EFFECT OF CIRCULATION RATIO IN BOILER DESIGN

Any Boiler design starts with the calculation of circulation ratio based on the drum working / operating pressure. The circulation ratio is immensely important for providing safe cooling of the heated tubes. Any adjustment & efforts for cost cutting & value engineering will leave a huge impact on boiler maximum operating life. A wrong calculation of circulation ratio (CR) OR reduction of desired circulation ratio shall start pressure parts failure after few months of operations in the forms of bulging, sagging, cracks, tube / pipe inside grooving, wrinkles, overheating rupture etc.

If steam generation is exceeded w.r.t. design consideration, fluid loses contact with tube wall and heat transfer is impacted. When tubes are positioned horizontally and inclined, then fluid flow will stratified and water flows along the bottom and steam along the top of the tube /pipe and flow pattern is rippled - wavy type, hence the top of the tube is alternately wet and dry.

A lower circulation ratio selection against the actual requirements will lead to less supply of water due to inappropriate sizing of downcomer, riser & other supply feeder pipes. This scarcity of water supply will start steam film formation leading to overheating and failures.

There is only one solution i.e. increase circulation ratio by correcting engineering faults OR reduce input heat absorption.

Higher circulation ratio means less steam formation in tubes and Lower CR means high steam formation in tubes.

CR = 100 / (percentage of steam in steam & water mixture)

Circulation Ratio is the percentage of steam formed which reduces the density of a column of water against the density of water in downcomers and hence propels circulation due to this thermal head.

Basic concept of CR calculation at drum working/operating pressure = (Saturated Water Density/Saturated Steam Density)

Steam drum length calculation is mainly dependent on Circulation Ratio, drum operating pressure, steam-flow, number of steam separator cyclones, high steam load variations, steam purity and steam space loading.

Finally, Downcomer, Feeder pipes & Riser Velocities is directly proportional to Circulation Ratio, if flow & area is constant. Further each waterwall panel & evaporator panel has their own circulation ratio.

M/s Unite Energy Corporation LLP is keen to provide the Spares, Sales & Services, Retrofit & Site Repairs of Boiler & Auxiliaries, Performance Evaluation, Shop & Site Fabrication, Erection & Commissioning, Design Modification & Feasibility Study, Consultancy & troubleshooting support to mitigate the irregularities in the plant, minimize breakdown & downtime and improvise design & system performance to improve the overall plant's health and performance.

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