CN112582147B

CN112582147B - Cooling device of transformer

Info

Publication number: CN112582147B
Application number: CN201910924563.2A
Authority: CN
Inventors: 李奎; 荣智林; 张志学; 何凯; 田恩; 冯钊赞; 马明; 梁好玉; 吴辉
Original assignee: CRRC Zhuzhou Institute Co Ltd
Current assignee: CRRC Zhuzhou Institute Co Ltd
Priority date: 2019-09-27
Filing date: 2019-09-27
Publication date: 2023-03-21
Anticipated expiration: 2039-09-27
Also published as: CN112582147A

Abstract

The invention relates to a cooling technology of a transformer, in particular to a transformer cooling device based on a liquid-passing capillary network. In the cooling device for the transformer provided by the invention, the transformer and the current transformation unit of the circuit are integrally installed in a sealed insulating box body. The cooling apparatus of the transformer includes: the capillary network is arranged in the insulating box body and covers the outer surface of the potting layer of the transformer; the radiator is arranged outside the insulating box body and used for radiating heat of the refrigerant in the capillary network; and the fluid pump is used for driving the refrigerant to circularly flow between the capillary network and the radiator. The invention can be used for cooling the transformer in a narrow space in the insulating box body, and reduces the heat dissipation amount to the inside of the insulating box body while taking away the heat of the transformer.

Description

Cooling device of transformer

Technical Field

The invention relates to a cooling technology of a transformer, in particular to a transformer cooling device based on a liquid-passing capillary network.

Background

A Transformer (Transformer) is a device that changes an alternating voltage using the principle of electromagnetic induction, and its main components include a primary coil, a secondary coil, and an iron core (magnetic core). When the transformer is operated, the coil winding and the core generate heat due to loss. This heat generated by the losses must be conducted out of the transformer in a timely manner so as not to cause overheating damage to the insulation of the coil.

The existing cooling technology of the transformer can lead heat generated by loss out of the transformer in an oil-immersed self-cooling mode and an oil-immersed air-cooling mode. In the oil-immersed self-cooling scheme, a coil winding of a transformer needs to be immersed in transformer oil, heat is brought to an oil pipe radiator by means of natural thermal circulation of the oil, and then natural ventilation cooling is performed through the oil pipe radiator. In the oil-immersed air-cooled cooling scheme, a coil winding of a transformer needs to be immersed in transformer oil, heat is brought to an oil pipe radiator by means of natural thermal circulation of the oil, and then cooling is carried out by blowing air through a fan.

However, in the practical application of the transformer in rail transit, it is usually necessary to integrally mount the transformer and the current transforming unit of the circuit in the same sealed insulated box. Therefore, the transformer is required to have high voltage breakdown resistance and a compact structure. In addition, since the heat resistance of devices such as the inverter unit and the capacitor integrally mounted in the insulating box is generally poor, the heat dissipation amount of the transformer cooling device to the inside of the insulating box must be limited while the transformer is cooled.

Therefore, in order to meet the above requirements of the transformer in the practical application of the rail transit, a cooling technology of the transformer is needed in the art for cooling the transformer in the narrow space inside the insulating box and reducing the heat dissipation amount to the inside of the insulating box while taking away the heat of the transformer.

Disclosure of Invention

The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.

In order to meet the requirements of the transformer in practical application of rail transit, the invention provides a transformer cooling device based on a liquid-flowing capillary network, which is used for cooling the transformer in a narrow space in an insulating box body and reducing the heat dissipation quantity to the inside of the insulating box body while taking away the heat of the transformer.

In the cooling device for the transformer provided by the invention, the transformer and the current transformation unit of the circuit are integrally installed in a sealed insulating box body. The cooling apparatus of the transformer includes: the capillary network is arranged in the insulating box body and covers the outer surface of the potting layer of the transformer; the radiator is arranged outside the insulating box body and used for radiating heat of the refrigerant in the capillary network; and the fluid pump is used for driving the refrigerant to circularly flow between the capillary network and the radiator.

