MartinLe
Civil/Environmental
- Oct 12, 2012
- 394
Hello all,
I'm trying to figure out the hydraulics of a screen (vertikal bar, like this: The application is wastewater, screen after initial headworks of a WWTP.
The usual formula is Kirschmer:
Xi = beta * (s/a)^4/3 * sin alfa
with Xi = loss factor of screen, beta = form factor of the bars (range 0.8 - 2.4); s thickness of and a space between bars and alfa the angle towards the horizontal of the screen.
With this, headloss is:
dh = Xi * v^2/(2*g)
v beeing flow velocity in the screen.
The confusing part ist that different manufactueres calculate v differently - same use water level before screen as basis, some after (which seems wrong since we expect significant losses in wastewater applications with fine screen).
Additionally, I've seen a modified Kirshcmer formula that include a term for the loading (f) of the screen:
dh = beta * (s/a + (1+f)^2)^4/3 * v^2/2g * sin alfa
however, loading goes also into v (via effective Area).
To make the confusion complete, one I've seen a modified Kirschemr that included a term for stagnating flow downstream the screen: say you have a stagnant flow layer height ls, with water level upstream the screen lu, the headloss fould be modified by a factor * 1 + ls/lu (all levels measured from channel bottom). How would I estimate this ls?
Looking at all this, it seems most logical for me to use the unmodified Kirschmer and account for loading in calculating the effective area:
A_eff = A * (1+sin alfa) * s/(s+a) * (1 - f), with A screen width * lu
I started writing this question somewhat more confused than I am now. What I would ask you to is
1) poke holes in what I've written above
2) point me to an athoritative source on the subject
Writing all this down helped my clear my confusion
I'm trying to figure out the hydraulics of a screen (vertikal bar, like this: The application is wastewater, screen after initial headworks of a WWTP.
The usual formula is Kirschmer:
Xi = beta * (s/a)^4/3 * sin alfa
with Xi = loss factor of screen, beta = form factor of the bars (range 0.8 - 2.4); s thickness of and a space between bars and alfa the angle towards the horizontal of the screen.
With this, headloss is:
dh = Xi * v^2/(2*g)
v beeing flow velocity in the screen.
The confusing part ist that different manufactueres calculate v differently - same use water level before screen as basis, some after (which seems wrong since we expect significant losses in wastewater applications with fine screen).
Additionally, I've seen a modified Kirshcmer formula that include a term for the loading (f) of the screen:
dh = beta * (s/a + (1+f)^2)^4/3 * v^2/2g * sin alfa
however, loading goes also into v (via effective Area).
To make the confusion complete, one I've seen a modified Kirschemr that included a term for stagnating flow downstream the screen: say you have a stagnant flow layer height ls, with water level upstream the screen lu, the headloss fould be modified by a factor * 1 + ls/lu (all levels measured from channel bottom). How would I estimate this ls?
Looking at all this, it seems most logical for me to use the unmodified Kirschmer and account for loading in calculating the effective area:
A_eff = A * (1+sin alfa) * s/(s+a) * (1 - f), with A screen width * lu
I started writing this question somewhat more confused than I am now. What I would ask you to is
1) poke holes in what I've written above
2) point me to an athoritative source on the subject
Writing all this down helped my clear my confusion
