Chemistry

Heat exchange

Heat exchange


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Cross and mixed flow guidance

With cross-flow heat transport, one of the fluids flows through pipes and channels that are perpendicular to the direction of flow of the surrounding second fluid. Calculating this cross-flow heat transport is considerably more difficult than calculating the direct and counter-flow, since the temperature conditions of the external fluid change, for example in a heat exchanger with several rows of tubes, as it flows past each row of tubes. For this reason, the mean logarithmic temperature difference becomesΔTmas with cocurrent or countercurrent, taking into account a correction factorεkcalculated for cross flow.


Agitator selection (process engineering)

The choice of stirrer depends on the stirring task, the viscosity of the medium, the shear strength (whether the medium has to be spared) or, conversely, on the stirrer power available or required for the process.

A distinction is made between the following essential stirring tasks, which in practice have to be carried out individually, simultaneously or in a chronological sequence one after the other in a stirred tank:

  • Homogenization: The equalization of differences in concentration of different media that can be mixed with one another
  • Dispersing liquid / liquid: The stirring of insoluble media into another fluid
  • Dispersing liquid / gaseous: The stirring of a gas phase into a liquid phase, e.g. during hydrogenation
  • Heat transfer, i.e. the targeted introduction of large amounts of heat into a fluid
  • Suspending: The fluidizing and mixing of solids in a liquid phase
  • Emulsification: The stirring of a liquid phase into a second liquid

For each of these stirring tasks, certain types of stirrers are better or worse suited. Since in practice, as described above, stirring tasks rarely occur in isolation, a compromise must often be made between the "optimal" stirring element for the most important process step and the general suitability for process steps that also occur but are less relevant for the yield and quality of the product . In some cases, several agitators, which can be operated independently of one another, are installed in one agitator tank in order to operate the best agitator for several process steps.

Multi-stage stirrers are often used in practice, as shown, for example, in the illustration on the right. While the lower level realizes a high power input with high shear and a residual quantity stirring function, the upper stirring element is suitable as an axial conveyor to achieve a good homogenization of the container contents.


Prévost's theorem

Of the Prévost's theorem is a concept of physics and is used in thermodynamics.

Pierre Prévost recognized in 1809 that the heat exchange between two different hot bodies A and B in a closed system proceeds as follows: The warmer body A radiates a certain amount $ S_A $ of radiant energy onto the colder body B. At the same time, however, the body A also receives a smaller amount $ S_B $ from it. $ S_A $ also represents the radiation energy absorbed by B. Since A emits more energy than it receives, it cools down slowly, while conversely B warms up until both have the same temperature. In this dynamic state of equilibrium, the exchanged heat quantities $ S_A $ and $ S_B $ are the same.

The designation Prévost's theorem or Prévost's theory of heat exchange only has historical significance, since the described connection forms the natural basis of the radiation laws today.


The correction for the heat exchange between a calorimeter and the environment

Freiburg i. Br., December 20, 1942. Thermochemical Research Center, University Medical Clinic.

Freiburg i. Br., December 20, 1942. Thermochemical Research Center, University Medical Clinic.

Abstract

King and Grover have asserted that the correction for heat exchange with the environment can only be calculated in rare cases on the basis of Newton's law of cooling. B. more complicated approaches would have to be made when working with the calorimetric bomb. That is denied. Even if the test conditions vary greatly, the simple cooling law “integration with the slide rule” means that matching values ​​are obtained. Even if the measurement accuracy is increased (platinum resistance thermometer), the simple law still applies to temperature differences of 8. Adiabatic and compensatory work, where the correction is completely or almost completely omitted, has delivered the same values ​​as poikilothermal measurements with correction according to Newton.


Heat exchange - chemistry and physics

Stefan Pietrusky, Learning Level Up

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When does a heat exchange occur? Which body gives off heat and which body absorbs heat? How can one make use of the heat exchange? What does the basic law of heat exchange say? When will there be no more heat exchange? You can find all the important information about heat exchange simply explained in this content.


