270NH3.MAN.REVD.062019
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Description and Principle of Operation
APPLICATION
The Ammonia (NH3) Converter is typically bolted to the side of, and plumbed to, a 270 series probe. The
assembly is designed to sample gas in stacks where ammonia is added and must be analyzed. There is an
unconverted sample output located immediately after the filter on the probe. A second sample connection is
made after the converter. NO is analyzed from samples at both locations, and the difference in concentration
is a measure of the amount of ammonia that has been converted.
All other standard model 270 probe options may be ordered along with the ammonia converter, including high
pressure blowback functions, calibration gas injection, and all standard flange sizes. Refer to the specific
model 270 manuals for more information regarding the probe specifications.
GENERAL DESCRIPTION, HEATED FILTER SECTION
The Heated Stack Filter Section of the NH3 Conversion Assembly consists of the 270 heated filter mounted
on the left side of the converter and contains a blowback solenoid valve to be activated by external means. An
acid blocking component is provided to protect the valve from acid in the sample. The solenoid valve can be
provided with any of the available solenoid coils to allow the external control system to provide the opening
coil power which is most convenient.
The NEMA 4X enclosure is fastened directly to the mounting flange which is sized to match the mating
sample flange on the stack. There is a heat shrink boot on the bottom of the enclosure to accept a heated
sample line. When the heated sample line is properly supported, the flange mounting of the enclosure is
sufficient to support the entire assembly.
A 125 watt Heater Assembly is mounted on the outside of the filter cavity. The heater holds the temperature
of the filter at 340°F. A bi-metallic thermal switch fused to the heater keeps the temperature from exceeding
340°F (171°C) by opening the circuit at that temperature. An optional independent thermocouple/RTD can be
provided as a means to measure and transmit or record the temperature of the filter. A second thermal switch,
set at 225°F (105°C.) is provided to be used as an alarm contact if the temperature drops below the switch
temperature.
Several types of filter elements can be supplied with the Model 270. The 2 μm ceramic filter is supplied as an
economical general purpose filter. A similar ceramic filter with an internal 0.1 μm coating is available for finer
filtration or to provide a surface to enhance the blowback capability where the fines have a tendency to fill the
pores of the 2 μm element. A 2μm, 316SS filter can be inserted for those applications where the ceramic filter
is determined to be unsuitable. Additional filtration materials and pore sizes are available on request.
The blowback accumulator is a 7” diameter stainless steel sphere. It is designed to be pressurized to a
maximum pressure of 125 psig compressed air. At that pressure there is a 1/2 cubic foot of air stored to flush
the particulate matter out of the filter and through the probe and into the stack.
Compressed air is used to clean the filter element. Compressed air supplied to the blowback assembly needs
to be clean and dry. Instrument quality air is preferred. The pressure should be as high as possible, up to 125
psig. High pressure air fills the accumulator and provides a substantial blast when the solenoid valve opens.
This loosens the particles on the filter surface and forces them back through the sample probe into the stack.
The period of time between blowback cycles should be commanded to occur before the pressure drop across
the filter begins to increase. This should be as often as every 15 minutes but no less frequently than once per
day. The time period between blowback cycles can be based on a calculation to estimate the amount of
sample required to deposit from 3 to 5 grams of solid in the filter element.
Instrument air usage is minimal and smoothed by the fact that the air accumulator is charged over a period of
time through a 1/4” instrument air line. The recharge time could be extended with a restriction in the air line, if
it were desired to reduce the pressure pulses on the instrument air supply and to consume instrument air
more slowly.
