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848 CHAPTER 54 1965 Guide And Data Book of the outlet grille at a distance of 2 in. and multiplying-; the average velocity by the gross area of.the grille. The' aae-^ ' mometer rating is based on the final temperature of the air leaving the grille while the unit is delivering outdoor-air and recirculated',air ini the proper:proportion, sand thejieiting element is supplying heat as spedfied. This method of rating is subject,.to error,due to differences in types and calibration of instruments and/grUteMesign^anfl1 to thVcare'witlj^hicli: the rating test ia'conducted. Because 'of'these variffEibns,"then standard air rating is used'by rating for unit ventilators. " Table 1 shows-typical air capacities as determined-by:the- two methods of rating. Heating elements are available for capacities higher aid lower than those given in Table !. ` HeatingCapdrity Requirements for Unit Ventilators ..... ttv` .. Rg.,; 3 . i.. TyP'cci^Ceillns-Mounted Unit VerifiJafor Since a unit'ventilator has the'dual-'fuhctio^'df iDtimiuong outdooriair-for (ventilation and- maintaining a specifiedroom temperature;'the beat required by>the unit maybe similarly divided' into- (1) heat' required' for.'ventilation; and (2)' surplus heat; -H',\ 'The- surplus Aeoi>is -'available1. for main taining room .temperature! If auxiliary radlatidn is-installedJ the.-surplus- Aeat.requirement -may be; reduced by!'a; corre sponding amount.5-The sum of fi.'and 'H. is.the total!heat,ff<;;tobe suppliedby, the unit-ventilator.' '.- ii i; '.'These quantities 'of heat- are1 related!-by .the .following equations (based;on the iise of lOO percent outdoor air)s r 'Y1 \}\0i24 Q' 'j/_ '/ 7-rT&(.- u) . Jf- r^<7:^. ";0-24 W ; `.o), 7,(2i Xv *-?. 1*"` - d.60Q " B, ^ 0.24'd m ^ d u!' .v.;(4) toftere ^ , .. d ** density, of air, pounds per cubic foot (0.075 L ) per cu . ;i. i : ft for Standard Air, by definition). jH, m surplus heat, BUi per hour. Mm " heat;required to warm air'for ventilation, Btu per ,f,--:hour. ...... * :Ht a total heat requirements for both beating and yen; tilation, Btu per hour. 1 Q " volume of air handled by ventilating equipment, . ;'cubic feet per minute.., : i t " temperature.,to be .... . . heit in the room, Fahren-,-i - t --outdoor temperate^ Fahrenheit. . . tftemperature of the air leaving the unit, Fahrenheit. W weight of sir circulated, pounds per hour. 0.24;6pecific heat of air-at constant'pressure (approximate .value). Example t: The heat'loss ;of a certain room is 24,000 Btu per hour,-->and the ventilating requirements are-1000 cfm.;If - the room temperature.is-to be.70 F, and all air is totp-n from the outdoors at,4) F,-what,will be the total lteat-demand bn the nnit. if if in raottiwwf #/i nmwiifa. frt. krt*k kn .n/J .unffl ........... r__ ________________ __ replace t heat loss of the roam:. , - H24,000 Btu-per hour Substituting in Equation'5: '' .`1|- ' Bt. "'-24,000 ,+.0.24 *X 6.075 X 60 X 1000 (70 - 6) ' / 1 - * ' . * 99,600 Btu per hour 24,000 + 70 - 92.2 F. . 0.24 X 0.075 X 60'X 1000 Unit'Ventilators and Unit Heaters 849 Table 1 . .Typical Air and.Heating Capacities of Unit Ventilators - Cvfatc f*d of air per mami* Heat for Vcafihxtian* Anemometer rating lln MSli Steam Total heat Hn MBh Surphn heat H,, MBh .1500 . 750 1000 1260 15G0 -, 1S60 9.45 14.2 15.9 236 28.3 , 40.6 59.8 76.5 96.6 117.7 31.2 45.6 57.6 72.0 86.4 p.--d ao 25 percent outdoor air at 0 P, 70 r room temperature.- Hoofing iinfiw Hot Water Total heat H MBh Surplus beat Hb MBh 32.0 48.0 64.0 80.0 96.0 22.5 33.8 45.1 56.4 67.7 Bectric Total heat Hi, MBh Surplus heat Hm MBh 31.6 47.6 63.9 79.6 95.5 22.2 33.4 45.0 56.0 67.2 ' 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 .would be: U, =>`24,000 + 0124 X 0.075 X 60 X 0.25 -X 1000 (70 - 0) - 42,900 24,000 .' ; - u ------------:--------- :----------:--------- 4--70 - 92.2 H 0.24 X 0.075 X 60 X 1000. . .. The only difference from Example1 is that the- ventilatioh load has been reduced. Heating Capacity Ratings Fluting and air capacity ratings for all unit ventilators are obtained in accordance with the ASIiRAE.Standard Code for Testing and Rating Steam Unit, Ventilators.1 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 tvoter unit ventilators have heating capacity ratings based on various entering water >temperatures, flow rates1 or temperature drops, and entering air temperatures. ..:Gas-fired unit ventilators have heating capacity ratings obtained in accordance with regulations established by the Amgriran Gas. Association. Heating capacities-include' the total heat input and the toted heat output, in Btu per hour; for various entering air temperatures. - Electric ;unit ventilators -ihave heating capacity ratings based on the heat input to the-electric heating .element,- in kilowatts or Btu per hour. The air capacities of hot water, gas-fired, and electric unit ventilators are obtained in accordance with the ASHRAE 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-' Inters are: -(l) unit air capacity, (2) percent minimum out-' door sir, (3) heating and cooling 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 num ber of occupants in the space. Other factors to be considered' are state and local code requirements, volume of the room, density of occupancy, and the usage of the room. The number of air changes required for a specific application also depends on window area, orientation, and the maximum outdoor temperature at which the unit will be expected to prevent overheating. Values approaching 6 air changes are associated with northerly oriented rooms having amallar window areas. Values approaching 9 air changes are required in southerly oriented classrooms having larger window areas. With very large window areas and southern exposures, as many as 12 air changes may be required. The air capacities listed are based on. preventing overheating-at outdoor temperatures up to about 55 F. For satisfactory, cooling, using outdoor air tem peratures up. to 60 F, the air.capacity provided should be in creased accordingly. The air capacities recommended above principally apply to classroom applications. In factories and kitchens, 30 to 60 air changes or more may be required. Office areas may-need from 10 to 15 air changes per hour. 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. In the absence of other criteria^-lO'cfarstandard air pfer occupant is frequently used as a minimum ventilation rate for classroom applications. Spaces such as laboratories, cafeterias, and meeting' rooms will usually require larger outdoor air quantities. The heating capacity of a unit to meet the heating require meat can be determined from manufacturers tables. Selec tion of heating capacity should always be made after selecting the unit air capacity for'mild weather cooling. Automatic Control of Unit Ventilators Many different cycles of control are available. The principal difference in the various cycles pertains to the amount of out door air delivered to the room. Usually, a room thermostat simultaneously controls a valve, damper, or step controller 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. Ail 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 of the thermostat, ventilation is accomplished by the partial or complete opening of the outdoor-air damper, according to the cycle used: Auxiliary hftftt.ing equipment is shut off. As the room temperature f-nntiniips to rise, the unit ventilator heat supply is throttled. Cooling and Ventilating Stage. When the room temper-