Document JmGb2ZZpKMKeLj7kzkM12J8e

268 CHAPTER 18 1960 Guide poising warm air over the cooler basement walls and floor. In areas of high prevailing dew points, however, this prac tice may cause objectionable dampness to develop in the basement. Complete summer air conditioning can be readily com bined with forced warm air systems either by using factorybuilt units incorporating both refrigeration and beating equipment, or in some cases by adapting separate refrigera tion equipment or well-water coils to existing warm air systems. A more complete treatment of residential summer air con ditioning is given in Chapter 46. GRAVITY WARM AIR SYSTEMS In gravity warm air heating systems the motive head pro ducing flow depends upon the difference in weight between theheated air leaving the top of the furnace casing and the cooled air entering the bonnet of the casing. Hence, the mo tive head is very small and the system must allow free flow. A gravity warm air furnace heating plant consists of a fuel-burning furnace or heater, enclosed inra casing of sheet metal, which is placed in the basement of the building. The heated air, foken from the bonnet on the top of the furnace casing, is distributed to the various rooms of the building through sheet-metal warm air pipes. The warm air pipes in the basement are known as leaders, and the vertical warm air pipes which are installed in the inside partitions of the building are called stacks. The heated air is discharged into the rooms through registers which are set in register boxes placed either in the floor or in the side wall, usually at or near the-baseboard. The air supply to the furnace is usually taken entirely from inside the building through one or more recirculating ducts, although in some cases an outside air supply duct is provided. Gravity warm air heating systems may be designed1* by following the procedures given in Manual 5 of the National Warm Air Heating and Air Conditioning Association. This publication also gives information on terminology, outlet and return locations, and standard sizes of pipe and fittings. The data underlying this information and the design procedure are given in a circular" issued by the University of Illinois, and are based on research conducted there. REFERENCES 1 Application Guide for Residential Central Aw Conditioning Systems--Winter and Year-Round (National Warm Air Beat-. ing and Air Conditioning Association, Manual 8, Fifth Edi tion, 1358). *A. P. Krata sod S. Konxo: Performance of a forced warmair heating system as affected by volume and temperature of air recirculated (ASHVE T&ansacrox3, Vol. 48, 1942, p. 393). * Manual for Adjusting Winter Air ConditioningSystems for Maximum Comfort (National Warm Air Heating and Air Conditioning Association, Manual 6, 3rd ed., 1957). * Warm Air Perimeter Heating (National Warm Air Heating and Air Conditioning Association, Manual 4, 6th ed., 1959). * Four-Inch Pipe Warm Air Perimeter Heating (National Warm Air Heating and Air Conditioning Association, Manual 10, 1956). * Code and Manual for the Design and Installation of Warm Aw Winter Air Conditioning Systems (National Warm Air Heating and Air Conditioning Association, Manual 7, 1953). 1 Standards for the Installation of Residence Type Warm Air Heating and Atr Conditioning Systems (National Board of Fire Underwriters No. 9QB and National Fire Protection As sociation No. 90B, 1956.) * Standard for the Installation of Atr Conditioning and Ventilating Systems of Other than Residence Type (National Board of Fire Underwriters No. 90A and National Fire Pro tection Association No. 90A, 1957.) * Criteria and Test Procedures for Combustible Materials Used for Warm Air Ducts Encased tn Concrete Slab Floors (Federal Housing Administration, 1956.) " Calculating Heat Losses (National Warm Air Heating and Air Conditioning Association, Manual 3,6th ed., 1957). 11S. Konzo, R. J. Martin, I>. 8. Levinson, and R. W. Roose.: Proposed design procedures for large mechanical warm air heating systems (ASHVE Transactions, Vol. 53, 1947, p. 177). " Code and Manual for the Design and Installation of Warm Atr Winter Air Conditioning Systems and Year 'Round Air Conditioning Systems (National Warm Air Heating and Air Conditioning Association, Manual 9, 6th ed., 1958). " Perimeter Warm Air Heating and Ventilating of Industrial, Commercial, and Public Buildings (National Warm Air Heat ing and Air Conditioning Asociation,. Supplement to Manual 9, 1955.) "S. Konzo and A. F. Hubbard: Automatic controls for forced-air heating systems (ASHVE Transactions, Vol. 40, 1934, p. 37). " Code and Manual for the Design and Installation of Warm Atr Ceiling Panel Systems (National Warm Air Heating and Air Conditioning Association, Manual 7-A, 1950, 3rd ed.). " Gravity Code and Manual for the Design and Installation of Gravity Warm Air Heating Systems (National Warm Air Heating and Air Conditioning Association, Manual 5, 1954). "A. P. Kratz and S. Konzo: Simplified Procedure for Se lecting Capacities of Duct Systems for Gravity Warm