1、4831 Efficacy of Intermittent Ventilation for Providing Acceptable Indoor Air Quality Max H. Sherman, PhD Fellow ASHRAE ABSTRACT Ventilation standards and guidelines typically treat venti- lation as a constant and spec its value. In many circum- stances, a designer wishes to use intermittent ventila
2、tion rather than constant ventilation, but there are no easy equiv- alencies available. Thispaper develops a model of eficacy that can determine how much intermittent ventilation is needed to get the same indoor air quality (IAQ) as the continuous value speclfied. This paper describes a simple relat
3、ionship between three dimensionless quantities: the temporal ventilation effec- tiveness-which will be called the eficacy-the nominal turn- over, and the underventilation fraction. This relationship allows the calculation of intermittent ventilation for a wide variety ofparameters and conditions. Th
4、e relationship can be used to dejne a critical time that separates the regime in which ventilation variations can be averaged from the regime in which variable ventilation is of low effectiveness. The paper shows that ventilation load-shifting, temporary protection against poor outdoor air quality,
5、and dynamic ventilation strategies can be quite effective in low-density buildings such as single-jiamily houses or ofice spaces. The results of this work enable ventilation standards andguidelines to allow this extra flexibility and stillprovide acceptable indoor air quality. INTRODUCTION A key ste
6、p in designing a building is determining the correct amount of ventilation and the optimal system with which to provide it. There is no shortage of guidance on how much ventilation to use. The standard of care for ventilation system design is probably the 62 series of ASHRAE stan- dards-62.1-2004 fo
7、r nonresidential buildings and 62.2-2004 for residential buildings (ASHRAE 2004a, 2004b). The reader can find a variety of books and-other publications with recom- mendations from ASHRAE (http:/www.ashrae.org/book- store). Ventilation is not an end in itself but is part of the system intended to pro
8、vide a desired level of indoor environmental quality. One of the key aspects of indoor environmental quality is controlling contaminants that can have adverse health impacts on the occupants. Health impacts can depend in a quite nonlinear way on the concentration of contaminants, but most commonly i
9、t is desirable to control the dose of indoor contaminants through ventilation. In principle, the ventilation and ventilation efficiency are only intermediates because it is the dose and associated health effects that are ofdirect interest. As a practical matter, however, an HVAC designer rarely know
10、s the sources, their emission rates, or the appropriate dose-response relationships and therefore is usually provided guidance in terms of the things that can be controlled, such as the ventilation and its efficiency. This is why standards such as 62 focus on ventilation. Ventilation rates are usual
11、ly stated in terms of air changes per hour, airflow rate per person, or airflow rate per floor area, and the most common assumption may be that there is a constant airflow during the entire period of interest. There are, however, a variety of reasons to design and operate the venti- lation system wi
12、th variable amounts of ventilation airflow. For example : There may be periods of the day when the outdoor air quality is poor and indoor air quality could be improved by reducing the amount of outdoor air entering the building. Economizer operation can overventilate a space com- pared to minimum ve
13、ntilation requirements; energy sav- Max Sherman is senior scientist at Lawrence Berkeley National Laboratory, Berkeley, Calif. 02006 ASHRAE. ings can be achieved by lower ventilation rates at other times by taking account of the overventilation. Demand charges or utiliy peak loads may make it advant
14、ageous to reduce ventilation for certain periods of the day. Some HVAC equipment may make cyclic operation more attractive than steady-state operation, such as resi- dential or small commercial systems that tie ventilation to heating and cooling system operation. Regardless of the reason, the design
15、er or decision maker needs a method to determine how intermittent ventilation compares to continuous ventilation for the purposes ofprovid- ing acceptable indoor air quality (IAQ). Both ASHRAE Stan- dard 62.1 and 62.2 address the issue, but in limited ways. In section 6.2.6 of ASHRAE Standard 62.1-2
16、004 there is some flexibility for varying operating conditions. The stan- dard allows the instantaneous ventilation rate to be small provided that the deviation is short enough and that the average ventilation rate meets requirements. Section 4.4 of 62.2-2004 has similar provisions but goes a step f
17、urther by providing prescriptive ventilation effectivenesses for underventilation periods that are longer. The purpose of this paper is to develop approaches for determining the equivalency of intermittent ventilation using an equivalent dose model and to show how efficient these approaches are over
