Liquid separator for an evaporator system

Abstract

Liquid separator (2), designed as a U-shaped pipe (5) and arranged essentially horizontally, for a plate heat exchanger evaporator (1) system for separation of liquid droplets from vapor transported from the evaporator to the separator.

Claims

1. A liquid separator for a plate heat exchanger evaporator system for separation of liquid droplets from vapour transported from the evaporator to the separator, wherein the separator is designed as a U-shaped separator pipe comprising two parallel pipes interconnected by a pipe portion, said pipes being arranged horizontally, each of said parallel pipes extending from the pipe portion and terminating at a respective free end, the two pipes including a first pipe possessing an end remote from the pipe portion and a second pipe possessing an end remote from the pipe portion, the end of the first pipe being connected to the evaporator to introduce into the separator the liquid droplets and vapour from the evaporator, the vapour separated from the liquid droplets and the liquid droplets separated from the vapour leaving the separator at the end of the second pipe.

2. The liquid separator according to claim 1, wherein the separator pipe has a bend of 180?.

3. The liquid separator according to any one of claim 1 or 2 wherein the length of the separator pipe is adjustable.

4. The liquid separator according to any one of claim 1 or 2, wherein the length and width of the separator pipe is configured to approximately correspond to the length and width of the evaporator.

5. The liquid separator according to any one of claim 1 or 2, wherein the separator pipe has a diameter between 100 and 400 mm.

6. The liquid separator according to any one of claim 1 or 2, wherein the entire U-shaped separator pipe is configured to be arranged on top of the evaporator.

7. The liquid separator according to any one of claim 1 or 2, wherein the separator is configured to be arranged on a side of the evaporator.

8. The liquid separator according to any one of claim 1 or 2, wherein the separator is provided with a demister.

9. The liquid separator according to any one of claim 1 or 2, wherein the separator is provided with an ejector injecting high pressure liquid into the separator.

10. The liquid separator according to any one of claim 1 or 2, wherein the separator is made of stainless steel.

11. The liquid separator according to any one of claim 1 or 2, wherein the separator is made of carbon steel with a surface treatment against corrosion.

12. A liquid separator together with an evaporator in a plate heat exchanger evaporator system for separating liquid droplets from vapour transported from the evaporator to the separator, the separator being a U-shaped separator pipe comprising two parallel pipes interconnected by a pipe portion, said pipes being arranged horizontally, one end portion of the U-shaped separator pipe being connected to the evaporator to receive the liquid droplets and the vapour from the evaporator and an opposite end portion of the U-shaped separator pipe also being connected to the evaporator to directly return to the evaporator the liquid droplets which have been separated from the vapour while the vapour which has been separated from the liquid droplets leaves at the opposite end of the U-shaped separator pipe to a compressor, each of said parallel pipes extending from the pipe portion and terminating at a respective free end.

13. The liquid separator according to claim 12, wherein the separator pipe has a bend of 180?.

14. The liquid separator according to claim 12, wherein the separator pipe possesses an adjustable length.

15. The liquid separator according to claim 12, wherein the separator pipe possesses a length and width approximately corresponding to the length and width of the evaporator.

16. The liquid separator according to claim 12, wherein the separator pipe has a diameter between 100 and 400 mm.

17. The liquid separator according to claim 12, wherein the entire U-shaped separator pipe is arranged on top of the evaporator.

18. The liquid separator according to claim 12, wherein the separator is arranged on a side of the evaporator.

19. The liquid separator according to claim 12, wherein the one end portion of the U-shaped separator pipe is connected to a top connection of the evaporator, and the opposite end portion of the U-shaped separator pipe is connected to a bottom connection of the evaporator.

20. The liquid separator according to claim 12, a first one of the two parallel pipes extending from the pipe portion and terminating at a first free end, and a second one of the two parallel pipes extending from the pipe portion and terminating at a second free end, the first free end of the first pipe being remote from the second free end of the second pipe.

