Patent Publication Number: US-2016231040-A1

Title: Refrigeration circuit with heat recovery module

Description:
Refrigeration circuits comprising in the direction of flow of a circulating refrigerant a compressor, a gas cooler/condenser, an expansion device and an evaporator are known in the state of the art. It is also known to provide a heat recovery module in the refrigeration circuit in order to recover at least some of the energy used for compressing and heating the refrigerant. 
     From a thermodynamic point of view such heat recovery modules are applied in a very wide range (high temp heating, low temp heating, . . . ) at maximum performance. As such a wide-range applicability requires a lot of different parts, especially differently sized heat exchangers (e.g. brazed plate heat exchangers “BPHE”) for the exchange of heat between the circulating refrigerant and an external heat recovery fluid, on stock to be used in the modules, the modules are very expensive, sometimes too expensive for customers. 
     Accordingly, it would be beneficial to provide a cooling system with an inexpensive heat recovery module comprising only a single heat exchanger instead of two or even more heat exchanger allowing to adjust the heat transfer provided by the heat recovery module as in the common wide-range modules, which avoids the problems related to the use of only a single heat exchanger, which may include the problem of partial condensation at the outlet of the heat recovery module. 
     A refrigeration circuit according to an exemplary embodiment of the invention, which is configured for circulating a refrigerant, comprises in the direction of flow of the refrigerant: at least one compressor; at least one heat recovery heat exchanger; a gas/liquid separator; at least one gas cooler/condenser; at least one receiver; and at least one evaporator with an evaporator associated expansion device fluidly connected upstream thereof. The gas/liquid separator comprises a refrigerant inlet, which is fluidly connected to an outlet side of the at least one heat recovery exchanger, a gaseous phase outlet, which is fluidly connected to an inlet side of the at least gas cooler/condenser, and a liquid phase outlet, which is fluidly connected to the receiver. 
     An exemplary method of operating a refrigeration circuit according to the invention comprises the steps of:
         compressing a refrigerant;   providing heat-exchange between the compressed refrigerant and an external heat recovery fluid;   separating the compressed refrigerant into a gaseous component and a liquid component, wherein the gaseous component is gas cooled and/or condensed and delivered into a receiver; and the liquid component is delivered directly into the receiver without being gas cooled/condensed before; and   expanding and evaporating the liquefied refrigerant taken from the receiver.       

     A method and a refrigeration circuit according to exemplary embodiments of the invention allow to use a single BPHE, instead of two or even more as in the common wide-range modules, in combination with a constant speed pump for flowing the external heat recovery fluid circuit through the BPHE, in order to account for the need of a cheaper version of the heat recovery module. 
     When only a single heat exchanger having a constant heat transfer capacity is used, partial condensation of the refrigerant having passed the heat recovery module may occur. The gas/liquid separation according to the invention, however, reliably avoids problems related to partial condensation of the refrigerant within or downstream of the heat recovery heat exchanger as according to exemplary embodiments of the invention the liquid component of the refrigerant is separated from the gaseous component and delivered directly to the receiver bypassing the gas cooler/condenser. This in particular avoids that condensed refrigerant is delivered into the gas cooler/condenser where it would deteriorate the gas cooler&#39;s/condenser&#39;s performance. It further avoids the need of lifting the liquid refrigerant to the level of the condenser/gas cooler, which may be provided at a level significantly above the level of the compressor(s) and the heat recovery module. 
    
    
     
       An exemplary embodiment of the invention is described in greater detail below with reference to the figures, wherein: 
         FIG. 1  shows a schematic view of a cooling system comprising a refrigeration circuit according to an exemplary embodiment of the invention; and 
         FIG. 2  shows an enlarged view of the gas/liquid separator comprised in the cooling system shown in  FIG. 1 . 
     
