MEASUREMENT OF VENTILATION RATES USING A TRACER GAS

J. J. Leonard, J. J. R. Feddes, andJ. B. McQuitty

Department ofAgricultural Engineering, University ofAlberta, Edmonton, Aha T6G 2H1.

Received 12 January 1983, accepted 28 November 1983.

Leonard J. J., J. J. R. Feddes and J. B. McQuitty.gas. Can. Agric. Eng. 26: 49-51.

Use of a tracer gas to measure ventilation rates in animal housing was evaluated utilizing three methods. The gasused was sulphur hexaflouride which was detected by an electron-capture chromatograph. The three tracer methodsinvolved whole-room constant-flow, whole-room decay, and fan-duct constant-flow techniques. Ventilation rates measured ranged from 70 L/sec to 1250 L/sec. When care was exercised in calibration of the detector and in the meteringof tracer gas, results were obtained that were within 5% of hot-wire anemometer measurements. All methods dependon complete mixing of tracer gas with the air in the ventilatedvolume under consideration.

1984. Measurement of ventilation rates using a tracer

INTRODUCTIONIn studies of the heatand moisture pro

duction of housed animals under commercial conditions, ventilation rates mustbedetermined inorder tocarry outenergyand mositure balances. Accurate measurement of ventilation rates is also requiredto determine the production rates of aircontaminants such as carbon dioxide andammonia. However, despite their importance, there is often considerable uncertainty associated with measurements ofventilation rates and a simpler, more reliable method would be desirable.

The difficulty of measuring ventilationrates arises from the fact that air velocitiesthrough inlets and outlets are variable and,despite the good intentions of designers,commercial buildings generally are notconstructed in an 'airtight' fashion. Oneconventional approach to the problem hasbeen to construct ducts, complete withflow-straightening vanes, downstreamfrom exhaustfans. The air-velocity profileof the duct cross-section, and hence thefan delivery, may then be determined inaccordance with established fan-engineering practice (Jorgensen 1961) using pitottube, hot wire, or thermistor anemometers(Feddes and McQuitty 1980). Althoughthis method has proved satisfactory despite problems with dustladen sensors, theuse of some form of tracer that could beinjected into the ventilation airstream appeared to have potential as an alternativemethod.

Consequently, a study was undertakenwith the objective of evaluating use of atracer gas as a technique for measuring airexchange rates in animal housing. For thisapplicationsulphur hexaflouride (SF6) wasselectedas the tracer gas. This gas is colorless, odorless, and non-flammable. It isnot a normal constituent of air and is sensitiveto electron-capture detection by virtue of the six fluoride atoms in the mole

repeated until any desired concentrationwasobtained. The finaldilutionstagewasalways carriedout by injecting the penultimate mixture into a known volume of aircontained in a Mylar bag. The instrumentpump could then withdraw the calibrationmixturefrom the bag at atmospheric pressure.

Whilst this method gave useable calibrationcurves, it was timeconsuming andsubject to introduction of errors at eachstage of dilution. The resulting scatter ofcalibration points led to the subsequentadoption of a technique using certified calibration mixtures. Three mixtures of SF6in air were used having SF6concentrationsof 1 ppm, 0.66 ppm and 0.33 ppm. Theuse of certified mixtures gave excellent repeatability, and one-to-one correspondence between sample concentration andinstrument reading could be obtained byadjustment of the bypass valve.

After zeroing the instrument in air, the1-ppm mixture was used to adjust thespan. This was done by adjusting the bypass flow rate until the meter reading was1 ppm. With the instrument so adjusted itinvariably gave true readings whenchecked with the other two calibrationgases. All measurements taken fell withinthe range of concentrations covered bythese gases.

cule. In the concentrations used, it posesno threat to the health of either animals oroperators (Kumar et al. 1979). The tracergas was guaranteed by the supplier to beat least 99.8% pure.

