Once-Through Evaporator Flow Stability via Dynamic Harp Control
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Solution Overview
Problem
Once-through evaporators experience flow instabilities and fluctuating thermal stress, leading to tube failures, particularly at partial load operations and low mass flow rates, due to inadequate pressure drop and steam buildup.
Innovation Solution
The implementation of a once-through evaporator system with multiple primary and secondary heat exchange stages, where valves are used to selectively control pressure drop and mass flow rate across heat transfer tubes, allowing for balanced flow distribution and minimizing steam buildup by adjusting the operation of individual harps and stages based on load conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If pressure drop across individual harps is increased to overcome buoyancy-controlled flow rates, then flow stability is improved, but device complexity increases due to additional orifices or reduced inlet diameters
Solution Approach 1:
The patent implements variable pressure drop control where the pressure drop across individual harps can be dynamically adjusted based on operating conditions. This is achieved through a control system that monitors flow rates and steam generation demands, then adjusts the pressure drop across specific harps accordingly. This dynamic approach allows the system to maintain flow stability across a wide range of operating loads without requiring fixed complex hardware modifications like permanent orifices or reduced inlet diameters.
2Loss of energy
If downcomer velocity is reduced at low mass flow rates, then energy consumption is reduced, but steam buildup occurs in the downcomer and outlet manifold causing flow instabilities
Solution Approach 1:
The patent employs a feedback control system that continuously monitors steam generation rates, water levels, and flow conditions in the downcomer and outlet manifold. When steam buildup is detected or anticipated at low load conditions, the control system adjusts the pressure drop distribution across harps to maintain adequate downcomer velocity. This feedback mechanism ensures flow stability is maintained while minimizing energy consumption by only increasing velocity when necessary to prevent steam accumulation.
3Ease of operation
If multiple primary evaporator stages are used to distribute two-phase flow, then flow distribution control is improved, but device complexity increases
Solution Approach 1:
The patent divides the evaporator system into multiple primary evaporator stages, each with its own outlet manifold and downcomer configuration. This segmentation allows independent control of two-phase flow distribution to different sections of the secondary evaporator stage. Each primary stage can be independently adjusted to optimize flow distribution based on local heat transfer requirements and steam generation demands, improving overall system control without requiring a complete redesign of the entire evaporator.
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
This approach stabilizes flow and thermal conditions across a wide range of operating loads, reducing tube failures and extending operational life by maintaining uniform exit quality and temperature, thus enhancing the reliability and performance of the evaporator.
Implementation Method 1
The flow of gas heats the fluid flowing through the primary arrays of tubes to form a two phase flow
Implementation Method 2
Mass flow rate within internal portions of the tubes 22 and 32 is controlled by buoyancy forces, for example, density differences induced by heat transfer to the fluid in the tubes
Data Source
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AI summary
An evaporator 100 for steam generation is presented. The evaporator 100 includes a plurality of primary evaporator stages 110 and a secondary evaporator stage 150. Each primary stage 110 includes one or more primary arrays of heat transfer tubes 120, 160, an outlet manifold 135 coupled to the arrays 120, 160, and a downcomer 137 coupled to the manifold 135. Each of the primary arrays 120 has an inlet for receiving a fluid and is arranged transverse to a flow of gas through the evaporator 110. The gas heats the fluid flowing through the arrays 120, 160 to form a two phase flow. The outlet manifold 135 receives the two phase flow from the arrays 120, 160 and the downcomer 137 distributes the flow as a component of a primary stage flow. One or more of the plurality of primary evaporator stages 110 selectively form the primary stage flow from respective components of the two phase flow, and provide the primary stage flow to inlets of the secondary evaporator stage 150.