    
    
       FIG. 1  shows a schematic view of an exemplary embodiment of a cooling system with a refrigeration circuit  1  comprising in the direction of the flow of a refrigerant, which is circulating within the refrigeration circuit  1  as indicated by the arrows A, a set  2  of compressors  2   a,    2   b,    2   c,    2   d  connected in parallel to each other, a heat recovery heat exchanger  4 , a gas/liquid separator  8 , a gas cooler or condenser  10 , a high pressure expansion device  12 , which is configured to expand the refrigerant from high pressure to a lower medium pressure, a receiver (refrigerant collector)  14 , an optional flash gas heat exchanger  16 , an evaporator associated expansion device  18 , which is configured to expand the refrigerant from medium pressure to low pressure, and an evaporator  20 . The outlet side of the evaporator  20  is fluidly connected to the suction (inlet) side of the compressors  2   a,    2   b,    2   c,    2   d  completing the refrigerant cycle. 
     Thus, the exemplary embodiment of a refrigeration circuit  1  shown in  FIG. 1  employs a one-stage compression by means of the compressors  2   a,    2   b,    2   c,    2   d  connected in parallel and a two-stage expansion by successive expansions by means of the high pressure expansion device  12  and the subsequent evaporator associated expansion device  18 . Such a two-stage expansion is in particular employed when CO 2  is used as the refrigerant. The skilled person, however, will easily understand that the invention may also applied to a refrigeration circuit  1  which employs a single-stage expansion by means of only an evaporator associated expansion device  18  arranged upstream of the evaporator  20  and in which the high pressure expansion device  12  is omitted. 
     An optional flash gas tapping line  21  fluidly connects an upper portion  14   a  of the receiver  14  to the inlet side of the compressors  2   a,    2   b,    2   c,    2   d  allowing flash gas collecting in the upper portion  14   a  of the receiver  14  to bypass the evaporator  20 . A flash gas expansion device  22  is arranged in the flash gas tapping line  21  in order to expand the flash gas delivered from the receiver  14 . Downstream of said flash gas expansion device  22  an optional flash gas heat exchanger  16  is provided in order to cool the expanded flash gas by means of heat exchange with the refrigerant supplied from the receiver  14  to the evaporator associated expansion device  18 . 
     Although the exemplary embodiment shown in  FIG. 1  comprises only a single gas cooler/condenser  10 , a single medium pressure valve  18  and a single evaporator  20 , respectively, it is self evident to the skilled person that a plurality of each of said components  10 ,  18 ,  20  respectively connected in parallel to each other may by provided in order to provide enhanced condensing and/or cooling capacities. In this case additional switchable valves may be provided in order to allow to selectively activate and deactivate one or more of the plurality of said components in order to adjust the condensing and/or cooling capacity to the actual needs. 
     Similarly, only a single compressor may be provided instead of the set  2  of a plurality of compressors  2   a,    2   b,    2   c,    2   d  as it is shown in  FIG. 1 . Said single compressor or at least one of the plurality of compressors  2   a,    2   b,    2   c,    2   d  may be a variable speed compressor allowing to control the cooling capacity provided by the refrigeration circuit  1  by controlling the speed of said compressor. 
     In operation the compressed refrigerant leaving the set  2  of compressors  2   a ,  2   b,    2   c,    2   d  passes a refrigerant circuit side  4   a  of the heat recovery heat exchanger  4  for transferring heat from the refrigerant to an external heat recovery fluid circulating within a heat recovery fluid circuit  6  and flowing through a heat recovery fluid circuit side  4   b  of the heat recovery heat exchanger  4 . The heated external heat recovery fluid may be used e.g. for heating the building and/or providing heated water. 
     Depending on the amount of heat transferred from the circulating refrigerant to the external heat recovery fluid at least a portion of the refrigerant may condensate within or downstream of the heat recovery heat exchanger  4 . As a result, a refrigerant liquid gas mixture is present in the outlet line  7  of the refrigerant circuit side  4   b  of the heat recovery heat exchanger  4 . 
     Liquid refrigerant entering the gas cooler/condenser  10 , however, will deteriorate the gas cooler&#39;s/condenser&#39;s  10  performance and when the gas cooler/condenser  10  is installed at a significant higher level, i.e. at a vertical distance d of up to 20 m with respect to the other components of the refrigeration circuit  1 , the liquid component of the refrigerant may not be able to be transferred completely to the outlet side of the gas cooler/condenser  10 , which will deteriorate the performance and efficiency of the refrigeration circuit  1  even further. 