INSTRUMENT

The instrumentthat wasacquiredfor thepurposeof tracer-gasmonitoringwas a detectorchromatograph manufactured by AIIndustrial, Cambridge, England. This instrument works on the basis of ion captureand is specifically designed for use withSF6. Although it may be run for shortperiods on a rechargeable battery pack, theinstrument was invariably connected inthis application to a 110-V supply whichpowered the detector and the vacuumpump that is part of the unit. Also mountedon the instrument chassis is a small cylinder containing argon, the carrier gas.The argon supply is controlled with a gasregulator which was set at the manufacturer's specifiedpressureof 15psi. Outputis in the form of an analogue display onthe front panel, or a 0- to 10-V analogueoutput that may be interfaced with a datalogger or chart recorder. A multi-positionswitch on the front panel is used to selectsensitivity ranges that give full scale readings from 0.1 to 2.0 ppm of SF6. Also located on the front panel is a bypass valvethat controls the amount of air passingthrough the detector from the pump.

CALIBRATIONInitially, calibration was carried out us

ing samplesof air intowhichknownquantities of tracer gas had been introduced bymeans of a multiple-dilution technique. Agas syringe was used to inject a small,measured quantity of SF6 into a knownvolume of air in a sealed container. Aftermixing, a measured quantity of the mixture was withdrawn and added to another

container of air. This procedure could be

MEASUREMENT SYSTEMFor a tracer gas to be used quantitatively

in air-flow measurement, its rate of injection must be measured and controlled easily. Also, if several fans were to be monitored, provision must be made to delivergas to all fan locations simultaneously.The system used to do this is illustratedschematically in Fig. 1. The flow of gasfrom the supply cylinder to each deliveryline was controlled by precision needlevalves and measured with soap-film me-

CANADIAN AGRICULTURAL ENGINEERING, VOL. 26, NO. 1, SUMMER 1984 49

",,Ua""fl Exhaust r

1I

Data Logger/Controller

_.!._ ChanRecorder

Figure 1. Block-diagram of system for measuring multiple fan flowrates.

ters similar to that described by Keule-mans (1959).

The sampling lines from each locationterminated in a bank of solenoid valves.

These were controlled by a microprocessor-based data logger which, together withpatch-wiring, enabled sampling of any location at desired intervals, and for longenough periods to allow for transportationlag and instrument settling time. The sample air was drawn from each valve to thechromatograph for analysis before beingvented into the surrounding air. The output from the instrument normally was recorded by the data logger at preset timeintervals or, continuously for short periods, on a chart recorder.

METHODS

Tracer gas may be used in a number ofways to measure air flow (American Society of Heating, Refrigerating and Air-Conditioning, 1981). Three methods thathave been used by these authors are described below:

(i) Whole-room, constant-flow: Thetracer gas is injected at a constant rate atthe air inlet and sampled at the air outletfrom the room. If complete mixing of theair in the room is assumed, then the ventilation rate may be calculated from the diluted concentration using Eq. 1.

Q = RIC (1)

where Q = air flow rate (L/sec); R = injection rate of SF6, (mL/sec); and (C =measured concentration (mL/L).

In practice, unless high-capacity circulation fans are in operation in the ventilated space, the assumption of complete

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mixing may be hard to justify (Barber andOgilvie 1982). In addition, both injectionand ventilation rates must be held constant

sufficiently long to allow the concentration of tracer gas to reach equilibrium inthe ventilated room.

(ii) Whole-room decay: The tracer gasis injected at a high rate into the roomwhere, again, complete mixing is assumed to occur. When the meter indicates

a suitable initial concentration, injection isstopped and the concentration is monitored as it decays. If the ventilation rate isconstant and mixing is complete, the decay of concentration with time, t, may bedescribed by the differential equation:

dCldt = C.Q/V (2)

where V = volume of the ventilated space(L).

The solution to Eq. 2 is:

ln(C/C„) = -t.Q/V (3)

where C0 = initial concentration of SF6(ppm).

