Document XaQk3r2qzB5M7VaQp35BmE3G

224 CHAPTER 15 1960 Guide method of rating is subject to error from differences in types and calibration of instruments, grille design, and the care with which the rating test is conducted. Because of this, the standard air reding is used by manufacturers to rate unit ventilators. Table 1 shows the air delivery capacities by the two methods of rating and the approximate room boating equiv alent in square feet of direct radiation (EDR) of an inter mediate size of heating element. Heating elements are avail able for higher and lower capacities. Heating Capacity Requirements for Unit Ven tilators Since a unit ventilator has the dual function of introducing outdoor air for ventilation and maintaining a specified room temperature, the heat required by the unit may be similarly divided into (1) heat required for ventilation H. and (2) surplus heat H,. The surplus heat is available for main taining room temperature. If auxiliary radiation is installed, the surplus heat requirement may be reduced by a corre sponding amount. The sum of H. and H. is the total heat Hi to be supplied by the unit ventilator. These quantities of heat are related by the following equations (based on the use of 100 percent outdoor air): B% 0.24 W (i -- U) B, - 0.24 W (i[, -- U) R, - H, -- H, = 0.24 W (t, - 0 a) (2) (3) W - d60Q (4) H, - H, + 0.24 dl 60 Q (1 - Q (5) Fig. 3.... Typical Ceiling-Mounted Unit Ventilator where d = density of air, pounds per cubic foot (0.075 lb per cu ft for Standard Air by definition). H, = surplus heat, Btu per hour. H, = beat required to warm air for ventilation, Btu per hour. H, -- total heat requirements for both heating and ven tilation, Btu per hour. Q = volume of air bandied by the ventilating equipment, cubic feet per minute. t " temperature to be maintained in the room, Fahren heit L = outdoor temperature, Fahrenheit. . If = temperature of the air leaving the unit, Fahrenheit. W *= weight of air circulated, pounds per hour. 0.24 -- specific heat of air at constant pressure (approximate value). Example 1: The heat loss of a certain room is 24,000 Btu per hour, and the ventilating requirements are 1000 fm. If the room temperature is to be 70 F and all air is taken from the outdoors at 0 F, what will be the total heat demand on the unit if it is required to provide for both the heating and venti lating requirements (combined system)? Solution: Since the surplus heat is available to replace the heat loss of the room. H, = 24,000 Btu per hour. - Substituting in Equation 5: B, * 24,000 + 0.24 X 0.075 X 60 X 1000 (70 - 0) = 99,600 Btu per hour ;24,000 b 0.24 X 0.075 X 60 X 1000 + 70 92.2 F Fig. 2 .... Typical Floor-Mounted Unit Ventilator-- Blow-Through Type Horizontal Position Unit Ventilators and Unit Heaters Table 1....Typical Capacities of Unit Ventilators for an Entering Air Temperature of 0 F Cvbk Foot of air per eJsvts Anemarn* Standard ter rating air rafing 760 1000 1260 1560 500 750 1000 1250 ToM capacity in square feet. qtrivefent direct radiation 214 320 427 534 Capacity avaBable for healing 8m naai air temperature equivalent direct radiation f 56 95 84 95 112 95 141 95 If in Example 1 a 1000 cfm (Standard Air) unit were required, but only 25 percent of the air introduced were outdoor air, the solution is: H, - 24,000 + 0.24 X 0.075 X 60 X 0.25 X 1000 (70 - 0) - 42,900 The only difference from Example 1 is that the ventilation load has been reduced. Heating Capacity Ratings Heating capacity ratings for steam unit ventilators are obtained in accordance with the ASHAE Standard Code for Testing and Rating Steam Unit Ventilators* Under this code the capacities are established for varying air entering temperatures in addition to the standard rating. Capacity ratings include the entering air temperature, the total heat ing capacity, the surplus or heating capacity, and the final air temperature. Hot water unit ventilators have heating capacity ratings based on various entering water temperatures, flow rates or temperature drops, and entering air temperatures. Gas-fired unit ventilators have heating capacity ratings obtained in accordance with regulations established by the American Gas Association. Heating capacities include the total heat input and the total heat output, in Btu per hour, for various entering air temperatures. Electric unit ventilatoiB 'have heating capacity ratings based on the heat input to the electric heating element, in kilowatts or Btu per hour. The air capacities of hot wateT, gas-fired, and electric unit ventilators are obtained in accordance with the ASHAE Standard Code for Testing and Rating Steam Unit Venti lators.1 Application of Unit Ventilators Items to be considered in the application of unit venti lators are: (I) unit air capacity, (2) percent minimum out door air, (3) heating capacity, (4) cycle of control, and (5) location of unit. The primary considerations in the selection of the unit air capacity are the mild weather cooling capacity and the number of occupants in the space. Other factors to be con sidered are state and local'code requirements, volume of the room, density of occupancy, and the usage of the room. A safe rule for determining unit air capacity is to allow a total air quantity of 30 cfm per person, or six to nine room air changes, per hour, whichever is greater. With this quantity of 225 air handled, it is possible to obtain satisfactory cooling in mild weather. Table 2 gives recommended room volume cir culation rates for different types of spaces served. .For rooms facing East, South or West, higher values should he used. The minimum amount of outdoor air for ventilation is determined after the total air capacity has been established. It may be governed by state or local codes or may be cal culated by the engineer to meet the ventilating air needs of the particular application. The heating capacity of a unit to meet the heating require ment can be determined from manufacturers tables. Selec tion of heating capacity should always be made after deter mining the basic size of unit by selecting the unit air capacity. Automatic Control of Unit Ventilators Many different cycles of control are available. The prin ciple difference in the various cycles pertains to the amount of outdoor air delivered to the room during normal periods of occupancy. Usually a room thermostat controls both a valve to regulate the heat supply and a damper to regulate the supply of outdoor air. An air-stream thermostat in the unit prevents the discharge of air below the desired minimum temperature. Unit ventilator control cycles provide the proper sequence for the following stages: Warm-up Stage. All control cycles function to provide rapid warm-up by having the units provide full heat with the outdoor damper closed. Thus 100 percent room air is recirculated and heated until the room temperature ap proaches the desired temperature level. Heating and Ventilating Stage. As the room temperature rises into the operating range o! the thermostat, ventilation is accomplished by the partial or complete opening of the outdoor air damper according to the cycle used. Auxiliary heating equipment is shut off. As the room temperature continues to rise the unit ventilatoi heat supply is throttled. Cooling and Ventilating Stage. When the room temper ature rises above the normal level; cool air is discharged into the room. The room thermostat accomplishes this by throt tling the heat supply, finally shutting it off, and opening the outdoor air damper to prevent overheating of the room. The air stream thermostat frequently takes control during this stage to prevent the discharge temperature from falling below a set level. - The three basic cycles of control commonly used are as follows: Cycle X. 100 percent of outdoor air is admitted at all times except during the warm-up stage. Table 2.... Recommended Outdoor Air Capacities for Mild Weather Cooling For North Exposure Type of ipocy Classrooms Laboratories, Forge Shops Metal Working Shops Offices Card Rooms, Lodges Private Offices Cafeterias 4 Kitchens Recommended outdoor air delivery--room volumes per hoar 6-9 6-8 6-8 6-8 6-9 6-9 4K-7 .