WO1998004872A1

WO1998004872A1 - A method for measuring ventilation in rooms with tracer gas technique

Info

Publication number: WO1998004872A1
Application number: PCT/SE1997/001315
Authority: WO
Inventors: Hans Stymne
Original assignee: Hans Stymne
Priority date: 1996-07-29
Filing date: 1997-07-28
Publication date: 1998-02-05
Also published as: SE9602884D0; EP0917635A1; SE510876C2; EP0917635B1; SE9602884L; DE69709120D1; WO1998004872A8; AU3715197A

Abstract

In a method for measuring the ventilation in zones in a zone-divided ventilated system, an amount of tracer gas proportional to the volume of the zone is spread in each zone. At the places where one is interested in determining the ventilation conditions, integrating samplers for the tracer gas are placed.

Description

A method for measuring ventilation in rooms with tracer gas technique

BACKGROUND

Ventilation measurement in its most general form implies measuring the supply rate of ambient air to a building and how this air is distributed within the building space.

Field measurement of ventilation can be performed with several known techniques. The most commonly used technique involves measuring the air flow rates in the ventilation equipment, ducts or supply- and exhaust devices. Such ventilation measurements can only be used in mechanically ventilated buildings. The methods also have some additional limitations:

Estimated air flows may be a mixture of ambient air and air which has previously circulated in the building (return air or air short-circuiting in ducts and heat exchangers).

Ambient air is also often infiltrated into the building through other routes than via ducts (windows, leakage etc.), a contribution which is neglected in measuring in ducts and supply- and exhaust devices).

No information is gained on how the supplied air is distributed within the building space, after it has left the ductwork or the supply devices.

In order to be able to measure the ventilation rate in buildings in which the supply of air wholly or partly occurs in other ways than by mechanical ventilation, one has to rely on tracer gas technique. Depending on which ventilation parameters are of interest, several different tracer gas techniques are available. Some of the most commonly used techniques are shortly described below.

The concept "ventilated system" involves all spaces, that are somehow connected in a ventilation sense, i. e. all spaces to which or from which air can be transferred from or to other parts of the system. The only air which can be supplied over the boundary of a ventilated system is ambient air.

1- DECAY TECHNIQUE

A tracer gas is mixed into all spaces of the ventilated system to the same initial tracer concentration, after which the decay as a function of time is registered in one or several points. Different ventilation parameters can be calculated from the decay course. Some different cases can be separated.

1a) Measurement of the specific ventilation flow rate

(previously called air change rate). In this case it is required that a thorough mixing is maintained in the whole system with the help of for example fans. The tracer gas concentration curve as a function of time will then ideally show an exponential decay. Evaluation of the specific ventilation flow rate is usually obtained from the slope of a plot of the logarithm of concentration versus time.

In many cases (especially when there are very large rooms or many rooms) it is difficult to achieve a satisfactory mixing. The decay will in that case not be entirely exponential. In the evaluation of the air change rate it is then common to utilise only the last part of the logarithmic decay curve, which usually is linear. This procedure is an approximation and a fairly good skill is required, in order to measure at the correct positions and judge what part of the decay curve is useful.

1 b) Measurement of the local mean age of air.

Also in this case, tracer gas is mixed to an even concentration in the whole of the ventilated system, but during the measurement of the decay, no effort is done to influence the mixing in the system. In this case different decay curves are usually ob- tained in different parts of the system, which reflects the different distribution of ventilation air. Well ventilated parts of the space shows a quicker decay than badly ventilated parts. The evaluation is done by measuring the area under the plot of concentration versus time from the beginning of the decay until all tracer gas has disappeared from the system. This area (integral) constitutes a direct measure of the mean age of air at the measuring point. The local mean age of air tells how long the air around the measuring point in average has stayed in the building, since it entered as ambient air. By measuring the de- cay at several positions in the system, it is possible to map the distribution of ventilation air. Spaces mainly ventilated with direct supply of outside air, show a shorter mean age than such mainly ventilated with air from other spaces.

The local mean age in the exhaust air is always equal to the inverted value of the specific ventilation flow rate. When it is possible to measure in the exhaust air this method makes it possible to determine the air change rate, even if the system is badly mixed.

2. CONSTANT EMISSION TECHNIQUE

In this case, tracer gas is injected into the system with a constant rate. After some time the concentration of tracer gas and its distribution in the system will attain a steady state. Also in this case some special cases can be distinguished.

2a) Determination of total ventilation flow rate

In this case the whole system shall be mixed by means of fans, so that the tracer gas concentration is equal everywhere. The evaluation is performed by measuring the steady state concentration and from that value calculate the total ventilation flow rate as the quotient between the rate of tracer gas emission and the tracer gas concentration.

When it is possible to measure in the exhaust air it is always possible to determine the total ventilation flow rate from the mentioned quotient, even if the system is badly mixed.

2b) Determination of the local mean age of air.

