Patent Publication Number: US-9410709-B2

Title: Multichannel condenser coil with refrigerant storage receiver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Provisional Patent Application No. 60/910,334 filed Apr. 5, 2007, hereby incorporated by reference in the entirety into this application. 
    
    
     BACKGROUND 
     This application generally relates to multichannel heat exchanger applications in heating, ventilation, and air-conditioning (HVAC) systems. The application relates more specifically to a refrigerant-storage refrigerant-storage vessel configuration for a multichannel heat exchanger coil of a condenser. 
     It should be noted that the present discussion makes use of the term “multichannel” tubes or “multichannel heat exchanger” to refer to arrangements in which heat transfer tubes include a plurality of flow paths between manifolds that distribute flow to and collect flow from the tubes. A number of other terms may be used in the art for similar arrangements. Such alternative terms might include “microchannel” (sometimes intended to imply having fluid passages on the order of a micrometer and less), and “microport”. Other terms sometimes used in the art include “parallel flow” and “brazed aluminum”. However, all such arrangements and structures are intended to be included within the scope of the term “multichannel”. In general, such “multichannel” tubes will include flow paths disposed along the width or in a plane of a generally flat, planar tube, although, again, the invention is not intended to be limited to any particular geometry unless otherwise specified in the appended claims. 
     In a typical multichannel heat exchanger or multichannel heat exchanger coil, a series of tube sections are physically and thermally connected by fins configured to permit airflow through the heat exchanger to transfer heat between the airflow and a circulating fluid such as water or refrigerant being circulated through the multichannel heat exchanger. The tube sections of the multichannel heat exchanger are oriented to extend either horizontally or vertically and each tube section has several tubes or channels that circulate the fluid. The outside of the tube section may be a continuous surface typically having an oval or generally rectangular shape. 
     Multichannel coils can offer significant cost and performance advantages compared to conventional round-tube condenser coils when used in an aircooled condenser. However, multichannel condenser coils have a much smaller internal volume than is available with conventional coils. ASHRAE 15-2004.9.11.4 states that “liquid receivers, if used, or parts of a system designed to receive the refrigerant charge during pump down shall have sufficient capacity to receive the pump down charge. The liquid shall not occupy more than 90% of the volume when the temperature of the refrigerant is 90° F. or 32° C.”. More particularly, the smaller internal volume in microchannel coils often requires a condenser that incorporate a refrigerant-storage refrigerant-storage vessel, which may be referred to as a receiver or a refrigerant-storage vessel, in order to hold the refrigerant change for pump down or servicing to meet this requirement. For examples of prior art related to receivers, see the ASHRAE Handbooks. 
     What is needed is a system and/or method that satisfies one or more of these needs or provides other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs. 
     SUMMARY 
     One embodiment relates to a refrigeration circuit with applications for heating, ventilation, and air-conditioning (HVAC) systems. In one embodiment, a chiller for use in an HVAC system is disclosed. The chiller includes a compressor, a condenser unit comprising at least one multichannel heat exchanger coil, an expansion device, and an evaporator. The HVAC system further includes a refrigerant-storage vessel configured to receive refrigerant from the multichannel heat exchanger coil. 
     Another embodiment relates to an HVAC system including a compressor, a condenser unit comprising at least one multichannel heat exchanger coil, an expansion device, an evaporator, and an air handling unit. The HVAC system further includes a refrigerant-storage vessel in fluid communication with a return header of the multichannel heat exchanger coil. 
     Another embodiment relates to a method of operating a refrigeration circuit including a compressor, a condenser unit comprising a multichannel heat exchanger coil, an expansion device, and an evaporator. The method further includes providing a refrigerant-storage vessel in fluid communication with the multichannel heat exchanger coil, and operating the refrigeration circuit under a normal operating condition. The refrigerant-storage vessel is configured to contain substantially all refrigerant vapor during normal refrigeration circuit operating condition. 
     Certain advantages of the embodiments described herein are improved liquid subcooling, which assures reliable performance of the expansion valve, better chiller control through the addition or subtraction of refrigerant charge, increased chiller cooling capacity, improved efficiency which meets ASHRAE 90.1, and cost reduction through reduced charge requirements. 
