AE 457 - HVAC Controls
Lab 2
Ashley Bistline
Marissa Caldwell
Anya Godigamuwe
Mike Giuliani
Kevin Ricart
PART 3. POST LAB EXERCISES
Digital I/O - Mike’s Section to be handed in on Monday
a. Why are 5V DC TTL digital I/O signals appropriate for use with some mechanical system
components but not others?
b. What other digital I/O voltages are commonly used in HVAC equipment? What are their
threshold values for switching between digital high and low states?
c. What standards organizations define digital I/O signals and their threshold values?
d. At Step 8 of Part 1 of the lab, what was the voltage input required to transition to a digital
high state? At what voltage level did the transition back to a digital low state occur? Look at
the digital input specifications in the manual for the cFP-DIO-550 module at
http://digital.ni.com/manuals.nsf/websearch/66912CC171939FD9862571860070846D
Does this make sense? Why or why not?
e. At Step 10 of Part 1 of the lab, what voltage did the digital high state occur? At what voltage
did the lamp turn off (i.e., a digital low state occur)? Are these transitions consistent with the
specification sheet of the Crydom TD1210 solid state relay used?
Damper Actuators
The fan and damper sections in Figure 1 for the 52.2 rig. include a New York Blower Co. backward
inclined single width, single inlet, size 18, PLR wheel, Class 3, counter-clockwise rotation centrifugal
fan with a top horizontal discharge in direct drive arrangement 4 with a 4000 cfm capacity (operating
at 15 in. H20 static pressure, 3500 rpm and 14.9 brake horse power). A Baldor Electric Co. model
EM2394T, 15 HP, 3-phase motor coupled to a Baldor Electric model IHH215-E variable frequency
drive is used to drive the fan. Three, Ruskin model CD80AF4, opposed blade, industrial control
dampers designed for ultra low leakage performance (~ 6 CFM/sq. ft.) are incorporated into the 12
gage stainless steel damper housing. Belimo Model AF24-MFT proportional damper actuators with
spring return failsafe action, are mounted to the ¾ in. damper blade axles. Three actuators are used
for each damper section. The actuators are configured to provide a 2-10V DC analog signal to track
the actuator position for control purposes.
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AE 457 - HVAC Controls
Lab 2
Figure 1
f.
Present a plot of analog output voltage against angle of rotation for the Belimo MFC damper
actuator you worked with in the lab. Be sure to plot both the ascending and descending
voltage vs. angle relationship. Use regression to fit curves to the data and present the
regressed curve equations along with statistical measures of best fit. Review the Belimo
specifications at their website for this damper actuator:
http://www.belimo.us/ishop/article/chapter/10079_10075/node/10075/article/59908_10084_1
0079_10075/subchapter/10084_10079_10075.xml. Is your plot what you would expect based
on your review of the documentation for the actuator?
***We were never given directions or the time to record the descending values.
The desired outcome of the damper is to achieve a proportional relationship between the voltage
and the angle of rotation. Our plot is what we would have expected on our review of the actuator
as it produced a regression value of .91. Such a regression value is considered to be very strong,
meaning that the results were accurate and desirable. In order to achieve this, the actuators must
be properly sized and mounted to the damper shaft with a universal clamp.
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Lab 2
g. Without considering details of duct sections after the fan and damper section shown in the
photos above or specific implementation details about the programmable automation
controller to which you would connect sensors, etc., how would you devise a laboratory
experiment to characterize the damper flow characteristics (i.e., to generate a plot of
maximum flow vs damper position as one x-axis and the control signal voltage as the other)?
Based on the specifications noted above, what range of differential pressure transducers
should be used? Where will pressure drops be measured using differential pressure
transducers to make this characterization? What model pressure transducers at
www.setra.com would you choose?