Atr Healing Plants (University of Illinois, Engineering Experiment Station Circular 45, December 1942). CHAPTER 19 CENTRAL SYSTEMS FOR AIR CONDITIONING Features of Systems, Zoning, Humidity Control, Cooling Load, Heating Load, Air Quantity and Temperature Differentia/, Unitary-Central Systems, High-Pressure Induction Convectors, High-Velocity Systems, Fan and Coil Units, Evaporative Cooling, Precooling, Sensible Cooling with Unwetted Coils, Selection of System, location of Apparatus THE term, central, applied to an air conditioning system heating or tempering coils are required to warm the enter implies that the equipment such as fans, coils, filters and ing outdoor air to a temperature above freezing, the heat their encasement are designed for assembly in the field ratherbeing supplied by means of hot water or steam. Cooling than in a factory as a unit. As a central system usually coils provide the necessary sensible cooling and dehumidifi serves several different rooms, or spaces, individual controls cation. The coils may be chilled by direct expansion of an are required for each room. approved refrigerant within the tubes, or by a pump-cir FEATURES OF CENTRAL SYSTEMS culated liquid such as water or brine. A water-tight drainage tank must be installed under the cooling coil and should One advantage of a central air-supply system is that one extend for a distance toward the fan. apparatus serving many rooms may involve a lower invest Reheater coils, utilizing steam or hot water, reheat the ment cost than that for a number of self-contained plants, air in warm weather for control purposes or bring the au each serving a single room. A central system may occupy to its final temperature in cold weather. For humidification basement of attic space that is relatively unimportant, in dry weather, water sprays are used either separately as whereas individual factory-assembled apparatus plfmed in shown or, combined with the cooling coils having the spray each room may occupy otherwise valuable space. Another nozzles directed against the coil surfaces. The spray water advantage of a central system is accessibility for servicing, since it is possible to provide doors in the`encasement for cleaning and inspecting all of the component parts in a manner usually superior to that practicable with compact factory-assembled equipment. In addition service is con centrated in one or just a few places. Central- air-conditioning systems usually are connected by ducts with the various rooms served, and preferably have exhaust fans that may effect complete removal and dis posal of any desired proportion of the air. The return-air fan may return air to the supply system for recirculation, as a measure of economy of fuel or refrigeration. . Central air-conditioning systems are served by heating and refrigerating equipment which may be located at some distance from the air-supply apparatus, and which may serve one or more central air-supply systems. Fig. 1 is a sketch of a typical year-round central system. Outdoor air enters from an intake preferably on that side of the building least exposed to solar heat, and not close to the ground or to a sun-heated or dust-gathering roof. The outdoor-air damper is split into two sections, minimum outdoor-air and maximum outdoor-air, with the marimum outdoor-air damper interlocked with the return-air damper in such a manner that as the outdoorrair volume increaf**? the return-air volume decreases. The return-air duct or may be circulated by a small pump from the water tank under the spray chamber. The general requirements for the control of the various components of the central station apparatus are covered in Chapter 43. The functioning of a typical set of controls as illustrated in Fig. 1 is as follows: connection could come from a return-air fan. The return-air and outdoor-air connections should be arranged for complete mixing of the two air streams to minimize the possibility of stratification through equipment. All the air passes through filters which must have ample room on both sides of the filters for servicing. The filters may be of a mechanically- Whenever the fan is started, solenoid air valve or relay E-l, actuated by the fan motor starter, opens minimum outdoor-air damper~D-l, places humidistat H in service, and allows duct thermostats T-3 and T-4 to control the maximum outdoor-air damper D-2 and the return-air damper D-3. When the fans stop, E-l is de-energized to close the out door-cur dampers and ala** to close humidifier valve V-4. cleaned type, a replaceable-cell type, or may be electronic as described in Chapter 24. The cleaned air passes to the equipment that changes its temperature and humidity. Except in very warm climates, Thermostat T-l positions steam valve V-3, on the reheater coil, to maintain a constant space temperature. As the space temperature rises, T-l positions rebeater valve V-3 to a closed or to a minimum open position, as determined by low limit discharge thermostat T-5. Duct thermostat T-6, in the pres