     Alternative exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The application will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements, in which: 
         FIG. 1  is an illustration of an exemplary environment using an exemplary HVAC system according to the disclosure. 
         FIG. 2  is a schematic of an exemplary refrigeration circuit. 
         FIG. 3  is a perspective view of an exemplary embodiment of a condenser. 
         FIG. 4  is an end view of the condenser of  FIG. 3  taken from direction B. 
         FIG. 5  is an end view of the condenser of  FIG. 3  taken from direction C. 
         FIG. 6  is an illustration of an exemplary two pass heat exchanger coil. 
         FIG. 7  is a partial view of a section of an exemplary heat exchanger coil. 
         FIG. 8  is a top perspective view of a section of the condenser shown in  FIG. 3  taken from direction D and having coil  6  removed. 
     
    
    
     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Before turning to the figures, which illustrate the exemplary embodiments in detail, it should be understood that the application is not limited to the details or methodology set forth in the following description or illustrated in the figures. It should also be understood that the phraseology and terminology employed herein is for the purpose of description only and should not be regarded as limiting. 
     Referring to  FIG. 1 , an exemplary environment using an HVAC system  10  according to the disclosure is shown. As shown in  FIG. 1 , the HVAC system  10  provides cooling to a commercial building  12 . In alternative embodiments, the HVAC system  10  may be used in commercial, light industrial, industrial, and in any other suitable applications for providing cooling in areas, such as a building, structure, and so forth. HVAC system  10  includes an air cooled packaged chiller (chiller)  14  and at least one air handling unit  22 . The HVAC system  10  further includes associated supply and return lines  24  in fluid communication between chiller  14  and at least one air handling unit  22 . Chiller  14  provides a cooled fluid, for example water, to at least one air handling unit  22  where it provides cooling to yet another fluid, most often building air, by conventional heat exchange methods known in the art, to provide cooling to building  12 . In alternative embodiments, the cooled fluid may be any fluid that may provide heat exchange with air handling unit  22 , for example a refrigerant. It should be appreciated by one of ordinary skill that chiller  14  is not limited to being disposed atop building  12 , but may be located outside building  12  at any location. In alternative embodiments, some components of chiller  14  may be located within building  12 . HVAC system  10  includes many other features that are not shown and/or described in  FIG. 1 , such as connective piping and electrical features. These features have been purposely omitted to simplify the drawing for ease of illustration. 
       FIG. 2  shows an exemplary refrigeration circuit  200 . Refrigeration circuit  200  includes a compressor  202 , a condenser  204 , an expansion device  206 , and an evaporator  208 . Circulating through refrigeration circuit  200  is a refrigerant, examples of which are discussed below, which completes a refrigeration cycle through refrigeration circuit  200 . 
     Compressor  202  compresses vapor refrigerant and delivers the vapor refrigerant to condenser  204  through a compressor discharge line  203 . Compressor  202  can be any suitable type of compressor. For example, compressor  202  may be a screw compressor, reciprocating compressor, centrifugal compressor, rotary compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable compressor as known in the art. The refrigerant may be any suitable refrigerant as is known in the art. For example, the refrigerant may be a hydrofluorocarbon (HFC) based refrigerant such as R-410A, R-407, or R-134a. Additionally, the refrigerant may be carbon dioxide (also known as R-744), CO 2 , ammonia (also known as R-717), NH 3 , HFO1234yf (CF 3 CF═CH 3 ) or other similar or equivalent compound or mixture of compounds that are suitable for use as a working fluid in a vapor-compression refrigeration cycle. 
     Compressor  202  is driven by a motor (not shown), which may be integral to the compressor  202 . The motor can be powered by a variable speed drive (VSD) (not shown) or can be powered directly from an AC or DC power source (not shown), as would be appreciated by one of ordinary skill in the art. For example, the motor can be a switched reluctance (SR) motor, an induction motor, an electronically commutated permanent magnet motor (ECM) or any other suitable motor type. The VSD, if used, receives AC power having a particular fixed line voltage and fixed line frequency from an AC power source and provides power to the motor having a variable voltage and frequency. In an alternate embodiment, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive compressor  202 . 
