Measurement and Analysis of Vitiation of Secondary Air in Air Distribution Systems (RP-1276)

Transcription

1 University of Nebraska - Lincoln From the SelectedWorks of David Yuill Spring May, 2008 Measurement and Analysis of Vitiation of Secondary Air in Air Distribution Systems (RP-1276) David P Yuill, University of Nebraska-Lincoln Grenville K Yuill Andrew H Coward Available at:

2 2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE s prior written permission. VOLUME 14, NUMBER 3 HVAC&R RESEARCH MAY 2008 Measurement and Analysis of Vitiation of Secondary Air in Air Distribution Systems (RP-1276) David P. Yuill Grenville K. Yuill, PhD Andrew H. Coward Member ASHRAE Fellow and Life Member ASHRAE Received July 23, 2007; accepted February 6, 2008 This paper is based on findings resulting from ASHRAE Research Project RP Appendix A of ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality, describes the recycling of unvitiated ventilation air in a recirculating air-handling system. Equation A-2 considers air delivered through the primary air path (central air distribution system) and secondary air paths, such as fan-powered boxes or transfer-air fans. It contains a variable,, that describes the extent to which the secondary air comes from the zone in question, as opposed to coming from average system return air. This paper describes the development of an equation that can be used to quantify, and shows the results of the first experimental measurements of. For these measurements, tracer gas experiments were carried out on an office building in Omaha, Nebraska. The study showed that the theoretical range of is from zero to infinity. Its range in the test building is from 0.74 to 1.01 as built, but could range from 0.14 to 1.13 if the fan-powered boxes had been located in different, but realistic, locations. These findings contradict the standard, which defines as ranging from 0 to 1. INTRODUCTION ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality, (ASHRAE 2007) specifies minimum ventilation rates and other measures intended to provide indoor air quality conditions that will be acceptable to occupants of buildings and that minimize the potential for adverse health effects. Standard 62 was first published in 1973 and has undergone many revisions, including substantial changes to the amount of outdoor air that must be provided to occupied areas, and to the procedure for calculating these amounts. One of these revisions was the introduction of the multiple spaces equation (MSE). The MSE can be applied to buildings that use a recirculating system one that recirculates some portion of air returned from the occupied zones in its supply air. The MSE is based on a mass balance. It accounts for the fact that only the critical zone a zone that requires the highest fraction of ventilation air in its supply air will produce fully vitiated air (where fully vitiated air is air from zones that receive exactly the minimum ventilation rate required by the standard, and ventilation air is a combination of outdoor air and recirculated return air that is not fully vitiated). All other zones receive more outdoor air than the minimum required, so that the return air from those zones contains some unvitiated air. This unvitiated air is recirculated, so the amount of new ventilation air that must be introduced is lower than if this effect were ignored. David P. Yuill is president and Andrew H. Coward is a research scientist at Building Solutions, Inc., Omaha, NE. Grenville K. Yuill is director of the Charles W. Durham School of Architectural Engineering and Construction, University of Nebraska, Lincoln, NE. 345

