r The effects of damper position and inlet pressure on

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1 LABORATORY TESTS OF THE NONLINEARITY OF OUTDOOR AIR PERCENTAGE AS A FUNCTION OF DAMPER POSITION AND INDUCED INLET PRESSURE P.S. Crtiss J.F. Kreider, Ph.D. ABSTRACT r The effects of damper position and inlet pressre on the ratio of otside to total spply air ere tested in an HVAC laboratory. It as fond that the ratio varies nonlinearly ith the damper position at lo otdoor airflos. With indced pressre at the inlet to the main air-handling nit (simlating a indy otdoor air condition), significantly more otside air as taken in than the damper position might sggest. The relationship beteen flo and damper actator signal or position is key to the proper control of the otside air dampe:..j / INTRODUCTION Recent concerns abot airborne polltants have placed mch emphasis on indoor air qality, particlarly in forced-<:irclation systems. The simplest ay to ensre acceptable air qality (in npollted areas, of corse) is to bring % otside air into a bilding. The cost of conditioning this air, on the other hand, makes this a rather expensive option, especially in extreme eather conditions. A more typical control scheme is to bring in the minimm amont of otside air necessary to satisfy local code reqirements (or ANSI/ASHRAE ), except hen ambient conditions permit the se of the economizer mode. The process of bringing otside air into a bilding is relatively simple. Dampers are modlated to either recirclate retrn air back into the main air-handling nit(s) or to expel it to the otdoors. In the latter case, fresh otside air is then dran into the bilding to make p for the exhasted air. In the case of minimm otside air intake, it is fairly ell established that the ratio of fresh air to total bilding spply air ill not be a linear fnction of the damper position, since the otside airflo ill increase rapidly as soon as the damper begins to open yet ill not sho mch change in flo once the damper is close to flly open. It is also generally assmed that this ratio ill remain a constant for a given damper position, regardless of the total spply airflo rate. Hoever, the test reslts presented here sho that this may not alays be the case. TEST FACILITY The experiments ere performed at an HV AC systems and controls laboratory. This facility consists of a single-deck variable-air-volme system that can spply 12, ACFM to for individal mnes (see Figre 1). The main air-handling nit (AHU-1) is a front vertical discharge system, and the retrn fan is an in-line axial fan. Both fan speeds are controlled sing variable-freqency drives. The temperatre and hmidity of the simlated otside air are maintained by the otside air-<:nditioning station (OACS) located pstream of AHU-1. The OACS is also sed to control the total pressre at the inlet to AHU-1. Retrn air can either be exhasted from the bilding or recirclated back into AHU-1. Both the recirclated and otside air dampers on AHU-1 are linked and are modlated by an electronic actator. The freeze protection and relief dampers are also controlled from the same signal that is sed to control the otside air damper. The damper anglar position is linear ith respect to the control voltage. Both the recirclated and otside air dampers are of parallel-blade configration. All data acqisition and control in the HV AC laboratory is handled by a netork of dataloggers and compters. More than 25 channels are monitored every 1 seconds, hile concrrent analog control signals are generated by a combination of timed digital otpts and plse-idth-modlator signal generators. Dct static and total pressres are recorded at several locations. The static pressres are measred sing differential pressre transdcers attached to for taps located on the dct alls. The total pressres are measred sing pilot tbes located in the center of the airstream. Air temperatres are measred sing platinm RTDs, each packaged ith individally calibrated transmitters. The sensor lengths are 12 or 24 feet, depending on the dct size, and are monted sch that they sense the average dct air temperatre. These sensors have a time constant of approximately one minte de to their metal sheath and a rated accracy of.1 F. Air velocities are measred ith a combination of Peter S. Crtiss is a research assistant and Jan F. Kreider is a professor and director of the Joint Center for Energy Management, University of Colorado, Bolder. THIS PREPRINT IS FOR DISCUSSION PURPOSES ONLY, FOR INCLUSION IN ASHRAE TRANSACTIONS 1992, V. 98. Pt. 1. Not to be reprinted In hole or in part ithot ritten penni&bion of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. 179) Tllle Circle, NE, Atlanta, GA 329. Opinions, findings, conclsions, or recommendations exprebbed In this paper are those of the athor(s) and do not necebbanly reflectthe vies of ASHRAE. Written qestions and comments regarding this paper shold be received at ASHRAE no later than Feb

