JPH09166390A - Equipment for recovering exhaust heat - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep the surface temperature of a heat transfer tube above a sulfuric acid dew point even when the tube is used in a low temperature range wherein the temperature of exhaust gas is a specific temperature or below. SOLUTION: A U-tube type heat exchanger having a large number of U-shaped heat transfer tubes 2 provided inside a can body 1 is divided into a plurality of stages, and air is preheated to a temperature of 70-100 deg.C by a U-tube type heat exchanger 6 provided on the upstream side of exhaust gas and then is led to the heat transfer tubes of a U-tube type heat exchanger 7 on the downstream side of exhaust gas. According to this constitution, a bent part 5 of the heat transfer tube 2 of the U-tube type heat exchanger 7 on the downstream side of exhaust gas is prevented from being at a low temperature of a sulfuric acid dew point or below.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an exhaust heat recovery apparatus used for recovering exhaust heat from exhaust gas from a waste incinerator or the like.

[0002]

2. Description of the Related Art As a heat exchanger for recovering exhaust heat from exhaust gas from a waste incinerator or the like, a U-tube heat exchanger shown in Japanese Utility Model Publication No. 62-1578 is known. As shown in FIG. 3, this U-tube heat exchanger is provided with a large number of U-shaped heat transfer tubes 2 inside a can body 1 through which exhaust gas flows. The end of each heat transfer tube 2 on the upstream side of the exhaust gas is the can body 1.
It is connected to the air manifolds 3 and 4 provided on the outer periphery of and has a structure in which air as a fluid to be heated is heated by exhaust gas while passing through the U-shaped heat transfer tube 2.

In this U-tube type heat exchanger, since the heat transfer tube 2 is U-shaped and only its base is fixed to the can body 1,
It has the advantage that it can freely expand thermally when it comes into contact with high-temperature exhaust gas, and there is no risk of damage due to thermal stress. Further, the heat transfer tube 2 should be arranged in parallel with the flow of the exhaust gas,
In addition, since a sufficient gas flow passage area is secured, there is an advantage that ash in the exhaust gas is less likely to be clogged due to accumulation in the heat transfer tube 2, and it is suitable for waste heat recovery of a waste incinerator. is there.

However, when this U-tube type heat exchanger is used in a low temperature range where the exhaust gas temperature is 500 ° C. or less, the surface temperature near the bent portion 5 of the metal heat transfer tube 2 is the acid dew point (about
It became less than 150 ° C) and it was found that there is a risk of corrosion. That is, as shown in FIG. 3, the bent portion 5 of the heat transfer tube 2
Is located on the most downstream side of the exhaust gas, the ambient exhaust gas temperature becomes low. For example, when the exhaust gas inlet temperature is 300 ° C and the outlet temperature is 200 ° C, the ambient temperature of the bent portion 5 of the heat transfer tube 2 is 200 ° C. It becomes about ℃. Moreover, since the air flowing inside the heat transfer tube 2 is not sufficiently heated at this position, the heat transfer tube 2 is cooled from the inside, and the surface temperature near the bent portion 5 of the heat transfer tube 2 becomes about 130 ° C. Of. Even if the direction of the air flow inside the heat transfer tube 2 is reversed as shown in FIG. 4, the surface temperature near the bent portion 5 of the heat transfer tube 2 is 120.
Only about ℃.

As described above, the surface temperature near the bent portion 5 of the heat transfer tube 2 is as low as 130 ° C. or lower, and is lower than the acid dew point (about 150 ° C.) at which the acid component in the exhaust gas is condensed, so that it is contained in the exhaust gas. It will be corroded by acids such as sulfuric acid. Therefore, it has been a problem that the conventional U-tube heat exchanger is used in a low temperature range where the exhaust gas temperature is 500 ° C or lower.

[0006]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned conventional problems, and even when exhaust heat recovery from exhaust gas in a low temperature range of 500 ° C. or less is performed, the bent portion of the heat transfer tube is corroded by acid. This is done in order to provide an exhaust heat recovery device that is free from the risk of receiving heat.

[0007]

The exhaust heat recovery apparatus of the present invention made to solve the above problems is a U-tube type in which a large number of U-shaped heat transfer tubes are provided inside a can body through which exhaust gas flows. The heat exchanger is divided into a plurality of stages, preferably two stages, and the U-tube heat exchanger provided upstream of the exhaust gas preheats the air to be heated, and the U-tube heat exchanger downstream of the exhaust gas. It is characterized in that it is guided to a heat transfer tube of and heated to a predetermined temperature. In addition, it is preferable to heat the air to 70 to 100 ° C. by a U-tube type heat exchanger provided on the upstream side.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described below. FIG. 1 is a cross-sectional view showing a first embodiment of the present invention, in which a conventional U-tube heat exchanger includes a U-tube heat exchanger 6 on the exhaust gas upstream side and a U-tube heat exchanger on the exhaust gas downstream side. It is divided into two parts. In this case, the sum of the lengths of the U-tube heat exchangers 6 and 7 divided into two is substantially equal to the length of a conventional U-tube heat exchanger having the same capacity. The U-tube heat exchangers 6 and 7 divided into these two may be arranged separately, but are preferably arranged adjacent to each other. Also U
The tube heat exchanger may be divided into two stages, or may be divided into a plurality of stages of three or more.

