Patent Publication Number: US-2009223232-A1

Title: Defrost system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a defrost system for defrosting components on which frost is formed, where the defrost system comprises at least one compressor, which compressor has a hot gas outlet, which is connected to condensing means, from where primarily liquid refrigerant is connected to pressure reduction means, from where the flashing refrigerant is led through evaporator means. 
     The present invention further relates to a method for defrosting a refrigeration system comprising at least one refrigeration system component on which frost is formed, where defrosting is performed by heating the refrigeration system component in periods of no operation of that refrigeration system component. 
     BACKGROUND OF THE INVENTION 
     Prior art shows that it is possible to use glycols or brines to defrost a refrigeration coil. The disadvantage of this solution is the associated problems with erosion if the liquid moves too fast through the pipes especially in the bends. A way to solve this problem would be to use traditional hot gas-defrost with a compressor also as seen in prior art. The disadvantage of this system is the oil management problems when working with many compressors at different suction pressures as in the European patent EP 1 409 936. A way to solve this problem could be to pump the CO2 into a vessel and then heat/evaporate this and use the generated hot gas to defrost the coil as in the patents U.S. Pat. No. 5,400,615 or GB 2,258,298. The simple solution to solve this problem is to use the brine circuit at a pressure designed for a high working pressure and with CO2 as the working fluid condensing at an appropriate temperature. This solution solves the problems found in the brine solution and eliminates the problems seen with oil management in the traditional hot gas solutions. It also eliminates the use of high-pressure pumps as seen in the boiling out system. 
     U.S. Pat. No. 6,588,221 describes a method of defrosting a refrigeration system having a main compressor connected by a main hot gas discharge line to a condenser, the condenser being connected by a main liquid line to thermal expansion valves and subsequent cooling coils, each thermal expansion valve and cooling coil being in parallel connection with the other thermal expansion valves and cooling coils, and each cooling coil being connected by a suction line to the main compressor. The defrosting method includes passing hot gas from the main hot gas discharge line through a selected cooling coil to defrost same, compressing cooled gas which has passed through the cooling coil by means of a separate dedicated defrost compressor, and returning the compressed hot gas to the main hot gas discharge line. 
     OBJECT OF THE INVENTION 
     The object of the invention is to perform effective defrosting by means of a separate defrost system thus avoiding oil management problems. A further object is to achieve a lower system cost. 
     DESCRIPTION OF THE INVENTION 
     This can be achieved if the defrost system is formed as an independent cooling system, where the condensing means are transmitting heat to the defrosting components. 
     It can hereby be achieved that the defrost system can operate completely independent of the refrigeration system. All negative effects with traditional defrost operation of refrigeration systems are overcome by this solution where the defrost system operates as a system without any influence from the refrigeration system. Even the working fluid can be different so that the refrigeration system can use CO2, the defrost system can operate with a traditional refrigerant like 134A. In this way, it becomes possible to build the defrost circuit with other pressure conditions than those of the refrigeration system. In fact, this defrost system is operating as a heat pump where the condensing heat is used for defrosting. The defrost system can only operate if the refrigerant after passing through the condensing means is sent through at least an expansion valve and evaporator means before the refrigerant is returned to a compressor. In this way, a waste of cooling energy is performed. This cooling energy could be used in combination with the refrigeration system. Depending on the environmental operating conditions, the evaporator from the defrosting system could be used as a part of an air condition system. Also in combination with refrigeration systems, the evaporator can be used in combination with condensing or subcooling of refrigerant. 
     Preferably, the evaporator of the defrost system can be cooperating with external cooling means or with the refrigeration system. This can lead to a reduction of the power consumption of the refrigeration system. 
     The defrost system can be operating in conjunction with a refrigeration system, where the condensing means of the defrost system is cooperating with cooling components cooled by the refrigeration system. It can hereby be achieved that for example an evaporator can comprise another circuit for circulating and condensing the defrost refrigerant. In this way, it is possible to perform defrost in non-operating periods of the refrigeration system. In a refrigeration system, evaporators can be cut out of operation, and defrost can be performed in different evaporators. In single evaporator systems, the refrigeration system can be stopped, and the defrost system can be started. Hereby, the refrigeration system can be defrosted without having to reverse the refrigeration system. 
     The defrost system can operate in conjunction with a refrigeration system, which refrigeration system is in standstill, where the condensing means of the defrost system is cooperating with cooling components cooled by the refrigeration system. This can lead to a fast defrosting of evaporators or other cooling means. 
     Preferably the defrost system comprises a liquid receiver, which receiver is connected to an expansion valve, where a gas connection from the upper part of the receiver can be connected to the evaporator through a modulating valve. The receiver can be installed either in the liquid line or in the compressor suction line. 
     It can hereby be achieved that if the amount of defrost fluid in gas form and under high pressure reduces the liquefying of defrost fluid, and pressure is still increasing in the liquid receiver, then it is possible to open the modulating valve and let some of the high pressure defrost gas flow towards the expansion valve. This high pressure defrost gas is mixed with the defrost fluid, which after passing the expansion valve will flash, and the cooling energy that will be delivered to the evaporator will in this way be reduced, but the defrost system can continue in normal operation. 
