i thought the same and use the valve on the outlet to control flow. i have seen both ways . interested in other comments

I would think that if the heat exchanger is designed correctly and plumbed correctly, the plates would always be covered with water no matter where you were throttling the flow. That is, the inlet will be near the bottom of the HX and the outlet will be near the top, for both the wort and the water/glycol.

Throttle the inlet

You want to keep the water pressure (or glycol) in the chiller as low as possible. Make sure your chiller is flooded with water but throttle the inlet.

If you throttle the outlet you create unnecessary pressure inside the chiller. This becomes and issue if you ever get a leak in one of your plates (it happens eventually) and the pressure will push the CL or glycol in to the wort. If you throttle the water inlet, the pressure will be higher on the wort side and the wort will leak into the cooling medium. Not great but much safer.

This is something we learned from putting in plate heat exchangers in to dairy plants where it is the law to keep the product at a higher pressure than the cooling medium. It is not a legal requirement in brewing but a very good practice.

This is great! Some good information. The funny thing about bottom inlets is, all of the chillers I have worked with have all had top inlets and bottom outlets. All of them Thremaline as well. So the main point being to balance the pressures to higher pressure on the wort side. I'm guessing that heatX's have optimum and maximum pressure ratings. Perhaps a better idea is to have a set of gauges, one on the chilling medium and one on wort, determine optimum knock out rate and adjust pressure balance from there. Also, as long as we throttle the outlet of the wort side and throttle the chilling medium inlet side we should have a higher pressure on the wort side. Sound right?

Pressure gauges are a good idea but you are right, if you throttle the water in and the wort out you should be fine.

Re: inlets and outlets on top and bottom. It depends on the the number of passes on the heat exchanger. The more passes in the heat exchanger the more effective the heat transfer will be.

If all four of the ports are on one end then you have a single pass heat exchanger. The wort will go in the top and out the bottom and the water the other direction. These are easy and cheap to build but not efficient.

For better heat exchange you build a heat exchanger with 2 or 4 passes (or more). The wort will go in at the front bottom and come out at the back bottom and the water the opposite direction. Inside the heat exchanger the plates are arrange so the water and wort pass each other twice (or four times) thus exchanging more heat. You use less water and get cooler wort.

Usually the connections will be on the bottom so you can drain the unit completely when CIPing it.