1 Understand the Basic concept of BMS

system
2 What is a BMS or Building Management System?
In a nutshell, BMS-System otherwise called as BAS or building automation
is computer-based control system which reduces the workforce, automate
the system, and saving the energy consumption in buildings by monitoring
and controlling the mechanical and electrical equipment in modern-day
buildings or any industrial plants.

Not only that but BMS helps to

 Increasing productivity.
 Increase the equipment lifetime and better performance.
 Identify the systems faults earliest.
 Manag the hotel tenants in an effective manner.

Nowadays any modern-day buildings built with BMS to support facilities

management to accomplish the maintenance and save the energy in the
building from one place of computers.

3 Essential Features of BMS software

 monitoring and controlling connected equipment in the building.
  The alarm should be a popup in operator workstation for any
critical faults in the system.
 Any types of equipment on, off status and alarm should be logged
or stored in PC to retrieve later.
 Schedule the equipment to on and off automatically by preset
time.
 User interface graphics should be available to visualise the field
equipment to monitor for BMS operator easily.

4 List of Equipment controlled by BMS or BAS in buildings

 HVAC (Heating, Ventilation, and Air-conditioning or all supply and
exhaust fans, ACs etc.).
 Lighting control system.
 Fire alarm system.
 Firefighting system.
 Security control system.
 CCTV system.
 Lift control system.
 Pumping system.
 Water tanks level.
 Irrigation system.
 Electrical meters.
 Water Leak detection system.
 Split units.
 VFD-Variable frequency drives.
 VRF/VRV-Variable refrigerant flow or volume (both are same but
each term copyrighted by a different vendor)
 And any other system which has provision for BMS to control and
monitor.

5 Main components of the BMS System

1. Hardware
 DDC-Direct digital controller
 Sensors
 Actuators
 Cables to connect sensors, actuators to DDC.
 HMI display-Human machine interface.
 PC Workstation
 Server to save the extensive database.
2. Software
  Programming or configuration tools.
 Graphics or User interface.
3. Networking protocols
 TCP/IP– Transfer control protocols/Internet Protocol.
 BACnet– Building automation controller network-ASHRAE
 Modbus
 LonWorks
 CANbus
 and numerous protocols available.
Don’t worry about the various protocols, this all protocol doing the same
task to transfer data from one device to another device. anyhow we will
discuss these each protocol in an upcoming post

6 BMS System architecture in the modern-day building

However, BMS System controls and monitor all the electrical and
mechanical systems in buildings from BMS workstation or HMI(Human
Machine Interfaces),

But not directly because each system has its dedicated functionality and
unique purpose like,

 HVAC System helps to facilitate and provide comfortable and

healthy air conditioning to tenants.
  The lighting control system which has a variety of lightings in
buildings that needs to be on and off effectively and save energy
while tenants not available.
 CCTV helps to facility management to secure the building
 Access control systems may also be used to control access into
certain areas located within the interior of buildings.
 A fire alarm system is the life safety system to warn people by
audio and visual to protect their lives from fires, smoke, carbon
mono oxide and other toxic elements for the human.
 In case of fire Firefighting system aims to protect human life and
property in the building by a large amount of water and other
gas.
 UPS is to provide to the uninterrupted power supply in the
building for electrical equipment.
 Pumping system used in the building to pump the water to the
required area.
 Still tons of systems evolved in the modern-day building to
facilitate the people.
All systems have their dedicated controllers and processing system due to
the different functionality of each system.

So BMS controllers or device designed for controlling and

monitoring the HVAC system and other small systems and integrate
all other systems through dedicated networking protocols like BACnet,
Modbus etc.

7 General BMS System architecture with Levels

  Management Level: This is the front end for the operator and
engineer used to visualise the graphics for controlling and
monitoring the systems which have computer workstation,
server, web browser, printers.

 Automation Level: BMS Router and other main controllers

connected in building network integrate the third-party system
and connect BMS devices.

Field devices Level: this is Level where BMS controllers connect

to field systems sensors, actuators, and other panel circuits to
monitor and control.
8 Real-Time example for BMS System

Any modern-day building client provides huge specifications for BMS

System,

Whereas here I am going to take simple requirement to monitor and control

the sequence of Air Handling Unit.
Let us see below the requirement of the client to monitor and control the
sequence in BMS System.

Before we go detailed about how to design the BMS System for the
requirement, let us see some basics components of the AHU-Air handling
unit.

