燃焼ガスからの潜熱回収交換器 高性能化

• Material Type: 博士論文
• Title: 燃焼ガスからの潜熱回収交換器 高性能化
• Author/Editor: 山下, 隼平
• Author Heading: 山下, 隼平
• Publication Date: 2014-03-26
• Publication Date (W3CDTF): 2014-03-26
• Alternative Title: High performance heat exchanger for latent heat recovery from flue gas
• Degree grantor/type: 横浜国立大学
• Date Granted: 2014-03-26
• Date Granted (W3CDTF): 2014-03-26
• Dissertation Number: 甲第413号
• Degree Type: 博士（工学）
• Conferring No. (Dissertation): 甲第413号
• Text Language Code: jpn
• NDLC: UT51
• Target Audience: 一般
• Note (General): ガス給湯器において，従来システムでは排ガスとして大気中に排出していた燃焼ガスには水蒸気が含まれており，その温度は200?C近い高温であるなど多くの熱量を保有している．近年のガス給湯器には，その排ガスの潜熱と顕熱を回収する熱交換器が搭載され，熱量回収効率が約15%向上している．しかし主熱交換器に加え，潜熱を回収する2次熱交換器の搭載により，機器の大型化や腐食性凝縮液の発生が問題となっており，よりコンパクトかつ凝縮液の高効率な排除方法が求められている． 本研究では，最も構造が簡単で汎用性の高いシェルチューブ型熱交換器の伝熱管を細径化することにより，伝熱性能の向上を図りコンパクト化の実現を図っている．すなわち，シェルチューブ型熱交換器においては，伝熱面積の拡大に有効な細管化が熱交換器のコンパクト化には効果的と考えられる．しかし従来方式のように，管内に水を流す場合には，細管化による水圧の増大，あるいは水抜きの困難さから寒冷条件において凍結の問題が発生するなど，コンパクト化には限界がある．ここでは，細管を用いるシェルチューブ型熱交換器における上記の課題を解決するため，従来と異なる，燃焼ガスを管内，シェル側を水とする新形式熱交換について検討を行っている．管内にガス流を与える方式にすることで，管内外の水抜きも容易に行うことが可能になり，凝縮液の効率的な排除も期待できる．このような熱交換器においては，管群のそれぞれの管の独立性が高いと考えられるため，その特性を模擬するには，単管の熱伝達特性を検討することが基本となる．すなわち，給湯器における条件では，冷却水の流量は充分に大きく，管壁温度は冷却液温度に近いことから，単管により熱交換器性能の予測が可能であると考えられる．また，流入ガスは温度が高く単相であるため，管群における流入量の偏りが小さいと考えられる．ガス給湯器の2次熱交換器に流入する実排ガスと，温度および湿度を調整した湿り空気の物性の差異は小さく，実排ガスの管内流れ熱伝達特性は，湿り空気を試料として用いることで模擬できると考えられる． 以上から，本研究は管内にガス，シェル側に水を流す方式のシェルチューブ型潜熱回収2次熱交換器の細管化による性能向上を目的としている．そのため単管実験において，燃焼ガスを模した湿り空気の熱伝達特性について実験的に検討し，その結果から熱交換器の性能について検討している．論文は全6章で構成され，各章の内容は以下の通りである． 第1章では研究の背景，従来の研究と本研究の目的を述べている． 第2章では実験装置および試験部の構成，構造，温度等の測定方法およびその精度について述べている．試験部において，熱交換後の湿り空気を分岐・合流，さらに外周を通すことで断熱性を向上させ，混合平均温度と同時に凝縮液量を測定して湿り空気の熱伝達特性を検討している．また試料に乾燥空気を用いて数値計算と実験値を比較することで，本実験系の熱交換における有効管長を決定している．また乾燥空気における混合平均温度の測定結果は数値計算の結果とよく対応しており，十分な測定精度が得られることを示している． 第3章では試料湿り空気における実験結果について述べている．当初，従来型の使用条件を基に，湿り空気の流入温度および流速を固定し，管内径1.0～5.0mmの範囲において管内径の影響を比較した．管内径の小さい場合は温度降下が急峻であり，有効管長に対する凝縮量の変化率が小さいことから，細管で伝熱性能が向上することを示している．次に湿り空気の流入温度を変化させることによる影響について検討し，温度範囲を広げても細管では伝熱性能の向上が確認された．また圧力損失の測定では，時間的変動と定常時の平均圧力損失について示している． 第4章では温度助走区間に注目し，温度助走区間が湿り空気の熱伝達に与える影響について検討している．流入時の状態を基に温度助走区間を決定し，有効管長と温度助走区間の比を用いた無次元整理により，管内径，流入条件によらず一定の温度降下割合および熱量の移動割合を表せることを明らかにしている．この変化割合から無次元整理式を提案し，実験結果との比較から，管内径または流入条件によらず，提案した無次元整理式が湿り空気の熱伝達特性を良く表すことを示している． 第5章では，本研究で提案する熱交換器の性能検討を行っている．従来の2次熱交換器の使用条件および出力値が同等な熱交換器について，無次元整理式を用いて必要な熱交換器体積を比較し，例えば管径を1mmとすることにより，基準となる5mmの場合と比べて約5％になるなど，管内流をガスとする本研究の方式が熱交換器のコンパクト化に極めて有効であることを示している．また圧力損失が熱交換器の性能に与える影響について検討し，本研究の条件における圧力損失は特に問題にならないことを示している． 第6章では各章の内容を総括し結論している．; In the conventional model of a household gas water heater, water is heated by using only the sensible heat of flue gases, after which the gases are discarded as exhaust gas. This combustive gas contains steam at approximately 200°C; therefore, it has a relatively large amount of heat. Recently, water heaters with heat exchangers heat exchanger that collect the latent heat of exhaust gas heat water in advance. As a result, the efficiency of heat utilization has improved by approximately 15%. However, installation of the 2nd heat exchanger has created problems in that the product size has increased and the condensate obstructs the heat exchange process. Therefore, it is necessary to produce a product that is more compact and where the condensate can be easily collected. The commonly-used secondary heat exchanger is the shell and tube type heat exchanger which the water flows in tubes and the gases flow in the shell. This type heat exchanger is simpler structure than others and has high flexibility. When making a heat exchanger compact, it is considered that using mini-tubes is effective for expansion of the heat transfer area. However, because the water flows into the tubes and the gases flow inside the shell in typical secondary heat exchanger, problems such as the hydraulic pressure, drainage and freezing may occur when using mini-tubes. Therefore, there is a limit when designing a compact heat exchanger of this type. In this study, a heat exchanger which flows exhaust gas in mini-tubes and cooling water in the shell is examined with the aim of developing a compact heat exchanger that has a large heat transfer area due to using narrow tubes. Drainage on the shell side can be easily achieved and efficient exclusion of condensation is expected by using the gas tube flow. In order to simulate the characteristics of the proposed heat exchanger, it is fundamental for investigating the heat transfer characteristics of the single tube. The performance of heat exchanger can be predict by characteristics of single tube because the wall temperatures of all tubes are similar each other and near the coolant temperature