How is heat transfer from bimetallic heating radiators? Types and which ones are better to choose? Step by step + Video

How is heat transfer from bimetallic heating radiators? Types and which ones are better to choose? Step by step + Video Heat transfer of bimetallic radiators: which is better? Many of those who have had to deal with the replacement and repair of batteries know firsthand that the most expensive of all available water heater designs (including steel, cast iron and aluminum) are bimetallic heating radiators.

For visual confirmation that bimetallic batteries are effects, there is a conditional heat transfer table, where data on bimetallic radiators, thermal conductivity of other metals and air temperature measurement are indicated. Is this device really that effective?

What is it?

At its core, bimetallic heating is a mixed type of construction that was able to embody the advantages of aluminum and steel heating systems.

It is on these elements that the heatsink device is based:

  • How is heat transfer from bimetallic heating radiators? Types and which ones are better to choose? Step by step + Video Heater, which consists of 2 cases - external (aluminum) and internal (steel).
  • Thanks to the strong inner shell made of steel, the body of the structure is not afraid of exposure to strong hot water, can withstand even high pressure and gives excellent indicators of the strength of the connection of each radiator section into a single battery.

  • The body of aluminum perfectly transfers and dissipates heat in the air, does not corrode from the outside.

To confirm what kind of heat transfer from bimetallic heating radiators, a comparison table was created. The closest and strongest competitor is a radiator made of CG cast iron, aluminum AL and AA, steel TS, but the BM bimetallic radiator has the best heat transfer rates, good operating pressure and corrosion resistance.

Interestingly, that almost all tables contain information from manufacturers about the level of heat transfer, which are standardized as a heatsink height of 0.5 m and a temperature difference of 70 degrees.

But in fact, everything is much worse, since recently 70% of manufacturers indicate the heat transfer of thermal power per section and per hour, that is, the data may differ significantly. This is done on purpose, the data is not specially cited to simplify the perception of the buyer, so that he does not have to calculate the data about a particular radiator.

Is a bimetallic radiator profitable and how much?

To confirm high heat transfer rates, data from tables are often cited.

Material from which the heating radiator is made Heat transfer rates (W / m * K)
Cast iron 53
Steel 66
Aluminum 230
Bimetal 380

Such information that stands out favorably against the background of "fellows" are often used for advertising as reliable data on the heat transfer of various water heating systems.Although the fact that the heat transfer of bimetallic radiators is higher than that of analogues is well known to everyone even without data from the reference book, can the difference really be up to 40%?

If we consider the table from the reference book, then it can be seen that the biggest difference in heat transfer is 10%, but not 40%.

What determines the heat transfer factor

Before trying to evaluate or compare the heat transfer efficiency of a bimetallic radiator, let us recall what determines the heat output of the heating system:

  • The heat head of the radiator plays the following role - the greater the difference between the air temperature and the average data of the surface temperature, the stronger the heat sweat that is transferred to the room air.
  • Thermal conductivity of the material from which the heatsink is made - the higher the thermal conductivity, the less the difference between the outer wall of the heatsink and the temperature of the carrier will be.
  • Dimensions of the heating system and the number of sections.
  • Pressure and temperature of the heating medium.

Please note, that in those heating systems where water is used, 98% of the heat transfer from the walls to the air is carried out due to the convention, therefore, in addition to dimensions, it is very important and the form.

But in practice, it is quite difficult to take into account all configurations, so only linear dimensions are used.

How is heat transfer from bimetallic heating radiators? Types and which ones are better to choose? Step by step + Video Thermal head is the first criterion that is calculated as the difference between half-sums and temperature indoor air. There is even a certain correction factor that helps to clarify the heat dissipation of the radiator when calculating the system power for a room.

According to the table of correction factors, it can be concluded that those data on the heat transfer of a bimetallic radiator will correspond to reality only during the first hour of operation of the heating system, since such data is possible only with a temperature difference in a cold room. Typically, heat transfer fluids rarely heat up higher than 85 degrees, which means that the maximum heat transfer is available at room 15 degrees.

Thermal conductivity of the radiator wall material - this is the second criterion, in which a radiator made of bimetal is much inferior to that of aluminum. The bimetal heating section shown in the diagram clearly shows that the walls are made of aluminum and steel. Even if the wall thickness is the same under similar conditions, a bimetallic housing cannot be better in terms of heat transfer than an aluminum heating system.

Usually the dimensions of these two heating systems are the same and are designed for installation under a windowsill. Note that the structure of aluminum and bimetal takes up more space than steel or cast iron models.

For this reason, heat transfer can be stronger than with a standard calculation based on the properties of metals alone - heat capacity and thermal conductivity. Now it remains to deal with the pressure and temperature of the coolant.

Ideal conditions for using bimetallic radiators

In many ways, the device and circuit of an aluminum bimetallic system are similar. Inside the section there is a main channel through which the heated coolant will move. The dimensions and shape of the channel will correspond to the section of the supply pipe, which means that the liquid will not be subject to additional turbulence and there will be no local overheating points.

From the tabular data, which we have already relied on above, it becomes clear that these two types of radiator structures are designed based on high pressure and high temperature of the coolant. In this case, all the advantages are obvious. To begin with, the temperature difference increases, and instead of the usual 70 degrees, the difference may be already 100. For example, at the entrance to the heating system, the pressure and temperature of the coolant are 18 bar and 110 degrees, and for steam systems, all 120 degrees. This means that we have a correction coefficient for the efficiency of heat transfer of 1.

2, which is equal to 20%.

And also, the greater the pressure of the coolant, the higher the coefficient of heat transfer and heat transfer from liquid to metal. Increasing the value due to increased pressure can increase the final reading by up to 7%. When all the conditions are summed up, it turns out that bimetallic radiators are perfect for heating high-rise buildings.

Although all manufacturers give the same service life and guarantees for two types of heat exchangers, in fact, only bimetal can work for a long time.

In the presence of various additives, hot water will still be damaging to aluminum. Alloy steel with nickel and manganese additives, the service life of which can be equal to 15 years, is another matter.

Conclusion

You can get high heat transfer on a bimetallic radiator not only when you apply high pressure. For all types of radiators, you can increase the heat transfer by at least 20% if you use antifreeze or antifreeze in your home boiler rooms. The pressure will remain unchanged, and the outlet temperature will be 97 degrees, which is a 20% increase in heat transfer.

In addition, antifreeze well preserves cast iron, aluminum, steel pipes and heat exchangers.

( 2 marks, average 1 of 5 )

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