Air separation purification device

Abstract

The utility model relates to an air separation purification device. Comprising a molecular sieve adsorber, an air inlet pipeline, an air outlet pipeline, a waste nitrogen heating pipeline and a waste nitrogen cold blowing pipeline are connected to the molecular sieve adsorber, and a hot air inlet of a refrigerating unit generator is connected with a waste nitrogen outlet of the molecular sieve adsorber through a waste nitrogen waste heat recovery pipeline. A hot air outlet of the refrigerating unit generator is connected with a waste nitrogen emptying pipeline, a cold water inlet of the refrigerating unit evaporator is connected with a cooling water pipeline, and a cold water outlet of the refrigerating unit evaporator is connected with a chilled water pipeline used for conveying chilled water obtained after cooling water is refrigerated to the air separation pre-cooling system. The waste heat of the waste nitrogen of the molecular sieve adsorber is recovered, the refrigerating unit recovers the waste heat to refrigerate, and the prepared chilled water is used by the air separation precooling device, so that the energy conservation and consumption reduction of the whole air separation device are realized.

Description

Technical Field

[0001] The utility model relates to the technical field of deep cooling air separation and purification, in particular to an air separation and purification device. Background Art

[0002] Air separation equipment obtains gases such as oxygen and nitrogen through cryogenic distillation, which is widely used in coal chemical industry and metallurgy, and is one of the main sources of energy consumption in the industrial sector. The purification unit is one of the core components of a typical cryogenic air separation unit, and the energy consumption of the air separation purification unit accounts for about 11% of the total energy consumption of the air separation unit.

[0003] The air separation equipment also includes an air separation precooling system. The compressed high-temperature air enters the air separation precooling system to cool down before entering the air separation purification device for treatment. The air separation precooling system is also one of the main energy consumption of the air separation equipment. The air separation purification device includes a molecular sieve adsorber. The desorption and regeneration of the molecular sieve adsorber is an endothermic process. Generally, dirty nitrogen is heated by a heater and then enters the molecular sieve adsorber for desorption and regeneration. In order to reach the working temperature of the molecular sieve adsorption, cold dirty nitrogen is required for cold blowing after desorption to reduce the temperature of the adsorber. The heat contained in the dirty nitrogen discharged during the cold blowing process is difficult to utilize and is usually directly discharged, resulting in a large amount of energy waste. Utility Model Content

[0004] In order to solve the above problems, the utility model provides an air separation purification device, including a molecular sieve adsorber, to which an air inlet pipe, an air outlet pipe, a dirty nitrogen heating pipe and a dirty nitrogen cold blowing pipe are connected, and also includes a refrigeration unit, the hot air inlet of the refrigeration unit generator is connected to the dirty nitrogen outlet of the molecular sieve adsorber through a dirty nitrogen waste heat recovery pipe, the hot air outlet of the refrigeration unit generator is connected to a dirty nitrogen venting pipe, the cold water inlet of the refrigeration unit evaporator is connected to a cooling water pipe, and the cold water outlet of the refrigeration unit evaporator is connected to a chilled water pipe for transporting chilled water after cooling the cooling water to an air separation precooling system.

[0005] Furthermore, the refrigeration unit is a lithium bromide refrigeration unit.

[0006] Furthermore, a steam heater and an electric heater are connected in series to the contaminated nitrogen heating pipeline, the contaminated nitrogen outlet of the steam heater is connected to the contaminated nitrogen inlet of the electric heater, and the contaminated nitrogen outlet of the electric heater is connected to the molecular sieve adsorber.

[0007] Furthermore, a bypass pipe is provided on the dirty nitrogen heating pipeline, two ends of the bypass pipe are respectively connected to the front and rear sides of the steam heater, and a bypass valve is provided on the bypass pipe.

[0008] Furthermore, the number of the electric heaters is two, and the two electric heaters are connected in parallel between the steam heater and the molecular sieve adsorber.

[0009] Furthermore, the molecular sieve adsorber is connected to a first main pipeline, and the air outlet pipeline, the contaminated nitrogen gas heating pipeline and the contaminated nitrogen gas cold blowing pipeline are all connected to the molecular sieve adsorber through the first main pipeline.

