Design Principles for Radiant Systems
Determining Thermal Losses for Radiant Systems
Thermal loss calculations for spaces to which radiant systems shall be applied are similar to those made for conventional systems. As specified in Turkish Standards, the calculation of thermal loss through transmission via external walls, roofs, and windows is performed in the same way as for conventional systems. The issue that should be handled carefully here is the fact that while indoor air temperature around the roof is assume to be 5°C higher than the ambient temperature for radiant systems, the temperature around the roof for conventional systems is 20 and even 30°C higher than the ambient temperature.
Since radiant systems are usually implemented for very large spaces, heat loss from air leakage is calculated instead of heat loss from infiltration. In these types of large buildings, leakages through seams between roofs and walls, corners, window profiles and walls cause a much larger heat loss than is calculated using the heat loss from infiltration method.
According to this, the quantity of air within the building that can be exchanges within 1 hour is important. The heat loss from leakage factor which does not have a theoretical basis for calculation is determined on experience based on the building’s location, insulation status, and the time during which doors remain open. However, as a coarse generality, this factor can be taken as around 0.3 for buildings considered to have a medium level of air leakage.
Adjustment Factors
Two different factors are considered in monitoring radiant systems:
1. RAF (Radiant Adjustment Factor)
2. HAF (Height Adjustment Factor)
Radiant Adjustment Factor (RAF)
Thermal output should be adjusted according to the heater efficiency. This means that less heating than the calculated amount will occur as a result of the adjustment. However, applications have shown that the heating calculated without applying an RAF is too much for the space, and in addition to the increased initial cost, there is the problem of the heaters operating for short and standing by for long stretches of time. A decrease in the number of heaters is preferable in terms of initial investment cost. In addition to this, since it is possible for the system to remain online for longer periods, to stop for short periods, or to operate on a continuous basis in case of insufficient heating load a reduction in the number of heaters with the application of RAF does not cause insufficiency in heating capacity. In indicating the burner efficiency;
If ? > 90 %, thermal output is reduced by 20% according to RAF.
If ? ? 90 % , thermal output is reduced by 15% according to RAF.
Height Adjustment Factor (HAF)
Rated powers of radiant heaters indicate the thermal output that can be achieved in the event that they are mounted at a height of 6 m. If the space where the system is to be installed has a higher ceiling than 6 m, the thermal output should be raised according to the calculated value. This adjustment is made using the HAF.
In spaces with ceilings higher than 6m, the installed thermal output shall be increased by 3.5% for each meter after 6 m.
System Selection
The shape of the system is determined according to the space to which radiant heating will be applied. Factors influencing this are are elements which restrict construction and may increase thermal losses such as crane rays, electrical cables, the locations of columns and beams, areas mostly occupied by people, and the placement of windows.
The system selection may be based on a small number of large capacity heaters or a large number of small capacity heaters. A large number of small capacity heaters will naturally lead to a more homogeneous radiant umbrella. However, considering the increase in the number of vacuum pumps and burners, it is obvious that such a selection will not be cost effective. Therefore the proper balance should be established between the homogeneity of the radiant umbrella and the number of heaters, i.e. cost-effectiveness. In the event that heaters of varying outputs are used for the space to which the system shall be applied, a very good analysis of cost effectiveness and homogeneous heating should be carried out, on which will be based the decision to select thermal outputs of heaters.
Another important issue for spaces to which radiant heating is to be applied is the layout of the heaters. The major points that should be considered for determining the layout of heaters are the following:
1. Rays coming from the heater heat the space that falls within a triangular prism with an apex angle of 120°. This is called the radiant umbrella. Too much overlap (more than 1 m) between the projection areas of umbrellas should be avoided, depending on the ceiling height.
2. Designers should ensure that the radiant umbrellas are homogeneous on both sides of the building.
3. The rays should be prevented from striking cold walls. In other words, if heaters are placed at perpendicular to walls, compounding the heat loss from these walls with the excess caused by direct heating will be avoided.
4. Windows should not receive infrared rays. This is because glass lets the rays pass through.
5. Infrared controlled devices (such as CNC machines) may be affected by high intensity rays. It is preferable to keep optic sensors of these devices outside the radiant umbrella.
6. Special conditions of the space should be taken into consideration. For example, it should be possible to individually operate the unit that heats an area where people frequently work on their night shift.
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