     At condenser  204 , the vapor refrigerant enters into a heat exchange relationship with a fluid, e.g., air, and undergoes a phase change to a liquid refrigerant as a result of the heat exchange relationship with the fluid. The refrigerant from condenser  204  is then provided by a refrigerant liquid line  205  to expansion device  206 , which reduces the pressure of the refrigerant before it is provided to evaporator  208  via an evaporator refrigerant inlet line  207 . 
     At evaporator  208 , the refrigerant enters into a heat exchange relationship with another fluid, which may or may not be the same type of fluid used for condenser  204 , and undergoes a phase change to a vapor refrigerant as a result of the heat exchange relationship with the fluid. For example, at evaporator  208 , the refrigerant may exchange heat with water. The refrigerant is provided from evaporator  206  to compressor  202  by a compressor suction line  209  to complete the refrigeration cycle. 
     As can be appreciated in light of the refrigerant system and circuit described herein, efficient heat exchange with secondary fluids outside of the circuit, for example, at the condenser  204 , is important to the overall efficiency of the refrigeration circuit and the overall efficiency of the refrigeration system described above. Additionally, it can be appreciated that refrigerant in liquid or vapor phase continuously occupies the circuit. Therefore, in order to allow refrigerant to be removed or pumped down from compressor  202 , refrigerant liquid line  205 , or evaporator  208  without removing refrigerant from the circuit, a device must be added to the circuit to temporarily contain the pumped down refrigerant. In addition to allowing for easier servicing of these components, pumpdown can be used to ensure that evaporator  208  contains little or no liquid refrigerant at start up, which reduces potential problems with liquid damage to compressor  202  during start-up conditions. 
     A control system (not shown) may be provided to control operation of compressor  202 . The control system may include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board. Preferably, the control system can execute a control algorithm(s) to control operation of compressor  202 . Additionally, the control system may provide other control operations and monitoring systems to refrigeration circuit  200 , as would be appreciated by one of ordinary skill in the art. While the control algorithm can be embodied in a computer program(s) and executed by the microprocessor, it is to be understood that the control algorithm may be implemented and executed using digital and/or analog hardware by those skilled in the art. If hardware is used to execute the control algorithm, the corresponding configuration of the control system can be changed to incorporate the necessary components and to remove any components that may no longer be required. 
     Compressor  202 , condenser  204 , expansion device  205 , and evaporator  208  form the major components of a refrigeration circuit of the chiller  14  ( FIG. 1 ). Chiller  14  may include one or more refrigeration circuits and each circuit may share one or more components, including the major components. 
       FIGS. 3-5  show an exemplary embodiment of chiller  14  according to the disclosure. Chiller  12  includes at least one compressor  302 , a condenser  304 , at least one expansion device  305 , at least one evaporator  308 , and controls  312 . At least one compressor  302  have been consecutively numbered  1  through  4  as shown. Two compressors  302 , designated as compressors  1   a  and  2   a , are connected as part of the first refrigerant circuit, and two other compressors  4   a  and  5   a , are connected as part of the second refrigerant circuit. For systems with scroll compressors, two or three compressor are normally used in each circuit to provide capacity control and to achieve a larger system capacity that would be available with a single compressor. Two or more refrigerant circuits are normally used with air-cooled chillers to allow for continued cooling in the event of a component failure in one refrigerant circuit. Multiple refrigerant circuits also allow for chiller capacities with more than three scroll compressor in a single circuit. Using more than three or four scroll compressors in a refrigerant circuit can result in low vapor velocity in the suction line if operated with a single compressor. The low velocity can lead to poor oil return from the evaporator, so it is generally preferable to use multiple refrigerant circuits instead of increasing the number of compressors beyond three or four in a single circuit. 
