Since the tubes in a HX are normally short i would not consider then except as a discrete loss. If you wish you could calculate the equivalent flow area of the whole HX and then isert a pipe section with this ID in your model?
I would suspect that you would not do the analysis through the HEX, but in its attached piping, then consider that the maximum pressure is transmitted throughout the entire HEX instantaneously.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
The plant is a few 100 meters of hot water piping then four HEXs in series then a few more 100m of the cooler water piping to a pump station then 1000m to a well.
Do a transient of the connecting piping and pump and pipeline to the wells, that is. I would think that most of the high pressures will be seen between pump and well and that not much will be seen in the suction piping. However "much" is a term relative to the design pressure. If the design pressure is low in the suction piping, any pressure increase might be more significant than if it happened in a higher pressure design segment, such as might be the case in the pump discharge and downstream pipeline.
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
