Calculation of heat transfer by convection from a heated surface

What is thermal conductivity

This term means the ability of various materials to exchange energy , which in this case is represented by heat. In this case, energy transfer passes from the hotter part to the colder part and occurs due to:

1. Molecules
2. Atoms.
3. Electrons and other particles of the metal structure.

The thermal conductivity of stainless steel will differ significantly from that of another metal - for example, the thermal conductivity of copper will be different than that of steel.

To indicate this indicator, a special value is used, called the thermal conductivity coefficient. It is characterized by the amount of heat that can pass through a material in a certain unit of time.

Indicators for steel

Thermal conductivity can vary significantly depending on the chemical composition of the metal. The coefficient of this value will be different for steel and copper. In addition, with an increase or decrease in carbon concentration, the indicator under consideration also changes.

There are other features of thermal conductivity:

1. For steel that does not have impurities, the value is 70 W/(m* K).
2. Carbon and high-alloy steels have much lower conductivity. Due to an increase in the concentration of impurities, it is significantly reduced.
3. The thermal effect itself can also affect the structure of the metal. As a rule, after heating, the structure changes its conductivity value, which is associated with a change in the crystal lattice.

The thermal conductivity coefficient of aluminum is much higher, which is due to the lower density of this material. The thermal conductivity of brass also differs from that of steel.

Knowledge base on 3D design in Pro/Engineer, Creo, Solidworks, electronics on STM32

CAD training. Important parameters of some materials used in thermal calculations

This table presents such important parameters as Thermal Conductivity Coefficient λ
and
Specific Heat Capacity ср
, which are necessary for thermal calculations according to the articles Creo 3. Calculation of a cooling radiator with forced ventilation and Solidworks 2013. Thermal calculation of a cooling radiator with forced ventilation in Solidworks Simulation.

The following table shows the convection coefficients h or α

(otherwise known as
Convective Heat Transfer Coefficients
and
Convective Heat Transfer Coefficients
) required for evaluation calculations

Source

Effect of carbon concentration

The carbon concentration in steel affects the amount of heat transfer:

1. Low carbon steels have a high conductivity index. That is why they are used in the manufacture of pipes, which are then used to create the heating system pipeline. The coefficient value varies from 54 to 47 W/(m* K).
2. The average coefficient for common carbon steels is a value from 50 to 90 W/(m* K). That is why such material is used in the manufacture of parts for various mechanisms.
3. For metals that do not contain various impurities, the coefficient is 64 W/(m* K). This value does not change significantly under thermal influence.

Thus, the considered indicator for alloyed alloys may vary depending on the operating temperature.

How to increase heat transfer?

Due to the existing ratio of pipe volume to its surface area, quite often there is a need to increase its ability to transfer heat. This is required for the most efficient heating of premises.

How to increase the heat transfer of a pipe has been known for a long time; the following methods have been and are being used in practice.

An example of an effective increase in heat transfer is a convector, which was used in heating systems back in Soviet times. It was a bent pipe (U-shaped) with plates welded perpendicular to it. This method is called fins, and it is also used in modern heating devices.

Importance in everyday life and production

Why is it important to consider thermal conductivity? A similar value is indicated in various tables for each metal and is taken into account in the following cases:

1. In the manufacture of various heat exchangers. Heat is one of the important carriers of energy. It is used to provide comfortable living conditions in residential and other premises. When creating heating radiators and boilers, it is important to ensure rapid and complete heat transfer from the coolant to the end consumer.
2. In the manufacture of outlet elements. You can often encounter a situation where you need to remove heat rather than supply it. An example is the case of heat removal from the cutting edge of a tool or gear teeth. To ensure that the metal does not lose its basic performance qualities, rapid removal of thermal energy is ensured.
3. When creating insulating layers. In some cases, the material should not conduct thermal energy transfer. For such operating conditions, a metal is selected that has a low heat conductivity coefficient.

The indicator under consideration is determined when testing under various conditions. As previously noted, the thermal conductivity coefficient may depend on the operating temperature. Therefore, the tables indicate several of its values.

Pipe insulation

If in heated rooms everything is done in order to take as much heat as possible from the pipe, then in the main lines there is a completely opposite need - to reduce heat transfer to the maximum.

For this purpose, pipe insulation is used.

The market for materials for these purposes is quite extensive, so there are no problems with choosing insulation. In addition to the cheapest fiberglass insulation, basalt wool, polyurethane foam, and expanded polystyrene are used.

