No real easy answers...

In TEMA (Tubular Exchanger Manufacturers Association) terms,
BEM is a typical shell and tube design wher you use a separation drum over the exchanger. Typically you will leave the top 2 or 3 rows out of the shell. The vessels will have 2, 3 or 4 risers to connect the shells. Riser design is critical.

One that has the disengaging area in the sheell can be a BEM that is only 1/2 full of tubes. Ans alternate design is a BEU or BKU, often refered to as a kettle type evaporator. The disadvantage of this type design is the extra refrigerant charge required to flood the tubes.

On either design, you set the liquid level controller to operate about 2 or 3 rows of tubes below the top of the tubes. The agitation from boiling keeps the top row of tubes wetted at full load and at reduced load, the slight loss of wetter surface doe not matter.

Next issue is the proper liquid entry into the evaporator...lots written on this, but I prefer a perforated distributor plate below the tubes. Larger vessels may require multiple entries.

Next is the proper number and design of vapor outlets from the exchanger.

There is very little written on the design of flooded evaporators and vendors guard their designs. You will probably need to hire an consultant to do the initial designs for you.

Ken