There is a direct proportionality between the position of the dampers and the
voltage of the system. When the voltage reaches 10 V, the damper is completely or
almost fully open, whereas the damper is considered to be nearly closed at 2 V. These
two parameters could both be plotted as x-axes to be plotted versus the pressure. In order
to run this experiment, you would need a depressurized, sealed duct to measure the
airflow. A device such as a pitot tube could be used to measure the pressure at different
damper positions that would correspond with an increasing voltage. The specifications
call for the fan to have a pressure drop of 15 in. WC and this would be measured at both
before and after the dampers. Based on the pressure transducer specs, we would chose
AccuSense Model ASL as the differential pressure transducer because it is used for a
range of 0-30 in. WC.
h. Based on your plot at a, are any hysteretic effects apparent for this actuator in a no flow
situation? How would you further measure and characterize the hysteresis in the actuator and
its linkage? How would you expect this to change with changes in the fan’s operation along
its curve? How would the controller need to compensate for hysteretic effects?
**We did not measure the descending voltages, so I cannot analyze the hysteretic effects
in the actuator.
In order to compensate for hysteretic effects in the actuator, it would be wise to
measure or control another contingency. For example, maintaining the pressure change or
measuring another data point could be used to reference the relationship measured
between the voltage and angle of rotation. Another possibility would be to hold two trials
and averaging the results to give a more accurate reference point.
i.
What are the typical consequences of poorly-tuned flow control loops employing damper
actuators?
If dampers actuators do not have properly-tuned flow control loops it can result
in inefficient systems that have hysteretic errors/inconsistencies, lateral movement can
occur, the system may not be properly programmed for overload protection, and the
dampers could be damaged.
j.
Why is the default control voltage for most Belimo damper actuators 2-10 VDC?
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Lab 2
Most Belimo damper actuators are between 2-10 VDC because they do not
include spring returns. Typically, the returns are not standard or necessary, but the
actuators can be customized if the spring returns are needed. Additionally, the dampers
are typically considered to be fully closed at 2 V and fully open at 10 V, so any other
range of voltages would result in data that is already recorded.
Electronic Proportional Regulator
1. Figure 2 is a plot of analog output voltage to the Parker P31PA92AD2VD1A electronic proportional
regulator under zero flow conditions. Review the specification sheet for the regulator at
http://www.parker.com/literature/Literature%20Files/pneumatic/Literature/Pneumatic%20Products%
20-%20PDN1000%20Catalog/Air%20Preparation%20Products/PDN1000US_P31P-P32P.pdf. This
particular model has an operating range of 0-145 psi for a 0-10 V input signal. However, only ~60
psi system air supplied by the university power plant was supplied to the inlet port of the regulator.
The first curve on the plot indicates the output pressure from the regulator through it’s entire 0-10 V
range of control signals sent to the actuator. Based on the specifications for this model, what is the
control signal expected to deliver 60 psi? How does this compare to what is plotted? If the 7 bar
model were used instead, what would be the expected output pressure at this control voltage? What
would the output be for the 2 bar model?
According to Figure 2, the control signal for 60 psi would be approximately 4.2 V. At
this voltage, the seven bar model would read 42 psi. In the 2 bar model, this voltage reading
would have an expected output of 12 psi.
2. What type of control scheme is used for the electronic proportional regulator? With reference to
Figures 2, 3 and 4 are there hysteretic effects apparent in the regulator? What provisions are made in
the electronic pressure regulator to compensate for hysteretic effects?
a. Electronic pressure regulators are used for controlling these devices. They take input signals
that are either 0-10 V or 4-20 mA. The output signal is given “of 24VDC” with the reference
resistance at 1 kOhm.
Figure 2 has only small hysteretic effects. Figures 3 & 4 have larger hysteresis
when the change in measured value is greater. However, a correction is applied to both at
6V that causes the readings to be adjusted.
A provision for compensate for these hysteretic effects can be to use two measuring
devices and to take the average output of the measurements as the actual condition.
