ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 28-2012 UNIT VENTILATORS UNIT HEATERS AND MAKEUP AIR UNITS.pdf

1、28.1CHAPTER 28UNIT VENTILATORS, UNIT HEATERS, AND MAKEUP AIR UNITSUnit Ventilators. 28.1Unit Heaters 28.4Makeup Air Units 28.8UNIT VENTILATORSHEATING unit ventilator is an assembly whose principalAfunctions are to heat, ventilate, and cool a space by introduc-ing outdoor air in quantities up to 100%

2、 of its rated capacity. Theheating medium may be steam, hot water, gas, or electricity. Theessential components of a heating unit ventilator are the fan, motor,heating element, damper, filter, automatic controls, and outletgrille, all of which are encased in a housing.An air-conditioning unit ventil

3、ator is similar to a heating unitventilator; however, in addition to the normal winter function ofheating, ventilating, and cooling with outdoor air, it is also equippedto cool and dehumidify during the summer. It is usually arrangedand controlled to introduce a fixed quantity of outdoor air for ven

4、-tilation during cooling in mild weather. The air-conditioning unitventilator may be provided with a various of combinations of heat-ing and air-conditioning elements. Some of the more commonarrangements include Combination hot- and chilled-water coil (two-pipe)Separate hot- and chilled-water coils

5、(four-pipe)Hot-water or steam coil and direct-expansion coilElectric heating coil and chilled-water or direct-expansion coilGas-fired furnace with direct-expansion coilThe typical unit ventilator is equipped with controls that allowheating, ventilating, and cooling to be varied while the fans operat

6、econtinuously. In normal operation, the discharge air temperaturefrom a unit is varied in accordance with the room requirements. Theheating unit ventilator can provide ventilation cooling by bringingin outdoor air whenever the room temperature is above the room setpoint. Air-conditioning unit ventil

7、ators can provide refrigeratedcooling when the outdoor air temperature is too high to be usedeffectively for ventilation cooling.Unit ventilators are available for floor mounting, ceilingmounting, and recessed applications. They are available with var-ious airflow and capacity ratings, and the fan c

8、an be arranged sothat air is either blown through or drawn through the unit. Withdirect-expansion refrigerant cooling, the condensing unit caneither be furnished as an integral part of the unit ventilator assem-bly or be remotely located.Figure 1A shows a typical heating unit ventilator. The heating

9、coil can be hot water, steam, or electric. Hot-water coils can beprovided with face-and-bypass dampers for capacity control, ifdesired. Valve control of capacity is also available.Figure 1B shows a typical air-conditioning unit ventilator with acombination hot- and chilled-water coil for use in a tw

10、o-pipe sys-tem. This type of unit is usually provided with face-and-bypassdampers for capacity control.Figure 1C illustrates a typical air-conditioning unit ventilatorwith two separate coils, one for heating and the other for coolingwith a four-pipe system. The heating coil may be hot water, steam,o

11、r electric. The cooling coil can be either a chilled-water coil or adirect-expansion refrigerant coil. Heating and cooling coils aresometimes combined in a single coil by providing separate tube cir-cuits for each function. In such cases, the effect is the same as hav-ing two separate coils.Figure 1

12、D illustrates a typical air-conditioning unit ventilatorwith a fan section, a gas-fired heating furnace section, and a direct-expansion refrigerant coil section.ApplicationUnit ventilators are used primarily in schools, meeting rooms,offices, and other areas where the density of occupancy requiresco

13、ntrolled ventilation to meet local codes.Floor-model unit ventilators are normally installed on an outerwall near the centerline of the room. Ceiling models are mountedagainst either the outer wall or one of the inside walls. Ceiling mod-els discharge air horizontally. Best results are obtained if t

14、he unitcan be placed so that the airflow is not interrupted by ceiling beamsor surface-mounted lighting fixtures.Downdraft can be a problem in classrooms with large windowareas in cold climates. Air in contact with the cold glass is cooledand flows down into the occupied space. Floor-standing units

15、ofteninclude one of the following provisions to prevent downdraft alongthe windows (Figure 2):Window sill heating uses finned radiators of moderate capacityinstalled along the wall under the window area. Heated air risesupward by convection and counteracts the downdraft by temper-ing it and divertin

16、g it upward.Window sill recirculation is obtained by installing the return airintake along the window sill. Room or return air to the unitincludes the cold downdrafts, takes them from the occupied area,and eliminates the problem.Window sill discharge directs a portion of the unit ventilator dis-char

17、ge air into a delivery duct along the sill of the window. Thedischarge air, delivered vertically at the window sill, is distributedthroughout the room, and the upwardly directed air combatsdowndraft.SelectionItems to be considered in the application of unit ventilators areUnit air capacityPercent mi

