The efficiency of refrigeration systems and heat pumps is denoted by its Coefficient Of Performance (COP). The COP is determined by the ratio between energy usage of the compressor and the amount of useful cooling at the evaporator (for a refrigeration instalation) or useful heat extracted from the condensor (for a heat pump). A high COP value represents a high efficiency. Most of the electric energy needed to drive the compressor is released to the refrigerant as heat. Therefore more heat is available at the condensor than is extracted at the evaporator of the heat pump. For a heat pump a COP value of 4 means that the addition of 1 kW of electric energy is needed to have a release of 4 kW of heat at the condensor. At the evaporator side 3,0-3,5 kW of heat is extracted. The additional heat is generated by the compressor. On the other hand: For a refrigeration system a COP of 4 indicates that 1 kW of electricity is needed for a evaporator to extract 4 kW of heat. Due to this important difference in COP definition, for a heat pump one often speaks of COPh.

In this abbreviation 'h' means heating. The efficiency of a heat pump, COPh, depends on several factors. Especially the temperature difference between waste heat source and potential user is an important factor. The temperature difference between condensation and evaporation temperature mainly determines the efficiency: the smaller the difference, the higher the COPh. The figure on the left shows the influence of this temperature difference on the COPh value. These values are based on figures from a Grasso 65HP compressor with the refrigerant Ammonia. The figure shows an increase in COPh with an increasing evaporation temperature. Futhermore it shows a decrease in COPh with a decreasing condensation temperature. In general the COPh decreases with an increase in temperature difference between condensation and evaporation. The figure below gives an indication of the dependence of the COPh of an Ammonia heat pump as a function of this temperature difference. Another important factor that influences efficiency is the applied refrigerant.

Ammonia, for example, is a very efficient refrigerant with a COPh of 6 for a evaportion temperature of 30 °C and condensation temperature of 70 °C. These same conditions only give a COPh of 4,5 for refrigerant R134A. Other factors that will effect the efficiency of a heat pump are system controls, efficiency of pheripheral equipement like fans, pumps, etc. The theoretical maximum efficiency of a heat pump is described by the Carnot-efficiency: The equation shows that the Carnot-efficiency depends on the condensation and evaporation temperature. With an ideal compression cycle without losses it is possible to achieve the Carnot efficiency. However, in practice there are a lot of parameters that have a negative influence on the efficiency. Therefore the real COPh is given by the product of the Carnot efficiency and the system efficiency: The system efficiency is usually 50% to 70%. With a transcritical heat pump the Carnot-efficiency can not be used, because there is no condensation temperature, but a temperature range in the gas cooler.

The theoretic maximum efficiency of a transcritical heat pump is described by the Lorentz efficiency.5 ton ac unit 14 seer Tm is the mean temperature in the gas cooler. wall air conditioning units frigidaireThis temperature is calculated from the temperature at the inlet and the outlet of the gas cooler:window ac units 15000 btu Similar to Carnot, the Lorentz efficiency will not be reached in practice due to all kind of losses. To determine the real COP, a system efficiency must be taken into account: Login for UK Access Management Federation Institutions (Shibboleth) USING SI UNITSSOME BASE AND DERIVED SI UNITSCONVERSION BETWEEN SI AND I-P UNITSCONVERSIONS OF TEMPERATURE, ENTHALPY, AND ENTROPYSOME IMPORTANT SI CONSTANTSREFRIGERANT PROPERTIESTWO KINDS OF PRESSURE: GAUGE AND ABSOLUTESATURATION TEMPERATURES AND PRESSURESDENSITY AND SPECIFIC VOLUMEENTHALPYSUPERHEATED VAPOR AND SUBCOOLED LIQUIDTHE PRESSURE-ENTHALPY DIAGRAMSATURATED LIQUID AND VAPOR LINESLINES OF CONSTANT TEMPERATURESPECIFIC VOLUMEENTROPYTHE IDEAL REFRIGERATION CYCLE-THE CARNOT CYCLEACHIEVING THE CARNOT CYCLE WITH A REAL REFRIGERANTEFFICIENCY OF A REFRIGERATION CYCLE—THE COEFFICIENT OF PERFORMANCECONDITIONS FOR HIGH COP OF THE CARNOT CYCLESTEADY-FLOW ENERGY EQUATIONSTATE OF REFRIGERANT EXPRESSED