3 346 HVAC&R RESEARCH Originally, the MSE accounted only for single-path air distribution systems. Warden (1995) pointed out that a secondary path one that does not pass through the central air handler could also be used to help ventilate the critical zone. He presented a general case of the MSE that accounted not only for the primary path, as the earlier MSE does, but also for secondary paths, such as dual-fan dual-duct systems, fan-powered terminal units, and transfer fans. Ke (1997) also developed a generalized MSE with a different form, but consistent with Warden s derivation. The general case of the MSE was included as Equation A-2 in the 2004 version of ASHRAE Standard To use the general case of the MSE for ventilation calculations requires knowledge of two additional variables not previously considered. In the nomenclature of the standard, these are E p (the primary air fraction to the zone, i.e., the ratio of primary airflow to total discharge flow) and. The first of these variables is straightforward, a fraction of two quantities typically known by a designer. The variable,, however, is more complex and is the subject of this paper. DEFINITION OF ASHRAE Standard defines as the fraction of secondary recirculated air to the zone that is representative of average system return air rather than air directly recirculated from the zone. No quantitative definition is provided, but guidance in selecting a value is given in a note: For plenum return systems with local secondary recirculation (e.g., fan-powered VAV with plenum return), 1.0. For ducted return systems with local secondary recirculation (e.g., fan-powered VAV with ducted return), typically = 0.0. The general case of the MSE (Equation A-2 in the standard) is shown in expanded form in Equation 1. ( E p + ( 1 E p ) ) + X s E p Z d [ 1 ( 1 E z ) ( 1 ) ( 1 E p )] E vz = E p + ( 1 E p ) (1) where E p = V pz / V dz E vz = the ventilation efficiency of the zone in question E z = the effectiveness of the zone air distribution system in delivering ventilation air to the breathing zone (zone air distribution effectiveness) V dz = discharge flow rate to the zone V ou = uncorrected outdoor air intake; required intake if system ventilation efficiency were 1.0 V oz = outdoor airflow rate to the zone V ps = primary airflow rate to all zones served by the system V pz = primary airflow rate to the zone X s = V ou / V ps Z d = V oz / V dz Most of these quantities are depicted in a schematic shown in Figure 1. With the exception of, all of the variables used in Equation 1 can be calculated from values known or defined in the system design process. Quantifying requires some assumptions to be drawn about the exact meaning of the qualitative definition given in the standard, as well as knowledge about the mixing and airflows in the region that the secondary air is drawn from. If the secondary air is a proportionate mix of return air from all zones in the system, then = 1.0. Note that vitiation, as discussed in this paper, does not relate to concentrations of pollutants. Rather, it refers to the amount of ventilation air in a sample that exceeds or falls short of the ventilation rate prescribed by the standard.

4 VOLUME 14, NUMBER 3, MAY Figure 1. Schematic of airflows used in the multiple spaces equation. To develop a quantitative definition of, let U represent the fraction of unvitiated air in a given sample. Consider the fraction of unvitiated air in the secondary air for the critical zone, U sz, and in the system return air, U rs. U sz can be evaluated by taking a weighted average of unvitiated outdoor air fractions in all of the constituents of the secondary air. i = 0m i U U i sz = n m sz (2) If the air density is assumed to be constant for each airstream, then volumetric flow rates can be used in place of mass flow rates, and the secondary airflow rate can be expressed using terms from ASHRAE Standard for a system with n zones contributing to the secondary air. n V U i = 0 iu i sz = = V sz n i = 0V iu i ( 1 E p )V dz (3) is defined by dividing the unused outdoor air fraction in the secondary air by the unused outdoor air fraction in the system s return air. U sz = U rs (4) The term U rs, the fraction of unused outdoor air in the system return air, can be expressed using the ventilation efficiency term, E v, from ASHRAE Standard Since E v is a function of, this is an iterative calculation. U rs = 1 E v (5)

5 348 HVAC&R RESEARCH The study described in this paper included field measurements of. To determine, an alternate equation was derived for use with tracer gas measurements. This derivation is based on a tracer gas mass balance at the inlet to the fan-powered box. The concentration at the inlet is denoted C fpb, the return air concentration is C ret, and the zone concentration is. C fpb ( 1 E p )V dz = ( 1 )( 1 E p )V dz + C ret ( 1 E p )V dz (6) This is simplified to C fpb = ( 1 ) + C ret. (7) Solving for gives C fpb = C ret (8) During this test the building was treated as a laboratory assumptions about occupancy and space usage could be made arbitrarily. Therefore, we assumed that all non-critical zones were equally vitiated. Note that this gives an identical outcome to a well-mixed return plenum with zones that are not equally vitiated. A less obvious but important fact is that the magnitude of the concentration of the other zones does not affect the result. This is shown in the discussion of validity, below. RANGE OF If we assume that representative of average system return air means having the same vitiation as average system return air, then can be greater than one. A value of greater than one occurs when secondary air is less vitiated than average system return air. If half of the zones in a building are less vitiated than the average and half are more vitiated, it is not unrealistic to draw return air from a location where it is less vitiated than the average system return air. In a small system, values much greater than one are possible. The following example illustrates a calculation of using Equation 8. In this example, is greater than one. Consider a three-zone building with a common return duct. If the air from all three zones is well-mixed at some point in the return duct, that air can be assumed to be representative of average system return air, so is 1.0 if the secondary air is drawn from this point. However, consider the case in which the secondary air is drawn from a location that has air less vitiated than the fully mixed return air, as in Figure 2. Assume the flow rates and zone vitiation percentages as shown in Figure 2. The total return flow is 3000 cfm (1416 L/s), of which 2000 cfm (944 L/s) comes from the critical zone. Weighted averages are calculated to determine the vitiated fraction of the secondary air (65%) the unvitiated fraction (U sz = 35%), and the system return air (88.3% vitiated), so U rs = 11.7%. Then is calculated with Equation 4 as follows: U sz = = = U rs (9) Using the tracer gas formulation for (Equation 8) gives the same result. C fpb = = = C ret (10)

