Dilo SF6 - Page 57

Dry filter revision | 3-899-R…
www.dilo-gmbh.com | C 1978-03 | Page 3 / 8
1. Description
The dry filter serves for adsorbing moisture and decomposition products out of SF
6
-gas.
An exchangeable filter cartridge is installed in the dry filter.
The filter cartridge consists of a tube filled with approx. 2.3 kg (5 lbs) desiccative. At the input side there
is the aluminium oxide, at the output side the molecular sieve.
1.7 kg (3.8 lbs) aluminium oxide Al
2
O
3
pore size 20-50 Angström, granular size 2 - 5 mm
0.6 kg (1.3 lbs) molecular sieve pore size 4 Angström, granular size 4 mm (0.08 to 0.2 in)
This filter mixture proved to be the best for the adsorption of moisture and decomposition products.
With the filter cartridge a dew point temperature of below -50 °C (-58 °F) can be obtained within one
drying process.
The throughput is one of the decisive factors for the optimum drying effect.
As the filtering is based on the adsorption the adsorbability for humidity and decomposition products
depends on the concentration of these substances on the input, on the contact time with the filter as well
as on the degree of saturation of the filter.
The throughput of a filter unit is raised by using several filter units in parallel, however not in series
because the filter material needs a minimum contact time with the gas, in order to enable an adsorption.
The desiccative can absorb approx. 120 g (4.2 ounzes) water without the residual moisture of the gas
behind the filter exceeding 10 ppm
m
(= -44 °C dew point temperature at p
a
= 1 bar).
(= -77.2 °F dew point temperature at atmosphere)
SF
6
gas molecules can decompose to fluoride and sulphur-fluoride compounds because of discharges or
high temperatures. Normally these substances recombine to SF
6
again. However, if water and other
substances exist at the same time the recombination is not complete and gaseous residues, the
decomposition products, are left behind. If metals exist also metallic fluorides can arise in form of dust.
The following gaseous decomposition products appear most frequently beside others in used SF
6
-gas:
HF hydrogen fluoride
SO
2
sulphur dioxide
SOF
2
thionyl fluoride
SO
2
F
2
sulfuryl fluoride
SF
4
sulfur tetrafluoride
With the dry filter 3-899 these gaseous decomposition products can be adsorbed.