AHU is an HVAC system which consists of the duct, fan, filter, cooling coil,
heating element,humidifier, sound attenuators, dampers, valves and many
more to regulate the air into the room by heating, ventilation and
conditioning to distributes the conditioned air through the building and
returns it to the AHU and also called as centralised AC in modern-day
building.

Duct – It is the collection of metallic tubes that interconnected and

distributes the heated/cooled air to the required rooms.
To monitor the duct air temperature in fresh, return and supply duct. We
have to install the duct temperature sensor in the duct.

Fan Motor– Blower is used to circulate the air from fresh and return duct
to the supply duct.

This fan motor controlled and monitored by the separate electrical panel by
the designed electrical circuit with the help of electrical relay
and contactor and providing an option to BMS system to
 On/Off the fan.
 Monitor the fan running status.
 Monitor the Fan motor overload fault status and many more.

Filter– It is one of the main components in AHU to prevent the dust and
dirt particles from entering in the AHU.
When the AHU fan motor started, the fresh outside air supplied into the
duct where filter components used to filter the dirty particles continuously
and to monitor the filter extreme dirty condition,

DPS switch is used to install across the filter and provide signals to BMS,

when the filter gets dirty(technically DPS-Differential pressure switch will

send the signal to BMS when the pressure reached more than pre-set
across the filter, and this same function can be used to monitor the fan
status.

Read More about How DPS used to monitor fan and filter status.

Heating/Cooling element- It is used to cool or heat the water that entered

in the coil so that air in the duct can be heated or cooled based on the user
requirement.

Either heating or cooling water enters into the coils are controlled and
monitored by valves on the pipe with the help of the valve actuator.

Dampers- An HVAC damper is a movable plate, located in the ductwork,

that regulates airflow and directs it to areas that need it most.

damper opening and closing position-controlled electrically with the help of

damper actuators, and these actuators have terminal for control from BMS
and terminal to monitor the feedback of position.

