Document DvDp07kk6L3VQd3zOwX9R0dRO

HEATING VENTILATING AIR CONDITIONING CUIDE 1942 Velocities through registers may be reduced by the use of registers larger than the connecting pipes. Some suggestions for equalizing veloci ties over the face area of the register by means of diffusers are illustrated in Fig. 4. Merely to use a larger register may not result in materially reduced velocities unless diffusers are used. Fig. 4. Diffusers in Transition Fittings to Equalize Velocities through Register Faces Dampers Suitable dampers are essential to any trunk or individual duct system, as it is virtually impossible to so lay out a system that it will be absolutely in balance without the use of dampers. Special care must be used in the design of any system to avoid turbulence and to minimize resistance. Sharp elbows, angles, and offsets should be avoided. (See chart giving loss of pressure in elbows, Chapter 32.) Volume D Squeeze Fig. 5. Three Types of Dampers Commonly Used for Trunk and Individual Duct Systems Three types of dampers are commonly used in trunk and individual duct systems. Volume dampers are used to completely cut off or reduce the flow, through pipes. (See A and B, Fig. 5.) Splitter dampers are used where a branch is taken off from a main trunk. (See C, Fig. 50 ( Squeeze dampers are used for adjusting the volume, of air flow and resistance through a given duct. (See D, Fig. 5.) It. is essential that a damper be provided for each main or duct branch. A positive locking device should be used with each type of damper. 384 CHAPTER 20. MECHANICAL WARM AIR FURNACE SYSTEMS Ducts The ducts may be either round or rectangular. The radii of elbows should be not less than one and one-half times the pipe diameter for round pipes, or the equivalent round pipe size in the case of rectangular ducts. AUTOMATIC CONTROLS Air stratification, high bonnet temperatures, excessive flue gas tem peratures, and heat overrun or lag in a properly designed system can be largely eliminated through proper care in the planning and installation of the control system3. The essential requirements of the control are: 1. To keep the fire burning when using solid fuel regardless of the weather. 2. To avoid excessive bonnet temperatures with resultant radiant heat losses into the basement. 3. To avoid the overheating of certain rooms through gravity action during off periods ol blower operation. 4. To have a sufficient supply of heat available at all times to avoid lag when the room thermostat calls for heat. 5. To prevent cold air delivery when heat supply is insufficient. 6. To avoid heat loss through the chimney by keeping stack temperatures low. 7. To provide quick response to the thermostat, with protection against overrun. 8. To provide for humidity control. 9. To provide a means of summer control of cooling. 10. To protect against fire hazards. The following controls are desirable: 1. A thermostat located at a point where maximum fluctuation in temperature can be expected, in order to secure frequent operation of fans, drafts, and burners. This location would be near an outside wall but not upon it, in a sun room, or in a room with some unusual exposure. The thermostat, of course, should not be located where it will be affected by direct radiant heat from the sun or from a fireplace, or by direct heat from any warm air duct or register. 2. A thermostatic blower switch located in the bonnet to permit blower operation only between the temperatures of 100 F and 150 F. In certain extreme cases it may be necessary, or weather conditions may make it advisable, to adjust the high limit to a higher temperature than that given. Another location sometimes used for the blower switch is in the main duct near the frame opening from the bonnet. 3. A protective limit control located in the bonnet to shut down the system inde pendently of the thermostat if the bonnet temperature exceeds 200 F. 4. On oil and gas burner installations, a control should be included which will shut down the system if the fire goes out or if there is a failure of the ignition system. 5. A humidistat to regulate the moisture supplied to the rooms. 6. On automatic stoker installations, a control is usually included which will start the operation regardless of thermostat settings whenever the bonnet temperature indicates that the fire is dying, or a time interval contactor is used that will start the stoker to run a predetermined length of time at predetermined intervals. METHOD OF DESIGNING FORCED-AIR HEATING SYSTEMS 1. Determine heat loss from each room in Btu per hour. (See Chapter 6.) 2. Locate warm air registers and return registers on plans of house, beginning with the upper story rooms. Automatic Controls for Forced-Air Heating Systems, by S. Konzo and A. F. Hubbard (A.S.H.V.E. Transactions, Vol. 40, 1934, p. 37). 385