The flash boiler design evolved around the tubing cross fitting from swagelok ( 4 coils) and a 6 " diameter tube to match the 6" cast iron burner. The tube chimney design was used to get natural draft circulation of the hot gasses through the boiler coils. The vertical upflow circulation of the water allows the air that seperates when the water is heated to flow up and not block water flow in tubes. Down flow circulation is more efficient but does not work at low flow rates used for sparge and steam generation, air bubbles block flow and boiler pops and snaps as water flashes on the hot spots. Increasing the number of coils will help heat transfer at higher flow rates for water heating, but increases the chances of boiling the liquid in the tubing at low flow rates. With both of the boilers built from 1/4" OD stainless tubing the efficiency is not what one could get if they were built using copper tubing but the ability to withstand the thermal abuse and availability of materials for experimenting were the reason for using stainless coils.