Whirlpool Tanks
The whirlpool tank is the most widely used method for hot trub removal, particularly in breweries that use hop pellets, powder, and extracts. Whirlpool tanks come in various designs, and opinions vary as to the best geometry. Some call for a conical bottom, some a slightly welled bottom, and still others prescribe a flat or even an inverted cone bottom. The original whirlpools were constructed to have a height-to-diameter ratio of 1:1. Although this ratio remains the more popular of geometries, later whirlpool designs have ranged from very shallow (0.6:2), to accommodate the trub associated with hop powders, to tall (3:2).
The wort usually enters the tank tangentially at speeds between 13 and 50 ft/s (see Figure 3); faster wort flows result in better trub separation (4). Placing a ping pong ball on the outside surface of the whirling wort will give you an idea of the speed your wort is moving. You can calculate the speed by first determining the inner circumference of your whirlpool tank (circumference = pi X diameter). By counting the number of revolutions in a fixed period of time (20 seconds, for example), you can calculate the distance covered per second (rate = [circumference X number of revolutions]/number of second measured).
In general, the effectiveness of whirlpools decreases as the original gravity of the wort increases because the relative difference in density between the trub particles and the wort decreases. Injecting carbon dioxide into the wort as it goes to the whirlpool has been shown to increase the amount of trub deposited (3).
To minimize oxygen uptake in a whirlpool, the inlet should be placed one-quarter to one-third of the way from the tank bottom (see Figure 3). The rotation about the vertical axis should continue for 20-40 min after the wort is in the tank. As the whirling action slows, the trub, which is heavier than beer, forms a fairly hard conical cake at the center of the bottom of the tank. If the time the wort is left standing is too short, separation will be incomplete; too long a stand, and the risk of infection rises and the breakdown of S-methyl-methionine continues, raising dimethyl sulfide levels.
The whirlpool tank is fitted with one or more take-off points (some with interchangeable standpipes that can be sized for particular beer styles), so that the clarified wort can be discharged to the heat exchanger, leaving the trub behind. By having more than one discharge point, the wort can be removed with minimal turbulence, leaving the trub cake intact and resulting in a clearer wort. The trub is then flushed down a center drain, fitted with a valve that is usually twice the size of the wort outlet.
Some breweries have a combination kettle-whirlpool, with essentially the same properties of the dedicated whirlpool. Unfortunately, the presence of kettle baffles or an agitation propeller will completely disrupt the whirling action, making a combination kettle-whirlpool impossible. If the kettle has no baffles and the kettle agitator is equipped with a high-power, high-speed motor with an inverter and a clutched gear drive, the wort can be whirled using the agitator. When the desired whirling action is reached, the agitator can be turned off, activating a clutch and allowing the propeller to turn freely with the wort. Alternatively, the wort is discharged from the bottom of the kettle and pumped back into the kettle tangentially. Pumping this way for 15-20 min will create a whirlpool. In either case, when the whirling subsides and the trub cake has formed, the clear wort can be pumped from a port above the trub cone.
Advantages of the combination system include reduced oxygen uptake, because no wort transfer is necessary. It also saves time and eliminates the need for a separate whirlpool tank. On the other hand, because of necessary engineering compromises, it is unlikely to be quite as effective as a dedicated whirlpool. For it to work at all, it must be carefully engineered.