1、358 2009 ASHRAEABSTRACTExcessive use of safety factors in the design of a dedicated outdoor-air system (DOAS) can result in significant over-ventilation and larger-than-necessary equipment. However, design decisions are often based on incomplete information and assumptions that may or may not be val
2、id, so safety factors are important tools for the design engineer. This paper discusses several of the common uses of safety factors in the design of a DOAS, and provides recommendations on how to design a system with “reserve capacity” to accommodate unexpected loads and the need for increased airf
3、low or dehu-midification capacity, while minimizing the impact on installed cost and energy use.INTRODUCTIONSafety factors are commonly used by engineers when designing various types of HVAC systems for use in all types of buildings. Excessive use of safety factors in the design process can result i
4、n larger-than-necessary equipment, inflated installed costs, and sometimes excessive energy use. This is especially true when safety factors are used during several steps of the design process, which “compounds” their impact along the way.However, many of the decisions made during the design process
5、 are based on incomplete information, assumptions that may turn out to be invalid, or valid assumptions that may no longer be valid one, five, or ten years after the system or equipment is installed. Therefore, safety factors are important tools for design engineers, allowing the HVAC system to be d
6、esigned with “reserve capacity” to accommodate unexpected loads and the need for increased airflow or dehumidification capacity.A dedicated outdoor-air system (DOAS) uses a separate piece of equipment to condition (filter, heat, cool, humidify, dehumidify) all of the outdoor air brought into the bui
7、lding for ventilation. This conditioned outdoor air is then delivered either directly to each occupied space or to local HVAC units serving those spaces. Meanwhile, the local units (such as fan-coils, water-source heat pumps, PTACs, small packaged units, VAV terminals, chilled ceiling panels, or chi
8、lled beams) located in or near each space provide cooling and/or heating to maintain space temperature (Coad 1999, Shank and Mumma 2001)Treating the outdoor air separately can make it easier to verify that sufficient ventilation airflow reaches each occupied space and can help avoid high indoor humi
9、dity levels. The latter is accomplished by dehumidifying the outdoor air to remove the entire ventilation latent load and most (or all) of the space latent loads, leaving the local HVAC units to primarily handle space sensible cooling loads. Some types of local HVAC equipment, such as chilled ceilin
10、g panels or chilled beams, must operate dry and avoid condensation. This limits their duty to handling sensible loads only.Figure 1 shows several example DOAS configurations. Some deliver the conditioned outdoor air (CA) directly to each zone (Mumma 2008), while other configurations deliver the air
11、to the intakes of local, single-zone units (such as fan-coils, water-source heat pumps, dual-duct VAV terminals, small packaged rooftop units, or single-zone air handlers) or to centralized, multiple-zone units (such as floor-by-floor VAV air handlers or self-contained units).In addition, there are
12、many types of dedicated outdoor-air equipment available (Figure 2). Dehumidification is usually provided by direct-expansion (DX) refrigeration, a chilled-water coil, a desiccant-based dehumidification device, or Role of Safety Factors in the Design of Dedicated Outdoor-Air SystemsJohn MurphyMember
13、ASHRAEJohn Murphy is an applications engineer with Trane Commercial Systems, La Crosse, WI. LO-09-033 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional rep
14、roduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 359Figure 1 Example dedicated OA system configurations.Figure 2 Example dedicated OA equipment types.360 ASHRAE Transactionssome combination of these
15、 technologies. Often, the dedicated outdoor-air unit includes an exhaust-air energy recovery device (such as a total-energy wheel, fixed-plate heat exchanger, coil runaround loop, or heat pipe), which can reduce energy use and allow for downsizing of the cooling and heating equipment. In fact, ASHRA
16、E Standard 90.1 requires the use of an exhaust-air energy recovery device for many DOAS applications (ASHRAE 2007).SAFETY FACTORS WHEN DETERMINING DESIGN AIRFLOWThis section discusses how safety factors impact the calcu-lation of the design airflow for the dedicated outdoor-air unit.Calculating the
17、Design Airflow of the Dedicated Outdoor-Air UnitIn most applications, the design airflow for a dedicated outdoor-air unit is dictated by the amount of ventilation air required by industry standard or local code (Stanke 2004). In some cases, the owner or design team may choose to deliver more than co
