BMS Field Devices & Their Functions — A Practical HVAC Guide A Building Management System (BMS) is an integrated monitoring and control system used to operate HVAC equipment, electrical systems, pumps, lighting, and other building services. Its purpose is to maintain occupant comfort, improve energy efficiency, monitor equipment performance, and generate alarms when operating conditions fall outside defined limits. In a hospital such as AKUH, a properly configured BMS is especially important for monitoring critical areas such as operating theatres, ICUs, isolation rooms, laboratories, and medical support facilities. 1. How BMS field devices work together Field sensors and input devices Temperature, humidity, CO₂, pressure, airflow and equipment status Measured signals DDC controller Reads inputs, executes control logic, compares values with setpoints Control commands Actuators and control devices Valve actuators, damper actuators, VFDs and equipment enable commands HVAC equipm...
Recommended Differential Pressure in Pharmaceutical Cleanrooms
Differential pressure (DP) is an important parameter in pharmaceutical HVAC systems. It helps control the direction of air movement between rooms and reduces the risk of airborne contamination spreading from one area to another.
The key principle: Air should generally flow from the cleaner, higher-pressure room toward the less-clean, lower-pressure room when the process requires a positive-pressure cascade. For hazardous or containment applications, the required airflow direction may be reversed.
1. Recommended pressure differential
Reference / application | Typical guidance or design approach |
|---|---|
EU GMP Annex 1 | A pressure difference of at least 10 Pa is a guidance value between adjacent rooms of different grades, where appropriate |
WHO pharmaceutical HVAC guidance | 5–20 Pa may be considered for certain applications, depending on the facility and risk assessment |
ISO 14644-4 | Pressure differentials should be designed to support the required airflow direction and contamination control; a 5–20 Pa range is commonly used as an engineering design range, not a universal requirement for every room |
These values should not be treated as interchangeable mandatory limits. The applicable edition, room classification, process risks, door arrangement and approved contamination-control strategy determine the actual design and acceptance criteria.
2. Understanding the pressure cascade
In a positive-pressure cleanroom arrangement, the pressure typically decreases from the cleanest protected area toward less-clean adjacent areas.
Illustrative example:
Grade B room
Higher pressure
+30 Pa
15 Pa differential
Grade C room
Intermediate pressure
+15 Pa
10 Pa differential
Less-clean corridor
Lower pressure
+5 Pa
Illustrative pressures relative to the same reference area—not prescribed values for these room grades. Actual room-to-room differences are determined by the approved design.
The pressure relationship matters more than an isolated room reading. Each room's reference point must be clearly defined so that the cascade can be verified.
3. How to measure and monitor differential pressure
Install a suitable differential pressure transmitter or gauge between the designated rooms or against an approved common reference.
Confirm the high-pressure and low-pressure connections are correctly connected.
Verify calibration, tubing integrity, zero reading and instrument range.
Record readings with doors closed under the defined test conditions.
Test the pressure cascade with the HVAC system operating in its specified normal mode.
Verify alarms and recovery after door opening, where required by the approved qualification protocol.
For pharmaceutical facilities, the pressure monitoring system should be calibrated and connected to alarms where appropriate. WHO guidance also emphasizes avoiding overlapping acceptable pressure ranges that could permit an unnoticed reversal of airflow.
Fifty-third report
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4. Common causes of low or reversed differential pressure
Problem | Possible cause |
|---|---|
Low positive pressure | Insufficient supply airflow or excessive exhaust/return airflow |
Negative pressure in a positive-pressure room | Fan failure, incorrect damper position or excessive extraction |
Unstable DP reading | Door movement, fluctuating airflow or faulty transmitter |
Pressure cascade reversed | Incorrect airflow balance, open doors or control failure |
Excessive pressure | Poor pressure relief or inadequate transfer-air path; doors may become difficult to open |
5. Practical engineering recommendation
For a pharmaceutical cleanroom, use the following approach:
Establish the pressure cascade from the contamination-control strategy and room classifications.
Select the design differential using the applicable GMP guidance, risk assessment and approved project requirements.
Define operating limits, alarm thresholds and allowable instrument tolerances.
Verify the cascade during commissioning and qualification.
Recheck it after HVAC balancing, filter replacement, room modifications or changes to supply and extract airflow.
Standards clarification: WHO's interpretive guidance explicitly discusses considering 5–20 Pa, while EU GMP Annex 1 identifies at least 10 Pa as a guidance value between adjacent rooms of different grades where appropriate. ISO 14644-4:2022 covers cleanroom design, construction and start-up but should not be cited as imposing a universal 5–20 Pa requirement.
The correct differential pressure is therefore the validated, risk-based setpoint that maintains the intended airflow direction and contamination control, not simply a number applied identically to every cleanroom.
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