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Figure 2
Proportional Valve
3. What’s the best Cv value you think for this test?
*At constant 3V Pressure and 8V on the valve
Cv=Qf/[k*sqrt(P1-P2/rho)]
-Density is expressed as specific gravity. Specific gravity and K are equal to unity
-Pressure is in psi
- Flow rate is in GPM
Cv=62.33/sqrt(.0331) = 342.6
Mass Flowmeter
4. What is the principle of operation for the Omega FMA 1745 mass flow sensor used in this lab
http://www.omega.com/Manuals/manualpdf/M1680.pdf?
a. What are typical disturbances that could affect the operation of a mass flow loop?
The mass flow meter only works for filtered gases, therefore liquids may not be
metered. The flow path may not be interrupted by debris or gas crystallization, therefore,
the filter must be cleaned occasionally, and debris must be removed as necessary. Since
the meter only works for gases, the main source of disturbances are from solids forming
from the gas, or contaminants passing through the filter.
b. Based on the specifications of the mass flow meter, what is the fastest practical loop rate that
could be used in a mass flow loop using its signal for feedback?
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The fastest practical loop rate that can be used is 50 L/min. This is because there
is a +/- 1.5% accuracy for the entire flow range (0-100%). The faster loop rate of 100
L/min provides an accuracy of +/- 1.5% for a flow range of (20-100%), but only provides
an accuracy of +/- 3% for a flow range below 20%. Therefore, 50 L/min provides a more
practical accuracy.
c. Using the barometric pressure and temperature of the ambient air, how would you correct the
flow to standard conditions of 29.92 in. Hg and 69.4 ºF? Is this necessary with the mass
flowmeter?
Ambient temperature is about 70 ºF, which means the difference between the
standard and ambient temperature is 0.6 ºF thus we must subtract this difference from
each temperature measurement to meet the condition. The ambient barometric pressure is
29.92 in. Hg also, so the measurements do not have to be adjusted since they already
meet the standard condition.
d. Comment on the position of the proportional valve and flowmeter used in the lab set up.
Why might you switch the position of the valve and flowmeter? What other concerns would
you have about the positioning of these devices in the air supply line to the duct?
The mass flowmeter was placed after the proportional valve in the lab set up. The
valve will create turbulent flow, the flowmeter sensor requires a laminar flow to have
accurate readings. If the valve is fully open, the large amount of air passing will create a
turbulent flow, however when the valve is partially closed, a laminar flow will form thus
allowing the sensor to make an accurate reading. If the valve is placed after the
flowmeter, the flowmeter will not measure the change in flow that the valve creates.
RTD Sensor - Mike’s Section to be handed in on Monday
5. How did the temperature you looked up in the Omega Engineering RTD Temperature vs. Resistance
Table using the resistance output you measured with the ohmmeter setting of the Fluke 289
multimeter compare with the temperature read using the cFP-RTD-122 module of the compact
FieldPoint system for the Omega Engineering model RTD-805 RTD in the duct section? List two
significant reasons for variations between the two methods.
Differential Pressure Sensor
6. Download the data sheet for the Setra Model 264 (p.n. 2641005WD11T1F) differential pressure
sensor at http://www.setra.com/downloads/9.htm. Using the data sheet, what is the accuracy,
pressure range and output range (voltage or current) of the device? What were the analog output
voltages sent to the proportional regulator and the proportional valve and what was the mass flow
read when you held the differential pressure output to about 1 in. H20? A cFP-AI-110 module is used
to acquire data from the sensor. The data sheet for the module can be found at
http://www.ni.com/pdf/products/us/cpf_fp_ai.pdf. If the sensor is connected and set up for use with the
module as a 4-20 mA signal, what is the expected resolution for acquired readings if only the 16 bit
specification for A/D conversions is considered in the calculation? What would be the expected
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Lab 2
reading in mA from the sensor when sensing a 1 in. H20 differential pressure? How would you
recommend calibrating this device and the cFP-AI-110 hardware module used with it to achieve the
maximum accuracy in your readings from the system?