18、nimum outdoor airHeating and cooling capacityCycle of controlLocation of unitMild-weather cooling capacity and number of occupants in thespace are the primary considerations in selecting the units aircapacity. Other factors include state and local requirements, volumeof the room, density of occupanc

19、y, and use of the room. The numberThe preparation of the sections on Unit Ventilators and Unit Heaters isassigned to TC 6.1, Hydronic and Steam Equipment and Systems. Thepreparation of the section on Makeup Air Units is assigned to TC 5.8,Industrial Ventilation.28.2 2012 ASHRAE HandbookHVAC Systems

20、and Equipment Fig. 1 Typical Unit VentilatorsFig. 2 Methods of Preventing Downdraft along WindowsUnit Ventilators, Unit Heaters, and Makeup Air Units 28.3of air changes required for a specific application also depends onwindow area, orientation, and maximum outdoor temperature atwhich the unit is ex

21、pected to prevent overheating.Rooms oriented to the north (in the northern hemisphere) withsmall window areas require about 6 air changes per hour (ach).About 9 ach are required in rooms oriented to the south that havelarge window areas. As many as 12 ach may be required for verylarge window areas a

22、nd southern exposures. These airflows arebased on preventing overheating at outdoor temperatures up toabout 55F. For satisfactory cooling at outdoor air temperatures upto 60F, airflow should be increased accordingly.These airflows apply principally to classrooms. Factories andkitchens may require 30

23、 to 60 ach (or more). Office areas may need10 to 15 ach.The minimum amount of outdoor air for ventilation is deter-mined after the total air capacity has been established. It may begoverned by local building codes or it may be calculated to meet theventilating needs of the particular application. Fo

24、r example,ASHRAE Standard 62.1 requires 7.5 to 10 cfm of outdoor air peroccupant (0.06 to 0.18 cfm/ft2) in lecture halls or classrooms, labo-ratories, and cafeterias, and 5 cfm per occupant in conferencerooms.The heating and cooling capacity of a unit to meet the heatingrequirement can be determined

25、 from the manufacturers data. Heat-ing capacity should always be determined after selecting the unit aircapacity for mild-weather cooling.Capacity. Manufacturers publish the heating and coolingcapacities of unit ventilators. Table 1 lists typical nominalcapacities.Heating Capacity Requirements. Beca

26、use a unit ventilator has adual function of introducing outdoor air for ventilation and main-taining a specified room condition, the required heating capacity isthe sum of the heat required to bring outdoor ventilation air to roomtemperature and the heat required to offset room losses. The venti-lat

27、ion cooling capacity of a unit ventilator is determined by the airvolume delivered by the unit and the temperature differencebetween the unit discharge and the room temperature.Example. A room has a heat loss of 24,000 Btu/h at a winter outdoordesign condition of 0F and an indoor design of 70F, with

28、 20% out-door air. Minimum air discharge temperature from the unit is 60F. Toobtain the specified number of air changes, a 1250 cfm unit ventilator isrequired. Determine the ventilation heat requirement, the total heatingrequirement, and the ventilation cooling capacity of this unit with out-door ai

29、r temperature below 60F.Solution:Ventilation heat requirement:qv= 60cpQ(ti to)whereqv= heat required to heat ventilating air, Btu/h = density of air at standard conditions = 0.075 lb/ft3cp= air specific heat = 0.24 Btu/lbFQ = ventilating airflow, cfmti= required room air temperature, Fto= outdoor ai

30、r temperature, Fqv= 60 0.075 0.24 1250 (20/100)(70 0) = 18,900 Btu/hTotal heating requirement:qt= qv+ qswhereqt= total heat requirement, Btu/hqs= heat required to make up heat losses, Btu/hqt= 18,900 + 24,000 = 42,900 Btu/hVentilation cooling capacity:qc= 60cpQ(ti tf)whereqc= ventilation cooling cap

31、acity of unit, Btu/htf= unit discharge air temperature, Fqc= 60 0.075 0.24 1250 (70 60) = 13,500 Btu/hControlMany cycles of control are available. The principal difference inthe various cycles is the amount of outdoor air delivered to the room.Usually, a room thermostat simultaneously controls both

32、a valveand either damper or step controller, to regulate the heat supply, andan outdoor and return air damper. A thermostat in the airstream pre-vents discharge of air below the desired minimum temperature. Unitventilator controls provide the proper sequence for the followingstages:Warm-Up. All cont

33、rol cycles allow rapid warm-up by having theunits generate full heat with the outdoor damper closed. Thus100% of the room air is recirculated and heated until the roomtemperature approaches the desired level.Heating and Ventilating. As the room temperature rises into theoperating range of the thermo

