Recommendations are not bad
The problem with recommendation questions is that they usually aren't trying to solve a specific problem, which means that there isn't a specific answer. Often someone asking a recommendation question actually has a problem that can be figured out through clarifications.
If someone asks, "What is the best book to learn about large-scale heat exchangers for oil refineries?" they may have a problem with a large-scale heat exchanger in an oil refinery that can be gleaned by asking questions like:
- Which process?
- What type of heat exchanger?
- Information on installing, maintaining existing exchangers, or troubleshooting them?, etc.
If you ask those clarification questions, it may very well turn out that "What is the best book to learn about large-scale heat exchangers for oil refineries?" was really a question about "How can I troubleshoot heat exchanger tube corrosion on a large-scale heat exchanger used in process X?" And maybe part of the answer to that will be "Heat exchanger corrosion is often caused by problems with water treatment. The first things you want to look at are X, Y, Z. For a more general overview of heat exchangers in that specific process, take a look at this book/this website."
That is a good recommendation, and one that will help people in the future who have the same problem (or a similar one).
Product Recommendation Requests
A product recommendation request may also be something similar. Someone may ask, "What is the best valve to discharge condensate from a 50 psi steam main in batch operation?" Again, there is probably a question hidden in there:
- What are you currently using?
- Are you having trouble with condensate backup?
- What schedule do you run batches on?
- Do you shut down overnight? etc.
So after clarifying, it may turn out that the question is actually, "I am having trouble with condensate backup in my 50 psi 1/2" steam pipe on first start-up every morning after 8 hours with the equipment off. How can I prevent condensate backup after a long shut down?" And again, part of a good answer may be "You want to look at a manufacturer's steam trap sizing diagram like this or like this, and match it with the amount of condensate built up in your pipes after a shut down using equations like this or this." Or alternatively, "This steam trap manufacturer recommends installing a bypass valve after long shut downs to manually purge the initial condensate"
These are also good recommendations, even though they are recommendations.
When recommendations are good
What both of these examples have in common is that they provide useful resources for further information on how to solve a specific problem. They do that while also answering the question for the person here on the site even if those links do end up dead because they explain why they are also recommending those resources.
I hope that when you see a recommendation question you don't reflexively close it as off-topic, but that you think about what sorts of underlying problems the person asking for the recommendation may have, and try to clarify the problem to salvage the question and leave a better resource for people going forward.
I wouldn't worry about product recommendations in answers too much either. If I ask a friend for advice on an engineering problem, and that friend gives me a specific product that has worked for them on the same issue in the past, that is a good thing. What we want to make sure is that those recommendations are organic, and not people trying to sell more of their product. See how not to be a spammer.