21. A liquid separator for a plate heat exchanger evaporator system for separation of liquid droplets from vapour transported from the evaporator to the separator, wherein the separator is a U-shaped separator pipe comprising two parallel pipes interconnected by a curved pipe portion, the pipes being arranged horizontally, each of the parallel pipes extending from the pipe portion and terminating at a respective free end, each of the two parallel pipes possessing a length, the length of each of the two pipes being equal, a first connection at an end of one of the two parallel pipes by way of which two-phase flow is introduced into the separator to flow along the separator and be separated into the liquid droplets and the vapour, and a second connector at an end of the other of the two parallel pipes and through which the liquid droplets separated from the vapour are transported out of the separator directly to the evaporator while the vapour leaves at the end of the other of the two parallel pipes to a compressor.

22. The liquid separator according to claim 19, wherein the opposite end portion of the U-shaped separator pipe is also connected to a compressor so that dry vapor separated from refrigerant liquid droplets is introduced into the compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention will now be described in more detail below, reference being made to the accompanying drawings, which by way of example illustrate currently preferred embodiments of the invention.

(2) FIG. 1 is a perspective view of a known separator for an evaporator system.

(3) FIG. 2 is side view of the separator of the invention.

(4) FIG. 3 is a top view of the separator in FIG. 2 of the invention.

(5) FIG. 4 is a perspective view of a first embodiment of the separator according to the present invention.

(6) FIG. 5 is another perspective view of the separator according to FIG. 2-4.

(7) FIG. 6 is a perspective view of a second embodiment of the separator of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(8) In FIG. 1 the general principle for a plate heat exchanger circulation evaporator system is shown. The evaporator 1 is connected to a vapour liquid separator 2 by two pipes 3, 4, a lower pipe leg 3 feeding the evaporator 1 and an upper pipe leg 4 which returns the partially evaporated liquid.

(9) In case of an insoluble oil heavier than the refrigerant present in the refrigeration system, e.g. oil/ammonia, the oil is drained from the separator bottom and is collected in a vessel (not shown) at the lowest part of the refrigeration system and may be drained from there. The primary duty of the oil in the system is to lubricate moving parts of the compressors and to seal in order to prevent refrigerant from leaking. It is important that oil is prevented from reaching the evaporator and the oil is therefore collected before entering the evaporator and returned to the compressor.

(10) The separator 2 always maintains a liquid level usually well above the top of the evaporator. The evaporator 1 is thus always filled with liquid and this type of evaporator is normally called a flooded flow evaporator.

(11) Depending on the driving force for the circulation, flooded flow evaporators are classified as thermosiphon evaporators, where the driving force is the natural density differences between the two legs of the system separator-evaporator, or forced flow evaporators, where the driving force is a pump or an ejector.

(12) The circulationthe ratio of the total refrigerant amount entering the evaporator and the amount evaporatedmay vary from about 5 to 10 for a shell- and tube heat exchanger to about 1.2 in a plate heat exchanger, of either the brazed or semi welded type. A smaller circulation rate earns smaller pipe work and separator and a reduction of the total refrigerant content of the plant.

(13) The heating surface is thus always wetted by the refrigerant. This is important, as the heat transfer is then two phase convective of a vapour in a liquid, i.e. high as compared to the direct expansion evaporator, where the heat transfer mode, at the end of the evaporation and the superheating of the vapour, is gas heat transfer, i.e. low.

(14) At large capacities, the relatively small size of the flooded flow evaporator is favoured compared to the dry expansion evaporator. In this case, the efficiency of the evaporator is more important than the extra cost of a separator.

(15) The smaller dry expansion brazed plate heat exchangers are thus normally not used but as auxiliary coolers like oil coolers in large plants. Larger brazed plate heat exchangers can be used as flooded flow evaporators for non ammonia refrigerants.