[0010] Furthermore, the molecular sieve adsorber is also connected to a second main pipeline, and the air inlet pipeline and the waste nitrogen waste heat recovery pipeline are both connected to the molecular sieve adsorber through the second main pipeline.

[0011] Furthermore, it also includes a contaminated nitrogen inlet pipeline for providing contaminated nitrogen, and the air inlet ends of the contaminated nitrogen heating pipeline and the contaminated nitrogen cold blowing pipeline are both connected to the contaminated nitrogen inlet pipeline.

[0012] Furthermore, regulating valves are respectively provided on the air inlet pipe, the air outlet pipe, the dirty nitrogen heating pipe, the dirty nitrogen cold blowing pipe and the dirty nitrogen waste heat recovery pipe.

[0013] Furthermore, the number of the molecular sieve adsorbers is two, and the two molecular sieve adsorbers are arranged in parallel.

[0014] Due to the adoption of the above technical solution, the utility model has the following beneficial effects compared with the prior art:

[0015] The air separation purification device provided by the utility model recovers waste heat from the dirty nitrogen gas discharged during the heating and cooling process of the molecular sieve adsorber, and refrigerates the waste heat of the refrigeration unit to obtain chilled water for use in the air separation precooling system, thereby achieving energy saving and consumption reduction of the entire air separation device; and the original air separation process is not damaged, and the impact on the original air separation process is very small, which is conducive to the transformation of the existing air separation system. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the utility model or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the utility model. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 This is a schematic structural diagram of the air separation and purification device provided by the utility model.

[0018] 1-first molecular sieve adsorber; 11-fifth regulating valve; 12-seventh regulating valve; 2-second molecular sieve adsorber; 21-sixth regulating valve; 22-eighth regulating valve; 3-air inlet pipeline; 31-first regulating valve; 32-second regulating valve; 4-air outlet pipeline; 41-third regulating valve; 42-fourth regulating valve; 5-dirty nitrogen heating pipeline; 51-steam heater; 52-electric heater; 53-bypass valve; 54-ninth regulating valve; 6-dirty nitrogen cold blowing pipeline; 61-tenth regulating valve; 7-refrigeration unit; 71-dirty nitrogen waste heat recovery pipeline; 72-cooling water pipeline; 73-chilled water pipeline; 74-dirty nitrogen venting pipeline; 8-dirty nitrogen inlet pipeline; 9-venting silencer. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of them. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model. In the drawings, the size and relative size of some parts may be enlarged for clarity.

[0020] In the description of the present invention, unless otherwise clearly specified and limited, the terms "connection" and "connected" should be interpreted in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integrated connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0021] In the description of the present invention, terms such as "upper", "lower", "left", "right", "front", "backward", etc., and the orientation or position relationship are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of description and simplification of operation, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0022] In addition, in the description of the present invention, the terms "first" and "second" are only used to distinguish in the description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. In addition, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0023] As the instruction manual Figure 1As shown, the utility model provides an air separation purification device, including a molecular sieve adsorber, to which an air inlet pipe 3, an air outlet pipe 4, a dirty nitrogen heating pipe 5 and a dirty nitrogen cold blowing pipe 6 are connected, and also includes a refrigeration unit 7, the hot air inlet of the generator of the refrigeration unit 7 is connected to the dirty nitrogen outlet of the molecular sieve adsorber through a dirty nitrogen waste heat recovery pipe 71, the hot air outlet of the generator of the refrigeration unit 7 is connected to a dirty nitrogen venting pipe 74, the cold water inlet of the evaporator of the refrigeration unit 7 is connected to a cooling water pipe 72, and the cold water outlet of the evaporator of the refrigeration unit 7 is connected to a chilled water pipe 73 for transporting chilled water after cooling the cooling water to the air separation precooling system.