Also in this case tracer gas is spread with a constant rate, but the emission is to be evenly (homogeneously) distributed in the whole space of the system. This technique (the homogeneous emission technique) is relatively new and is practically performed by dividing the system space in smaller zones, in each of which tracer gas is emitted with a rate, which is proportional to the volume of the zone. The equilibrium concentration in a zone is a direct measure of the local mean age of air (= concentration divided by the emission rate per volume unit). By measurement of the concentration in many parts of the system, it is possible to map the distribution of the ventilation air.

If it is possible to measure in the exhaust air, the specific ventilation rate of the system can be calculated from the inverted value of the mean age of air at that point. 2c) Determination of air flows between zones.

By using several different types of tracer gas simultaneously (e. g. different tracer gases in the different zones in a zone-divided system) all air flows to and from each zone can be calculated. Such multi-tracer gas technique is relatively seldom used in research and field measurements). With the so-called passive tracer gas technique, simultaneous use of 2-3 different tracer gases is relatively common.

3. CONSTANT CONCENTRATION TECHNIQUE

This technique can be used in a zone-divided system and implies that an automatic dosing device injects tracer gas to the different zones, in a way that all zones achieve the same tracer gas concentration. The technique requires a relatively complicated equipment, with feedback between the measured tracer gas concentration and the injection rate. The direct supply of ambient air to each zone can be estimated in this way.

4. PULSE TECHNIQUE.

This is a relatively unusual technique, which is sometimes used in larger buildings with mechanical ventilation. A certain amount of tracer gas is injected into the supply air and the time response of the concentration is measured in the exhaust air. The total ventilation flow rate can be calculated from the injected tracer amount, divided with the integral under the concentration response in the exhaust air. From the "first moment" (the integral of the product of concentration and time) for the concentration plot, the local mean age of air can be determined. Problems with field measurements

The method, which this invention refers to, is meant to facilitate ventilation measurement with tracer gas in field work. I will therefore shortly describe the problems of applying the different known techniques in field work and demonstrate how the new invention can solve these problems.

All techniques require analysis of (low) concentrations of tracer gas. Sampling equipment as well as tracer gas analysis equipment is therefore needed. Accurate tracer gas devices are expensive equipment, which has to be handled by experts. The expensive tracer gas equipment as well as the expertise are tied up during the time period, during which measurements are made. It is therefore out of question that tracer gas measurement with analysis equipment in the field can be commonly used in field work.

One solution to this problem has been to take air samples in the field (with bag, syringe etc.) and bring the samples to a laboratory for later analysis and evaluation. In order to be able to make accurate evaluation of the decay technique, several samples taken with a time delay at each sampling position are necessary. In order to get a correct equilibrium value with the constant emission technique, first a long waiting period is necessary after which several samples must be taken.

The air mixing constitutes a special problem with method 1 a and 2a. Artificial mixing is impractical in occupied premises.

In recent years the so called passive tracer gas technique has grown in use. In this technique tracer gas is spread with a constant rate through diffusion from miniature containers, which have been positioned in the measurement object. Small diffusive air samplers are also positioned at suitable locations in the measurement object. The samplers accumulate tracer gas from the air with a rate which is proportional to the concentration. After the measurement period (from a few days to months) the samplers are sent to a laboratory for analysis of the accumulated amount of tracer gas. One advantage with the passive tracer gas technique is that staff is only needed for positioning the equipment and that no expensive equipment is tied up during the measurement. The technique is used for determining the total ventilation flow rate according to technique 2a, and since some time also for determining the local mean age of air according to 2b, as adjustable passive tracer gas sources have been available. Some investigations are also performed according to 2c.

This passive tracer gas technique has many advantages for field measurements. It yields among other things the average ventilation performance during an extended time of normal occupancy. This technique can however occasionally be less desired. One such occasion is when a short term measurement is desired, for example during a working day, due to the fact that the ventilation may be decreased during off-work time. Another occasion is when it is desirable to have the ventilation measurement rapidly and non-expensively concluded, for example for a routine check of the ventilation performance, in which case it is allowed to accept a value, which is less representative for an extended time period.

It is problematic to make quick measurements with the passive technique. The air sampling shall namely be performed during "steady state conditions". If the air has a mean age of 2 hours or more (which is normal in dwellings), it is a delay of 8-10 hours to approach this state after positioning the tracer gas sources. When measuring extended times, this build up period can be neglected, but with short term measurement it is of es- sential importance. The samplers must not be opened before a considerable time period has elapsed, which complicates the handling.

The invention solves these problems.

The manner to investigate the ventilation with tracer gas technique, for which I apply a patent, can be characterised in the following way:

Using an equipment for tracer gas emission, an amount of tracer gas is spread in each zone in a zone-divided ventilated system, in such a way that the amount of tracer gas is proportional to the volume of the zone. Before then, integrating sampling devices for the tracer gas have been placed at those positions at which it is of interest to know the ventilation performance. The sampling devices are active until essentially all tracer gas has disappeared from the system.

