A schematic illustration of the continuous flow condensation particle counter UF-02. 

Context in source publication

Context 1

... p d is the saturation vapour pressure on the droplet surface, p s is the saturation vapour pressure, δ is the activity coefficient, m f is the mole fraction of the solute, γ is the surface tension, v is the molar volume of the liquid, R is the gas constant, and T is the abso- lute gas temperature in K [20]. Equation is derived for vapour condensed on liquid droplets of the same mate- rial or on insoluble particles with surface wettable for the working fluid. There are two basic types of existing CNCs: mixing and cooling ones. The cooling type CNCs reach supersaturation by cooling a previously saturated particle- containing sample stream. For a mixing type CNCs, supersaturation is determined by the difference of the incoming sample flow and the growth chamber temperatures. While some particles immediately exit the mixing region and enter the detector, other particles continue to circulate inside the CPC and randomly exit at some later time, leading to an exponentially decaying distri- bution of delay times between the particle entrance into CPC and its detection. Thus, application of such type of CPCs is impractical for obtaining fine particle size distributions. A newly developed condensation particle counter UF-02 [14] was approbated and tested. The scheme of the CPC model UF-02 is shown in Fig. 1. Inside a saturator block, working fluid ( n -butanol) is evapo- rated from a liquid pool to the passing air flow. The liquid level within the saturator is maintained with a peri- staltic pump, which transfers n -butanol from an outside reservoir on demand. When the counter is switched off, the pump removes all the butanol from the pool to the reservoir. This protects the optics from flooding with working fluid, e. g., during the relocation of the UF-02 counter. The high carrier inlet flow of the CPC is 1929 cm 3 / min. From the carrier flow, the aerosol flow (274 cm 3 / min) is extracted with the capillary tube. Furthermore, the aerosol flow is divided into two flows, one (sheath) flow passing through a high performance filter and a saturator block (242 cm 3 / min) and the other (aerosol sample flow) directly to a growth tube (32 cm 3 / min). The sheath flow is drawn through a heater where it becomes saturated with vapour. The working liquid is at the bottom of the cylindrical saturator. A liquid level sensor controls the amount of the liquid inside the saturator. The top part of the saturator is connected with the condenser through a special plastic tube. The bottom part of the condenser is cylindrical and the top part is conical. Both parts are cooled (10.45 ± 0.08 ◦ C) thermoelectrically. The aerosol sample flow enters the cleaned vapour-sheath flow near the inlet of the condenser at its centreline and the saturated sheath air rotates around the aerosol flow. A short, heated section at this juncture allows vapour to diffuse into the aerosol before entering the cooled condenser. As the aerosol sample flow is confined near the centreline of the condenser, it experiences high supersaturation and negligible losses to the wall. For better mixing of the cold aerosol flow and the hot saturated air the turbine is used. It is between the plastic tube and the bottom part of the condenser. The turbine is a slender disk with special formed wings. Thus, the mixing of hot and cold flows and cooling of the condenser create high supersaturation and the particles grow into droplets, which can be detected by optical methods. The exit nozzle of the top part of the condenser causes the droplets to accelerate and pass through a measurement volume in the optical system. The particle number concentration is calculated using the lifetime counting method. Internal optics focuses the laser light to a thin ribbon just above the aerosol focusing nozzle. Droplets are counted individually with the concentrations up to 10 5 particles per cubic centimetre as they scatter light onto a photomultiplier. The continuous flow is cleaned by a filter (the fall of pressure is low) and controlled volumetrically. Particles are carried upward the condenser to the optics through the saturated air swirl created by the turbine. The saturated air stream through the turbine con- tacts tangentially with the inner surface of the chamber. Thus, a weak vortex is produced in the space above the turbine. The swirl velocity increases towards the centre due to conservation of the flow angular momentum cre- ating uniform supersaturation in the central part of the chamber. This inward motion of the swirl reduces par- ticle losses to the inner surfaces of the supersaturation chamber and allow reaching supersaturation very close to the homogeneous nucleation limit. Experimental set-up was made to determine the par- ticle counting efficiency and the concentration range of the condensation particle counter UF-02 (Fig. 2). For this purpose, silver particles were generated with the Carbalite Furnaces generator. The particles were charged ( 241 Am source) and size-segregated with a dif- ferential mobility analyzer (DMA). The sample was di- luted in both the counting efficiency and concentration range experiments. A diluted monodisperse aerosol sample was transferred to the CPC UF-02 and a ref- erence condensation particle counter TSI 3025A. The minimum detectable concentration is deter- mined by false background counts in the optics. For these investigations, a HEPA filter was connected with the inlet of the CPC UF-02. The measurements of concentrations were made during one hour. The aver- age number concentration during the measurement pe- riod was calculated. The results show that the false particle number concentration background was only 0.002 cm − 3 , when the temperature difference between the saturator and the condenser was set at 32.5 ◦ C. The existence of this background can be explained by the particle production due to homogeneous or ion-induced nucleation inside the CPC or by the noise of the elec- tronics. However, as recently shown by Kulmala [21], homogeneous nucleation under these conditions is im- probable. The maximum measured number concentra- tion was investigated by comparing the number concentration measured with the UF-02 to that of the ref- erence CPC TSI 3025A. The experiments were performed using monodisperse aerosol with a ...