     In this exemplary embodiment, evaporator  306  is partitioned to provide separate heat exchange zones (not designated) for the first and second refrigerant circuits. However, in alternative embodiments, one or more evaporators  306  may be used and configured as necessary as would be appreciated by one of ordinary skill in the art to provide heat exchange between the refrigerant and the cooling fluid provided to at least one air handling unit  22  ( FIG. 1 ). Pump  316  may be provided with chiller  14  that provides for the flow of the cooling fluid between evaporator  308  and at least one air handling unit  22 . In alternative embodiments, pump  316  may be separate from chiller  14 . 
     Condenser  304  includes at least one multichannel heat exchanger coil (coils)  314 , at least one refrigerant-storage vessel  315 , and at least one blower unit  317 . Refrigerant-storage vessel  315  may also be referred to as a receiver. Coils  314  are heat exchangers configured to exchange heat between a refrigerant flowing within coils  314  and a fluid passing over and/or through coils  314 . For example, coils  314  may be microchannel heat exchanger coils or other similar heat exchanger coils as are known in the art. 
     In this exemplary embodiment, condenser  304  includes six coils  314 , which have been consecutively numbered  1  through  6  as shown. Furthermore, in this exemplary embodiment, three coils  314 , designated as coils  1 ,  2  and  3 , are connected as part of a first refrigerant circuit, and three other coils  314 , designated as coils  4 ,  5  and  6 , are connected as part of a second refrigerant circuit. In alternative embodiments, condenser  304  may include one or more coils  314  configured in one or more refrigerant circuits, the number and configuration of coils  314  depending upon the cooling demand of chiller  12 . 
     At least one blower unit  317  draws air into condenser  304  and exhausts air from condenser  304  in direction A. In this exemplary embodiment, chiller  14  includes six blower units  317 . However, in alternative embodiments, more or less than six blower units  317  of varying size and configuration may be used as determined by the cooling demand of chiller  14 . The condenser  304  includes end panels  320  and a bottom panel  322  (see  FIG. 8 ) to assist in channeling substantially all of the cooling air drawn into condenser  304  by blower units  317  through coils  314 . 
     A schematic representation of a two pass flow design coil (design coil)  614  is shown in  FIG. 6 . Header feed line  616  provides refrigerant vapor to a header  618  for distribution to rows of tubes (not shown) that span across an upper section  620  of design coil  614 . Upper section  620 , which may also be referred to as a de-superheat section, is configured to provide for a first pass of the refrigerant across design coil  614 . During this first pass, the vapor refrigerant exchanges heat with cooling fluid, such as air, and is cooled. The refrigerant may also condense in upper section  620 . After the refrigerant completes the first pass, the refrigerant is collected in a return header  622 , which is configured to collect the refrigerant from upper section  620  and distribute the refrigerant to rows of tubes (not shown) in a lower section  630  of design coil  614 . Lower section  630 , which may also be referred to as the sub-cooling section, is configured to provide a second pass for the refrigerant through other rows of tubes (not shown) for further exchange of heat with the cooling fluid. Header  618  collects the refrigerant from the rows of tubes (not shown) forming the second pass, and provides the refrigerant to a refrigerant liquid line  634 . Header  618  and return header  622  preferably are formed of a single tube with an internal partition that separates incoming flow of refrigerant vapor from outgoing flow of refrigerant liquid. Alternatively header  618  and return header  622  may be formed from physically separate tubes providing distribution and collection of refrigerant. It should be appreciated by one of ordinary skill, that the relative proportions of upper and lower sections  620 ,  630 , respectively, and corresponding tubes (not shown) forming the first pass and return pass of the refrigerant, may vary based on application. Additionally, while design coil  614  of this exemplary embodiment is configured to provide a two-pass flow, a single pass or more than two-pass configuration may be used in condenser  614 . 
       FIG. 7  shows a partial section view of an exemplary configuration of a header  718  and tubes  720  for carrying refrigerant across a coil (not shown). Header  718  may be a feed, return or discharge header. Tubes  720  include passageways  722  that carry the refrigerant through tubes  720  where the refrigerant exchanges heat with air or another cooling fluid passing over tubes  720 . In alternative embodiments, other suitable fluid distribution systems or structures may be used to distribute the refrigerant to tubes  720 . 
