Heat Exchangers – The difference between passes and sections.
Donald Byres | Jun 9, 2020
It is common to get confused about the difference between sections and passes in a plate heat exchanger, so I will try to shed some light on it for you. Simply put, each section does a different job and within each section there could be multiple passes.
A single section - single pass heat exchanger is the simplest design and it will look like this with all four connections on one end.
These are commonly used where there is a big temperature difference, (AKA “Delta T” or ∆T) such as heating domestic water with steam in a commercial plant. They are not efficient for an application where the ∆T is small, such as wort cooling in a brewery. For a brewery application, you would use a single section heat exchanger but with multiple passes.
Above is a two pass heat exchanger but you commonly see a heat exchanger with 4 or 5 passes and sometimes even more. The more passes you add to the heat exchanger the more efficient it is in terms of heat transfer, however, there is a trade off because as you increase the passes the pressure drop will go up.
When you have a more complex application such as a dairy pasteurizer or even cooling beer wort to lagering temperatures, you need a multiple section heat exchanger.
A heat exchanger for cooling wort for a lager would have two sections, the first to cool the wort with water and a second section to cool further with glycol. You could do this with two separate heat exchangers but it is more convenient to do it with one two section heat exchanger.
This example above has two passes in the left hand (water) section and a single pass in the right (glycol) section.
When you look at larger heat exchangers for pasteurizers you could have up 3, 4 or 5 sections with any number of passes in each one. The design of these gets quite complicated.
So the way to think about sections and passes is that each section does something different, and each pass increases cooling efficiency. Each section can have a single, or multiple passes. For brewing, you’re almost always going to use a multi-pass heat exchanger, but if you need to get temperatures down below the temperature of city water, you’re going to need a secondary stage cooling with something cooler than city water, such as glycol.