2 RETURK DUCT STATIC PRESSURE. POST FAK '\ _ RETURN AIR f"lov RATE RELIEF LOIJVER RELIEr DAMPER RECIRCULATED A JR DUC'l' STATIC: PRE:SSURt.... RETURM FAN RECIRCULATED A IR flov RATE 4 RETURN AIR TEMPERATURE FULL SIZE ZONE RETURN DUCT 'rotal PRESSURE. PRE-FAN RETURN OAHPER FREEZE PROTECTION DAnPER OUTSIDE AIR \ \ l nixed AIR TEnPERATURE l'a ". ;i ;;:;: :>" OVERED ftlxing BOXES l...l '---41,., SlnULATED OA TEHPERATURE SINULATED OA. PRESSURE SUPPLY AIR FLO" RATE srrt.y AIR DUCT STATIC PRESSURE Figre 1 HVAC laboratory air system schematic (shoing placement of sensors). averaging pitot tbe arrays and differential pressre transdcers. The volmetric airflos are then calclated by mltiplying the dct area by the air velocity after making corrections for the atmospheric pressre and the air temperatre at the location of the airflo station. The conversion eqation from differential pressre to airflo is given by here Vair = volmetric flo rate of air, Adcr area of dct, AP pressre differential across the pitot tbe array, and p density of air (a fnction of the atmospheric pressre and air temperatre). The airflo stations are periodically recalibrated by comparing pitot tbe traverses to the otpt from the pressre transdcers. The pressre transdcers have been shon to be qite accrate ithin their specified pressre range, ith any significant error coming solely from sensor drift over time. Measrement errors cold arise from an neven velocity profile in the dct. The pressre differential from the higher airspeeds old tend to dominate, and the overall reading old be too high. An additional sorce of error is the decrease in effective dct area de to the (1) pitot tbe grid hardare. This hardare can occpy as mch as 2 % of the total dct cross section. The airflo stations ere fond to have poor accracy and slightly nonlinear response. While e are able to compensate for mch of this intrinsic error in the compter-based conversion eqations, the accracy is still only good to ithin approximately 7 % to 1 % of the total volmetric airflo reading. There are 11 airflo stations in the lab, one of hich is located donstream of AHU-1 (hich measres the total spply airflo rate) and one in the recirclated air dct. The amont of otside air dran into AHU-1 is the difference beteen these to airflos. A redndancy check is made on the ratio of otside air to total spply air by sing an energy balance based on the temperatres of the retrn, mixed, and otside airstreams. The ratio of otside air to total spply air, based on the energy balance approach, is given by here % OA ratio of otside airflo to total spply airflo, T = otside air temperatre, Tma mixed air temperatre, and Tra retrn air temperatre. (2)

3 TEST PROGRAM Sixteen separate tests ere performed. The goal of each test as to qantify the ratio of otside air to total spply air for different spply airflo rates. Dring each test, the compters continosly increased the spply fan speed from 2% to % fll speed at a rate of 2% per minte. The chilled-ater system as in operation dring these tests to ensre a relatively large temperatre differential beteen the otside and retrn airstreams. Wann otside air as broght in nconditioned, hile the airhandling nit and individal load coils in the zones ere sed to loer the retrn air temperatre. The 16 tests arose from varios combinations of experimental parameters that ere fixed for the dration of each test. We anted to examine the effects of (1) different otside air damper positions, (2) a positive pressre on the otside air inlet lovers, and (3) the retrn fan speed control. The experimental parameters ere as follos: Retrn fan speed control To methods ere sed to control the retrn fan speed. The first involved a simple tracking scheme, here the retrn fan operated at 5 % fll scale less than the spply fan; i.e., if the spply fan as at 8% of its maximm 9 rpm, then the retrn fan as at 75 % of its maximm rotational speed of 1,5 rpm. The second method had the retrn fan operating so as to maintain a retrn dct static pressre of -.1-in. eqivalent ater pressre, measred beteen the retrn risers from zone simlators 1 and 2. OACS fan control Normal operation of the OACS fan is sch that atmospheric pressre is maintained at the inlet to AHU-1. When sing this scheme, the fan is necessary only to overcome the pressre drop associated ith the dampers and coils in the OACS. Half the tests ere performed this ay. Dring the other tests, the OACS fan maintained a.16-in. ater colmn eqivalent pressre at the inlet of AHU-1. This pressre corresponds to that prodced by a steady ind of 2 mph impinging on the otside air intake lovers of AHU-1. Otside air/recirclated air damper position A different test as performed for each of several settings of this damper. The settings sed ere 1 %, 2 %, 3 %, and 4 % fll open. RESULTS Data ere taken every 1 seconds, then averaged over 6 seconds and ritten to a data file for later analysis. Each test took approximately one hor to perform---4 mintes to ramp the fan speed p and 2 mintes to let the system air temperatres stabilize for a ne damper position and indced inlet lover air pressre. The OACS fan speed as controlled sch that the indced inlet air pressre at AHU-1 as maintained at the desired setpoint ( in. or.16 in..c.) ith a standard deviation of ±.5 in. ater colmn, hile the typical maximm deviation as no more than.15 in. ater colmn. The ratios of otside air to total spply air, as calclated by the airflo rates and from the temperatres (Eqation 2), agreed fairly ell throghot all the tests; a slight discrepancy beteen the to as noted, typically 4 % fll scale bt sometimes as mch as 1 % at lo spply airflos hen the OA percentage as qite high. This error most likely arises from airflo station calibrations. Figre 2 shos the airflo readings from the data set hen the retrn fan as controlling the retrn dct static pressre, the AHU-1 inlet lover damper pressre as at in..c., and the OA damper as at 1% fll open. Figre 3 shos the dct air pressres for the same test. Figres 4 throgh 7 sho the calclated airflo as a fnction of the total spply airflo. The calclated airflo is based pon the temperatre energy balance from Eqation 2 becase the accracy of the airflo stations sffers at very lo flos. A slight periodic variation in the otside air percentage is noticed in some of the data sets. This is de to the consistent cycling of the chiller sed to maintairi. a large temperatre difference beteen the otside and retrn airstreams. No provisions ere made for actal otside air pressres against the relief dampers. This lover empties to an alleyay, hoever, and faces east (aay from the prevailing inds), so it is assmed that actal otside indced pressres old not affect the experiments significantly. Figre 4 shos the reslts sing a z.ero inlet damper pressre and the retrn fan tracking the spply fan. The airflo is increased from 3, to abot 11, ACFM (roghly 2 % to % fll-scale otpt). The retrn airflo rate dring these tests matched the spply airflo qite ell,.demonstrating that the system is in good balance sing this techniqe. The otside/total airflo ratio does not increase at the same rate as the increase in damper position and, in fact, remains fairly constant for each of the for tests. At 1 % damper position, the ratio stays beteen 1 % and 15 %, hile at 4 % damper position, the ratio is beteen 16 % and 23 %. Figre 5 gives the same data as in Figre 4, except ith a positive inlet pressre at AHU-1. Comparison of these to figres shos an obvios difference in the behavior of the airflos. The actal otside airflo rate appears to remain basically constant over the entire range of spply airflo rates, despite the fact that the recir-

