Principle and application of aspirating smoke detectors (ASDs) with an innovative method for very early fire alarms – Part 1

From the earliest laser to particle-counting methods, air-sampling aspirating smoke detectors (ASDs) for very early fire warning has made an essential contribution to property protection. Part One of the article introduces the particle-charging method’s principles and evaluates the charging performance using a sample prototype using the particle-charging method.

Smoke detectors based on air-sampling technology have realised precise detection and very early warning for fire hazards. At the smouldering phase of the fire, a large number of microparticles （below 150 nanometres in diameter） are released into the surrounding environment. The size of released particles gradually increases as they bond together over time to form macroparticles with diameters over 150 nanometres. Meanwhile, very few materials release macroparticles with very low concentrations at the smouldering phase of the fire.

There are two fundamental detection methods for very early smoke detection: laser scattering and particle-counting methods. Laser scattering methods measure the intensity of light reflection or deflection of the aerosol particles. The particle-counting method is based on cloud-chamber technology. Charged aerosol particles are enlarged to form liquid droplets in the cloud chamber when interacting with the supersaturated water vapour. This process increases the size of the aerosol particles making them visible to the optical detection system. 

ASDs based on the particle-charging method collect and analyse the charges on the surface of aerosol particles. The total number of charges, depending on the sum of the particle surfaces area, is the critical sensitivity criteria for detection.

Due to the limitation of the optical system, the sensitivity of the laser scattering method only covers particles with diameters larger than 150nm. Particle-counting ASDs have limited sensitivity to the smouldering of particular materials that release particles at low concentrations but with large sizes.

With a unique charging mechanism, the particle-charging method allows aerosol particles with different particle concentrations and diameters to be detected effectively.

The particle-charging method delivers the sampling air to a particle charger to conduct the unipolar charging process. The charged particles are then subjected to the charge collection and neutralisation process in the collector. During the charge collection process, the charged particles quickly migrate toward the receiving electrodes under the impact of a biasing voltage. The current signal generated by the neutralisation process is regulated to a voltage signal as the sensitivity measurement for the particle charging method.

Principles of the particle-charging method

Particle charger

One of the commonly used principles for particle charging is corona discharge. There are two fundamental discharge techniques: negative corona discharge and positive corona discharge. Positive corona discharge has advantages over negative corona discharge, as it produces a much higher concentration of positive ions at the outer region of the discharging field. Figure 1 demonstrates a general geometric configuration of the positive corona charger. Generally, the aerosol particles’ characteristics, including particle surface area, equivalent electromobility diameter and concentration, could impact the charging performance of the positive corona discharge, as well as the charging mechanisms that are applied to the aerosol particles. Ion deposition on the particle surface by electric field is the field charging. Ion attachment due to thermal impact is called diffusion charging.¹

The former mechanisms target particles below 100nm. The amount of charge on the particle can be calculated by the following equation (1):^2,3

where 

ε₀ = the permittivity of vacuum

E₀ = the electric field strength

ε = the particle relative permittivity

d_p = the particle diameter (m)

The latter mechanisms target particles above 1,000nm. The number of electrical charges can be calculated by equation (2): ^2,4

where 

K = the Boltzmann constant

T = the Kelvin temperature

N₀ = the concentration of ions 

U_max = the maximum velocity of oxygen ions in an electric field 

t = the particle residence time (s).

For particle sizes between 100nm and 1,000nm, both mechanisms affect the final charging performance in varying degrees.⁵ Table 1 shows the calculation of charges per particle combining Equations (1) and (2). The conclusion can be drawn as the charge quantity of the two charging mechanisms increases with particle diameter. 

Charge collector

After being charged, the aerosols are delivered to the collector for charge quantity analysis. The collector consists of a collecting chamber and two electrodes whose primary function is to collect and neutralise the charged particles. Figure 2 illustrates the cross-section geometric of the charge collector. Charged particles in the collecting chamber migrate to different locations of the receiving electrodes depending on their electric mobility in the electric field. A current signal relating to the number of neutralised ion particles is generated. Microparticles fall on the receiving electrode A. While macroparticles migrate to the receiving electrode B. A series of amplification circuits are connected to the receiving electrodes for signal regulation and amplification. This dual-electrode configuration categorises charged particles and analyses particle distributions in terms of their equivalent electromobility diameter. 

