BMS Control Strategies for Enthalpy and Air Quality

The assignment requires the identification of control strategies to be adopted by the BMS for enthalpy control and air quality control. It also involves managing conflicting demands between efficient space heating and maintaining good air quality. The solution proposes adjustable fan speed, prioritizing energy efficiency over ventilation requirements, and using a psychometric chart to maintain humidity levels. A detailed description of the constraints and how the BMS outstation/controller serves the AHU is provided.

MSc Building Services Engineering – Building Management Systems
Brunel
University
MSc in Building Services
Engineering
Building Management and Control
Systems
Building Management Systems Assignment
2017/18
Part 1
Part 1.1
Indicate in a table how you would specify the inputs and outputs to achieve effective control
of the AHU by the BMS. An example of the type of table required is illustrated below.
Controller
interface
Points
Field device e.g.
sensor type/actuator etc.
Input value or control action
Universal Input 1 Inlet Low temperature sensor Detecting low temp values and activate the actuator
Universal Input 2 High temperature sensor Detecting high temp values
Universal Input 3 Hot water Flow sensor Excites flow in the hot water line
Universal Input 4 Volume sensor Detects volume of air allowed in the chambers
Universal Input 5 Voltage controller Controls source voltage hence optimized performance
Universal Input 6 AHU damper inlet actuator Regulates the amount of air entering damper
Universal Input 7 Air pressure control at inlet Regulates air pressure at the inlet
Universal Input 8 Self adjusting valve actuator Automatic closing and opening of valves
Universal Input 9 Chilled water flow sensor Excites the flow in the chilled water line
Universal Input
10 Fan actuator Allows fan to draw in air when required
Analogue Output
1 Voltage sensor in input 1 Regulated current to sensor 1
Analogue Output
2 Voltage sensor 2 Regulated current to sensor 2
Analogue Output
3 Voltage sensor 3 Regulated current to sensor 3
Analogue Output
4 Voltage sensor 4 Regulated current to sensor 4
Analogue Output
5 Voltage sensor 5 Regulated current to sensor 5
Analogue Output
6 Voltage sensor 6 Regulated current to sensor 6
Analogue Output
7 Voltage sensor 7 Regulated current to sensor 7
Analogue Output
8 Voltage sensor 9 Regulated current to sensor 9
Part 1.2 (10%)
This AHU is used to implement an optimum start and stop energy saving strategy for the
building. Describe how this would be implemented and how the BMS outstation/controller
would interact with the AHU, i.e. the sensor data used, the control actions over the course of
a working day in the winter months (northern hemisphere) to implement this strategy. It is
important to also describe the way in which the BMS outstation/controller might be
programmed to allow it to decide when to operate plant.
Optimum performance is often desired such that the system is only used when needed. And
this is implemented as follows:
- During extreme weather conditions, when temperatures are very low, the low temperature
sensor detects this change and transmits signal to the BMS. Necessary control actions

MSc Building Services Engineering – Building Management Systems
such as sending of information to the high temperature sensor to allow more heat and air
into the space are activated (Electronic design, 2018).
Part 1.3 (5%)
A key field device in this BMS is the temperature sensor. A typical sensor type used in this
application is the platinum resistance thermometer (PRT) that can be used in the form of a
probe into the AHU or the associated ductwork.

MSc Building Services Engineering – Building Management Systems
The relationship between temperature sensed and resistance generated for a PRT can be
defined by the following equation over a limited range:
Rt= Ro(1+αt)
R0 = Resistance at 0°C (Ω) given as 100 Ω
α = temperature coefficient for platinum (K-1)= 3.9 x10-3
t = temperature sensed (°C)= 109oC
If the PRT is defined as a PT100 (Resistance at 0°C = 100 Ω) and the value of temperature
coefficient for platinum is 3.9 x 10-3 per degree K.
If a PRT PT100 sensor is used to measure the air temperature in the return air duct of the air
handling unit and it reads 109 Ω at the sensor, what is the air temperature in the duct as
measured by the PRT to the nearest degree Celsius?
Solution:
We simply substitute in the given equation:
Rt =100(1+3.9x10-3 x 109)
= 143oC
Part 4
The dampers used to divert air in the AHU would be operated by an actuator controlled by
the BMS outstation/controller. List and describe the elements of the specification that you
would consider in selecting a suitable actuator to control the inlet air damper.
-The damper inlet valve must control air inflow appropriately and in real time fashion
-

MSc Building Services Engineering – Building Management Systems
Figure 1

Part 2.1
(7%)
Part 2 (25%)
The central air-handling unit (AHU) is in a plant room. Analogue signals are transmitted
using cables between sensors and the BMS outstation/controller located within this plant
room. What is the appropriate form of topology for :
I. Sensor cabling between BMS outstation/controller and the various sensors
-Interoperability
Figure 1: Bus topology for interoperability
II. Cabling between this outstation/controller and another outstation/controller
elsewhere in the building controlling lighting.
-Linking optimization and modulation
Part 2.2
(9%)
How does the risk of signal error arise in the transmission of these analogue signals, and what
can be done to minimize these errors?
-Error in transmission occurs during modulation, transit and demodulation. There is need to
harmonize these three phases in transmission so that frequencies are matched.
-During modulation, the transmitted data becomes more vulnerable to distortion due to
external disturbances such as noise (Sherman and Walker, 2009)
Part 2.3
A sensor is installed to measure the outside air temperature with an accuracy of ± 0.5 C. The
BMS will convert the analogue signal from this sensor to a digital signal. The temperature
measurement is limited to a range of -10 to 50 C.