     The evaporator of the defrost system can cooperate with the refrigeration system by forming the evaporator in a second heat exchanger which is heated by partly or fully liquefied refrigerant of the refrigeration system. It is hereby achieved that all the cooling effect that is achieved by the defrost operation is transmitted to the refrigeration system as cooling energy. This can lead to a very effective combination of a refrigeration system and a defrost system. The cooling energy can be transmitted independently of the fact that the two systems can operate quite differently in pressure and also in the type of refrigerant. 
     The upper part of the receiver in the defrost system can be used as a liquid separator, where the top of the receiver is connected through a first heat exchanger for liquefying the gas, which liquid is led back towards the receiver, where the first heat exchanger is part of a cascade heat exchanger, which is part of the refrigeration system. It can hereby be achieved that if the defrost operation is not led to a complete condensation of the refrigerant; a relatively high amount of gas will enter the receiver. This can lead to a pressure increase in the receiver, and it is therefore necessary to condensate a part of that gas. This condensation process can take place in a heat exchanger so that the extra heat is transported by a cascade refrigeration system to the refrigeration system where the extra heat simply is sent directly to existing condensing means. By reducing the amount of gas in the receiver, the pressure is reduced, and as such the defrost system is operating more efficiently. 
     The receiver can comprise a heat-exchanging coil in the upper part, which coil is connected to the liquid outlet from the receiver, where the coil is reducing the temperature of the gas in the top of the receiver. This is an alternative method to condensate the amount of gas that might end in the receiver in certain operation situations. This coil will automatically lead to condensation of the gas, which also reduces the pressure. 
     The invention comprises one or more compressors, one or more coils to be defrosted with a separate defrost circuit designed for the refrigerant used, a liquid receiver, an expansion valve and an evaporator in thermal connection to a heat source that can be air or any type of liquid e.g. from other processes or heat loads coming from cooling other products. The refrigerant can be any refrigerant pure or mixtures of HFC e.g. R134a or CO2. 
     The advantage of the system is the fact that the defrost system is operating independently of the primary and secondary refrigeration system operating as a cascade refrigeration system. The defrost system can be thermally connected whenever the design gives an opportunity to optimise the process. The interaction is not limited to the refrigeration system itself, but it can be connected to other processes as well. 
     An advantage is also that defrost can take place regardless of whether the refrigeration system is running or not. The defrost system can also be used with a one-evaporator system without requiring the refrigeration system to work. Another advantage of the system is the freedom of choice of refrigerant. All refrigerants can be used, but the preferred refrigerant is CO2. 
     The defrost method can be performed by an independent defrost system, which defrost system comprises at least compressing means for compressing and heating a defrost gas, which defrost gas is heating the refrigeration system components by condensing the defrost gas, which defrosting is performed in periods of no supply of refrigerant to the refrigeration system component from the refrigeration system, where the defrost system comprises a closed circuit for defrost fluid without connection to the refrigeration system. 
     This method can lead to a very energy effective defrost because the heat is generated from the defrost system operating as a heat pump, and the defrost fluid can be expanded in evaporation means connected to the refrigeration system. Because the defrost system is independent of the refrigeration system can different refrigeration media be used. 
    
    
     
       DESCRIPTION OF THE DRAWING 
         FIG. 1  shows an example of how a CO2 system can be connected to the defrost coil, 
         FIG. 2  shows an example of how the defrost system and the cooling system can be combined in operation, 
         FIG. 3  shows an alternative embodiment for the invention, 
         FIG. 4  shows an external load as the evaporator, 
         FIG. 5  shows a receiver comprises a coil, and 
         FIG. 6  shows an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an example of how a CO2 system can be connected to the defrost coil in an air-cooled evaporator.  FIG. 1  shows a defrost system  2  and a refrigeration system  4 . The defrost system  2  comprises a compressor  10  which has a gas outlet line  12 , 14 , where the line  12  is connected to a solenoid valve  20  from where a line  22  leads to condenser  30  placed in conjunction with a cooling system. From here, a line  32 , which is combined with another line  34 , ends up in a line  36 , which leads to a liquid receiver  40 . This liquid receiver has a liquid outlet  42 , which leads to an expansion valve  50 , from where a line  52  leads to an evaporator  70 . From the upper part of the receiver  40 , a line  44  leads to a magnetic modulating valve  60  from where a line  62  leads to the line  52  near the inlet to the evaporator  70 . From the evaporator  70 , a line  72  is arranged, which leads to the inlet of the compressor  10 . 
     The refrigeration system  4  comprises a compressor  80  which has a hot gas outlet line  82  connected to a cascade condenser  90 . The cascade condenser  90  is by a line  92  connected to a receiver  100  from where a line  102  over a control valve  110  is connected to a main receiver  120 . The main receiver  120  has an outlet line  122  connected to pumping means  130 , from where a line  132  continues in not shown lines  134 . The line  132  is connected to an expansion valve  140  from where a line  142  leads to the evaporator  150 . From the evaporator  150 , a line  152  is combined by a line  154  to a line  156 , which leads into the main receiver  120 . From the top of the receiver  120 , a line  124  connected to the suction side of the compressor  80  is arranged. 