(16) Ammonia, favours the use of flooded flow evaporators and the recent development of fusion bonded plate heat exchangers could increase their use here. The separator according the present invention favours ammonia to be used for lower capacities than is common today

(17) For economic reasons the thermosiphon system is often considered the best solution for a flooded flow evaporator since the cost for a pump and its operation is saved. The circulation rate is, however, very dependent on the heat transfer and pressure drops in the various parts of the system, which in their turn are dependent on the circulation, i.e. there is an integration and interdependence between pressure drop, circulation rate and heat transfer in the circulating system.

(18) By definition thermosiphon means circulation owing to density differences between the fluids in two connecting legs. When the unit in FIG. 1 is not operating but filled with liquid refrigerant and both valves are open, the refrigerant level in the separator is the same as in the evaporator.

(19) When liquid enters the evaporator 1 on the other side, the refrigerant heats up and slowly bubbles start to form. The channels are then partly filled with ascending bubbles. Thus, the mean density in the pipe leg 3 formed by the evaporator is much lower than in the pipe leg 4 formed by the separator and the descending pipe.

(20) Accordingly, the two pipe legs 3, 4 are not in balance and the refrigerant gradually enters the evaporator 1 from the separator 2 via the descending pipe 4. At the top of the evaporator 1, a two-phase mixture is pushed into the separator 2 and the liquid and vapour separate. At the bottom of the evaporator 1, the entering refrigerant heats up and finally starts to boil. Thus, a two-phase mixture is always maintained in the channels.

(21) As the circulation rate increases, the various pressure drops increase and finally the driving force is balanced by retarding forces. The system is then in balance and a constant refrigerant flow enters the evaporator while a certain fraction of the flow evaporates.

(22) When the liquid-vapour mixture enters the separator 2, it is saturated. The liquid droplets separates from the vapour and the refrigerant enters the evaporator 1 again but it is now not saturated. The temperature is the same as in the separator 2, but the pressure is higher, increased with the static head from the liquid level to the inlet, i.e. the refrigerant is sub-cooled.

(23) This means that in the first part of the heat exchanger there will be no boiling, just a temperature increase. However, as the refrigerant rises, it will decrease in pressure, reducing the sub-cooling.

(24) These two effectsincreasing temperature and decreasing pressuremean that after a while boiling is reached and the refrigerant starts to boil, albeit at a higher temperature than at the exit. The pressure continues to decrease because of the changing height and the pressure drop, and the refrigerant, now saturated, will continue ascending with decreasing temperature until it reaches the separator and the loop is closed.

(25) FIG. 2-6 show the liquid separator 2 according to the present invention which is designed as a U-shaped pipe 5 comprising two elongated essentially parallel pipes 5A interconnected by means of a pipe portion 5B, said pipes 5A being arranged essentially horizontally. Of course, the separator pipe 5 may be made in one piece. The pipes 5A, 5B may be standard pipes with a diameter between 100 and 400 mm. The length of the separator pipe 5 may be varied depending on the capacity and the evaporation temperature of the evaporator. Preferably the separator pipe 5 has a length and width approximately corresponding to the length and width of the evaporator 1. The two-phase refrigerant flow leaves the evaporator 1 from a top connection 6 of the plate heat exchanger evaporator 1 and enters the separator 2 into a first end 7 of the separator 2. The refrigerant liquid droplets are separated from the vapour in the separator 2 and the liquid refrigerant is circulated back from a second end 8 of the separator to a bottom connection 9 of the plate heat exchanger evaporator 1. From the separator 2 the dry vapour leaves at the second end 8 to a compressor.

(26) By arranging the separator pipe 5 mainly horizontally a very efficient separation is achieved and the liquid droplets can separate from the vapour during the transportation of the vapour through the length of the separator pipe 5. A small radius of curvature of the U-shaped pipe further improves the liquid separation and makes the separator compact.

(27) Optionally an oil drain (not shown) may be provided before the liquid refrigerant enters the bottom connection 9 of the evaporator 1. In this case it is important that the oil drain is arranged at the lowest point of the evaporator system.

(28) Also, a demister 10 may be located at the end, near and before a suction nozzle with the function in operating conditions to aggregate any small, possibly remaining droplets to larger droplets that will separate to the bottom at a higher speed than small droplets. This prevents liquid from entering through the suction nozzles to the compressor and is used for systems with sudden load variations.

