Whirlpool hops are primarily used not for bitterness but instead for extracting volatile hops oils. While some hops bitterness is isomerized when the wort is still near boiling temperature, hops utilization falls off quickly once the temperature drops below about 176°F (80°C). A long-boil hops addition is a much better way to add bitterness to your hops if that is your goal.
This brings us back to the hops oils. The four primary hops oils are myrcene, humulene, caryophyllene, and farnesene. The largest of these is myrcene, which makes up 40–60 percent of the hops oils in most varieties. Myrcene has an herbal note that can be described as green, balsamic, or hoppy and can also have piney/citrusy hints as is the case with many American varieties. Cascade, for instance, has a high myrcene content of around 50 percent. Noble hops, in contrast, have low myrcene content.
Humulene is the classic noble hops oil and is widely used in the perfume industry. It has a strong herbal component and has a boiling point just below that of water and tends to produce a spicy flavor if boiled long. Caryophyllene is the counterpoint to humulene, providing often spicy, earthy, woody tones. It is strong in many English varieties such as East Kent Goldings and Northdown. Finally farnesene, which is typically a factor only in noble hops, can provide flowery, green apple, or woody notes.
While some of these oils have a boiling point above that of water, many are volatile or not soluble and not easily retained or degrade in an active boil. To preserve these hops oils, you need to steep hops at a temperature below boiling. That’s where whirlpool hops additions come into play.
Whirlpool additions can be done at different temperature points. The first effective range is what I call the high isomerization range, which is roughly 185–210°F (85–99°C). In this range, the temperature is high enough that some hops alpha acids are still being isomerized, adding bitterness. This is the range you might use if you still want to add some IBUs to your beer while also adding some hops oils. At the higher temperature, however, you do risk vaporizing a larger portion of hops oils.
The medium whirlpool range is roughly 160–170°F (71–76°C). In this range, there is little isomerization going on, so you will not be adding much bitterness. Also, volatilization is reduced, so you will preserve more hops oils. Fortunately, the wort is still hot enough to allow good solubility for the oils. This is a good compromise range to use if you are looking to preserve hops oils with minimal whirlpool time, and it is probably a good range for homebrewers to work in since it does not require an excessively long whirlpool.
Below that you have the low whirlpool range, which is 150–160°F (60–66°C). In this range, you will volatilize the fewest hops oils, but your hops will also require a longer whirlpool time due to lower solubility at the lower temperature. This is a range you might want to try when trying to preserve oils such as myrcene, which has low solubility and is also highly volatile, as it is less likely to vaporize. This is also a range where many pro brewers have the advantage due to the longer whirlpool times and ability to more accurately control their whirlpool temperature.
One final note: there is some debate about whether to cover your pot during the whirpool. Obviously, you should not cover your pot during the boil, as it will trap volatile off-flavors such as dimethyl sulfide (DMS), which can ruin your beer. But if you are doing a whirlpool/steeped hops stand after boiling, there may be some advantages to covering the pot. Specifically, many hops oils such as myrcene are volatile and tend to vaporize easily. Covering the pot can trap some of these oils, preserving them. The counterpoint is that covering your pot also traps some DMS, which is still being generated, albeit at a low rate, in the hot wort.
My personal experience is that a typical homebrew whirlpool/steep stage is not long enough to generate much DMS, so I prefer covering the pot once I reach my steep temperature and add my whirpool hops to preserve the volatile oils. I’ve not had problems with DMS using this method for typical 10–45 minute hops stands.
To summarize: assuming your goal is to preserve hops oils and not add much bitterness, the medium temperature range listed above (160–170°F or 71–76°C) is probably the sweet spot for the average homebrewer. Good results can be achieved within a shorter whirlpool time (15–25 minutes), and you avoid the extremes of either volatilizing your hops oils or requiring a drawn-out low-temperature whirlpool stage. You can also preserve hops oils with dry hopping, but that’s really an entirely separate discussion.