10. Cooling via perspiration is affected by latent and sensible heat. In vapour absorption refrigerator, the compression of refrigerent is avoided. A High Suction gas density and a Low Compression ratio. More heat can be absorbed and ejected in each cycle of refrigeration. Why manholes in vessel are elliptical in shape. Why high latent heat of vaporization is desirable in refrigerant ? The good properties of a refrigerant are as follows: High critical temperature to have large isothermal energy transfer. The latent heat of freezing has the same numerical value as the latent heat of melting, but heat is released as a result of the change in phase from liquid to solid. Upload or insert images from URL. June 14, 2014. Trouton’s rule shows that the latent heat of vaporization will be high for refrigerants having lower molecular weight.   Pasted as rich text. 1. Otherwise mass flow rate will be high. 3. Inside the evaporator, there is latent heat of vaporization as heat conducts into the coil and boils the refrigerant. It results in a high refrigerating effect and a low rate of refrigerant in circulation. I am wondering what exactly is it that makes R134a … Latent Heat vs Sensible Heat . However, it's much harder to cool down in a sunny location than in a shady one because sensible heat from absorbed sunlight competes with the effect from evaporation. The important properties such as latent heat of vaporization and specific heat depend on the molecular weight and structure of the molecule. refrigerant with a lower latent heat of vaporization will absorb less energy during evaporation than those with higher hfg. I have been comparing values of latent heat of vaporization - R12 is about 166 kj/kg and r134a is 215kg/kg, which means for R134a that for any given liquid amount in the evaporator, it can absorb more energyfrom the incoming air from the blower until it evaporates compared to R12, which would mean it can cool more air for a longer time. A high latent heat of vaporization is desirable in refrigerant, because it results in high refrigerating effect per unit weight of the refrigerant and low rate of circulation of the refrigerant. Every kilogram of liquid refrigerant vapourised at the evaporator coil should take away a large amount of heat, i.e. Otherwise mass flow rate will be high. It should be oil miscible. Latent heat energy storage is a near-isothermal process that can provide significantly high storage density with smaller temperature swings in comparison with sensible storage systems. High latent heat of vaporization is desirable in a refrigerant. Refrigerant 1. The latent heat of vaporization is a measure of the heat per pound that the refrigerant can absorb from an area to be cooled. A refrigerant should have a high latent heat of vapour at the evaporator temperature.   You cannot paste images directly. Why high latent heat of vaporization is desirable in refrigerant ? Thermal Conductivity: Thermal conductivity of the refrigerant should be high for faster heat transfer during condensation and evaporation. The difference will give you the latent heat for the mixture. Every kilogram of liquid refrigerant vapourised at the evaporator coil should take away a large amount of heat, i.e. ; Example - Boiling Water at 100 o C, 0 bar (100 kPa) Atmospheric Pressure. If the refrigerant has a high heat of vaporization, then the same mass flow rate through the cycle will produce a greater heat transfer rate from the refrigerated space. Cost is always a factor in selecting a refrigerant. High latent heat of vaporization. A High Suction gas density and a Low Compression ratio. 2. The specific heat of liquid should be as small as possible and of vapour should be as high as possible to give less superheating of vapour. When a refrigerant has a high latent heat of vaporization, it can effectively move more heat when it boils and condenses within the target ranges. The specific heat of refrigerant is related to the structure of the molecule. When a material in liquid state is given energy, it changes its phase from liquid to vapor; the energy absorbed in this process is called heat of vaporization. High latent heat of vaporization is an asset for refrigerants, because most of the heat absorption and heat rejection in the vapor-compression refrigeration cycle occurs as a phase change. It should be non-flammable, non-explosive and chemically stable. When a refrigerant has a high latent heat of vaporization, it can effectively move more heat when it boils and condenses within the target ranges.   Your previous content has been restored. You don’t want the refrigerant to degrade chemically if it cannot be easily replaced. Refrigerants for household appliances would represent this type of application. La- tent heat is the amount of heat required to change the refrigerant from a liquid to a gas. Latent heat is the heat absorbed or released as the result of a phase change. •Thermal conductivity :- It should have high thermal conductivity. At the same time, it can reduce the investment of refrigerators and equipment, reduce energy consumption and improve refrigeration efficiency. When the refrigeration system obtains the same cooling capacity, the large latent heat of vaporization can reduce the refrigerant circulation. × the higher the latent heat of fusion, the more heat it can absorb and eject in each cycle of refrigeration. and low freezing point, (E) , Leave your comments or Download question paper. Enthalpy of vaporization: To ensure the maximum heat absorption during refrigeration, a refrigerant should have a high enthalpy of vaporization. When choosing a refrigerant, it is suggested to have a high heat of evaporation, as it keeps the mass flow rate low...why is this? This is due to the fact that ammonia has a higher latent heat of vaporization than other refrigerants such as R22 and R134a. It should be oil miscible. 