9 System Description
The variable speeds Air Handling Units are used to serve air conditioning
need for all area of
buildings
10 The Air Handling Unit comprises
 Variable Speed Supply Fan
 Chilled water coil with the 2-Way modulating control valve
 Duct mounted supply air pressure sensor
 Outdoor & re-circulating air modulating damper
  Carbon dioxide sensor.
 Supply and Return Air temperature sensors
 Supply air differential pressure switch
 Differential pressure switches for 2 set of filters
11 System Monitoring and Alarm
1. Software alarms shall be generated at the operator workstation
whenever the run status of the supply fan (with differential
pressure switch) does not match the current command state.
2. A failure alarm shall occur when the run status of the load shows
no operation, and the load has been commanded to be on.
3. An advisory alarm shall occur when the run status of the load
shows operation and the load has been commanded to be off. All
alarms shall be recorded in an alarm log for future review.
Provide 15 seconds (adjustable) time delays before generating an
alarm.
12 The sequence of Operation
a. Auto Mode:
When the AHU start is in AUTO mode (i.e. selector switch installed in the
MCC must be in Auto Position), the unit is started and stopped from the
BMS via a time schedule or BMS override command. When the start for the
AHU is initiated, the control program residing in the controller follows the
following sequence
Start-Up:
The following sequence follows with a preset time interval per interlock
equipment start-up:
1) Check Supply fan trip signal – Normal State
2) Supply Air Damper –Open Position
3) Outdoor Air Damper –Open Position
4) Return Air Damper – Open Position
5) Once the above conditions are satisfied, AHU is enabled to start in Auto
mode or using a plant enable button on the graphics in manual mode by the
operator. Once enabled, BMS will automatically command the supply fan to
start.
6) Supply Fan shall start, and it’s associated Interlock equipment in
sequence. Through the signal from the Diff. Airflow Switch, if airflow is
detected, the System will continuously run, if No airflow is detected by the
DP Switch, the Supply Fan will de-activated and send an Alarm to the DDC
– for “No Airflow” and shut down the whole system including its associated
interlocks. If the Airflow switch signal is proved ‘ON’ then BMS will enable
control loops.
b. Shutdown Mode:
When the shutdown command for the AHU is initiated, the control program
residing in the
controller follows the following sequence.
1) Send Stop command to stop the supply fan
2) The outdoor air, return and supply air damper move to close
3) Move chilled water valve to close position
b. Manual (Hand) Mode:
When the AHU is the manual mode, the fans are started and stopped from
the AHU control panel. Other control except for fan on/off control shall
function as per the Auto mode.
c. Fire / Smoke Mode:
Fire condition is determined by the Fire Alarm Control Panel. AHU will
automatically shutdowns the whole system with associated interlocks.
4. AHU Control
The control program, on the feedback of air handling unit operation,
initiates the control
algorithm. This algorithm consists of three controls. Each temperature,
pressure and ventilation control has its own control loop. The pressure
control loop is used to modulate the speed of the supply air fan hence
supply airflow. The control loops design to function as per the following
explanation
a. Temperature Control loop:
The supply air temperature installed in the duct will relay the measured
signal(temperature) to the DDC controller, the DDC controller compares this
signal with set-point (adjustable by the operator from BMS central) and
generates an analog output to the 2-way modulating cooling valve. Based
on the difference between the two values, a proportional-integral program
will determine the percentage of the cooling coil valves opening to achieve
the desired condition. The default set-point value for the supply air
temperature is 13ºC (Adjustable).
b. Pressure Control loop:
The supply air pressure sensor shall be installed in the duct will relay the
measured signal (static pressure) to the DDC controller, the DDC controller
compares this signal with the set-point (adjustable by the operator from
BMS central) and generates an analog output to the variable frequency
drive (VFD) of the supply air fan. Based on the difference between the two
values, a Proportional-Integral program will determine the percentage of
the fan speed to achieve the desired pressure. The set-point value for the
supply air pressure for each AHU shall be adjusted.
c. Ventilation Control loop:
Demand control ventilation employs return air carbon dioxide controlling
strategy.
A single carbon dioxide sensor sense carbon dioxide concentration in the
return air duct and sent to the DDC controller, the DDC controller compares
the signals with return air carbon dioxide concentration (Default carbon
dioxide level difference value 400 ppm ).
Then DDC controller generates an analogue output to the outside air
dampers and returns air damper to modulate, based on the difference
between the values, the Proportional integral program will determine the
percentage of the modulation of outdoor and return air dampers.
Minimum outdoor air quantity shall be governed either by building
pressurisation requirement (Input from Building differential pressure
sensor) or 20% of the Maximum outdoor demand of the AHU.
5. Alarms:
The following minimum alarms shall be generated on BMS
1) Filter Dirty Alarm: This is generated when the pressure drop on each
filter exceeds the set value to indicate dirt accumulate at filters.
2) Fan Trip Alarm: A normally open “NO” volt free contact at the MCC panel
when closed will generate an alarm at the BMS indicating that the fan is
tripped
3) Fan Fail: In case the supply air fan fails to start or if the differential
pressure switch across
the supply fan is not giving the signal according to the command due to any
reason then an alarm shall be generated. In case of a fan fail alarm on the
BMS, due to abnormal behaviour, the DDC controller will latch the alarm.
The operator has to acknowledge (reset) the alarm on the BMS once the
trouble has been checked and removed. The operator shall not be able to
start the AHU until the alarm s acknowledged and reset.
4) Temperature High & Low: Temperature HIGH and LOW alarms shall be
generated if the supply/return air temperature rises above or falls below the
supply /return air temperature alarm limit.

13 A list of Input and output points are required for the above-
discussed sequence of operation for AHU
Some basic terms of digital electronics

 Analog Input: Analog inputs can come from a variety of sensors

and transmitters. You can measure a whole bunch of different
things. The job of the sensor or transmitter is to transform that
into an electrical signal. Here are a few of the things you can
measure with analogue sensors:
 Level
 Flow
 Distance
 Viscosity
 Temperature
 Digital Input: It allows a microcontroller to detect logic states
either 1 or 0, otherwise called as VFC-Volt free contact.
 Analog Output: In automation and process control applications,
the analogue output module transmits analogue signals (voltage
or current) that operate controls such as hydraulic actuators,
solenoids, and motor starters.
 Binary Output: it is nothing but relay output from the controller to
trigger on and off any equipment.
Now it’s time to choose the DDC controllers based on the above input and
output point list.
Any BMS controllers manufacturer must have the basic controllers types of
analogue input-output, binary input, and output controllers either dedicated
controllers or mixing of all types in a single controller.

for the above applications, we need to choose controllers that should

accommodate 17 AI, 6 BI, 5 AO, and 1 BO(Note that temperature and
humidity are two different analogue inputs)

Once controllers are designed, we need to calculate the power load for each
controller (available in controller datasheet) and field devices to choose the
right transformer rating for our DDC panel.

The next things are to write a program for our controllers to accomplish the
above sequence,
First, we need to change English words into the flowchart then we can
change it later on to the different programming languages that required for
BMS vendors either ladder logic or functional block or plain English and etc.

whatever it is any BMS program functionality that will not go beyond the
basic digital logic gates.