Richmann's rule of mixing

the Richmann's rule of mixing is a rule for determining the Mixing temperaturethat occurs when two (or more) bodies of different temperatures are brought together. It is named after its discoverer Georg Wilhelm Richmann. & # 911 & # 93

Under the condition that there is no change in the aggregate state and the system of the bodies is closed (in particular only heat exchange between the bodies is possible), the following applies:

The resolved formula according to the mixture temperature:

$ T_ = frac cdot c_ <1> cdot T_ <1> + m_ <2> cdot c_ <2> cdot T_ <2>> cdot c_ <1> + m_ <2> cdot c_ <2>> $

  • m1, m2 stands for the mass of bodies 1 and 2,
  • c1, c2 stands for the specific heat capacity of bodies 1 and 2,
  • T1 stands for the temperature of the body 1, which gives off heat, i.e. the warmer one is,
  • T2 stands for the temperature of body 2, which absorbs heat, i.e. the colder one,
  • Tm represents the common temperature of both bodies after mixing.

After recognizing the conservation of energy, the mixing rule could be derived from the conservation of thermal energy.


Table of contents

Calorimetric measurements are carried out in a calorimeter. In most cases, heat is added to or withdrawn from the calorimeter and the temperature change is observed. If a thermometer is used, it should be a Beckmann thermometer or a digital thermometer (reading accuracy up to 0.01 K). There are many different calorimeters. They can be divided into the following groups.

Anisothermal calorimeters

The calorimeter is thermally insulated from the environment. The heat exchange takes place with a liquid (liquid calorimeter) or with a metal (metal block calorimeter). This type of device is the most common in calorimetry. With clean work, accuracies of up to 0.01% are possible. This procedure is used when the heat exchange takes a maximum of 20 minutes.

Liquid calorimeter

It consists of a double-walled copper container, the space between which is filled with water and is intended to ensure a temperature-constant environment in the inner calorimeter. The calorimeter vessel made of thin sheet metal is placed on a heat-insulated surface. Ordinary water is used as the calorimeter liquid, but other liquids can also be used. A stirrer, the speed of which must remain constant, ensures better heat exchange. The change in temperature is measured with a thermometer. see also: Bomb calorimeter to determine the calorific value.

Adiabatic calorimeters

With these devices, the temperature difference between the calorimeter liquid and the vessel jacket is constantly compensated by heating or cooling. Both processes must take place at the same speed. The slower the heat transfer to the calorimeter, the easier it is to achieve this (20 to 60 minutes).

Isothermal calorimeter

With these devices, the amount of heat is taken from certain substances, which undergo a phase change in the process. The temperatures therefore remain constant during the experiment. These devices are also known as phase change calorimeters. They are used for slow reactions that take several hours.

Ice calorimeter

For measurements of heat quantities at 0 ° C, this calorimeter is one of the most accurate. In the picture is the room b with dist. Filled with water and is in contact with the mercury at the bottom of the vessel. The mercury is in the capillary to the point m filled. Around the bottom of the pipe a An ice coat is created by placing a cold mixture in a thin-walled sample tube in a introduces. The entire calorimeter is protected from external temperature influences by an ice pack and additional insulation. The amount of heat to be measured is taken from the room V fed and given there to the ice coat, which partially melts. As a result, a certain change in volume occurs, which is a measure of the amount of heat given off and from the displacement of the mercury thread in the capillary m can be calculated.

Condensation calorimeter

This calorimeter, often also called a steam calorimeter, is mainly used to determine the specific heat capacity of a substance between 100 ° C and 20 ° C. Steam is used as the condensing gas. The body K to be examined is suspended from a sensitive scale by means of a fine wire and is located inside the calorimeter. If one suddenly introduces saturated water vapor, which has been freed from dripping liquid, into this space, a certain amount of vapor will condense on the initially cold body until the body has assumed the temperature of the vapor. An amount of heat of ΔQ = r · m has passed to the body (ΔQ = amount of heat r = heat of condensation m = mass of condensed steam).

A thin-walled platinum bowl attached to the bottom of the body protects against water dripping. The buoyancy that occurs due to the steam flow must be taken into account. The method can provide very precise values.

Heat exchange calorimeter

In the case of reactions that extend over several hours to a few months, a quick and complete exchange of heat with the environment is ensured. The speed is measured as a function of time.