18、 a broad range of parameters from low- density residential settings to high-density commercial ones. Such an approach would, for example, enable standards such as 62.1 and 62.2 to expand their existing options and provide more options for the designer. Background Ventilation is principally used to m
19、aintain acceptable indoor air quality by controlling indoor contaminant concen- trations-and, hence, doses-and minimizing occupant expo- sures to the contaminants. Whole-building ventilation dilutes Contaminants in the indoor air with air that (ostensibly) does not contain those contaminants and is
20、normally used for controlling unavoidable, generic, or nonspecific contami- nants.* When specific contaminant sources can be identified, they are best dealt with directly through source control meth- ods such as local exhaust. For example, bathroom and cooking contaminants including water vapor are
21、best addressed by exhaust fans in those spaces. Volatile organic compounds In this paper ventilation will refer to any form of outdoor air exchange that can provide dilution, including mechanical venti- lation, natural ventilation, and infiltration. When outdoor air contains significant amounts of c
22、ontaminants it cannot successfully be used for dilution and the indoor concen- trations cannot be reduced below background levels without air cleaning. For the purposes of this paper, however, the outdoor air will be assumed to contain no significant contaminants of concern. (VOC) are often best add
23、ressed by changes in composition or use of specific materials. If ventilation rate and contaminant concentration were linearly related, the average concentration would be propor- tional to the average ventilation, and straightforward methods could be used to determine the effectiveness of intermitte
24、nt ventilation. Unfortunately, ventilation and concentration are dynamically and inversely related through the mass continuity equation, which leads to a nonlinear relationship between ventilation and concentration. Solutions to the continuity equation always involve an air change rate appropriate t
25、o the problem at hand. Although most people are more accustomed to dealing with ventilation in terms of specific airflow rates, the efficacy of intermittent ventilation will depend on the air change rate, so it is important to keep typical rates in mind for some specific but common occupancies. A re
26、lated parameter of interest is the turnover time, which is the inverse of the air change rate. It is the char- acteristic time in which the concentration of a contaminant responds to a change in ventilation rate. Sandberg and Sjorberg (1984) developed much of the language related to spatial ventilat
27、ion effectiveness, including the concept of turnover time. They, among others, have used turnover time to quantifi issues in mixing and other aspects of spatial distribution of both supply air and contaminants. Spatial effectiveness issues are beyond the scope of this paper; the interested reader sh
28、ould consult the 2005 ASHRAE Hand- book-Fundamentals, Chapter 27, “Ventilation and Infiltra- tion,” as well as Chapter 33, “Space Air Diffusion.” This paper focuses on temporal rather than spatial issues and assumes that the two can be treated independently. Accordingly, the spaces being addressed a
29、re treated as perfectly mixed from a spatial standpoint, but the development itselfwould work just as well for displacement ventilation, etc. The typical air change rates and turnover times can be found from literature and from standards such as the ASHRAE 62 series using specific ventilation rates,
30、 typical occupant densities, and typical geometry of the space in question. The inverse of the air change rates in Table 1 vary from 15 minutes to six hours, indicating that different occupancies will behave quite differently in a variety of configurations. The use of such quantities to explore the
31、spatial dependency of venti- lation is also important for large spaces but will not be discussed here. Sandberg and Sjorberg (1984) developed much of the nomenclature used in this field to deal principally with spatial variation. Shermanand Wilson (1986) and Yuill(l986, 1991) have already solved the
32、 continuity equation for the general case of equivalent linear dose and have defined the temporal ventila- tion effectieness, E, as a measure of how good a given, time- varying ventilation pattern is at providing acceptable IAQ. ASHRAE Standard 136 (ASHRAE 1993) uses this kind of 3. The term eflcacy
33、 is used as a synonym for temporal ventilation efectiveness. 94 ASHRAE Transactions: Research Table 1. Prototypical Air Change Rates (ACHY and Turnover Times ACH Turnover l/h) Time h Description 0.15 6.67 0.25 4.00 0.3 3.33 0.5 2.00 0.7 1.43 I 1 2 0.50 4 0.25 Infiltration rate of new homes from Stan