[0024] Specifically, the compressed high-temperature air is cooled by the air separation precooling device and then enters the molecular sieve adsorber through the air inlet pipe 3, where H2O, CO2 and part of C n H m After that, it is transported to the next process for treatment through the air outlet pipe 4, and the next process is the cold box system and the expander system; after the molecular sieve adsorber has worked for a period of time, the molecular sieve therein needs to be regenerated for standby use, and the analytical regeneration of the molecular sieve is an endothermic process. After the contaminated nitrogen is heated to a certain temperature through the contaminated nitrogen heating pipe 5, it enters the molecular sieve adsorber for analytical regeneration of the molecular sieve adsorber. After the regeneration is completed, the molecular sieve adsorber is cooled by the contaminated nitrogen in the contaminated nitrogen cold blowing pipe 6. After the molecular sieve adsorber is cooled to the set temperature, the air purification operation can continue; the contaminated nitrogen after the molecular sieve adsorber is analyzed and the contaminated nitrogen after the cold blowing has a certain temperature, and both can enter the refrigeration unit through the contaminated nitrogen waste heat recovery pipe 71. 7 is used for waste heat recovery in the generator; the cooling water pipe 72 is connected to the inlet end of the evaporator of the refrigeration unit 7, and the chilled water pipe 73 is connected to the outlet end of the evaporator of the refrigeration unit 7. After the normal temperature water enters the refrigeration unit 7 through the cooling water pipe 72, it is refrigerated by the refrigeration unit 7 to obtain chilled water, which is sent to the air separation precooling device through the chilled water pipe 73 to provide cooling capacity for the air separation precooling device. The refrigeration unit prepares chilled water, which can replace the chiller that consumes electrical energy, thereby reducing the energy consumption of the entire air separation system; the waste heat of the dirty nitrogen gas can be effectively used for refrigeration to reduce energy consumption; the outlet end of the generator of the refrigeration unit 7 is connected to a dirty nitrogen gas venting pipe 74, and the dirty nitrogen gas venting pipe 74 is provided with a venting silencer 9.

[0025] In an optimized implementation mode, the temperature of the chilled water after being cooled by the refrigeration unit 7 is 5-10°C.

[0026] In the optimized implementation mode, the refrigeration unit 7 is a lithium bromide refrigeration unit, preferably a lithium bromide absorption refrigeration unit. The lithium bromide refrigeration unit can use the heat source of the recovered hot dirty nitrogen to complete cooling water refrigeration. The hot air inlet of the lithium bromide refrigeration unit generator is connected to the dirty nitrogen waste heat recovery pipeline, the hot air outlet of the lithium bromide refrigeration unit generator is connected to the dirty nitrogen venting pipeline, the cold water inlet of the lithium bromide refrigeration unit evaporator is connected to the cooling water pipeline, the cold water outlet of the lithium bromide refrigeration unit evaporator is connected to the chilled water pipeline, and the other end of the chilled water pipeline is connected to the air separation precooling system for cooling the air separation precooling system; the hot dirty nitrogen enters the heat exchange coil of the lithium bromide refrigeration unit generator, heats the lithium bromide dilute solution to produce water vapor and lithium bromide high concentration solution. The temperature of the hot dirty nitrogen is reduced and discharged from the hot air outlet. Water vapor enters the condenser, is cooled by the cooling water in the condenser, and condenses to become high-pressure and low-temperature liquid water; when the water in the condenser enters the evaporator through the throttle valve, it expands rapidly and vaporizes, and absorbs a large amount of heat from the refrigerant in the evaporator during the vaporization process, thereby achieving the purpose of cooling. The cooling water at about 12-20℃ is cooled in the evaporator of the lithium bromide refrigeration unit, and its temperature is reduced to 5-10℃ before entering the chilled water pipeline and being discharged to the air separation precooling system for cooling. The low-temperature water vapor enters the absorber and is absorbed by the lithium bromide aqueous solution in the absorber. The concentration of the solution gradually decreases, and then it is sent back to the generator by the circulating pump to complete the entire cycle.

