Here is a snippet from reply to a customer that historically had problems with whirl pooling (to get a nice trub cone prior to transfer out to fermenter) and also wondered how our trub dam worked:
Your practice of using a bag for your hops is fairly common in home brewing. . . but really never in professional brewing. Pro brewers (I own a brewery with a 20bbl brewhouse) depend on the whirlpool to concentrate the trub (hops and hot break) into the center of the kettle.
I’m guessing that your whirlpool didn’t last long enough to allow the heavy material to move to the center of the kettle and fall out. As a home brewer for many years, I can tell you that a proper whirlpool will cause the trub to fall to the center and hold a nice cone.
The reason for the trub dam, as opposed to a simple pickup tube is all physics. Think about the flow rate of the wort coming out of your kettle drain valve. . . if you have a similar opening on the dip tube, you create quite a high velocity at the entrance of the tube. . . that causes a lot of trub to be picked up, and pulled through (high speed flow vs very slow flow). The Trub Dam, separates the entry into the two sides, where each side is about an inch wide. . . so the relative speed of wort flowing into this area is ~ 1/8 as fast as the tube pickup point. Long story, but this allows the trub to “stay put”.
Creating a strong whirlpool (something that will still be spinning for a good 5+ minutes) and allowing it to settle will create a good cone, and allow you to separate trub from wort. The time your wort spins is just as key as the time you allow it to settle. The spin time, causes the heavy materials to move to the center, the wait time allows the heavy materials to fall to the bottom of the kettle.
Having said that, some trub entering your chiller will always occur. . . the point is to minimize it.
By the way, you should always “back flush” your chiller with PBW or similar caustic. Then, once every 4-5 brews, put the chiller in the oven at 300F for 30 minutes. . . this will carbonize any organic material .. after it cools, flush it with water.
Hope that helps