What is a Whirlpool Chiller and how does it work? What are the advantages or disadvantages?
As brewchez mentions, whirlpool chilling typically uses an immersion chiller, however you can whirlpool chill with counterflow and plate chillers also. The principle of recirculating the wort is the same - the wort is pumped into the plate or counterflow chiller and back into the kettle, where it creates a whirlpool.
When plate and counterflow chillers are used without recirculating (the "normal" way), the cold break forms in the chiller, and has nowhere to go but end up in the fermentor. By recirculating the wort, the cold break ends up back in the kettle, and a good whirlpool can ensure most of it stays there.
An advantage of circulating the wort with these chillers is that each drop of wort is chilled very quickly, typically dropping from near boiling to 75F/23C in the time it takes to pass through the chiller. This is beneficial since a rapid chill forms a good cold break, which helps reduce chill haze. Yet, since the wort is then recycled, the wort is chilled as a whole. Chilling the whole wort rather than a small part of it is beneficial as it reduces production of DMS and locks in more hop aroma.
The combination of rapid chill and whole-wort chilling is unique to whirlpooling with plate chillers and counterflow chillers.
Whirlpool chilling utlilizes a pump and an immersion chiller. Many brewers that have an immersion chiller will find that an upgrade to a pump for other uses allows them to get better chilling from the immersion chiller.
A whirlpool chiller uses a pump to pull wort from the base of the kettle, then returns the wort to the top of the kettle. The return is just beneath the surface of the wort in the kettle to minimize aeration effects while the wort is hot. The returned wort is pumped under the surface at an angle that is closer to parallel with the surface. This creates a sideways force which makes the entire wort volume whirlpool.
The whirlpooling action increases contact volume of the wort with the chiller surface. Whirlpool chilling stirs the wort over the immersion chiller coil and reduces the total chill time as compared to just an immersion chiller alone.
The second advantage after chill time is the whirlpool action itself. The forces created by the whirlpool help to create a cone of hop debris and break material in the center of the kettle. After the chilling is complete, one can rack or draw off from the side of the kettle, normally without pulling much of this material into the fermentor. Getting clearer wort for fermentation is an advantage of whirlpooling used by the pros.
A downside to whirlpool chilling is the requirement of a pump. However, a pump often becomes a standard upgrade for more serious brewers eventually. The second down side is the water usage with the immersion chiller. Plate chillers tend to be better on water. However, using a whirlpool setup does chill faster than an immersion chiller alone, hence reducing some water useage.