     Therefore the gas/liquid separator  8  is provided downstream of the refrigerant circuit side  4   b  of the heat recovery heat exchanger  4  in order to separate the liquid portion of the refrigerant leaving the heat recovery heat exchanger  4  from its gaseous component. 
     An enlarged view of such a gas/liquid separator  8  is shown in  FIG. 2 . 
     The gas/liquid separator  8  may be formed by a vessel or pipe having a significant larger cross-section/diameter than the outlet line  7  of the heat recovery heat exchanger  4 , which also forms the inlet line  7  of the gas/liquid separator  8 , in order to considerably reduce the velocity of the refrigerant allowing the liquid component of the refrigerant to separate from the refrigerant&#39;s gaseous component and to collect at the bottom  8   a  of the gas/liquid separator  8 . In an exemplary embodiment the cross-section or diameter of the gas/liquid separator  8  is four to five times larger that the diameter of the outlet line  7  and the velocity of the refrigerant is reduced from approx. 9 m/s at the outlet of the compressors  2   a,    2   b,    2   c,    2   d  to approx. 0.3 m/s within the vessel or pipe of the gas/liquid separator  8 . 
     The gaseous component of the refrigerant leaves the gas/liquid separator  8  via the gas/liquid separator&#39;s  8  gaseous phase outlet line  9  fluidly connecting the top  8   b  of the gas/liquid separator  8  to the inlet side of the gas cooler/condenser  10 . The gaseous component of the refrigerant is gas cooled and/or condensed within the gas cooler/condenser  10 , expanded by the high-pressure expansion device  12  fluidly connected to the outlet side of the gas cooler/condenser  10  and delivered into the receiver  14 , which is fluidly connected to the outlet side of the high-pressure expansion device  12 . 
     A liquid phase outlet line  13  of the gas/liquid separator  8  is fluidly connected to the bottom  8   a  of the gas/liquid separator  8  allowing to transfer liquid refrigerant, which has collected at the bottom  8   a  of the gas/liquid separator  8 , into the receiver  14 . 
     At least one switchable valve  24  is arranged in the liquid phase outlet line  13  allowing to selectively open and close the fluid connection between the gas/liquid separator  8  and the receiver  14 . 
     The gas/liquid separator  8  is further provided with a liquid level sensor  26 , which is configured for sensing the level of liquid refrigerant collected at the bottom  8   a  of the gas/liquid separator  8 . The liquid level sensor  26  is functionally connected to a control unit  28 , which is configured for opening the at least one switchable valve  24  when the level of liquid refrigerant collected in the gas/liquid separator  8  exceeds a first predetermined level and for closing the at least one switchable valve  24  when the level of liquid refrigerant collected in the gas/liquid separator  8  falls below a second predetermined level, which is equal or lower than the first predetermined level. Closing the at least one switchable valve  24  avoids that gaseous refrigerant may bypass the gas cooler/condenser  10  via the liquid phase outlet line  13  in case the level of liquid refrigerant collected at the bottom  8   a  of the gas/liquid separator  8  is so low that it allows gaseous refrigerant to enter the liquid phase outlet line  13 . 
     The liquid level sensor  26  may be a mechanical or electromechanical flush type fluid indicator or an electronic level sensor. 
     In a first embodiment the switchable valve  24  may be a simple on/off-valve, having only a closed state and a single open state. 
     In an alternative embodiment the switchable valve  24  may have two or more different open states providing at least two different open cross-sections allowing to regulate the flow of liquid refrigerant form the gas/liquid separator  8  to the receiver even more accurately. In another embodiment the open cross-section of the switchable valve  24  may be controlled continuously for allowing an even finer control of the flow of liquid refrigerant out of the gas/liquid separator  8 . 
     In yet another embodiment, at least one additional switchable valve  25  is connected in parallel to the switchable valve  24 . Providing two or more switchable valves  24 ,  25  connected in parallel allows to control the amount of liquid refrigerant flowing out of the gas/liquid separator  8  by selectively opening one or more of said switchable valves  24 ,  25 . The switchable valves  24 ,  25  may have the same or different open cross-sections (Kv-values). 