Alternatively, in the cooling device for a transformer provided by the present invention, the capillary network may be formed by a plurality of parallel capillaries, and two ends of each capillary may be connected to a header. The refrigerant is divided by one of the headers and flows into the plurality of capillary tubes, and the refrigerant is converged by the other header and flows out of the plurality of capillary tubes.

Optionally, in the cooling device for a transformer provided by the present invention, the capillary tube may be made of a flexible heat conductive material for conforming to the outer surface shape of a plurality of different potting layers.

Alternatively, in the cooling device for a transformer provided by the present invention, the two headers may be provided on the same side of the transformer. A plurality of said capillaries may extend from one said header to the other side of said transformer and bend back on the other side of said transformer to connect to another said header.

Optionally, in the cooling device for a transformer provided by the present invention, a gap between a plurality of the capillaries in the capillary network may be determined according to a bending radius of the capillaries.

Alternatively, in the cooling device for a transformer provided by the present invention, two headers may be provided on both sides of the transformer. A plurality of said capillaries may extend from one of said headers to the other side of said transformer and connect to the other of said headers on the other side of said transformer.

Alternatively, in the cooling device for a transformer provided by the present invention, the capillary tube may be made of an insulating flame-retardant material, and the capillary tube may be thermally fused to the header.

Optionally, in the cooling device for a transformer provided by the present invention, the capillary network may be formed by a plurality of capillaries with a diameter of millimeter.

Optionally, in the cooling device for a transformer provided by the present invention, a pipe network compression structure may be further included, and the pipe network compression structure may be configured to fix and attach the capillary network to an outer surface of the potting layer of the transformer.

Optionally, in the cooling device of the transformer provided by the present invention, a pipe network gap filling pad may be further included, where the pipe network gap filling pad may be made of a flexible insulating and heat conducting material, and is disposed between the plurality of capillaries of the capillary network, and is used to fix a gap between the plurality of capillaries.

Drawings

The above features and advantages of the present disclosure will be better understood upon reading the detailed description of embodiments of the disclosure in conjunction with the following drawings. In the drawings, components are not necessarily drawn to scale, and components having similar relative characteristics or features may have the same or similar reference numerals.

Fig. 1 shows an installation schematic of a transformer cooling apparatus provided according to an aspect of the present invention.

Fig. 2A illustrates a schematic front view of a transformer cooling apparatus provided according to an embodiment of the present invention.

Fig. 2B shows a schematic side view of a transformer cooling arrangement provided in accordance with an embodiment of the invention.

FIG. 3 shows a schematic cross-sectional view of a web gap filling pad provided in accordance with one embodiment of the present invention.

Fig. 4 shows a schematic front view of a transformer cooling arrangement provided according to an embodiment of the invention.

Fig. 5 shows a schematic deployment of a capillary network provided according to an embodiment of the present invention.

Reference numerals:

11. a transformer;

111. an iron core;

12. a current transformation unit;

13. an insulating box body;

21. a capillary network;

211. a capillary tube;

212. 213 header pipes;

22. a heat sink;

23. a fluid pump;

31. a pipe network compression structure;

32. filling a pipe network gap with a pad;

41. a capillary network;

411. a capillary tube;

412. 413 header.

Detailed Description

The following description of the embodiments of the present invention is provided for illustrative purposes, and other advantages and effects of the present invention will become apparent to those skilled in the art from the present disclosure. While the invention will be described in connection with the preferred embodiments, there is no intent to limit its features to those embodiments. On the contrary, the invention is described in connection with the embodiments for the purpose of covering alternatives or modifications that may be extended based on the claims of the present invention. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The invention may be practiced without these particulars. Moreover, some of the specific details have been left out of the description in order to avoid obscuring or obscuring the focus of the present invention.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

Additionally, the terms "upper," "lower," "left," "right," "top," "bottom," "horizontal," "vertical" and the like as used in the following description are to be understood as referring to the segment and the associated drawings in the illustrated orientation. The relative terms are used for convenience of description only and do not imply that the described apparatus should be constructed or operated in a particular orientation and therefore should not be construed as limiting the invention.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, regions, layers and/or sections, these elements, regions, layers and/or sections should not be limited by these terms, but rather are used to distinguish one element, region, layer and/or section from another element, region, layer and/or section. Thus, a first component, region, layer or section discussed below could be termed a second component, region, layer or section without departing from some embodiments of the present invention.