(29) A check valve 11 may be located at the bottom liquid inlet connection 9 to the evaporator 1 and after the oil drain separator. The valve 11 is open at normal operation conditions with a minimum pressure drop of the supplied liquid refrigerant. At start up and sudden load variations the liquid may flow backwards up the liquid supply or drop leg and reach a level inside the separator U-Turn pipe. This reduces the cross flow area in the U-shaped separator pipe 5 which should be avoided due to a reduction of the separator capacity under such circumstances. The check valve function prevents liquid to return backwards up the liquid feed drop leg.

(30) When there is a pressure increase in the evaporator because of start up or sudden load changes the valve 11 will close. The valve has no external regulation or control. It remains closed by gravity at stop condition, open in normal operation and closed at back flow tendencies.

(31) Liquid may be injected into the separator 2 from the high pressure side of the refrigeration system by means of an ejector having the effect of increasing the static suction pressure to the compressor (not shown). A higher suction pressure to the compressor results in a lower power consumption. The relatively small diameter of the U-shaped separator pipe 5 and a direction of the injected high pressure refrigerant co currently with the separating two phase flow results in an increased pressure in the separator 2 after the injection. Injection can also be effected in the wet return line between the evaporator exit and the separator where the ejector effect is utilized further due to the smaller diameter of this pipe.

(32) In a preferred embodiment of the present invention a number of fixed lengths of the pipes are provided to each of the different types of plate heat exchanger evaporators. Consequently, each plate heat exchanger evaporator model is provided with a separator of a fixed diameter but the length of the separator pipe may vary depending on the temperature and the capacity of the evaporator.

(33) In another preferred embodiment of the present invention the U-shaped separator pipe 5 has a bend of preferably 180?. The U-formed shape of the separator 2 increases the separation efficiency in comparison with traditional separators since the separation length is longer than in a traditional separator. For the larger separator diameters the 180? turn can be made of two T-pieces with end-cups to reduce the distance between the two separator legs. A narrow curvature of the U-shaped separator pipe 5 enhances the separation of liquid.

(34) Preferably the separator 2 according to the invention is arranged principally horizontally or slightly inclined on top of the evaporator. In a preferred arrangement the separator is supported with supports 12 on the frame 13 or connection plate and with one support on the plate heat exchanger support column 14. By this construction the separator pipe of the invention is allowed to stretch and shrink with the temperature variations.

(35) In an alternative embodiment of the present invention the separator is arranged on the upper side of the separator, see FIG. 6. The two separator pipes 5A are arranged one above the other with a connecting pipe portion 5B.

(36) The separator can be made of stainless steel or other alloyed material in order to avoid corrosion without the need for surface treatment and surface coating. As an alternative, it can be made of common carbon steel grades with surface treatment.

(37) By the mainly horizontal arrangement of the liquid separator consisting of to essentially parallel pipes interconnected by a pipe portion into a U-shaped form an efficient liquid separation can be achieved due to the increased length of the separator compared to previously known liquid separators.

(38) Due to the compact design of the separator of the invention a low weight separator is achieved which is less space consuming than previously known liquid separators.

(39) A further advantage with the liquid separator according to the present invention is that it can operate at part loads with maintained cooling efficiency owing to a short wet return line with small pressure drop and adapted driving liquid levels.

(40) Furthermore, the separator 2 of the invention also functions as a connector for safety valves, feed expansion, valves, liquid level control, suction pressure control, oil separation, drip tray and as a lift from floor levels. The liquid level control for traditionally made separators with large diameters is made of a parallel liquid pipe connected to the separator and the liquid drop leg with two stop valves, a level glass and transmitter to control a regulator valve. The invention with small separator diameters allows for inductive transmitters internally mounted. Such level control transmitters allow for lower weight, few components and a reduced leak risk.

(41) It will be readily apparent to one skilled in the art that various substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention.