Patent Publication Number: US-7724368-B2

Title: Condensation particle counter

Description:
RELATED APPLICATION 
   The present application is based on, and claims priority from, Korea Application Number 10-2007-0067518, filed Jul. 5, 2007, the disclosure of which is hereby incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to a condensation particle counter. Especially, the present invention relates to a condensation particle counter using water as a working fluid for efficiently measuring the number and size of fine particles. 
   BACKGROUND OF THE INVENTION 
   A condensation particle counter comprises a saturator, a condenser and an optical particle counter (OPC) and is widely used in measuring the number and size of fine particles. The saturator of the condensation particle counter contains working fluid to saturate an aerosol, i.e., a particle-suspended gas. Examples of the working fluid include alcohol-based fluid such as alcohol, butanol, isopropyl alcohol or the like and organic compound fluid such as ethylene glycol or the like. 
   The saturator is heated by a heater and kept at a temperature higher than an ambient temperature, as a result of which the working fluid contained in the saturator is evaporated. If the particle-suspended gas is introduced into the saturator, it is saturated by the working fluid and turned to a saturated gas which in turn is supplied to the condenser from the saturator. In case of using alcohol as the working fluid, the saturator is kept at a temperature of 35° C. and the condenser is kept at a temperature of 10° C. As the temperature of the condenser drops, the saturated gas is turned to a supersaturated gas. Condensation of the saturated gas occurs in such a fashion that liquid droplets grow bigger around fine particles as their nuclei. The liquid droplets thus grown are supplied to the optical particle counter. If the saturated gas is diffused faster than the heat transfer speed in the condenser, the saturated gas is not condensed around the fine particles but condensed only on the wall surface of the condenser. The optical particle counter is designed to count the number and size of the fine particles by detecting the liquid droplets. 
   There are provided many advantages if water is used as the working fluid of the condensation particle counter. This is because water is not harmful to the human body and does not generate any odor or pollutant. In case water is used as the working fluid in the conventional condensation particle counter, however, the water vapor is condensed only on the wall surface of the condenser that remains at a low temperature and there occurs no condensation that uses fine particles as nuclei. Therefore, the fine particles are discharged as they are. This poses a problem in that the fine particles cannot be detected by means of the optical particle counter. 
   For the reasons mentioned above, the conventional condensation particle counter makes use of an organic compound as the working fluid. The organic compound is detrimental to the human body and gives off a strong smell. In addition, the organic compound is highly difficult to handle because it is flammable. Particularly, if an alcohol-based organic material is used as the working fluid of the condensation particle counter in a semiconductor manufacturing process, the organic material acts as a pollutant and therefore becomes a cause of defect. Inasmuch as the organic compound tends to absorb moisture contained in a gas, the condensation particle counter suffers from degradation in performance if the organic compound is used for a long period of time. Thus, there is a need to periodically replace the organic compound, which task is onerous. 
   The conventional condensation particle counter has a fixed aerosol measuring capacity. Therefore, several condensation particle counters have to be used in combination in order to measure a large quantity of aerosol in one place, which is cumbersome and inconvenient. Furthermore, in case of using several condensation particle counters at one time, it is very difficult to uniformly control the temperature of the saturator and the condenser of the individual condensation particle counters. If a great deviation exists in the temperature of the saturator and the condenser, the data obtained by measuring the fine particles become less reliable. 
   SUMMARY OF THE INVENTION 
   In view of the above-noted and other problems inherent in the prior art, it is an object of the present invention to provide a condensation particle counter that uses water as working fluid by forming an inner surface of a condenser tube with a hydrophilic layer. 
   With these objects in view, the present invention provides a condensation particle counter, comprising: 
   a saturator generating a saturated gas by saturating a gas in which fine particles are suspended with working fluid; 
   a condenser connected to a downstream side of the saturator and condensing the saturated gas so that liquid droplets grow around the fine particles; and 
   an optical particle counter connected to downstream sides of the condenser and optically detecting the liquid droplets supplied from the condenser; 
   wherein the condenser has a condenser tube interconnecting the saturator and the optical particle counter, the condenser tube provided with a hydrophilic tube installed inside surface of the condenser tube. The hydrophilic tube is made of a titanium oxide (TiO 2 ). The working fluid includes water. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of a preferred embodiment, given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a section view showing a configuration of a condensation particle counter in accordance with the present invention; 
       FIG. 2  is a sectional view illustrating a configuration of a condensation particle counter in which a hydrophilic tube is installed inside surface of the condensation tube according to the present invention; 
       FIG. 3  is an enlarged view illustrating a state that a liquid droplet is formed around a fine particle as a nucleus within a condenser tube of the present condensation particle counter; 
       FIG. 4  is a sectional view illustrating a configuration of a condensation particle counter in which a porous tube is installed inside surface of the condensation tube according to one example of the hydrophilic tube; 
       FIG. 5  is a graph plotting a performance test result of the present condensation particle counter against that of a conventional condensation particle counter which makes use of butanol as working fluid; 
       FIGS. 6 and 7  are graphs plotting performance test results of the present condensation particle counter against those of a conventional condensation particle counter which makes use of water as working fluid; 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of a condensation particle counter in accordance with the present invention will now be described in detail with reference to the accompanying drawings. 
