ENGR 260 Section 6'1 6'4

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ENGR 260Section 6.1 6.4
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So this wont work?
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First Law of Thermo

• What is the inadequacy of the first law of
  thermodynamics?
• It doesnt tell us if a process will take place.

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Energy Flow!
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Thermal Energy Reservoirs

• A hypothetical body with relatively large thermal
  energy capacity that can supply or absorb
  infinite amounts of heat without undergoing a
  temperature change.

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Thermal Reservoirs

• Atmosphere
• Large rivers/lakes/ocean
• Controlled furnace
• A two phase system
• Any physical body whose thermal energy capacity
  is large relative to the amount of energy it
  supplies or absorbs

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Heat Sources and Heat Sinks
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Thermal Efficiency
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Net Power Production of a Heat Engine

• Heat is transferred to a heat engine from a
  furnace at a rate of 80 MW. If the rate of waste
  heat rejection to a nearby river is 50 MW,
  determine the net power output and the thermal
  efficiency for this heat engine.

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Fuel Consumption Rate of a Car

• A car engine with a power output of 65 hp has a
  thermal efficiency of 24. Determine the fuel
  consumption rate of this car if the fuel has a
  heating value of 19,000 Btu/lbm (that is 19,000
  Btu of energy for each lbm of fuel burned).

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Can we save Qout?
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Kelvin-Planck Statement

• The Second Law of Thermodynamics
• It is impossible for any device that operates on
  a cycle to receive heat from a single reservoir
  and produce a net amount of work.

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Refrigeration
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Components of a Refrigeration

• Compressor
• Condenser
• Expansion Valve
• Evaporator

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Refrigeration efficiency

• Coefficient of Performance

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Heat Pumps

• Same cycle as refrigeration but different
  objectives
• Transfers heat from a low temperature medium to a
  high temperature medium
• Absorb hear from a low temperature source (water
  or air) and transfer it to a high temperature
  medium such as a house

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Heat Pump Efficiency

• Coefficient of Performance

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More Heat Pump Efficiency

• Most heat pumps operate at a COPHP between 2 and
  3.
• COPHP decreases when the outdoor temperature
  decreases.
• HSPF (Heating Seasonal Performance Factor )

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Air Conditioners

• Refrigerators whose refrigerated space is the
  house.
• Air condition in the summer Heat Pump in the
  winter with a reversing valve.

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Air Conditioner Efficiency

• EER Energy Efficiency Rating (Steady State)
• BTUs per Watt Hour
• Air conditioner EER between 8 and 12.
• Variable speed fans and compressors allow for
  increased efficiency.

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Refrigeration Efficiency

• 2.6 3.0 Cutting Rooms or Prep Rooms
• 2.3 2.6 Meat, Dairy, or Deli
• 1.2 1.5 Frozen Food
• 1.0 1.2 Ice Cream

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Practice

• An air conditioner removes heat steadily from a
  house at a rate of 750 kJ/min while drawing
  electric power at a rate of 6 kW.
• Determine the COP of the air conditioner
• Determine the rate of heat transfer to the
  outside air.

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More Practice

• A house heated by an electric resistance heaters
  consumed 1200 kWh of electrical energy in a
  winter month. If the house were heated by a heat
  pump with an average COP of 2.4, determine how
  much the home owner would have saved. Assume
  that electricity costs 8.5/kWh.

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Heat Engine
Heat Pump
Refrigerator