Document 71v8v0NL5JaBpJ9bOX4KVG8zE

HEATINC VENTILATINC AIR CONDITIONING GUIDE 1942 USE OF CENTRAL SYSTEMS Several factors must be considered in deciding on whether or not to use a central system and in deciding on the type of central system and modifications required. These factors are: 1. Comparative effectiveness. 2. Characteristics and requirements of the load. 3. Space requirements. 4. Initial cost. 5. Operating costs and maintenance. The comparative effectiveness of the type of system and modification plays a major part in the choice and is to a large extent affected by the other factors. One great advantage provided by the central system lies CHAPTER 21. CENTRAL SYSTEMS FOR COMFORT AIR CONDITIONING on the same zone presents a problem to be studied. Ability to maintain conditions during the intermediate seasons without the use of refrigera tion must be considered at all times, particularly in those applications where a high internal load exists. The matter of space requirements may rule out one type of system or another. The avoidance of the use of rentable and usable space for equipment and ductwork, in office buildings, stores, etc., is most im portant since the loss of revenue is directly chargeable to the operating cost of the system. Consideration of the spaces available with respect to the type of system and method of distribution used and their effect on overall cost is essential. Initial cost is given first consideration all too frequently. While elaborately designed installations are seldom justifiable, proper con sideration must be given to operating costs and maintenance, performance required, and the life of the system; The increase in cost incurred'by in its ability to diffuse odors and smoke which may occur in parts of the system, so that the outside air is determined by the average instead of the sum of the peak requirements. However, caution must be used where odors are apt to be objectionable even if greatly diluted. In such cases a positive exhaust to the outdoors or a separate treatment for the particular locality is recommended. The averaging ability both as to odors and thermal effects of the system is one item to be considered in studying the comparative effectiveness, particularly where adequate zoning is to be provided. Full advantage of diversity and non-simultaneous peak load requirements can be taken in determining the dehumidified air quantity where adequate zoning is provided. The characteristics and requirements of the load frequently are the deciding factors. Wide, non-simultaneous variations in load between spaces or parts of the same space, indicate the necessity of zoning. Isolated spaces having a short time occupancy or brief load duration may be handled by units advantageously at times. The occurrence of simultaneous heating and cooling requirements in spaces having the same exposure or 396 Fig. 5. Summer Central System with Zoning by Recooling suitable zoning can be offset partially by reduced quantities of dehumidi fied air. with smaller air handling apparatus, and partially by reduced refrigeration requirements. In the end it may prove less expensive than an unsatisfactory single zone system. A small increase in initial cost to provide better access to equipment, better airflow and distribution, and better zoning, usually will pay for itself. Operating costs often receive too little attention. Proper relationship between equipment selected and the load to be carried must be obtained. Cooling systems in general operate at maximum capacity less than 20 per cent of the time and heating systems operate under design conditions but a few. days in the year. Good partial load performance is essential for low operation costs. Proper zoning, good arrangement and selection of equipment and type of system all tend to reduce operation costs. Central systems having die bulk of equipment in a central location properly arranged, and having only the equipment required for zoning distributed through the conditioned space or spaces, will have relatively low main tenance costs. Care must be used to provide ease of access to important equipment and those items requiring servicing. 397