A water heater uses 100?kW to heat 60 gallons of liquid water from an initial temperature of 20°C. The average density and specific heat of the water are 970?kg/m3 and 4.18?kJ/kg•K, respectively. According to the service restrictions of the ASME Boiler and Pressure Vessel Code (ASME BPVC.IV-2015, HG-101), water heaters should not be operating at temperatures exceeding 20°C at or near the heater outlet. Determine the heating duration such that the final water temperature that would exit the water heater would be in compliance with the ASME Boiler and Pressure Vessel Code.
What will be an ideal response?
A water heater uses 100 kW to heat 60 gallon (0.2271 m3) of liquid water initially at 20°C. Determine the heating duration such that the water exiting the heater would be in compliance with the ASME Boiler and Pressure Vessel Code (ASME BPVC.IV-2015) service restrictions.
Assumptions1 Heating of the heater material is negligible (i.e. 100 kW is for heating the water only). 2 Constant properties are used for the water. 3 No water flowing out of the heater during the heating.
Properties The average density and specific heat of water are given to be 970 kg/m3 and 4.18 kJ/kg•K, respectively.
Analysis The mass of the water in the heater is
Discussion It takes about 15 minutes at 100 kW to heat 60 gallons of liquid water from 20°C to the ASME Boiler and Pressure Vessel Code service restrictions temperature of 120°C. If the heating duration is more than 15.3 minutes, then the final temperature of the water that would exit the heater would be higher than 120°C, which exceeds the code restrictions.
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