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5-1 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e Chapter 5 THERMOSTATS
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5-2 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e Functions of a thermostat 1.To regulate the temperature of a substance or space by controlling the flow of gas to the burner. 2.To add to the convenience, utility, economy and, in some cases, safety of the appliance.
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Types of thermostats 1.Rod and tube thermostats 2.Liquid expansion thermostats 3.Vapour pressure thermostats 4.Bimetal thermostats 5-3 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Thermostat function Snap acting Gas flows at full rate then shuts off when temperature is reached. Cooling may take place before thermostat comes back on. Modulating Heat gradually increases and will begin to reduce gas flow as temperature is reached. Gas flow will then maintain temperature. 5-4 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Rod and tube thermostats (1) Operation Rod and tube has different coefficient of expansion. Outer tube and inner rod fixed at one end. 5-5 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Rod and tube thermostats (2) 5-6 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Liquid expansion thermostats (1) Liquids expand when heated and contract when cooled. A bellows is opened and closed by this expansion and contraction. By attaching a gas valve to the bellows gas flow can be controlled. 5-7 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Liquid expansion thermostats (2) 5-8 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Vapour pressure thermostats Sensing bulb of thermostat is filled with gas. When temperature increases, the gas pressure in the bulb increases. This increases the bellows acting on the gas valve. 5-9 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Bimetal thermostats If two different metals are bonded together and an increase in temperature takes place, the expansion will cause the bimetal strip to bend upwards or downwards depending on the location of the metal with the higher expansion rate. Bimetal thermostats are: (i) used to control the fan of a duct air heater; or (ii) used as room thermostats. 5-10 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Bimetal thermostats—central heaters 5-11 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Bimetal room thermostats 5-12 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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Thermostats—Summary Purpose: To regulate the temperature of a substance or space by controlling the flow of gas to the burner. Thermostat types 1. Rod and tube 2. Liquid expansion 3. Vapour pressure 4. Bimetal Thermostat function –Snap acting –Modulating 5-13 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Puffett, Hossack, Stone, Burn & Miles, Gasfitting 2e
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