The accuracy of the Setra Model 264 is +/- 1% F.S., the pressure ranges from 0-100 in.
WC, and the output range is 0-5 VDC. The voltages that were sent to the regulator and the valve
were: 0, 1.97, 3.95, 8.93, 7.91, 9.89 and the mass flow rate at approximately 1 in was about 500
scfh.
The expected resolution for a 16 bit resolution module set to a signal ranging from 4-20
mA would be 5-.66 Hz and 0-10 V with 1 in 65,536. In order to achieve the maximum accuracy
in the readings for the system, we would recommend calibrating the devices by depressurizing the
area and isolating the transmitter. The calibrator will then be zeroed and a measurement will be
tested to see if the pressure reading is accurate based on the input of the machine. The error can
be measured for the current and the pressure, and adjustments can be made using that error or
physical adjustments and recalibrations can be attempted to achieve a more accurate reading.
Additionally, running the test multiple times, with two sensors can help to find a more precise
instrument and measurement that should be achieved.
Air Leakage System Control
7. A plot of analog output voltage to the proportional valve versus displayed mass flow in scfh and
differential pressure in in. H20 for various proportional regulator voltage settings is given in Figures 3
and 4 for both ascending and descending operation.
a. Based on these curves, and the specification sheets referenced above, evaluate the suitability
of the combination of proportional regulator, proportional valve, flowmeter, and differential
pressure sensor for developing an automatic control system for evaluation of duct air leakage
at various levels of differential pressure. What would be the expected controllable range?
How would you further characterize the equipment to develop a feedback controller for the
system? What type of automatic control mode would you expect to work well for this
application, i.e., P, PI or PID? How might you go about tuning the control loop(s) that you
would employ for the system?
Figure 3 is representative of the Setra Model 264. This sensor is advertised as
having “the accuracy necessary for proper building pressurization and air flow control”
meaning it will measure pressure [psi] and air flow [scfh] well. It uses changes in
capacitance because of pressure changes to a diaphragm as its measurement technique.
Pairing this with its curves in Figure 3 lead to the conclusion that this device will be
effective for its use. The graph particularly has nearly no hysteretic effects between 0 and
4 V input. The control range for this device is between 0 to 175 F.
Figure 4 is representative of the National Instruments cFP-AI-110 model. This
device is more accurate than the previous and is advertized for use in “battery-pack
monitoring, [and] fuel-cell testing”. These variances are depicted in Figure 4 by the large
and constantly occurring hysteretic effects. The acceptable control range of this product
is -40 to 70 C. These applications are more advanced than those required for building
pressure and airflow measurements. Therefore this device is overqualified and is not
recommended for use.
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Lab 2
Both these systems will require a known temperature measurement, likely from an ice
bath, to evaluate its accuracy.
b. How well do you think that you could control differential pressure in an open loop manner
using any fitted equations you might develop through the plotted data in Figure 3 and or 4 to
obtain settings for the proportional valve and proportional regulator needed to maintain a
specific differential pressure? How would you deal with hysteretic effects? What would you
expect to be the likely result of open loop operation over time for this system?
Controlling differential pressure in an open loop manner would be difficult. For
more accurate and precise control of differential pressure, it is recommended to use more
iterative processes. The fitted equations from Figure 3 would be the most useful in this
application because the instrument in question already measures differential pressure.
Hysteretic effects within this system can be dealt with by inserting redundancies through
the installation of a second or third sensor to verify the values output by the first. If the
system is an open loop operation, over time an error will develop in the form of a drift.
