Title:
HEAT EXCHANGER USING CORRUGATED SHEETS
Kind Code:
A1


Abstract:
In an MVR evaporator, a heat exchanger core comprises several condensing units. Each unit comprises front and back corrugated sheets, arranged trough-to-trough. The units are arranged side-by-side, in peak-to-trough configuration, creating sinuous passageways between the units. Steam is fed into the units through port-pipes, and the units are physically supported via the port-pipes from steam manifolds.



Inventors:
Forstmanis, Talivaldis (Kitchener, CA)
Application Number:
12/471509
Publication Date:
12/03/2009
Filing Date:
05/26/2009
Primary Class:
International Classes:
F28F13/00
View Patent Images:



Primary Examiner:
MANOHARAN, VIRGINIA
Attorney, Agent or Firm:
ANTHONY ASQUITH (WATERLOO, ON, CA)
Claims:
1. Heat exchanger apparatus, wherein: the apparatus includes several heat transfer units, and in respect of each unit: the unit includes front and back corrugated sheets, which are fastened together in a trough-to-trough configuration; the unit includes sheet-spacers, and the sheet-spacers hold adjacent corrugation troughs of the front and back corrugated sheets in a spaced-apart relationship, a distance D1 apart; the unit includes peripheral seals, which straddle sealingly between the spaced-apart sheets, thereby creating a sealed and enclosed interior space between the sheets and between the seals; the unit includes inlet and outlet ports, through which fluid can be introduced into, and discharged from, the interior space; the several heat-transfer units are arranged side by side, and form a core of the heat exchanger apparatus; the apparatus includes a support, in which the several units of the core are mounted, and the support is structured to so mount adjacent heat-transfer units, in the core, that: the front sheet F-U1 of one of the heat transfer units U1 faces the back sheet B-U2 of the next adjacent unit U2; the said front and back sheets F-U1 and B-U2 face each other in a peak-to-trough configuration; the support includes unit-spacers, and the unit-spacers hold the units U1 and U2 in spaced-apart relationship, a distance D2 apart; the distance D2 between the sheets F-U1 and B-U2 is large enough to create and define a passageway between those sheets, which is large enough to enable fluid to pass between those sheets; the distance D2 is small enough that fluid, in passing through the passageway between the units U1 and U2, and in encountering the peaks and troughs of the sheets F-U1 and B-U2, is forced by such encounters to undergo respective substantial changes of direction; whereby the passageway created between adjacent heat-transfer units can be characterized as sinuous.

2. As in claim 1, wherein the arrangement of the apparatus is such that fluid passing through the sinuous passageway encounters at least three peaks each of the corrugated sheets F-U1 and B-U2, being at least six peaks in aggregate.

3. As in claim 1, wherein: in respect of a path-line that traces the shortest path that fluid can take in passing through the passageway; the path-line passes from a peak P1-F-U1 of the sheet F-U1 to a peak P1-B-U2 of the sheet B-U2, and then to a peak P2-F-U1 of the sheet F-U1, and then to a peak P2-B-U2 of the sheet B-U2, and so on; the portion of the path-line joining peak P1-F-U1 to peak P1-B-U2 lies at an angle A to the portion of the path-line joining peak P1-B-U2 to peak P2-F-U1; the angle A is no more than about 150 degrees.

4. As in claim 1, wherein: the corrugations of sheet F-U1 are of the same pitch and the same peak-to-trough height, being PTH centimetres, as the corrugations of sheet B-U2; the distance D2 between the sheets F-U1 and B-U2 is smaller than the height PTH, whereby the peaks of sheet F-U1 overlap the peaks of the sheet B-U2; and preferably D2 is about one-half of PTH.

5. As in claim 1, wherein the sheet-spacers that hold the sheets the distance D1 apart in the unit include spacer-strips of thickness D1, fasteners go through them, at the troughs, clamp sheets onto spacer-strips.

6. As in claim 1, wherein: the peripheral seals include left and right masses of filler-sealant; and each mass straddles between the front and back sheets.

