Axial thermal mass flowmeter

Thermal mass flowmeter comprising a conduit defining a flow path and an elongated thermal mass flow sensor. The thermal mass flow sensor is coupled to the conduit with the longitudinal axis of the thermal mass flow sensor being aligned substantially parallel to the general direction of the fluid flow within the conduit. A method of measuring flowrates provides the same axial alignment of the thermal mass flow sensor.

TECHNICAL FIELD 
This invention relates to an improved thermal mass flowmeter, as well as a 
method of using thermal mass flowmeters, particularly in hostile 
environments where particulate deposition is otherwise a problem. 
BACKGROUND OF THE INVENTION 
A conventional thermal mass flowmeter 10 is shown in FIG. 1. The flowmeter 
10 comprises a thermal mass flow sensor 12 that monitors the mass flowrate 
of a fluid flow, and a temperature sensor 14 that monitors the fluid's 
temperature. The sensors 12, 14 are mounted in a probe shaft 16, and 
sensor wires 28 lead from the flowmeter to an analog or digital sensor 
drive. The sensors 12, 14 typically include a resistance temperature 
device whereby a temperature output is available from each sensor 12,14. 
In use, the flowmeter 10 is immersed in a fluid flow which passes 15 
transversely over the sensors 12, 14. The thermal mass flow sensor 12 is 
heated to a temperature above the flow temperature, either by a current 
passed directly through its resistance temperature device, or by means of 
a separate heating element. 
The sensor drive electronics normally operates the thermal mass flow meter 
either in a "constant-temperature" mode in which the electronics maintains 
constant the temperature differential between the thermal mass flow sensor 
12 and the temperature sensor 14, or in a "constant-power" mode in which 
the electronics maintains constant the electrical power used to heat the 
thermal mass flow sensor 12. 
In the constant temperature mode, the electrical power needed to maintain 
the temperature differential between the sensors 12,14 is related to the 
mass flowrate in a known manner, while in the constant power mode, the 
temperature differential resulting from the constant power supplied to the 
mass flow sensor 12 is related to the mass flowrate in a known manner. 
Thus, the heat transfer between the thermal mass flow sensor and the fluid 
flow is the parameter which is used to determine the fluid flowrate, and 
anything which affects this heat transfer will have a corresponding effect 
on the output of the thermal mass flow meter. 
In certain applications of thermal mass flowmeters, for example in power 
station coal fired gas stacks, particles entrained in the gas flow can 
deposit on the sensors 12 and 14. While this was previously believed not 
to effect the operation of the thermal mass flowmeter, Applicant has 
determined that such particle deposition can result in serious degradation 
of the accuracy of a thermal mass flowmeter operating in such an 
environment. 
Also, where the thermal mass flowmeter is used in a conduit where the 
temperature of the fluid flow differs substantially from the conduit wall 
or other mounting to which the thermal mass flow meter is mounted, heat 
transfer between the conduit wall and the thermal mass flow sensor can 
cause inaccuracies in the temperatures measured by the sensors 12 and 14. 
Accordingly, there is a need for a thermal mass flowmeter and method of 
using a thermal mass flowmeter which is less sensitive to particle 
deposition, and is less sensitive to temperature differences between the 
fluid flow and the surrounding equipment. 
SUMMARY OF THE INVENTION 
For a better understanding of the advances made the invention, reference is 
now made to FIG. 2 which illustrates an end view of three thermal mass 
flow sensors 22, 24 and 26 positioned in a transverse fluid flow 28. 
The thermal mass flow sensor 22 illustrates the flow around the sensor 22 
under normal operating conditions in an environment where particulate 
deposition is not a problem. The flow 28 passes around the sensor 22, and 
vortices 30 form at the rear side of the sensor 22. Heat is transferred 
convectively away from the sensor 22 by the flow 28. 
The thermal mass flow sensor 24 illustrates the conventional 
misunderstandings regarding the effects of particulate deposition on a 
thermal mass flow sensor. In this figure, particles have deposited on the 
sensor 24 to form a substantially cylindrical particulate buildup 32. 
This particulate buildup 32 typically does not function either as a thermal 
insulator, nor does it function as a thermal conductor to draw heat 
conductively away from the sensor in any manner different from before. In 
other words, the thermal conductivity of the sensor 24 remains essentially 
unchanged by the buildup 32, and accordingly the heat flow from the 
sensor, and hence the flowrate readings will be substantially unchanged. 
