BS EN 779:2002 pdf download
BS EN 779:2002 pdf download.Particulate air filters for general ventilation- Determination of the filtration performance.
7.3 Aerosol generation
7.3.1 DEHS Test Aerosol
The test aerosol described shall consist of untreatea or unaiiutea Uh Any otner aerosoi proven to give equiva. lent performance may be used. Test aerosol of DEHS (DiEthylHexylsebacate) produced by a Laskin nozzle Is widely used In performance testing of HEPA and ULPA filters.
Figure 4 gives an example of a system for generating the aerosol. It consists of a small container with DEHS liquid and a Laskin nozzle, The aerosol is generated by feeding compressed particle-free air Through the Laskii nozzle. The atomised droplets are then directly introduced into the test hg. The pressure and air flow to the nozzle are varied according to the test flow and the required aerosol concentration, For a test flow of 0,944 m% the pressure is about 17 kPa. corresponding to an air flow of about 0,39 dm3/s (1.4 m3lh) through the nozzle.
Any other generator capable 01 producing droplets in sufficient concentrations in the size range of 0,2 pm to 3.0 i,am can be used. One such generator is specified In the French standard NF X 44-060 and Consists of two pressunsed containers and a sonic atomlser fed by compressed a.
Before testing, regulate the upstream concentration to reach steady state and to have a concentration below the coincidence level of the particle counler
7.3.2 Neutralisation (conditioning) of aerosol
The test aerosol shall be brought to a Bo4tzmann electrostatic charge distribution by contact with a beta or gamma radiation generator with an activity of at least 185 MBq (5 mCI), or by a corona discharge ionizer. The corona disd’large ionizer shall have a minimum corona current of 3 pA and shall be balanced to provide equal amounts d positive and negative lons
7.4 Aerosol sampling system
Two rigid sample lines of equal length and equivalent geometry (bends and straight lengths) shall connect the upstream and downstream sampling heads to the particle counter. The sample tubes shall be electrically conducting or have a high selectric constant and have a smooth inside surface (steel, lygon etc).
Tapered sampling probes are placed in the centre of the upstream and downstream measuring sections. The sampling heads shall be centrally located with the inlet tip facing the inlet of the ñg parallel to the air flow. The sampling shall be isokinetic within 10 % at a test flow rate of 0,944 m3fs. lsokinetic sampling is also recommended at other test flows.
Three one-way valves make it possible to sample the aerosol upstream or downstream 01 the tiller under lest. or to have a blank” suction through a HEPA filler. These valves shall be of a straight-through design. Due to possible particle losses from the sampling system, the first measurement after a valve is switched should be ignored.
The flow rate can be maintained by the pump in the counter in the case of a Particle counter with a hgh flow rate (e.g. 0.47 dma/s) or by an auxiliary pump in the case of a counter with smaller sample flow rates. The exhaust line shall then be fitted with an isokinetic sampling nozzle directly connected to the particle counter to achieve isokinetic condltns within a tolerance of ± 10 %.
Particle losses will occur in the lest duct, aerosol transport Irnes and particle counter. Minimisation of particle losses is desirable because a smaller number of counted particles will mean Larger statistical errors and thus less accurate results. The influence of particle losses on the result is mirNmised it the upstream arid downstream sampling losses are made as near equal as possle.
7.5 Flow measurement
Flow measurement shall be made by standardised flow measuring devices in accordance with EN ISO 5167-1. Examples are orifice plates, nozzles, Venturi tubes, etc.
The uncertainty of measurement shall not exceed 5 % of the measured value at 95 % confidence level.
