JP2016121811A - Heat exchanger and water heater with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat exchanger capable of eliminating disadvantage that a heat recovery amount by heat transfer pipes near an exhaust port is reduced even if a total number of heat transfer pipes stored in a case is many.SOLUTION: A heat exchanger HE1 has the first and second air supply ports 21A, 21B acting as air supply ports for heater gas arranged at a case 2 storing a plurality of heat transfer pipes 4. The first air supply port 21A is arranged at the opposite position to an exhaust port 22 while holding the plurality of heat transfer pipes 4, the heater gas supplied from the first air supply port 21A into the case 2 can reach to the exhaust port 22 after passing through a substantial entire arrangement region of a plurality of heat transfer pipes 4. The second air supply port 21B is arranged near the exhaust port 22 rather than the first air supply port 21A, the heater gas supplied from the second air supply port 21B into the case 2 can reach the exhaust port 22 after passing a certain arrangement region of a plurality of heat transfer pipes 4.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a heat exchanger of a type that performs heat recovery from a heating gas such as combustion gas using a heat transfer tube, and a hot water device such as a hot water supply device equipped with the heat exchanger.

The present applicant has previously proposed, for example, one described in Patent Document 1 as a specific example of a heat exchanger for a hot water apparatus. The heat exchanger described in the document has a configuration in which a plurality of heat transfer tubes are arranged in a case, and a heating gas supply port and an exhaust port are provided on the rear wall portion and the front wall portion of the case. Is provided.
According to such a configuration, heat exchange is performed between the heating gas and each heat transfer tube in the process in which the heating gas flowing into the case from the air supply port advances toward the exhaust port, The hot and cold water that circulates is efficiently heated.

  However, the prior art still has room for improvement as described below.

That is, when the total number of heat transfer tubes is increased in order to increase the hot water supply capacity of the hot water device, among many heat transfer tubes, a high-temperature heating gas can act on the heat transfer tubes close to the air supply port, It is difficult to cause a high-temperature heating gas to act on the heat transfer tube close to the exhaust port. This is because the temperature of the heating gas that has flowed into the case from the air supply port decreases in the process of sequentially acting on many heat transfer tubes and moving forward. For this reason, although many heat transfer tubes are provided in the case, the amount of heat recovered by the heat transfer tubes closer to the exhaust port becomes smaller. For this reason, conventionally, there is still room for improvement in increasing the heat recovery amount of the entire heat exchanger and sufficiently increasing the heat exchange efficiency.
As a means for increasing the amount of heat recovered by the heat transfer tube near the exhaust port, a constriction part for restricting the flow path of the heating gas is provided in the case, and the heating gas acts on the heat transfer tube near the exhaust port. It is conceivable to increase the flow velocity at that time. However, even if such a means is adopted, it is difficult to sufficiently increase the heat recovery amount because the heating gas acts on the heat transfer tube near the exhaust port while the temperature of the heating gas is lowered. In addition, there is a disadvantage that exhaust resistance increases.

JP 2007-333343 A

  The present invention has been conceived under the circumstances as described above, and even when the total number of heat transfer tubes accommodated in the case is increased in order to increase the amount of heat recovered from the heating gas. An object of the present invention is to provide a heat exchanger capable of eliminating the disadvantage that the amount of heat recovered by the heat transfer tube near the exhaust port is reduced and obtaining high heat exchange efficiency, and a hot water device equipped with the heat exchanger. .

  In order to solve the above problems, the present invention takes the following technical means.

The heat exchanger provided by the first aspect of the present invention includes a case in which an air supply port and an exhaust port for a heating gas are formed, and a plurality of heat transfer tubes accommodated in the case. The heat exchanger is provided with first and second air supply ports as the air supply ports, and the first air supply ports are arranged on the exhaust port with the plurality of heat transfer tubes interposed therebetween. And the heating gas supplied into the case from the first air supply port passes through the substantially entire arrangement region of the plurality of heat transfer tubes and then enters the exhaust port. The second air supply port is provided at a position closer to the exhaust port than the first air supply port, and is used for heating supplied into the case from the second air supply port. The gas is allowed to reach the exhaust port after passing through a partial arrangement region of the plurality of heat transfer tubes. It is set to.