[0027] In an optimized implementation mode, a steam heater 51 and an electric heater 52 are connected in series to the contaminated nitrogen heating pipeline 5, the contaminated nitrogen outlet end of the steam heater 51 is connected to the contaminated nitrogen inlet end of the electric heater 52, and the contaminated nitrogen outlet end of the electric heater 52 is connected to the molecular sieve adsorber.

[0028] Preferably, it also includes a contaminated nitrogen inlet pipeline 8 for providing contaminated nitrogen, wherein the contaminated nitrogen inlet pipeline 8 is respectively connected to the contaminated nitrogen heating pipeline 5 and the contaminated nitrogen cold blowing pipeline 6 to provide contaminated nitrogen.

[0029] Preferably, a bypass pipe is provided on the contaminated nitrogen heating pipeline 5, two bypass points of the bypass pipe are arranged at the front and rear sides of the steam heater 51, and a bypass valve 53 is provided on the bypass pipe.

[0030] In an optimized implementation manner, the number of the electric heaters 52 is two, and the two electric heaters 52 are connected in parallel between the steam heater 51 and the molecular sieve adsorber, and the two electric heaters 52 are used for heating the contaminated nitrogen gas, one for backup and the other for use.

[0031] Specifically, the heating process of the contaminated nitrogen gas is as follows: the contaminated nitrogen gas from the cold box system is heated to 165°C from the contaminated nitrogen gas inlet pipe 8 through the contaminated nitrogen gas heating pipe 5, and the temperature of the dry contaminated nitrogen gas at the inlet of the molecular sieve adsorber is 150°C, passing through the molecular sieve adsorber from top to bottom. The contaminated nitrogen gas is first heated by the steam heater 51, and then passes through two electric heaters 52. The two electric heaters 52 are one for use and one for backup, and can complete the regeneration of the molecular sieve adsorber independently. When the saturated steam pressure in the steam heater 51 is lower than 0.5MPa.G, the electric heater 51 can be used in series to complete the regeneration of the molecular sieve adsorber. When the saturated steam pressure in the steam heater 51 is not lower than 0.5MPa.G, the steam heater 51 can complete the regeneration of the molecular sieve adsorber independently. The steam used as the heating source is heated by the steam heater 51, and the heat is transferred to the contaminated nitrogen gas and then condensed into water, which is discharged to the pipe network or the ditch through the steam trap. The condensate discharge outlet needs to be lower than the outlet height of the steam heater 51 to avoid the condensate water being retained in the steam heater 51. The electric heater 51 is used to make up for the defect of low-pressure steam heating temperature. The heating element is made of high resistivity material. Under the action of electric heating effect, the high-temperature contaminated nitrogen in the contaminated nitrogen heating pipeline enters the molecular sieve adsorber for regeneration, and H2O, CO2 and C n H m The waste nitrogen gas is then sent to the lithium bromide refrigeration unit through the waste heat recovery pipeline.

[0032] Preferably, the steam heater 51 is provided with regulating valves on the contaminated nitrogen inlet and outlet, and the electric heater 52 is provided with regulating valves on the contaminated nitrogen inlet and outlet, which can adjust the flow of the contaminated nitrogen according to the heating conditions to ensure that the contaminated nitrogen is heated to the set temperature.

[0033] Preferably, a ninth regulating valve 54 is provided on the contaminated nitrogen heating pipeline 5 , and a tenth regulating valve 61 is provided on the contaminated nitrogen cold blowing pipeline 6 .

[0034] In an optimized implementation mode, a first main pipeline is connected to the molecular sieve adsorber, and the air outlet pipeline 4, the dirty nitrogen heating pipeline 5 and the dirty nitrogen cold blowing pipeline 6 are all connected to the molecular sieve adsorber through the first main pipeline. Preferably, the dirty nitrogen heating pipeline 5 and the dirty nitrogen cold blowing pipeline 6 are collected to the dirty nitrogen inlet main pipeline and then connected to the first main pipeline.

[0035] In an optimized implementation mode, the molecular sieve adsorber is also connected to a second main pipeline, and the air inlet pipeline 3 and the waste nitrogen waste heat recovery pipeline 71 are both connected to the molecular sieve adsorber through the second main pipeline.