3. The heat of vaporization of … Therefore, a refrigerant with a larger heat of vaporization is desirable. In vapour compression refrigeration, the vapour is drawn in the compressor cylinder during its suction stroke and is compressed adiabatically during the compression stroke. 5. Therefore the refrigerant vapors should be easily condensible. Confused and have questions? It results in a high refrigerating effect and a low rate of refrigerant in circulation. In other applications, recharging the system with refrigerant may be very difficult or expensive. Higher latent heat of vaporization of the refrigerant will result in lower mass flow rates according to the Heat transfer equation. 3. It results in high ... Refrigerant Latent heat of vapour at 15 º C (Kcal/kg) R-12 (CCl 2 F 2) 37.97 R-717 (NH 4) 314.42 R.744 (CO 2) 65.44 R-764 (SO 2) 94.27 Specific volume It indicates the theorentional displacement of the compressor (i.e volume of suction vapour to compressor). EQ-COMP software is showcased on www.eq-comp.com website and the results of the software can be … Some refrigerants are well-suited to maintaining very cold spaces while others are especially well-suited to moderate temperature applications, such as air-conditioning. The specific heat of liquid should be as small as possible and of vapour should be as high as possible to give less superheating of vapour. High heat of vaporization: For every kilogram of refrigerant that gets vaporized at the evaporator, it should take away a large amount of heat from the refrigerator.This is important because the higher the value of heat taken by the refrigerant, the greater the cooling effect accomplished. black radiation full radiation total radiation all of these ⇒ The refrigerant for a refrigerator should have high sensible heat high total heat high latent heat low latent heat The latent heat of solidification is the heat involved in the phase change of a substance from liquid to solid. Latent heat is the amount of heat added to or removed from a substance to produce a change in phase. Hence why the heat absorption and heat rejection heat exchangers are called the evaporator and the condenser respectively. When the energy of a system changes because of a temperature difference between the system and its surroundings, we say that energy has been transferred as heat (q). the vaporized refrigerant can be made to give up the latent heat of vaporization that it absorbed in the evaporator is by cooling and condensing it. Because high specific heat decreases the refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the … R290 refrigerants, namely, propane, is a kind of natural hydrocarbon refrigerants can directly obtain from liquefied petroleum gas. Latent heat of vaporization A refrigerant should have a high latent heat of vapour at the evaporator temperature. It results in high refrigerating effect per kg of refrigerant and which reduces the mass of refrigerant to be circulated per ton of refrigerants. It should be non-corrosive, non-toxic and non-poisonous. 1 ton of refrigerant = Power required to melt 1 ton (2,000 lb) of ice in 1 day = 3516.8 Watts. When latent heat is added, no temperature change occurs. 2. Therefore dielectric strength should be high to avoid short circuits. Therefore ammonia cannot be used for small refrigeration systems. 1) 1 bar abs = 0 bar gauge = 100 kPa abs = atmospheric pressure Vacuum steam is the general term used for saturated steam at temperatures below 100°C. 7. ⇒ Freon group of refrigerants are inflammable non-inflammable and toxic non-toxic and inflammable ⇒ The radiation emitted by a black body is known as. Why ice floats. Heat transfer takes place from high temperature to low temperature, which is according to a temperature gradient. 5. ‘Specific Enthalpy of Vaporisation’ (latent heat) of refrigerant should be high. High critical temperature to have large isothermal energy transfer. Because of the relatively high temperature of the available cooling medium, the only way to make the vapor condense is to compress it. 4. 8. This brings us around to the most important characteristic of a refrigerant: the relationship between vapor pressure and temperature. It results in high refrigerating effect per kg of refrigerant and which reduces the mass of refrigerant to be circulated per ton of refrigerants . Flip the page to see why this is so important. Since a refrigerant’s goal is to move heat, an ideal refrigerant should have a high latent heat of vaporization. Solution (By Examveda Team) -The refrigerant should have low boiling point and low freezing point. This temperature change during a phase change is due to the energy released from the potential energy stored in the bonds between the particles, and not due to the energy release from the kinetic energy of the particle. REFRIGERANTS By :- ENGINEER IN ... •Latent heat of vaporization :- It should have high latent heat of vaporization. Latent heat of vaporization A refrigerant should have a high latent heat of vapour at the evaporator temperature. •Critical pressure and temperature :- It should be above the condensing pressure and temperature. Most of the heat transfer occurs because the refrigerant changes state. That internal temperature is fixed so long as the pressure remains the same across the coil and the refrigerant is a single component or azeotropic (no glide). You can share the question You can get updates of new questions on Facebook linkedin twitter & google plus Evaporative cooling. Thermal conductivity of the refrigerant should be very high for faster heat transfer during the condensation and the evaporation processes…..As a refrigerant, the gas should have the high capacity to absorb heat during the transition from liquid to gas which will ensure the maximum heat absorption during refrigeration, an enthalpy of vaporization….. Most of the heat transfer occurs because the refrigerant changes state. Latent heat of vaporization. Therefore the refrigerant vapors should be easily condensible. the vaporized refrigerant can be made to give up the latent heat of