Boiling_and_Condensation, Energy Heat, Heat Transfer.pptx

Understanding Condensation - What is condensation? If you have ever gone outside to breathe fresh morning air, you must have seen water droplets sticking to the windows of your house or motorized vehicles and leaves at dawn when dawn has not yet arrived. The water droplets that seem to stick are what we usually call dew.

A natural phenomenon that is natural in the form of a beautiful natural process and is always something that is calming and comfortable to look at. For those of you who like to climb mountains, it is common to see dew in the morning sticking to trees at an altitude of several thousand meters above sea level.

Especially if you are in a hamlet that is still surrounded by natural scenery in the form of forests, hills, and tea plantations, then you will easily find so much morning dew resting on tree trunks and leaves.

When we were children, we must have unknowingly played with the dew that stuck to the leaves, not only because of its unique appearance, the dew that is present in the morning also indicates that the air feels fresher and clearer than any water and especially that in the bathroom tub, let alone the boiled water in the cup at home.

On average, small children prefer to find ways to move this dew to the surface from the other side of their hand without trying to break it. Sadly, because dew is basically made from water, the dew that was previously attempted to be maintained in its round shape suddenly easily breaks into water droplets.

So, when discussing the issue of morning dew, have you ever tried to think, where do these small round dew droplets come from? In short, the round grains that we call morning dew appear to be caused by a process called condensation which is carried out naturally by nature.

Hearing the term condensation, you must be very unfamiliar with this term. Indeed, condensation is not a term that many people know, so it is rare for anyone to know it. Worse, maybe your friends who used to go to school majoring in natural sciences may not know at all or no longer remember this term, or the worst situation is that they have never studied and understood the context of condensation in their learning at school before?

If you yourself are one of the many people who also do not understand the context of condensation, you are very lucky because in this topic we will try to discuss it. So the big question is, what is condensation? Let's look at the following explanation of what condensation is! Condensation is the process by which a gas changes state to liquid. Condensation, also called dew and the opposite of evaporation or the evaporation process. For example, if a glass with a glass lid is filled with hot water, the glass lid used will gradually condense. Condensation occurs when vapor cools to a liquid, as explained in the book Bioenergy and Biorefinery.

However, it can also occur when vapor is compressed to a liquid (i.e., pressure increases) or when it undergoes a combination of cooling and compre

1. Boiling and Condensation

2. Many familiar engineering applications involve condensation and boiling heat transfer. In a household refrigerator, for example, the refrigerant absorbs heat from the refrigerated space by boiling in the evaporator section and rejects heat to the kitchen air by condensing in the condenser section ( the ling coils behind the refrigerator). Also, in steam power plants, heat is transferred to the steam in the boiler where water is vaporized, and the waste heat is rejected from the steam in the condenser where the steam is condensed. Some electronic components are cooled by boiling by immersing them in a fluid with an appropriate boiling temperature. Introduction

3. i - Boiling Paradigm Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid evaporates from its surface, at the boiling point vapour bubbles come from the bulk of the liquid. For this to be possible, the vapour pressure must be sufficiently high to win the atmospheric pressure, so that the bubbles can be "inflated". Thus, the difference between evaporation and boiling is "mechanical", rather than thermodynamically.

4. Nucleate Boiling Transition Boiling Film Boiling Boiling Stages

5. Nucleate Boiling Nucleate boiling is characterized by the growth of bubbles on a heated surface, which rise from discrete points on a surface, whose temperature is only slightly above the liquid’s. In general, the number of nucleation sites are increased by an increasing surface temperature. An irregular surface of the boiling vessel (i.e. increased surface roughness) can create additional nucleation sites, while an exceptionally smooth surface, such as plastic, lends itself to superheating. Under these conditions, a heated liquid may show boiling delay and the temperature may go somewhat above the boiling point without boiling.

6. Transition Boiling Transition boiling may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling. The formation of bubbles in a heated liquid is a complex physical process which often involves cavitations and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.

7. Film Boiling If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapor , which has low thermal conductivity insulates the surface. This condition of a vapor film insulating the surface from the liquid characterizes film boiling.