8. As in claim 1, wherein the inlet and outlet ports of the unit comprise respective inlet and outlet port-pipes.

9. As in claim 8, wherein the port-pipes are embedded in the filler-sealant.

10. As in claim 1, wherein: the apparatus includes an inlet manifold; the inlet manifold is formed with pipe-holes, the walls of which are suitably sized to receive the port-pipes of the several units; the walls of the pipe-holes in the inlet manifold are so positioned relatively that, when the port-pipes of the several heat transfer units are received therein, the walls of the pipe-holes constrain the port-pipes of one unit against movement towards and away from the other units; whereby the walls of the pipe-holes of the inlet manifold serve as the said unit-spacers.

11. As in claim 10, wherein the walls of the pipe-holes in the inlet-manifold are so positioned relatively as to support the several units, the port-pipes of which are received therein, in the said peak-to-trough configuration.

12. As in claim 1, wherein: the HE apparatus is a component of an evaporator; in the evaporator, incoming water containing a dissolved contaminant at a dilute contamination is evaporated, whereby, in the outgoing final water, the contaminant is more concentrated; the arrangement of the evaporator is such that, in use, steam entering the inlet ports of the units is condensed upon passing through the units, and water passing through the passageways is evaporated.

12. As in claim 1, wherein: the evaporator is an MVR evaporator; the evaporator includes a chamber, in respect of which: the contaminated water is circulated through the sinuous passageways; steam is drawn, and compressed, and then fed into the inlet ports of the heat transfer units.



Description:

This invention relates to heat exchangers, and to a cost-effective manner of construction thereof.

BACKGROUND TO THE INVENTION

The invention will be described herein mainly as it applies to evaporators. Evaporators are used in the treatment of industrial wastewaters, in that water is evaporated from the in-stream of wastewater, thereby concentrating the contaminant in the final-water out-stream. Evaporators are used when the savings in disposal costs more than outweigh the costs of evaporation.

The invention is especially applicable to mechanical vapour recompression (MVR) evaporators. Typically, in an MVR evaporator, water and steam are present together in a state of pressure-temperature equilibrium, at a pressure below atmospheric, in a vacuum chamber of a container vessel. Steam is extracted from the vacuum chamber, and is compressed in such manner that the mechanical energy of compression serves to raise the temperature, as well as the pressure, of the steam. The now-hotter steam passes through a condenser, in which it is condensed into liquid water. The condensate is collected and conveyed away.

The liquid water from the vacuum chamber serves as the coolant in the condenser, i.e as the coolant that is used to condense the (hot, compressed) steam. The coolant is, of course, heated by its passage through the condenser, and some of the liquid water turns into steam. In fact, in equilibrium, the liquid water coolant, as it passes through the condenser, turns into steam at the same rate at which steam is extracted from the vacuum chamber to be compressed.

An MVR evaporator typically is used in the cleanup of wastewater from industrial processes. Depending upon the temperature at which the wastewater is received, the wastewater might have to be pre-heated, but basically, the MVR evaporator requires the the energy input (i.e the energy needed to operate the evaporator) be only in the form of mechanical energy. That is to say, the steam is heated by being compressed, rather than by direct heating.

The MVR process is, or can be, efficient, easy to control, and economical. The collected condensate is basically distilled water, a by-product which can be useful in various industrial processes.

A component of the MVR evaporator is the condenser. Here, heat is extracted from the compressed and heated steam at a sufficient rate to condense the steam. The steam is only a few degrees hotter than the coolant liquid water. The heat transfer relies on utilizing the latent heat of water, rather than on utilizing a large temperature differential. As such, the heat exchanger (HE) that is the condenser should be thermally efficient. Also, the HE should be mechanically robust, in that the HE needs to be designed to handle through-flows of fluids at higher rates than is usual in other traditional HE designs. Also, the HE should be easy to clean, because scaling can be a problem.

The design of HE that is the subject of this specification is aimed at combining those desiderata in a cost effective manner.

LIST OF THE DRAWINGS

The technology will now be further described with reference to the accompanying drawings, in which:

FIG. 1 is a sectioned side elevation of a condensing unit of a HE.

FIG. 1a is a (diagrammatic) end elevation of the condensing unit.

FIG. 2 is a pictorial view, partly cut away, of the condensing unit of FIG. 1.

FIG. 3 is a diagram showing a HE that incorporates some condensing units. The HE is shown incorporated into an MVR evaporator.

FIG. 4 is a sectioned side elevation of part of the condensing unit shown in FIG. 3, with some associated components.