     When the defrost cycle is required, one or more defrost compressors  10  and the solenoid valve  20  in front of defrost coil  30  that is to be defrosted start/open. In the beginning of the process, there will be a lot of defrost liquid returning to the liquid receiver  40 . Later in the defrost cycle, there is a larger amount of defrost gas returning to the receiver  40 , and the pressure will increase. The surplus defrost gas can be removed by a modulating valve  60  and led to the evaporator. The efficiency of the evaporator will deteriorate but the main object of the evaporation is to generate warm defrost gas for the defrosting of the coil  150 . The defrost system can be designed with CO2, R744 as two phase defrost agent. It must be ensured that the pressure in the receiver  40  is controlled by means of cooling by air or by the circuit itself. The defrost system can be equipped with an additional condenser allowing it to act as an independent refrigeration system when not used for defrosting. Then it can be used for other purposes like air conditioning or as a cooling process. The heat can when not needed be used for heating with an air coil or heating of water for other purposes when there is no need for the defrost capacity, or if parts of the full capacity are not needed. The compressors  10  are built into a centrally based system. The capacity on each rack can be adjusted to fit all sorts of capacities. In a total plant, there can be a need for many sizes of evaporators and some can have a high need when the defrost starts while some evaporators has a lower need. This can require special control strategy that can be handled from a central processing unit. 
     The system comprises:
         1. A closed refrigerant loop designed only to defrost with a compressor, defrost coil/condenser, expansion valve and evaporator and may be a modulating bypass valve.   2. The evaporator can be heated by the main system or other heat load.   3. 1 and 2 the refrigerant can be the same as in the refrigerant used in the main system   4. 1 and 2 the refrigerant can be different from the refrigerant used in the main system   5. 3 with the same medium as in the main system, there can be an equalising connection for charge exchange/equalisation.   6. The defrost cycle can be part of another process and defrost on demand enhancing the running conditions of the process because the condensing pressure might be lowered.       

       FIG. 1  shows an example of how the defrost system and the cooling system can be combined in operation. Most of the components are equal to the description of  FIG. 1 , and the following description will only concern features not mentioned before. The first difference is that the liquefied refrigerant from the refrigeration system  4  in the line  102  passes through a heat exchanger  210  where this liquid refrigerant is heat exchanged with the defrost refrigerant from the line  52  which is expanded in the expansion valve  50  where this refrigerant is evaporated in the heat exchanger  210  before the defrost refrigerant is led through the line  72  towards the compressor  10 . The sub-cooled refrigerant is from the heat exchanger  210  by line  212  led to a control valve  110  from where it is sent to the main receiver  120 . 
     A second difference to  FIG. 1  is a heat exchanger  200 , which is placed as part of the cascade heat exchanger  90  for heat exchanging to a primary refrigerant, where the heat exchanger  200  is connected to the top of the receiver  40  by a line  202  and where a line  204  leads primary liquid defrost refrigerant into the line  36  towards the receiver  40 . Gas with high temperature and high pressure can hereby be sent to condensing in the cascade heat exchanger  200 . 
     The main idea about this cycle is to use the cooling capacity for sub-cooling the liquid used in the main system. The disadvantage of this solution is that defrost can only take place when the main system is working, and the capacity is dependent on the liquid available. 
       FIG. 3  shows an alternative embodiment for the invention, where it is assumed that all or nearly all refrigerant is condensed when returned to the receiver. Adding an air-coil to the defrost circuit will help giving the capacity even if the main system is not running 
       FIG. 4  shows nearly the same system as shown in  FIG. 1  and the only difference is that an external load  220  is shown as the evaporator. 
     In this case, the cooling capacity is used for cooling an external load. This solution requires a constant load on the cooling side of the defrost cycle. 
     In  FIG. 5 , another receiver  240  is shown where this receiver comprises a coil  242  for internal heat exchange in the top of the receiver  240 . The outlet  42  from the receiver  240  leads to a regulation valve  244  from where a line  245  leads to the coil  242 . The outlet from this coil  242  leads to the expansion valve  50 . 
     The expansion valve  50  keeps up the pressure in the defrost coil, and the valve  60  controls the superheat on the air-coil. The valve  60  opens only in case of an increase in pressure of the receiver  40 . 
       FIG. 6  describes an alternative embodiment of the invention. The same numbers are used as in previous figures so only the differences will hereafter be described. The cooling energy generated by the defrost system is now transmitted by a heat exchanger  370  into the cascade condenser  390  of the refrigeration system  4 . A receiver  340  is operating as previously described, and from here, liquid defrost fluid is sent through the expansion valve  50  towards the heat exchanger  370 . 
     This leads to a very effective evaporation of the defrost liquid, and waste heat from the cascade condensing unit is used for this evaporation.