6 VOLUME 14, NUMBER 3, MAY Figure 2. Schematic of flow paths for example calculation. This example shows that can be greater than one. In fact, the upper limit of is unbounded. The extreme case, where all of the return air from the other zones is used as secondary air in the critical zone gives an infinite value of. Quantitatively, this is because there is no unvitiated air in the return air (U rs = 0). Qualitatively, this is because if the secondary air has some quantity of unvitiated air, it is infinitely cleaner than the fully vitiated return air. (If the return air from the other zones has no unvitiated air, then all zones are critical zones, and is undefined.) A final observation about is that it is defined only for the fully vitiated critical zone; thus, its lower bound is zero. If were applied to a noncritical zone, it is possible that the secondary air could be more vitiated than the zone, in which case would be negative. EXPERIMENTAL SETUP The variable was measured with the use of tracer gas tests in a five-story office building located in Omaha, Nebraska. This building is 130,000 ft² (12,100 m²) and is served by two large, single-duct, variable-air-volume air handlers with a design capacity of 65,000 cfm (30,700 L/s) each. The air handlers serve 134 pressure-independent air terminal units (ATUs), of which 87 are parallel fan-powered boxes (FPB). The zones have a shared ceiling-plenum return path (i.e., unducted on each floor). This building was built in Four zones in the building were selected to serve as nominal critical zones. Each of these zones is an enclosed office or conference room, and is served by an FPB. Sulfur hexafluoride (SF 6 ) tracer gas was released into the test zone through a mixing and dispersal system. To eliminate the confounding variable of stratification due to room air distribution imperfections, the test zone was fully mixed with several large mixing fans (the mixing was confirmed experimentally). Measurements were made periodically over several hours as the concentration rose and while it was at steady state. The variable was calculated by evaluating Equation 8 using arithmetic averages of seven concentration measurements at steady state. During the tests the air handlers were configured to use 100% outdoor air. This prevented another confounding variable: imperfect mixing of all of the non-test zones. Since the essence of the experiment is to determine how much of the air drawn into the FPB comes from the critical

7 350 HVAC&R RESEARCH zone, it is necessary that there be only two zonal concentrations: the critical zone concentration and the other zones concentration. During the test the other zones concentrations were zero, as confirmed by measurements. Three test conditions were studied. 1. All adjacent zones at minimum flow. Zones that are served by FPB have the fans running. 2. All adjacent zones at design flow with no FPB fans running. The critical zone is at minimum flow with its FPB fan running. 3. Identical to (1) except that no FPB fans are running except the critical zone s. This condition was only tested for one zone. Conditions 1 and 2 were tested for each of the four test zones. Some of these tests were repeated once or twice to evaluate experimental repeatability. A total of 13 tests were conducted. During this experiment several additional parameters were measured. These data were intended to allow other analyses, to detect or troubleshoot experimental problems, and to provide redundant, but less certain, results. They include airflow measurements, pressure differentials across each test zone s walls and partitions, temperature and humidity of indoor and outdoor air, wind direction and speed, and tracer concentrations in several other locations. Since these data are not used to calculate the results in this paper, they are not reported here. EXPERIMENTAL RESULTS AND DISCUSSION An example of the raw data from one test is shown in Figure 3. In this figure the concentration in the test zone reaches steady state after about one hour. Error bars represent the uncertainty associated with tracer gas measurements (3%), as stated by the manufacturer of the analyzer. Concentration measurements were made in the breathing zone (a location four feet [1.2 m] above the floor near the center of the room) in the return grille, which should be similar to the breathing zone if the room is well-mixed, and in the secondary air inlet, where the FPB draws air from the ceiling plenum. Figure 3. Example of SF 6 measurements.