According to such a configuration, the following effects can be obtained.
That is, the first air supply port corresponds to the air supply port provided in the case in the prior art, and the heating gas supplied from the first air supply port into the case is directed toward the exhaust port. In the process of moving forward, it acts on almost all of the plurality of heat transfer tubes sequentially. For this reason, originally, when the total number of the plurality of heat transfer tubes is large, the temperature of the heating gas in the portion close to the exhaust port is low. On the other hand, according to the present invention, the heating gas can be supplied from the second air supply port into the case at a position closer to the exhaust port than the first air supply port. A high-temperature or relatively high-temperature heating gas can be applied to the heat transfer tube near the exhaust port. As a result, compared to the conventional technology in which only the low-temperature heating gas is acting on the heat transfer tube near the exhaust port, the amount of heat recovery in that part can be increased and the heat exchange efficiency of the entire heat exchanger can be improved. it can.
Thus, if the heat exchange efficiency can be increased, the total number of heat transfer tubes can be reduced as compared with the prior art, and the entire heat exchanger can be reduced in size and the manufacturing cost can be reduced.

  In the present invention, it is preferable that an opening area of the second air supply port is equal to or smaller than an opening area of the first air supply port.

  According to such a configuration, it is solved that the amount of the heating gas flowing into the case from the first air supply port becomes insufficient, and the heating gas is efficiently supplied to the entirety of the plurality of heat transfer tubes. In terms of action, it is more preferable.

  In the present invention, it is preferable that the plurality of heat transfer tubes constitute first and second heat exchanging sections in which hot water flows in and out individually by being connected to a header for flowing in and out of hot water. These first and second heat exchanging portions are provided in an arrangement lined up in the flow direction of the heating gas from the first air supply port to the exhaust port.

  According to such a configuration, the hot gas efficiently acts on both of the first and second heat exchange units while flowing hot water individually to each of the first and second heat exchange units. And hot water heating can be performed efficiently.

  In the present invention, preferably, the second air supply port is provided so as to be able to supply a heating gas toward a gap region between the first and second heat exchange portions.

  According to such a configuration, the heating gas that has flowed into the case from the second air supply port can be appropriately applied to the entire or substantially entire second heat exchange section.

  In the present invention, preferably, each of the first and second heat exchange units has a configuration in which a plurality of spiral heat transfer tubes having different sizes are arranged in a substantially concentric wrapping manner.

According to such a structure, while suppressing the enlargement of the size (winding diameter or winding width) of the helical heat transfer tube constituting each of the first and second heat exchange units, the entire heat transfer tubes The heat transfer area can be increased and the overall manufacturing cost can be reduced. As a means for increasing the entire heat transfer area of the plurality of heat transfer tubes, for example, it is conceivable to increase the number of lap windings of the plurality of spiral heat transfer tubes as described in Patent Document 1, for example. When such a means is employed, the size (winding diameter or winding width) of the spiral heat transfer tube is considerably increased, and the manufacturing cost is increased. The manufacturing cost of the spiral heat transfer tube increases as the size increases. On the other hand, according to the said structure, it becomes a preferable thing to suppress the enlargement of a helical heat exchanger tube and to suppress manufacturing cost. Note that the helical heat transfer tube is advantageous in increasing the heat transfer area of the entire heat transfer tube while reducing the number of heat transfer tubes, for example, as compared with a straight heat transfer tube.

  The hot water device provided by the second aspect of the present invention is a primary heat exchanger for recovering sensible heat from a heating gas to perform hot water heating, and for heating after passing through the primary heat exchanger. A hot water apparatus comprising a secondary heat exchanger for recovering latent heat from a gas and performing hot water heating, provided as the secondary heat exchanger according to the first aspect of the present invention. A heat exchanger is used, and the heating gas after passing through the primary heat exchanger is configured to flow into the first and second air supply ports of the secondary heat exchanger. It is characterized by being.

  According to such a configuration, the same effect as described for the heat exchanger provided by the first aspect of the present invention can be obtained, the amount of latent heat recovered by the secondary heat exchanger can be increased, and the entire hot water apparatus It is possible to increase the heat exchange efficiency.

  Other features and advantages of the present invention will become more apparent from the following description of embodiments of the present invention with reference to the accompanying drawings.