18、de-minimum ventilation airflow to improve indoor air quality or to earn the “Increased Ventilation” credit when certi-fying a project using the LEED Green Building Rating System (USGBC 2009). Finally, in applications with very low ventilation requirements or very high indoor latent loads, the design
19、 engineer may chose to increase the airflow delivered by the dedicated outdoor-air unit so that the conditioned outdoor air can be deliv-ered at a higher dew point (not as dry).Table 6-1 of ASHRAE Standard 62.1-2007 (ASHRAE 2007) prescribes two ventilation rates for each occupancy cate-gory: one for
20、 people-related sources of contaminants (Rp) and another for building-related sources (Ra). Equation 6-1 from ASHRAE 62.1 is used to determine the minimum outdoor airflow (Vbz) that must be delivered to each breathing zone:(1)where= outdoor airflow required in the breathing zone of the occupiable sp
21、ace, cfm (L/s)= outdoor airflow rate required per person, cfm/person (L/sperson)= largest number of people expected to occupy the zone during typical usage= outdoor airflow rate required per unit area, cfm/ft2(L/sm2)= occupiable floor area of the zone, ft2(m2)Next, Equation 6-2 and Table 6-2 from AS
22、HRAE 62.1 are used to account for zone air distribution effectiveness (Ez), and to calculate the design outdoor airflow for the zone (Voz). This is the outdoor airflow that must be provided to the zone by the air distribution system (that is, through the supply-air diffusers).Finally, for a 100% out
23、door-air system in which one air handler supplies only outdoor air to one or more zones, Equation 6-4 from ASHRAE 62.1 is used to calculate the system-level outdoor air intake flow (Vot), by summing the zone outdoor airflows of all zones served by the dedicated outdoor-air unit:(2)In some system con
24、figurations, the dedicated outdoor-air unit provides conditioned OA to the intakes of local or centralized HVAC units, rather than directly to each zone. In these configurations, the dedicated OA unit must be sized to deliver the sum of the outdoor air intake flows (Vot) required by each of the syst
25、ems being served. If the condi-tioned OA (CA) is delivered to local HVAC units that each serve a single zone (bottom left configuration in Figure 1), Equation 2 above still applies since Equation 6-3 from ASHRAE 62.1 defines Vot equal to Voz for single-zone ventilation systems. However, if the condi
26、tioned OA (CA) is delivered to central air handlers that each serve multiple zones (bottom right configuration in Figure 1), Vot must be determined for each air handler using Section 6.2.5 (Multi-ple-Zone Recirculating Systems) of ASHRAE 62.1 (Stanke 2005) and then summed together to determine the a
27、irflow delivered by the dedicated OA unit.Makeup Air Applications. In some applications, the design airflow for the dedicated outdoor-air unit is dictated by the need to replace air that is being exhausted from the build-ing. This is common in laboratories, commercial kitchens, or other applications
28、 with very large exhaust requirements. In this case, the design airflow is the sum of all exhaust airflows plus any air needed for positive building pressurization.Safety Factor for Zone PopulationThe first common use of a safety factor occurs when determining Pz, the number of people expected to oc
29、cupy the zone. The definition of this term in ASHRAE 62.1 states that this is “the largest number of people expected to occupy the zone during typical usage.” This is not the largest number of people that could conceivably be in the zone under any special circumstance (such as a tornado warning or a
30、 brief retirement or birthday celebration).If actual zone population is not known, or if the owner and design team are not comfortable estimating it, Table 6-1 from ASHRAE 62.1 includes default values for occupant density. A note included under Table 6-1 clarifies that design engineers are not requi
31、red to use this default, but may choose to if the actual occupant density is not known.Some engineers use an overly-conservative estimate for Pz as a safety factor. In addition, some engineers or code offi-cials use occupant load (or exit population), which is intended for use in designing a means o
32、f egress to comply with the local fire code. Occupant load is typically much higher than the expected zone population used for designing the ventilation system (Table 1). Vbz RpPzRaAz+=VbzRpPzRaAzVot all zonesVoz=ASHRAE Transactions 361Using a larger-than-necessary value for zone popula-tion can res
33、ult in significant over-ventilation and excessive energy use. In addition, as explained later in this paper, a larger-than-necessary value for zone population also impacts the calculation of the space latent cooling load, and the resulting dehumidification capacity of the dedicated OA unit.The recom
34、mendation of this author is to avoid applying safety factors to zone population (Pz), instead using the best estimate for expected occupancy during typical usage. Hold off on applying safety factors until later in the process, as will be explained later in this paper.Safety Factor for Future Expansi