As before, the flow 28 passes around the sensor 24, and vortices 34 form at 
the rear side of the sensor 24. 
Applicant however has determined that, contrary to conventional wisdom, 
particulate buildup does degrade the accuracy of flowrate outputs 
available from thermal mass flowmeters, and that the root of the problem 
is not an alteration of the thermal conductivity of the sensor by the 
buildup, but rather that the buildup alters the thermal convection 
characteristics of the sensor. 
This situation is illustrated by the thermal mass flow sensor 26 in FIG. 2. 
In this Figure, particulate buildup 36 is seen to be taking place on the 
trailing edge of the thermal mass flow sensor 26. This buildup is caused 
by the vortices 30 which are generated behind the clean sensor when it is 
first inserted into the particulate flow. Particles entrained in the fluid 
flow enter this vortex system behind the sensor 26, where they impact and 
remain attached to the sensor 26. Over a short period of time, the 
particles accumulate to form the buildup 36. This buildup 36 transforms 
the shape of the sensor from a cylinder to a more streamlined wing shape 
as shown. This in turn alters the nature of the flow around the sensor 26, 
and in particular, the initial vortex system 30 at the trailing edge is 
found to be no longer present. 
This vortex system 30 was originally responsible for creating a large 
proportion of the heat transfer occurring between the sensor 26 and the 
flow 28. By reducing the turbulence around the sensor 26, the convective 
heat transfer is substantially altered, with a corresponding degradation 
in the flowrate readings obtained from the thermal mass flowmeter. 
The exact nature of the particulate buildup occurring on the thermal mass 
flow sensor depends on a number of factors including the adhesive quality 
or "stickiness" of the particles, the composition of the particles, the 
velocity of the flow and hence the speed of the particles. 
Under some circumstances, no buildup may occur, while in other 
circumstances, the particles may adhere to the leading edge of the sensor. 
In the case where the particles adhere to the leading edge of the sensor, 
a streamlining effect is also found causes a corresponding degradation in 
the flowrate reading obtained from the thermal mass flowmeter. 
To overcome this problem, the current invention provides a thermal mass 
flow meter wherein an elongated thermal mass flow sensor is axially 
aligned with the fluid flow. Under such conditions, the component of the 
flow normal to the wall of the sensor is small, which reduces the particle 
impact energy to nearly zero along the walls of the sensor. This 
substantially eliminates the buildup of particles on the sensor walls, and 
the convective interaction between the sensor and the fluid flow therefor 
remains substantially unchanged. 
The only affected impact area of the sensor in this configuration is the 
tip which faces into to fluid flow. In use, a conical or rounded 
particulate buildup forms on the tip within a relatively short time, and 
any effects of this buildup on the convective heat transfer, which are 
small to begin with, stabilize and remain constant from then on. 
To anticipate this buildup, the thermal mass flow sensor may be provided 
with a conical or rounded tip. 
More particularly, according to the invention there is provided a thermal 
mass flow meter comprising: 
a conduit defining a fluid flow path having a general direction along which 
fluid flows in use: 
an elongated thermal mass flow sensor having a longitudinal axis, the 
thermal mass flow sensor being coupled to the conduit with the 
longitudinal axis of the thermal mass flow sensor being aligned 
substantially parallel with the general direction of flow; and 
a temperature sensor mounted in the conduit for providing an output of the 
temperature of the fluid flow in use. 
Also according to the invention there is provided a method of determining 
the mass flowrate of a fluid flow comprising the steps of: 
providing an elongated thermal mass flow sensor having a longitudinal axis; 
mounting the thermal mass flow sensor in the fluid flow with the 
longitudinal axis of the thermal mass flow sensor substantially aligned 
with the general direction of the fluid flow; and 
operating the thermal mass flow sensor under the control of processing 
means for obtaining a measurement of the fluid flowrate. 
Other features of the present invention are disclosed or apparent in the 
section entitled: "BEST MODE FOR CARRYING OUT THE INVENTION."

BEST MODES FOR CARRYING OUT THE INVENTION 
As thermal mass flowmeters are well-known in the art, in order to avoid 
confusion while enabling those skilled in the art to practice the claimed 
invention, this specification omits many details with respect to known 
items. 
