Moderately high evaporator pressure boosts the compressor suction pressure thus reduces the power costs. In hermetically sealed compressors refrigerant vapor contacts with motor windings and may cause short circuits. The oil and refrigerant should be miscible to assure that the oil is transported out of the refrigeration circuit and not accumulated in the system. Pressure to which compressor has to compress the drawn gases, to convert them back into liquid at the condenser, should be low. g) It should not be toxic. Whenever we choose them for particular use Properties of Refrigerants also plays a vital role in economic and environmental friendly application. The method has been applied to the refrigerant HCFC‐22, alternative refrigerants HFC‐32, HFC‐125, HFC‐134a and refrigerant mixtures HFC‐32/125 [35/65, 50/50 (HFC‐410A), 68/32 wt%], HFC‐32/125/134a [23/25/52 (HFC‐407C) wt%]. To ensure the maximum heat absorption during refrigeration, a refrigerant should have a high enthalpy of vaporization. Desirable Properties of a Primary Refrigerant. 3. The refrigerants should not react with the materials used in refrigeration cycle like evaporators, condenser tubes, compressors, control valves etc. Vapor produced after vapourisation of the liquid at the evaporator coil should occupy minimum volume, to keep pipeline diameter, compressor size, etc. Chemical Engineers spend their life in making the world a […]. Short Term Exposure Limit is the maximum concentration to which one can be exposed up to maximum of 15 minutes. The moisture inside the system will starts freezing at low temperature zones and clogs and chokes the system. High thermal conductivity, 7. In a negative pressure evaporator Atmospheric air or Moisture will Leak into the system. The refrigerant should be chemically stable. j) It should have satisfactory heat transfer coefficient. The liquid at the evaporator coil should therefore vapourise easily, otherwise the compressor will have to create too much of vacuum to cause the liquid to vapourise. Water for example cannot be used below 0 Deg C. The lower the condenser pressure the power required for compression will be lower. The refrigerant is a heat carrying medium which during their cycles (i.e. It should not freeze during application. Leak outside the system results in refrigerant loss and it can be identified easily and refrigerant loss can be topped up. The year which will be remembered in the history has come to an end. This means that copper's high thermal conductivity allows heat to pass through it quickly. Easy to handle and easy to control its flow. Temperatures can be controlled by controlling the flow. Continued use of this site indicates that you accept this policy. Thus, less insulating material will be required. If the oil density is less than the refrigerant used and it if it is immiscible, the oil will float on the surface of the refrigerant. Desirable Properties of an ideal Refrigerant . Refrigerants are widely used in Refrigeration cycle where cooling effect below the atmospheric temperatures are needed. Therefore cost of the refrigerants is normally high when compared to other chemicals. … CH3Cl reacts with Aluminium. A transient short‐hot‐wire method has been proposed to simultaneously measure the thermal conductivity and thermal diffusivity of liquids. Temperatures can be controlled by controlling the flow. the thermal conductivity of several refrigerant mixtures such as R125+R134a, R125+R32, R125+R152a, R134a+R32, R152a+R32, R134a+R143a, and R125+R143a. Therefore dielectric strength should be high to avoid short circuits. Refrigerants should be readily available near the usage point. Every kilogram of liquid refrigerant vapourised at the evaporator coil should take away a large amount of heat, i.e. This thermal conduction gives substantial influenece to the cooling efficiency. If Freon is present blue flame turns into green. Author(s) : LIN J. Y., PATE M. B. Presence of air will reduce the partial pressure of refrigerant and the condensation temperature will rise. Required fields are marked *. Global Warming Potential (GWP) is a relative measure of how much heat a refrigerant traps in the atmosphere. It is normally mentioned as 20 years or 100 years period. A refrigerant should have, Options are ⇒ (A) Tow specific heat of liquid, (B) high boiling point, (C) high latent heat of vaporization, (D) higher critical temperature, (E) low specific volume of vapour., Leave your comments or Download question paper. Desirable characteristics for insulation materials for fish holds. The desirable properties of a refrigerant are as follows: 1. Due to the above disadvantages, Positive evaporator pressure is preferred. Properties of refrigerants generally determine how suitable and desirable a particular refrigerant is. Refrigerants should have low freezing point than the normal operating conditions. Desirable properties of secondary refrigerants mainly are its specific gravity, freezing point, thermal conductivity, viscosity and specific heat. What is Chemical Engineering all about? Refrigerant Thermal Conductivity at Low and Moderate Levels of Density. An estimation method is proposed for the thermal conductivity estimation of the refrigerant fluids in the liquid state along or near the saturation line. h) It leak should be easily detectable. Your email address will not be published. Therefore oil separators are to be employed. THERMAL CONDUCTIVITY OF HFO-1234ze(E)+HFC-32 MIXTURE . The toxicity depends upon the concentration and exposure limits. 