It is a schematic sectional drawing which shows an example of the hot water apparatus which concerns on this invention. (A) is a plane sectional view of the secondary heat exchanger used for the hot water apparatus shown in Drawing 1, and (b) is a IIb-IIb sectional view of (a). It is a principal part schematic sectional drawing which shows the other example of the hot water apparatus which concerns on this invention. (A) is side sectional drawing which shows the other example of the heat exchanger which concerns on this invention, (b) is IVb-IVb sectional drawing of (a). It is sectional drawing which shows the other example of the heat exchanger which concerns on this invention. (A) is a plane sectional view showing other examples of the heat exchanger concerning the present invention. It is side surface sectional drawing which shows the other example of the heat exchanger which concerns on this invention.

  Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the drawings.

The hot water device WH1 shown in FIG. 1 is configured as a hot water supply device, and the overall basic configuration is the same as that of the hot water supply device described in Patent Document 1. Therefore, the overall basic configuration will be briefly described.
The hot water device WH1 includes a fan 31, a combustor 3, a primary heat exchanger 1, and a secondary heat exchanger HE1.
The secondary heat exchanger HE1 corresponds to an example of a heat exchanger to which the present invention is applied. The primary heat exchanger 1 does not correspond to a heat exchanger to which the present invention is applied.

  The combustor 3 is a gas burner, for example, and is disposed in the burner case 30. Combustion air is supplied upward from the fan 31 into the burner case 30. The primary heat exchanger 1 is for recovering sensible heat from the combustion gas generated by the combustor 3. For example, the heat exchanger tube 11 having a plurality of fins 12 is arranged in the can body 13. is there.

The secondary heat exchanger HE1 is for recovering latent heat from the combustion gas whose sensible heat has been recovered by the primary heat exchanger 1, and is disposed on the can body 13 via the auxiliary can body 5 (collecting cylinder). It is installed. The secondary heat exchanger HE1 includes a case 2 and a plurality of heat transfer tubes 4 arranged in the case 2. The plurality of heat transfer tubes 4 constitute first and second heat exchange portions H1 and H2 arranged in the front-rear width direction of the case 2 with a gap 29 therebetween.

  More specifically, as clearly shown in FIG. 2, each heat transfer tube 4 is a spiral heat transfer tube formed in a substantially rectangular shape or a substantially oval shape in plan view, and the first and second Each of the heat exchanging portions H1 and H2 has a configuration in which a plurality of spiral heat transfer tubes 4 of different sizes are arranged in a substantially concentric lap shape (spiral heat transfer tube lap winding structure portion). The plurality of spiral heat transfer tubes 4 have a horizontally long direction in which the width in the left-right horizontal width direction of the case 2 is longer than the width in the front-rear direction, and both end portions thereof penetrate the side wall portion 20 d of the case 2. And connected to the header 6A, 6B for inflow and outflow of hot water. A pair of headers 6A and 6B are individually connected to the first and second heat exchanging parts H1 and H2, and the inflow and outflow of hot water to the first and second heat exchanging parts H1 and H2 are as follows: It can be done individually. However, hot water flowing through the hot water supply pipe 90 is branched and supplied to the two hot water inflow headers 6A, and the first and second heat exchanging portions H1 and H2 are not heated. Hot water of the same temperature flows in. On the other hand, the hot water discharged from each of the two hot water outflow headers 6B merges into the hot water discharge pipe 91, and is supplied to a desired hot water supply destination in a state where the hot water is mixed.

The case 2 has a substantially rectangular parallelepiped shape, and first and second air supply ports 21A and 21B are provided in the bottom wall portion 20c. As shown in FIG. 1, the auxiliary can 5 is provided with two openings 50a and 50b that allow passage of combustion gas to the first and second air supply ports 21A and 21B. An exhaust port 22 is provided in the front wall portion 20 a of the case 2.
The first air supply port 21A is provided on the rear side of the case 2 (the right side in FIG. 1 and the upper side in FIG. 2A) with respect to the first and second heat exchange portions H1 and H2. The combustion gas that has flowed into the case 2 from the first air supply port 21A passes through both the first and second heat exchange portions H1 and H2 and reaches the exhaust port 22. On the other hand, the second air supply port 21B opens toward the gap 29 between the first and second heat exchange portions H1 and H2, and the case 2 is connected to the second air supply port 21B. The combustion gas that has flowed into the exhaust gas passes through the second heat exchange part H2 and reaches the exhaust port 22. The 1st heat exchange part H1 does not pass. The opening areas A1 and A2 of the first and second air supply ports 21A and 21B have a relationship of A1 ≧ A2. Preferably, the width La of the gap 29 and the width Lb of the second air supply port 21B have a relationship of La ≧ Lb.