The tracer gas emission is best done in the form of a short pulse of a known amount of tracer gas, which is then preferably mixed into the zone volume. Due to the fact that the integrating samplers are active already from the beginning of the tracer gas emission, the spreading of tracer gas in the different zones may be performed in any pace, which is convenient. Any time delay between the injections into the different zones is without importance. The integrating samplers should have the characteristics, that their rate of tracer gas sampling is directly proportional to the concentration of trace gas in the air. This is essentially true for samplers of the diffusive type and for all "pumped" sampling.

After the sampling the samplers are inactivated and sent to a laboratory for analysis of the amount of sampled tracer gas (M). Knowing the "equivalent air sampling rate (K)" of the samplers the integral:

can be evaluated, from which the local mean age of air τ can be calculated

Cdt m / V (m l V) - YL

where m/V is the amount of injected tracer gas per volume unit of the space.

EXAMPLE

Below, an example is given of how a ventilation measurement may be performed, using the methodology, for which a patent is applied. The example should not be regarded as a limitation of the general character of the patent.

a) An inspector is visiting a naturally ventilated dwelling, in which the ventilation and the distribution of ventilation air in the different rooms are to be estimated. In each room the inspector mounts an open diffusive sampler on the wall at a height of 1.7 m.

A diffusion sampler may consist of a 5 cm long glass tube with an inner diameter of 4.3 mm, closed in one end. In the glass tube there is an adsorbent bed with approx. 100 mg activated charcoal, which extends from 17 mm below the open end of the tube. Through diffusion the constituents of the air are transported through the open end of the glass, down to the adsorption bed, in which most contaminants in the air, including the tracer gas are adsorbed. The sampling rate of tracer gas is determined from Fick's first law of diffusion, which implies that the sampling rate is directly proportional to the concentration of the compound in the air, if the adsorption is 100% effective, which is generally true for small amounts of adsorbed compounds. The activated charcoal in the sampler retains all, which is adsorbed and the diffusion sampler therefore functions as a integrating sampler, which means that all amounts are added on the charcoal.

b) The controller then visits the different rooms, in which he estimates the volume of the room space and adjusts the tracer gas injection equipment in such a way that he can deliver an amount of tracer gas, which is adjusted to the room volume in a pulse. After the tracer gas pulse, he slightly mixes the tracer gas into the room air, whisking with a piece of paper board. When he has finished all rooms this way, he presents a return envelope to the occupants together with instructions to seal the sampling tubes after approx. 20 hours, use the enclosed plastic caps, put the samplers in the envelope and post it to the laboratory.

The tracer gas may be of the perfluorocarbon type, which are non-toxic compounds, which have the property of not being adsorbed on commonly occurring indoor materials and that they can be analysed at extremely low concentrations. The total amount injected in a dwelling is less than a thousand of a gram.

c) The laboratory receives the labelled samplers and transfers the activated charcoal to a sampler flask, into which a millilitre of solvent is also added. The solvent extracts the adsorbed compounds from the charcoal. A small portion (a microlitre) of the solution is then injected, by means of an automatic device, into a gas chromatograph for separation and analysis of the tracer gas amount in the sample.

Gas chromatography is a separation technique, based on the fact that different compounds are transported with different speed through a capillary column which may have 0.2-0.5 mm diameter and 5-50 m length. A carrier gas, which is flowing through the capillary column carries the compounds injected into it. The adsorption of the compounds on the column wall delays the different compounds different times, so that the compounds leave the column on different times after injection. After the separation column there is a so-called electron capture detector, which has special sensitivity for the tracer gas (in this case fluorinated hydrocarbons). The output signal from the detector is depending on the amount of tracer gas passing, which allows the amount to be calculated from a calibration plot.

d) From the analysed amounts, the local mean ages of air can be calculated for each sample in the points at which the sampling were made, according to the equation stated earlier. Besides the analysed amount, it is necessary for the calculation to know the air sampling rate for the sampler (known from calibration) and the injected amount of tracer gas per volume unit in the measurement object (reported by the controller). Thereafter, the laboratory writes a report to the controller.

The term "zone" used above and in the claim may be equivalent to a "room" in premises or dwellings but must not be so.

Claims

1. A method to measure the ventilation in a zone-divided ventilated system, in which tracer gas is spread in the zones and integrating samplers for the tracer gas are used for measurement of the ventilation conditions, characterised in that in each zone an amount of tracer gas is spread, which is proportional to the volume of the zone, and that the integrating samplers are used for sampling the content of tracer gas in the air, from the moment at which tracer gas spreading begins in the system until essentially all tracer gas has been ventilated away from the ventilated system.

2. A method according to 1., characterised in that tracer gas is spread in the different zones during a time period, which is short, compared to the time, during which the integrating sampling is performed.

3. A method according to 1. or 2., characterised in that the sampling is performed using diffusion samplers (so called passive sampling).