REFRIGERATION, HEAT PUMP CYCLES

REFRIGERATORS AND HEAT PUMPS

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

THE REVERSED CARNOT CYCLE

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

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

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 because

Process 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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HISTORY

- In Egypt (2 century) – cooling effect - vaporization water

- 1755 - William Cullen – produced ice using vacuum pumps and phase transformation

- 1777 Walther Hermann Nerst – added to water H2SO4

- 1834.a. Jacob Perkins – the first prototype as today we use

- 1844.a. Jon Corien…air conditions…

- 1864.a. absorber effect, Littman

- ………………………

THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLEThe 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.

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

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 exitSubcooled liquid at condenser exitPressure drops in condenser and evaporator

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The Compressor

• The compressor is the heart of the system. The compressor does just what it’s name is. It compresses the low pressure refrigerant vapor from the evaporator and compresses it into a high pressure vapor.

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The Condenser

• The “Discharge Line” leaves the compressor and runs to the inlet of the condenser.

• Because the refrigerant was compressed, it is a hot high pressure vapor (as pressure goes up – temperature goes up).

• The hot vapor enters the condenser and starts to flow through the tubes.

• Cool air is blown across the out side of the finned tubes of the condenser (usually by a fan or water with a pump).

• Since the air is cooler than the refrigerant, heat jumps from the tubing to the cooler air (energy goes from hot to cold – “latent heat”).

• As the heat is removed from the refrigerant, it reaches it’s “saturated temperature” and starts to “flash” (change states), into a high pressure liquid.

• The high pressure liquid leaves the condenser through the “liquid line” and travels to the “metering device”. Sometimes running through a filter dryer first, to remove any dirt or foreign particles.

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Metering Devices

• Metering devices regulate how much liquid refrigerant enters the evaporator .

• Common used metering devices are, small thin copper tubes referred to as “cap tubes”, thermally controller diaphragm valves called “TXV’s” (thermal expansion valves) and single opening “orifices”.

• The metering device tries to maintain a preset temperature difference or “super heat”, between the inlet and outlet openings of the evaporator.

• As the metering devices regulates the amount of refrigerant going into the evaporator, the device lets small amounts of refrigerant out into the line and looses the high pressure it has behind it.

• Now we have a low pressure, cooler liquid refrigerant entering the evaporative coil (pressure went down – so temperature goes down).

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Thermal expansion Valves

• A very common type of metering device is called a TX Valve (Thermostatic Expansion Valve). This valve has the capability of controlling the refrigerant flow. If the load on the evaporator changes, the valve can respond to the change and increase or decrease the flow accordingly.

• The TXV has a sensing bulb attached to the outlet of the evaporator. This bulb senses the suction line temperature and sends a signal to the TXV allowing it to adjust the flow rate. This is important because, if not all, the refrigerant in the evaporator changes state into a gas, there could be liquid refrigerant content returning to the compressor. This can be fatal to the compressor. Liquid can not be compressed and when a compressor tries to compress a liquid, mechanical failing can happen. The compressor can suffer mechanical damage in the valves and bearings. This is called” liquid slugging”.

• Normally TXV's are set to maintain 10 degrees of superheat. That means that the gas returning to the compressor is at least 10 degrees away from the risk of having any liquid.

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The Evaporator

• The evaporator is where the heat is removed from your house , business or refrigeration box.

• Low pressure liquid leaves the metering device and enters the evaporator.

• Usually, a fan will move warm air from the conditioned space across the evaporator finned coils.

• The cooler refrigerant in the evaporator tubes, absorb the warm room air. The change of temperature causes the refrigerant to “flash” or “boil”, and changes from a low pressure liquid to a low pressure cold vapor.

• The low pressure vapor is pulled into the compressor and the cycle starts over.

• The amount of heat added to the liquid to make it saturated and change states is called “Super Heat”.

• One way to charge a system with refrigerant is by super heat.

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Refrigerant

A liquid that has a low boiling point.

´There are several refrigerant manufacturers.

Heat pumps still use R22 refrigerants. R22 performs well over

the range of temperatures that heat pumps operate at.

R22 is known as a hydrochlorofluorocarbon (HCFC) refrigerant

and has an ozone depletion (ODP) factor of 0.05.

Many heat pumps today use R-407C or R-410A, which are hydrofluorocarbons (HFC). Both R-407C and R-410A have zero ozone depletion potential (ODP), and slightly lower global warming potential (GWP) in the case of R-407C, than R-22. R410A has a slightly higher GWP than R22.

Performance (heating capacity and efficiency) is about the same with R-407C and about 4% better with R410A compared to R-22.

R-22 will be phased out for new equipment by January 1, 2010.

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 (it is toxic).• 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.

• 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.

HEAT PUMP SYSTEMS

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.

INNOVATIVE VAPOR-COMPRESSION REFRIGERATION SYSTEMS

• The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and it is adequate for most refrigeration applications.

• The ordinary vapor-compression refrigeration systems are simple, inexpensive, reliable, and practically maintenance-free.

• However, for large industrial applications efficiency, not simplicity, is the major concern.

• Also, for some applications the simple vapor-compression refrigeration cycle is inadequate and needs to be modified.

• For moderately and very low temperature applications some innovative refrigeration systems are used. The following cycles are generally employed:

• Cascade refrigeration systems

• Multistage compression refrigeration systems

• Multipurpose refrigeration systems with a single compressor

• Liquefaction of gases

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Gas for Heat Pumps

• Heat pumps fired by natural gas have been commercially developed.

• One type uses the absorption cycle, where the energy for refrigerant compression is provided by a gas burner.

• Another variation is the engine-driven heat pump cycle. Here a natural gas engine is used to drive the compressor. During operation, heat is recovered from the engine jacket cooling water and engine exhaust.

Gas heat pumps are less common than electric heat pumps.

• Performance compared to electric heat pumps is lower, with lower COPs for both absorption and engine-driven units than for conventional electric heat pumps.

• They promise to reduce global warming through more efficient conversion of natural gas and reduced emissions from electric power plants as they do not use electricity to drive the heat pump.

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Gas engine driver

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District heating and heat pump

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