for the sake of large flow rate of coolant under the gas water heaters conditions. It is considered that the variation of inlet of gas flow rate of single tube is little since because the inlet gas is in single phase due to its higher temperature. Since the differences in density and composition ratio between flue gas and moist air are small, it is adequate that the moist air is used as a test gas instead of flue gas. In this study, the heat transfer in a shell & tube type heat exchanger in which the moist air flows inside tubes and the cooling water in the shell-side is investigated for further improvement of heat exchanger performance. In order to investigate the heat exchangerperformance, the heat transfer characteristics of single tube are clarified using improved test sections. The performance of heat exchanger with thin tubes is investigated by using characteristics of single tube. This thesis consists of 6 chapters: Chapter 1 describes the background to the study, previous studies, and the purpose of the present study. In chapter 2, the experimental system, the experimental methods, and the measurement precision are described. The temperature of moist air after heat exchanging with coolant is measured as the bulk mean temperature to bifurcate and converge the moist air. Furthermore, the condensation rate from moist air is also measured in parallel and the heat transfer characteristics is investigated by using these experimental results. Measurement precision is verified and the effective tube length for heat exchanging in this experimental system is decided by comparing measurement value and theoretical value by using dry air as sample because the test section used for this study has a non-cooling region for fixing the test section into the measurement apparatus. In chapter 3, the experimental results in moist air are described. First, the influence of inner diameter (di = 1.0 ~ 5.0 mm) is investigated by fixing the inlet temperature and velocity conditions which is based on the typical conditions of use for a secondary heat exchanger. The inlet gas velocity was the same for all di, the moist air temperature at outlet Tgo varied significantly for different di. TB varied significantly for smaller di and shorter L, which confirms that heat transfer is improved when using thinner tubes. Second, the influence of inlet temperature of humidified air is investigated and the heat transfer is also improved by thinner tube even if the range of inlet temperature is extended. Additionally, in the measurement pressure loss, the average and temporal variation of pressure loss are explained. In chapter 4, by focusing on thermal entrance region, it is explained about the effect which the thermal entrance region has on heat transfer of moist air. The thermal entrance region is estimated from the inlet condition of moist air, and it is understood that all measured data reveal very close trends in the variations of the non-dimensional temperature difference and specific enthalpy against the non-dimensional length that are both qualitatively and quantitatively independent of tube diameter or inlet conditions. The equations which proposed from the non-dimensional temperature difference and specific enthalpy against tube length correlate well with the experimental data for the temperature variations and enthalpy changes against effective tube length for all tube diameters of di = 1-5 mm. In chapter 5, it is explained about the heat exchanger performance which proposed inthis study. The heat exchanger which the same usage conditions and performance are the same as conventional secondary heat exchanger is investigated and compared by using non-dimensional equations which is shown in chapter 4. As a result, it is elucidated that a new type heat exchanger which gas flows in mini-tubes is remarkably effective to reduce the size of heat exchanger. And the effect that the pressure loss has on the heat exchanger performance is investigated. Additionally, the optimum configuration of high performance secondary heat exchanger for gas water heater is proposed from the relationship of heat transfer coefficient to pressure loss. Chapter 6 summarizes the results described in the all chapters.
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• Collection (particular): 国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文; https://dl.ndl.go.jp/collections/A00014
• Acquisition Basis: 博士論文（自動収集）
• Date Accepted (W3CDTF): 2016-08-04T09:59:51+09:00
• Date Created (W3CDTF): 2016-09-16
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• Periodical Title (URI): http://hdl.handle.net/10131/8556; https://ynu.repo.nii.ac.jp/records/4666
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