【作者】 张艳乔；

【导师】 韩吉田；

【作者基本信息】 山东大学 ， 制冷及低温工程， 2008， 硕士

【摘要】 快速式换热器能够对由于流体流量的变化导致的流体温度变化进行快速调节而保持在设定的温度范围内。与普通换热器不同,快速式换热器不是单一的换热器,而是一个由基于前馈控制原理的温度控制阀和热交换器所组成的换热系统。由于快速式换热器具有体积小、重量轻、安装方便、占地面积小、无需常用的感温装置、反应速度快、节约能源等优点,广泛应用于石化、采暖、制冷空调、洗浴、生产及生活热水供应等系统中,是宾馆饭店、办公楼、小区、企业等部门常用的换热设备之一。快速式换热器的核心部件—温度调节混合阀是采用前馈式控制原理对温度进行控制的。它使用一种带有压差感应装置的混合阀替代传统的温度控制阀和温度传感器来实现温度的自动控制。在使用过程中通过感应混合阀出口的压力来调节冷、热水的流量,通过调节冷、热水的混合比对出口水的温度进行调节,无温度响应的时间延迟。因而能够快速的满足温度调节的要求,并具有较高的温度调节精度。本文的主要目的是对快速式换热器的传热和流动特性进行研究,并探讨螺旋管换热器的曲率比、倾斜度等参数对换热器传热和流动特性的影响。采用传热有效度一传热单元数(ε—NTU)的方法建立了快速式换热器的传热和流动数学模型,通过MATLAB的SIMULINK建立了相应的仿真模型,对传热和流动模型进行模拟分析,得到了换热器的出口温度和压降随流量的变化关系以及热水温度和压降随流量及进入混合阀的冷、热水混合比的变化关系,并进一步分析了快速式换热器的传热特性。快速式换热器模拟分析的结果表明,在水需求负荷保持一定和发生变化时,换热器出口混合水的温度随着进入换热器的冷水流量及进入混合阀的冷、热水的流量和混合比之间具有确定的函数变化关系。对换热器流动特性的分析表明,混合阀进出口的压降与进入换热器的冷水流量之间具有确定的函数变化关系。因此,得到了换热器出口混合水的温度与质量流量和混合阀进出口压降之间的函数关系。当水的需求量(负荷)发生变化或进入换热器的水流量变化而使快速式换热器的混合水温偏离设定值时,可以通过混合阀进出口的压降变化对流量进行自动调节,从而实现对混合水温度的快速自动调节。研究结果表明,基于前馈原理的换热器温度控制技术可以实现流体混合温度的快速自动调节。本文为快速式换热器的传热和流动特性及温度控制技术提供了一种比较实用有效的分析方法,为快速式换热器的优化设计分析与安全经济运行提供了基本数据和分析依据。更多还原

【Abstract】 Instantaneous steam-water is a completely engineered system designed to provide accurately-controlled temperatures for hot water applications. Instantaneous steam-waters provide instant hot-water supplies,always at the preset temperature ranges.The heat exchanger transfers thermal energy from steam of low pressures and delivers hot water with accurate temperatures. Different from the ordinary water heaters,the instantaneous steam-water is actually a heat-exchanging system composed of a compact heat exchanger and a temperature regulating device of feed-forward control mechanism instead of a single heat exchanger.It has been widely employed in a variety of applications, such as petroleum and chemical engineering,food processing,heating, refrigeration and air-conditioning,hot water supplies for domestic and official buildings,and many others.The heart of the instantaneous water heater is a unique blending device that automatically proportions hot and cold water to produce the hot water of desired outlet temperature.The temperature blending valve based on feed-forward control mechanism is actuated by the demand for hot water.An impulse line to the top of the blender’s differential sensing head comes from a connection to the cold water supply line.Blended water is sensed under the diaphragm through ports in the blending valve.The feed-forward blending device ensures that the hot and cold water blend ratio is accurately proportioned to maintain the set temperature for all flow demands.Water is blended instantly automatically,with virtually no lag in response time to demand,and increased efficiency is obtained by blending the water instantly.The heat exchanger, blending valve and intermediate piping act to produce an engineered pressure drop so that each increment in flow represents a specific water pressure differential.The main objective of this paper is to study the heat transfer and flow characteristics of the instantaneous steam-water and the effect of curvature ratios and other relevant parameters on the heat transfer and flow characteristics.A mathematical model is developed for the heat transfer and flow characteristics of the instantaneous steam-water by using theε-NTU method. The effect of blending ratio of cold and hot water and the cold water mass flow rates of the heat exchanger on the heat transfer and flow characteristics are investigated in detail.By numerical simulating with the SIMULINK in MATLAB,the variation of the final blended hot water temperature of blending valve is determined with different water flow and the blending ratio of cold and hot water when the blended hot water demand is changed or maintained at a fixed level.The variation of the outlet pressure drop of the blending valve is can be also obtained with the blending ratio of cold and hot water.As a result, the empirical correlation of temperature,mass flow rate,pressure drop can be further developed.When the final blended hot water or the incoming cold water into the heat exchanger is changed,the water flow is adjusted according to the pressure drop across the bleeding valve to regulate the blended hot water temperature to the desired temperature range.The developed simulating method obtained results can be used for the investigation of heat transfer and flow characteristics of instantaneous water heaters and the optimal design and safe and effective operations.更多还原