Patent Publication Number: US-8978400-B2

Title: Air cooled helium compressor

Description:
This invention relates generally to helium compressor units for use in cryogenic refrigeration systems, operating on the Gifford McMahon (GM) cycle. More particularly, the invention relates to a means of air cooling the compressor that has ecological and economic benefits. 
     BACKGROUND OF THE INVENTION 
     The basic principal of operation of a GM cycle refrigerator is described in U.S. Pat. No. 2,906,101 to McMahon, et al. The GM cycle has become the dominant means of producing cryogenic temperatures in small commercial refrigerators primarily because it can utilize mass produced oil-lubricated air-conditioning compressors to build reliable, long life, refrigerators at minimal cost. GM cycle refrigerators operate well at pressures and power inputs within the design limits of air-conditioning compressors, even though helium is substituted for the design refrigerants. Typically, GM refrigerators operate at a high pressure (Ph) of about 2 MPa (300 pounds per square inch absolute, psia), and a low pressure of about 0.8 MPa (117 psia). The cold expander in a GM refrigerator is typically separated from the compressor by 5 m to 20 m long gas lines. The expanders and compressors are usually mounted indoors and the compressor is usually cooled by water, most frequently water that is circulated by a water chiller unit. Some compressors are air cooled, mounted indoors and cooled by air conditioned air, or mounted outdoors and cooled by outdoor air. 
     Air-conditioning compressors are built in a wide range of sizes and several different designs. Means of providing additional cooling to adapt these compressors to compressing helium are different for different compressors. For example, compressors that draw approximately 200 to 600 W are typically reciprocating piston types which are cooled by adding air cooled fins to the compressor shell. Between about 800 to 4,500 W, the most common compressor is a rolling piston type with low pressure return gas flowing directly onto the compression chamber. In rolling piston compressors, oil flows into the compression chamber along with the helium and absorbs heat from the helium as it is being compressed. Most of the oil separates from the helium in the compressor shell which is at high pressure. U.S. Pat. No. 6,488,120 to Longsworth describes the cooling of helium, oil, and the compressor shell by wrapping a water cooling tube around the shell, and further wrapping a helium cooling tube and an oil cooling tube over the water tube. Cooled oil is then injected into the return helium line. In effect, the compressor serves as an oil pump. Scroll compressors that draw between 3,000 W and 15,000 W, and screw compressor that draw between 15 kW and 50 kW have been used for compressing helium, but at present the largest GM cycle refrigerators draw about 15 kW. The small reciprocating compressor has intake and exhaust valves and the rolling piston compressor compressor has a discharge valve. These valves limit the flow rate of oil that can be tolerated to flow with the oil to about 0.5% of the displacement while the scroll and screw compressors that don&#39;t have valves can pump oil that is typically about 2% of the displacement. This is sufficient to absorb about 75% of the heat from the compressor while the balance flows into the helium. Both streams flow from the compressor to be cooled external to the compressor and there is no need to remove heat from the compressor shell as is done with the smaller compressors that have valves. 
     Published patent application US 2007/0253854 describes a horizontal scroll compressor manufactured by Copeland Corp. which has been adapted by the same assignee as this application for compressing helium. The adaptation to flowing several times as much oil as is needed for air-conditioning refrigerants is done by having the excess oil by-pass the motor and flow directly into the scroll inlet. The Copeland compressor requires an external bulk oil separator to remove most of the oil from the helium. Heat is removed from the oil and helium in a water cooled heat exchanger, the oil is returned to the compressor and the helium passes through a second oil separator and an adsorber before flowing to the expander. 
     Prior art for converting this to being air cooled would replace the water cooled heat exchanger with an air cooled heat exchanger as shown in  FIG. 1 . This works acceptably well if the compressor is in an indoor air-conditioned environment where the air temperature is between 15° C. and 30° C. Experience has shown that heat loads of up to about 3 kW are acceptable for end users but for larger heat loads it is preferred to reject the heat to outdoor air if cooling water is not available. Designing a helium compressor to operate in an outdoor environment where temperatures can range from −30° C. to +45° C. present many challenges. The oil circulation rate is set high enough to keep the maximum discharge temperature below about 85° C. This is within an acceptable limit for the compressor but oil outgases contaminants that are adsorbed in it, principally water