A hop back is a small vessel used after the boil kettle and ahead of a counter flow chiller to add "hop brightness" to the brew. Ours has a clear lid in order to see the level of the liquid running into it, and a screen in the bottom. It will hold at least 2 ounces of whole hops. Normally you would gravity feed into the hopback and pump out. We have found it very easy to match the flow rates and keep a constant level. It is 7.5" wide from coupler to coupler and 8.5" tall.