Dear all,
For the double pass sieve trays calculations, the liquid flow rate will be divided by 2 as it is splitted into 2 downcomers. So what about the vapor flow rate can I keep it as it is ? or should i divide it by 2 aswell ?
Hi,
Understand the basics. The calculations or correlations really don't understand the number of passes but the absolute values. Let me explain it for you, the geomterical outputs such as downcomer top and bottom area, weir length, active area, net free area are the one which will be used for checking the hydraulic capacity.
In your case it's a two pass tray so calculation sequence will be,
1. Assume a diameter of column - estimate col cross section area (A - m2)
2. Assume downcomer widths for side and central downcomer and estimate chord length (outlet weir length) (WL), downcomer inlet area (Dci)
3. Estimate the active area (column area minus 2*downcomer area)
4. Net free area is column area minus one DNCR area
Once you get these geomtrical parameters, perform hydraulic calculations to check the vapor and liquid capacity
1. Downcomer choke flood - refer Henry Z Kister Distillation Design Book to get the correlations like Koch, Nutter which are based on downcomer residence time. Downcomer choke flood is nothing but the ratio of liquid inlet velocity to maximum allowable velocity. Liquid flowrate and downcomer area will give inlet liquid velocity and max allowable velocity with the correlation. Use appropriate system factor if applicable to derate the capacity.
2. Jet Flood - refer same handbook to estimate the jet flood by Fair's correlation or Kister-Haas correlation.
3. Downcomer Backup Flood - it's simple pressure balance i.e. liquid from top tray is flowing towrads higher pressure and hence it should build up inside downcomer to overcome tray delta P and other resistances
4. Tray delta P - it is mainly sum of dry drop, wet drop and vapor head plus minor loss to form the vapor bubble (surface tension)
Once you get the active area, choose the open area which is nothing but the hole area (e.g. 10 mm size total 300 sieve holes) and estimate the dry pressure drop by simple orifice equation. Wet pressure drop is weir height plus liquid crest. Liquid crest will be calculated with some correlations given in Kister book which is function of weir load - m3/h-m (liquid flow in m3/h divide by weir length in meter)
it's really simple. All the best.
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