vapor, at a higher rate than lower temperature oil. This loads the adsorber faster and necessitates more frequent replacement of the adsorber. At low outdoor temperatures the oil becomes very viscous and makes starting the compressor difficult. This problem has been solved in the past by putting the compressor in a small shed that has adjustable louvers and a fan both of which are thermostatically controlled to keep the shed near room temperature. One or both of these features can also be incorporated in the compressor cabinet. A heater is needed to warm up the compressor before it is turned on, then the heat from the compressor keeps the shed or cabinet warm. The assignees of this application manufacture helium air-cooled compressors for operation indoors, Model CSA-71, which uses an Hitachi scroll compressor, and operation outdoors, Model CNA-61, which uses a Sanyo rolling-piston compressor. Both use prior art cooling means. 
     The Hitachi Corporation makes several models of scroll compressors that have been adapted to compressing helium. They draw between 5 and 9 kW. The Hitachi scroll compressors differ from the horizontal Copeland compressor in being oriented vertically and having return gas and oil flow through separate lines directly into the scroll. Helium and oil together are discharged into the shell at high pressure. Most of the oil separates from the helium and collects in the bottom of the compressor, similar to the rolling piston compressor described above. Unlike the smaller compressors, for this type of compressor, cooling the shell with a water cooling tube wrapped around it is not effective. Here, heat from the helium and oil is removed by an after-cooler that is external to the compressor shell, which is either air or water cooled. The Hitachi scroll is used to illustrate the principals of this invention because it does not need a separate bulk oil separator and the piping circuit is thus simpler. 
     SUMMARY OF THE INVENTION 
     The objective of this invention is to keep the oil separator(s) and adsorber, which are components in an air cooled, oil lubricated, helium compressor, in an indoor air conditioned environment while rejecting most of the heat from the compressor outdoors during the summer. The present invention is designed to be used with a GM or Pulse Tube cycle cryogenic refrigerator and will reject at least 65% of the heat produced by the compressor to outdoor air during the summer, with the balance being rejected to the indoor air conditioned air. This is accomplished by circulating hot oil at high pressure to an outdoor air cooled heat exchanger and returning cooled oil to the compressor inlet, while hot high pressure helium is cooled in an air cooled heat exchanger in an indoor assembly that includes the compressor, one or more oil separators, an oil adsorber, and other piping and control components. 
     It is a further objective to offer the option of rejecting the heat from the oil to the indoor space during the winter to save on the cost of heating the indoor space. 
     This invention will probably be favored for compressor systems that draw between about 4 to 12 kW and will reject about 1 to 3 kW of heat into air conditioned space in the summer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of an oil-lubricated helium compressor system that would use prior art to replace the standard water cooled after-cooler with the air cooled after-cooler shown as  6  with fan  27 . 
         FIG. 2A  is a schematic diagram of a compressor system in which oil is circulated to an air cooled oil-cooler  9  that is mounted outdoors while the rest of the system, including an air cooled helium-cooler  12 , is located indoors in accordance with the present invention.  FIG. 2A  also includes a helium/oil heat exchanger  11  which minimizes the amount of heat transferred to the indoor air by cooler  12 . 
         FIG. 2B  is a schematic of a variation of  FIG. 2A  in which heat exchanger  11  is omitted. 
         FIG. 3A  is a schematic diagram of a compressor system which shows the option of adding a second air cooled oil-cooler  10  and fan  29  which are mounted indoors. Solenoid valves  48  and  49  are used to circulate the hot oil to outdoor oil-cooler  9  during the summer or indoor oil-cooler  10  during the winter. 
         FIG. 3B  is similar to  FIG. 3A  except that oil-cooler  10  is mounted with helium-cooler  12  and shares the same fan  30 . 
         FIG. 4  shows a system similar to  FIG. 2A  with the addition of by-pass line  25  and oil temperature regulator  24 . When the outside air temperature is very low, regulator  24  allows hot oil to flow through by-pass line  25  to mix with cold oil from oil-cooler  9  and maintain a return temperature greater than about 10° C. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Parts that are the same or similar in the drawings have the same numbers and descriptions are usually not repeated.  FIG. 1  is a schematic diagram of an oil-lubricated helium compressor system that is presently being manufactured by the assignee of this invention except that after-cooler  6  is water cooled rather than air cooled. It shows a horizontal scroll compressor manufactured by Copeland Corp. which requires bulk oil separator  5  connected to compressor discharge line  21  to separate most of the oil from the helium. Subsequent drawings use the vertical scroll compressor manufactured by Hitachi to illustrate the compressor because the helium/oil mixture is discharged from the scroll into the compressor shell, which serves as a bulk oil separator. 
