A boiler with a mass of 10?kg uses 50?kW to heat 20 gallons of liquid water initially at 15°C. The average specific heats for the boiler material and the water are 0.48?kJ/kg•K and 4.18?kJ/kg•K, respectively. The average density of the water is 850?kg/m3. According to the service restrictions of the ASME Boiler and Pressure Vessel Code (ASME BPVC.IV-2015, HG-101), hot water boilers should not be operating at temperatures exceeding 120°C at or near the boiler outlet. Determine whether the final water temperature that would exit the boiler after 30 min of heating at 50?kW would be in compliance with the ASME Boiler and Pressure Vessel Code.

Question

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Answer 1

A boiler (10 kg) is used to heat 20 gallon (0.07571 m3) of liquid water with 50 kW for 30 minutes. Determine whether this operating condition would be in compliance with the ASME Boiler and Pressure Vessel Code (ASME BPVC.IV-2015) service restrictions.

Assumptions1 Heat loss from the boiler is negligible. 2 Constant properties are used for both the boiler and the water. 3 The raise in temperature for the boiler and the water is equal. 4 No water flowing out of the boiler during the heating.

Properties The average specific heats are given to be 0.48 kJ/kg•K for the boiler material and 4.18 kJ/kg•K for the water. The average density of the water is 850 kg/m3.

Analysis We take the boiler and the water in it as a closed system. The energy balance in this case can be expressed as



Discussion The final temperature of the water after 30 minutes of heating at 50 kW is 224°C greater than the ASME Boiler and Pressure Vessel Code service restrictions of 120°C. Thus, the operating condition would not be in compliance with the code. A temperature control mechanism should be implemented to ensure that the water exiting the boiler stays below 120°C.