     Tubes  720  can have a cross-sectional shape in the form or a rectangle, parallelogram, trapezoid, ellipse, oval or other similar geometric shape. Passageways  722  in tubes  720  can have a cross-sectional shape in the form of a rectangle, square, circle, oval, ellipse, triangle, trapezoid, parallelogram or other suitable geometric shape. In one embodiment, passageways  730  in tubes  720  can have a size, e.g., width or diameter, of between about a half (0.5) millimeter (mm) to about three (3) millimeters (mm). In another embodiment, passageways  730  in tubes  720  can have a size, e.g., width or diameter, of about one (1) millimeter (mm). 
     Connected between tubes  720  may be two or more fins or fin sections (not shown). In one embodiment, the fins can be arranged to extend substantially perpendicular to the flow of refrigerant in the tube sections. However, in another embodiment, the fins can be arranged to extend substantially parallel to the flow of refrigerant in the tube sections. The fins can be louvered fins, corrugated fins or any other suitable type of fin. 
     Tubes  720  can be of any suitable size and shape, including, but not limited to, generally rectangular, square, round, oval, triangular or other suitable geometric shape. Fins, plates or other similar heat exchange surfaces (not shown) may be disposed between or used in conjunction with tubes  720  to increase heat transfer efficiency from tubes  720  to the surrounding environment as is known in the art. 
     Referring to  FIG. 4 , condenser  304  further includes compressor discharge lines  410  that supply refrigerant vapor to inlet headers  418  by way of vapor feed lines  416 . Compressor discharge lines  410  are in fluid communication to receive refrigerant vapor from at least one compressor  302 , and in fluid communication to deliver refrigerant to vapor feed lines  416 . Vapor feed lines  416  distribute refrigerant vapor to inlet headers  418  of coils  314 . Inlet headers  418  are configured to provide refrigerant vapor to an upper portion (not shown) of coils  314  for a first pass through tubes (not shown) of coils  314 . After the refrigerant makes the first pass, the refrigerant is collected by return headers  522  (see  FIG. 5 ), which are located at the opposite of coils  314  from the inlet header  418 . Return headers  522  distribute the refrigerant to a lower portion (not shown) of coils  314  for a second pass across other tubes (not shown) of the coils  314 . After the refrigerant completes the second pass, the refrigerant is collected by liquid headers  420  that provide the refrigerant to liquid lines  422  configured to provide refrigerant to at least one expansion device  305  ( FIG. 3 ). 
     As shown in  FIGS. 1 and 5 , condenser  304  further includes a refrigerant-storage vessel  315  in fluid communication with return headers  522  of coils  314  through refrigerant lines  530 . Refrigerant-storage vessel  315  is also in fluid communication with a compressor discharge line  203  ( FIG. 2 ) through hot gas lines  532 . Compressor discharge line  203  ( FIG. 2 ) provides vapor refrigerant to refrigerant-storage vessel  315 . In alternative embodiments, hot gas lines  532  may be in fluid communication with other refrigerant lines containing vapor refrigerant. Refrigerant-storage vessel  315  provides for additional refrigerant circuit volume to provide for pump down refrigerant volume from other components of the refrigeration circuit. 
     The introduction of refrigerant vapor from hot gas lines  532  to refrigerant-storage vessel  315  vaporizes any liquid refrigerant present in refrigerant-storage vessel  315  during normal operating conditions, but permits liquid refrigerant from the refrigerant circuits to flow into refrigerant-storage vessel  315  during pump down operations. 
     The geometry of the hot gas lines  532  is important for proper control of refrigerant in refrigerant-storage vessel  315 . For example, hot gas lines  532  may have an optimum nominal diameter of roughly ¼ to ⅜ inches for copper line that are several feet long. Hot gas lines  532  of significantly larger diameter can introduce an excessive quantity of warm refrigerant vapor to refrigerant-storage vessel  315 , which may adversely affect the performance of the condenser  304  by introducing an excessive amount of refrigerant vapor to coils  314  through refrigerant lines  530 . Hot gas lines  532  having a larger diameter may also raise the temperature of the walls of the refrigerant-storage vessel  315  to a high temperature that interferes with flow of liquid refrigerant into refrigerant-storage vessel  315  during pumpdown. Hot gas lines  530  having a smaller diameter may allow excessive amount of refrigerant liquid to remain in the refrigerant-storage vessel  315  during start-up or operating conditions, especially at lower ambient temperatures. 