As mentioned above, in practical applications of the transformer in rail transit, it is usually necessary to integrally mount the transformer and the current transforming unit of the circuit in the same sealed insulated box. Therefore, the transformer is required to have high voltage breakdown resistance and a compact structure. In addition, since the heat resistance of devices such as the inverter unit and the capacitor integrally mounted in the insulating box is generally poor, the heat dissipation amount of the transformer cooling device to the inside of the insulating box must be limited while the transformer is cooled.

Referring to fig. 1, fig. 1 is a schematic view illustrating an installation of a transformer cooling apparatus according to an aspect of the present invention.

As shown in fig. 1, the transformer 11 may include an encapsulation layer for wrapping the coil winding, and a core 111 surrounding the encapsulation layer. The transformer 11 may be mounted integrally with the current transforming unit 12 of the circuit inside a sealed insulating casing 13. The converter unit 12 includes, but is not limited to, a rectifier module, an inverter module, a capacitor, and a weak current control board of the rail vehicle. Since the heat resistance of the converter unit 12 is generally poor, the transformer cooling device must limit the amount of heat dissipated from the transformer 11 into the insulating box 13 while cooling the transformer 11, so as to prevent the converter unit 12 of the circuit from being damaged due to overheating.

In one embodiment of the present invention, the cooling device of the transformer may include a capillary network 21 disposed inside the insulation case 13, a heat sink 22 disposed outside the insulation case 13, and a fluid pump 23 disposed outside the insulation case 13.

The capillary network 21 may be formed by a plurality of parallel capillaries. The plurality of parallel capillaries may be uniformly attached to the outer surface of the potting layer of the transformer 11. The plurality of parallel capillaries may have a flowing refrigerant flowing therein for absorbing heat generated from the potting layer of the transformer 11 and the iron core 111 of the transformer 11. The refrigerant includes, but is not limited to, water, oil, or other liquid refrigerant. In one embodiment, the plurality of parallel capillaries in the capillary network 21 may be capillaries with a diameter of millimeter, preferably capillaries with a diameter of less than 5mm, and are used for cooling the transformer 11 in a small space inside the insulating box and reducing heat dissipation to the inside of the insulating box 13 while taking away heat of the transformer 11.

Compare and offer the scheme of cooling tube groove on transformer 11's embedment layer in prior art, the above-mentioned capillary network 21 that will absorb heat is attached in the scheme of the surface of transformer 11's embedment layer in order to form the cooling surface, can keep the integrality of 11 solenoid of transformer and embedment layer to can install again after 11 main parts of transformer complete, be favorable to reducing the installation degree of difficulty and reduce manufacturing cost.

The heat sink 22 may be used to dissipate heat from the refrigerant in the capillary network 21. Specifically, the radiator 22 may be composed of one heat exchanger and one fan. Such heat exchangers, including but not limited to plate heat exchangers and plate fin heat exchangers, may be used to conduct heat from the refrigerant. The fan can continuously generate cooling air flow to the heat exchanger, and the low-temperature air is used for taking away heat on the surface of the heat exchanger, so that the refrigerant in the heat exchanger is cooled to realize cooling of the transformer 11. In one embodiment, the temperature of the refrigerant inside the insulating tank 13 can be controlled by adjusting the heat dissipating capacity of the heat sink 22, thereby achieving control of the temperature of the outer surface of the transformer 11. The heat dissipation capacity of the heat sink 22 can be adjusted by changing the rotation speed of the fan.

The fluid pump 23 includes, but is not limited to, a positive displacement pump and a vane pump, and may be used to drive the refrigerant to circulate between the capillary network 21 and the radiator 22. Specifically, the fluid pump 23 may continuously drive the high-temperature refrigerant, which has absorbed heat of the transformer 11, to the radiator 22 for heat dissipation, and continuously drive the low-temperature refrigerant, which has completed heat dissipation, to the capillary network 21 for cooling the transformer 11. The particular selection of fluid pump 23 may be determined based on the particular characteristics of the refrigerant. In one embodiment, the temperature of the refrigerant inside the insulation case 13 may be controlled by adjusting the flow rate of the refrigerant in the fluid pump 23, thereby achieving control of the temperature of the outer surface of the transformer 11.

