An iron boiler of mass 150 kg contains 590 kg of water at 12 degrees Celcius. A heater supplies energy at the rate of 9.7×104 kJ/h.a) How long does it take for the water to reach the boiling point?b) How long does it take for the water to all have changed to steam (including the time to reach the boiling point)?
Let's transfer some heat!!! Concepts: heat is in constant flux. It moves in the direction of lower temperature. In order to heat up the entire system (iron boiler and water) to the point at which each additional kilojoule into the system generates a unit mass of steam, you'll have to heat up the metal and water to the saturation boiling point. In Celsius, that's 100 degrees. Since you didn't mention it in the problem, are we neglecting to calculate the required energy to heat up the boiler to the same temperature as the boiling point of water? I'll have to assume both cases and give you two different times. You need heat capacities of the iron and the water: iron = 0.45 J / g / K, and water is 4.2 J / g / K You also need a heat rate. You're given that. Okay so let's take the iron boiler and the water separate and calculate two different duties. Heating up the metal to the boiling point of water: 150,000 grams iron * 0.45 J / g / K * ( 100 -12 K) = 5,940,000 Joules 590,000 grams water * 4.2 J / g / K * (100 - 12 K) = 218,064,000 Joules What's going to happen is that the heater will heat up the water to 100 degrees and at the same time the metal. Both bodies will have to be at 100 degrees before steam is being produced. Now: you need time; you have the rate; divide the duties by the rate to get time. 5,940 kilojoules / 97,000 kJ / h = 0.061 hours 218,064 kilojoules / 97,000 kJ / h = 2.24 hours Roughly 2 hours and 19 minutes to make steam!
In the UK washing machines often have both hot and cold feeds- thats for our USA cousins. You are right in thinking that the machine will heat the water if that coming in is not hot enough. However, if you only have cold feed available you must use a T or Y junction and feed both machine inlets with cold water. For some programmes the machine will open the hot feed only as it is going to do a hot wash and then adjust the water temperature if not hot enough- if no connection it will not fill. For cooler washes it will open the cold feed only and heat the water as there is a good chance your hot feed is too warm. The junctions are available at all DIY outlets. The heating is done by a mini immersion heater fitted below the drum.
