Titus AeroCross User guide

AG-AeroCross-00
March 4, 2002
Application Guide
Flow Sensor

AG-AeroCross-00
March 4, 2002
1
Table of Contents
Introduction 2
Description 2
Performance 2
Amplification 2
Multi-point Center Averaging 3
Inlet Sensitivity 4
Sound Performance 4
K-factors 5
Suggested Specifications 6

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Introduction
Accuracy of airflow control is critical to terminal unit performance, because of its impact on many
important aspects ranging from acoustics to occupant comfort. The key to accurate flow control from
a terminal unit is well-designed and repeatable airflow sensor. While there have been many
improvements to both flow transducers and controller software/algorithms from the HVAC controls
industry, all are dependent on an accurate flow signal from a flow sensor. A flow sensor that can
measure accurately regardless of inlet conditions simplifies and takes much of the guesswork out of
the balancing and commissioning process.
The Titus multi-point center averaging flow sensor is the number one sensor on the market because it
is accurate regardless of inlet conditions. A flow sensor should provide flow signal amplification and
immunity from poor inlet conditions, while keeping pressure drop and sound levels to a minimum. In
addition, a flow sensor should have a high degree of repeatability and sturdy construction.
The original flow sensor design required a trade-off between amplification (accuracy) and pressure
drop (sound), resulting in a very accurate sensor that was occasionally louder than the competitions’
sensors. Using Computational Fluid Dynamics (CFD) software, we were able to design a new flow
sensor that maximized amplification and minimized pressure drop.
We are happy to introduce the AeroCrossäflow sensor.
Description
The flow sensor is the most important component of a VAV terminal. The flow sensor measures the
total and static pressure in a terminal so that a controller can calculate the cfm through the unit. If the
flow sensor does not provide accurate information to the controller, the controller cannot determine
actual cfm, and therefore cannot maintain comfort in the space.
The AeroCrossäis a multi-point center averaging flow sensor. The new sensor has a narrower profile
than the original sensor. To visibly differentiate the new sensor, the AeroCrossäwill be blue. The
flow label on the terminal will also be blue to allow for easy identification from the outside of the
unit. Like the original Titus flow sensor, the AeroCrossäis injection molded out of a high impact
plastic material.
Performance
Amplification:
Amplification is the ability of a flow sensor to produce a signal greater than the velocity pressure.
Pitot tubes read true velocity pressure, which requires 4005 FPM to produce a 1” wg signal. Velocity
pressure is the difference between total pressure (taken from the tip of the probe) and static pressure
taken from the side of the probe). Amplified flow sensors improve upon this signal by taking the
difference between total pressure (from the front of the probe) and a reduced static pressure (from the
rear of the probe). Amplification is critical to accurate control of minimum flow rates. While many

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digital controllers have made great gains in processing low pressure signals accurately, a sensor
should be capable of providing a signal of sufficient magnitude for any type of controller to monitor
easily.
The AeroCrossäsets the standard for amplification, with performance ranging from 1.7 to 2.9,
depending upon the inlet size.
Multi-point Center Averaging:
Multi-point center averaging flow sensors are more accurate than linear averaging flow sensors.
Linear averaging sensors are not always linear in shape. They come in a variety of shapes such as
round and diamond. Multi-point center averaging flow sensors take the pressure readings at the center
of the sensor. (See Figure 1.) Center averaging sensors are not affected by poor inlet conditions as
linear averaging sensors are.
Figure 1. AeroCrossäMulti-point Center Averaging Flow Sensor
As you can see from figure 1, each total pressure port has the same “weight” in determining the
pressure reading. In a linear averaging sensor, the port closest to the point where the reading is taken,
will have a higher “weight” than the port farthest away from the reading. Tests have shown linear
averaging sensors can be up to 30% incorrect with poor inlet conditions. Figure 2 shows a typical
linear averaging flow sensor.
4 total pressure sensing
ports on the front
3 static pressure sensing ports on
the back (not visible) in figure
Pressure reading is taken at
the center of the sensor