2. k) It should have high thermal conductivity. h) Thermal conductivity: Thermal conductivity in both liquid as well as vapour phase should be high for higher heat transfer coefficients i) Viscosity: Viscosity should be small in both liquid and vapour phases for smaller frictional pressure drops The thermodynamic properties are interrelated and mainly depend on Solution (By Examveda Team) -The refrigerant should have low boiling point and low freezing point. Al 2O3 reacts with water & heat is generated . R-22 for example, has an ODP of 0.055. 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. According to Wikipedia, Chemical engineering is a branch of engineering that applies physical sciences (physics and chemistry), life sciences (microbiology and biochemistry), together with applied mathematics and economics to produce, transform, transport, and properly use chemicals, materials and energy. i) It should have positive condensing pressure. GWP of Carbon Dioxide is 1.0. Let us look back on the journey so far for […], Copyright © 2016-2020 - Chemical Engineering Site, Advertise | Testimonials |Sitemap | Privacy Policy | Disclaimer. Desirable properties of secondary refrigerants mainly are its specific gravity, freezing point, thermal conductivity, viscosity and specific heat. Copper has many desirable properties for thermally efficient and durable heat exchangers. Therefore ammonia cannot be used for small refrigeration systems. Higher condenser pressure will result in high operating costs. Low boiling and freezing point, 2. CCl2F2, CH4Cl can form HCL with water which dissolves the copper from condenser tubes and deposits them on compressor pistons and deteriorates the life of the machinery. It increasers the condenser pressures and thereby the power consumption for the compressor will also rise. In practice, the required weight of RED TEK 12a needed to give the same refrigeration capacity and performance as R12 or R134a is 35% and 40%, respectively. Summary. Thermodynamic Properties 2. Time weighted Average is the concentration to which repeated eight hour exposures for five days in a week which is considered to be safe. The cumulative correction … We continue to change in order to grow and really do shine. e) It should not have corrosive action with system material. The thermal conductivity of a specific material is highly dependent on a number of factors. High critical temperature to have large isothermal energy transfer. 2. Refrigerant oil should be stored in sealed containers. We solve numerically the Ornstein-Zernike (OZ) equation using the ypernetted H Chain (HNC) approximation for binary fluid mixtures and obtain the pair correlation functions. Desirable properties of an Ideal Refrigerant : A refrigerant is said to be ideal if it has all of the following properties: 1. The free water apart from the dissolved water in refrigerant freezes below 0 Deg C and chokes the narrow orifice of expansion valve. Low specific heat of the liquid, and high specific heat of vapor, 5. Excellent way of fully understanding the engeeniring system. A modified steady-state hot-wire method was used for these measurements. High latent heat of vaporization to get more refrigeration effect. Methane, butane and other hydrocarbons are flammable. Strong smelling chemicals like acrolein may be added to refrigerant for easy leak detection. This makes the equipments compact and reduced the operating cost. Here are some of the desirable properties of refrigerant explained in detail. Ammonia will form explosive mixture when the concentration in air is between 16 to 25 %. This may also cause bursting of the tubes. Change is the only option to survive and grow! Thermal Conductivity: Thermal conductivity of the refrigerant should be high for faster heat transfer during condensation and evaporation. If the mass flow is very small it is difficult to control the flow rates. In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid.Convection includes both advection (fluid motion) and diffusion (conduction). This will cause corrosion and deteriorates valves, Seals and Metallic parts. It may accumulate in evaporator. When recharging the system with refrigerant, always add the new oil. Department of Mechanical Engineering, Saga University . Water, for example, has a thermal conductivity of 0.326 BTU/hr-ft-° F. at 32° F., whereas ice has a thermal conductivity of 1.18 BTU/hr-ft-° F. at the same temperature. The conductive component is measured under the same conditions as … Similarly if it is very low industry professional will not take necessary action to control the leaks. The refrigerants should be non poisonous to humans and food stuff. Home » General Engineering Knowledge » Refrigeration » Desirable Properties of a Primary Refrigerant. (PIR) Polyisocyanurate -185°C to +140°C Petrochemical equipment, buildings, refrigerated vehicles, tankers Higher heat transfer coefficient requires smaller area and lower pressure drop. CuO & ZnO can be dangerous for health. It consists of an alcohol lamp that emits a blue flame. If the oil is not miscible in the refrigerant used and it is heavier it will settle down in the evaporator and reduces the heat transfer. 