BY QUALITYANALYSIS OF A CARNOT CYCLE USING REFRIGERANT ENTHALPIESREFRIGERATION CAPACITY EXPRESSED IN I-P UNITS—TONS OF REFRIGERATIONDRY VERSUS WET COMPRESSIONTHROTTLING VALVE VERSUS AN EXPANSION ENGINETHE STANDARD VAPOR-COMPRESSION CYCLEHORSEPOWER PER TONVARIATIONS IN THE STANDARD VAPOR-COMPRESSION CYCLEUSEFULNESS OF THE THERMODYNAMIC FOUNDATION Please sign in to view the rest of this entry. REFRIGERANT PROPERTIES, REFRIGERATION CYCLES AND SI UNITS10109001011100REFRIGERANT PROPERTIES, REFRIGERATION CYCLES AND SI UNITSUSING SI UNITS The Metric Conversion Act of the United States, signed into law in December 1975, encouraged the adoption of the SI (Systeme International d’Unites) but permitted the conversion to be voluntary. Nations that had traditionally used the inch-pound (I-P) system, such as Canada, the United Kingdom, and Australia, made the change to SI mandatory a… REFRIGERANT PROPERTIES, REFRIGERATION CYCLES AND SI UNITS, Chapter

Refrigeration technology is commonly used in domestic and industrial applications. This video gives a detailed and logical introduction to the workings of refrigerators using the vapor compression cycle. An elaborated webpage version of the video is given below. The basic principle of refrigeration is simple. You simply pass a colder liquid continuously around the object that is to be cooled. This will take heat from the object. In the example shown, a cold liquid is passed over an apple, which is to be cooled. Due to the temperature difference, the apple loses heat to the refrigerant liquid. The refrigerant in turn is heated due to heat absorption from the apple. It is clear that, if we can produce cold liquid refrigerant continuously, we can achieve continuous refrigeration. This simple fact forms the core of the refrigeration technology. We will next see how this is achieved. An inside view of a refrigerator is shown. It has 4 main components: compressor, condenser, evaporator, and throttling device.

Of these components, the throttling device is the one that is responsible for the production of the cold liquid. So we will first analyze the throttling device in a detailed way and move on to the other components. The throttling device obstructs the flow of liquid; cold liquid is produced with the help of this device. In this case, the throttling device is a capillary tube. The capillary tube has an approximate length of 2 m and an inside diameter of around 0.6 mm, so it offers considerable resistance to the flow. For effective throttling at the inlet, the refrigerant should be a high-pressure liquid. The throttling device restricts the flow, which causes a tremendous pressure drop. Due to the drop in pressure, the boiling point of the refrigerant is lowered, and it starts to evaporate. The heat required for evaporation comes from the refrigerant itself, so it loses heat, and its temperature drops. If you check the temperature across the throttling device, you will notice this drop.

It is wrong to say that the throttling is a process. We know only the end points of throttling, that is, the states before and after throttling. We don’t know the states in between, since this is a highly irreversible change. So it would be correct to call throttling a phenomenon rather than a process. The next phase is simple: this cold liquid is passed over the body that has to be cooled. As a result, the refrigerant absorbs the heat. During the heat absorption process, the refrigerant further evaporates and transforms into pure vapor. A proper heat exchanger is required to carry the cold refrigerant over the body. This heat exchanger is known as an evaporator. So we have produced the required refrigeration effect. If we can return this low-pressure vapor refrigerant to the state before the throttling process (that is the high-pressure liquid state), we will be able to repeat this process. So first step, let’s raise the pressure. A compressor is introduced for this purpose.

The compressor will raise the pressure back to its initial level. But since it is compressing gas, along with pressure, temperature will also be increased. Now the refrigerant is a high-pressure vapor. To convert it to the liquid state, we must introduce another heat exchanger. This heat exchanger is fitted outside the refrigerator, and the refrigerant temperature is higher than atmospheric temperature. So heat will dissipate to the surroundings. The vapor will be condensed to liquid, and the temperature will return to a normal level. So the refrigerant is back to its initial state again: a high-pressure liquid. We can repeat this cycle over and over for continuous refrigeration. This cycle is known as the vapor compression cycle. Refrigeration technology based on the vapor compression cycle is the most commonly used one in domestic and industrial applications. You can find more details on refrigerator components here. Evaporators and condensers have fins attached to them.

The fins increase the surface area available for convective heat transfer and thus will significantly enhance heat transfer. Since the evaporator is cooling the surrounding air, it is common that water will condense on it, forming frost. The frost will act as an insulator between the evaporator heat exchanger and the surrounding air. Thus it will reduce the effectiveness of the heat removal process. Frequent removal of frost is required to enhance the heat transfer. An automatic defrosting mechanism is employed in all modern refrigerators. Apart from raising the pressure, the compressor also helps maintain the flow in the refrigerant circuit. Usually, a hermetically sealed reciprocating type compressor is used for this purpose. You might have noticed that, your household refrigerator consumes a lots of electricity compared to the other devices. In a vapor compression cycle, we have to compress the gas; compressing the gas and raising pressure is a highly energy intensive affair.