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Figure 2. Linear Averaging Flow Sensor
Inlet Sensitivity:
Inlet sensitivity is a measure of flow sensing accuracy that can be lost to ‘less than ideal’ inlet
conditions. Although SMACNA recommends a minimum of three duct diameters of straight duct in
front of any flow-measuring device, this is often not the case. Obstructions such as plumbing, conduit,
and structural members result in jogs and turns in both rigid and flexible supply ductwork. Real world
conditions require that a good flow sensor is able to read air volume to a +/-5% accuracy, no matter
what the inlet conditions may be. This is critical to guarantee the accuracy of factory calibrated
controls, that would otherwise require a field calibration. It be noted that if excessive inlet sensitivity
results in a reduced flow signal for a given flow volume, the benefit of amplification has been lost.
No controller, regardless of its sophistication, can overcome less than adequate accuracy from a flow
sensor under common field conditions.
The AeroCrossähas less inlet sensitivity than any flow probe on the market, with no more than +/-
5% error regardless of inlet condition. While other center-averaging designs are capable of +/-10%
error, linear averaging designs can range from +/-10% to 35% depending upon the exact condition
and angle of approach.
Sound Performance:
The greatest impact of the AeroCrossäflow sensor is the effect on sound performance of the terminal
units. The narrower profiles resulted in a reduced pressure drop for the flow sensor. This resulted in
significant sound reductions in the many of the terminals.
The greatest effect is in the ESV product line. Table 1 shows the comparison between the NC values
for the original flow sensor and the new AeroCrossäsensor for the ESV.
Total pressure sensing
ports on front
Static pressure sensing ports on
back (not visible in figure)
Pressure reading is taken
at end of sensor

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Table 1. NC Comparison
As you can see from table 1, the sound reduction was significant in many sizes and cfm ranges.
K-factors:
The AeroCrossäsensor has different k-factors than the original sensor. Titus will provide the major
controls companies with the new k-factors to update their software. Until the software updates are
made, the control contractors will need to be aware of the new constants. Table 2 shows the new k-
factors.
Table 2. K-factors
Inlet Size K-Factor
4273
5360
6448
7667
8904
91167
10 1436
12 1891
14 3015
16 3839
24 x 16 7176
AeroCrossäK-Factors
Sound Noise Criteria (NC)
0.5" 1.0" 2.0" 3.0" 0.5" 1.0" 2.0" 3.0"
4175
- +1 -1 -2 - -2 -3 -4
5300- -1 -3 -3 - -1 +1 +2
300 - - 0 +1 - -4 -3 -2
500 -1-4-10-7-5-30
350 - - -1 -2 - - -5 -4
650 - -6-4-3-2-4-2 0
450 - - --1- --3-5
800 - -1-4-2-4-8-9-6
600 - - -2-6 - -3-3-1
1000 - -1-3-2-8-11-6-3
700 ---+2-3-5+1+3
1400 - -3-5-4-16-13-7-2
1000 ---0--3-20
2000 - - +2 +3 -11 -9 -6 -3
1500 ---+1-8-11-6-4
3000 - - -4 -4 -26 -23 -17 -14
2000 ------4-5-4
4000 - - -6 -5 -9 -8 -6 -4
4000 -6-10-12-11+4+3+2+1
8000 NA -18 -14 -13 NA +2 0 -1
Radiated
DPs
Inlet Size cfm
Dischar
g
e
DPs
6
7
8
9
24 x 16
10
12
14
16

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Because the k-factors are different between the AeroCrossäand original sensor, replacing an original
sensor with an AeroCrossäsensor will result in different airflow readings. Table 3 compares the
AeroCrossäk-factors to the original sensor k-factors and shows the % difference in cfm reading
between the two sensors.
For example, if you replaced a size 8 original sensor with a size 8 AeroCrossäsensor, the same
pressure reading would result in 2.6% lower cfm calculation in the controller, if the k-factor was not
updated to the AeroCrossäk-factor.
Table 3. K-factor Comparison
Suggested Specification
The following is the suggested specification for the AeroCrossäflow sensor.
Differential pressure sensor shall be cross shaped multi-point center averaging type. Single axis
sensor shall not be acceptable for duct diameters 6" or larger. A sensor that delivers the differential
pressure signal from one end of the sensor is not acceptable. The sensor shall output an amplified
differential pressure signal that is at least 1.5 times the equivalent velocity pressure signal obtained
from a conventional pitot tube. Balancing taps and airflow calibration charts shall be provided for
field airflow measurements.
Inlet Size
A
eroCrossä Original Sensor % Difference
4 273 269 -1.5%
5 360 404 10.9%
6 448 474 5.5%
7 667 625 -6.7%
8 904 881 -2.6%
9 1167 1094 -6.7%
10 1436 1371 -4.7%
12 1891 1931 2.1%
14 3015 2795 -7.9%
16 3839 3677 -4.4%
40 7176 6986 -2.7%