Chapter 5 - Introduction to Radiant Panels

1.1 Radiant Panel Basics

A radiant heating panel, normally a flat surface, is defined as a surface from which at least 50 percent of energy transfer is accomplished through radiation. The balance of surface or panel face heat transfer occurs by convection. Radiant heating panels include concealed and visible panels that cover or encompass a large or small fraction of a wall, floor, or ceiling. Panel form ranges from discrete ceiling units to cove, recessed, or surface-mounted wall units, and embedded floor, ceiling, or wall hydronic tubing, or electric cables, wire, and mats.

Research results and physical governing laws prove that the radiative emission from a surface is independent of surface orientation. However, it is well documented that convection heat transfer is a strong function of surface orientation. For example, Nusselt number (Nu) correlation for a heated horizontal plate facing downward versus the same heated horizontal plate facing upward leads to substantially different convective heat transfer coefficients. As long as the surface temperature and other characteristics remain the same, the radiation heat transfer rate is unchanged. However, the varying heat transfer coefficient changes the percentage of the total energy that is represented by the radiant energy emitted from the surface.

For example, a heated plate facing downward with a surface temperature of 370 K exposed to an air temperature of 290 K results in a natural convection Nu of 98, leading to a convective heat flux from the surface of 110 W/m².The same plate exposed to the same conditions—but facing upward—results in a natural convection Nu of 491, almost five times larger, equating to a fivefold increase in the convective heat transfer rate—552 W/m².

For this plate at the given temperature, the radiative emission is 1063 W/m². Consequently, changing the orientation of the heated plate from horizontally facing downward to horizontally facing upward changes the percentage of the radiative emission from about 90 percent to 48 percent for these example conditions.

Large-percentage variation significantly impacts heating system design. The orientation can be adjusted to achieve the desired level of occupant thermal comfort, which may be determined from a combination of convective and radiant heating. The level of occupant thermal comfort obtainable from surface conditions and orientations is explained in Sec. 8 of this Handbook, where the BCAP design methodology is demonstrated.

Thermal radiation is transmitted at the speed of light, travels in straight lines that can be reflected, and elevates the temperature of solid objects by absorption but does not perceptibly heat the air through which it travels.All bodies in the built environment exchange thermal radiation continuously. The rate at which heat transfers depends on the following four factors:

1. Emitting surface and receiver temperature
2. Radiating surface emittance
3. Receiver reflectance, absorptance, and transmittance
4. Emitting surface and receiver view factor—viewing angle of the occupant to the radiant source

All types of radiant panels can be successfully designed if the preceding factors are taken into consideration in the analysis. The impacts of the physics involved often determine which type of radiant panel or heater is more appropriate for a given application as well as the operational efficiency of the installation.

1.1 Radiant Panel Basics

A radiant heating panel, normally a flat surface, is defined as a surface from which at least 50 percent of energy transfer is accomplished through radiation. The balance of surface or panel face heat transfer occurs by convection. Radiant heating panels include concealed and visible panels that cover or encompass a large or small fraction of a wall, floor, or ceiling. Panel form ranges from discrete ceiling units to cove, recessed, or surface-mounted wall units, and embedded floor, ceiling, or wall hydronic tubing, or electric cables, wire, and mats.

Research results and physical governing laws prove that the radiative emission from a surface is independent of surface orientation. However, it is well documented that convection heat transfer is a strong function of surface orientation. For example, Nusselt number (Nu) correlation for a heated horizontal plate facing downward versus the same heated horizontal plate facing upward leads to substantially different convective heat transfer coefficients. As long as the surface temperature and other characteristics remain the same, the radiation heat transfer rate is unchanged. However, the varying heat transfer coefficient changes the percentage of the total energy that is represented by the radiant energy emitted from the surface.

For example, a heated plate facing downward with a surface temperature of 370 K exposed to...