- Thermal physics basics
- Convection component of the structure
- Materials and their thermal capacity
- Design pressure, lining
- Displacement, conventional thermal power
In our educational program for you, we tried to highlight in as much detail as possible one of the fundamental issues in the design of water heating systems. What are the radiators, what are their fundamental differences and what type is right for you, what to look for when buying.
Thermal physics basics
To understand how a particular type of radiator will behave in a specific heating system, you need to understand a number of aspects of the operation of liquid heating. In particular, the chain of heat conversion from the heating source to the direct air in the room is important..
Heat transfer is carried out the faster, the higher the contact area of two bodies with different temperatures and the greater the difference in these temperatures. Therefore, the option is considered optimal when water of an extremely low temperature enters the boiler heat exchanger, so most of the heat from the combustion products will be literally “absorbed” into the coolant.
In reality, this approach is used only in those systems where the power of the heating unit is not regulated, as is the case with solid fuel boilers. Most modern heating equipment has a sufficiently flexible automation system, which allows you to quickly reach a balanced operating mode. In this case, the boiler gives off exactly as much energy as the system’s radiators can dissipate..
This is the main misconception of novice designers: it is most obvious that a radiator must heat the air, although in fact its main task is to cool the coolant. The heat exchange rate can be simply adjusted by forced convection and its local regulation. Therefore, when choosing and calculating the number of heating radiators, the main task is to replenish the heat loss in the premises, and not fight over balancing the heat exchange circuit.
Convection component of the structure
The high efficiency of modern radiators is due to the first factor affecting the rate of heat transfer – surface area. Numerous ribs made of thermally conductive material ensure very fast cooling of the coolant, the effect is enhanced by the high speed of the air flow passing through the radiator.
The indisputable leader in this regard is bimetallic and aluminum radiators. In addition to the numerous air ducts, they have a bend at the top of the ribs, which effectively redirects the air flow to the center of the room and loop it. Therefore, this type of radiator is installed mainly under windows with wide window sills or inside technological niches..
Panel steel radiators have a slightly less effective convection ability. One to three ribbed panels provide high-quality heat transfer, but the heated air is directed mainly vertically. Cast iron radiators have the least heat transfer. Among the batteries of the old model, the best indicators in this regard are characterized by sections with oblique internal inserts, modern cast-iron radiators are slightly superior to them. But as you will see later, cast iron has other advantages..
Materials and their thermal capacity
Let us consider in more detail the variant of the system operation at average values of the coolant temperature, when it does not have time to cool down to the air level in the room. In this case, both the upper and lower parts of the radiator will have a slight temperature difference, and the boiler will only slightly heat up the coolant..
The massiveness of cast-iron radiators allows operation in this mode. Preheated to 50-60? С, they manage to give enough heat to the room air and at the same time provide stable heating without noticeable differences. This is the main difference from the operating mode of aluminum radiators: they operate in a cyclic mode, either heating the room at maximum power, or quickly cooling down.
Cast iron and steel radiators have another interesting feature: they transfer heat not only by convection, but also due to direct radiation. However, to heat such a mass of metal, time is required, sometimes quite long. In terms of the speed of reaching the mode and the amount of heat given off per unit of time, cast iron radiators are inferior to other types. It is beneficial to install them in a private house with good insulation, but when connected to a centralized heating system, there will be no tangible benefit from cast iron. Considering the fact that the number of sections and the space for their installation in apartments is very limited, it is better to give preference to steel or metal.
Design pressure, lining
An additional limitation when choosing heating radiators is always the quality of the coolant and the technical parameters of the system. Aluminum radiators are not suitable for replacing old radiators in an apartment, they are simply not designed for pressures above 6–8 atm. As a replacement, at least bimetallic ones are suitable, but it is better to give preference to steel ones. They can be safely called the most unpretentious among other varieties..
The quality of water or other heating medium also severely limits the use of aluminum radiators. Ensure that the water does not contain high concentrations of dissolved ions. It is also important to exclude any manifestation of electrical emission, which is possible when grounding / grounding to metal elements of the heating water supply. In such conditions, aluminum corrodes extremely quickly, which is accompanied by pronounced gas formation and airing of the system..
Cast iron batteries are not at all sensitive to aggressive substances, and rather wide duct channels allow a significant content of mechanical impurities. Steel radiators can be somewhat susceptible to the deposition of undissolved particles on the inner walls, so the water for them needs to be filtered and softened.
Displacement, conventional thermal power
Heat transfer and the potential ability to regulate the flow locally depend on how much volume passes through the radiator per unit of time. For cast iron radiators, a slightly larger pipeline throughput is required than for steel and aluminum. And this means a deliberate overstatement of the boiler power and the size of the expansion tank..
Large displacement and power reserve for a certain period remain a reasonable investment. In this mode, the heating system consumes the service life much more slowly, there is a small increase in the efficiency of the heating unit, and the high intrinsic heat capacity smoothes temperature drops. However, it is not worth infinitely increasing the internal volume of the system, at least because of the undesirable inertia of the system, besides, the final task remains to heat the air in the room, and not the water in the pipes.
The modern method of calculating the heating system implies the reverse order of calculations. First, it is determined how many radiators need to be installed to replenish heat losses, and then a heating boiler is selected for a certain total power. In this case, the calculation in each room should be carried out using a redundancy factor from 1.1 to 1.5, depending on climatic conditions, insulation features and the density of radiators.
Note that the power dissipation values are purely relative values. This is how the manufacturer indicates how much heat the radiator can dissipate in principle, as if it were connected to an ideal heating system. In fact, the operating modes always differ from the ideal ones, and therefore special amendments should be taken, noting the real temperature level from the design one. The indicated values of the heated area also take into account the convection capacity of the radiators..
When choosing radiators, it is also necessary to take into account the installation conditions: will it be possible to fit a heat exchanger with the required dissipation power into the available space. For convenience, the concept of energy density can be introduced here: it will be the highest for aluminum radiators, followed by bimetal, then steel and cast-iron batteries are the least profitable. It is quite easy to assess the possibility of placing a certain type of radiator, since they are well standardized.
The simplest problem is with bimetallic and aluminum radiators. They are assembled from sections, each of which has a set indicator for power dissipation, displacement and heated area. The standard width of the section is 80 mm, the height can vary from 13.5 to 117.5 cm in increments of about 10 cm. There are six types of sections in depth, depending on the number of flow channels (columns). There is a directly proportional relationship between the dimensions of the section, its thermal and convection efficiency..
Steel radiators, in addition to their dimensions, have an additional two-digit marking. The first is the number of dissipation panels, the second is the number of heat exchange coils. Depending on this, the depth of the radiator changes: from 47 to 155 mm. Steel radiators are not assembled from sections, and therefore their length is determined individually for each product in the range from 40 cm to 3 meters. In height, steel radiators can be either 300mm or 500mm with rare exceptions.
Cast iron radiators have the least clear standardization in terms of dimensions. Some manufacturers adhere to the dimensions generally accepted for aluminum and bimetallic radiators, some products correspond to the dimensions of the sections of old cast-iron batteries: 90×580 mm at a depth of 90 or 140 mm.