Understand the Basic concept of BMS system
In a nutshell, BMS otherwise called as BAS or building automation is computer-based control system which reduces the manpower, automate the system, and saving the energy consumption in building 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.
Any BMS software or system must provide the following facility to the operator
- 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.
- Scheduling the equipment to on and off automatically by preset time.
- User interface graphics should be available in order to visualize the field equipment to monitor for BMS operator easily.
BMS or BAS system
monitor and/or controls the following system 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.
- UPS 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.
Main components of the
BMS System
· DDC-Direct digital controller
· Sensors
· Actuators
· Cables to connect sensors, actuators to DDC.
· HMI display-Human machine interface.
· PC Workstation
· Server to save the large database.
- Software
- Networking protocols
· Graphics or User interface.
· TCP/IP– Transfer control protocols/Internet Protocol.
· 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.
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 own 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 its own 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.
General BMS System
architecture with Levels
- Management Level: This is the front end for operator
and engineer used to visualize 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 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.
Simple 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.
In order 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.
- 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 to enter 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 in order to monitor the filter extreme dirty condition,
DPS switch is used to install across the filter and provide a signal 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.
Now we Read 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 help of 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 this actuators have terminal for control from BMS and terminal to monitor the feedback of position.
System Description:
The variable speeds Air Handling Units are used to serve air conditioning need for all area of buildings
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
System Monitoring and
Alarm:
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:
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 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 Air flow switch signal is proved ‘ON’ then BMS will enable control loops.
b. Shutdown Mode:
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
c. 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.
d. Fire / Smoke Mode:
Fire condition is determined by the Fire Alarm Control Panel. AHU will automatically shutdowns the whole system with associated interlocks.
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 air flow. The control loops design to function as per 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 pressurization requirement (Input from Building differential pressure sensor) or 20% of the Maximum outdoor requirement of the AHU.
Alarms:
1) Filter Dirty Alarm: This is generated when 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
supply
fan is not giving the signal according to the command due to any reason then
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.
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 analog 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 its 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 mixed 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 input)
Once controllers are designed, we need to calculate power load for each controller (available in controller datasheet) and field devices to choose the right transformer rating for our DDC panel.
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 the different programming language 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.
Flowchart for AHU
Control sequence of operation
