Model 270S Heated Sample Probe

The Model 270S is a Heated Stack Filter Assembly consisting of the filter body mounted in an aluminum protective housing. An optional blowback system is provided to allow a blowback to occur on command from a computer, data logger, PLC or external switch mounted in a safe area.

A 125 watt heater is mounted on the outside of the filter cavity. The heater holds the temperature of the filter near 340°F (171°C). A bi-metallic thermal switch mounted to the filter body keeps the temperature from exceeding 350°F (177°C) by opening the circuit before that temperature. An optional independent thermocouple 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.

Several types of filter elements can be supplied with the Model 270S. 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 2 μm sintered 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 (8.8 barg). 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 set to occur before the pressure drop across the filter begins to increase. 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 air line. The recharge time could be extended with a restriction in the air line if it is 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 insure that calibration gas does not leak into the sample while the sample is being drawn through the filter.

If an independent measurement and display of the oven temperature is desired, the 270S can be assembled with a thermocouple or RTD installed in the heater tube assembly.