FIG. 5 is a pictorial diagrammatic view of some of the components of the MVR evaporator of FIG. 3

FIG. 6 is a diagram of a pair of condensing units, illustrating some of the nomenclature used herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIGS. 1, 1a, 2, a heat transfer unit—in this case a condensing unit 20—is made up from front and back sheets 23F, 23B of corrugated material. In FIG. 1, the front sheet 23F has been removed. The corrugated sheets 23F, 23B are attached together, trough-to-trough, the troughs being held a small distance apart by spacer-strips 25. Fasteners 27 clamp the troughs of the two sheets together, onto the spacer-strips 25. The fasteners can be bolts, rivets, etc, as appropriate.

It is emphasized that the troughs of the corrugated sheets 23F, 23B do not actually make touching contact with each other, being held a distance D1 apart (FIG. 6) by the spacer-strips 25. Thus, in the areas between the spacer-strips 25, there is open space between the troughs, through which fluids can travel (i.e travel downwards) between the corrugated sheets.

Top and bottom of the sheets 23F, 23B, channel-strips 28 sealingly close off the longitudinal edges of the sheets. In use, the pressure inside the unit 20 is higher than the pressure outside, and the designers should see to it that the channel-strips 28 remain firmly attached to the sheets, despite the pressure differential.

At the left and right ends of the condensing unit 20, the space between the corrugated sheets 23F, 23B is filled with respective masses 29L, 29R of a sealant-filler. In FIG. 2, the lines 30 indicate the distance to which the sealant-filler penetrates into the spaces between the corrugated sheets. FIG. 1 also shows the extent to which the masses penetrate into the interior space between the sheets.

Embedded in the sealant mass 29L are two ports, the upper 32 being a steam-inlet port, and the lower 34 being a water-outlet port. The ports comprise respective short lengths of metal or plastic pipe. The port-pipes 32, 34 communicate with the interior space between the corrugated sheets. Apart from the ports, the interior space is completely sealed.

FIG. 3 shows a core of condensing units 20 that together make up the HE 36. It will be observed that, in FIG. 3, while the front and back corrugated sheets 23F, 23B of the individual condensing units 20 are assembled trough-to-trough, the units 20 are assembled to each other in peak-to-trough configuration. That is to say, the corrugations between adjacent units are—preferably exactly—out of phase with each other.

The individual condensing units are spaced from their adjacent neighbouring units, and it will be observed, again from FIG. 3, that assembling the condensing units 20 in peak-to-trough configuration creates sinuous passageways 38 between adjacent condensing units. These passageways, though sinuous, are of uniform horizontal width (dimension D2 in FIG. 6) along their vertical lengths.

Preferably, the corrugated sheets are regular, i.e all the peaks are pitched the same distances apart, and all the peaks and troughs are of the same amplitude. Preferably also, all the sheets used in the HE are of the same profile of regular corrugation. When that is so, the units do not need to be specially matched and aligned.

Each condensing unit 20 is a rigid structure, in itself. Preferably, the inlet and outlet ports of the unit are duplicated at the left and right ends of the condensing unit (as may be seen in FIG. 2). Then, the condensing unit can very readily support itself, simply as a result of the port-pipes 32, 34 being inserted into suitable apertures or pipe-holes located at the ends of the unit 20. Furthermore, the whole core of several condensing units can be supported on suitably-placed respective holes.

It is not essential that the condensing units 20 be supported on their port-pipes, although that is the preferred manner of support. The condensing units are easy to support, in that the units have many downwards-facing surfaces, which can engage, and rest on, e.g pegs carried in a suitable unit mounting means.

As will be understood from FIGS. 3, 4, 5, the condensing units 20 are supported on the pipe-holes 42 formed in left and right steam-inlet-manifolds 40L, 40R. Mechanically, the manifolds support all the condensing units, and hold the units in their correct relative positions. Thus, the steam-inlet manifolds do double duty: they also collect steam and feed it into the ports 32, and thence into the interior spaces of all the condensing units. Steam enters the steam-inlet-manifold 40L via a manifold inlet-pipe 41.

Left and right water-outlet-manifolds 43L, 43R also provide mechanical support from the condensing units 20, and serve to collect and convey liquid water out of the units. In FIGS. 3, 5, each condensing unit is provided with six steam-inlet ports 32 and two water-outlet ports 34 (steam occupying much more volume than water, of course).

