Radiators can be divided into two groups. Steel devices of panel type, popular in many countries and radiators of sectional type, which are made from aluminum, steel, cast iron, or a combination of several materials (such devices have the name «bi-metallic»).
Cast Iron Radiators. Cast iron is a material traditionally used for making heating devices. The advantages of cast iron radiators primarily relate to high resistance to corrosion. The maximum operating pressure is 6 bars; for the Russian radiator MS140, it is 9 bars. If you make a one-time test of the heating system or hydraulic shocks, the cast iron radiators can withstand short-term pressure of 16 bars. Their appearance is known to everyone. Cast iron radiators are characterized by heaviness and low mechanical strength. These devices are also characterized by high thermal inertia that makes it almost impossible to use automatic thermal regulators on them. But they are indispensable in volatile central heating. If there are jumps in the temperature of the warmth carrier, the radiators do not get cool for a long time because of their warmth inertia. This advantage can be also a disadvantage of cast iron radiators and in large houses it is impossible to use climate control devices. For example, an automatic device gives orders to reduce the temperature of the warmth carrier and to open a three-way mixer or to dampen the flame of the boiler, but the cast iron radiators seem not to notice that the warmth carrier has become cooler and continue to give heat and the opposite — if the outdoor temperature decreases, the automatic device decreases the temperature of the warmth carrier, but the cast iron radiator needs time to get hot. If an automatic device continually reacts to weather changes by increasing or decreasing the temperature of the warmth carrier only by several degrees, the cast iron radiators will not notice at all, and continue to work in a steady manner. But they work quite well in heating schemes with natural circulation where, without any automatic devices, self-regulation of warmth carrier temperature occurs — this is the same «low paced» (inertial) process as that which occurs in cast iron radiators.
Aluminum Radiators have a more attractive appearance. The relatively high mechanical properties of aluminum makes it possible to create radiators from it with sections of large surface. In addition to the external differences of aluminum radiators of different models and manufacturers, there are differences in their production technology. The most common method of casting under pressure is from silumines — alloys from aluminum and silicon with a silicon content up to 12%. As a rule such radiators are designed for a working pressure of 6 bars.
Aluminum radiators, unlike cast iron ones, the capacity of the sections of which is 2,5–3,5 litres, have a section capacity of less the 0,5 litres. Reducing the total capacity of the heating system entails many benefits and amenities. Lower capacity causes a decrease in warmth carrier volume and diameters of the pipelines and also of the power supply of circulating pumps and, as a result, reduces the financial cost. Moreover, the system becomes less inert and therefore reacts more easily to automatic management.
Bi-metallic radiators (see Figure 81). To improve the characteristics of aluminum radiators a combination of aluminum and steel is used, as being a more durable construction material (bi-metallic radiators). In such radiators, either the channels connecting high and low collectors are made of steel or the whole internal part of the sections (channels + collectors) is made of steel, and, as a result, there is no contact of the warmth carrier with the material of the aluminum ribs.
Steel panel radiators have become widely spread because of their relatively low cost and a variety of options — for height, length, depth, and thermal power. Another of their features is the possibility of modifications with integrated thermostatic valves.
Heated towel rails. Steel radiators for bathrooms or heated towel rails form a separate group. In their construction, they have steel tubular radiators in which the horizontal curved or straight pipes are welded into vertical collectors. These devices are designed for heating auxiliary rooms — bathrooms, lavatories, corridors, kitchens, etc. In Russia, heated towel rails are connected to the circulational pipe of the hot water supply system, so that the water remains hot even if the hot water tap has been closed for some time. Abroad, they are connected to the heating system. Because of this, imported heating towel rails are made of pipes with less thick walls.
Compact-radiators. The basis of compact-radiators is steel sheet panels which can be fitted with vertical ribbing. Thereby, convection heat transfer of these devices is increased to 75% and the metal content and warmth inertia are reduced. This makes it possible to use compact-radiators in low-temperature heating systems with reduced parameters of the warmth carrier (55/35 degrees C) which are more effective from the energy point of view. Compact-radiators can serve as an addition for the floor heating systems.
While heating rooms with radiators, there is always a choice: either install small radiators and increase the heat output from them by increasing the temperature of the warmth carrier (high temperature heating) or the opposite, try to get the same heat output by increasing the radiator size but with lower surface temperature (low temperature heating). If the heating is high temperature, then radiators become too hot and cannot be touched (see Figure 82). This is uneconomical and this system has no regulation margin. Besides, if the temperature on the radiator is high, decomposition of organic dust, which is in every room, occurs. The product of the this decomposition goes into the air and then gets inhaled by people. In low temperature heating, the radiators are slightly warm, but the room is warm. It is comfortable, safe, and more economical. Studies have shown that the most comfortable radiator temperature for a person is 37 degrees C.