What is the minimum number of digits required to represent the temperature value while
ensuring that the error band is less than 0.5 C?
The uncertainty in this case would be: 0.5/(50+10)= 0.00833 approximately 0.008
Hence there will be at least 4 digits (including the whole number part)
Part 3
The user of the BMS wishes to log the outside air temperature data from the sensor Part 2.3
above. The temperature values are digitally encoded using the number of bits calculated in
Part 2.3.
Part 3.1
Suggest, with a justification, a suitable sampling interval for the outside air temperature data.
-A suitable interval would be 0.005 given the calculated uncertainty
Part 3.2 (5%)
If the data were stored as digital information in bytes, what is the minimum number of bytes
of memory capacity required to store the data for 30 days?
= 30 x 0.005= 0.15GB
Part 3.3 (5%)
If one hour’s data were transmitted using a simple Hamming Code, how many check bits
would be used?
0.15x1/(30x24)x1000= 0.2083, hence 4 check bits

Part 4 (15%)
To control the capital costs, the BMS (of which the AHU outstation controller forms part)
is required to share data cabling with other ICT systems in the building.
Part 4.1 (10%)
Describe the steps you would take in specification of the BMS components to ensure
interoperability with the other systems making careful references to existing international
standards, protocols, etc. You will need to demonstrate your knowledge of one of the leading
protocols in reasonable depth.
-A good example is the supervisor to supervisor protocol in which at the level of
management, the distinct management systems interact. The standard used in this case is the
BACNet
- Normally it is used to manage communication sessions such that departmental exchanges are
facilitated and prompt response to emergency such as fire outbreak is appropriately
coordinated. These exchanges of information would normally be in the form of multiple back-
and-forth transmissions between two nodes.
Part 4.2
What approach would be taken to cabling in the building if was to be constructed on a
university campus with an existing BMS and ICT network. You should consider and describe
the approach taken with reference to current international standards.
The major functional approach would be interoperability where the systems are designed to
complement each other in a manner that optimizes the overall performance.
Part 5.1
Part 5
If the BMS were to be required to provide enthalpy control of the conditioned space what
control strategies would be implemented. Include in your response a detailed description of
the typical constraints and how the conflicting demands of enthalpy control and air quality
control would be managed by the BMS outstation/controller serving the AHU in Fig 1.
(Assume that the supply and extract fans are controlled by variable speed drives).
The following are the control strategies to be adopted:
- Ventilation air quality control
- Relative humidity within the designated space
Notably, these two strategies somehow present a conflicting scenario in which efficient space
heating required would mean less amount of air is to be allowed by the damper inlet;
However, to increase the air quality, more air would be needed to maintain the inside
conditions. This then requires some sought of balance between the two strategies and it is
achieved as follows:

- The fan speed is made adjustable so that it can admit either less or more air per unit time.
Normally, the set points are established prior to the building enthalpy control, where the
exact objectives are formulated. The BMS would calculate the ventilation ratio such that if
found to be lower than the set point value, a signal is sent to the fan to increase its speed for
more outside air to be allowed. However, at times enthalpy control could be prioritized
where energy efficiency overrides the ventilation requirements.
Continuous humidity maintenance is facilitated by the psychometric chart embedded in the
system. This chart provides real-time indication on the humidity situation of the inside air
and signals are sent to the humidity sensors for appropriate actuation. This works together
with the temperature sensor (HVAC, 2018).
Lastly, the BMS would calculate the ventilation ratio which is the ratio between the outside
to inside air in a bid to optimize the quality of air within the space and ensure it is consistent
with the humidity and temperature conditions.
References
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Electronic Design. Available at:
http://www.electronicdesign.com/communications/understanding-modern-digital-modulation-
techniques [Accessed 13 Mar. 2018].
Goleniewski, L. (2018). Analog and Digital Transmission | Telecommunications Technology
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http://www.informit.com/articles/article.aspx?p=24687&seqNum=5 [Accessed 13 Mar. 2018].
HVAC, H. (2018). DDC Control Module for an Air Handling Unit | HVAC Control. [online]
High Performance HVAC Heating & Cooling. Available at:
https://highperformancehvac.com/ddc-control-module-air-handling-unit/ [Accessed 13 Mar.
2018].
Knx.org. (2018). KNX Association - KNX Association [Official website]. [online] Available at:
https://www.knx.org/knx-en/index.php [Accessed 13 Mar. 2018].
Reginald, A. (2015). Integrating BIM with BMS in Energy Performance Assessment.
International Journal of 3-D Information Modeling, 4(1), pp.19-44.
Sherman, M. and Walker, I. (2009). Measured Air Distribution Effectiveness for Residential
Mechanical Ventilation. HVAC&R Research, 15(2), pp.211-229.
Users.cs.fiu.edu. (2018). Calculating the Hamming Code. [online] Available at:
https://users.cs.fiu.edu/~downeyt/cop3402/hamming.html [Accessed 13 Mar. 2018].
YouTube. (2018). AHU(air handling unit) working through BMS system (hvac stream). [online]
Available at: https://www.youtube.com/watch?v=DaCmHEjz6p4 [Accessed 13 Mar. 2018].