I am building an recirc arm for my chilling set up. Should my recirc have a gentle bend in it to 90degrees or would it be OK to just use a 90degree elbow of a piece of straight pipe under the wort surface?
I'm not a physicist, but I'd imagine you'd rather have a graceful bend in it. With a 90, I could see the wort slowing down at the bend. If I didn't have flexible pipe from which I could make a gentle bend, I'd consider using two 45's with a small piece in between.
Full disclosure: I don't have a wort chiller; I'm just theorizing.
A 90 degree elbow would restrict the flow a little bit, but I'm not sure that you would notice any difference. It also depends on the flow you're trying to achieve. Here's a link to a guy that did use a 90 degree arm (not sure if this is exactly what you're trying to do or not). If you're thinking about bending it by hand to achieve the gradual 90 degree elbow, you'll have a much higher risk of restricting the flow by partially flattening out the tube. In my opinion, the 90 degree elbows look a lot cleaner and would provide a little more strength on the joint.