Heat Transfer And Flow Characteristics And Temperature Control Of Instantaneous Water Heater

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.