Thus, if the natural logarithm of fractional concentration, C/C0, is plottedagainst time the result will be a straightline with a negative slope of Q/V, whichis the number of air changes per unit time.This method suffers from the same de

pendence on mixing as the previousmethod. Nevertheless, if air is sampledfrom a number of locations in the room to

check on the mixing, good results may beobtained.

(iii) Fan-duct, constant-flow: Thismethod is very similar to the whole room,constant-flow method except that thetracer gas is introduced at the exhaust fan

inlets and air is sampled at the end of ductsconstructed downstream from the fans.

The flow rate through each fan is calculated using Eq. 1.

Although the third method requires injection and sampling lines for each individual fan, it was used by the authors inrecent studies of animal environments on

the basis that complete mixing of the tracergas was more likely to occur, or could beencouraged more easily, in the limitedconfines of a duct rather than in an animal

building as a whole.Measurement of the concentration pro

file was carried out on a 16-point gridacross the square duct cross-section. Thesemeasurements were facilitated greatly bythe use of a two-pen chart recorder, onechannel of which could be connected to a

fan-speed sensor. Using this equipment,fan flow-rates were calibrated against fanspeeds which were monitored continuously during heat and moisture balanceexperiments. Such calibrations are onlyvalid if the static pressure differential between the inside and outside of the venti

lated space remains constant.

RESULTS AND DISCUSSION

Tracer gas techniques were used first,by the authors, in an experiment on heatand moisture production of growing pigs(Feddes et al. 1983). In this experiment,pigs were housed in small (5-m x 2-m x2.5-m) rooms supplied by a common conditioned-air duct. The ventilation rate of

each room was determined by using several methods. With the first method, theair velocity profile in each exhaust ductwas measured with a hot-wire anemome-

CANADIAN AGRICULTURAL ENGINEERING, VOL. 26, NO. I, SUMMER 1984

ter. The ventilation rate was then determined from the mean velocity and thecross-sectional area of the duct.

Secondly, the whole-room constant-flow method was used with SF6 being introduced at the air inlet grill and sampledat the end of the exhaust duct. Each roomhad a circulation fan to ensure mixing ofthe tracer gas, which was injected at approximately 0.1 mL/sec. Airwas sampledin the exhaust ducts of each room whichwere also instrumented with thermistoranemometers. Steady and repeatable readings were obtained from the SF6 meterinthis application, indicating that completemixingwastakingplace in the room. Mixing was also checked by sampling tracergas concentrations throughout the roomvolume. These measurements were foundto be uniform with those at the exhaustduct. There was noevidence ofabsorptionof the tracer gas by the animals or thestructure.

Thirdly the condition of the inlet andexhaust air were measured while only thewall-mounted stripheaterswereprovidingheat to the rooms. The ventilation rate wasthen determined by carrying out a simpleheat balance. As a further check, the airvelocity profile at the inlet grill was usedto calculate the inlet flow rate.

The ventilation rates derived from thelast three methods were within 5% of eachotherbut differedfrom that obtained usingthe first method by almost 20%. This suggested that, despite careful construction ofthe rooms, the positive-pressure ventilation system was giving rise to air leakagethrough openings other than the exhaustducts. This was confirmed when the SF6was then introduced at the exhaust duct

inlet and the flow rates derived from tracer

gas measurements were found to be within5-8% of those derived from the hot-wire

anemometer.

The whole-room decay method also wasutilized in the same experiment. The SF6 *was injected at the air inlet grill until themeter was indicating full scale. Injectionthen was stopped and the decay of gasconcentration was monitored at the ex

haust duct. Good agreement (5%) with themethods described above was achieved

but, since the decay method is inherentlytime consuming, it is of limited value incontinuous monitoring applications. The

range of ventilation rates measured usingthe above techniques was 60 L/sec to 85L/sec.

