Title: Prepare report on refri- Geration.
Description:
The term refrigeration means cooling a space, substance or system to lower and/or maintain its temperature below the ambient one. In other words, refrigeration is artificial cooling. Energy in the form of heat is removed from a low temperature reservoir and transferred to a high temperature reservoir.
Outcomes Refrigeration:
Definition : the process of removing heat from an enclosed space or from a substance for the purpose of lowering the temperature.
4 Type of refrigeration system:
1) Ice refrigeration
Ice has been used for cooling for a very long time. The use of ice as a cooling medium offers several advantages over mechanical refrigeration: It has a large cooling capacity for a given weight and involves little or no complication in the design and operation of the storage space
In this method the ordinary ice is used for keeping the space at temperature below the surrounding temperature. The temperature of ice is considered to be 0 degree Celsius hence it can be used to maintain the temperatures of about 5 to 10 degree Celsius. To use the ice for refrigerating effect a closed and insulated chamber is required. On one side of the chamber ice is kept while on the other side there is a space which is to be cooled where some material to be cooled can be placed. If the temperature below 0 degree Celsius is required, then the mixture of ice and salt is used. This method of cooling is still being used for cooling the cold drinks, keeping the water chilled in thermos, etc
2) Evaporative refrigeration
The main difference between an evaporative air cooling system and AC is that a cooler uses the hot air in the room and water in order to produce cool air leading to less energy consumption as it only has two major components which need powering; a water pump and a fan. A constant supply of water is needed in order to keep the pads wet and cool the air.
The cooler is also known as Evaporative air cooler, Swamp cooler, Desert cooler or Water Air cooler.
Evaporative air cooling is the natural process of cooling, similar to wind flowing across a lake. Evaporative air cooler delivers 100% fresh filtered cool air at much lower cost than a refrigerated air conditioner. Thus it is the most viable & economic option for cooling large & open areas. Moreover ,There is no need to shut doors & windows. In fact, when doors & windows are left open, it works more efficiently.
3) Vapour refrigeration system
Vapor-compression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressor, a condenser, a metering device or thermal expansion valve (also called a throttle valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor[2] and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air flowing across the coil or tubes.
The superheated
vapor then passes through the condenser. This is where heat is transferred from the circulating refrigerant to an
external medium, allowing the gaseous refrigerant to cool and condense into a liquid. The rejected heat is
carried away by either the water or the air, depending on the type of
condenser.
4)Refrigeration by throttling of gas
Gas throttling is reduction of pressure in a gas flow forced through a throttle – local hydrodynamic resistance point (orifice choke, valve, tap, check), accompanied by temperature change.
Gas throttling is used for liquefying and deep refrigeration of gases. The latter is performed in low temperature separation devices during gas treatment before long-distance transportation. In addition, gas throttling is used in pipeline gas transmission for gas flow rate and pressure regulation.
Throttling can cause frosting of locking, regulating and metering devices, as well as forming of gas hydrates in pipelines. As a result of gas throttling the gas temperature in gas trunklines can decrease below the ambient temperature.
Refrigeration cycle :
1. BELL coleman cycle
The Bell Coleman Cycle (also called as the Joule or "reverse" Brayton cycle) is a refrigeration cycle where the working fluid is a gas that is compressed and expanded, but does not change phase. Air is most often this working fluid. As there is no condensation and evaporation intended in this gas cycle, components corresponding to the condenser and evaporator in a vapour compression cycle are the hot and cold gas-to-gas heat exchangers.
This cycle may be thought of as a modification of reversed Carnot cycle, as the two isothermal processes of Carnot cycle are replaced by two isobaric heat transfer processes.
2 . Vapour compression refrigeration cycle
Step 1: Compression
The refrigerant (for example R-717) enters the compressor at low temperature and low pressure. It is in a gaseous state. Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters to the condenser. Since this process requires work, an electric motor may be used. Compressors themselves can be scroll, screw, centrifugal or reciprocating types.
Step 2: Condensation
The condenser is essentially a heat exchanger. Heat is transferred from the refrigerant to a flow of water. This water goes to a cooling tower for cooling in the case of water-cooled condensation. Note that seawater and air-cooling methods may also play this role. As the refrigerant flows through the condenser, it is in a constant pressure. One cannot afford to ignore condenser safety and performance. Specifically, pressure control is paramount for safety and efficiency reasons. There are several pressure-controlling devices to take care of this requirement
Step 3: Throttling and Expansion
When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Because of these changes, the refrigerant leaves the throttle valve as a liquid vapor mixture, typically in proportions of around 75 % and 25 % respectively. Throttling valves play two crucial roles in the vapor compression cycle. First, they maintain a pressure differential between low- and high-pressure sides. Second, they control the amount of liquid refrigerant entering the evaporator.
Step 4: Evaporation
At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. Therefore, it evaporates and absorbs latent heat of vaporization. Heat extraction from the refrigerant happens at low pressure and temperature. Compressor suction effect helps maintain the low pressure. There are different evaporator versions in the market, but the major classifications are liquid cooling and air cooling, depending whether they cool liquid or air respectively.
2. absorption refrigeration cycle
Air conditioning may seem like an aspect of the build environment that is inextricably bound to the increased carbon intensity of our building stock due to energy input requirements and the utilization of refrigerants that are, in aggregate, an enormous contributor to increasing global carbon emissions.
Yet, there is one type of refrigeration cycle that has the potential to provide clean, low-carbon air conditioning: absorption refrigeration.
Classification of refrigerants:
Primary Refrigerants:
Primary refrigerants are those fluids, which are used directly as working fluids, for example in vapour compression and vapour absorption refrigeration systems. When used in compression or absorption systems, these fluids provide refrigeration by undergoing a phase change process in the evaporator. As the name implies, secondary refrigerants are those liquids, which are used for transporting thermal energy from one location to other.
Secondary Refrigerants:
Secondary refrigerants are also known under the name brine’s or antifreeze’s. Of course, if the operating temperatures are above 0°C, then pure water can also be used as secondary refrigerant, for example in large air conditioning systems. Antifreeze’s or brine’s are used when refrigeration is required at sub-zero temperatures.
Unlike primary refrigerants, the secondary refrigerants do not undergo phase change as they transport energy from one location to other. An important property of a secondary refrigerant is its freezing point. Generally, the freezing point of a brine will be lower than the freezing point of its constituents. The temperature at which freezing of a brine takes place its depends on its concentration. The concentration at which a lowest temperature can be reached without solidification is called as eutectic point. The commonly used secondary refrigerants are the solutions of water and ethylene glycol, propylene glycol or calcium chloride. These solutions are known under the general name of brine’s.
Applications of Refrigeration in Industries
· District Cooling.
· Electricity Production.
· Chemical and Petrochemicals.
· Pharmaceutical.
· Food & Beverages.
· Data Centres.
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