thermodynamics 2_chapter 5 part1

Chapter 5…Part 1REFRIGERATION CYCLES

(Kitar Penyejuk)

Duty, Honor and Integrity

Thermodynamics II (EMM3503)Chapter 5- REFRIGERATION CYCLES

Delivered by: Ir Khalid

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Objectives• Introduce the concepts of refrigerators and heat pumps and the

measure of their performance.

• Analyze the ideal vapor-compression refrigeration cycle.

• Analyze the actual vapor-compression refrigeration cycle.

• Review the factors involved in selecting the right refrigerant for an application.

• Discuss the operation of refrigeration and heat pump systems.

• Evaluate the performance of innovative vapor-compression refrigeration systems.

• Analyze gas refrigeration systems.

• Introduce the concepts of absorption-refrigeration systems.



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REFRIGERATORS AND HEAT PUMPSDuty, Honor and

Integrity


The Purpose of Refrigeration

The fundamental reason for having a refrigerator is to keep food cold. Cold temperatures help food stay fresh longer. The basic idea behind refrigeration is to slow down the activity of bacteria (which all food contains) so that it takes longer for the bacteria to spoil the food.

The basic mechanism of a refrigerator works like this:

1.The compressor compresses the refrigerant gas. This raises the refrigerant's pressure and temperature (orange), so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization. 2.As it cools, the refrigerant condenses into liquid form (purple) and flows through the expansion valve. 3.When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates (light blue). In evaporating, it absorbs heat, making it cold. 4.The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.

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A heat pump is a device that uses a small amount of energy to move heat from one location to another. Heat pumps are usually used to pull heat out of the air or ground to heat a home or office building, or they can be switched into reverse to cool a building.

REFRIGERATORS AND HEAT PUMPS…cont.Duty, Honor and

Integrity


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REFRIGERATORS AND HEAT PUMPS…cont.

The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium.

The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.

for fixed values of QL and QH



Note: The capacity of a refrigeration system that can freeze 1 ton of liquid water at OC in 24 hours is said to 1 ton.1 ton of refrigeration is equivalent to 211 kJ/min

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THE REVERSED CARNOT CYCLE

Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.

The Carnot cycle is an idealized thermodynamic cycle that describes a perfect heat engine. In the Refrigeration system we need cooling effect, so it has to operate in opposite nature to produce the cooling effect. So we run the carnot cycle reversely in the refrigeration system. So we call the Refrigeration cycle called as REVERSED CARNOT CYCLE.The reversed Carnot cycle is the most efficient refrigeration cycle operating between TL and TH.However, it is not a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical becauseProcess 2-3 involves the compression of a liquid–vapor mixture, which requires a compressor that will handle two phases, and process 4-1 involves the expansion of high-moisture-content refrigerant in a turbine.



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THE REVERSED CARNOT CYCLE…cont

Both COPs increase as the difference between the two temperature decreases, that is, as TL rises or TH falls.



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THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE

The vapor-compression refrigeration cycle is the ideal model for refrigeration systems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device.

Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.

This is the most widely used cycle for refrigerators, A-C systems, and heat pumps.



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An ordinary household refrigerator.

The P-h diagram of an ideal vapor-compression refrigeration cycle.

The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems. Replacing the expansion valve by a turbine is not practical since the added benefits cannot justify the added cost and complexity.

Steady-flow energy balance



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ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

Schematic and T-s diagram for the actual vapor-compression refrigeration cycle.

An actual vapor-compression refrigeration cycle differs from the ideal one in several ways, owing mostly to the irreversibilities that occur in various components, mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings. The COP decreases as a result of irreversibilities.

DIFFERENCESNon-isentropic compressionSuperheated vapor at evaporator exit Subcooled liquid at condenser exitPressure drops in condenser and evaporator



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SELECTING THE RIGHT REFRIGERANT• Several refrigerants may be used in refrigeration systems such as

chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point).

• R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market.

• The industrial and heavy-commercial sectors use ammonia due to low cost, higher COPs, higher heat transfer coefficients and no effect to ozone layer. The major drawback is toxicity which makes it unsuitable for domestic used.

• R-11 is used in large-capacity water chillers serving A-C systems in buildings.

• R-134a (replaced R-12, which damages ozone layer) is used in domestic refrigerators and freezers, as well as automotive air conditioners.

• R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.



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SELECTING THE RIGHT REFRIGERANT…cont.

• R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.

• R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.

• CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the ozone layer. Refrigerants that are friendly to the ozone layer have been developed.

• Two important parameters that need to be considered in the selection of a refrigerant are the temperatures of the two media (the refrigerated space and the environment) with which the refrigerant exchanges heat.



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HEAT PUMP SYSTEMSDuty, Honor and

Integrity


A heat pump is a machine or device that moves heat from one location (the 'source') at a lower temperature to another location (the 'sink' or 'heat sink') at a higher temperature using mechanical work or a high-temperature heat source. The difference between a heat pump and a normal air conditioner is that a heat pump can be used to heat a home as well as cool it.Heat pumps are a unique kind of heating system, because they can do the work of both a furnace and an air conditioner.

http://home.howstuffworks.com/home-improvement/heating-and-cooling/heat-pump1.htm

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HEAT PUMP SYSTEMS…cont

A heat pump can be used to heat a house in winter and to cool it in summer.

The most common energy source for heat pumps is atmospheric air (air-to- air systems). Water-source systems usually use well water and ground-source (geothermal) heat pumps use earth as the energy source. They typically have higher COPs but are more complex and more expensive to install.

Both the capacity and the efficiency of a heat pump fall significantly at low temperatures. Therefore, most air-source heat pumps require a supplementary heating system such as electric resistance heaters or a gas furnace.

Heat pumps are most competitive in areas that have a large cooling load during the cooling season and a relatively small heating load during the heating season. In these areas, the heat pump can meet the entire cooling and heating needs of residential or commercial buildings.



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HEAT PUMP SYSTEMS…cont.Duty, Honor and

Integrity


Heat Pumps are usually characterized by a coefficient of performancewhich is the number of units of energy delivered to the hot reservoir per unit work input.

thermodynamics 2_chapter 5 part1

Documents

heat ql

heat pumpscont

heat pumpsduty

heat qh

transfer of heat

heat of pressurization

ton of refrigeration

heat pump systems