United Sensor Gas Sampling
probes have been designed for accurate sampling of burner
exhaust gases primarily used in efficiency studies of combustion
and they have been endurance tested in routine test work.
An internal static tap is located in the inner flow passage
of the probe near the sample inlet as shown in Fig. 1 so the
static pressure of the sampled gas can be measured. In order
to obtain a true sample, it has been found necessary to draw
off the gas at the same velocity as is flowing in the duct.
If the sample is accelerated or decelerated as it is drawn
into the probe, it will not be a true average since, in the
first case, a disproportionate share of low density components
will be drawn in, and in the second case, too much of the
high density components will be sampled.
This would not be true in a perfectly homogeneous mixture
of gases, but in the region most sampling is done the mixture
is not homogeneous since the fuel particles are usually still
burning or have just completed burning and the products of
combustion are concentrated in "slugs" of gas interspersed
by lighter, unburned air.
The gas velocity in the duct is proportional to:
Where Pt, Ps, and d are total pressure, static pressure, and
density, and K is a constant.
The gas velocity in the probe inlet is proportional to:
Pt1 will automatically equal Pt2 since no work is done on
the gas in drawing it into the probe so if Ps1 is made equal
to Ps2 it can be seen that V1will equal V2. Since the static
pressures are equal and there is no change in temperature,
d1 and d2 will also be equal.
By measuring the static pressure Ps1 with a static tap in
the wall of the duct at the same cross section and Ps2 with
the internal static tap on the probe, the two can be equalized
by adjusting the sampling suction.
In some cases, the resulting volume of gas flow through the
sample probe is too high or too low for the gas sampling apparatus.
The orifice inlet diameter is picked to be high enough to
give a flow at the lowest duct velocity V1 sufficient for
analysis purposes and the excess gas sampled at higher duct
velocities is by-passed around the analyzer by a bypass line
United Sensor Sampling probes are made in three types:
Type A is uncooled, of welded construction, suitable for use
up to 2000¡Æ and is the smallest.
Type B can be used either as a heated or cooled probe. The
cooling passages are especially designed for unrestricted
flow of cooling or heating fluids. For some applications,
it is important to keep the sampled gas above500¡Æ to the analyzer
to prevent condensation of evaporated hydrocarbons. In this
case, the probe is heated by blowing hot air through the cooling
passages. This is more satisfactory than trying to heat it
with anelectric coil because the variation of heat loss conditions
from the surface of the probe makes it almost impossible to
maintain an even temperature. For higher temperatures, the
probe can be kept hot enough to prevent condensation but cool
enough not to melt by blowing unheated shop air through the
coolant passages. Above 3000¡Æ even with water cooling the
stainless steel tip on the Type B probe it would burn out.
Therefore, Type C is recommended.
Type C probes have a copper tip, which has a high enough conductivity
to prevent burning out at the highest temperatures and velocities.
Since the tip is easily damaged, it is not recommended for
lower temperature applications.