34、stat, the outdoor air damper par-tially or completely opens to provide ventilation, depending onthe cycle used. Auxiliary heating equipment is shut off. As theroom temperature continues to rise, the unit ventilator heat supplyis throttled.Cooling and Ventilating. When the room temperature risesabove

35、 the desired level, the room thermostat throttles the heatsupply so that cool air flows into the room. The thermostat grad-ually shuts off the heat and then opens the outdoor air damper.The airstream thermostat frequently takes control during thisstage to keep the discharge temperature from falling

36、below a setlevel.The section on Air Handling, under the Air Systems section, inChapter 47 of the 2011 ASHRAE HandbookHVAC Applicationsdescribes the three cycles of control commonly used for unit venti-lators:Cycle I. 100% outdoor air is admitted at all times, except duringwarm-up.Cycle II. A minimum

37、 amount of outdoor air (normally 20 to 50%)is admitted during the heating and ventilating stage. Thispercentage is gradually increased to 100%, if needed, during theventilation cooling stage.Cycle III. Except during warm-up, a variable amount of outdoorair is admitted, as needed, to maintain a fixed

38、 temperature of airentering the heating element. The amount of air admitted is con-trolled by the airstream thermostat, which is set low enough (oftenat 55F) to provide cooling when needed.Air-conditioning unit ventilators can include any of the threecycles in addition to the mechanical cooling stag

39、e in which a fixedamount of outdoor air is introduced. The cooling capacity is con-trolled by the room thermostat.For maximum heating economy, the building temperature isreduced at night and during weekends and vacations. Severalarrangements are used to accomplish this. One arrangement takesTable 1

40、Typical Unit Ventilator CapacitiesAirflow,cfmHeating Unit VentilatorTotal Heating Capacity,Btu/hA/C Unit VentilatorTotal Cooling Capacity,Btu/h500 38,000 19,000750 50,000 28,0001000 72,000 38,0001250 85,000 47,0001500 100,000 56,00028.4 2012 ASHRAE HandbookHVAC Systems and Equipment advantage of the

41、 natural convective capacity of the unit when thefans are off. This capacity is supplemented by cycling the fan withthe outdoor damper closed as required to maintain the desired roomtemperature.UNIT HEATERSA unit heater is an assembly of elements, the principal function ofwhich is to heat a space. T

42、he essential elements are a fan and motor,a heating element, and an enclosure. Filters, dampers, directionaloutlets, duct collars, combustion chambers, and flues may also beincluded. Some types of unit heaters are shown in Figure 3.Unit heaters can usually be classified in one or more of the fol-low

43、ing categories:Heating Medium. Media include (1) steam, (2) hot water, (3) gasindirect-fired, (4) oil indirect-fired, and (5) electric heating.Type of Fan. Three types of fans can be considered: (1) propeller,(2) centrifugal, and (3) remote air mover. Propeller fan units maybe arranged to blow air h

44、orizontally (horizontal blow) or verti-cally (downblow). Units with centrifugal fans may be small cab-inet units or large industrial units. Units with remote air moversare known as duct unit heaters.Arrangement of Elements. Two types of units can be consid-ered: (1) draw-through, in which the fan dr

45、aws air through theunit; and (2) blow-through, in which the fan blows air through theheating element. Indirect-fired unit heaters are always blow-through units.ApplicationUnit heaters have the following principal characteristics:Relatively large heating capacities in compact casingsAbility to projec

46、t heated air in a controlled manner over a considerable distanceRelatively low installed cost per unit of heat outputApplication where sound level is permissibleThey are, therefore, usually placed in applications where the heatingcapacity requirements, physical volume of the heated space, or both,ar

47、e too large to be handled adequately or economically by othermeans. By eliminating extensive duct installations, the space isfreed for other use.Unit heaters are mostly used for heating commercial and indus-trial structures such as garages, factories, warehouses, showrooms,stores, and laboratories,

48、as well as corridors, lobbies, vestibules, andsimilar auxiliary spaces in all types of buildings. Unit heaters mayoften be used to advantage in specialized applications requiring spotor intermittent heating, such as at outer doors in industrial plants orin corridors and vestibules. Cabinet unit heat

49、ers may be used whereheated air must be filtered.Unit heaters may be applied to a number of industrial processes,such as drying and curing, in which the use of heated air in rapidcirculation with uniform distribution is of particular advantage.They may be used for moisture absorption applications, such as re-moving fog in dye houses, or to prevent condensation on ceilings orother cold surfaces of buildings where process moisture is released.When such conditions are severe, unit ventilators or makeup airunits may be required.SelectionThe following factor