8 VOLUME 14, NUMBER 3, MAY The return concentration, C ret, is absent from this list. The return concentration was calculated, rather than being measured, for two reasons. The first is that the clean return air from the dozens of other zones diluted the tracer gas to a very low concentration in the system return air. The concentration is below the accurate range of the gas chromatograph that was used to measure the concentration. The second reason is that the return air is highly stratified, even after passing through the return fan, so a measurement would be meaningless. This was confirmed by experimentation. Any attempt to mix the return air would interfere with the normal return path, damaging the validity of the experiment. Instead of measuring the return concentration, it was calculated by multiplying the concentration in the test zone,, by the portion of the total discharge air that comes from the test zone. C ret = V dz,test V dz (11) The discharge airflow rates to all zones were set to constant values during testing. The research team recalibrated the flow stations on all ATUs prior to conducting the experiment. Since C ret is very low, the result is very insensitive to C ret, so that any error associated with this approach has no significant effect. The results of the tests are shown in Table 1. These results reference the four test zones that are located as shown in Figure 4. The variable ranges from 0.74 to One question that was considered is: does the status of the other ATU (for example, whether they are on maximum or minimum flow, and whether FPB fans are running) affect the results? Table 2 has the results of similar tests grouped together for easier comparison. Although the data set is too small for definitive conclusions, there appears to be no correlation between adjacent zones ATU status and. In Zone A, is higher when the adjacent zones are on design flow, but the opposite is true for Zone C. In Zones B and D, is essentially unaffected by ATU status. These results are not surprising because depends on the airflow patterns in the ceiling plenum, which vary depending on the physical layout of each zone. For a Table 1. Test Results Test ID Zone Adjacent Zones Er 1 C Minimum a 0.85 Er 2 C Minimum 0.94 Er 3 C Minimum 0.82 Er 4 C Design 0.94 Er 5 D Minimum 0.92 Er 6 D Design 1.01 Er 7 B Minimum 0.74 Er 8 B Design 0.84 Er 9 A Minimum 0.74 Er 10 A Design 0.91 Er 11 A Minimum 0.75 Er 12 A Design 0.99 Er 13 A Minimum 0.79 a Adjacent zone FPB fans were off for Test 1. In all other minimum flow cases, they were on.

9 352 HVAC&R RESEARCH Figure 4. Test zone locations. Table 2. Grouped Results of Similar Tests Test ID Zone Adjacent Er 10 A Design 0.91 Er 12 A Design 0.99 Mean 0.95 Er 9 A Minimum 0.74 Er 11 A Minimum 0.75 Er 13 A Minimum 0.79 Mean 0.76 Er 8 B Design 0.84 Er 7 B Minimum 0.74 Er 4 C Design 0.94 Er 2 C Minimum 0.94 Er 3 C Minimum 0.82 Mean 0.88 Er 6 D Design 1.01 Er 5 D Minimum 0.92

10 VOLUME 14, NUMBER 3, MAY Table 3. Standard Deviation of Data Test ID Zone Adjacent Er 10 A Design 0.91 Er 12 A Design 0.99 Mean 0.95 Std. Dev % Er 9 A Minimum 0.74 Er 11 A Minimum 0.75 Er 13 A Minimum 0.79 Mean 0.76 Std. Dev % Er 2 C Minimum 0.94 Er 3 C Minimum 0.82 Mean 0.88 Std. Dev % given zone, therefore, we can conclude that may or may not vary with adjacent ATU status, and, if it varies, it may increase or decrease. In Table 3 the standard deviation is calculated for each set of data from tests that were made under identical conditions. This gives an indication of the repeatability of the testing. The standard deviation is also presented as a percentage of the average value. The most meaningful repeatability data come from tests 9, 11, and 13 because there are three data. In this case the standard deviation is 4% of the average value. To extend the results of this study, a second kind of test was developed. In this test, referred to as a grid map test, tracer gas was released in one test zone, as in previous tests, but tracer gas samples were taken from approximately 100 locations in the nearby ceiling plenum that are potential locations of an FPB. The locations are laid out in a 4 ft (1.2 m) grid pattern in the region near the test zone, and in an 8 ft (2.4 m) grid in further regions.the values of were calculated for each of these sets of concentration measurements, and then bi-linearly interpolated to create the grid maps shown in Figures 5 and 6. Figure 5 shows the results of a test with adjacent ATU at minimum and all FPB fans on, superimposed on a mechanical system plan. Figure 6 shows the results of a test with adjacent ATU at design flow. In these figures solid black regions indicate barriers to flow that are present in the ceiling plenum. In Figure 5, an FPB placed in a region near the test zone would have values as low as A beam runs from the top of the drawing downward, starting at the column located between the test zone and the adjacent office. This beam appears to cause return air from the test zone to collect before dissipating downstream toward the return air duct inlet. In this test, the repositioning of an FPB by only 10 ft (3 m) could cause to increase from 0.1 to 0.9 in some areas.