     Location of the refrigerant-storage vessel  315  is preferably in the air stream leaving the coils  314 . This location helps to keep the refrigerant-storage vessel  315  at a temperature that is near the refrigerant saturation temperature in the condenser  304 . Other locations are also possible and do not prevent acceptable operation of the system. 
     Refrigerant lines  530  are preferably connected between a bottom of refrigerant-storage vessel  315  and a lower portion of return header  522 . For example, a line nominal diameter of approximately ⅜ inch is sufficient to allow adequate flow of refrigerant between refrigerant-storage vessel  315  and coil  314 . In alternative embodiments, multiple refrigerant lines  530  may be used for each refrigerant-storage vessel  315 . In general, the bottom of the refrigerant-storage vessel  315  should be connected to coil  314  at location that is intermediate between vapor feed lines  416  and liquid lines  422 . 
     While these embodiments show coils  314  having two refrigerant passes, other coil pass configurations are possible. For example, more than two refrigerant passes may be used. Depending on the details of the coil geometry and design conditions, three or more passes may be preferred. In this case, the preferred connection location for refrigerant line  530  to coil  314  is at a header at an entrance to a second or higher pass. 
     Connection to inlet header  418  is not preferred because of two important factors. First, liquid refrigerant cannot be present at this location until the coil is nearly full of liquid, which can result in at least one compressor  302  shutting down on high discharge pressure before a pumpdown is complete. A second factor is that there is almost no refrigerant pressure drop to drive a flow of refrigerant vapor to the refrigerant-storage vessel  315  at this location, which can result in liquid refrigerant accumulating refrigerant-storage vessel during normal chiller operation. 
     Furthermore, connection of the refrigerant line  530  at an outlet of coil  314  is also not preferred. The problem is that any refrigerant vapor that leaves refrigerant-storage vessel  315  goes directly into liquid line  530 . This configuration may result in reduced subcooling and even vapor entering at least one expansion device  305 , which can penalize system performance and can even create reliability issues unless a valve or other active control device is included in the hot-gas line  532  to prevent excessive flow of refrigerant vapor out of refrigerant-storage vessel  315 . 
     In this exemplary embodiment, the condenser  304  includes two refrigerant-storage vessels  315  designated as a first refrigerant-storage vessel  315   a  and a second refrigerant-storage vessel  315   b , as shown in  FIG. 5 . Refrigerant lines  530  are in fluid communication with return headers  522  proximate to where return headers  522  provide refrigerant to a lower section (not shown) of coils  314  at a location where return headers  522  contain substantially liquid refrigerant during normal condenser operations. Refrigerant lines  530  are also in fluid communication with the bottom of refrigerant-storage vessels  315   a ,  315   b  so as to be in fluid communication with any liquid refrigerant present in refrigerant-storage vessels  315   a ,  315   b.    
     First refrigerant-storage vessel  315   a  is in fluid communication with coil  1  to provide pump down volume for the first refrigerant circuit, and second refrigerant-storage vessel  315   b , is in fluid communication with coil  6  to provide pump down refrigerant volume for the second refrigerant circuit. Connecting refrigerant-storage vessels  315   a ,  315   b  to coils  1 ,  6 , respectively, at only one return header location eliminates the possibility of pulling liquid into refrigerant-storage vessels  315   a ,  315   b  because of pressure differences between different return headers  522  ( FIG. 5 ). In this exemplary embodiment, refrigerant lines  530  are connected to coils  1 ,  6  because in this condenser configuration, coils  1  and  6  have improved access to cooling air drawn by the blower units  317  flow compared to coils  2 ,  3 ,  4  and  5 . The improved air flow access results in improved cooling and subcooling to coils  1 ,  6 , which results in the refrigerant in the return headers  522  of coils  1 ,  6  is more likely to be liquid. In an alternative embodiment, a refrigerant-storage vessel  315  may be connected to any coils  1  through  6 , and one or more than two refrigerant-storage vessels  315  may be used. 
