Understanding Negative Pressures and Their Impact on overall Ventilation System

As designer firm of industrial ventilation system and being responsible for maintaining and optimising the operation of ventilation systems, it is crucial to be aware of the complex interplay of pressures within buildings especially when the ventilation systems being constructed dictates the pressure on the enclosures. Among these, the negative pressures being generated by the ventilation system often goes unnoticed but can lead to various unsatisfactory or even hazardous conditions. This blog entry aims to shed light on the concept of negative pressures and their potential impact on objectives of deploying ventilation systems.

I. The Basics of Pressure

Before delving into negative pressures, let's review the fundamentals of pressure within buildings:

1. Positive Pressure: This occurs when there is an excess of air supply relative to air exhaust within a space. Positive pressure can be employed to prevent the infiltration of contaminants, allergens, or unwanted outdoor air. The positive air will become the drive for exfiltration. From the aspects of energy balance, the supply flow rate will be the dominating one being higher than the exhaust (if there is any exhaust service constructed).

2. Neutral Pressure: When the supply and exhaust of air are balanced, a neutral pressure environment is achieved. This is ideal for maintaining consistent quality of air within the enclosure.

3. Negative Pressure: Negative pressure results from an insufficient supply of air relative to the exhaust. This is usually due to failure realising the impact of having exhaust ventilation alone without make up air. The excessive pressure variance will create significant amount of infiltration, air tightness, scarcity of air for exhaust fan to operate at the highest efficiency and etc. ergens, or unwanted outdoor air. The positive air will become the drive for exfiltration. From the aspects of energy balance, the exhaust flow rate will be the dominating one being higher than the supply (if there is any make up or supply service constructed).  

II. Causes of Negative Pressures

Negative pressures can be caused by various factors, including:

1. Exhaust Systems: Local Exhaust Ventilation Systems in occupational settings or other areas with imminent air borne hazards can create negative pressures if not adequately balanced with supply air. A good practise is always have a pressure variance of 15% between the two services. Any variance above this range will have significant impact on infiltration of exfiltration of the air.

2. Improper Ventilation Design: Inadequate ventilation, or improperly designed ventilation systems, can lead to negative pressures especially when the boundary of the enclosure and the energy balance is not properly defined. The improper balance of pressure and poor planning on air distribution will cause accumulation of stale air, stuffiness creating very uncomfortable and unsafe working environment.

3. Leaks and Envelope Issues: Building envelope leaks or breaches can allow outdoor air to infiltrate spaces, creating negative pressures. This can result from gaps in windows, doors, or damaged building materials. Excessive negative pressure will also cause issue to open doors.

4. Improper addition of LEV system : Addition of Local Exhaust Ventilation System without checking the impact of the overall flow rate Operating HVAC systems at high fan speeds or addition of excessive amount of LEV systems can create excessive negative pressures, particularly in small or closed spaces.

III. Impact of Negative Pressures

Negative pressures can give rise to a range of unsatisfactory conditions within buildings:

1. Backdrafting: In process areas with emission of air borne hazards of chemicals with high vapour pressure (evaporate easily), negative pressure can cause exhaust vapours or gaseous from point of generation to be drawn into other work areas spreading the occupational health risk from point source to other areas sporadically.  

2. Contaminant Infiltration: Negative pressures can pull in outdoor pollutants, allergens, or even foul odors, compromising the safety of occupational settings. This can impact occupational health and product contamination too.

3. Temperature and Humidity Variations: Improperly balanced pressure can result in temperature and humidity variations, leading to discomfort for occupants. Apart from that imbalance in cooling will great affect the function of the HVAC system in providing controlled temperature and humidity witting an enclosure.

4. Excess Energy Consumption: The excessive workload based on addition of LEV system without checks will generate high flow rate exhaust from any enclosure and excessive internal draft. The extreme negative pressure will force the fan to operate harder to overcome the air tightness. This will incur more electricity as the fan will be running at higher amps. Apart from that, the extreme infiltration will also increase the cooling load resulting to overall higher energy consumption.

5. Air tightness : Excessive pressure will cause difficulties in opening doors or any openings. Extreme negative pressure will not only impact fan performance and generate excessive cooling load, it will also create difficulties to access to the extreme negative pressure enclosures. For instance at 0.05 to 0.1 inwc of negative pressure, the doors will be very difficult to be opened.

6. Poor performance of other fans : Excessive negative pressure will create a scarce air in the enclosure causing the operation of other fans incorporated in the same enclosure not to meet the actual fan curve performance. This is because without the air, the fan will not meet the duty point. The air tightness will increase the static pressure losses creating imbalance to the overall design objectives.

IV. Mitigation Strategies

As ventilation system designers, it is our responsibility to identify and mitigate negative pressures to ensure optimal indoor environments:

1. Balanced Ventilation Systems: Ensure exhaust systems are properly balanced with supply systems to prevent negative pressures. A clear derivation of pressure variance in terms of percentage between positive and negative pressure must be made. Apart from that, the overall flow rate and draft in the room generated by the dominating flow rate must not be excessive. To achieve this the system designers must be well versed in various implications of their design basis.

2. Sealing and Maintenance: Regularly inspect and maintain building envelopes, including windows, doors, and structural components, to prevent air leakage either via infiltration and exfiltration.

3. Placement of diffusers and exhaust registers based on desired air flow pattern covering entire enclosures or contaminant pathway : Improper deployment of number of ventilation systems and poor placement of air intake and exhaust registers will create a non homogeneous air flow into an enclosure. A designer must work on a design based on a basis of air movement either to dilute or remove contaminant or heat or any other form of the ventilation system’s objective. Setting a right flow rate but making the wrong selection of hood or air registers place meant will never enable the achievement of the system’s objectives.

4. Performance monitoring and air quality monitoring: Deployment of differential pressure gauges are crucial to control the negative pressure in an enclosure. Upon making a good design, the performance in the aspect of room pressurisation must be established. Any imbalance in pressure should trigger for correction action. On top of this action, implementation of air quality monitoring systems to detect and address issues promptly can also be established.

Understanding the impact of negative pressures is fundamental to our role as ventilation system designers in maintaining satisfactory occupational settings. By adding room pressurisation as one of the most important element in ventilation design, we can ensure that occupation settings are safe, comfortable, and energy-efficient. Moreover, our work contributes to the overall well-being and productivity of building occupants, underscoring the importance of managing pressures within structures.