Heat Gain from Lights

Heat gain from light contribute to heat load and may have major impact on the air condition system

The heat gained from lights in a modern office or production area may be of a significant amount. The heat emitted to a room depends on the

• preferred light level in the room
• type of lights and their construction
• location of the light equipment

Light Level - Illuminance

The preferred light level in a room depends primarily on the type of activity. For common office work the level may be in the range of 500 - 1000 lux.

• Recommended Light Levels in Rooms

Necessary Installed Electric Power for Lights

The electric power to the light equipment is at the end converted to heat emitting to the room (unless special arrangements is used as local cooling or air outlets through the lighting equipment). Electric power to achieve a recommended light level can be expressed as:

P = b / (η_e η_r l_s) (1)

where

P = installed electric power (W/m² floor area)

b = recommended light level (lux, lumen/m²)

η_e = light equipment efficiency

η_r = room lighting efficiency

l_s = emitted light from the source (lumen/W)

Emitted Light From Source - l_s

The purpose of a lamp is to convert electrical power (Watts) into light (lumens). Different lamps do this with varying efficiencies and the light emitted from a source depends on the type of source.

The typical efficiency of different lamp types can be found in the table below:

• A typical incandescent GLS light bulb emit approximately 10 lumen/Watt.
• A typical fluorescent tube emit up to approximately 60 lumen/Watt.

Light Equipment Efficiency - η_e

The light equipment efficiency express how much of the light is really emitted from the light equipment to the room.

A bare fluorescent tube emits 100% to the room. A shielded tube emit less - between 50% to 80% is common.

Room Lighting Efficiency - η_r

The room lightning efficiency express how much of the light is absorbed by the room before entering the activity area.

Light Equipment Efficiency and Room Lightning Efficiency influence each other. Common values of the product η_e η_r are in the range 0.3 - 0.6.

Example - Heat Load from Lights

1,000 lux is recommended light level in a office where detailed drawing work is performed. The room and lightning equipment efficiency an be set to 0.5.

Using standard GLS bulbs - the electric power for lightning bgcolor="#FFFFCC"can be calculated as:

P = (1,000 lumen/m²) / (0.5 (10 lumen/W))

= 200 W/m²

Using standard fluorescent tubes - the electric power for lightning can be calculated as:

P = (1,000 lumen/m²) / (0.5 (60 lumen/W))

= 33.3 W/m²

Due to the high energy consumption an the major impact on the air conditions system, standard GLS bulbs is not an alternative for high illuminance applications.

The table below shows the installed electrical effect at different light levels:

Note! Datasheets from manufacturers should always be consulted before detail engineering. The numbers above will do for rough preliminary calculations.

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