     Compressor system components that are common to all of the figures are: compressor shell  2 , high pressure volume  4  in the shell, oil separator  7 , adsorber  8 , compressor scroll  13 , drive shaft  14 , motor  15 , oil return port  16 , helium return line  17 , helium/oil mixture discharge from the scroll  19 , high pressure hot oil line  22  to an after-cooler, oil flow control orifice  23 , oil in the compressor sump, high pressure helium to an air cooled after-cooler  31 , high pressure oil  32  from cooler  6  or  9 , gas line  33  from oil separator  7  to adsorber  8 , gas line  34  from oil separator  7  to internal relief valve (IRV)  35 , adsorber gas couplings  36 , high pressure helium gas supply line  37  that connects the compressor to the expander (not shown), return gas coupling  38  that connects gas returning from the expander at low pressure through line  39  to the compressor, atmospheric relief valve (ARV)  40 , oil return line  41  from separator  7  to the compressor through orifice/filter  42 , oil pump  47  which is integral to drive shaft  14 , and line  50  that connects helium from the after-cooler to oil separator  7 . 
     Compressor system  100  in  FIG. 1  shows the conventional way of converting a water cooled unit to an air cooled unit by replacing the water cooled after-cooler with an air cooled after-cooler  6  and fan  27 , and mounting the entire system outdoors. The oil flow rate is set to keep the maximum temperature at an acceptable value and the heat exchanger and fan are sized to reject the heat from the oil and helium at a maximum air temperature of about 45 C. Helium will flow to oil separator  7  at about 5° C. Unless there is a mechanism to cool it closer to room temperature, about 25 C, it will carry a high rate of contaminants, mostly water, that outgas from the hot oil, and collect in the adsorber. The adsorber thus needs to be replaced more frequently than if the compressor, helium after-cooler, oil separator(s), and adsorber are kept in an air conditioned environment per the present invention. 
     Compressor system  100  has the following differences from subsequent systems; return gas flows from line  17  into the shell of the compressor on the inlet side of the scroll thus most of the volume in shell  2  is at low pressure, 3. Oil in sump  26  is at low pressure and mixes with low pressure helium as it flows into the scroll at  18 . Discharge line  21  contains the same helium/oil mixture that leaves the scroll, 19.  FIG. 1  shows TSG  44  which is a temperature switch that shuts down the compressor if the discharge temperature is too high. TSM  45  is a temperature switch that shuts down the compressor if the motor temperature is too high. Most compressor systems have these two protectors. 
       FIG. 2A  is a schematic diagram of compressor system  200 . It shows a vertical Hitachi compressor which is constructed such that helium returning through line  17  flows directly into scroll  13  as does the return oil through line  16 . A mixture of hot compressed helium and oil exit from the scroll,  19 , and most of the oil drops to sump  26  in the bottom of the compressor. Hot compressed helium with a small amount of oil exits the compressor through line  20  into heat exchanger  11 , which transfers some of the heat from the hot helium to the returning oil. Helium then flows through helium cooler  12  which is cooled by indoor air driven by fan  30 . Most of the heat from the compressor is rejected outdoors in oil-cooler  9 . 
     Table 1 provides an estimate of the temperatures of the helium and oil in the systems shown in the figures for a summer outdoor temperature of 45° C. and a winter temperature of −30° C. Indoor temperatures are assumed to be 27° C. in the summer and 21° C. in the winter. The oil circulation rate is set by fixed orifice  23  to limit the maximum oil temperature in line  22  to be 85° C. It is assumed that this flow rate remains the same at lower ambient temperatures but in reality the flow rate drops with temperature. The calculations are done for a scroll compressor operating at 60 hz that has a displacement of 98 mL and draws 8.0 kW of power when compressing helium from 0.9 MPa to 2.3 MPa. The fan speeds are assumed to be variable so, for example, the outdoor air flow is reduced in the winter to prevent the oil from getting too cold. Lines to the outdoor heat exchanger are assumed to be insulated. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Outdoor T - C. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 45 
                 −30.0 
                 45 
                 −30.0 
                 45 
                 −30.0 
                 −30.0 
                 −30.0 
                 −30.0 
               