11 354 HVAC&R RESEARCH Figure 5. grid map with adjacent zones ATU on minimum flow and FPB fans on.

12 VOLUME 14, NUMBER 3, MAY Figure 6. grid map with adjacent zones ATU on maximum flow and FPB fans off.

13 356 HVAC&R RESEARCH Figure 6 shows that a very different flow pattern exists when the adjacent zones are at maximum flow. These tests give a general sense as to the size of the mixing plume downstream of a particular zone in a ceiling plenum. Most importantly, they show that carefully locating the FPB is critical to achieving an efficient ventilation design. One might conclude from an examination of the grid map results that the practical minimum value of is 0.1. However, if the flow of return air from the critical zone passed through a constriction, such as a transfer boot, a plume of concentrated return air could likely cause values much lower than 0.1 if the FPB were located in this plume. VALIDITY Vitiation of Other Zones Since the building was unoccupied during experimental testing, any assumption about occupancy could be made. We assumed that occupants, or other sources of vitiation, were distributed such that the return air from all zones except the critical zone was equally vitiated. This assumption allowed the experiment to be conducted using a single tracer gas. It also allows airflow rates to be used to determine concentrations and vice versa. It seems that tracer gas in the test zone and concentrations of zero in adjacent zones could not accurately represent a real building in which there are concentrations of pollutant everywhere. The following example shows that this experiment gives the same result as if there were tracer gas sources (emulating pollutants) evenly distributed throughout the building. Assume 1000 cfm (472 L/s) of primary air is delivered to the critical zone, and 9000 cfm (4250 L/s) is delivered to all other zones. The critical zone s FPB draws 1000 cfm (472 L/s) of secondary air, of which 30% is from the critical zone. The critical zone is 100% vitiated, and the other zones are 0%. Therefore, C fpb is 30%, and C ret is 20%. Evaluating Equation 8 for this scenario gives C fpb = = = C ret (12) Now assume the other zones are 90% vitiated instead of 0%. This means that C fpb is 93%, and C ret is 92%. Evaluating Equation 8 for this scenario gives C fpb = = = C ret (13) The result, 0.875, is unaffected by the concentration in the other zones because in a two-concentration system, a ratio of airflows is a perfect proxy for concentrations. Unless the airflows change, remains constant. The general case of, in Equation 4, gives identical results in this example. SUMMARY AND CONCLUSIONS Two equations for have been derived. The first is based on a mass balance of unvitiated air in the critical zone; the second is based on a mass balance of tracer gas at the inlet to an FPB. The theoretical range of for a critical zone has been shown to be from zero to infinity. Experiments on an existing building have been conducted, and the resulting values of ranged from 0.74 to 1.01 as built, but could have ranged from 0.14 to 1.13 if the FPB had been located in different, but feasible, locations. The findings of this study contradict ASHRAE Standard , which defines as ranging between 0 and 1. It is now clear that

14 VOLUME 14, NUMBER 3, MAY values greater than one are possible, since locally recirculated air can be less vitiated than average return air. ACKNOWLEDGMENTS The authors would like to acknowledge the great efforts and insight of the monitoring subcommittee for this project: Steve Taylor, Craig Wray, Iain Walker, and Jim Reardon. We are also grateful to Dennis Stanke for his careful review of the results. REFERENCES ASHRAE ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Ke, Y.P Optimum ventilation control for variable-air-volume systems. PhD thesis, Department of Mechanical Engineering, Pennsylvania State University, State College, PA. Warden, D Outdoor air Calculation and delivery. ASHRAE Journal 37(6):54 63.