Redundant Lab Exhaust Systems: Are You Selecting the Best Option ...

Safety. Reliability. Ensure proper design and installation of systems to keep a laboratory open and operational. Laboratories must meet these requirements. However, labs have stringent ventilation needs when compared with other applications.

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Many labs work on critical research with potential for infection or contamination of laboratory researchers. Laboratory exhaust fan systems provide the necessary ventilation containment for safety and reliability, keeping the laboratory facility in operation. Because of this, most laboratories use redundant fan exhaust systems to protect lab occupants and maintain integrity of research projects.

Redundancy in a multiple laboratory exhaust fan system refers to the ability to maintain containment and proper ventilation in certain situations. N is typically used to designate the number of fans used per fan system during Normal (N), and most common, operation. This could be 1, 2, 3, … etc. as the actual number of fans is tailored to each project application and situation. On multiple fan systems the redundancy question refers to how the system will be controlled in a situation where a fan is not operable. The choice of redundancy will impact not only the operation of the lab and the availability, sizing and performance of the fan selections.

There are three choices for redundancy selecting lab fans and determining their operation: None (no redundancy), N + 1, and N-1.

None – No Redundancy

The system design for a redundancy of None has a total airflow equally divided between the number of fans selected (Figure 1) with no provision to supplement airflow if a fan is not working. Such a system would operate with reduced system airflow in that situation (Figure 2). The benefit of this system is having a lower first cost because the fans are not oversized to compensate for the decreased airflow, nor are there extra fans to substitute if one fan becomes inoperable.

Example:

Total quantity of fans: 3
Redundancy type: None
Total exhaust flow: 12,000 cfm
Flow through each normally operating fan: 4,000 cfm

 Figure 1 – Redundancy of None during normal operation - all fans splitting flow equally  Figure 2 – Redundancy of None with single fan not operation – reduced lab exhaust volume

N+1 – Fan on Standby

N+1 is also referred to as having a fan on standby. The design of the exhaust system has one completely redundant fan (Figure 3). All fans in this system never operate at the same time. The redundant (standby) fan in the exhaust system is a fan that sits idle until needed (Figure 4). It is available in the event of maintenance to the other existing fans to prevent system downtime.

Individual fan airflow is calculated by taking the system total airflow and dividing by the number of fans minus the one on standby (in case another fan becomes inoperable). It is a preferred practice when controlling this system to keep equal run time on all fans. Therefore, the standby fan is not always the same. It is common to rotate the standby fan each week, so all fans receive consistent run time. This configuration provides 100% backup and allows the laboratory to remain fully operational. 

Example:

Total quantity of fans: 3
Redundancy type: N+1
Total exhaust flow: 12,000 cfm
Flow through each normally operating fan: 6,000 cfm

 Figure 3 – Redundancy of N+1 during normal operation - all but one fan splitting flow  Figure 4 – Redundancy of N+1 with single fan down - all but one fan splitting flow equally

N-1 – Equal Airflow but Sized to Compensate

The design of an N-1 system has all the fans operating at the same time and divides the airflow equally between each (Figure 5). However, fans are sized with the capacity to compensate for the loss of a fan and operate at 100% airflow with the remaining fans (Figure 6). This solution is quieter and more efficient during normal operation because of reduced running speed and a lower nozzle discharge velocity. A secondary advantage is that operational run time is equal for all the fans reducing the complexity of controls systems that maintain equal runtime between all fans. The N-1 redundancy option provides 100% backup and allows the laboratory to maintain operations.

Example:

Total quantity of fans: 3
Redundancy type: N-1
Total exhaust flow: 12,000 cfm
Flow through each normally operating fan: 4,000 cfm

 Figure 5 – Redundancy of N-1 during normal operation - all fans splitting flow equally  Figure 6 – Redundancy of N-1 with single fan down – remaining fans splitting flow equally

Selecting the required redundancy for a laboratory exhaust fan system depends on the laboratory type, preferences of the facility owner/operator and their level of risk avoidance. Each exhaust system design has advantages and disadvantages as illustrated in the examples. Additional considerations for control and operation of multiple exhaust fans in laboratories should include staging fans (on/off) based on exhaust demand and controlling fan flow as an energy reduction strategy.

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Laboratory Exhaust

2.1. Ventilation

Ventilation moves outdoor air into a building or a room, and distributes the air within the building or room. The general purpose of ventilation in buildings is to provide healthy air for breathing by both diluting the pollutants originating in the building and removing the pollutants from it (Etheridge & Sandberg, ; Awbi, ).

