Fig.3 shows the experimental setup for investigating the collection efficiency of the ion collector. It consisted of an ionizer, a DC voltage power supply, a Faraday cup, an electrometer, a high efficiency particulate-free air (HEPA)filter, and a flow system. A high concentration of ions was generated by corona discharge with the corona-needle ionizer developed by Intra and Tippayawong [14,15] as shown in Fig.4. The ion concentration at the ionizer outlet was measured by the Faraday cup electrometer. Fig.5 shows the number concentration of positive and negative ions as a function of the corona voltage at the ionizer outlet, usually greater than 10 13 ions/m 3
, at operating flow rate between 1 and 5 L/min and the corona voltage between 2.3 and
3.4 kV[15]. As shown in Fig. 5, the ion concentration at the ionizer outlet was found to be relatively high with the same corona voltage at higher airflow rates, because the free ions can be more easily drawn off the ionizer by faster flowing air[14e16].