Chapter11

# Chapter11 - Chapter 11 Refrigeration Cycles Study Guide in...

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Chapter 11 Refrigeration Cycles Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach , 5th edition by Yunus A. Çengel and Michael A. Boles

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2 The vapor compression refrigeration cycle is a common method for transferring heat from a low temperature to a high temperature. The above figure shows the objectives of refrigerators and heat pumps. The purpose of a refrigerator is the removal of heat, called the cooling load, from a low- temperature medium. The purpose of a heat pump is the transfer of heat to a high- temperature medium, called the heating load. When we are interested in the heat energy removed from a low-temperature space, the device is called a refrigerator. When we are interested in the heat energy supplied to the high-temperature space, the device is called a heat pump. In general, the term heat pump is used to describe the cycle as heat energy is removed from the low-temperature space and rejected to the high-temperature space.
3 The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as COP Q W COP Q W R L net in HP H net in = = = = = = Desired output Required input Cooling effect Work input Desired output Required input Heating effect Work input , , Both COP R and COP HP can be larger than 1. Under the same operating conditions, the COPs are related by COP COP HP R = + 1 Can you show this to be true? Refrigerators, air conditioners, and heat pumps are rated with a SEER number or seasonal adjusted energy efficiency ratio. The SEER is defined as the Btu/hr of heat transferred per watt of work energy input. The Btu is the British thermal unit and is equivalent to 778 ft-lbf of work (1 W = 3.4122 Btu/hr). An EER of 10 yields a COP of 2.9. Refrigeration systems are also rated in terms of tons of refrigeration. One ton of refrigeration is equivalent to 12,000 Btu/hr or 211 kJ/min. How did the term “ton of cooling” originate?

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4 Reversed Carnot Refrigerator and Heat Pump Shown below are the cyclic refrigeration device operating between two constant
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Chapter11 - Chapter 11 Refrigeration Cycles Study Guide in...

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