How can chimney tray pressure drop be estimated?
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08/09/2016
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S Banik, Centre for High Technology, sbdr@rediffmail.com
To a great extent this has already been answered. I would like to add that pressure drop of chimney tray becomes important for vacuum system as this pressure drop is a dead loss without contributing to separation . Typically in a refinery vacuum column 1-2 mmHg pressure drop could be allowed. Therefore cross section of the tower needs to be judiciously allocated for chimney tray, as certain hold-up of liquid is to be maintained as process requirement. Chimneys are basically ducts with a cover at top. One can estimate the pressure drop by using equivalent diameter and usual frictional pressure drop approach. There are free softwares available in the internet which could be used also. Empirical methods are also available in standard fluid flow books. However you need to ensure that there is no entrainment of liquid in the packed bed. In that case you need to correct vapour density for entrainment.
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27/08/2016
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Leonardo Leite Garcia de Souza, Petrobras SA / Duque de Caxias Refinery, llgs394041@yahoo.com.br
Chimney tray pressure drop should be negligible if the device is well designed. The cross-sectional area of the chimneys should be sized considering critical velocity for vapor/liquid disengaging and the scape area between the top of the chimney and the hat should be, at least, equal to the cross-sectional area. Having the vapor flow rate and specific gravities you should be able to size the required cross-sectional area and then choose a proper distance for the gap between the chimney and its hat. I have designed two chimney trays for an atmospheric crude tower revamp, using this approach, and both showed negligible pressure drop after star-up. You should take a look at the PTQ of September (Revamps special edition) where I present a case study of a successful revamp of two crude towers where, among other things, it will be shown how to properly design chimney trays.
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17/08/2016
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Nagesh Bejgam, Reliance Industries Limited, nagesh.bejgam@ril.com
Agreed. The inlet loss will be contraction loss and exit loss is the expansion loss. I presume frictional loss in straight portion will be negligible. But, the contraction and expansion loss will have loss coefficients which are function of diamter change. Are there any typical numbers for these coefficients?
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16/08/2016
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
You can estimate it as an entrance loss + an exit loss. Note that the cross sectional areas will be different for the two calculations. The inlet loss is across the area of the chimney itself. The exit loss will use the area of the gap between the chimney and the hat (perimeter of the chimney times the spacing between the chimney and the hat).
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