction and the side direction of the compressor to the oil separator vessel (not shown). In this arrangement, the compressor driveline can be installed at least partially above the oil separator vessel. As further shown in FIG. 5, evaporator 12 is positioned above condenser 11 and arranged near the compressor driveline and separator vessel. In other embodiments (not shown), evaporator 12 and condenser 11 may be positioned in different arrangements with respect to each other and/or to the compressor driveline and oil separator vessel. In another arrangement, such as shown in FIG. 6, the compressor suction is directed vertically, such that compressor 9 is installed on top of evaporator 12, with compressor discharge from one side of the compressor to oil separator vessel 1. In another embodiment (not shown) the compressor suction may extend outwardly from the side, or in yet another embodiment, the compressor suction may extend between a vertical and a side orientation with respect to the oil separator vessel 1. In this arrangement, compressor 9 can be installed at least partially above evaporator 12. As further shown in FIG. 6, oil separator vessel 1 is shown positioned laterally beside compressor 9 and on top of condenser 11. In other embodiments, other arrangements between the compressor, oil separator vessel, condenser and evaporator may be utilized.
This use of filter pads is especially advantageous for use with heat pumps using halogenated refrigerants like HFC's or HFO's, and when the evaporation temperature is significantly greater than evaporator temperatures normally associated with air conditioning applications (e.g., 5° C.). For such heat pumps, the evaporation temperature can be up 30° to about 40° C. with HFC refrigerant R-134a or possible equivalents, and even higher temperatures associated with lower pressure refrigerants such as R-245fa.
In another embodiment, the refrigerants may include hydrocarbons such as R-290 or R-1270.

Claims

What is claimed is:

1. A refrigerant circuit using a vapor compression cycle, the circuit usable for air conditioning, refrigeration or heat pump purposes, comprising a lubricated compressor connected to an oil separator vessel separate from the compressor, a falling film or hybrid falling film evaporator and a condenser, wherein the oil separator vessel extends substantially horizontally, the oil separator vessel separated into a primary space and a secondary space by a filter pad configured to substantially remove entrained oil droplets of about 5 μm and larger from the refrigerant entering the oil separator vessel, the primary space being in fluid connection with a discharge of the compressor; the secondary space being in fluid connection with an inlet of the condenser, the circuit having an oil entrainment flow discharge of lubricant from the compressor of at least about two percent by mass relative to refrigerant flow.

2. The refrigerant circuit of claim 1, wherein a bottom of the oil separator vessel is used as an oil sump, and the oil sump is in fluid connection with oil supply orifices of the compressor.

3. The refrigerant circuit of claim 1, wherein the compressor has a discharge directed substantially downwards and suction from above or sideways from the compressor, and wherein a compressor driveline is positioned at least partially above the oil separator, and the evaporator is positioned at least partially above the condenser.

4. The refrigerant circuit of claim 1, wherein the compressor has a downwardly directed suction and a discharge from a side of the compressor, and wherein the compressor is positioned at least partially above the evaporator, and the oil separator positioned at least partially above the condenser.

5. The refrigerant circuit of claim 1, wherein an inlet of the oil separator vessel is bifurcated, the oil separator vessel separated into three spaces by two filter pads.

6. The refrigerant circuit of claim 5, wherein refrigerant flow is bifurcated.

7. The refrigerant circuit of claim 5, wherein the three spaces include two secondary spaces that are not adjacent to each other.

8. The refrigerant circuit of claim 1, wherein the filter pad is located perpendicular to a longitudinal axis of the oil separator vessel.

9. The refrigerant circuit of claim 8, wherein the filter pad is located at about mid-length of the longitudinal axis of the oil separator vessel and including a second filter pad at one end of the oil separator vessel.

10. The refrigerant circuit of claim 1, wherein the filter pad is positioned non-perpendicular of the longitudinal axis of the oil separator vessel.

11. The refrigerant circuit of claim 10, wherein the filter pad is composed of two or more portions arranged at an angle to each other.

12. The refrigerant circuit of claim 11, wherein the two or more portions are of unequal length.

13. The refrigerant circuit of claim 1, wherein the filter pad permits between about 50 to about 100 PPM of oil entrained in refrigerant flowing from the oil separator vessel.

14. The refrigerant circuit of claim 1, used as a heat pump, and using a halogenated fluid as the refrigerant.

15. The refrigeration circuit of claim 14, wherein the halogenated fluid is an HFC or an HFO as the refrigerant.

16. The refrigeration circuit of claim 1, wherein the refrigerant is a hydrocarbon.