The following equation can calculate the theoretical voltage output TH∆V:

where 

I_A, I_B = the current signal generated at electrode A and B respectively;

R = the pull-down resistor, 5 x 109 (ohm);

β₁, β₂ = the amplification coefficient of stage one and two amplifiers, which are 100 and 10, respectively;

Q = the airflow rate in the collector 47.16 (cm³/s), 2.83 L/mins;

dp = the particle equivalent electromobility diameter (nm);

C_dp = the particle concentration at inlet of the collector (number/cm³);

N_e = the number of charges carried by the aerosol particle with diameter dp nm; and

N_c = the number of electrons per ampere-current, 6.24146 x 10¹⁸.

Leaving the particle size dp unchanged, we can find the relationship between the voltage signal TH∆V and the particle concentration Cdp according to equation (3), in which Cdp is acquired by NanoScan SMPS Nanoparticle Sizer 3910 (TSI SMPS 3910).

Performance evaluation using standard PSL monodisperse nanoaerosols

Test apparatus

To evaluate the actual charging performance of the particle-charging method, a sensitivity evaluation was introduced to the evaluation process using the standard PSL nanoaerosols test method, where nanoaerosols with diameters ranging from 20nm to 250nm are delivered into the sample prototype. For nanoaerosol generation with particle diameters below 100nm, a TSI 3480 Electrospray Aerosol Generator is used. A commercially available aerosol generator is used for nanoaerosol generation with particle diameters above 100nm. One of the crucial factors that could influence the evaluation result is the airflow rate; therefore, it must be maintained at a constant 2.83L/mins for all subsequent tests.

Test results

The results of the sensitivity evaluation are shown in Figure 3 and Figure 4, where the aerosol particle concentration measurement of TSI SMPS 3910 is added as a comparison.

Figures 3a and 3b illustrate the sensitivity evaluation results of 20nm and 100nm PSL nanoaerosols. On the one hand, the voltage signal of receiving electrode B, indicating macroparticle characteristics, is dominated by white noise. On the other hand, spikes can be observed from receiving electrode A, providing particle concentration of microparticles. The declining trend of both electrodes A and B match the TSI particle concentration curve. This suggests that the sample prototype based on the particle-charging method can detect 20nm nanoparticles effectively.

Figures 4a to 4b show the sensitivity test results for 150nm and 250nm PSL nanoaerosols. The maximum voltage signal on both receiving electrodes A and B increases with increasing particle size. Furthermore, the decreasing tendency of both receiving electrodes A and B matches the particle concentration measurement of TSI 3910. In conclusion, we can see that the particle-charging method delivers high detection performance for nanoaerosols down to 20nm and is also very sensitive to variations of particle concentration.

Charging efficiency

Charging efficiency is the leading performance evaluation criterion for any particle-charging device. The charging efficiency refers to the fraction of the actual charged particle concentration ∆V with respect to the total concentration TH∆V of the particles.

By combining TH∆V and ∆V, we can determine the charging efficiency η:

Table 2 shows that the particle-charging method has been subjected to five different sensitivity evaluations in terms of particle size. The average charging efficiency remains at 50% to 60% with increasing particle diameter.

Performance evaluation

The diameter of monodisperse nanoaerosol covers most of the combustion particles released at the very early stage of the fire. The result of the sensitivity evaluation shows good agreement in general trend of TSI measurement. A clear decline in both electrode voltage readings can be observed for all cases, suggesting that the particle-charging method can effectively detect microparticles down to 20nm. Furthermore, the charging efficiency for macroparticles is maintained at a relatively high level.

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References

1. P. Intra and N. J. J. o. E. Tippayawong, ‘Progress in unipolar corona discharger designs for airborne particle charging: A literature review’, vol. 67, no. 4, pp. 605-615, 2009.
2. W. C. Hinds, Aerosol technology: properties, behavior, and measurement of airborne particles. John Wiley & Sons, 1999.
3. P. T. Sardari, H. Rahimzadeh, G. Ahmadi, and D. J. J. o. A. S. Giddings, ‘Nano-particle deposition in the presence of electric field’, vol. 126, pp. 169-179, 2018.
4. Y. Y. Cao et al., ‘Design and performance evaluation of a small unipolar aerosol charger system’, (in English), IOP Conference Series. Earth and Environmental Science, vol. 69, no. 1, Jun 2017 2017.
5. D. Park, M. An, and J. J. J. o. A. S. Hwang, ‘Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution’, vol. 38, no. 4, pp. 420-430, 2007.