Document 26Dodpo7vXEmZK8LxN1zN2xg

568 CHAPTER 24 1952 Guide 25 percent of the air introduced were outdoor air, the solution would be as follows:- '!' : , Ht = 24,000 + 0.24' X0.075 X 60 X 0.25 X 1000 (70 - 0) = 42,000 .. ...... 24,000 t, =.---- ---- -------------------^---------- h 70 = 92.2. 0.24 X 0.075 X 60 X 1000 1 The only difference from Example 1 is that, in the latter case, the ventila tion load has been reduced. . Applications of Unit Ventilators . Items to be considered in the application of unit ventilators are: (1) combination with other means of heating, (2) selection of unit ventilator size, (3) cycle of control, (4) location of units, and (5) method of venting and exhausting.' . .` In a split system the unit ventilator heat output is supplemented by that of additional radiators.or convectors, and consequently a correspond ing reduction in. required'uhit ventilator heating capacity may be made. The combined system employs a unit ventilator with sufficient heating 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 so that a fixed minimum of outdoor air is introduced at all times. Theamount of outdoor airmay be governed by state or local codes; or may be calculated by -the engineer to meet the need of the application.. In some cases, a variable aihouht of outdoor air is introduced depending on outside temperatures. This is.done in order to conserve fuel, .with some sacrifice of ventilation. 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.' If the ventilation require ments are less than this value, some; air. may be recirculated from the room. With this quantity of air handled, it is possible to obtain satisfactory cooling in mild weather,;. Since coohng is an important function, the unit ventila tor- 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 front 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: Cyde 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 oc.cupancy. . . ..; . . .. , 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- Unit 'Heaters and Unit Ventilators 569 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 preven+ioverheating, 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-rrA 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 of 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 Fig. 11. Typical Window Ventilator of the room.. It is difficult to obtain proper air distribution if` the unit is installed either on an inside wall or in a corner 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 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