Figure 3
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Figure 4
VAV Terminal Box Simulation
8. Develop a single plot with series of differential pressure data collected at the flow sensor of the
Trane Varitrane VCCF-8 VAV box along one y-axis, fan control signal along another y-axis, and the
measured air velocity (cfm) along the x-axis for each damper opening. How does your fully open
curve compare with that presented by Trane in their specification sheet for the VAV box? Please
comment on the relationships among flow rate, fan power, and damper openings. If we want to
provide the widest range of flow rates with the terminal box, how should we control the fan and
damper opening? In a VAV system with multiple terminal units, why would it be advantageous to
locate the static pressure sensor downstream from the fan at a location that will ensure that the
terminal farthest away from the fan maintains a proper static pressure level? If sufficient time were
available in the lab, how would you develop a minimum flow and full flow system characteristic
curve for the VAV terminal unit along with a system operating curve between these conditions and
intersecting various damper openings in the VAV box?
The fully open curve developed experimentally is fairly similar to the manufacturers
curve given its linear nature; however, the manufacturers curve is less steep. For example, the
manufacturers curve reads that at 200 scfh the differential pressure is .05 in w.g, while the
experimentally derived graph reads .08 in w.g. As shown on the graph Fan Voltage, Air velocity
and Differential Pressure are all proportional. As Fan voltage increases, so does air velocity and
differential pressure.
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For the widest range of flow rates, the damper voltage should be set to 10V, while
varying the fan voltage. It would be advantageous to locate the sensor downstream from the fan
due to the static pressure drops caused by friction in the ducts and fittings. The static pressure will
inevitably be highest closest the the fan. In order to ensure that downstream boxes are receiving
adequate static pressure it is best to locate the sensor downstream.
There was not sufficient time to develop minimum and maximum characteristic curves.
Filter Pressure Drop
9. Develop an x-y plot with the acquired differential pressure (in. w.c.) upstream of the filter in the zone
of the simulated VAV terminal box (from the data file saved during your work with the VAV
terminal box) along the y-axis and the measured air velocity (cfm) along the x-axis. Go to Aeolus
Filter’s website to download the performance data sheet for the MERV 14 Model SMP-9024242
panel filter used in this lab. How well does the data you collected map into the curve presented for
this filter?
The velocity measurements we made in the lab are lower compared to those used in the
performance data sheet for the MERV 14 Aeolus Filter Model. Although they are at a smaller
scale, the data values fall on the same slope. The figure below, Differential Pressures in Filters,
displays the technical data on the same graph as the measurements when the damper is at 10 V,
and when it is at 7 V. The velocity measurements from the damper at 4 V were close to zero.
Therefore, the values did not give an accurate estimate to the slope. Measurements at 2 V were
found to be at a velocity of 0, so they were not used.
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Flow Control Valve
10. Was the control valve normally open or normally closed? What is the pressure range through which
the valve is responsive to pneumatic signals? Using the links for the Research Control Valve Model
No: 1002GCN36SVCSALN36 to aid you, discuss the details, including inherent valve characteristic
behavior for this valve.
The control valve is normally open, and responds to a pressure range from 3 psi (fully
open) to 14 psi (fully closed), as discovered in our laboratory experiment. The valve is a ½” valve
with a globe cast body type and NPT end fitting. The body bonnet material is 316 S/S with CV
ring packing. The actuator type is ATC standard. The trim size and characteristic is “A” Linear.
The trim material is the same as the body material.
11. Plot the recorded data on an x-y graph with air flow rate (scfh) as the y-axis and control signal (psi) as
the x-axis. Plot ascending data in one series and descending data in another series. Do the two curves
overlay perfectly or can hysteretic effects be observed? Can you discern whether the valve operating
characteristic is quick opening, linear, or equal percentage?) How does your “operating”
characteristic curve differ from the inherent characteristic curve for the valve developed by the
manufacturer? Why do they differ?
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The two curves overlay nearly perfectly. Very little hysteretic effects can be observed. The curve
appears to indicate a linear valve. The experimentally found characteristic curve matches the inherent
characteristic curve developed by the manufacturer quite closely. The manufacturer provides their curve
with % opening rather than the control signal in psi.
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