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12. Heat Exchangers Chemical engineering 170
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Explain the different mechanisms of heat transfer:
Review: Heat Transfer Explain the different mechanisms of heat transfer: Conduction Convection Radiation
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Review: energy balances
Where would you use each equation? π = π C P Ξπ π = π Ξ π»
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Heat exchangers In industrial settings, we transfer heat using a heat exchanger.
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Heat Duty Heat exchangers have a hot side and a cold side
πΈ π
πππ : heat that flows from one side or the other
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Finding heat duty In an oil refinery, an oil stream is heated to a higher temperature using a heat exchanger with steam being condensed to water as the heat source. You have complete information about both streams. Describe two different ways of finding the heat duty (two different equations you could use).
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Finding heat duty Oil (Cold Stream) Water (Hot Steam)
In an oil refinery, an oil stream is heated to a higher temperature using a heat exchanger with steam being condensed to water as the heat source. You have complete information about both streams. Describe two different ways of finding the heat duty (two different equations you could use). Oil (Cold Stream) Water (Hot Steam) Mass flow rate: 960 π π π /πππ Heat capacity: 0.74 π΅ππ πππ Β°πΉ Inlet temperature: 35 Β°πΉ Outlet temperature: 110 Β°πΉ Mass flow rate: 57.6 π π π /πππ Heat of vaporization: 925 π΅ππ πππ Vaporization temperature: 35 Β°πΉ
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Sizing heat exchangers
π ππ’π‘π¦ = π π π΄Ξ π ππ£π Overall heat transfer coefficient Heat transfer area Temperature difference between hot and cold streams (averaged)
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Typical units π ππ’π‘π¦ = π π π΄Ξ π ππ£π πΎ ? π π π² Β°π π©π»πΌ ππ π π π
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Typical units π ππ’π‘π¦ = π π π΄Ξ π ππ£π πΎ π π π² π©π»πΌ ππ π π π Β°π
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Average temperature difference
Why do we need to average the temperature difference? π» πππ,ππ π» ππππ
,πππ π» ππππ
,ππ π» πππ,πππ 1 2
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Log Mean Temperature difference
For a simple heat exchanger (the only kind weβll be using): Ξ π πππππππ = Ξ π 1 βΞ π 2 ln Ξ π 1 Ξ π 2 Ξ π 1 = π βππ‘,ππ’π‘ β π ππππ,ππ Ξ π 2 = π βππ‘,ππ β π ππππ,ππ’π‘
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Finding heat duty A chemical process creates a very hot waste stream. We will use a stream of cold water to cool it before it can be released. We have the following information about the two streams:
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Finding heat duty Waste Stream (Hot) Cooling Water (Cold)
A chemical process creates a very hot waste stream. We will use a stream of cold water to cool it before it can be released. We have the following information about the two streams: Waste Stream (Hot) Cooling Water (Cold) Inlet temperature: 50 Β°πΆ Outlet temperature: 30 Β°πΆ Heat capacity: 2 ππ½ ππ Β°πΆ Density: 10 ππ π 3 Flow rate: 0.5 π 3 /π Inlet temperature: 20 Β°πΆ Outlet temperature: 25 Β°πΆ Heat capacity: 4.17 ππ½ ππ Β°πΆ Density: 997 ππ π 3 We also know that for this heat exchanger, π π =284 π π 2 Β°πΆ . Find (a) the exchangerβs heat duty, (b) Ξ π πππππππ , (c) the required heat transfer area.
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