  Next, the effect | action of above described hot water apparatus WH1 is demonstrated.

  First, the combustion gas generated by the combustor 3 passes through the primary heat exchanger 1 and then enters the case 2 of the secondary heat exchanger HE1 from both the first and second air supply ports 21A and 21B. Inflow. The combustion gas that has flowed into the case 2 from the first air supply port 21A sequentially passes through the first and second heat exchanging units H1 and H2, as described above. For this reason, the combustion gas in a relatively high temperature acts on the first heat exchanging part H1, and the helical heat transfer tube 4 (circulating in the helical heat transfer pipe 4) constituting the first heat exchanging part H1. The temperature difference between the hot water and the combustion gas can be increased. Here, the amount of heat transferred between the two materials is proportional to the temperature difference between them. Therefore, it is possible to increase the amount of heat recovered from the combustion gas by the first heat exchange unit H1.

On the other hand, the combustion gas flowing into the case 2 from the second air supply port 21B immediately acts on the second heat exchange part H2. At this time, the combustion gas acts on the second air supply port 21B in a state of being mixed with other combustion gas that has passed through the first heat exchange section H1 from the first air supply port 21A, The combustion gas that has flowed into the case 2 from the air supply port 21B is a relatively high-temperature combustion gas that has not yet been subjected to latent heat recovery. For this reason, as with the first heat exchange unit H1, a relatively high-temperature combustion gas is applied to the second heat exchange unit H2, and the amount of heat recovered by the second heat exchange unit H2 is increased. A lot can be done. Unlike this embodiment, when the combustion gas is caused to flow into the case 2 only from the first air supply port 21A, when the combustion gas reaches the second heat exchange section H2, the temperature is increased. Significantly decreases, and the amount of heat recovered in the second heat exchanging portion H2 may be considerably reduced. On the other hand, according to this embodiment, it is possible to appropriately eliminate such a concern and increase the overall heat exchange efficiency of the secondary heat exchanger HE1.

  Since the opening area A2 of the second air supply port 21B is set to be equal to or smaller than the opening area A1 of the first air supply port 21A, an excessive amount of combustion gas is introduced into the case 2 from the second air supply port 21B. Inflow can be prevented. This prevents the flow rate of the combustion gas from the first air supply port 21A to the first heat exchanging portion H1 from becoming insufficient, and the first and second heat exchanging portions H1 and H2 respectively. It is preferable for the combustion gas to act at an appropriate flow rate and temperature. Further, since the second air supply port 21B is provided so as to allow the combustion gas to flow toward the gap 29 formed between the first and second heat exchange portions H1 and H2, It is possible to prevent the combustion gas immediately after passing through the second air supply port 21 </ b> B from colliding with the spiral heat transfer tube 4 and not proceeding further upward on the case 2. This is preferable in that the relatively high-temperature combustion gas that has flowed into the case 2 from the second air supply port 21B acts efficiently on a wide region of the second heat exchange unit H2.

  As for the secondary heat exchanger HE1, as a means for increasing the total heat transfer area of the plurality of spiral heat transfer tubes 4 and increasing the amount of heat recovery, the two spiral heat transfer tube overlapping winding structures described above are used. Adopting a means of arranging. Here, the actual condition is that the manufacturing cost of the spiral heat transfer tube 4 increases as the size thereof increases. On the other hand, according to this embodiment, the increase in the size of each helical heat transfer tube 4 is suppressed by using two spiral heat transfer tube overlapping winding structures. Therefore, it is possible to reduce the manufacturing cost. Further, according to the present embodiment, as described above, since the heat exchange efficiency of the secondary heat exchanger HE1 can be increased, the number of the helical heat transfer tubes 4 is reduced by that amount, thereby producing There is also an advantage that the cost can be further reduced.

  3 to 7 show other embodiments of the present invention. In these drawings, elements that are the same as or similar to those of the above-described embodiment are denoted by the same reference numerals as those of the above-described embodiment, and redundant description is omitted.