The heat transfer of steel pipes can be most effectively reduced in factory conditions. The production of pipes with a layer of insulation and polyethylene is constantly increasing; today, installing heating lines from such materials is one of the best ways to reduce heat loss.

As you can see, knowledge of actual heat transfer is necessary to solve many technical problems associated with the construction of hot water supply and heating systems. Therefore, when designing these systems, be sure to perform such calculations, or even better, entrust this to a specialist.

We calculate the return for 1 m of product

It is easy to calculate the heat transfer of 1 m of pipe made of steel. We have a formula, all we have to do is substitute the values.

Q = 0.047*10*60 = 28 W.

• K = 0.047, heat transfer coefficient;
• F = 10 m2, pipe area;
• dT = 60° C, temperature difference.

It's worth remembering

Do you want to make a heating system correctly? You should not select pipes by eye. Heat transfer calculations will help optimize construction costs. In this case, you can get a good heating system that will last for many years.

Source

Comparison of indicators: analysis and table

In addition to the material from which the device is made, the power factor is affected by the center-to-center distance - the height between the axes of the upper and lower outlets. also has a significant impact on the efficiency .

What is a bimetallic radiator

In fact, a bimetallic heater is a mixed design that embodies the advantages of steel and aluminum heating systems. The radiator design is based on the following elements:

• The heater consists of two housings - internal steel and external aluminum;
• Due to the inner shell made of steel, the bimetallic case is not afraid of aggressive hot water, withstands high pressure and ensures high strength of connection of individual sections of the radiator into one battery;
• The aluminum body best transfers and dissipates heat flow in the air, and is not afraid of corrosion of the outer surface.

A comparison table can be used to confirm the high heat transfer of the bimetallic housing. Among its closest competitors - radiators made of ChG cast iron, TS steel, AA and AL aluminum, the BM bimetallic radiator has one of the best heat transfer rates, high operating pressure and corrosion resistance.

In reality, things are even worse; most manufacturers indicate the amount of heat transfer in the form of thermal power per hour for one section. That is, the packaging may indicate that the heat dissipation of the bimetallic section of the radiator is 200 W.

This is done forcedly, the data does not result in a unit of area or a temperature difference of one degree, in order to simplify the buyer’s perception of the specific technical characteristics of the heat transfer of the radiator, while simultaneously making a small advertisement.

Theory, algorithms, literature.

Pipes in heat supply systems can perform two functions - transport the coolant to the place of its use and serve as a heating device (register) themselves.

When implementing any of the above functions, it is necessary to quantitatively assess the effectiveness of its implementation.

The main indicators for thermal energy transport systems are defined by regulatory documents SO 153-34.20.523-2003 in 4 parts.

In any case, there is a need for prompt and accurate calculations:

• parameters of heat exchange between the pipe and its environment;
• energy costs for transporting the coolant (water) through the pipe.

Heat transfer from a bare pipe

The parameters, the knowledge of which allows one to calculate thermal processes in the “water-pipe-air” system, are collected and shown in the source data block of the table from the previous part of the article.

The figure below shows the equivalent heat transfer diagram of a bare pipe.

When calculating the heat transfer of a pipe, it is convenient to use the method of analogy between thermal engineering and electrical engineering, assuming:

• temperature difference dt = t water - t air , as the difference in electrical potentials;
• heat flow q as electric current;
• thermal resistance Rt is like electrical resistance.

By analogy with Ohm's law, we obtain the following equation:

q = dt / Rt =( t water - t air )/( R in + R tr + R out ) , W.

Thermal resistance between two media - water and air - prevents all forms of heat exchange between them:

Each of the listed forms of heat transfer has its own specifics and is described by corresponding analytical expressions.

1. Convective heat exchange between moving water and a solid cylindrical wall

R in =1/(αin · F in ) – thermal internal resistance, °C/W, where:

• αin – average heat transfer coefficient along the length of the pipe from moving water on the inner surface of the pipe, W/(m²·°С);
• F int - area of ​​the wetted inner wall of the pipe, m².

Calculation of pipe heat transfer in Excel.

To perform calculations, you must enter the source data into the MS Excel table. There are 13 of them. These are the physical parameters of the coolant (water), the ambient temperature, the geometric dimensions of the pipe and the thermal insulation layer, the thermal conductivity of the materials and the degree of blackness of the outer surfaces of the pipe and insulation.

The results cells automatically display the value of the heat output power of the pipe in Watts for four options, and the cooling temperature of the water in degrees Celsius during movement along a given section of the pipeline.

All 22 user functions involved in this Excel calculation program are each recorded in their own Module in the Modules folder. Access to the folder is in the Visual Basic Editor.