[0036] In an optimized implementation mode, there are two molecular sieve adsorbers, which are respectively denoted as a first molecular sieve adsorber 1 and a second molecular sieve adsorber 2. The first molecular sieve adsorber 1 and the second molecular sieve adsorber 2 are used in a standby manner. When one molecular sieve adsorber is working, the other molecular sieve adsorber is regenerated and used in a standby manner. The first end of the first molecular sieve adsorber 1 is connected to an air outlet pipe 4, a dirty nitrogen heating pipe 5 and a dirty nitrogen cold blowing pipe 6 through a first main pipe, and the second end of the first molecular sieve adsorber 1 is connected to an air inlet pipe 3 and a dirty nitrogen waste heat recovery pipe 6 through a second main pipe. The connection method of the second molecular sieve adsorber 2 is the same as that of the first molecular sieve adsorber 1, which will not be repeated here.

[0037] The air is divided into two branches from the air inlet pipe 3, which are respectively connected to the first molecular sieve adsorber 1 and the second molecular sieve adsorber 2. The air purified by the first molecular sieve adsorber 1 and the second molecular sieve adsorber 2 is collected to the air outlet pipe 4 through the two branches.

[0038] Preferably, the two branches of the air inlet pipe 3 are respectively provided with a first regulating valve 31 and a second regulating valve 32, the two branches of the air outlet pipe 4 are respectively provided with a third regulating valve 41 and a fourth regulating valve 42, the two branches of the waste nitrogen gas inlet main pipe are respectively provided with a fifth regulating valve 11 and a sixth regulating valve 21, and the two branches of the waste nitrogen gas waste heat recovery pipe 71 are respectively provided with a seventh regulating valve 12 and an eighth regulating valve 22.

[0039] The regeneration of the molecular sieve adsorber is carried out in four steps: pressure reduction, heating (purging the adsorbent with heated dry gas), cooling (purging the adsorbent with unheated dry gas), and pressure increase.

[0040] The details are as follows:

[0041] Pressure reduction is the process of discharging the pressurized air in the container of the molecular sieve adsorber at the end of its working cycle, which takes 6 to 10 minutes to complete.

[0042] Heating: The molecular sieve adsorber is purged with dirty nitrogen gas heated by the dirty nitrogen gas heating pipeline 5, and the purged dirty nitrogen gas enters the dirty nitrogen gas waste heat recovery pipeline 71 to the lithium bromide refrigeration unit; when the steam pressure is lower than 0.5MPa.G, the valve 61 is opened, and the series electric heater (optional one) is operated to complete the regeneration of the molecular sieve adsorber. When the saturated steam pressure is not lower than 0.5MPa.G, the valve 61 is closed, the electric heater does not need to work, and the steam heater alone completes the regeneration of the molecular sieve adsorber.

[0043] Blowing cold: After the analysis is completed, the unheated dirty nitrogen cold blowing pipeline 6 blows cold to the molecular sieve adsorber. At the end of the blowing cold, the temperature of the dirty nitrogen out of the molecular sieve adsorber can drop to 5-10°C higher than the working temperature. After sufficient blowing cold, the molecular sieve adsorber restores its adsorption capacity. The dirty nitrogen pipeline after cold blowing is also connected to the dirty nitrogen waste heat recovery pipeline 71 to the lithium bromide refrigeration unit.

[0044] Pressure boost: Close the corresponding heating and cooling valves to allow the air in a working molecular sieve adsorber to be charged into another adsorber that is about to be regenerated. The completion time is 12 to 22 minutes.

[0045] The first molecular sieve adsorber 1 is working and the second molecular sieve adsorber 2 is regenerating. When the first molecular sieve adsorber 1 is performing adsorption, valves 31 and 41 are opened, valves 32 and 42 are closed, and air flows through the channel of the first molecular sieve adsorber 1. At the same time, the second molecular sieve adsorber 2 is performing regeneration and cold blowing, valves 21 and 22 are opened, valves 11 and 12 are closed, and the contaminated nitrogen flows through the channel of the second molecular sieve adsorber 2. When the second molecular sieve adsorber 2 is performing regeneration, valve 54 is opened, valve 61 is closed, and the contaminated nitrogen is heated; when the second molecular sieve adsorber 2 is performing cold blowing, valve 61 is opened, valve 54 is closed, and the contaminated nitrogen is directly cold blown.