In subsequent work on the monitoringof broiler barns (Feddes et al. 1983) complete mixing of the barn air could not beassumed. Consequently, in these cases,the fan-duct method was applied to thecalibration of theexhaust fans. Mixing oftracergas within the duct was checked bya 16-point sampling of tracergas concentrationacross the duct. Mixing, however,was found to be incomplete in the ductlength used (3.5 m) when the tracer gaswas injected at a single point. Completemixing was obtained when injection wascarried out through a manifold, situatedupstream of the fan and having 16 portsarranged on a 100-mm-square grid. In allcases there was agreement within 5% between flow rates calculated using thetracer gas and those determined with a hotwire anemometer. The flow rates measured ranged from 800 L/sec to 1250 L/sec.

Experience gained to date with the useof SF6 as a tracer gas for measuring ventilation rates in animal housing suggeststhat it offers an acceptable alternative tohot-wire or heated-thermistor anemometertechniques. However, since the accuracyof the three tracer gas methods is highlydependent on the effectiveness with whichthe gas is mixed in air, the use of SF6 tomeasure ventilation rates in commercialanimalhousing, wherecompletemixingismost unlikely, is of doubtful value unlessthe fan-duct, constant flow method isused. This method has been found to beconvenient to use since, provided that theSF6 is injectedupstreamof the fan througha suitable manifold, only one samplinglocation is necessary for measurement of thegas concentration in the downstream airflow. In comparison, a duct traverse involving multiple velocity readings is required with anemometer techniques. The

*method also may be adapted readily tocontinuousmonitoringandrecordingof airflows through a fan or fans.

CONCLUSIONS

Sulphur hexafluoride tracer gas can beused as a convenient and accurate meansof measuring ventilation flow rates in animal housing provided that the followingpoints are observed:

CANADIAN AGRICULTURAL ENGINEERING, VOL. 26, NO. 1, SUMMER 1984

(1) The gas detector should be calibrated carefully over the desired operatingrange before use. The calibration shouldbe checked from time to time during anyextended periods of monitoring.

(2) Provision must be made for the accurate metering of the tracer gas. This requires anaccurate regulator onthe supplybottle, precise metering valves that are notsubject to thermal drift, and flow measuring devices that are accurate at the lowflow rates used in these applications.

(3) Care must be exercised in selectinginjection and sampling points to ensureadequate mixing of the tracer gas, andsampling of air that is representative of theventilated air space.

ACKNOWLEDGMENTSThe authors wish to acknowledge financial

support for this study from the Alberta Agricultural Research Trust, the Alberta Pork Producers Marketing Board and the Natural Sciences and Engineering Research Council ofCanada. The assistance of Mr. D. Travis,Technician, Department of Agricultural Engineering, University of Alberta, also is gratefully acknowledged.

REFERENCES

AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR CONDITIONING ENGINEERS. 1981. Fundamentalshandbook. ASHRAE, Inc., New York.

BARBER, E. and J. R. OGIL-VIE. 1982. Incomplete mixing in ventilated airspaces. Part 1. Theoretical considerations. Can. Agric. Eng. 24: 25-29.

FEDDES, J. J. R. and J. B.MCQUITTY. 1980. Design of an air-speedsensor system. Can. Agric. Eng. 22: 97-99.

FEDDES, J. J. R., J. J. LEONARD, and J. B.MCQUITTY. 1983. The influence of selected management practices on heat, moisture and air quality in swine housing. Can.Agric. Eng. 25: 175-179.

FEDDES, J. J. R., J. J. LEONARD, and J. B.MCQUITTY. 1984. Broiler heat andmoisture production under commercial conditions. Can. Agric. Eng. 26: 57-64.

JORGENSEN, R. 1961. Fanengineering. 6thed. Buffalo Forge Company, Buffalo, N.Y.

KEULEMANS, A. I. M. 1959. Gas chromatography. 2nd ed. Reinhold PublishingCorporation, New York.

KUMAR, R., A. D. IRESON, and H. W.ORR. 1979. An automatic air infiltrationmeasuring system using SF6 tracer gas inconstant concentration and decay methods.Trans. ASHRAE 85: 385-395.

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