4 12 9. BODD...?ODD. 1 hooo 5 4 AHU - 1 SUPPLY AIR rlow HA'rE RETURN AlR FLOW HATE RECIRCULATED AIR PLOW _,_..,,.._.-r-r--r-r tzso ) OD 13:5 tl: ta : ts Figre 2 Typical aiiflos. Tl/1!: ( HH 1111] BO SOPPLY AIR DUCT S'T'A'T'IC PRESSURE RETURN DUCT STA.TIC PRESSURf:, POST-FAN... RECIRCULATED AIR STATIC: PRESSURE Sll'IULA.'T'ED OA PRESSURE RETURN DUCT STATIC PRES PRE-t'A.tl l, 6 1 o x 1. 2 ' JS.-.,-,-',-...,-..,.r 12 rn 1Z so ,-. 1) tj { Figre 3 Typical air pressres. TinI: [HB : ntt] AMJ'1En AT 4- OPEN OAMPE AT 3% OPEN 8 OAMflC::R Al 2 OPEN CJ DAMPER AT 1% OPEN <; 7 D iii,_ z 6 - so 4 JO 2U l()ial SYSTEM Al no lµl::\.fcl:ni FULL SCALE) ' 8 Figre 4 Otside air percentage ith retrn fan tracking spply fan and atmospheric pressre at inlet lover of AHU-1.

5 9 8 7 A () DAMPER AT 4 9' DAMPER AT 31: OAMrER AT 2tl..UPER fl T 1'11: 6 sn r ,---1r---r ; TOTAL SYSTl::M AIR FLOW [PERCENT FULL SC.O.LE] Figre 5 Otside air percentage ith retrn fan tracki11g spply fan and positive pressre at inlet lover of AHU-1. IOO , 9 DAMPER AT 4tl OAt.APE AT 3 8 DAMPER AT 2Utl O CAMPER AT 1% 7 GO so 4 JO DO inn TOTAL SYSTEM AIR FLOW (PERCENT FULL SCALE1 Figre 6 Otside air percentage ith retrn fan controlling retrn dct static pressre and atmospheric pressre at inlet lover of AHU- 1. clated air dct static pressre is noticeably higher than the indced otside air pressre. As a reslt, at lo demand for bilding air (i.e., lo total airflo) the ratio of otside air increases dramatically. This is the case for all for tests. As in the mro inlet pressre tests, the overall OA ratio does not vary mch ith damper position; rather, the main case of variation of this ratio is changes in the total airflo rate. Figre 6 shos the effect of increasing the spply airflo rate hen the indced inlet pressre is at atmospheric and the retrn fan is controlling to a negative pressre in the retrn dct. Here the OA percentage at lo spply airflo is roghly tice the percentage at maximm spply flo. Figre 7 gives the experimental data for a positive inlet pressre and a retrn fan controlling to a negative retrn dct pressre. Dring these tests, the OA percentage appears to increase as the spply airflo rate increases, albeit very slightly. This increase falls ithin the range of experimental error, hoever, and shold not be vieed as a tre effect. CONCLUSIONS For all the tests, it as noticed that the ratio of otdoor air to total spply air did not vary mch for a given particlar fan speed, despite damper positions beteen 1 % and 4 % fll open. An interesting reslt is that the OA percentage remains relatively constant over the entire range of fan speeds hen (1) there is an atmospheric pressre at the inlet to AHU-1 and the retrn fan tracks the spply fan, and (2) there is positive pressre at the inlet to AHU-1 and the retrn fan is controlling to a negative retrn dct pressre. For the other to cases, the OA percentage as greater (often dramatically so) at loer spply airflo rates. This is an important finding, since air handlers are sally operated in the range of 1% to 3% otdoor air. Of particlar interest is the case of positive inlet pressre and the tracking retrn fan (Figre 5). Since the recirclated air dct static pressre is higher than the indced inlet pressre, one might expect the OA percentage to be very lo, yet the airflo data sho jst the