   Referring first to  FIG. 1 , a condensation particle counter of the present invention includes a saturator  10  for generating a saturated gas by allowing working fluid to saturate a gas in which fine particles P is suspended, a condenser  20  for condensing the saturated gas and an optical particle counter  30  for optically detecting the fine particles P to calculate the number and size of the fine particles P. 
   The saturator  10  is provided with a pool  11  that contains working fluid W. The pool  11  is connected to an inlet port  12  of the saturator  10  through which an aerosol, i.e., a gas in which fine particles P are suspended, is introduced. A saturator tube  13  for guiding the stream of the saturated gas is connected to one side of the pool  11 . The saturator tube  13  has an outlet port  14  through which the saturated gas is discharged. The level of the working fluid W is kept lower than the inlet port  12  and the outlet port  14  so that the gas stream can be guided across the level surface of the working fluid W. A heater  15  is attached to the outside of the saturator  10 . The heater  15  serves to heat and evaporate the working fluid W contained in the pool  11 . Attached to an inner surface of the saturator tube  13  is an absorption member  16  that absorbs the working fluid W contained in the pool  11  to thereby accelerate evaporation of the working fluid W. Water, alcohol-based fluid or the like may be used as the working fluid W. Use of water is preferred. 
   Referring to  FIGS. 1 to 3 , the condenser  20  is connected to the downstream side of the saturator  10 . The condenser  20  serves to condense the saturated gas so that liquid droplets D can be formed around nuclei, i.e., the fine particles P supplied from the saturator  10 . The condenser  20  includes a condenser tube  21  connected to the outlet port  14  of the saturator  10 . Mounted to an outer surface of the condenser tube  21  is a thermoelectric cooler  22  that serves as a cooling means for reducing the temperature of the condenser tube  21 . Alternatively, the cooling means may be formed of a cooling device that includes a cooling chamber surrounding the outer surface of the condenser tube  21  and a refrigerating cycle for reducing the temperature of the condenser tube  21  by supplying coolant into the cooling chamber. 
   A hydrophilic tube  25  that allows water to be used as the working fluid W is formed on an inner surface of the condenser tube  21 . The hydrophilic tube  25  can be made of a titanium oxide (TiO 2 ) or a hydrophilic polymer. Alternatively, the hydrophilic tube  25  can be formed by coating a hydrophilic material, e.g., titanium oxide (TiO 2 ), on the inner surface of the condenser tube  21  or by using a plasma surface modification. With the plasma surface modification, a titanium oxide layer is formed on the inner surface of the condenser tube  21  by use of plasma generated in a well-known plasma surface modification apparatus. 
   Referring to the  FIG. 4 , the hydrophilic tube  25  is formed by a porous tube having a plurality of air hole  25   a . The porous tube is made of the titanium oxide (TiO 2 ), the hydrophilic polymer, or Zeolite. 
   As can be seen in  FIG. 1 , the optical particle counter  30  is connected to the downstream side of the respective condenser  20 . The optical particle counter  30  serves to calculate the number and size of the fine particles P by optically detecting the liquid droplets D supplied from the condenser tube  21 . The optical particle counter  30  includes a housing  31 , a light source  32 , a first lens array  33 , a second lens array  34 , a photo detector  35  and a computer  36 . 
   The housing  31  has an inlet port  31   b , an outlet port  31   c  and a sensing volume  31   a  arranged between the inlet port  31   b  and outlet port  31   c . The condenser tube  21  is connected to the inlet port  31   b  of the housing  31 . The light source  32  is mounted to one side of the housing  31  and is designed to emit light which in turn is irradiated into the sensing volume  31   a  of the housing  31  through the first lens array  33 . The light irradiated into the sensing volume  31   a  of the housing  31  is collected by means of the second lens array  34  attached to the other side of the housing  31 . The light collected by the second lens array  34  is detected by means of the photo detector  35 . 
   The photo detector  35  is designed to input optical signals to the computer  36 . The computer  36  calculates the number and size of the fine particles P by processing the optical signals inputted from the photo detector  35  with a pre-stored program. The photo detector  35  is formed of an image sensor for acquiring positional data of the liquid droplets D, e.g., a charge coupled device (CCD) camera or a quadrature detector. The computer  36  may include a signal processor that calculates the number and size of the fine particles P by processing the optical signals inputted from the photo detector  35 . A flowmeter  40  for measuring the flow rate of the gas and an air pump  41  for drawing the gas are serially connected to the outlet port  31   c  of the housing  31 . 
   Description will now be made regarding an operation of the present condensation particle counter configured as above. 
   Referring again to  FIGS. 1 and 2 , water as the working fluid W is contained in the pool  11  of the saturator  10 . If the temperature of the saturator  10  is increased to about 60° C. to 70° C. by the operation of the heater  15 , the water is evaporated to generate water vapor. If the air pump  41  is operated to exert a (inhaling) vacuum force, the aerosol, i.e., the gas in which the fine particles P are suspended, is introduced into the pool  11  through the inlet port  12  of the saturator  10 . The gas is saturated into a saturated gas by the water vapor and then discharged through the outlet port  14 . 
