Document ExKyNDEaD5Z5gm9Znw3dav2Z0

588 CHAPTER 25 1954 Guide capacity for both ventilation and normal heat losses. In such a case no direct radiation is required. The cost of installation of a combined system is usually less than that of a split system. The unit ventilator is normally arranged to circulate outdoor air constantly or to circulate room air con stantly up to full capacity; or to circulate mixed and variable quantities of room air and outdoor air automatically, depending on the thermal require ment of the room. The minimum amount of outdoor air for ventilation purposes may be governed by state or local codes or may be calculated by the engineer to meet the ventilating air needs of the particular application. Selection of Unit Ventilator Size The primary consideration in the selection of the size of unit ventilator is the number of occupants in the space. Other factors to be considered are state and local code requirements, volume of .the room, density of occu pancy, and the usage of the room. A safe rule for determining air capacity is to allow a total air quantity of 30 cfm per person, or six to nine room air changes through the unit, whichever is greater. With this quantity of air handled, it is possible to obtain satisfactory cooling in mild weather. Since cooling is an important function, the unit ventilator must be selected to supply adequate air quantities. After selecting the basic size of unit, the coil capacity to meet the heating requirement can be determined from the manufacturer's tables. Control of Unit Ventilators Three cycles of control are available for use with unit ventilators. These cycles of control determine the sequence of operation of the dampers and heating element as follows: Cycle X--All Outdoor Air. During the heating-up period, the damper remains closed to outdoor air and the unit ventilator recirculates room air. Just before the desired room temperature is reached, the room thermostat operates the damper to admit only outdoor air. No recirculation takes place during the periods of room occupancy. Cycle Y--A Variable Quantity of Outdoor Air with a Fixed Minimum. During the heating-up period, the damper remains closed to outdoor air and the unit venti lator recirculates room air. Just before the desired room temperature is reached, the room thermostat causes the damper to open to admit the desired minimum quantity of outdoor air, the balance of the air being taken from the room. As long as the minimum quantity of outdoor air is sufficient to prevent overheating, the damper . remains in this minimum position. If more outdoor air is needed for cooling, the damper, under control of the room thermostat and the air stream thermostat, is operated to increase the proportion of outdoor air as needed, up to the maximum. Cycle Z--A Variable Quantity of Outdoor Air without a Fixed. Minimum. During the heating-up period, the damper remains closed to outdoor air and the unit venti lator recirculates room air. Just before the desired room temperature is reached, the air stream thermostat, placed ahead of the heating element to control the mixture ot room air and outdoor air, assumes full control over the damper. Thereafter, this instrument positions the damper to maintain a predetermined constant-temperature mixture of indoor and outdoor air during all occupied periods. Meanwhile, the heat ing element, under the control of the room thermostat, adds just sufficient heat to the air mixture to maintain the desired room temperature. Location of Unit Ventilator The location of the unit ventilator in a room is important. Wherever possible it should be placed against an outside wall and on the center line Unit Heaters and Unit Ventilators 589 of the room. It is difficult to obtain proper air distribution if the unit is installed either on an inside wall or in a comer of the room. Standard units discharge the air stream upward, but for special cases units may be installed to discharge air horizontally.' Units may be set against the wall or par tially recessed into the wall to save space without materially affecting the results. Air Exhaust Vents and Flues The size and location of the air exhaust vent6 outlet are important and in many cases, are regulated by laws for public buildings. Where no codes govern, the location and size of vents are left to the discretion of the en gineer. Best results have been obtained with a velocity through the vent open ings nearly equal to that at which the air is introduced into the room,, thus Fig. 12. Typical Window Ventilator maintaining a slight pressure in the room. Calculated velocities at the vent openings of from 600 to 800 fpm produce the best diffusion results from this system. Many states, however, have regulations that will not permit velocities as high as 800 fpm. If a vent opening at or near the floor is near a desk or place where a person is seated, a velocity of 800 fpm in the vent opening will produce an objectionable draft. In such a case the veloc ity in the vent opening should not exceed 400 to 450 fpm, although duct velocities may be maintained at 600 to 800 fpm if codes permit. In school buildings provided with wardrobes or cloakrooms, the vents may be so located that the air passes through these spaces, ventilating them with air which otherwise would be passed to the outside without being used to the best advantage. Many state and local codes for ventilation of public buildings make this arrangement mandatory. WINDOW VENTILATORS A window ventilator illustrated in Fig. 12 consists of filter and switch controlled motor-driven fans enclosed in a cabinet to be mounted on the window sill. Such units accomplish ventilation, air cleaning, and air cir culation, but have no means of heating the air. The direction of air dis charge is manually adjustable for seasonal operation.