This invention relates to a compact, high efficiency, and reliable condensation nucleus counter for counting aerosol particles and a method of using such counter. Specifically, the invention relates to the measurement of particles suspended in a gas, which is referred to as an aerosol. The most common gas is air, but other gases may also be the media for particle suspension. The particles can be solids, liquids, or a mixture of both. In all cases, a gas containing suspended particles is referred to as an aerosol, with no limitation being made as to the chemical nature of the particles and that of the gas, and their respective physical states.
Aerosols consisting of small particles suspended in air or other gases are widely encountered in nature and in the human environment. A widely used method for counting aerosol particles is the condensation nucleus counter (CNC), which is also referred to as a condensation particle counter. In a typical CNC, the aerosol first passes through a saturator to heat the gas and saturate the gas with the vapor of a working fluid. The gas is then cooled in a condenser to produce supersaturation. The supersaturated vapor then condenses on the particles to form droplets, which are counted by a light-scattering droplet counter.
A prior art CNC is schematically depicted in FIG. 1. The saturator comprises a porous plastic block placed in a heated liquid reservoir containing the working fluid in liquid form. A gas-flow passageway in the porous plastic allows the aerosol to flow through and be heated and saturated with the working fluid vapor. The condenser comprises a tubular passageway in a metal block kept at a low temperature. As the gas flows through the condenser passageway, it cools by transferring heat from the flowing gas stream into the cold passageway walls, thereby reducing the gas temperature and causing the gas to become supersaturated. The supersaturated vapor then condenses on the particles carried in the gas to form droplets. The aerosol flows into a conventional optical particle counter which then provides a droplet count, and hence indirectly the particle-count.
The condenser block is kept at a low temperature by a thermoelectric cooler. The heat rejected by the thermoelectric cooler is partly used to heat the liquid reservoir to the desired temperature and to heat the optics block of the optical particle counter to prevent vapor condensation therein, with the remaining heat being dissipated to the ambient air via a heat sink.
The most commonly used working fluid in a CNC is butyl alcohol. The saturator is usually heated to about 35xc2x0 C., and the condenser is usually cooled to about 5xc2x0 C. The prior art CNC of FIG. 1 is capable of detecting particles as small as 8 nanometers (nm) in diameter. With special designs, particles as small as 3 nm can be detected by the CNC.
In the prior art CNC depicted in FIG. 1, the heated aerosol flow passageway in the saturator block has a large rectangular cross-section to reduce the gas-flow velocity; thereby increasing the gas residence time for heating and saturating the gas with vapor. For the same reason, several tubular passageways are provided in the condenser block to reduce the gas flow velocity in each passageway and increase the residence time of the gas needed for cooling and vapor condensation on particles to form droplets. A CNC with these design features is described in U.S. Pat. No. 4,790,650.
The airflow velocity used in the prior art CNC is on the order of a few centimeters per second. The typical residence time is on the order of a second. The low airflow velocity causes the gas flow in the saturator and condenser passageways to be mostly laminar, or streamlined, in nature.
In laminar, or streamlined flow, gas passing through the condenser at different radial distances from the center of the passageway will move at different gas velocities. At the same time, heat and mass transfer by molecular diffusion across the gas flow will cause a temperature and vapor concentration gradient to develop, with the lowest gas temperature and vapor concentration being at the tube walls, and the highest, at the passageway centerline.
As particles at different radial distances flow through the condenser, the particles experience different temperature and vapor supersaturation conditions depending on the radial position of the particles. In general, particles passing near the center of the passageway would experience the highest supersaturation while particles passing through near the passageway walls would encounter lower supersaturation because of vapor depletion and direct vapor condensation on the cold passageway walls.
For this reason, to count very small particles, it is necessary to confine the gas flow containing particles to be detected to within a narrow region near the center of the tubular passageway. This is usually done by introducing the aerosol into the condenser passageway through a small hypodermic needle along the axis of the passageway. The commercially available ultrafine CNC for detecting particles down to 3 nm is based on this principle as described in xe2x80x9cAn Ultrafine Aerosol Condensation Nucleus Counterxe2x80x9d, M. B. Stolzenberg and P. H. McMurry, Aerosol Science and Technology, Vol. 14, pp. 48-65, 1991.
In the ultrafine CNC, the aerosol flow through the hypodermic needle is typically 10% of the total gas flow through the condenser. Therefore, the effective aerosol flow rate of the ultrafine CNC is reduced by a factor of 10 from that of a conventional CNC designed to detect larger particles. Because the ultrafine CNC needs two airflow streams in the condenser, the device is complicated.
Another issue related to the operation of a conventional CNC is that under high humidity conditions, gas flowing through the condenser may cool below the gas dew point to cause the moisture in the gas to condense. The condensed water then flows down the passageway walls to the porous saturator block, along with the condensed vapor of the working fluid.
When condensed water reaches the porous plastic in the saturator, it tends to accumulate in the saturator pores, thereby displacing the organic working fluid, from the porous material. Over time, sufficient water may accumulate in the porous material to cause the performance of the CNC to degrade, leading to improper functioning of the device, and giving rise to faulty and unreliable data. A method to separate the condensed water from the working fluid has been described in U.S. Pat. No. 5,118,959.
The present invention relates to improvements in handling an aerosol in a CNC to improve performances. The saturator has a porous metal insert forming the gas passageway with controlled pore size so that the saturator can be placed in any orientation, other than the traditional horizontal, or near horizontal position.
An appropriate pore size is selected so that a higher pressure difference can be sustained across the porous material without the liquid being blown out from the pores.
The flow passageways are designed to cause turbulent eddies to increase the rate of heat and vapor transfer in the saturator and condenser to make the device smaller, with improved overall performance.
The working fluid is kept in one reservoir, while the condensate (working fluid and water) is collected in a separate reservoir in order to eliminate problems associated with condensed water permeating through the porous material of the saturator.
Also, the present invention comprises a multi-channel CNC that would allow several sample streams to be counted simultaneously by the CNC.