34、- dard 62.2 (2004) Infiltration rate of commercial buildings from Persily (1999) Ventilation requirement of almost empty commercial buildings from Standard 62.1 (2004) Ofice space requirement from Standard 62.1 (2004), also large home from Stan- dard 62.2 (2004) Ventilation requirement for small hom
35、es from Standard 62.2 (2004) Infiltration rate of older home from Sherman and Matson (1997) Conference room requirement from Standard 62.1 (2004) High-density space (e.g., a theater lobbv) from Standard 62.1 2004 1) ful. For example, ifthe ventilation requirement is 0.5 ACH and there is 0.2 ACH when
36、 the ventilation system is off, there can When the critical time is short comparedto the cycle time, the nominal turnover is large and the nonlinearities cannot be be a notch that is almost eight hours long for only a 50% increase in ventilation system capacity. 96 ASHRAE Transactions: Research Notc
37、h Ventilation at Various Air Change Rates 5 - O 2 4 6 Under-Ventilation Time (h) 8 Figure 2 Fractional increase in ventilation capacity necessary to accommodate an underventilation (notch) period, with and without 0.2 ACH of injltration. Pulse Ventilation In air distribution systems, the heating and
38、 cooling are often interconnected with the ventilation in such a way that it is not always possible to automatically get the right amount of ventilation for any heating or cooling demand. In commercial buildings, this appears in the form of minimum stops for vari- able air volume (VAV) systems. In r
39、esidential systems, this appears in the form of extra controls on systems with outside air inlets into return plenums. Conversely, a nominally fully recirculating system can “accidentally” induce air exchange either through duct leak- age or by differentially pressurizing various spaces and thereby
40、inducing extra infiltration. If, for a period of time, the HVAC system only provides this accidental ventilation, any underventilation could be made up for by adding a pulse of ventilation for a while to provide acceptable indoor air quality. To consider a pulse ventilation strategy, the ventilation
41、 capacity must go beyond the steady-state ventilation rate. Factors of three for excess ventilation capacity are common in residential buildings and are not unreasonable in commercial buildings if diversity is adequately taken into account. A pulse ventilation strategy can have advantages for HVAC s
42、ystems that cannot fully and independently control the ventilation and thermal conditioning, such as VAV systems or a supply air inlet system. Additional controls and extra ventilation capac- ity may, however, be required. Operational Approaches Some of the previous examples can be fully utilized in
43、 the HVAC design of the building of interest, but some approaches can only be realized with control systems that can respond to operational issues. For example, when a building uses an economizer, it will be ventilating more than is necessary to meet minimum venti- lation requirements. When it is no
44、 longer energetically favor- able to run the economizer, the ventilation rate can be reduced below the steady-state level for a period of time and thus save energy. Equations 1 through 3 can be used to reset the venti- lation rate, but that calculation will need to be made in real time as a function
45、 of the length of time the economizer was on, its average rate, and the time remaining in the occupancy cycle. Example: Protecting a House from Smog In order to get a better handle on how this works, a prac- tical example is in order. Consider the case in which a house is in a non-attainment area be
46、cause of urban smog and the designer is tasked to find a ventilation design that minimizes exposure to these outdoor contaminants by being off for the six worst hours of the day and on for the other eighteen. This is an fr, of 25%. The house is a 2000 ft2 (200 m2) three bedroom ranch. Using ASHRAE S
47、tandard 62.2-2004, there must be 50 cfm (25 Us) of mechanical ventilation if it is continuous in addition to some infiltration, resulting in an N of approximately 3. Using Equation 6, the efficacy comes out to about 87%. From Equation 1, the “high” ventilation rate necessary to provide acceptable in
48、door air quality is then 77 cfm (38 Us). DISCUSSION As can be inferred from the examples above, various intermittent ventilation strategies can be employed effectively but most will quickly become impractical once the efficacy begins to drop. This model of efficacy can be used to show which combinat
49、ions of parameters are likely to work and which are not. Figure 3 displays contour lines of efficacy as a function of the two key dimensionless parameters: underventilation frac- tion and nominal turnover. Figure 3 helps identify the target range of nominal tum- over and underventilation fraction. The left-most curve in the figure is the 90% efficacy contour. Anything to the left of or below that curve operates at a sufficiently high efficacy that the nonlinearities inherent in the problem are minimal. If the efficacy is high, the average ventilation over the period will be close t