35、on or Change of UseThe other common use of a safety factor when determin-ing the design airflow of the dedicated outdoor-air unit occurs when accounting for future expansion or a change in use of the facility. If the facility is expanded in the future, and the dedi-cated OA unit will be expected to
36、serve the expansion, it would be prudent to select a unit with some amount of extra (or reserve) airflow capacity. Or, if the use of the facility is likely to change in the future (from an office space to a group of meeting rooms or a retail area, for example) the ventilation requirements for the zo
37、nes served by the dedicated OA unit may change. Again, it would be prudent to select a unit with some amount of extra airflow capacity.The recommendation of this author is to calculate the design airflow as accurately as possible, without using safety factors. Then select a dedicated outdoor-air uni
38、t that has reserve airflow capacity, rather than a unit operating near its maximum allowable airflow. The impact of this will be discussed later in this paper.SAFETY FACTORS WHEN DETERMINING DEHUMIDIFICATION CAPACITYThis section discusses how safety factors impact the calculation of required dehumid
39、ification capacity of the dedi-cated outdoor-air unit.Calculating the Dehumidification Capacity of the Dedicated Outdoor-Air UnitThe required dehumidification capacity of a dedicated OA unit (Figure 3, Equation 3) is dictated by the design airflow (Vot), humidity ratio of the entering outdoor air (W
40、oa), and humidity ratio of the conditioned air leaving the unit (Wca):(3)where,= required dehumidification capacity, lb/hr (kg/hr)= design outdoor airflow, cfm (L/s)Table 1. Comparison of Occupant Density and Occupant LoadDefault Occupant Density,1people/1000 ft2(people/100 m2)Minimum Occupant Load,
41、2people/1000 ft2(people/100 m2)Classroom 35 50Office space 5 10Retail sales 15 331 Table 6-1, ASHRAE Standard 62.1-2007 (ASHRAE 2007) 2 Table 1004.1.1, 2006 International Fire Code (ICC 2006)Figure 3 Dehumidification provided by a chilled-water or DX dedicated OA unit.qL4.5 Vot Woa Wca() 7000 gr/lb=
42、qL4.32 Vot Woa Wca 1000 g/kg =()qLVot362 ASHRAE Transactions= humidity ratio of the entering outdoor air, grains/lb (g/kg)= humidity ratio of the leaving conditioned air, grains/lb (g/kg)Note: In Equation 3, 4.5 (4.32) is not a constant, but is derived from the density of air at “standard” condition
43、s: 69F (21C) dry air at sea level has a density of 0.075 lb/ft3(0.0012 kg/L). Air at other conditions and elevations will cause this factor to change.Since the previous section discussed the influence of safety factors on the design airflow (Vot) of the dedicated OA unit, this section will focus on
44、the remaining two variables in Equation 3: the humidity ratio of the entering outdoor air (Woa) and the humidity ratio of the leaving conditioned air (Wca).Safety Factor for Woa (Humidity Ratio of the Entering Outdoor Air)The ASHRAE HandbookFundamentals (ASHRAE 2005) is a popular source for climatic
45、 data that represents the outdoor design conditions for various locations. Historically, design engineers have used the design dry-bulb temperature and mean coincident wet-bulb temperature when calculating the required capacity of cooling systems. However, the highest outdoor humidity ratio does not
46、 occur at the same time as the highest outdoor dry-bulb temperature.Since 1997, the ASHRAE Handbook has included the design dew-point temperature and mean coincident dry-bulb temperature for use when calculating the required capacity of dehumidification systems. Table 2 lists the 0.4% outdoor design
47、 conditions for Jacksonville, Florida (ASHRAE 2005). Notice that the outdoor humidity ratio is 32% higher at the design dew-point condition than at the design dry-bulb condi-tion. For this reason, it is very important to use the design dew-point condition when determining the required dehumidifica-t
48、ion capacity of a dedicated outdoor-air unit.The first common use of a safety factor when determining the required dehumidification capacity of a dedicated OA unit occurs when selecting the design humidity ratio of the entering outdoor air. The ASHRAE Handbook includes three sets of design condition
49、s: 0.4%, 1%, and 2%. These indicate the percentage of hours during a year when with outdoor condi-tions are expected to exceed the tabulated design value. Table 3 lists the 0.4%, 1%, and 2% design dew point conditions for Jacksonville, Florida (ASHRAE 2005).Many engineers tend to use the most conservative (0.4%) design condition, but this often results in the selection of a larger dedicated outdoor-air unit. The impact of this decision on equipment capacity will be demonstrated and discussed later in this paper.Calculating the Humidity Ratio of the Conditioned Air Leaving
