Right, the evaporators were a way of keeping water/ice in your air lines from being as big of a problem.
Now most trucks use air dryers, often (or always?) the desiccant type, and not many use the evaporator any more.

But I too don't know much about them, other than I decided on the dryer and not the evaporator for my truck.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-