To improve the efficiency of a steam boiler system which generates 1000 psia steam for refinery process use, the boiler feed water (BFW) is to be preheated to 500oF from 100oF before it enters the boiler. This preheat will be accomplished by installing a heat transfer coil in the boiler flue gas stack, recovering heat energy that would otherwise be exhausted to the atmosphere. This BFW is a "compressed liquid water." How much heat energy, expressed as MBTU/hr. is required to effect this BFW preheat at 250gal/min BFW rate?