8. Boiling Applications 1- Distillation Is a method of separating mixtures based on differences in their boiling points. Distillation is a unit operation, or a physical separation process, and not a chemical reaction. Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its components—notably oxygen, nitrogen, and argon—for industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with a higher alcohol content.

9. Boiling Applications (con.) 2- Boiling for Water Sterilization Boiling can be used as a method of water disinfection but is only advocated as an emergency water treatment method, or as a method of portable water purification in rural or wilderness settings Bringing water to the boil is effective in killing or inactivating most bacteria, viruses and pathogens.

10. i i - Internal Condensation The spectrum of flow processes associated with condensation on a solid surface are almost a mirror image of those involved in boiling. Thus drop condensation on the underside of a cooled horizontal plate or on a vertical surface is very analogous to nucleate boiling. The phenomenon is most apparent as the misting up of windows or mirrors. When the population of droplets becomes large they run together to form condensation films, the dominant form of condensation in most industrial contexts.

11. Condensation Stages 1- Film Condensation Here, the liquid film covers the entire condensing surface, and under the action of gravity the film flows continuously from the surface. This is characteristic of clean uncontaminated surfaces.

12. Condensation Stages (con.) 2- Dropwise Condensation In terms of maintaining high condensation and heat transfer rates, droplet formation is superior to film formation. It is therefore common to practice to use surface coatings that inhibit wetting, and hence simulate dropwise condensation. It is often difficult to maintain the condition of dropwise condensation. For these reasons, condenser design calculations are often based on the assumption of film condensation.

13. Condensation Stages (con.) 3- Condensation Inside Horizontal Tubes Two fluids are separated by a tube wall. One fluid condenses on the inner surface of the tube and releases evaporation enthalpy. This heat is transferred to the wall, conducted through the wall and transferred to a second fluid, the cooling fluid. Figure 3.1 shows a typical temperature profile for a tube wall of a condenser tube.  

14. Condensation Stages (con.) 3- Condensation Inside Horizontal Tubes  

15. Condensation Stages (con.) 3- Condensation Inside Horizontal Tubes Firstly the condensing heat is transferred to the tube wall. The inner thermal resistance 1/λ results in a temperature difference between the saturated fluid and the inner tube wall. Secondly the heat is conducted through the tube wall with a thickness s and a thermal conductivity l. The thermal resistance of the tube wall s/l results in a temperature difference between the inner and the outer tube wall. The third compound of the total thermal resistance is the outer thermal resistance 1/λ that causes a temperature difference between the outer tube wall and the cooling fluid.  

16. Condensation Stages (con.) 4- Condensation Inside Mirco -Fin Tubes Since the end of 1970 condensation heat transfer inside horizontal tubes in heat exchangers is enhanced by finned tubes. Micro-fin tubes play a very significant role in modern high-efficiency refrigerators and air conditioners with their significantly enhanced heat transfer coefficients and low pressure loss characteristics. Low-fin tubes, that are characterized by a fin height hf greater than 0.04 times the inside tube diameter Dft , show high pressure drops and heat transfer coefficients. On the other side micro-fin tubes that have a lower fin height ratio result in high heat transfer coefficients with lower pressure drop.

17. Condensation Stages (con.) 4- Condensation Inside Mirco -Fin Tubes (con.) .Micro-fin tubes are typically made of copper and have an outer diameter from Do = 4 mm to 15 mm, a number of fin ranging from nf = 50 to 70 spiral fins with helix or spiral angles from b = 6° to 30°. Usually the fins are triangular or trapezoidal shaped and have a fin height from hf = 0.1 mm to 0.25 mm. Apex angles vary from g = 25° to 90°. Currently tubes with axial, helical, crosshatched and herringbone patterns are available. This chapter presents definitions for micro-fin tubes, results from earlier studies and correlations for calculation of HTC in micro-fin horizontal tubes.  

18. Condensation Stages (con.) 4- Condensation Inside Mirco -Fin Tubes (con.)  

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