FIG. 4 illustrates how the upper and lower manifolds 40L, 43L engage with the several condensing units 20, at the left end of the HE 36. The pipe-holes 42 are provided with seals 44, which seal the port-pipes 32, 34 with respect to the manifolds.

Once the several condensing units 20 have been assembled between the manifolds 40L, 43L and 40R, 43R, the manifolds are fastened to a cover 45. The cover 45 is formed from sheet metal, and is arranged to rest on a floor or platform 47 inside a chamber 49. The cover holds the manifolds in place, and the manifolds in turn brace the cover into its configuration as shown.

The cover 45 is basically open at the ends, and underneath, and is not airtight or watertight. A bracket 50 braces the sides of the cover 45, and holds them correctly spaced over its length. Of course, the designer should see to it that such brackets, between the sides of the cover 45, do not interfere with the HE core.

The cover 45 as shown is made of sheet material; however, the cover can alternatively be formed as a framework or skeleton. The floor 47 of the chamber 49 is open, i.e water can pass downwards, from the HE, through the floor.

The cover 45 is equipped with a number of spray-heads 52, suitably pitched in the roof portion of the cover. Water from the spray-heads passes down the sinuous passageways 38 between the condensing units 20. Thus, water from the spray-heads 52 makes vigorous contact with the outside surfaces of the corrugated sheets 23 of the condensing units.

As a result of the vigorous contact, heat is transferred from the steam inside the condensing units 20 into the down-flowing water. Consequently, the steam inside the units 20 condenses, and the water in the sinuous passageways 38 evaporates.

Generally, the designers will prefer that the water does not actually boil, when in contact with the corrugated sheets. Formation of physical bubbles on the metal surfaces would adversely affect heat transfer rates. Preferably, therefore, the water passing through the passageways 38 is collected and pumped back and rapidly re-circulated through the passageways, to keep bubble-formation to a minimum. Preferably, the circulation rate of the liquid water (as generated by the water-pump 58), measured as a flowrate of F kg/min, should be about five, or more, times the rate in kg/min at which the water evaporates.

FIG. 3 contains some exemplary values of the magnitudes of the pressure and temperature of the water as it passes through the MVR evaporator.

Contaminated water enters the evaporator through the entry-port 56. This water is typically received at a temperature of 65° C. (If the water is cold, it can be economical to pre-heat the water to this temperature.) The incoming dirty water is pumped (by water-pump 58) to the spray-heads 52. The sprayed water passes down through the sinuous passageways 38 between the condensing units 20, picking up heat from the steam inside the units.

Much of the falling water remains as liquid, and falls into the pool 60 of water resting on the bottom of the chamber 49. But some of the falling water evaporates, and passes into the upper areas of the chamber 49. The still-contaminated water collected in the pool is re-pumped back to the spray-heads 52, whereby the dirty water is circulated and re-circulated through the HE core, some of its water content being evaporated each pass.

The contaminant in the water does not evaporate. (The operators should be sure to keep the temperature inside the chamber below the temperature at which the contaminants might be volatile.) Therefore, the contaminant concentration in the water in the pool 60 is considerably stronger than in the incoming water in the entry-port 56.

The water in the pool 60 is discharged from the evaporator, through the concentrate-discharge port 65, as concentratedly-contaminated water, the disposal of which is more economical than the direct dispoal of the dilutely-contaminated water entering through the dirty-water entry-port 56.

Typically, the pressure inside the chamber 49 is maintained at about 0.3 bar (atmospheres), i.e well below atmospheric pressure. At this (low) pressure, water boils at 70° C. The steam inside the chamber 49, at a pressure of 0.3 bar and at a temperature that can be equated to 70° C., is drawn into a steam-blower 63. Here, its pressure is increased—in this example to about 0.6 bar. The compression produces an increase in temperature, in the steam—to about 80° C. in this example.

This steam, which is now at 80° C. and 0.6 bar, passes into the steam-inlet manifolds 40, and thence into the steam inlet ports 32 of the condensing units 20. Inside the condensing units 20, the steam condenses, and drips down into the lower regions of the units. The condensate water is drawn through the water-outlet ports 34, the water-outlet manifolds 43, and the manifold outlet-pipe 66, and is discharged from the evaporator via the condensate-discharge port 67.