     For example, in this exemplary embodiment, the configuration of refrigerant-storage vessels  315   a ,  315   b  as shown in this exemplary embodiment may permit refrigerant subcooling of about 15° F. to about 20° F. in condenser  314  without a significant amount of liquid refrigerant being present in refrigerant-storage vessels  315   a ,  315   b . In other words, during normal refrigeration system operating conditions, refrigerant-storage vessels  315   a ,  315   b  contain substantially all vapor refrigerant. 
       FIG. 8  shows a side perspective view of a section of condenser  304  having coil  6  removed to view internal detail. As can be seen in  FIG. 8 , refrigerant-storage vessel  315  has a generally cylindrical geometry. Refrigerant-storage vessel  315  is a hollow cylinder, preferably with an internal diameter of less than six inches so as to be exempt from the ASME code for pressure vessels. Refrigerant-storage vessel  315  may be provided with an insulating outer layer  805 , but in a preferred embodiment, the refrigerant-storage vessel has no insulating outer layer  805 . Refrigerant-storage vessel  315  is supported by end walls  320  and an interior wall  812  disposed therebetween as shown in  FIG. 8 . However, in alternative embodiments, refrigerant-storage vessel  315  may be supported by any similar configuration of walls and supports. 
     At least one refrigerant-storage vessel  315  are configured to hold liquid refrigerant from a refrigeration circuit when a component of that refrigeration circuit is pumped down. Pumpdown is normally initiated immediately before shutdown of at least one compressor  302  in a refrigerant circuit. Pumpdown normally starts with controls  312  closing a liquid-line solenoid valve (not shown) located in the refrigerant circuit between condenser  304  and at least one expansion device  305 . Closing the liquid-line solenoid valve stops the flow of refrigerant liquid out of condenser  304 , which causes liquid refrigerant to back up into condenser  304 . At least one compressor  302  continues to operate and to pump refrigerant vapor from at least one evaporator  308  to condenser  304 . As the liquid refrigerant starts to accumulate in condenser  304  the heat-transfer surface area that is available for condensing refrigerant decreases, which causes a rapid rise in the condenser refrigerant pressure. The rapid increase in pressure causes liquid refrigerant to flow from return headers  522  of condenser  304  through refrigerant lines  530  that connect to at least one refrigerant-storage vessel  315 , which allows liquid refrigerant to accumulate in at least one refrigerant-storage vessel  315 . A pressure transducer (not shown) on compressor discharge line  203  ( FIG. 2 ) in combination with controls  312  may reduce compressor capacity during the pumpdown process to prevent excessively high discharge pressures. A suction pressure transducer (not shown) in combination with controls  312  terminates the pumpdown process when the compressor suction pressure falls below a predetermined minimum value, which corresponds to a condition with little or no liquid refrigerant in at least one evaporator  308 . The configuration of at least one refrigerant-storage vessel  315  in condenser  304  allows controls  312  to operate in a manner that is very similar to that for convention round-tube condenser coils that have sufficient internal volume to hold refrigerant liquid without a separate refrigerant storage vessel. 
     For storage of refrigerant for servicing or shipping, pumpdown may be initiated by manually closing service valve (not shown) located on the refrigerant liquid line  205  ( FIG. 2 ). The service valve is normally located on refrigerant liquid line  205  between the condenser  304  and the liquid-line solenoid valve (not shown). Closing the service valve will cause liquid refrigerant to move into the condenser  304  and at least one refrigerant-storage vessel  315  in a process that is similar to that described above, except that the liquid-line solenoid valve remains open during the process. Controls  312  would normally closed the liquid-line solenoid valve only after the suction pressure drops below the specified minimum valve, which corresponds to the shutdown of at least one compressor  302 . 
     While the exemplary embodiments illustrated in the figures and described herein are presently preferred, it should be understood that these embodiments are offered by way of example only. Accordingly, the present application is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims. The order or sequence of any processes or method steps may be varied or re-sequenced according to alternative embodiments. 
     While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (For example, variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (For example, temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (For example, those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.