Chemical Engineering February 2014 - 49

Table 1. Typical specificaTions for
Thermal dispersion mass flowmeTers
specification
Gases
Two-temperature system four-temperature system
Most clean gases, including
air, methane, Ar, CO2,
He, N2, O2, C3H8, and mixtures
of these components
In-line flow body
sizesa
In-line meter mass
flowrate range for air
Insertion meter massvelocity
range for air
0.25, 0.5, 1.0, 1.5, 2.0, 4.0
in.; DN6 to DN100
0.0001 to 1.5 kg/s; 0.1 to
2,600 scfmb
1.4 to 140 normal m/s; 300
to 30,000 standard ft/minc
0.01 to 16 bara
Most clean gases, including
air, methane, Ar, CO2,
He, N2, O2, C3H8, and
mixtures of these components
0.25,
0.5, 1.0, 1.5, 2.0, 4.0
in.; DN6 to DN100
0 to 1.5 kg/s; 0 to 2,600
scfmb
0 to 140 normal m/s; 0 to
30,000 standard ft/minc
Temperature ranged -40 to 200°C; -40 to 392°F -40 to 200°C; -40 to 392°F
Pressure range
Accuracye
0.01 to 16 bara
1% of reading plus 0.5% of
full scale (FS)
Rangeability
Repeatability
Time responsef
Stability
100 : 1
0.2 % of full scale
3 s (constant power operation);
1.2 s (constant
ΔT operation)
1 year; typical drift 1 to 2%
per year
1% of reading from 10 to
100% of FS; 1% of reading
plus 0.5% FS from 0 to
10% FS
100 : 1
0.15 % of full scale
2 s
10 years; typical drift
0.1% per year
Notes to Table 1: (a) Some manufacturers offer sizes up to 12.0 in. (DN300); (b) Based on the point
mass-velocity range for insertion flowmeters cited below; (c) " Normal " conditions are 0°C and 1
atm, and " standard " conditions are 70°F and 1 atm; (d) High-temperature models are available
up to approximately 450°C = 842°F; (e) FS = full scale; and (f) Time response is the time required
to reach 63% of the final value (that is, the 1 sigma value).
independent variable and the electrical
power, W, shown as the dependent
variable, whereas in the
system they have reversed roles.
The three figures reflect the strong
direct dependence the electrical
power has on the thermal conductivity
of the gases. Thus, Figure 4a
results from the fact that kmethane
> kair > kargon, and Figures 4b and
4c result from the fact that thermal
conductivity increases as gas temperature
and pressure increase,
respectively. The fact that thermal
conductivity, and therefore W, increases
with gas pressure as shown
in Figure 4c is a phenomenon that
has heretofore been ignored, but
for higher accuracy applications
should be included.
Figures 4 a-c also reveal the
non-linear, logarithmic nature of
the output. A log versus log plot of
these figures will reveal a nearly
straight line over approximately
1 to 150 standard m/s. This logarithmic
property is responsible for
the exceptional rangeability and
low-velocity sensitivity of TD mass
flowmeters. A rangeability as high
as 100:1 is common. Even higher
rangeabilities are achieved with
multi-range flow calibration. Detectable
minimum-point mass velocities
as low as approximately 0.1
standard m/s (20 standard ft/min)
are reported by some manufacturers.
In the early days of analog electronics,
it was difficult to linearize
the output of TD mass flowmeters.
But now, with microprocessor-based
electronics, it is not a problem, and
the non-linear, logarithmic nature of
the output bears only advantages.
Figures 5 a-c show further results
of the four-temperature microprocessor-based
system. Figure
5a reveals how the temperature distribution
T1(x) of the heated section
of the velocity sensor undergoes
major changes as Vs increases from
0 to 100 standard m/s. Figures 5a
and 5b show, for air and methane,
the excellent comparison between
results calculated via the fourtemperature
microprocessor-based
system and actual flow calibration
data. Comparisons for other gases
are likewise as good. Algorithims
exist that make the output of the
system match flow calibration data
even better.
Specifications
Table 1 shows specifications for currently
available TD mass flowmeters.
Specifications for the column
labeled " Two-temperature system "
refer to the flow sensor shown in
Figure 2a and may vary from manufacturer
to manufacturer. Specifications
for the column labeled
" Four-temperature system " refer
to the microprocessor-based system
with the four-temperature flow
sensor having the dry velocity sensor
shown in Figure 2b. Accuracy
specifications in Table 1 may apply
to gas temperatures and gas pressures
that lie within bands around
their respective values at flow calibration.
Table 1 is also useful in
selecting and sizing the proper TD
mass flowmeter for the application.
Installation
In all cases, specifications and instructions
provided by the manufacturer
should be followed in sizing
and installing in-line and insertion
TD mass flowmeters. Ref. 3 is an
excellent source for details regarding
sizing, installation, safety and
flow calibration.
Figure 6 is helpful in the flowmeter
selection process for in-line
TD mass flowmeters. The permanent
pressure loss shown in Figure
6 applies to an in-line meter with
a built-in flow conditioner consisting
of two upstream separated perforated
plates. In sizing TD mass
flowmeters, as with all flowmeters
used for gas flow applications, the
Mach number of the gas flow should
be kept under approximately 0.3 to
avoid compressibility effects. At an
absolute gas pressure of 1 bara (approximately
1 atmosphere), the permanent
pressure loss is about 0.1
bar (approximately 2 psi) at all fullscale
mass flowrates. This is one or
two orders of magnitude less than
that required by Coriolis mass flowmeters
for gas flow applications. Almost
all of the permanent pressure
loss shown in Figure 6 is due to the
built-in flow conditioner.
Insertion flowmeters have very
low permanent pressure loss, especially
for larger line sizes. In-line
flowmeters without a built-in flow
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Chemical Engineering February 2014

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