I'm about to write a technical description for an industrial process. The description will follow logical blocks, or blocks from the PFD of the process. This a sales document aimed at (mostly) engineers. My idea is to have a short paragraph at the start of every chapter, that will explain the purpose of the block to be described and notable inputs and outputs. My thinking is that the document will probably not only be read in one sitting, but that someone will come back to it to look up specific things. So I want to help the readers to orient themselves. The only downside I see is that's a little extra text. Should I do this, is there a better way to make my description accessible?
Yes, you should have introductory text for every chapter in a technical work. However, you don't necessarily need to have a dedicated paragraph.
Ideally, you would use the chapter's title to clearly indicate what the chapter is about. And if each chapter describes a step in a process that has distinct inputs and outputs, you may want to include a plainly labeled "Inputs and Outputs" section (or sections) in each chapter.
I think an abstract or quick summary isn't a bad idea at all, especially if your audience is not necessarily familiar with the process or the parts, or if there's a lot of jargon involved.
I may not remember the difference between the Widgetizer and Widget Processing, but if you explain "The Widgetizer is the machine which inserts the ding-dong into the hoo-ha" and "Widget Processing is the process of moving fully interlocked widgets off the assembly line into the shipping boxes," I'll know which chapter I need.
How thick/voluminous will this document be? I think the answer depends on the overall length as well as the length of each chapter.
A summary is not necessary if the chapter is only a few paragraphs. But if the whole documentation had textbook length (800+ pages), an introduction, summary and even a table of contents for each section could be of great help to your readers.
Go to a library or bookstore and look at textbooks for students of the natural sciences to see how they break down their books into sections, then chapters, add an overview or table of contents and an introduction to all parts, add summaries, conclusions and a bibliography or study questions to the end of each chapter, and how they lay out the book, taking out sections of text into separate highlighted blocks on each page, etc. There are even books that have a short abstract for each subsection of a chapter, sometimes only a few paragraphs long!
Of course those textbooks are designed in this way to facilitate learning, but since you write a manual that you want to be used as reference, then maybe a textbook is not so completely different.
Publishers like Pearson create well designed textbooks. Take them as an example and take what you find useful.