            
           
           
               
               
            
               
                   
                 Indoor T - C. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 27 
                 21 
                 27 
                 21 
                 27 
                 21 
                 21 
                 21 
                 21 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 FIG. 
                 1 
                 1 
                 2A 
                 2A 
                 2B 
                 2B 
                 3A 
                 3B 
                 4 
               
               
                 System 
                 100 
                 100 
                 200 
                 200 
                 201 
                 201 
                 300 
                 301 
                 400 
               
               
                 Helium 
                 Summer 
                 Winter 
                 Summer 
                 Winter 
                 Summer 
                 Winter 
                 Winter 
                 Winter 
                 Winter 
               
               
                 T20, compr out - C. 
                   
                   
                 85 
                 70 
                 85 
                 70 
                 70 
                 70 
                 70 
               
               
                 T31 HX in - C. 
                 85 
                 68 
                 60 
                 38 
                 85 
                 68 
                 38 
                 38 
                 38 
               
               
                 T50, HX out - C. 
                 50 
                 33 
                 32 
                 24 
                 32 
                 24 
                 24 
                 24 
                 24 
               
               
                 T37, Ads out - C. 
                 50 
                 33 
                 32 
                 24 
                 32 
                 24 
                 24 
                 24 
                 24 
               
               
                 T17, return line - C. 
                 27 
                 21 
                 27 
                 21 
                 27 
                 21 
                 21 
                 21 
                 21 
               
               
                 Oil 
               
               
                 T22, cmpr out - C. 
                 85 
                 70 
                 85 
                 70 
                 85 
                 70 
                 70 
                 70 
                 70 
               
               
                 T32, cooler out - C. 
                 53 
                 38 
                 53 
                 37 
                 59 
                 42 
                 34 
                 34 
                 −7 
               
               
                 T43, He/oil HX in - C. 
                 53 
                 38 
                 53 
                 37 
                 59 
                 42 
                 34 
                 34 
                 20 
               
               
                 T16, compr in - C. 
                 53 
                 38 
                 59 
                 40 
                 59 
                 42 
                 42 
                 42 
                 38 
               
               
                 Heat to Indoors - % 
                 0 
                 0 
                 19 
                 11 
                 34 
                 30 
                 100 
                 100 
                 21 
               
               
                 Vol Oil/Disp Vol - % 
                 2.0 
                 2.0 
                 2.1 
                 2.1 
                 2.1 
                 2.1 
                 2.1 
                 2.1 
                 2.2 
               