     In an embodiment the refrigeration circuit comprises at least one switchable valve configured for selectively opening and closing the liquid-outlet of the gas/liquid separator. The refrigeration circuit further comprises a liquid level sensor, which is configured for detecting the amount of liquid refrigerant present in the gas/liquid separator, and a control unit, which is configured for operating the at least one switchable valve based on the amount of liquid refrigerant detected by the liquid level sensor. The control unit may be integrated with the liquid level sensor or the switchable valve or may be provided as a separate unit. 
     Controlling the at least one switchable valve based on the level of liquid refrigerant collected within the gas/liquid separator allows liquid refrigerant to flow from the gas/liquid separator into the receiver, but avoids gaseous refrigerant from flowing from the gas/liquid separator into the receiver bypassing the gas cooler/condenser. 
     In an embodiment the switchable valve is switchable between a closed state and at least two different open states. Providing a switchable valve having at least two different open states allows to control the flow of liquid refrigerant from the gas/liquid separator into the receiver more accurately. In one embodiment the switchable valve may by controlled continuously which allows an even finer control of the flow of liquid refrigerant from the gas/liquid separator into the receiver. 
     In an embodiment the refrigeration circuit comprises at least two switchable valves, which are connected in parallel. Selectively opening and closing one or more of the at least two switchable valves allows to control the flow of liquid refrigerant from the gas/liquid separator into the receiver more accurately. The switchable valves may have the same or different open cross-sections/Kv-values. Providing switchable valves having different open cross-sections/Kv-values provides additional options for regulating the flow of refrigerant. 
     In an embodiment the at least one gas cooler/condenser is arranged at a level above the level of the heat recovery heat-exchanger in order to improve a flow of air flowing through the gas cooler/condenser, which will enhance the gas cooler&#39;s/condenser&#39;s gas cooling capabilities. The gas cooler/condenser may in particular be arranged up to approximately 20 meter above the level of the heat recovery heat-exchanger, e.g. on top of the building housing the refrigeration circuit. 
     In an embodiment the refrigeration circuit comprises at least one additional (high-pressure) expansion device fluidly connected between the gas cooler/condenser and the receiver in order to provide, in combination with the evaporator associated expansion device arranged upstream of the evaporator, a two-stage expansion enhancing the performance and efficiency of the refrigeration circuit. 
     In an embodiment the refrigeration circuit comprises a flash gas tapping line fluidly connecting an upper portion of the receiver to the inlet side of the at least one compressor. A flash gas expansion device may be arranged within the flash gas tapping line. The bypass-line may further comprise a flash gas heat-exchanger arranged downstream of the flash gas expansion device which is configured for heat-exchange of the flash gas with refrigerant flowing from the receiver to the evaporator. Such a flash gas tapping line may help to enhance the performance and efficiency of the refrigeration circuit. 
     In an embodiment at least one compressor is configured as a variable speed compressor allowing to control the cooling capacity provided by the refrigeration circuit by controlling the speed of said compressor. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt the particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore it is intended that the invention is not limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. 
     REFERENCE NUMERALS 
       1  refrigeration circuit 
       2  set of compressors 
       2   a,    2   b,    2   c,    2   d  compressors 
       4  heat recovery heat exchanger 
       4   a  refrigerant circuit side of the heat recovery heat exchanger 
       4   b  heat recovery fluid circuit side of the heat recovery heat exchanger 
       6  heat recovery fluid circuit 
       7  inlet line of the gas/liquid separator/outlet line of the heat recovery heat exchanger 
       8  gas/liquid separator 
       8   a  bottom of the gas/liquid separator 
       8   b  top of the gas/liquid separator 
       9  gaseous phase outlet line of the gas/liquid separator 
       10  gas cooler/condenser 
       12  high-pressure expansion device 
       13  liquid phase outlet line of the gas/liquid separator 
       14  receiver 
       14   a  upper portion of the receiver 
       16  flash gas heat exchanger 
       18  evaporator associated expansion device 
       20  evaporator 
       21  flash gas tapping line 
       22  flash gas expansion device 
       24  switchable valve 
       25  additional switchable valve 
       26  liquid level sensor 
       28  control unit