Document npYdqwRK9pr23xx24bG5VN0Nm

288 CHAPTER 13 1957 Guide sky radiation and from outdoor-indoor temperature differential for glass areas and exterior, walls and roofs, modified by periodic heat flow or lag factors depending on the type of structure; (2) load due to heat gain through interior partitions, ceilings and floors; (3) load due to ventilation, either natural or mechanical; (4) load due to heat sources within the conditioned space such as people, lights, power equipment, and appliances; (5) load due to moisture transfer through permeable building materials; (6) miscellaneous heat sources. C. Determination of Air Quantity and Apparatus Dew Point. -- These factors will be discussed in turn. The material presented leads to an illustrative procedure for a cooling-load calculation, and a numerical example is given to demonstrate the calculations involved. DESIGN CONDITIONS Indoor Conditions Indoor air conditions for human health and comfort have been and con tinue to be the subject of much discussion and research. The effective temperature index, explained in Chapter 6 is probably the best available source of design criteria for comfort air conditioning systems for buildings or enclosures in which the air and inside surface Table 1 Design Room Conditions Usually Specified fob Summer Average Peak Load in Comfort Air Conditioning* Type of Installation Dby-Bulb Temp Wet-Bulb Temp*5 Relative Humidity Peb Cent Grains Pub Lb Effective Temp0 . 78 65 50 72.7 72.2 80 67 51 78.5 74.0 82 68 49 80.0 75.3 a Values in Table 1 are for peak load conditions. It is general practice to operate a syBtem at approxi mately 76 F and 50 percent relative humidity at other than peak load. ** Psychrometric data for standard barometric pressure. Fig. 10, Chapter 6, air movement 15 to 25 fpm. temperatures remain substantially equal; a condition that can safely be assumed for most ordinary comfort air conditioning installations. Other sources of design specifications are to be found in the requirements of codes and ordinances, and in the varied long-term experiences of manufacturers, contractors, and engineering specialists. Past experience, cumulative over many years, indicates that indoor der sign conditions for which summer air-conditioning equipment is selected, should not exceed a temperature of 80 F or a relative humidity of 50 per; cent for the average job in the United States. If these conditions are exceeded, complaints of discomfort may be expected, especially with con,' tinuous occupancy. For very brief occupancy only, a slightly higher peak.' load design temperature may be employed. In regard to the lower linpt of humidity, complaints are not encountered for store installations oper ated down to 35 percent relative humidity or, for office jobs, somewhat lower. These observations apply to normal commercial practice in country only; for extremes, such as tropical or very hot regions, it is re garded as more practicable to design for a peak-load outdoor-indoor tem perature difference of about 15 to 20 F. ' Table 1 offers typical design conditions for average requirements en countered. The values in line 1 would also apply in general for localiti?.; having a summer outdoor design temperature of 90 F or less; and IS to 40 min occupancy values, or even somewhat higher dry-bulb tem-,', Cooling Load 289 peratures, would indicate acceptable conditions for very hot localities. Table 1 is to be used with good judgment, for there is no universal rule whioh may be applied to indoor design conditions. Guarantees of conditions to be maintained for summer operation are based upon a definite set of load conditions. At other than the guarantee load, the conditions produced by a system are determined by the balance of imposed load and equipment capacity, and by the method adopted for regulating the system operation. Complete specifications of indoor design conditions would include part-load and overload operation, particularly from the viewpoint of economy. In the field of industrial air conditioning, indoor design conditions are established by the requirements of goods and processes, in addition to the comfort and efficiency of the workers. Many typical indoor design condi tions for products in industrial air conditioning are given in Chapter 45. The load calculations for- either comfort or industrial air conditioning are usually made in accordance with a guarantee. In comfort applica tions, for example, this is frequently 80 F and 51 percent relative humidity or 78 F and 50 percent relative humidity. The system is normally operated at 75 or 76 F and approximately 50 percent relative humidity at all times as long as the equipment has capacity to maintain these conditions. Dur ing very hot weather, the room temperature will rise above the control point and the equipment will operate continuously. Normally, industrial jobs are operated at design conditions at all times if the optimum condi tions are selected for the benefit of the product. The indoor design conditions suggested have had reference to conditions to be maintained at the level of occupancy. For extremely high ceilings in public or industrial buildings, only the zone from 10 to 15 ft above the floor may be cooled to the full extent. The air temperature at the ceiling would be much higher, and this should be kept in mind when calculating the convective portion of the roof heat gain. A reduction of outdoor-toindoor air temperature differential may be assumed in such instances; radiation from the inner roof surface is not diminished. Outdoor Conditions Summer climatic conditions and suggested design wet-bulb and drybulb temperatures are given in Table 2 for various locations in the United States. The highest temperature ever recorded is for the period of record shown. In some cases it should be noted that this period of record is com paratively short, and higher temperatures may be expected. In making comparisons for other localities than those shown in Table 2, due considera tion must be given to elevation. Column 6 of Table 2 indicates the design dry-bulb temperature suggested the A.S.H.A.E. Technical Advisory Committee on Weather Design Conditions. This temperature is the maximum hourly outdoor tempera ture which has been equalled or exceeded 2% percent of the total hours of 'hrae, July, August and September for the period of record, in this case ne 5-year period 1935-1939, and should not be confused with the period 01 reord given in Column 4 which applies only to highest temperature ever recorded. Since all of these data (Column 4) are based on airport ecords, they are not necessarily applicable to cities. Ch^6 Pven hi Columns 7 and 8 were obtained from local A.S.H.A.E. rm^P^er Secretaries, and represent the design temperatures in local use. /-esuch information was not available, it was taken from a publication he A.R.I.i an(j from various other sources.