MAN.270SF.REVB.102018
Page 8 of
18
5.0
Description and Principle of Operation
APPLICATION
The Universal Analyzers Model 270SF Extractive Gas Sample Probe Assembly is designed to be installed on
sample stacks containing non-hazardous gases. The 270 may be installed in an unclassified location. Filter
changes can be made in less than one minute.
The 270SF filter will mount by means of a pipe flange to a mating flange on the stack. The size of the flange
can be specified by the user and can include 2”, 3”, 4”, 6” or DIN type. A 1/2” NPT female threaded
connection is provided for the sample probe to mount inside the mounting flange. Stack temperatures and
corrosive gases will determine the material and design of the probe to be used.
Sample flow rates of up to 20 liters per minute can be extracted and filtered through the Model 270 with a
minimal amount of pressure drop.
GENERAL DESCRIPTION, HEATED FILTER SECTION
The Model 270SF is a Extractive Gas Sample Probe Assembly consisting of the filter body mounted in a
NEMA 4X Fiberglass Enclosure or Stainless Steel Enclosure. An optional blowback system is provided to
allow the blowback to occur on command from a computer, data logger, PLC, UAI timer card or external
switch mounted in a safe area.
Two 63 watt heaters are mounted in an aluminum sleeve around the filter cavity. The heater holds the
temperature of the filter at 340°F (171°C). A hermetically sealed bi-metallic thermal switch mounted to the
filter body keeps the temperature from exceeding 350°F (177°C) by opening the circuit at that temperature.
An optional independent temperature sensor 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 (107°C) is provided to be used as an
alarm contact, if the temperature drops below the switch temperature. In colder climates, an additional heater
can be supplied for the enclosure. This heater will maintain 150°F (65°C) inside the enclosure.
Several types of filter elements can be supplied with the Model 270. The 2μm ceramic filter is supplied as an
economical general-purpose filter. A similar ceramic filter with an internal 0.1μm coating is available for finer
filtration or to provide a surface to enhance the blowback capability where the particulates have a tendency to
fill the pores of the 2μm 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.
Blowback air is used to clean the filter element. Compressed air supplied to the blowback assembly needs to
be clean and dry (-40°F/°C recommended). Instrument quality air is preferred. The pressure should be as
high as possible, up to 125psig (8.8barg). High pressure air fills the accumulator (a 7” (178mm) diameter SS
sphere) and provides a substantial blast when the high flow solenoid valve opens. This loosens the particles
on the filter surface and forces them back through the sample probe into the sample point. The period of time
between blowback cycles should be set to occur before the pressure drop across the filter begins to increase
beyond acceptable limits. By installing and monitoring a vacuum gauge ahead of the sample pump, a
maintenance interval can be established. This can be as often as every fifteen 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 three to five grams of solids 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 airline. 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.
The calibration gas is injected into the chamber ahead of the filter. This is close to the sample source, as is
required by many EPA officers. A back-pressure check valve (set at 3 psig (0.2 barg) is provided in the cal
gas injection path to ensure that calibration gas does not leak into the sample while the sample is being
drawn through the filter.