Building ventilation has three basic elements:

• ventilation rate — the amount of outdoor air that is provided into the space, and the quality of the outdoor air (see Annex D);
• airflow direction — the overall airflow direction in a building, which should be from clean zones to dirty zones; and
• air distribution or airflow pattern — the external air should be delivered to each part of the space in an efficient manner and the airborne pollutants generated in each part of the space should also be removed in an efficient manner.

There are three methods that may be used to ventilate a building: natural, mechanical and hybrid (mixed-mode) ventilation.

2.1.1. What is natural ventilation?

Natural forces (e.g. winds and thermal buoyancy force due to indoor and outdoor air density differences) drive outdoor air through purpose-built, building envelope openings. Purpose-built openings include windows, doors, solar chimneys, wind towers and trickle ventilators. This natural ventilation of buildings depends on climate, building design and human behaviour.

2.1.2. What is mechanical ventilation?

Mechanical fans drive mechanical ventilation. Fans can either be installed directly in windows or walls, or installed in air ducts for supplying air into, or exhausting air from, a room.

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The type of mechanical ventilation used depends on climate. For example, in warm and humid climates, infiltration may need to be minimized or prevented to reduce interstitial condensation (which occurs when warm, moist air from inside a building penetrates a wall, roof or floor and meets a cold surface). In these cases, a positive pressure mechanical ventilation system is often used. Conversely, in cold climates, exfiltration needs to be prevented to reduce interstitial condensation, and negative pressure ventilation is used. For a room with locally generated pollutants, such as a bathroom, toilet or kitchen, the negative pressure system is often used.

In a positive pressure system, the room is in positive pressure and the room air is leaked out through envelope leakages or other openings. In a negative pressure system, the room is in negative pressure, and the room air is compensated by “sucking” air from outside. A balanced mechanical ventilation system refers to the system where air supplies and exhausts have been tested and adjusted to meet design specifications. The room pressure may be maintained at either slightly positive or negative pressure, which is achieved by using slightly unequal supply or exhaust ventilation rates. For example, a slight negative room pressure is achieved by exhausting 10% more air than the supply in a cold climate to minimize the possibility of interstitial condensation. In an airborne precaution room for infection control, a minimum negative pressure of 2.5 Pa is often maintained relative to the corridor (CDC, ).

2.1.3. What is hybrid or mixed-mode ventilation?

Hybrid (mixed-mode) ventilation relies on natural driving forces to provide the desired (design) flow rate. It uses mechanical ventilation when the natural ventilation flow rate is too low (Heiselberg &Bjørn, ).

When natural ventilation alone is not suitable, exhaust fans (with adequate pre-testing and planning) can be installed to increase ventilation rates in rooms housing patients with airborne infection. However, this simple type of hybrid (mixed-mode) ventilation needs to be used with care. The fans should be installed where room air can be exhausted directly to the outdoor environment through either a wall or the roof. The size and number of exhaust fans depends on the targeted ventilation rate, and must be measured and tested before use.

Problems associated with the use of exhaust fans include installation difficulties (especially for large fans), noise (particularly from high-power fans), increased or decreased temperature in the room and the requirement for non-stop electricity supply. If the environment in the room causes thermal discomfort spot cooling or heating systems and ceiling fans may be added.

Another possibility is the installation of whirlybirds (whirligigs or wind turbines) that do not require electricity and provide a roof-exhaust system increasing airflow in a building (see ).

2.2. Assessing ventilation performance

Ventilation performance in buildings can be evaluated from the following four aspects, corresponding to the three basic elements of ventilation discussed above.

• Does the system provide sufficient ventilation rate as required?
• Is the overall airflow direction in a building from clean to dirty zones (e.g. isolation rooms or areas of containment, such as a laboratory)?
• How efficient is the system in delivering the outdoor air to each location in the room?
• How efficient is the system in removing the airborne pollutants from each location in the room?

Two overall performance indices are often used. The air exchange efficiency indicates how efficiently the fresh air is being distributed in the room, while the ventilation effectiveness indicates how efficiently the airborne pollutant is being removed from the room. Engineers define the local mean age of air as the average time that the air takes to arrive at the point it first enters the room, and the room mean age of air as the average of the age of air at all points in the room (Etheridge & Sandberg, ). The age of air can be measured using tracer gas techniques (Etheridge & Sandberg, ).

The air exchange efficiency can be calculated from the air change per hour and the room mean age of air (Etheridge & Sandberg, ). For piston-type ventilation, the air exchange efficiency is 100%, while for fully mixing ventilation the air exchange efficiency is 50%. The air exchange efficiency for displacement ventilation is somewhere in between, but for short-circuiting the air exchange efficiency is less than 50%.