Each U-tube type heat exchanger 6, 7 is provided with a large number of U-shaped heat transfer tubes 2 inside the can body 1 through which exhaust gas flows, as in the conventional case. Further, the end of the U-shaped heat transfer tube 2 on the upstream side of the exhaust gas is connected to the air manifolds 3 and 4, and air as a fluid to be heated flows inside the heat transfer tube 2. In the example of FIG. 1, the air that has flowed through the heat transfer tube 2 of the U-tube heat exchanger 6 on the upstream side of the exhaust gas and has been preheated is guided to the U-tube heat exchanger 7 on the downstream side of the exhaust gas.

By introducing the preheated air to the U-tube type heat exchanger 7 on the downstream side of the exhaust gas, the surface temperature near the bent portion 5 of the heat transfer tube 2 where the exhaust gas temperature becomes the lowest becomes higher than the acid dew point. Can be maintained. Further, since the exhaust gas temperature is high in the U-tube heat exchanger 6 on the upstream side of the exhaust gas,
Even if the temperature of the air flowing inside the heat transfer tube 2 is low, the heat transfer tube 2
There is no possibility that the surface temperature in the vicinity of the bent portion 5 becomes lower than the acid dew point. In addition, it is possible to set the temperature of the recovered air high to prevent the acid dew point and eliminate the need to install an air preheater.

The preheating temperature of the air by the U-tube type heat exchanger 6 on the upstream side of the exhaust gas is preferably about 70-100 ° C. This is because if the preheating temperature is lower than 70 ° C., the surface temperature near the bent portion 5 of the heat transfer tube 2 of the U-tube heat exchanger 7 on the downstream side of the exhaust gas may be below the acid dew point. On the contrary, if the preheating temperature is set to 100 ° C. or higher, the exhaust gas temperature in the U-tube heat exchanger 6 on the upstream side of the exhaust gas greatly decreases, and the U-tube heat exchanger 7 on the downstream side of the exhaust gas becomes
This is because the temperature of the exhaust gas becomes low and the temperature difference between the air and the gas becomes excessively large, and the surface temperature near the bent portion 5 of the heat transfer tube 2 may be lower than the acid dew point.

FIG. 2 is a sectional view showing a second embodiment of the present invention. In this example, the air preheated to 70 to 100 ° C by the U-tube type heat exchanger 6 on the upstream side of the exhaust gas is pipe 8
Is guided to the lower end of the U-tube heat exchanger 6 on the upstream side of the exhaust gas. Therefore, the preheated air flows through the inside of the air manifold 10 to the upstream side, is heated by the exhaust gas during this time, enters the heat transfer tube 2, is further heated by the exhaust gas, and is finally taken out from the air manifold 9. With the structure of FIG. 2 as described above, the surface temperature near the bent portion 5 of the heat transfer tube 2 is lower than that of the structure of FIG. 1, but there is no fear of becoming lower than the acid dew point. Therefore, the structure of FIGS. 1 and 2 can be selected according to the case.

[0013]

Next, examples of the present invention will be described. Exhaust gas flow rate is 50
00m ³ / h, exhaust gas inlet temperature 300 ℃, outlet temperature 200
The conventional exhaust heat recovery device having the structure shown in FIG. 4 and the exhaust heat recovery device of the present invention having the structure shown in FIG. 1 were manufactured under the conditions of ℃, air inlet temperature of 20 ° C., and outlet temperature of 200 ° C. . In order to satisfy this condition, the conventional exhaust heat recovery device had a diameter of 1.8 m and a total length of 4 m, and the surface temperature near the bent portion of the heat transfer tube was 120 ° C., which was below the acid dew point.

On the other hand, the exhaust heat recovery apparatus of the present invention shown in FIG. 1 has a diameter of 1.8 m and a total length of 5 m, which is longer than before, but the surface temperature near the bent portion of the heat transfer tube is 170.
The temperature became ° C, which was higher than the acid dew point. Therefore, the exhaust heat recovery system of the present invention has no risk of corrosion due to acid in the exhaust gas.

[0015]

As described above, in the exhaust heat recovery apparatus of the present invention, the U-tube heat exchanger is divided into two stages, and the air preheated by the U-tube heat exchanger provided on the upstream side of the exhaust gas is used. Since it has a structure that guides the heat transfer tube to the heat transfer tube of the U-tube heat exchanger on the downstream side of the exhaust gas, the surface temperature of the heat transfer tube can be kept above the acid dew point even when the exhaust gas temperature is used in the low temperature range of 500 ° C or less. it can. Therefore, there is an advantage that acid corrosion of the conventional heat transfer tube can be prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of the present invention.

FIG. 2 is a sectional view showing a second embodiment of the present invention.

FIG. 3 is a sectional view showing a conventional example.

FIG. 4 is a sectional view showing another conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 can body, 2 heat transfer tube, 3 air manifold, 4 air manifold, 5 bent part of heat transfer tube, 6 U-tube type heat exchanger on exhaust gas upstream side, 7 U-tube type heat exchanger on exhaust gas downstream side, 8 pipe, 9 air manifolds, 10
Air manifold

Claims (3)

[Claims] 1. A U-tube heat exchanger in which a large number of U-shaped heat transfer tubes are provided inside a can body through which exhaust gas flows, is divided into a plurality of stages, and the U-tube heat exchanger is provided upstream of the exhaust gas. An exhaust heat recovery apparatus which preheats air, which is a fluid to be heated, by the above, and guides it to a heat transfer tube of a U-tube heat exchanger on the downstream side of exhaust gas to heat it to a predetermined temperature. 2. The exhaust heat recovery device according to claim 1, wherein the U-tube heat exchanger is divided into two stages, an exhaust gas upstream side and an exhaust gas downstream side. 3. The exhaust heat recovery device according to claim 2, wherein the air is heated to 70 to 100 ° C. by a U-tube type heat exchanger provided on the upstream side.