(lb/hr)= 0.43 x flow(GPM) x ∆ T (ºF)
(kg/hr) = 0.092 x flow(LPM) x ∆ T (ºC)
200 GPM x 50ºF temperature rise x 0.43 = 4,300 lb/hr of steam. You would require a 6X50-3 which has a maximum rated steam capacity of 5,000 lb/hr, and a maximum water flow rate of 230 GPM with threaded connections (500 GPM with welded flanged connections).
Variable Flow Heaters are normally used on water service and are offered with cast iron housings as standard. Piping, fittings, and valves are iron, bronze, and steel. Alternate alloys/materials are available upon request. Water piping 3" dia. and smaller is threaded. 4" dia. and larger provided with 125# FF companion flanges and fittings.