Referring now to FIG. 3, the best mode in-line thermal mass flowmeter 40 
comprises a conduit 42 defining a fluid flow path, a probe body 44 and a 
thermal mass flow sensor 46. Located next to the thermal mass flow sensor 
46 is a temperature sensor 48. This can be seen more clearly in FIG. 4 
which shows a schematic transverse cross section of the flowmeter 40 taken 
just upstream of the sensor pair 46, 48. For purposes of clarity, many of 
the features of the flowmeter 40 have been omitted from schematic FIG. 4. 
The conduit 42 comprises a series of tubular sections 52, 54, 56 and 58 of 
varying diameter, and located upstream of the sensors 46, 48 are two 
conventional flow conditioners 60 and 62. 
The probe body 44 serves to position the sensor pair at substantially the 
center of the flow stream 50. The probe body 44 has a bore 64 defined 
therein which provides a route for the sensor wires (not shown) leading 
from the sensors 46, 48 to exit the thermal mass flow sensor 40. 
The thermal mass flow sensor 46 and the temperature sensor 48 are mounted 
to the probe body 44 by means of a sensor plug 66. 
The elongated thermal mass flow sensor 46 and elongated temperature sensor 
48 are aligned with their longitudinal axes substantially parallel to the 
general direction of flow 50 in use. 
In the preferred operation of either of the two embodiments of the 
invention disclosed herein, the sensors are aligned within .+-.5.degree. 
of the direction of the fluid flow, but applicants believe that the 
alignment of the sensors can be within about .+-.20.degree. of the 
direction of the fluid flow and still retain advantages associated with 
the invention. The phrase "substantially parallel" should be interpreted 
accordingly. 
This alignment has two advantages, firstly, less particulate deposition on 
the walls of the sensors 46, 48 as described above, and secondly, the heat 
transfer between the sensors 46, 48 and the fluid flow is less affected by 
any temperature differences between the fluid flow and the conduit 42 or 
probe body 44. 
The thermal mass flow sensor 44 is conventional in structure, and may for 
example be a Sierra Instruments, Inc model no. 43-0121-01-1 resistance 
temperature device sensor. The temperature sensor is also conventional in 
structure and may for example be a Sierra Instruments, Inc model no. 
43-0120-01-1 temperature sensor. 
In use, the thermal mass flow meter 40 is placed into a fluid line such 
that the conduit 42 replaces a section of the fluid line, in what is known 
in the art as an in-line arrangement. After calibration using conventional 
techniques, the thermal mass flow sensor 46 and temperature sensor are 
operated in a conventional manner well known in the art, using 
conventional sensor drive electronics, such as Sierra Instruments, Inc. 
model no. 640. 
FIG. 5 illustrates the best mode insertion thermal mass flowmeter 70, which 
is particularly suited for use in large diameter flow passages such as 
power station stacks. 
The flowmeter 70 comprises a probe body 72, a junction box 74, a flange 76, 
a thermal mass flow sensor 78 and a temperature sensor 80. 
The elongated sensors 78, 80 project transversely from the probe body 72, 
and their longitudinal axes are aligned in use substantially parallel to 
the general direction of flow as shown. Located on either side of the 
sensor pair 78, 80 is a guard 82. The guards 82 serve to protect the 
sensors 78, 80 from damage during handling of the flowmeter 70. 
The probe body 70 serves to position the sensors 78, 80 at a desired 
location in a flow passage. The flowmeter 70 is mounted to the wall of the 
flow passage by means of the flange 76, after the flowmeter has been 
inserted through an aperture in the flow passage. Sensor wiring passes 
from the sensors 78, 80 through the probe body 72 and to the junction box 
74 where the wires are connected to terminals which are available to lead 
the sensor outputs to a sensor drive. 
As in the previous embodiment, the thermal mass flow sensor 78 and the 
temperature sensor 80 are conventional in structure and may for example be 
Sierra Instruments Inc. model nos. 43-0121-01-1 and 43-0120-01-1 
respectively. In use, the flowmeter 70 is mounted to a flow passage wall 
such that the thermal mass flow sensor 78 is positioned in the fluid flow 
with the longitudinal axis of the thermal mass flow sensor aligned 
substantially parallel to the general direction of the fluid flow, and the 
thermal mass flow sensor 78 is operated under the control of conventional 
sensor drive electronics for obtaining a measurement of the fluid 
flowrate. 
It will be appreciated that the invention is not limited to the embodiment 
of the invention described above, and many modifications are possible 
without departing from the spirit and the scope of the invention.