Book - School of Science and Technology

Miscellaneous heat gains 91Table 3.10 Approximate heat gain from lighting (including control gear, if any)Approximate electrical loading (W/m 2 )Level ofillumination(lux)Tungsten filament White fluorescent HPdischargeOpen With diffuser Open With diffuser Recessed Mercury Sodium250 45 55 10 15 20 ± ±500 80 ± 15 25 30 20 10750 ± ± 25 35 40 30 15absence of specific details from a manufacturer, it may be assumed that the output to thespace of 50% for a conventional luminaire is reduced to about 20% for a type having airhandlingfacilities. The balance of the output in the two cases, 50 and 80%, respectively,passes to the ceiling void and, although no longer a heat gain to the space, remains to bedealt with by the central cooling plant in circumstances where some or all of the airextracted is recirculated.Heat from office machinery and computersThe rate of heat dissipated from these sources may be high, typically 15 W/m 2 and up to25 W/m 2 in a commercial office accommodating a large number of items of electronicequipment. Equipment loads in offices are generally specified in W/m 2 without priorknowledge of who will occupy or at what density and care should be taken not to overspecify with the resultant inefficiencies this creates. In areas devoted wholly to dataprocessing or financial dealing facilities, a peak of 70 80 W/m 2 may occur. Output ismainly convective and reference should be made to manufacturers' data to establishaccurate information.The level of heat gain from office equipment is likely to continue to change in theforeseeable future under sometimes opposing influences. It is predicted that there will be afurther increase in the use and that the machines will continue to be more powerful. It islikely, however, that there will be significant improvements in the efficiency of machinesthereby reducing energy consumption, flat screen technology being an example of this.The short-term effect is that power requirements may increase from the current typicalmaximum of about 280 to around 350 W per person, but with a suggested fall in thelonger term to 150 W per person.Heat gains in excess of 600 W/m 2 may be expected in computer rooms and those areashousing support equipment. Such spaces will certainly need to be air-conditioned in orderto maintain the electronic equipment within the limits of temperature and humidity setdown by the makers and to provide comfort conditions for operators. It is interesting tonote signs of return to a practice adopted for early computers whereby certain processingunits incorporated a cooling coil and a closed circuit air circulation within the casing.Manufacturers' literature in such cases requires only that a supply of chilled water,specified as to quantity (possibly in US gallons) and temperature, be provided.Heat from machines and process equipmentElectrical energy provided to motor drives is for the most part converted wholly into heatat the time it is consumed, the exceptions being where work is done to produce potential