Document Rabp1q7jDq4qE3ep4wmGMrrwX

American Society of Heating and Ventilating Engineers Guide, 1930 Gravity systems obtain the necessary pressure difference by using chimneys which are called vent flues, and which become more and more effective as the difference between the outside and inside temperatures increases. As long as a temperature difference of not less than 40 deg. between indoors and outdoors exists, these systems can be depended upon for a definite result. With slight temperature differences, the effective ness of gravity systems decreases rapidly. Gravity ventilating systems must usually operate, except when the rooms are on the windward side of the building, at somewhat lower atmos pheric pressure than that out-of-doors. This is due to the necessity for pressure-difference to cause air to enter the rooms on the leeward side. Ejector type ventilators and inlet hoods will cause very effective move ment of air while the wind blows, proportional to the wind velocity, but the results are not always dependable since they are so very profoundly affected by wind movement. Direct-Indirect Gravity Ventilating Systems Direct-indirect gravity ventilating systems have small openings through the outside walls below windows, with encasement of the super imposed radiator sections and are sometimes used in school building gravity ventilation. This system is very difficult to control, and becomes less and less effective as the temperature difference between indoors and outdoors decreases. The windward rooms may receive more air supply than can be warmed, while simultaneously, the leeward rooms are losing heated air due to reverse air currents through the direct-indirect heaters, due to the low pressure zone which always occurs in the lee of any obstruc tion during a high wind. Indirect Systems of Gravity Ventilation Indirect systems of gravity ventilation with convectors in the base ment may operate satisfactorily so long as their air supply comes from , windward and the temperature conditions inside and out permit the con vectors or radiators to be heated. As soon as the heat is shut off from these convectors and the ducts become equalized as^to temperature, air move ment ceases. For the first hour or so of school on a cold morning, a well-designed gravity ventilating system will function well. After that period and in comparatively mild weather, gravity ventilating systems cease to function since heat must be shut off from the radiator and the temperature dif ference which causes air movement must, without heat, decrease rapidly. Gravity ventilating systems are not well adapted to the installation of humidifiers and air cleaners, and their control and upkeep with such systems is difficult. - Studies of buildings which operate with gravity ventilating systems indicate that the windward rooms usually may, with care, be provided with satisfactory air movement and temperature control, but that during'' this time the leeward rooms get too warm and usually cannot be made to receive any air supply except that which comes through the windward rooms and the corridors. 120 Chapter 4--Systems of Ventilation MECHANICAL VENTILATING SYSTEMS With mechanical systems of ventilation the circulation of air is main tained positively and uniformly regardless of outside air conditions and when properly designed and operated, mechanical systems will furnish any desired temperature or humidity, under automatic control. In all situations where controlled ventilation is desired, mechanical ventilation is necessary. In all situations where temperature control and freedom from odors or dangerous gases or dust is required, mechanical ventilation is necessary. In all situations where air cooler than that in the rooms must be handled, as with dehumidifying and cooling systems, mechanical ventilation is necessary. Central Supply Systems Central supply systems of mechanical ventilation may operate inde pendently, or in conjunction with the heating of the building. In some cities, for instance, the schools for many years have been heated and ventilated satisfactorily with combined heating and ventilating, no heaters being placed in the classrooms. Temperature is controlled by the auto matic varying in proportions of the cool air and of the warm air. The heating, ventilation and cooling of many large banks and theaters is accomplished with combined heating and ventilating systems. With this system it is necessary to operate the fans in order to warm the building and to control the conditions. There are obvious advantages in such an arrangement for certain buildings. It tends to insure proper maintenance and intelligent operation. When the heating is accomplished with radiators, heat conductors or local heaters, separate from the ventilation, the scheme is called a split system. This type of mechanical ventilation is used for many schools. With the split system the building can be heated and conditions can be controlled to some extent even though the fans are shut down. This arrangement is advantageous for after-hours occupancy, and also for free dom from danger of shut-down in case the power for fan operation fails, etc. The outstanding advantage of central supply systems of mechanical ventilation is that they are especially adapted to control of the air tem perature and moisture and cleanliness. Unit Supply Systems Unit supply systems of mechanical ventilation also may operate to warm the air sufficiently to care for the heating of the building, or may be accompanied by independent heating apparatus. Each unit has a fan and an electric motor and a heater, and each unit should have a ther mostat which insures positive control of the temperature of the air delivered. It is possible to provide viscous dust filters with unit venti lating machines, but the cleaning and maintenance of such filters in the constricted areas which exist in many unit ventilating machines is an arduous and often neglected task. Unit ventilating machines in some what larger sizes than the conventional schoolroom types are available with built-in humidifying arrangements, including water tanks, pumps, eliminators and automatic humidity control. They may easily be con- 121