A boiler supplies hot water to a commercial dishwasher through a pipe with a surface temperature of 50°C. The hot water exits the boiler at 95°C, and it is transported in a pipe that has an outside diameter of 20 mm. The distance between the boiler and the dishwasher is 20 m. The section of the pipe between the boiler and the dishwater is exposed to convection with a heat transfer coefficient of 100 W/m2•K at an ambient temperature of 20°C. The hot water flows steadily in the pipe at 60 g/s, and its average specific heat is 4.20 kJ/kg•K. The National Sanitation Foundation standard for commercial warewashing equipment (ANSI/NSF 3) requires the final rinse water temperature to be at least 82°C. Under these conditions, does the hot water entering the dishwasher meet the ANSI/NSF 3
standard? If not, discuss some possible ways to increase the water temperature entering the dishwasher.
What will be an ideal response?
A boiler supplies hot water to a dishwasher through a pipe at 60 g/s. The pipe dimensions are given. The water exits the boiler at 95°C. The pipe section between the boiler and the dishwasher is exposed to convection. The water temperature entering the dishwasher is to be determined whether it meets the ANSI/NSF 3 standard.
Assumptions1Constant properties are used for the water.2Steady operating conditions. 3 Surface temperature of the pipe is uniform.
Properties The average specific heat of water is given to be 4.20 kJ/kg•K.
AnalysisFrom energy balance, the rate of heat loss from the pipe is equal to the heat transfer rate by convection on the pipe surface:
Solving for the water temperature entering the dishwasher T2, we have
Discussion The hot water entering the dishwasher is 2°C lower than the temperature required by the ANSI/NSF 3 standard. To increase the water temperature entering the dishwasher, one or combination of the following steps can be taken: (a) add insulation on the pipe wall to reduce the heat loss from the pipe surface; (b) increase the water mass flow rate; (c) reduce the pipe distance between the boiler and the dishwasher; and (d) increase the water temperature coming out from the boiler.
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