Ventilation effectiveness can be evaluated by either measurement or simulation (Etheridge & Sandberg, ). In simple terms, the ventilation flow rate can be measured by measuring how quickly injected tracer gas is decayed in a room, or by measuring the air velocity through ventilation openings or air ducts, as well as the flow area. The airflow direction may be visualized by smoke. Computational fluid dynamics and particle image velocimetry techniques allow the air distribution performance in a room to be modelled (Nielsen, ; Chen, ; Etheridge & Sandberg, ).

2.4. Mechanical versus natural ventilation for infection control

The decision whether to use mechanical or natural ventilation for infection control should be based on needs, the availability of the resources and the cost of the system to provide the best control to counteract the risks.

For example, in the United Kingdom, the National Health Service policy tends to limit the adoption of mechanical ventilation to the principal medical treatment areas such as airborne infection isolation rooms, operating theatres and associated rooms. Patient wards are usually not required to be mechanically ventilated and natural ventilation through opening windows is usually the most common solution (Mills, ). Mills () also states that “One of the major energy users in hospitals is air treatment. The low-energy hospital study identified this as an area for saving by naturally ventilating all ‘nonclinical’ areas, and current NHS guidance has adopted this conclusion.” Conversely, in the American Society of Heating, Refrigerating and Air-Conditioning Engineers design guide (ASHRAE, a, b) all areas are required to be ventilated mechanically.

Mechanical ventilation is expensive to install and maintain in isolation rooms. It often does not deliver the recommended ventilation rate and may fail to maintain negative pressure (and may even be under positive pressure). For example, Pavelchak et al. () evaluated 140 designated airborne infection isolation rooms in 38 facilities during to and found that unwanted directional airflow out of the patient room was observed in 38% of the facilities. Primary factors that were associated with the incorrect operation of the airborne infection isolation rooms included:

• ventilation systems not balanced (54% of failed rooms)
• shared anterooms (14%)
• turbulent airflow patterns (11%)
• automated control system inaccuracies (10%).

In addition, a number of problems related to the use of mechanical ventilation can arise from the lack of active collaboration between medical and technical personnel, which can also occur with natural ventilation. For example (ISIAQ, ):

• building repair, without adequate control, may adversely affect nearby areas with high cleanliness requirements;
• sophisticated and expensive ventilation systems are often not properly integrated into the building design, and then maintained, or even used; and
• medical staff often have poor knowledge of the intended operational performance of ventilation systems, even with regard to their protective functions; systems that were originally properly designed can be misused to the extent that the intended functionality is reduced, leading to increased risks.

Other problems with mechanical ventilation include the loss of negative pressure differential in isolation rooms due to the opening of the doors; clogged filters; and adjacent, negatively pressurized spaces (Fraser et al., ; Dahl et al., ; Sutton et al., ; Pavelchak et al., ; Rice, Streifel & Vesley, ).

In response to the severe acute respiratory syndrome (SARS) outbreak, the government of Hong Kong SAR constructed 558 SARS isolation rooms with more than beds in 14 hospitals. The negative pressure, airflow path, air-change rate and local ventilation effectiveness were measured in selected isolation rooms in nine major hospitals (Li et al. ). Of the 38 rooms tested, 97% met the recommended negative pressure difference of 2.5 Pa between corridor and anteroom; and 89% of the 38 rooms tested met the same requirement between anteroom and cubicle. Although no leakage of air to the corridor was found, 60% of the toilets/bathrooms were operated under positive pressure. More than 90% of the corridor-anteroom or anteroom-cubicle doors had a bi-directional flow when the door was open. Of the 35 cubicles tested, 26% had an air-change rate less than 12 air changes per hour (ACH).

Most of these problems can also occur with natural ventilation.

A comparative analysis of mechanical and natural ventilation systems looked at eight hospitals in Lima, Peru (Escombe et al., ). Five of the hospitals had an “old-fashioned” design (built before ) and three had a “modern” design (built from to ). Seventy naturally ventilated clinical rooms for infectious patients were studied. These rooms were compared with 12 mechanically ventilated, negative-pressure respiratory isolation rooms built after . The analysis found that:

• opening windows and doors provided a median ventilation of 28 ACH — more than double the recommended 12 ACH in mechanically ventilated, negative-pressure rooms, but relies on correct door and window operation; none of the rooms were normally operated with windows and doors open; and
• facilities built more than 50 years ago, characterized by large windows and high ceilings (larger values of the volume to patient ratio), with windows and doors open, had greater ventilation than modern, naturally ventilated rooms (40 ACH versus 17 ACH).

However, these results should be used with caution. The ventilation rates in the analysis were reported without detailed information on climatic conditions, such as wind velocity and direction. The ventilation rate measurements were also affected by the carbon dioxide measurement device, and the fact that measurements were taken in buildings with multiple, inter-connected spaces, which would have affected the mixing conditions within the measured interior space.

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