By providing the compact capillary network 21 inside the insulating case 13 and providing the radiator 22 and the fluid pump 23 outside the insulating case 13, the transformer 11 can be cooled in a small space inside the insulating case 13. By forming a cooling surface by covering the outer surface of the potting layer of the transformer 11 with the capillary network 21 that absorbs heat and disposing the heat sink 22 that releases heat outside the insulating case 13, it is possible to reduce the amount of heat radiated into the insulating case 13 while cooling the transformer 11.

Referring to fig. 2A-2B in combination, fig. 2A is a schematic front view illustrating a transformer cooling apparatus according to an embodiment of the invention. Fig. 2B shows a schematic side view of a transformer cooling arrangement provided in accordance with an embodiment of the present invention.

As shown in fig. 2A-2B, two ends of the plurality of parallel capillaries 211 in the capillary network 21 can be connected to a header 212-213, respectively. The refrigerant is divided by one header 212 into a plurality of capillary tubes 211 and converged by the other header 213 out of the plurality of capillary tubes 211 by the driving of the fluid pump 23, thereby achieving the flow of the refrigerant in the capillary network 21. Since the capillary network 21 includes the plurality of capillaries 211 and the plurality of capillaries 211 are connected in parallel, the flow rate of the refrigerant in the capillary network 21 can be controlled to 1m/s or less. Therefore, by adopting the above-described structure of the capillary network 21, the flow velocity of the refrigerant can be effectively reduced, and the flow resistance of the refrigerant can be reduced to reduce the power loss of the fluid pump 23 for driving the refrigerant to flow. In addition, by adopting the mode of uniformly attaching the capillary network 21 to the outer potting layer of the transformer 11, the flexible and bendable characteristics of the capillary network 21 can be utilized to form a cooling surface with large heat dissipation area and uniform temperature distribution, so that the cooling effect of the transformer cooling device is improved.

In one embodiment of the present invention, two headers 212-213 may be provided on both sides of the transformer 11. A plurality of parallel capillaries 211 in the capillary network 21 may extend from one header 212 to the other side of the transformer 11 and connect to another header 213 on the other side of the transformer 11. The plurality of parallel capillaries 211 may be connected to the two headers 212-213 by a heat fusion. By adopting the connection mode of the hot melt connection, the plurality of parallel capillaries 211 and the two headers 212 to 213 do not need to add extra connecting pieces directly, so the whole capillary network 21 has light weight, and has the advantages of small required installation space, good insulation and low production cost.

Specifically, the plurality of capillaries 211 in the capillary network 21 may be made of a flexible heat-conducting material for conforming to the outer surface shape of the potting layer of the transformer 11. Such flexible heat conductive materials include, but are not limited to, nylon, polyvinyl chloride (PVC), and Polyurethane (PU). The capillary network 21 may be flexible in that the plurality of capillaries 211 in the capillary network 21 are made of a flexible, thermally conductive material. Therefore, the capillary network 21 can be attached to the outer surface shapes of various potting layers without any processing, thereby expanding the application range of the cooling device of the transformer provided by the invention.

Optionally, in one embodiment, the flexible heat conductive material may also preferably have the property of insulating and flame retardant. By adopting the capillary network 21 made of the insulating flame-retardant material to replace a heat dissipation plate made of metal and the like in the prior art, the influence on the electromagnetic parameters of the transformer 11 can be effectively reduced, and the negative influence on the voltage withstand grade of the transformer 11 is avoided.