The condensate water in the condensate-discharge port 67 is still hot, and its heat can be utilized for example to pre-heat the incoming dirty water (e.g in a suitable HE apparatus). The condensate water is more or less pure H2O, in which state it can be useful in many industrial processes.

As mentioned, it can be advantageous to thoroughly drench the HE core with the contaminated liquid water, and to circulate and recirculate the dirty water very vigorously through the HE. This vigorous drenching action of the water can be mechanically demanding on the structure of the HE.

Therefore, the structure of the core needs to be robust. In the designs as depicted herein, the individual condensing units, constructed as described, are remarkably strong and rigid, in themselves. That being so, all that is needed by way of mechanical support for the units is, basically, something for them to rest on; as a result, the supports are not required to contribute much by way of rigid structural support.

Another factor the designers should have in mind is that the spaces between the individual condensing units should be accurately maintained. It would not do for the sinuous passageway behind the unit to be wider than the one in front of the unit. The condensing units should be supported in such manner as to render the spacing between them uniform.

Another factor in the design of the support for the units is that, when the HE is dismantled (e.g for cleaning or de-scaling), the design should be such that the technicians can reassemble the units in exactly the same mutual spacing relationship with each other.

The support for the units as depicted herein, in which the units are supported from their port-pipe, and the port-pipes are simply inserted into holes in the manifolds, can provide the required ease of assembly and disassembly, and the required accuracy of positioning of the units.

If the several individual condensing units were so designed that they had to be, for example, bolted together for assembly, that would not be preferred, when compared with the illustrated designs, not least because removing and replacing many (threaded) fasteners is time consuming and labour intensive.

By contrast, the condensing units 20 do not have to be (and cannot be) dismantled, e.g for cleaning the interiors thereof. The more elaborate and non-dismantlable construction of the units, with their spacer-strips, many fasteners, and their filler-sealant masses, is acceptable because that construction can be done, and finished, on an in-factory basis.

The corrugated sheet-metal, from which the front and back walls of the condensing units are formed, serves as a thermal partition. The walls keep the fluids apart, but permit heat to pass between the fluids. As such, the walls should be of thin metal, for efficient heat transfer. A thickness of between about 0.3 and 1.0 millimetre is preferred. Even though the walls are thin, the shape and configuration of the units, as described, gives them sufficient stiffness and strength for the units to be highly suitable for performing the described functions.

Typically, the designers will prefer to use stainless steel for the metal components of the HE, as the presence of hot contaminated water can exacerbate corrosion problems.

Besides being very suitable for use in MVR evaporators, the heat exchanger as described herein is also suitable for use in other applications, especially where the fluids between which heat is exchanged both pass through gaseous and liquid phases.

The described HE is also efficient enough to be very suitable for use when the available temperature differentials are only a few degrees. Thus, the HE is very suitable for use with low-grade heat.

The sinuous passageways are effective in ensuring good heat transfer, in that movement of the water down the sinuous passageways serves to mix and stir the water very thoroughly, whereby any temperature differences and gradients, measured on a drop-to-drop basis, in the falling water, are quickly dissipated. Also, the falling water impinges vigorously the metal surfaces many times during travel through the passageways, which assists in heat transfer.

Regarding the seals around the periphery of the corrugated sheets 23, i.e the seals that define the perimeter of the interior space created between the sheets: of course, preferably, the seals should not leak. However, expensive leakproofing is not required, in that a small leakage is of little consequence, in that the leakage would be outwards, since the pressure inside the unit is higher than the pressure outside, so there would just be a leakage of steam and/or condensed water back into the chamber, and the condensate likely would not be subjected to contamination. In any case, it is generally an easy matter to make the seal complete.

The masses 29 of filler-sealant can be of injectable expanding foam, which sets solid. The port-pipes 32, 34 are first positioned between the front and back sheets, and then the foam is injected around them. The channel-strips 28 also are easy to seal, again simply being filled with expandable foam as they are assembled. The seals as described are robust enough to support fluid pressure inside the interior space 35 of the unit. The sealed condensing unit is permanent, in the sense that it is not dismantlable.

It is a simple matter to take the HE out of the chamber, for cleaning and de-scaling. The cover 45 is arranged simply to slide out of the chamber 49, over the open platform 47, with very little dismantling being required before that can be done. With the HE outside the chamber, it is a simple matter to separate the manifolds from the cover, and then the manifolds can simply be pulled off the left and right ends of the HE core. The individual condensing units can then be handled separately, for cleaning.

