Selection of heaters
The basis for selecting the surface of radiators for a heated room is the knowledge of the heat demand. When calculating thermal needs, attention should be paid to the method of laying heating risers in rooms. If these risers are not guided in furrows, but along the walls, and are exposed, then they constitute an additional heating element for these rooms. Means, that the heat flow through the walls of these risers is a heat gain, by which the nominal thermal needs should be reduced. The value of such profits depends on the parameters of the heating medium and the height of the room (vertical floors) and the diameter of the risers. For typical heating parameters of 90/70/20 ° C, room height 2,5 m and the nominal diameter of the riser 32 Mm, these gains can range from 90-110 In. This is a small value, but in rooms with low heat losses it may reduce the surface of the radiators. Under nominal conditions, the surface of the radiators will most often be determined from the relationship:
where Qp is the thermal needs for heating and ventilation for a given room, Qz are heat gains from risers led on the walls (In), ti is the internal temperature (C), and Ug is the heat transfer coefficient of a given type of radiator (W / m2K), with supply temperatures t1 or tz (°C) and return t2 or tp (°C). However, it should be emphasized, that risers should be insulated, which is a basic condition for heat saving and control of its consumption (Ie. Qz ≅ 0).
The surface of the radiator also depends on the method of its installation, the use of additional housings and location in the room. In practice, it consists in introducing dimensionless correction coefficients to the relationship, with values depending on the above-mentioned conditions. For a heater with known nominal values of the heat transfer coefficient, it is also important to take into account the variability of this coefficient with the change of operating conditions determined by the average temperature difference. This is mainly due to the implemented change in the operating parameters of the central heating installation (t1 / t2 the tz / tp) as the thermal needs of the room change, follows changes in the outside temperature, therefore from the necessary change in the heat output of the radiator.
An example of the dependence of the thermal power correction factor for a heater installed in a traditional way in a room with a temperature of t; = + 20 ° C from temperature difference (A9) and the exponent ng, Marks: Qg0, Qgx – the amount of heat emitted by the radiator under nominal and altered conditions, respectively, Δℑgo, Δℑgx – temperature differences between the average temperature of the radiator and the temperature in the room.
Comparison of correction factors to the thermal efficiency of radiators as a result of changes in temperature differences (for ΔIgo = 60 K)
Summary of correction factors for the thermal efficiency of radiators (and their surface) shown in the table, for sectional radiators, plate and single-layer convectors with a power factor ng = 1,3, in installations with typical parameters, for which it is possible to use the arithmetic temperature difference. The catalogs of most currently available radiators usually contain tables of the discussed correction factors for other than nominal parameters of the heating system..