Optionally, in an embodiment, the cooling device of the transformer may further include a pipe network compressing structure 31 and a pipe network gap filling pad 32. The network compression structure 31 may be a strapping made of an insulating material for securing and adhering the capillary network 21 to the outer surface of the potting layer of the transformer 11. The pipe network gap filling pad 32 may be made of a flexible insulating and heat conducting material, and is disposed between the plurality of capillaries 211 of the capillary network 21, for fixing the gap between the plurality of capillaries 211 in cooperation with the pipe network compressing structure 31. The gap between the plurality of capillaries 211 may be determined according to the specific size of the transformer 11 and the actual cooling requirement of the transformer 11, and may preferably be 20mm.

Referring to fig. 3, fig. 3 is a schematic cross-sectional view of a gap filling pad for a pipe network according to an embodiment of the present invention.

As shown in fig. 3, the tube network gap filling pad 32 may be a flexible insulating heat-conducting pad such as a silicone pad or a silicone pad, and is provided between the plurality of capillaries 211 of the capillary network 21 and the binding tape 31. The network gap filling pad 32 can compress the plurality of parallel capillaries 211 in the capillary network 21 under the binding action of the binding tape 31, so as to generate concave deformation inwards. At this time, the pipe network gap filling pad 32 and the plurality of parallel capillaries 211 in the capillary network 21 can be fixed and attached to the outer surface of the potting layer of the transformer 11 by the binding tape 31, thereby forming a complete cooling surface together. The capillaries 211 may be spaced apart from each other by an undeformed portion of the pipe network gap filling pad 32, thereby fixing the gap between the plurality of capillaries 211.

It will be appreciated by those skilled in the art that the above-described network gap filling pad 32, which is disposed between the capillary network 21 and the potting layer of the transformer 11, is only one example provided by the present invention, and is primarily intended to clearly demonstrate the concept of the present invention and to provide a practical solution for the convenience of the public, and is not intended to limit the scope of the present invention.

Alternatively, in another embodiment, based on the concept of the present invention, the above-mentioned pipe network gap filling pad 32 may also be disposed between the plurality of capillaries 211 of the capillary network 21 and the potting layer of the transformer 11. The plurality of parallel capillaries 211 in the capillary network 21 can press the network gap filling pad 32 under the binding action of the binding tape 31, so that the network gap filling pad 32 is deformed concavely inwards. At this time, the plurality of parallel capillaries 211 in the capillary network 21 can be fixed and adhered to the outer surface of the potting layer of the transformer 11 by the binding tape 31 through the network gap filling pad 32. The capillaries 211 may be spaced apart from each other by an undeformed portion of the pipe network gap filling pad 32, thereby fixing the gap between the plurality of capillaries 211.

Alternatively, in other embodiments, the pipe network gap filling pad 32 may be a plurality of separate insulating heat conducting pads disposed between the plurality of capillaries 211 of the capillary network 21 based on the concept of the present invention. The plurality of separated insulating heat-conducting pads 32 and the plurality of parallel capillaries 211 in the capillary network 21 can be fixed and attached to the outer surface of the potting layer of the transformer 11 under the action of the strapping tape 31, so as to jointly form a complete cooling surface. The capillaries 211 may be spaced apart by the plurality of spaced apart insulating thermal pads 32, thereby fixing the gap between the plurality of capillaries 211.

It will be appreciated by those skilled in the art that the above-described arrangement of two headers 212-213 on each side of the transformer 11 is only one example of the invention, and is provided primarily for clarity of illustration of the inventive concept and to provide a practical solution for the convenience of the public, and not for limiting the scope of the invention.

Referring to fig. 4 and 5 in combination, fig. 4 is a schematic front view illustrating a transformer cooling apparatus according to an embodiment of the present invention. Fig. 5 shows a schematic deployment of a capillary network provided according to an embodiment of the present invention.

As shown in FIG. 4, in one embodiment of the present invention, the two headers 412-413 of the transformer cooling apparatus may be disposed on the same side of the transformer 11. The plurality of capillaries 411 in capillary network 41 may extend from one header 412 to the other side of transformer 11 and bend back on the other side of transformer 11 to connect to another header 413.