               
                 Outdoor Fan Speed - % 
                 100 
                 20 
                 100 
                 20 
                 100 
                 21 
                 Off 
                 Off 
                 11 
               
               
                 Oil By-Pass - % 
                   
                   
                   
                   
                   
                   
                   
                   
                 60 
               
               
                   
               
            
           
         
       
     
     Helium, and the other monatomic gases, get much hotter than other gases when being compressed, so the oil that is injected with the helium at the compressor inlet is substantial. Table 1 shows that that for system  100  the volume of oil that is injected occupies 2.0% of the displaced volume for this example. System  100 , which represents prior art, rejects 100% of the compressor heat outdoors. System  200 , which illustrate the present invention, rejects 81% of the heat outdoors, 19% indoors, on the hottest day assumed, and 89% of the heat outdoors, 11% indoors, on the coldest day assumed. For the 8.0 kW of input power used in this example the maximum heat load on the air conditioning system is 1.5 kW. 
     The most important aspect of this invention is that keeping all of the compressor system indoors, except the oil-cooler, results in the helium flowing through separator  7  and adsorber  8  to be much cooler than for system  100 . Table 1 shows the helium out of the adsorber to be 32° C. for system  200  compared with 50° C. for system  100 . 
       FIG. 2B  is a schematic of a variation of  FIG. 2A  in which heat exchanger  11  is omitted. Table 1 lists temperatures for system  201  that are comparable to system  200 . System  201  rejects 66% of the heat outdoors, 34% indoors, on the hottest day assumed, and 70% of the heat outdoors, 30% indoors, on the coldest day assumed. For the 8.0 kW of input power used in this example the maximum heat load on the air conditioning system is 2.7 kW. System  201  illustrates that the cost savings of not having heat exchanger  11  are offset by significantly higher indoor heat loads on the air conditioning system. 
       FIG. 3A  is a schematic diagram of compressor system  300  which shows the option of adding a second air cooled oil-cooler  10  and fan  29  which are mounted indoors. Solenoid valves  48  and  49  are used to circulate the hot oil to outdoor oil-cooler  9  during the summer or indoor oil-cooler  10  during the winter. Gas line couplings  51 , which would typically be self sealing, enable this subassembly to be sold as an option. During the summer, solenoid valve  48  is open while 49 is closed so, the temperatures are the same as system  200 . During the winter oil flows through oil-cooler  10  which is indoors so all of the heat from the compressor heats the interior space. 
       FIG. 3B  shows compressor system  301  which is similar to  FIG. 3A  except that oil-cooler  10  is mounted with helium-cooler  12  and shares the same fan  30 . 
       FIG. 4  shows compressor system  400  which is similar to  FIG. 2A  with the addition of by-pass line  25  and oil temperature regulator  24 . When the outside air temperature is very low, regulator  24  allows hot oil to flow through the by-pass line to mix with cold oil from oil cooler  9  and maintain a return temperature greater than about 10° C. System  400  might have an advantage over system  200  in being able to start faster in the winter. Rather than waiting for a heater to warm the oil in outdoor cooler  9 , oil by-pass line  25  in system  400  can circulate oil immediately and cold oil from outside can mix while the compressor is warming up. It may be desirable to leave fan  28  off initially. 
     It is within the scope of this invention to replace air cooled He Cooler  12  with a water cooled heat exchanger. Nothing herein is meant to limit the present invention. It is understood that the present invention may be used with other horizontal scroll compressors or other compressors such as screw, reciprocating, centrifugal, and rotary vane types, as well as the compression of any monatomic gas. Helium/oil heat exchanger  11  is optional in any of the systems. 
     While this invention has been described, it will be understood that it is capable of further modification, uses and/or adaptations, following in general the principal of the invention, and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the essential features herein before set forth, as fall within the scope of the invention or the limits of the appended claims. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     It is also understood that the following claims are intended to cover all of the generic and specific features of the invention described herein.