   The condenser tube  21  of the condenser  20  connected to the outlet port  14  of the saturator  10  is kept at a temperature lower than the temperature of the saturator  10  by means of the thermoelectric cooler  22 . The condenser tube  21  is maintained at a temperature of about 20° C., which is about 10° C. higher than the temperature available in the conventional condensation particle counter that uses alcohol-based fluid as the working fluid. Thus, the water vapor is condensed around the fine particles P suspended in the gas, thereby generating liquid droplets D, i.e., water droplets. At this time, the saturator  10  is kept at a temperature of about 60° C. to 70° C. and the condenser tube  21  is maintained at a temperature of about 20° C. This helps optimize the generation of the liquid droplets D while assuring increased energy efficiency. 
   The liquid droplets D generated on the inner surface of the condenser tube  21  are readily moved down along the surface of the hydrophilic inner surface under the gravity force. In a case that the inner surface of the condenser tube  21  is hydrophobic, the liquid droplets D would be unable to move down along but adhere to the inner surface of the condenser tube  21 . The liquid droplets D adhering to the inner surface of the condenser tube  21  hinders heat transfer, consequently making uneven the temperature distribution within the condenser tube  21 . Therefore, the degree of super saturation within the condenser tube  21  becomes very uneven, which obstructs growth of the liquid droplets D around the fine particles P as their nuclei. 
   For the case that a hydrophilic tube  25  is installed inside surface of the condenser tube  21 , the liquid droplets D are generated on the inner surface the hydrophilic tube  25  because the hydrophilic tube  25  is contact with the inside surface of the condenser tube  21 . Therefore, the liquid droplets D are readily moved down along the surface of the hydrophilic tube  25  under the gravity force. Especially, for the case that a porous tube is installed inside surface of the condenser tube  21  as shown in  FIG. 5 , the liquid droplets D are generated on the inner surface the porous tube because the porous tube is contact with the inside surface of the condenser tube  21 . The plurality of air hole  25   a  of the porous tube absorbs the liquid droplets D. 
   Referring again to  FIGS. 1 and 3 , the liquid droplets D is introduced into the sensing volume  31   a  of the optical particle counter  30  through the condenser tube  21  and then discharged to the outside of the housing  31  from the sensing volume  31   a  through the outlet port  31   c . The light source  32  irradiates light into the sensing volume  31   a  via the first lens array  33 . The light thus irradiated is scattered by the liquid droplets D flowing through the sensing volume  31   a . The scattered light is sent to the photo detector  35  via the second lens array  34 . Upon detecting the light, the photo detector  35  generates optical signals. The computer  36  calculates the number and size of the fine particles P by processing the optical signals inputted from the photo detector  35  with a pre-stored program. The number and size of the fine particles P thus calculated is displayed on a display device such as a monitor or the like. The fine particles P and the liquid droplets D discharged to the outside through the outlet port  31   c  of the housing  31  are removed by means of a filter. 
   Performance tests were conducted for the present condensation particle counter and the conventional condensation particle counter.  FIG. 5  is a graph plotting a performance test result of the present condensation particle counter against that of the conventional condensation particle counter which makes use of butanol as working fluid. The present condensation particle counter differs from the conventional condensation particle counter in that a hydrophilic tube including a titanium oxide is formed on the inner surface of the condenser tube. Water was used as the working fluid in the present condensation particle counter, while butanol was used as the working fluid in the conventional condensation particle counter. It can be seen in  FIG. 5  that the number of the fine particles measured by the present condensation particle counter is substantially equal to that measured by the conventional condensation particle counter until the concentration of the fine particles having particle diameters (D p ) of 20 nm, 40 nm and 60 nm reaches 10,000 pieces/cm 3 . It can also be appreciated that the numbers of the fine particles measured by the present condensation particle counter and the conventional condensation particle counter nearly coincide with a theoretical line. 
   Performance tests were conducted for the present condensation particle counter and the conventional condensation particle counter, both of which make use of water as working fluid.  FIGS. 6 and 7  are graphs plotting the performance test results of the present condensation particle counter against those of a conventional condensation particle counter. The graph shown in  FIG. 6  illustrates the result of test for the fine particles having particle diameters (D p ) of 20 nm, 40 nm and 60 nm, which test was conducted at an aerosol flow rate of 1.01 pm (liter per minute). The graph shown in  FIG. 7  illustrates the result of test conducted at different aerosol flow rates of 1.01 pm, 0.81 pm, 0.51 pm and 0.31 pm. It can be seen in  FIGS. 6 and 7  that the particle counting efficiency achieved by the conventional condensation particle counter is as low as about 10% with respect to a theoretical line obtained by the present condensation particle counter. 
   As described hereinabove, the present condensation particle counter can use water as working fluid and also can optically measure fine particles in an easy and accurate manner by forming or installing an inner surface of a condenser tube with a hydrophilic tube. 
   The embodiment set forth hereinabove have been presented for the illustrative purpose only and, therefore, the present invention is not limited to the foregoing embodiment. It will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention defined in the claims.