6 a Vi 9 8 JO "!Z 4 " ii' 3 O DAMPER AT 4 DAMPER AT 3' DAMPER AT 2 O DAMPER AT 1' GO TOTAL SYSTEM AIR FLOW (PERCENT FULL SCALE) 8 Figre 7 Otside air percentage ith retrn fan controlling retrn dct static pressre and positive pressre at inlet lover of AHU-1. opposite. Also dring these tests, the balance of the system is thron off, ith the retrn airflo rate mch less than the spply flo. It is believed that the elevated pressre in the recirclation dct is cased not by the retrn fan by rather by the indced pressre at the inlet to AHU-1. This old also case an elevated pressre rise across the retrn fan for any given fan speed, thereby loering the total flo rate as compared ith the analogos data from the tests at zero indced pressre. This old also explain the overall rise in system pressre; the only occrrence of positive retrn dct pressres is dring this series of tests. In or system, hich is fairly typical of crrent commercial constrction, the amont of otside air entering the main air-handling nit varies nonlinearly ith respect to the damper position. Energy management systems designed to minimize energy se hile maintaining comfort conditions cold not rely on the damper position as an accrate indicator of the otdoor air ratio. Similarly, this ratio also depends on the relative pressre against the inlet lovers. A positive pressre similar to that cased by ind as shon to indce a mch larger otdoor airflo than damper position might sggest. Design and placement of inlet dampers shold consider prevailing ind directions and speeds. For example, a VA V system ith reheat may be talcing in more otside air than expected on cold days. This cold case large loads on the heating plant. Finally, it as shon that the accrate measrement of otside air percentages is a difficlt task, even nder laboratory conditions sing ell-maintained eqipment. This raises the possibility of inaccrate vales of OA flo as they are measred in typical bilding HV AC systems from the se of single-point temperatre references, misplaced sensors, etc. With the increased demand for sch data arising from the ne standards for fresh air in bildings, it becomes apparent that mch more research is needed in this field. BIBLIOGRAPHY Brandemehl, M.J., and J.F. Kreider Design and constrction of a niversity laboratory for test of commercial-bilding-scale HV AC systems and their controls. ASHRAE Transactions 96(2). Kreider, J.F., and A. Rabi Heating and cooling of bildings: Design for efficiency. Ne York: Mc Gra-Hill (in press). Moser, S The design of an HV AC systems and control laboratory. JCEM Report No. JCEM/TR/ Bolder: University of Colorado. APPENDIX DATA ACQUISITION AND CONTROL SYSTEM COMPONENTS The eqipment sed in the data acqisition and control system is manfactred by Helett-Packard, inclding the folloing: HP3497 A Data Acqisition and Control Unit (2) HP3456A Precision Mltimeters (2) HP9 Series Compters (6, varios models) HP Shared Resorce Management System (1) HP48 RTU for OACS experimental control (1). Sensors sed inclde the folloing: Airflo - Air Monitor Company mltiple pitot tbe arrays calibrated ith pitot tbe traverses; MKS model 223BD pressre transdcers sed ith arrays. Atmospheric Pressre (sed in airflo calclations) - MKS model 122AA absolte pressre transdcer.

7 Dct Air Pressres - MKS model 223BD pressre transdcers. Dct Air Temperatres - Omega Engineering platinm resistance temperatre detectors (-ohm nominal resistance); MAMAC Systems TE-211 platinm averaging RTD. Actators inclde the folloing: Fan Speed - Allen-Bradley variable-freqency drives on all fans. Damper Motion - Belimo SM24-SR electronic damper actator. Signal Generator - MAMAC Systems PW6 plse idth modlator.