Prevention of Pressure Shocks caused by Boiling in District Heat Exchangers
Björkqvist, Jelka (2022)
Björkqvist, Jelka
2022
Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2022052538661
https://urn.fi/URN:NBN:fi-fe2022052538661
Tiivistelmä
In order to combat climate change, the global energy sector needs to transition from fossil-based systems of energy production, including coal and natural gas, to renewable energy sources, such as wind, solar, and bioenergy. While the transition is proceeding well within the power sector, the progress within the heating and cooling sector remains inadequate. The decarbonisation of the heating and cooling sector is vital, as it represents the largest energy sector and the largest source of energy-related carbon emissions. District heating systems are an important part of the decarbonisation of the heating sector as they enable the integration of renewable energy sources and waste heat into the energy mix.
A hot water boiler plant is designed for the production of district heat and can be connected to the district heating network either directly or indirectly. Boilers utilizing fluidized bed technology feature a high combustion efficiency and low emissions and have wide fuel flexibility, including biomass and waste-derived fuels. In Valmet’s typical boiler plant, the district heating water is heated by absorbing heat from boiler water in a plate-and-shell heat exchanger. The temperature of the boiler water is, however, high enough to induce nucleate boiling of the district heating water. Boiling in the district heating system is detrimental, as it causes pressure shocks that may result in pipe or equipment failures, leakages, and even human injuries.
The purpose of this thesis is to identify the problem cases that will cause boiling in the district heating system and evaluate solutions to prevent these situations. To predict the wall temperature in the heat exchanger, a model of the plate-and-shell heat exchanger is developed using the programming language MATLAB and the thermophysical property database CoolProp. In principle, the district heating water starts to boil as the wall temperature reaches the saturation temperature corresponding to the pressure of the district heating water. However, the wall temperature must be somewhat above the saturation temperature to sustain bubble formation. No field studies about the incipience of boiling in district heating systems have been published, which contributes to the credibility of the model predictions.
Several solutions to the boiling problem are presented and evaluated in the thesis. Boiling in the district heat exchanger can be avoided by increasing the minimum system pressure or decreasing the fluid inlet and outlet temperatures. Furthermore, the flow configuration of the heat exchanger significantly affects the risk of boiling. New control loops could also be developed to better protect the district heating system from boiling in the case of a sudden drop in system pressure or mass flow rate.
Prior research on plate-and-shell heat exchangers is lacking, and further work on the subject could focus on predicting the wall temperature in the heat exchanger more accurately. This would evidently require CFD analyses, as local wall temperatures are heavily dependent on the plate geometry and velocity profiles of both fluids. The wall temperature could also be measured using temperature sensors placed across the plate. Further work could also focus on experimentally developing heat transfer correlations for plate-and-shell heat exchangers that fit a wider range of volumetric flow rates than those in currently available research.
A hot water boiler plant is designed for the production of district heat and can be connected to the district heating network either directly or indirectly. Boilers utilizing fluidized bed technology feature a high combustion efficiency and low emissions and have wide fuel flexibility, including biomass and waste-derived fuels. In Valmet’s typical boiler plant, the district heating water is heated by absorbing heat from boiler water in a plate-and-shell heat exchanger. The temperature of the boiler water is, however, high enough to induce nucleate boiling of the district heating water. Boiling in the district heating system is detrimental, as it causes pressure shocks that may result in pipe or equipment failures, leakages, and even human injuries.
The purpose of this thesis is to identify the problem cases that will cause boiling in the district heating system and evaluate solutions to prevent these situations. To predict the wall temperature in the heat exchanger, a model of the plate-and-shell heat exchanger is developed using the programming language MATLAB and the thermophysical property database CoolProp. In principle, the district heating water starts to boil as the wall temperature reaches the saturation temperature corresponding to the pressure of the district heating water. However, the wall temperature must be somewhat above the saturation temperature to sustain bubble formation. No field studies about the incipience of boiling in district heating systems have been published, which contributes to the credibility of the model predictions.
Several solutions to the boiling problem are presented and evaluated in the thesis. Boiling in the district heat exchanger can be avoided by increasing the minimum system pressure or decreasing the fluid inlet and outlet temperatures. Furthermore, the flow configuration of the heat exchanger significantly affects the risk of boiling. New control loops could also be developed to better protect the district heating system from boiling in the case of a sudden drop in system pressure or mass flow rate.
Prior research on plate-and-shell heat exchangers is lacking, and further work on the subject could focus on predicting the wall temperature in the heat exchanger more accurately. This would evidently require CFD analyses, as local wall temperatures are heavily dependent on the plate geometry and velocity profiles of both fluids. The wall temperature could also be measured using temperature sensors placed across the plate. Further work could also focus on experimentally developing heat transfer correlations for plate-and-shell heat exchangers that fit a wider range of volumetric flow rates than those in currently available research.