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376 Chapter 20 1945 Guide ------In -the foregoing1 it-has-been-'assumed-that summer reheating is derived -from-an extraneous source of heat such as steam, electric heaters or hot water. The economics of reheating as outlined may be improved by the use of sources of heat that are available within the equipment used. Where certain types of refrigeration cycles are used, auxiliary refrig erant condensing coils-can be placed in the air stream. At partial loads these can be used as additional refrigerant condensing surface and improve the performance of the refrigeration plant. This is generally known as hot gas reheating. Less effective is the use of coils in the air stream through which liquid refrigerant from the condenser is passed . before being delivered to the evaporator, sub-cooling the liquid refrigerant and improving the performance of the refrigeration cycle. The use of refrigeration condenser water as a source of reheating provides definite economies. If the condenser water is passed through coils in the air stream before being used for refrigerant condensing purposes, the lowering of the condenser water temperature accomplished by reheating the air will result in savings in the refrigeration plant power consumption and increased refrigeration capacity. The latter two methods are at some disadvantage in that the amount of reheat available decreases as the need for reheating increases, particularly where evaporative condensers or cooling towers are used. Zoning Zoning consists of an arrangement of equipment or a division of equip ment into sections that will permit individual control of the temperature and. humidity of those spaces or groups of spaces that do not have simul taneous variations in sensible or latent heat load. The equipment is so , arranged or divided that air can be supplied to spaces or groups of spaces in accordance with the individual load requirements of that space or group of spaces. Solar heat gain is one of the major causes of the zoning requirement since its effect and amount vary with the season, time of day and exposure. Other sources of heat gain, subject to variations, such as large changes in the number of occupants in one space with a constant occupancy in another indicate the necessity of zoning. Some of the various methods of zoning are: 1. Separate equipment. 2. Reheating or recooling. (See Figs. 4 and 5.) 3. Multiple fans with individual by-pass. (See Figs. 6 and 7.) 4. Volume control. (See Fig. 8.) 5. Dual duct system. (See Fig. 9.) 6. Combinations of the above methods. Zoning by separate equipment represents the extreme in zoning. In dividual conditioners, fans, heaters, controls, distributing duct work, etc., are provided for each zone and are separate from those of other zones. Each assembly of equipment is arranged to operate at full or partial load according to the requirements of that zone. In extreme cases an individual refrigeration plant may be provided for each zone. In general, this method of zoning is uneconomical since each piece of equipment is large enough to handle the full load requirements of that zone and no advantage can be taken of the fact that while one zone is at its full load others may be operating at considerably less than full capacity. This applies both to the initial cost of a system and the operating cost. There are, however, many cases where this method of zoning when used to a limited degree or combined with other methods is most desirable. Central Systems for Comfort Air Conditioning 377 Zoning by reheat is one of the more simple methods of approachirig-tKe~ problem of differing variations in load. Reheating has been discussed in ' the~fofegoing. Heating coils located in the distributing duct work or apparatus connections which supply only those spaces having sub stantially the same load variations will, by adding heat when the cooling load falls off (or the reverse in the case of a heating load), maintain the desired temperature conditions'. It has been shown previously that where the amount of reheating required is not excessive and that heat for this purpose is available from an economic standpoint this method of zoning may be entirely practical and even highly desirable. Zoning by recooling is literally the reverse of zoning by reheating. It is not used often but there are many cases where it is desirable. Its most practical application is limited to a sensible heat removal function where the latent heat requirements are handled by another source such as a dehumidifier or dehydrator, and where the recooling equipment (usually cold water cooling coils) can be utilized for reheating (usually with hot water) in the heating season. In using this method of zoning, when the cooling load of a given space is reduced, the cooling effect produced by the recooler is also reduced. The use of multiple fans with individual by-passes presents a simple and sometimes inexpensive method of zoning. Two or more fans may be arranged so that each draws its treated air from the same conditioner. The connection between the conditioner and fans is divided or partitioned in such a manner that a by-pass connection can be made to each fan. See Fig. 6. In this way the amount of by-pass air can be regulated accord ing to the requirements of the zone served by that fan. In this application a face damper must be used for each segment of the conditioner, arranged to close as the individual by-passes open or an unbalanced system may result. A much better adaptation of this principle, though slightly higher in initial cost, is obtained by using a single conditioner or dehumidifier, having a central conditioned air fan delivering the treated air to the necessary zone fans where the conditioned air is mixed with return air according to the requirements of the zone. See Fig. 7. With this method of zoning, the zone fans are provided with casings to which both returnair and conditioned air are delivered, their proportions being regulated by dampers working in opposite directions. If conditioned air is delivered to the casing at slight positive pressure the return air damper may.be omitted. Reheating can be effectively combined with either of these for year 'round' use or for winter use only. The reheater is usually located in the air stream to the zone fan. Where a central conditioned air fan is used to deliver conditioned air to a number of zones, a static pressure regulator controlling a volume damper or inlet vanes on this fan should be in stalled. to prevent unbalancing the system, when one or more zone fan conditioned air dampers are throttling. This is one of the better methods of zoning and is particularly effective when used in combination with reheating for winter or year .'round conditioning as previously outlined. Volume control is the least expensive and most frequently used method of zoning. It is usually obtained by placing a throttling damper in the supply duct feeding a particular zone and operating the damper to restrict flow of air as the heating or cooling load is reduced, The damper may be operated manually or automatically. Volume control, however, has two serious disadvantages. The first of these is that any large reduction in the quantity of air supplied may impair the ventilation. The second is that a large reduction of the air supply may entirely upset the distribution from the room outlets causing dead pockets, stratification,