Optimisation of Natural Circulation Boilers during Warm Start-Up: A mathematical model describing the dynamic phase transition of water
Translated title
Optimering af Naturlig Cirkulations Kedler under Varm Opstart
Author
Bilde, Kasper Gram
Term
4. term
Education
Publication year
2019
Submitted on
2019-05-31
Pages
78
Abstract
Opstart af en varm kedel er vanskelig, fordi vandet i rørene allerede er tæt på kogepunktet (mætning). Når der under opstart blæses varm luft gennem kedlen, kan det udløse hurtig kogning i både fordamperen og økonomiseren, to varmevekslingssektioner. Denne kogning presser vand ud af rørene og op i damptromlen, beholderen hvor damp adskilles fra vand, så vandstanden der kan stige hurtigt. Afhandlingen præsenterer en matematisk model af en kedel med naturlig cirkulation, hvor vand og damp bevæger sig på grund af tæthedsforskelle i stedet for ved hjælp af en pumpe. Kedlens geometri er forenklet og beskrevet en-dimensionelt for at vise, hvordan forholdene ændrer sig over tid under opstart. Modellen forklarer, hvordan den damp, der dannes i fordamperen og økonomiseren, fortrænger vand i rørene og hæver vandstanden i damptromlen. På baggrund af modellen præsenteres en sammenhæng mellem det relative starttryk og hvor stor en del af damptromlens volumen der er fyldt med vand. Dermed kan man vælge et minimum starttryk, der holder vandstanden inden for den tilladte grænse og begrænser risikoen for høj vandstand under opstart.
Starting up a warm boiler is challenging because the water in its tubes is already near the boiling point (saturation). During start-up, warm air is blown through the boiler, which can trigger rapid boiling in both the evaporator and the economizer—two heat-exchange sections. This boiling pushes water out of the tubes and into the steam drum, the vessel where steam is separated from water, so the water level there can rise quickly. This thesis presents a mathematical model of a natural-circulation boiler, where water and steam move because of density differences rather than a pump. The boiler’s geometry is simplified and represented in one dimension to show how conditions change over time during start-up. The model explains how steam formed in the evaporator and economizer displaces water in the tubes and raises the steam-drum water level. Based on the model, the thesis provides a correlation between the relative starting pressure and the fraction of the steam drum’s volume filled with water. This makes it possible to choose a minimum starting pressure that keeps the water level within the allowable limit and reduces the risk of excessive levels during start-up.
[This abstract was generated with the help of AI]
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