Equalizing Chamber for HRSG Evaporator Tube Stress
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Solution Overview
Problem
Current once-through evaporator technology in large heat recovery steam generators (HRSGs) experiences non-uniform distribution of water/steam mixtures, leading to differential thermal stresses in tubes, which can cause fatigue and potential failure due to varying steam and water fractions across adjacent tubes.
Innovation Solution
The introduction of an equalizing chamber within the evaporator stages to blend steam/water fractions, promoting uniform temperature distribution and reducing differential thermal stresses by mixing flows before they enter the secondary evaporator tubes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a parallel array of heat transfer tubes is used in the evaporator, then heat exchange efficiency is improved, but non-uniform distribution of water/steam mixture occurs leading to differential thermal stresses
Solution Approach 1:
An equalizing chamber is introduced as an intermediary component between the lower header and upper header. This chamber receives two-phase flow from multiple lower tubes and redistributes it to upper tubes, acting as a mixing zone that equalizes steam and water fractions before distribution. The chamber volume is specifically designed to provide sufficient mixing while maintaining compact overall dimensions.
Solution Approach 2:
The evaporator is segmented into distinct functional zones: a lower section for two-phase flow generation, an intermediate equalizing chamber for flow mixing and distribution, and an upper section for continued heat exchange. This segmentation allows each zone to perform its specific function optimally, with the equalizing chamber specifically addressing the non-uniform distribution problem.
2Ease of operation
If buoyancy forces control internal mass flow rate in vertical tubes, then natural circulation is achieved, but non-uniform flow distribution occurs across adjacent tubes
Solution Approach 1:
Multiple two-phase flow streams from different lower tubes are merged within the equalizing chamber, allowing buoyancy-driven flows to combine and mix. This merging process creates a more uniform mixed flow that is then distributed to multiple upper tubes, maintaining the benefits of natural circulation while achieving uniformity.
Solution Approach 2:
The equalizing chamber serves as a mediator that receives non-uniform buoyancy-driven flows from lower tubes and transforms them into uniform flows for upper tubes through passive mixing mechanisms within the chamber.
3Productivity
If secondary evaporator inlet receives two-phase flow from primary evaporator, then steam generation continues, but differential thermal stress develops in tubes
Solution Approach 1:
The equalizing chamber performs preliminary mixing and equalization of the two-phase flow before it enters the upper evaporator tubes. By pre-distributing uniform flow to all upper tubes, the system prevents the development of differential thermal stresses during steam generation operations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The equalizing chamber ensures more uniform temperature distribution at the inlet of the secondary evaporator, reducing thermal stresses and extending the lifespan of evaporator tube-to-header connections during startup and load ramps.
Implementation Method 1
The introduction of an equalizing chamber within the evaporator stages to blend steam/water fractions, promoting uniform temperature distribution and reducing differential thermal stresses by mixing flows before they enter the secondary evaporator tubes.
Implementation Method 2
The first stage produces steam/water mixture. The second stage evaporates the water to dryness and superheats the steam.
Implementation Method 3
The second stage evaporates the water to dryness and superheats the steam.
Implementation Method 4
internal mass flow rate is controlled by buoyancy forces, and is proportional to the heat input to each individual tube
Implementation Method 5
The thermal expansion of an individual evaporator tube is determined by the integral of the temperature rise of the internal fluid along the length of the tube.
Data Source
AI summary
An evaporator 10 for evaporating a liquid includes a plurality of harps 20 disposed within a duct or chamber such that a heated fluid flow 22 (e.g., heated gas or flue gas) passes through each successive row of harps 20 of the evaporator 10. Each of the harps 20 includes a lower header 24, a plurality of lower tubes 26, an intermediate equalizing chamber 28, a plurality of upper tubes 30, and an upper header 32. The lower tubes 30 are in fluid communication with the lower header 24 and extend upward vertically from the lower header. The upper ends of the lower tubes 26 are in fluid communication with the equalizing chamber 28. The upper tubes 30 are in fluid communication with the equalizing chamber 28 and extend upward vertically from the equalizing chamber. The upper ends of the upper tubes 30 are in fluid communication with the upper header 32.