You need to take a close look at your mesh. Especially with a model like this, where you have a high level of fine detail. Make sure your mesh accurately captures the geometry around the pipes. Use mesh cut plots and be sure to turn off the "Use CAD geometry" in the plot options so you can see exactly what your mesh looks like.
To be honest I'm not sure you will be able to do a complete analysis of this geometry, the required mesh cell count must be in the dozens of millions. I would try to simplify the model somehow, maybe a porous medium in place of the tubes.
what you're doing is mesh convergence and the effect you are seeing is expected as you improve the capturing of the flow and geometry
how big was the change? at some point you get much longer solution times with little improvement in the results
like amit said, this is going to take lots and lots of cells to solve, porous would be the way to go if you can. really depends on your goals.