  In the hot water device WH2 shown in FIG. 3, the first air supply port 21A is provided in the rear wall portion 20b of the case 2 of the secondary heat exchanger HE2, and the auxiliary can 5 is a primary heat exchanger. Part of the combustion gas that has passed through 1 can be guided to the first air supply port 21A. Even with such a configuration, it is possible to cause the combustion gas flowing into the case 2 from the first air supply port 21A to act on the first and second heat exchanging units H1 and H2 sequentially.

  In the heat exchanger HE3 shown in FIG. 4, a meandering heat transfer tube is used as the heat transfer tube 4A. Each of the first and second heat exchanging parts H1 and H2 includes a plurality of meandering heat transfer tubes 4A arranged at appropriate intervals in the front-rear width direction of the case 2, and both end portions of the plurality of meandering heat transfer tubes 4A. However, it is set as the structure connected with the headers 6C and 6D for hot water inflow and outflow. Although the structure of this embodiment changes the structure of the heat exchanger tube 4A (4) compared with the structure shown in FIGS. 1-3, the effect which this invention intends also in such a structure. It is possible to obtain.

In the heat exchanger HE4 shown in FIG. 5, a straight tubular heat transfer tube is used as the heat transfer tube 4B. Of the plurality of straight tubular heat transfer tubes 4B, both ends of some of the plurality of straight tubular heat transfer tubes 4B are connected to a hot water inflow header 6E and a hot water return header 6F, and other plural other Both ends of the straight tubular heat transfer tube 4B are connected to a header 6F for returning hot water and a header 6G for discharging hot water. In the present invention, as the heat transfer tube, a straight tubular heat transfer tube 4 as shown in FIG.
A configuration using B can also be adopted.
As understood from the above embodiment, the type of the heat transfer tube used in the heat exchanger of the present invention is not limited to the spiral heat transfer tube, but a meandering heat transfer tube, a straight tubular heat transfer tube, or these It is possible to use a heat transfer tube having a shape or structure other than the above.

In the heat exchanger HE5 shown in FIG. 6, a plurality of spiral heat transfer tubes 4 are accommodated in the case 2, but the plurality of spiral heat transfer tubes 4 constitute one heat transfer tube overlapping winding structure. However, unlike the configurations of FIGS. 1 and 2, the two heat transfer tube overlapping structures are not configured. In other words, the first and second heat exchange units referred to in the present invention are not configured in the present embodiment. However, the first air supply port 21A is provided on the rear side of the case 2 with respect to the plurality of spiral heat transfer tubes 4, and the second air supply port 21B is on the inner side of the innermost spiral heat transfer tube 4. The bottom wall portion 20c is provided to face the space portion.
According to the present embodiment, a relatively high-temperature combustion gas that has passed through the first air supply port 21 </ b> A is applied to the rear region R of the case 2 among the plurality of spiral heat transfer tubes 4. In addition, a relatively high-temperature combustion gas that has passed through the second air supply port 21B can be applied to the front-side region F of the case 2. Therefore, the effect of increasing the amount of heat recovered by the entirety of the plurality of helical heat transfer tubes 4 can also be expected.

  In the heat exchanger HE6 shown in FIG. 7, as in the embodiment shown in FIG. 6, the plurality of heat transfer tubes 4 are provided without being divided into the first and second heat exchange portions H1, H2. As the heat transfer tube 4, for example, a meandering heat transfer tube as shown in FIG. 4 or a straight tubular heat transfer tube as shown in FIG. 5 is used. The bottom wall portion 20c of the case 2 is provided with a second air supply port 21B located closer to the exhaust port 22 than the first air supply port 21A. However, a part of the heat transfer tube 4 is located immediately above the second air supply port 21B, and a gap region having a width larger than the opening width L3 of the second air supply port 21B is provided. Absent.

  In the present embodiment, although there is a disadvantage that a part of the combustion gas that flows into the case 2 from the second air supply port 21B and rises collides with the heat transfer tube 4, among the plurality of heat transfer tubes 4, the second It is possible to cause the combustion gas having a relatively high temperature to act on the portion closer to the exhaust port 22 than the air supply port 21B, and to obtain the operation intended by the present invention. In the present invention, preferably, for example, the gap 29 shown in FIG. 1 is provided, and the second air supply port 21B is opposed to the gap 29. As understood, the second air supply port only needs to be provided closer to the exhaust port than the first air supply port, and the heat transfer tube exists in a region facing the second air supply port. It does not matter if it is said.