vaporization that it absorbed in the evaporator is by cooling and condensing it. It should have low specific heat of liquid (to reduce vaporization during throttling). There isn’t a single best refrigerant for all applications. •Critical pressure and temperature :- It should be above the condensing pressure and temperature. The latent heat of vaporization of refrigerants is larger. •Thermal conductivity :- It should have high thermal conductivity. Steel tubes are used for ammonia systems and copper tubes are used for Freon systems. Which of the following best describes the principle of latent heat of vaporization? Question is ⇒ Pick up the wrong statement. When there is a breeze, evaporative cooling is highly effective. Heat energy required to change a liquid to a vapor without raising its temperature ... A. low refrigerant level B. high refrigerant level C. contamination caused by desiccant breakdown ... C. latent heat. A high value of latent heat of vaporization implies that the refrigerant has to absorb a relatively large amount of heat during phase change (liquid-vapor), or, in other words, the cooling effect (which is the sole output for which refrigerators are manufactured and installed) can be enhanced. 6. High latent heat of vaporization to get more refrigeration effect. A good refrigerant should have: (a) High latent heat of vaporization and low freezing point [IES-1992] (b) High operating pressure and low freezing point (c) High specific volume and high latent heat of vaporization. High latent heat of vaporization is desirable in a refrigerant. A. More heat can be absorbed and ejected in each cycle of refrigeration. The important properties such as latent heat of vaporization and specific heat depend on the molecular weight and structure of the molecule. Trouton’s rule shows that the latent heat of vaporization will be high for refrigerants having lower molecular weight. Since a refrigerant’s goal is to move heat, an ideal refrigerant should have a high latent heat of vaporization. In the case of the latent heat of fusion it is the heat required to change a substance from a solid (ice) to a liquid (water) or vice versa while the latent heat of vaporization from a liquid (water) to a gas (steam) or vice versa. Latent heat of solidification. Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process. Join us (login) to get full access : Please sign up to connect and participate. Thanks in advance :) 8. The specific heat of … It should be non-flammable, non-explosive and chemically stable. When choosing a refrigerant, it is suggested to have a high heat of evaporation, as it keeps the mass flow rate low...why is this? You can answer this question.   Your link has been automatically embedded. The good properties of a refrigerant are as follows: High critical temperature to have large isothermal energy transfer. The gas refrigerant within the condenser rejects its latent heat of … The difference will give you the latent heat for the mixture. Latent heat of vapourisation of water at 1 bar, $100^\circ C$ is $2257 \frac{kJ}{kg}$. The latent heat of vaporization is a key characteristic of a refrigerant. •Latent heat of vaporization :- It should have high latent heat of vaporization. The desirable properties of a refrigerant are as follows: 1. ‘Specific Enthalpy of Vaporisation’ (latent heat) of refrigerant should be high. 5. High Latent Heat of Vaporization. It should be environment-friendly. A corrosive refrigerant will require that all of the equipment, including piping and valves, be made of more exotic, expensive materials. © B-Cubed, 2003, 2005, 2006, 2014, 2018. Water has latent heat of vaporization of 540 calories per gram, the amount of heat energy that is necessary to convert 1 g of liquid water at 100°C to steam at 100°C, or 40.71 kJ/mol or about 2,260 kJ/kg water. In that case the chemical stability of the refrigerant may be an important property. As pressure on molecules increases they require more heat to overcome the pressure force acting or to escape and thus latent heat required is more. These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and liquid to gas. The latent heat of fusion and vaporization both involve the heat required to change the state of a substance without a change in temperature. High latent heat of vaporization. The liquid refrigerant in the evaporator absorbs its latent heat of vaporization, and in the process changes from a liquid to a vapor. Latent heat of vaporization A refrigerant should have a high latent heat of vapour at the evaporator temperature. 3.2.3. Large conductivity to reduce size of condenser and evaporator. Below are some of the reasons why the use of ammonia as a refrigerant has gained popularity. Heat is dissipated away from the body due to the high latent heat of vaporization of water. High latent heat of vaporization to get more refrigeration effect. It is therefore a measure of the cooling potential of the refrigerant circulated through a refrigeration system. Thermal Conductivity: Thermal conductivity of the refrigerant should be high for faster heat transfer during condensation and evaporation. Latent heat of vaporization is a physical property of a substance. Advantages. When a material in liquid state is given energy, it changes its phase from liquid to vapor; the energy absorbed in this process is called heat of vaporization. × When the air moving over the evaporator has more moisture in it and therefore a higher RH and dewpoint the surface temperature of the coil is increased so long as the coil temperature is below the air dewpoint. Higher latent heat of vaporization of the refrigerant will result in lower mass flow rates according to the Heat transfer equation. Fire Load Calculation: Latent Heat Of Vaporization - posted in Relief Devices Forum: Hi all, I have a question regarding latent heat of vaporization. High latent heat of vaporization to get more refrigeration effect. 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