SMS Mevac UK Ltd - CFD Analysis of a Fixed Fume Extraction Hood

SMS Mevac UK Limited required a Computational Fluid Dynamics (CFD) analysis of a Fixed Fume Extraction Hood in order to understand the flow inside the hood, with a view to optimising its design.

Company Profile

The term "Mevac" in the company name comes from two sources: Standard Messo, founded in 1960, and Vacmetal, founded in 1969, both providing pioneering solutions for secondary metallurgical processes. They merged to form Mevac in 1999.

SMS Mevac deliver highly efficient, innovative technologies and services for the refining of liquid steel. To achieve this goal, they balance process technologies, market requirements, customer demands and environmental responsibilities. This includes engineering, equipment supply, supervision of erection and commissioning, as well as training and after sales services.

SMS Mevac have a world-wide leading market position based upon their unique metallurgical and process expertise.

SMS Mevac UK Ltd - CFD Analysis of a Fixed Fume Extraction Hood

Background 

A fume hood is a local ventilation device designed to limit exposure to hazardous or noxious fumes, vapours or dusts. Two main types exist, ducted and recirculating. The principle is the same for both types: air is drawn in from the open side, and either expelled outside the building or made safe through filtration and fed back into the workspace. 

Most fume hoods for industrial purposes are ducted.   Hazardous or noxious vapours, gases or dusts are removed from the workspace and dispersed into the atmosphere.

The correct flow of air from the workspace into the hood is critical to its function. Much of fume hood design and operation is focused on maximising the correct containment of the air and fumes within the fume hood.

Analysis

SMS Mevac UK Limited required a Computational Fluid Dynamics (CFD) analysis of a Fixed Fume Extraction Hood in order to understand the flow inside the hood, with a view to optimising its design.  

Two CAD models of the fume extractor hood were supplied by SMS Mevac. The models had different sized slots in the horizontal and upper sections.  The geometry was defeatured appropriately for CFD analysis and a fluid domain was derived for the inner portions of the hood. Some of the external/internal features of the hood which would not affect the flow were removed in order to mesh the model. 

A boundary layer with an appropriate thickness was employed for the fluid domain walls to help capture the flow variations near the wall. In addition, a finer resolution mesh was placed on surfaces where there was a rapid change in fluid parameters or the geometric dimensions were relatively small.

A steady state, single fluid, non-buoyant, incompressible and isothermal CFD simulation was carried out in ANSYS CFX.

SMS Mevac UK Ltd - CFD Analysis of a Fixed Fume Extraction Hood
Defeatured Geometry

Boundary Conditions

The boundary conditions applied to the two models were as follows: 

  • The inlet was specified as an 'opening' boundary condition which allowed the fluid (in this case, air) to enter as well as leave the domain.
  • A volumetric suction flow rate of 50,000m3/h was applied at the main outlet of the hood. 
  • Wall boundary conditions were given to all other surfaces through which there is no flow. 

The fluid was modelled as air at standard conditions.

The flow rate through the inner pipe and the conical hopper was not modelled.  These were  included as obstacles to the incoming fluid from below.  An opening condition was also set around the outside of the ladle cover, in order to allow ‘external’ fluid to be sucked inside the hood.

SMS Mevac UK Ltd - CFD Analysis of a Fixed Fume Extraction Hood
Graph - Vertical Slots

The graphs below show the flow through each slot for both the horizontal (below left) [Image 3] and vertical slots (below right) [Image 4], for both the original and optimised configurations.   It can be seen quite clearly that the optimised configuration showed an improvement in flow distribution.

Design Benefit

Due to the relative distances between the slots (both horizontal and vertical) and the exhaust duct, it was noticed that there was a non-uniform distribution of the flow through these slots. Using the results obtained from the original design of the fume hood, the area of both the vertical and horizontal slots were modified and a more uniform mass flow distribution was obtained.  Apart from the horizontal slot no.1 and the vertical slot no.9, all the other slots responded to the variation of their sectional area.  The different behaviour of the horizontal slot no.1 and the vertical slot no.9 was attributed to their position relative to the exhaust duct.

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