  The present invention is not limited to the contents of the above-described embodiment. The specific configuration of each part of the heat exchanger and the hot water apparatus according to the present invention can be variously modified within the range intended by the present invention.

The position where the exhaust port is provided is not limited to the front wall portion of the case. For example, it may be provided at a position near the front portion of the upper wall portion of the case. Further, the first and second air supply ports are not limited to the bottom wall portion and the rear wall portion of the case, and can be provided on other wall portions. Of course, it is also possible to provide the exhaust port on the rear side of the case and provide the first air supply port on the front side of the case. The number of the first and second air supply ports is not limited to one each. For example, only one first air supply port is provided, and the second air supply ports are distributed at a plurality of locations. It is also possible.
About a some heat exchanger tube, it is also possible to set it as the structure further provided with the 3rd heat exchange part in addition to the 1st and 2nd heat exchange part. The present invention is suitable for increasing the number of heat transfer tubes and configuring a heat exchanger with a large amount of heat recovery, but the specific number of heat transfer tubes is not limited. As described above, the specific shape of the heat transfer tube is not limited.

The heat exchanger referred to in the present invention is not limited to recovering latent heat, but can be used for recovering sensible heat.
The hot water device as used in the present invention means a device having a function of generating hot water, and is used for various types of hot water supply devices for general hot water supply, bath hot water supply, heating, snow melting, and the like, and hot water supply. It is a concept that includes a wide range of equipment for producing hot water. As the heating gas, for example, exhaust gas from a gas engine for cogeneration or a fuel cell can be used other than the combustion gas.

WH1, WH2 Water heaters HE1-HE6 Heat exchangers H1, H2 First and second heat exchange sections 1 Primary heat exchanger 2 Case 21A, 21B First and second air supply ports 22 Exhaust port 29 Clearance 4 Spiral Heat transfer tube (heat transfer tube)
4A Serpentine heat transfer tube (heat transfer tube)
4B Straight tubular heat transfer tube (heat transfer tube)

Claims (6)

A case where a heating gas supply port and an exhaust port are formed; and
A plurality of heat transfer tubes housed in the case;
A heat exchanger comprising:
As the air supply port, first and second air supply ports are provided,
The first air supply port is provided at a position opposite to the exhaust port across the plurality of heat transfer tubes, and the heating gas supplied into the case from the first air supply port is: The exhaust port can be reached after passing through a substantially entire arrangement region of the plurality of heat transfer tubes,
The second air supply port is provided at a position closer to the exhaust port than the first air supply port, and the plurality of heating gases supplied from the second air supply port into the case The heat exchanger is configured to be able to reach the exhaust port after passing through a partial arrangement region of the heat transfer tube. The heat exchanger according to claim 1,
The heat exchanger is configured such that an opening area of the second air supply port is equal to or less than an opening area of the first air supply port. The heat exchanger according to claim 1 or 2,
The plurality of heat transfer tubes constitute first and second heat exchanging units in which inflow and out of hot water are individually performed by being connected to a header for inflow and out of hot water,
These first and second heat exchanging portions are heat exchangers provided in an arrangement lined up in the flow direction of the heating gas from the first air supply port to the exhaust port. The heat exchanger according to claim 3,
The second air supply port is a heat exchanger provided to be able to supply a heating gas toward a gap region between the first and second heat exchange units. The heat exchanger according to claim 3 or 4,
Each of the first and second heat exchange units is a heat exchanger in which a plurality of spiral heat transfer tubes having different sizes are arranged in a substantially concentric wrapping shape. A primary heat exchanger for recovering sensible heat from a heating gas and heating with hot water;
A secondary heat exchanger for recovering latent heat from the heating gas after passing through the primary heat exchanger and performing hot water heating;
A hot water device comprising:
The heat exchanger according to any one of claims 1 to 5 is used as the secondary heat exchanger,
The hot water apparatus is configured such that the heating gas after passing through the primary heat exchanger flows into the first and second air supply ports of the secondary heat exchanger. .

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