Electrical Heating SAMPLE. Learner Workbook. Version 1. Training and Education Support. Industry Skills Unit. Meadowbank. Product Code: 5639

Transcription

1 Learner Workbook Version 1 Training and Education Support Industry Skills Unit Meadowbank Product Code: 5639

2 Contents Introduction Heat Energy... 7 Practical exercise: Heating efficiency... 9 Review questions Heat Control Practical exercise: Operation of a thermostat Review questions Water Heaters Practical exercise: Over-temperature cut-out thermostat Review questions Appliances (Cooking) Review questions Space Heating Review questions Industrial Heating Review questions Faults in Heating Devices and Circuits Exercises Review questions Sample test Theory test Theory test Answers Review questions Theory test Theory test Appendix - Summary of Technical Information TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

3 1. Heat Energy Purpose In this topic you will learn about the concepts of heat energy, temperature, heat capacity and heat transfer. Objectives At the end of this topic you should be able to: Define the term heat energy and its unit Define temperature and the various temperature scales Describe the difference between heat and temperature Calculate the heat energy (in joules) needed to cause a temperature rise in a heating process State the specific heat capacities of known materials Define thermal conductivity and its dependent factors Describe and name the methods of heat transfer Define the term thermal stability Calculate the heat energy (in kwh) in a simple heating process Measure the temperature change and the time taken in the heating of a given mass Calculate the heat energy input and heat energy output for the above process. Technical information You will find the information to undertake this topic in the following references. At least one reference text should be used. References Jenneson, J.R., 2010, Electrical Principles for the Electrical Trades, 6th Ed., McGraw-Hill, Sydney. Hampson, J., 2011, Electrical Trade Principles - A Practical Approach, 2nd Ed., Pearson Education, Sydney. Phillips, P., 2012, Electrical Principles, 2nd Ed., Cengage Learning, Melbourne. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 7 of 116

4 Exercises 1. How many joules of heat energy are developed when 1 litre of water is raised in temperature from 21 C to boiling point (100 C). The specific heat of water is 4180 joules/kg.k? If the time taken for the water to reach boiling point is 5 minutes, What is the power rating of the heating element? (Assume the heating device is 100% efficient.) 2. How long in minutes, would it take a 1350 watt, 240 volt aluminium electric frypan to obtain a pre-set temperature of 200 C from an ambient temperature of 20 C. The aluminium electric frypan has a mass of 1.5 kg. The specific heat of aluminium is 900 Joules/kg.K 3. A thermostatic controlled mains pressure hot water system operates for 6 hours over an off peak period from 10 p.m. to 6 a.m. using 28.8 units of electricity. One unit of electricity is one kilowatt hour. What is the kw rating of the heating element? Page 8 of 116 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

5 Practical exercise: Heating efficiency Task To determine the efficiency of a simple water heater from measured results. Objectives At the completion of this exercise you should be able to: Calculate the electrical energy input to a water heater using measured values of voltage, current and time. Calculate the quantity of heat energy output of a water heater using measured values of temperature and the equation: H = mc )t. Calculate the losses and efficiency of a water heater. Risk Assessment Identify any hazards, list the supervision level (D, G or B), list the risk class (A, B or C) and list control measures required in the table below: Hazard Identification Supervision Level Risk Class Control Measures Equipment Your teacher will provide you with the specifications of the equipment to be used. D.C power supply Specifications Analog ammeter Type Range Analog voltmeter Type Range Single pole switch Rating Resistor: Ω W TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 9 of 116

6 Thermometer Range Beaker ml Stirring stick Stop watch Connection leads Procedure Figure Arrange the equipment on the bench in a neat and logical manner and connect the circuit as shown in Figure The resistor should be suspended at about 5 mm above the bottom of the beaker and towards one side of the beaker. 3. The thermometer should be positioned in the lower third of the beaker as shown in Figure 1.1. Ensure that there is adequate clearance between thermometer and resistor. 4. Fill the beaker with tap water to the level specified by your teacher. 5. Throughout the practical, gently swirl the water with the stirrer, especially just before taking temperature readings. Leave the stirrer in the beaker at all times. Page 10 of 116 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

7 Specific heat capacity for water is Ambient temperature for water is 4180 J/kg.C t 1 = C. Table 1 Time t seconds Voltage V volts Current I amperes Water temp. t 1 C Temp. change t 2 - t 1 Heat energy input Joules Heat energy output Joules Efficiency %. t 1 = 6. Read the cold temperature (t 1 ) and record the value in Table Note the time start, switch the supply on, close the circuit switch and quickly adjust the current to amperes. Record the starting time in Table 1. Your teacher will specify the current value. 8. Record the circuit voltage in Table At time intervals specified by your teacher, record the temperature in Table 1. Note: 1. Remember to stir water prior to taking each reading. 2. Ensure that the current remains constant throughout the practical. 10. When the final temperature reading has been taken, open the circuit switch and turn off the power supply. 11. Disconnect and return all the equipment to its proper place. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 11 of 116

8 Observations 1. Calculate the temperature change for each time period, using t 2 - t 1 2. Calculate the heat energy input for each time period, using Q = VI.t. Record the values in Table Calculate the heat energy output for each time period, using Q = mc(t 2 - t 1 ) Record the values in Table Calculate the efficiency for each time period, using Efficiency = Record the values in Table 1. output energy x input energy Page 12 of 116 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012

9 5. On each graph paper provided show the relationship between water temperature, heat energy input, heat energy output and efficiency against time and answer the following questions Graph 1: Showing the relationship between water temperature and the time. (a) Heat energy input in Joules Time in seconds Examination of Graph 1, shows the change in the water temperature against time, can the curve be described as linear or non-linear. (b) From Graph 1, what is the change in temperature for the heating time? TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2012 Page 13 of 116