ELECTRICAL HEATING –PROBLEMS IN POWER FREQUENCY HEATING
PROBLEMS – 01 - An electric bread toaster consists of two resistance elements each of 200 ohms. Calculate the power drawn from 230 V a.c supply, when the filaments are connected in (i) parallel and (ii) series.
PROBLEM – 02 – A cubic water tank has surface area of 8m^2 and is filled to 90% capacity 5 times daily. The water is heated from 20ºC to 75ºC. The losses per square metre of tank surface per 1ºC temperature difference is 6.5 watts. Find the loading in kW and the efficiency of the tank. Assume – specific heat of water – 4186 J/kg/ºC and one kWh – 3.6 MJ.
PROBLEM – 03 – A cylindrical water tank has radius of 0.25 m and length of 1.5 m and is filled to 93% 6 times daily. The water is heated from 20ºC to 75ºC. The losses per square metre of tank surface per 1ºC temperature difference is 6.3 watts. Find the loading in kW and the efficiency of the tank. Assume – specific heat of water – 4186 J/kg/ºC and one kWh – 3.6 MJ.
PROBLEM – 04 – The resistance elements of each phase of a 3-phase delta-connected resistance oven consists of two groups of elements in parallel. If the operating temperature of the elements when supplied at normal voltage under these conditions is 1150 degree centigrade. Find the temperature of the oven when the two groups are connected in series then in delta and the two groups are connected in series and then in star.
PROBLEM – 02 – A cubic water tank has surface area of 8m^2 and is filled to 90% capacity 5 times daily. The water is heated from 20ºC to 75ºC. The losses per square metre of tank surface per 1ºC temperature difference is 6.5 watts. Find the loading in kW and the efficiency of the tank. Assume – specific heat of water – 4186 J/kg/ºC and one kWh – 3.6 MJ.
PROBLEM – 03 – A cylindrical water tank has radius of 0.25 m and length of 1.5 m and is filled to 93% 6 times daily. The water is heated from 20ºC to 75ºC. The losses per square metre of tank surface per 1ºC temperature difference is 6.3 watts. Find the loading in kW and the efficiency of the tank. Assume – specific heat of water – 4186 J/kg/ºC and one kWh – 3.6 MJ.
PROBLEM – 04 – The resistance elements of each phase of a 3-phase delta-connected resistance oven consists of two groups of elements in parallel. If the operating temperature of the elements when supplied at normal voltage under these conditions is 1150 degree centigrade. Find the temperature of the oven when the two groups are connected in series then in delta and the two groups are connected in series and then in star.