4. - It must have low specific heat and high latent heat. 1. It makes white fumes of ammonium sulfide. This follows the same trend as observed for the thermal conductivities of the … Advantages of the secondary refrigerants over the direct cooling system are: 1. If the refrigerant velocity is not sufficient, then it cannot carry all oil back into the compressor. small and compact. This fact, combined with the higher latent heat and thermal conductivity, has the desirable effects of reducing the weight of refrigerant required to extract a specified amount of heat. At a given density, the differences in thermal conductivity between the refrigerant mixtures are on the order of (10–30) %. Most organic liquids (and therefore all refrigerants) have a thermal conductivity in the range of 0.100–0.200 W m −1 K −1 ( IUPAC, 1987 ). Refrigerants with High vapor density/ Low specific volume will require a smaller compressors and velocity can be kept small and so the condenser tubes used will also be in smaller diameter. 3. The liquid at the evaporator coil should therefore vapourise easily, otherwise the compressor will have to create too much of vacuum to cause the liquid to vapourise. Easy to handle and easy to control its flow. Question is ⇒ Pick up the wrong statement. Water for example cannot be used below 0 Deg C. Low Condensing Pressure. Advantages of the secondary refrigerants over the direct cooling system are, 1. has second highest thermal conductivity with no significant health risks but is second most expensive option. 3. Freon leak can be detected by a halide torch. A refrigerant should have (a) Tow specific heat of liquid (b) high boiling point (c) high latent heat of vaporisation (d) higher critical temperature (e) low specific volume of vapour. The quantity of refrigerant used in any industry is very small. 2. Since polyatomic gas refrigerants have internal degrees of freedom, the unity between translational and internal modes has a significant influence on the thermal conductivity of these gases. Critical pressure of the refrigerant should be higher than the condenser pressures. For Methane GWP is 72, it means that if the same mass of CH4 and CO2 were introduced into the atmosphere, that methane will trap 72 times more heat than the carbon dioxide. 4. If Boiling Point is Low, High Condenser Pressure – Reciprocating Compressor is used. The lubricating oils must be soluble in Refrigerants. To see this page as it is meant to appear, please enable your Javascript! Unprecedented pandemic has taught many lessons to everyone of us. This model requires speed of sound information obtained theoretically for a liquid mixture and the Carnahan-Starling-DeSantis (CSD) equation of state. When HFOs are mixed with HFCs in equal proportion, the ordering of thermal conductivity in the resulting mixture is given by the following sequence of HFC components: R125 < R134a < R143a. The refrigerant should not make combustion mixture in Air. f) It should not be flammable & explosive. Dry stagnant gas is one of the best insulating materials. Atmospheric air ingression inside the system may sometime results in explosions if the flammability values of the refrigerants are in wide range. The liquid has to vapourise at the evaporator coil to cause cooling. We use cookies to ensure that we give you the best experience on our website. Following are the desirable properties of a primary refrigerant. Thermal conductivity. Refrigerants with low boiling points will have high condenser pressure and high vapor density. Insulation of windings in hermatic compressors will also get damaged. ODP of R11 is fixed as the maximum value of 1.0. This phenomenon is called Oil logging. Atmospheric air ingression into the system will occupy the heat transfer area and results in poor heat transfer rates. Once the evaporated gas is compressed, the temperature of seawater should be low enough (below critical temperature of the refrigerant) to be able to condense these gases to liquid form. This is the most important property of refrigerant. High latent heat of vaporization, 4. A thermal conductivity prediction method for refrigerant mixtures in the liquid phase. Otherwise the zone of condensation decreases and the heat rejection occurs. Eg: R11, R13 & R114 etc. Ammonia and SO2 can be detected by their characteristic smell. Sorry, you have Javascript Disabled! The properties of refrigerants may be grouped into three categories: 1. It should not freeze during application. Higher latent heat of vaporization of the refrigerant will result in lower mass flow rates according to the Heat transfer equation. Oil has low thermal conductivity whereas refrigerant has very low thermal conductivity. R-134a is an HFC (hydrofluorcarbons) used to replace the chlorinated refrigerant R- 12. High critical pressure and temperature, 3. Most of the refrigerants form acids or bases in the presence of water. Therefore overflow drain is to be provided to remove oil. 2. Dielectric strength: Special pressure pumps can be used to pump oil in the low parts of the system. Your email address will not be published.
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