Description Heat flow (convection)

Anton bathes in a lake on a beautiful summer day. Then he is frightened in the water. I explain to him that he was startled by a flow of heat. We first repeat the terms “thermal energy” and “heat”. I would also like to remind you of heat sources as energy converters with a few examples. Although we have almost understood it, let's now explain what the heat flow has to do with it. Now we also understand how warm and cold air move in a heated room. In the penultimate section I give you an explanation of the Gulf Stream. Finally, we will discuss how a cooling tower works in a power plant. Have fun watching the video!

Transcript Heat flow (convection)

Hello and welcome. This video is called “Heat Flow (Convection)”. You already know heat, energy converters, thermal energy. Afterwards you can explain what heat flow, convection is. And you can describe examples of it. 1. The shock of bathing. It's a hot summer day. Anton bathes in a swimming lake. “Oh, that's nice. Refreshing and not too cold. Oops, what's that? Someone poured cold water into the lake? " “No, you came into a cold current. Stupid current. " - “Oh, you know, even if it was cold, that was a flow of heat. It is also called convection. " - "That sounds interesting, tell me more about it." - "Willingly." 2. Thermal energy and heat. What is it? I've brewed a huge tea here. He's here in the air. The tea is warmer than the air. The tea gives off heat to the air. The ability of a body to give off heat to the colder environment is called thermal energy. The symbol E is used for thistherm. Let's say we have a warm body. Around him is the environment. The environment should be cold. Then heat is transferred from the body to the environment. This shows that the body has thermal energy at its disposal. 3. Heat sources as energy converters. You can already see that we are repeating a little. The red light lamp is a heat source and an energy converter. Try Anton once. It's nice and warm. Electrical energy is converted into heat. And there is also light. The sun is also a source of heat. It converts nuclear energy into heat. And the earth is also a source of heat. It converts light energy from the sun into heat. One last example. A thermal power plant. Here chemical energy is converted into heat. 4. Heat transfer by heat flow. One also says convection. The candle heats air. The warm air rises now. This is because their density is relatively small. On the way up, the air cools down a little. Heat is transferred from warmer to cooler air. The cold air now goes down again. Because their density is relatively large. Once at the bottom, heat is transferred from the candle to air and so on. The process starts all over again. In liquids and gases, the heat can be transferred by heat flow, convection. Thermal energy is carried along with the flowing gas or the flowing liquid. 5. We heat. This is our room. And that's the heater. The heater warms the air and the air becomes lighter, the density decreases. Now the air rises. Right Anton, the warm air doesn't last long upstairs. The air is still moving a bit up in the room. Exactly. It gives off heat to the colder environment and thus becomes cold itself. Its density increases, it becomes heavier and it sinks to the ground. The cold air now moves to the heater. That's right Anton. This happens because warm air is carried away by the heater itself. Heat is exchanged upstairs in the room, between warm and cold air. There is also an exchange of heat between the warm heating and the cold air. 6. Ocean currents. There are different ocean currents in the world's oceans. The most important ocean current for us Europeans is the Gulf Stream. The Gulf Stream is an ocean current from America to Europe. It's relatively cold in Europe. The cold water sinks to the bottom. Right Anton. Because its density is quite large. This creates a pull between Europe and America. It is warm at the beginning of the Gulf Stream. The warm water reaches Europe via the suction. Besides, America is pushing it away. Because its density becomes smaller, it expands. The cold is moving to America. A gradual warming takes place, the movement also comes about through a suction, because the warm water moves from America to Europe. As you can see, for this to work there must be very deep water, like in the ocean. 7. Cooling towers. Cooling towers are of great importance in industry. For the most part, they are used in power plants. Their function is to dissipate heat. And something like this is what a cooling tower looks like there. It has the shape of a tapered hollow cylinder. He stands at a height of one meter, above the ground, on pillars. Below the tower is a basin with cooling water. Distribution pipes are attached at a suitable distance above. There are nozzles on these. Hot water is sprayed from the nozzles into the water basin. This causes the air above the water basin to heat up. The air expands and rises together with water vapor in the cooling tower. There is suction. Cold air is now sucked in at the edges of the cooling tower. This is called the chimney effect. So-called droplet separators are located on the inside of the cooling tower. Part of the water vapor condenses and flows back into the tower. This is called trickling. The rest of the water vapor leaves the tower and forms clouds. The water returns to the earth as rain. That was another film by Andre Otto. Also many thanks to Anton. Bye. I wish you all well and good luck. Bye.


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