4. A steam boiler with capacity 30 m3 contains saturated liquid water and saturated vapor in equilibrium at 7 bar, initially the liquid and vapor occupy equal volumes. During a certain time interval saturated vapor is withdrawn from the boiler while simultaneously 15000 kg of liquid water at 40°C are added to the boiler. During the process the boiler maintained at constant pressure of 7 bar by the addition of heat. At the end of the process the saturated liquid remaining in the boiler occupies one-quarter of the boiler volume, and the saturated vapor fills the remainder. How much heat was added to the boiler during the process?