FIG. 6 shows two of the heat transfer units in their side-by-side peak-to-trough configuration, and illustrates some of the nomenclature used herein.

A peak viewed from one side of the sheet is, of course, a trough when viewed from the other side of the sheet. Herein, a peak is a peak when viewed from the outside of the heat-transfer (condensing) unit. Thus, in FIG. 1a, etc, the corrugated sheets are fastened together in a trough-to-trough configuration.

The sinuous passageway 38 preferably is sinuous enough that there is no straight path through which water could fall. In passing downwards, preferably the steam/water traverses, and changes direction at, at least six peaks—i.e three on each side of the passageway. In FIG. 3, for example, the falling water encounters (and changes direction at) fourteen peaks.

In FIG. 6, a path-line 69 traces the shortest path that fluid can take in passing through the passageway. The path-line 69 passes from a peak P1-F-U1 of the sheet F-U1 to a peak P1-B-U2 of the sheet B-U2, and then to a peak P2-F-U1 of the sheet F-U1, and then to a peak P2-B-U2 of the sheet B-U2, and so on. The portion of the path-line 69 joining peak P1-F-U1 to peak P1-B-U2 lies at an angle A to the portion of the path-line joining peak P1-B-U2 to peak P2-F-U1. Preferably, the angle A is no more than about 150 degrees.

The sinuousness of the passageway 38 may be expressed in another way. The peak-to-trough height of the corrugations, being PTH centimetres, preferably is larger than the distance D2 between the sheets F-U1 and B-U2, whereby the peaks of sheet F-U1 overlap the peaks of the sheet B-U2. This being so, steam/water cannot follow a straight path through the passageway. More preferably, the spacing D2 is about one-half of PTH.

The apparatuses as depicted include sheet-spacers, being the spacer-strips 25 as described, which hold the front and back sheets of the respective condensing units the distance D1 apart. Equally, the depicted apparatuses include unit-spacers, which hold the adjacent units the distance D2 apart. The unit-spacers are, in the drawings, provided by the pipe-holes of the manifolds, which receive the respective port-pipes of the several condensing units. The pipe-holes in the manifolds are so positioned, relative to each other, that the several units are thereby held firmly in the required side-by-side, peak-to-trough configuration.

The HE as shown is arranged horizontally, and that is preferred for convenience. Alternatively, the HE can be arranged to operate in other orientations.

Some of the components and features in the drawings have been given numerals with letter suffixes, which indicate front, back, etc, versions of the components. The numeral without the suffix is used herein to indicate the component generically.

Terms of orientation, such as “above”, down”, “left”, and the like, when used herein are intended to be construed as follows. When the terms are applied to an apparatus, that apparatus is distinguished by the terms of orientation only if there is not one orientation into which the apparatus, or an image of the apparatus, could be placed, in which the terms could be applied consistently.

The scope of the patent protection sought herein is defined by the accompanying claims. The apparatuses and procedures shown in the accompanying drawings and described herein are examples.

The numerals used in the drawings are:

  • 20 condensing unit
  • 23F,B front and back corrugated sheets
  • 25 spacer-strips
  • 27 fasteners
  • 28 top and bottom channel-strips
  • 29L,R left and right masses of filler-sealant
  • 30 extent of sealant masses
  • 32 upper port=steam-inlet port-pipe
  • 34 lower port=water-outlet port-pipe
  • 35 sealed interior space of 20
  • 36 heat exchanger (HE) core
  • 38 sinuous passageways between condensing units 20
  • 40L,R left and right steam-inlet manifolds
  • 41 steam manifold inlet-pipe
  • 42 pipe-holes
  • 43L,R left and right water-outlet manifolds
  • 44 seals for 42
  • 45 cover
  • 47 floor or platform of 49
  • 49 chamber
  • 50 bracket (brace) for 45
  • 52 spray-heads
  • 56 dirty water entry-port
  • 58 water pump
  • 60 pool of water in 49
  • 63 steam-blower
  • 65 concentrate-discharge port
  • 66 manifold condensate outlet-pipe
  • 67 condensate-discharge port
  • 69 shortest path-line through 38