Lighting heat

The electrical power of fluorescent lamps is converted into light and, predominantly, heat (see figure).

Where ventilated luminaires are operated in a low-pressure ceiling with air exhausts instead of directly connected to an air extraction channel system, it can be assumed that the heat flow removed with the exhaust air corresponds to /_A + ̇/_z. Since ̇Q_R3 from the room below also enters the room in consideration, the sum of heat flows Q_R1 + Q_R2 + Q_R3 is the overall effective heat emitted by lighting in the room QB, which, for luminaires without air exhausts, corresponds to the overall electrical power P of the luminaires.

The heat emitted by lighting  ̇Q_B is determined using the following equation:

The coincidence factor l₁ accounts for the fact that in many cases at the time of maximum cooling load (e.g. due to intense sun irradiation) not all luminaires are switched on. This applies e.g. for large rooms, where a portion of the luminaires close to windows are switched off in the presence of daylight. The coincidence factor must be coordinated with the builder.

The storage factor s_B accounts for the temporal delay in room air heating up due to the storage effects of the room boundary surfaces. For luminaires without air exhausts, buildings are subdivided into construction type I and II. For luminaires with air exhausts, storage factors are mostly independent from the construction type, since a large portion of luminaire heat is directly removed and thus contributes less to stored heating.

The residual heat factor l₂ specifies which portion of the luminaires’ electrical power must be factored into the cooling load determination in stationary operation. For luminaires without air exhausts, the residual heat factor l₂ = , i.e. the overall connected load is cooling load. For luminaires without air exhausts, the residual heat factors depend on the air volume flow ̇V (in m³/h) as well as air exhaust system type.

In the case of air flow in low-pressure ceilings or through uninsulated exhaust ducts, a portion of the heat contained in the exhaust air is returned to the room via ceiling and floor. The residual heat factors of such systems are therefore slightly higher than those of systems with air flow in insulated exhaust ducts.

Figure contains a diagram which facilitates the determination of residual heat factors based on air volume flow of a luminaire (parameters for the straight line are luminaires for 1 or 2 lamps, 36 W or 58 W with ECG) depending on exhaust type. For example, the portion of luminaire heat burdening the cooling installation can amount to as little as 1/3 in ventilated luminaire operation within a low-pressure ceiling, depending on air volume flow (see marked example in Figure).

The method of luminaire calorimetry, which was used to determine heat flow with and without exhaust air on a luminaire-specific level with immense equipment effort has lost some of its significance since practice proved that usefulness and precision of the values determined using luminaire calorimeters stood in no relation to the required measurement efforts. To depict heat flow comprehensively, the entire room including its flow conditions must be considered. In special test assemblies, instead of calorimetric measurement at individual luminaires, the heat balance of entire room groups is determined for large projects. In most cases, the values depicted in figure provide sufficiently precise design data for cooling load calculations with ventilated luminaires.