Specifically, as shown in fig. 5, two ends of a plurality of parallel capillaries 411 in the capillary network 41 may be connected to one of headers 412 to 413, respectively. Header 412 may be provided on one side of transformer 11. A plurality of capillaries 411 may extend from header 412 to the other side of transformer 11 and bend at the other side of transformer 11 to return to the side where header 412 is located, thereby connecting headers 413 located on the same side of transformer 11. The refrigerant can be branched by one header 412 into a plurality of capillaries 411 and converged by the other header 413 out of the plurality of capillaries 411 by the driving of the fluid pump 23, thereby realizing the flow of the refrigerant in the capillary network 41. Since the capillary network 41 includes the plurality of capillaries 411 and the plurality of capillaries 411 are connected in parallel, the flow rate of the refrigerant in the capillary network 41 can be controlled to be 1m/s or less. Therefore, by adopting the above-described structure of the capillary network 41, the flow velocity of the refrigerant can be effectively reduced, and the flow resistance of the refrigerant can be reduced to reduce the power loss of the fluid pump 23 for driving the refrigerant to flow.

The gap between the plurality of capillaries 411 may be determined according to the specific size of the transformer 11 and the actual cooling requirement of the transformer 11. The cooling effect of the transformer cooling apparatus can be enhanced by narrowing the gap between the plurality of capillaries 411. Because the range of the gap between the capillaries 411 is limited by the bending radius of the capillaries 411, the bending radius of the capillaries 411 is determined by the specific material of the capillaries 411. Therefore, by selecting the capillary 411 made of a soft material, the gap between the plurality of capillaries 411 can be reduced, thereby enhancing the cooling effect of the transformer cooling device.

In one embodiment, the plurality of capillaries 411 in the capillary network 41 may be made of a flexible heat conductive material for conforming to the outer surface shape of the potting layer of the transformer 11 and reducing the gaps between the plurality of capillaries 411. Such flexible heat conductive materials include, but are not limited to, nylon, polyvinyl chloride (PVC), and Polyurethane (PU). Since the plurality of capillaries 411 in the capillary network 41 are made of a flexible, thermally conductive material, the capillary network 41 can be flexible. Therefore, the capillary tube network 41 can perfectly fit the outer surface shapes of various encapsulation layers without any processing, thereby improving the application range of the cooling device of the transformer provided by the invention.

The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A cooling device for a transformer, said transformer being mounted integrally with a converter unit of an electric circuit inside a sealed insulating casing, said cooling device comprising:

the capillary network is arranged in the insulating box body and covers the outer surface of an encapsulating layer wrapping the coil winding of the transformer to form a cooling surface; the radiator is arranged outside the insulating box body and used for radiating heat of the refrigerant in the capillary network; and

and the fluid pump is used for driving the refrigerant to circularly flow between the capillary network and the radiator.

2. A cooling apparatus as set forth in claim 1, wherein a header is connected to each of both ends of a plurality of said capillary tubes, and said refrigerant is branched from one of said headers to flow into a plurality of said capillary tubes and branched from the other of said headers to flow out of a plurality of said capillary tubes.

3. The cooling apparatus according to claim 2, wherein two of said headers are provided on the same side of said transformer,

the plurality of capillary tubes extend from one of the headers to the other side of the transformer and are bent back at the other side of the transformer to connect to the other header.

4. A cooling apparatus according to claim 3, wherein the gap between the plurality of capillaries in the capillary network is determined according to the bend radius of the capillaries.

5. The cooling apparatus according to claim 2, wherein two of said headers are provided on both sides of said transformer,

a plurality of said capillaries extend from one of said headers to the other side of said transformer and connect to the other of said headers on the other side of said transformer.

6. A cooling apparatus as claimed in claim 2, wherein said capillary tube is thermally fused to said header.

7. A cooling device according to claim 2, wherein said capillary network is formed by a plurality of capillaries of millimeter-diameter.

8. The cooling apparatus of any one of claims 1-7, further comprising a network of tubes hold-down structure for securing and holding the capillary network against an outer surface of a potting layer of the transformer.

9. The cooling apparatus as claimed in claim 8, further comprising a pipe network gap filling pad made of a flexible, insulating and heat conductive material, disposed between the plurality of capillaries of the capillary network, for fixing the gap between the plurality of capillaries.