[0046] Preferably, a heat accumulator containing heat storage materials such as quartz or porcelain balls can be set in the regeneration system, and the electric heater is kept in a normally open state. When the device is started, 25% of the regeneration gas is first used to store heat in the heat accumulator. During the heating period, only 75% of the heating gas needs to be added; during the cold blowing, pressure increase and pressure reduction period, 25% of the regeneration gas can be charged into the heat accumulator to improve the regeneration efficiency of the molecular sieve adsorber.

[0047] Those skilled in the art will appreciate that the present invention can be implemented in many other specific forms without departing from the spirit and scope of the present invention. Although the embodiments of the present invention have been described, it should be understood that the present invention should not be limited to this embodiment, and those skilled in the art can make changes and modifications within the spirit and scope of the present invention as defined by the attached claims.

Claims

1. An air separation purification device, comprising a molecular sieve adsorber, to which an air inlet pipe, an air outlet pipe, a contaminated nitrogen heating pipe and a contaminated nitrogen cold blowing pipe are connected, characterized in that: It also includes a refrigeration unit, wherein the hot air inlet of the generator of the refrigeration unit is connected to the dirty nitrogen outlet of the molecular sieve adsorber through a dirty nitrogen waste heat recovery pipeline, the hot air outlet of the generator of the refrigeration unit is connected to a dirty nitrogen venting pipeline, the cold water inlet of the evaporator of the refrigeration unit is connected to a cooling water pipeline, and the cold water outlet of the evaporator of the refrigeration unit is connected to a chilled water pipeline for transporting chilled water after cooling the cooling water to the air separation precooling system.

2. The air separation purification device according to claim 1, characterized in that: The refrigeration unit is a lithium bromide refrigeration unit.

3. The air separation purification device according to claim 1, characterized in that: A steam heater and an electric heater are connected in series to the contaminated nitrogen heating pipeline, the contaminated nitrogen outlet of the steam heater is connected to the contaminated nitrogen inlet of the electric heater, and the contaminated nitrogen outlet of the electric heater is connected to the molecular sieve adsorber.

4. The air separation purification device according to claim 3, characterized in that: The waste nitrogen heating pipeline is provided with a bypass pipe, both ends of which are respectively connected to the front and rear sides of the steam heater, and the bypass pipe is provided with a bypass valve.

5. The air separation purification device according to claim 3, characterized in that: The number of the electric heaters is two, and the two electric heaters are connected in parallel between the steam heater and the molecular sieve adsorber.

6. The air separation purification device according to claim 1, characterized in that: The molecular sieve adsorber is connected to a first main pipeline, and the air outlet pipeline, the contaminated nitrogen heating pipeline and the contaminated nitrogen cold blowing pipeline are all connected to the molecular sieve adsorber through the first main pipeline.

7. The air separation purification device according to claim 1, characterized in that: The molecular sieve adsorber is also connected to a second main pipeline, and the air inlet pipeline and the waste nitrogen waste heat recovery pipeline are both connected to the molecular sieve adsorber through the second main pipeline.

8. The air separation purification device according to claim 1, characterized in that: It also includes a contaminated nitrogen inlet pipeline for providing contaminated nitrogen, and the air inlet ends of the contaminated nitrogen heating pipeline and the contaminated nitrogen cold blowing pipeline are both connected to the contaminated nitrogen inlet pipeline.

9. The air separation purification device according to claim 1, characterized in that: The air inlet pipe, the air outlet pipe, the dirty nitrogen heating pipe, the dirty nitrogen cold blowing pipe and the dirty nitrogen waste heat recovery pipe are respectively provided with regulating valves.

10. The air separation purification device according to claim 1, characterized in that: The number of the molecular sieve adsorbers is two, and the two molecular sieve adsorbers are arranged in parallel.

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