Electric lighting

Luminous intensity is defined, by international agreement, in terms of the brightness of molten platinum at a temperature of 1755°C, and the unit adopted for its expression is the candela. A point source of light delivers a flow of luminous energy which is expressed in lumens; the quotient of this luminous flux and the solid angle of the infinitesimal cone, in any direction, is the intensity of illumination expressed in candela. The density of luminous flux is the amount of luminous energy uniformly received by an area of one square metre. Thus, illumination, or density of luminous flux, is expressed in lumens per square metre, or otherwise termed lux.

Electric lighting is usually chosen to produce a certain standard of illumination and, in doing so, electrical energy is liberated. Most of the energy appears immediately as heat, but even the small proportion initially dissipated as light eventually becomes heat after multiple reflections and reactions with the surfaces inside the room.

The standard of illumination produced depends not only on the electrical power of the source but also on the method of light production, the area of the surfaces within the room, their colour and their reflective properties. The consequence is that no straightforward relation exists between electrical power and standard of illumination. For example, fluorescent tube light fittings are more efficient than are tungsten filament lamps. This means that for a given room and furnishings, more electrical power, and hence more heat dissipation, is involved in maintaining a given standard of illumination if tungsten lamps are used. Table 7.15 gives some approximate guidance on power required for various intensities of illumination.

Table 7.15 Typical total heat emissions for various illuminances and luminaires

The table is based on a room of dimensions 9 m x 6 m, in plan, with light fittings mounted 3 m above floor level, for tungsten lamps and fluorescent fittings, and of dimensions 15mx9mx4m high, for mercury lamps. Light-coloured decorations and a reasonably clean room are assumed in each case. Generally speaking, larger rooms require fewer W m“2 than smaller rooms do, for the same illumination.

The efficiency of fluorescent lamps deteriorates with age. Whereas initially a 40 W tube might produce 2000 lux with the liberation of 48 watts of power, the standard of illumination might fall to 1600 lux with the liberations of 48 watts at each tube after 7500 hours of life.

A further comment on fluorescent fittings is that the electrical power absorbed at the fitting is greater than that necessary to produce the light at the tube. A fitting which has 80 watts printed on the tube will need 100 watts of power supplied to it; the surplus 20 watts is liberated directly from the control gear of the fitting as heat into the room.

As a rough guide it can be assumed that a typical modern lighting standard of 500 lux in an office involves a power supply of about 14 to 20 W m-2 of floor area. Thus an office measuring 5 m x 6 m will require about 600 W at its fluorescent light fittings to produce an illuminance of 500 lux. Six tubes each of 80 W (liberating 100 W) will be needed.

The heat liberated when electric lights are switched on is not felt immediately as a load by the air conditioning system since the heat transfer is largely by radiation. As with solar radiation, time must pass before a convective heat gain from solid surfaces causes the air temperature to rise. With an air conditioning system running for 12 hours a day and a floor slab of 150 kg irf2, the decrement (storage) factor, according to the Carrier Air Conditioning Company (1965), to be applied to the heat gain from lights, is about 0.42 immediately and 0.86 one hour after the lights have been turned on, for projecting light fittings. If the light fittings are recessed, the corresponding factors are about 0.4 and 0.81. The decrement becomes 0.93 after 4 hours.

This can be advantageous if the lights are recessed and the ceiling space above is used as part of the air extract system. Under these circumstances the storage factors are 0.34 initially, 0.72 after one hour and 0.97 after 8 hours. If the light fitting itself is used as an air outlet, for the extraction of air from the room, then the effect is a long-term one and a permanent allowance can be made for the heat liberated at the light and carried away by the extracted air, provided adequate information is available from the manufacturers of the light fitting. If all the extracted air is discharged to the atmosphere then full value is credited for the reduction of the air conditioning load. However, if as is more likely, a good deal of the extracted air is recirculated, the full effect is not felt and due allowance must be made by increasing the temperature of the mixture air (outside air plus the recirculated air). Since the maximum gain is usually some time after the lights are first switched on, the storage factor for lights should usually be taken as unity, when calculating the sensible heat gains.

When the ceiling void is not used as an extract air path the full emission from the electric lighting and its control gear is a heat gain to the conditioned space. If the ceiling void is used as a return air path then, for the older type of fluorescent tubes, Westinghouse (1970) argue that only 60 per cent to 70 per cent of the heat is liberated into the conditioned room. If the luminaire is an extract-ventilated type then 50 per cent to 60 per cent of the heat from the lights and control gear is given to the room. These figures take account of the fact that the ceiling void gets warmer and some heat is retransmitted through the suspended ceiling into the treated space and some flows upwards to the room above. No improvement is achieved by connecting ducts directly to the extract luminaires but there must be a basic extract duct system to convey the air back to the plant, for recirculation or discharge to waste. Such a basic duct system should have a dampering arrangement that ensures reasonably uniform extract air balance among the extract ventilated luminaires. In principle, no dampered extract duct spigot, in such a basic duct system, should be further than about 18 m from any extract luminaire.

Before using extract-ventilated luminaires it is essential to consult the manufacturers of the fluorescent tubes and obtain their approval.

Posted in Engineering Fifth Edition