Question

3. 8 kg water in a dishwasher with a 6 kW resistance heater is to be heated up from 15°C to 65°C. 130 kJ of heat is lost to the surroundings during the heating. The specific heat capacity of water is 4.1813 kJ/kg.K. The atmosphere pressure is 100kPa, no boiling has taken place. Determine the required heating time.

Answer 1

Given data. mass of water (m) = 8 kg. Power of heater that will beat the water from 15°c to 65°C Creater 6 kwi -Loss of Heat during heating to the surrounding (Sout) = -130 47. (As going out to from the system so (-rej] Heat Capauty of water (Cp) = 4.18 13 431 ng k. Herre Onu we re ony o we have to do energy balance How Heat required for water, to increase temparature from 15°c to 65°C the Отели • mw Pш . • 8 x A-181 ( 9 - 15) 8x 4.1813850 o 1672.52 KJ As heat loss during treating to the surrounding bour - - 130 ng. so wer heat required (Bnet) a 1672.52-(-1301 (oner = 1802.52 kg.) This is the her heat reauired after beat transferring to the surrounding from for hearing water from 15% to 65°C. But Heatr power = 6 kw a 6 hile in 1. second Heater will heat bug for reauired heat time required (t) = 1802.52 =300.42 sec ie approukat og so Reauired heating time (t) = 300.42 sec or (5 min) hus) toger