What are pharmaceutical fume hoods?

Fume hoods are enclosures that ventilate toxic gases, vapors, and fumes away from the working area. The fume hood's ventilation system is separated from the area or central HVAC system, with separate opening and piping infrastructure.

Due to its separate installation and working design, the vented toxic gases are not re-used with the HVAC ventilation system.

They maintain negative pressure air inside the hood by drawing air from outside the hood to into it. It effectively prevents air escape into the area, and instead, air escapes through the ventilation ducts into the environment. 

The main aim of a fume hood is to protect its user from the gases used during the process. It is only designed to pull air away from the air or environment, and do not contain any temperature-affecting components or mechanisms, so inside environmental condition is not effected or changed.

In most designs, the enclosure has a working bench where the process occurs at an average person's working height, where they sit and perform the required process. On the upper side of the fume hood, a blower is installed to blow air away from the person's working area. 

The air enters into the hood from the person's face side, flowing through the working area and exits from the upper side. Upon exiting, the air carry away any chemicals, fumes, and vapors in the working area. 

Exhaust air flows through filters to prevent hazardous and toxic components from entering the environment. The airflow can be controlled according to the requirements by adjusting the blower speed. 

A physical barrier between personnel and the processing

The use of toxic, reactive, and potent chemicals in pharma is common, and becomes more critical during the R&D and testing phase. Because these chemicals change their states and very often increase their reactive ability.

Additionally, chemicals in vapor and fumes states are more dangerous because they can be inhaled without being noticed. 

Fume hoods provide a physical barrier between the process and humans, providing a safe environment for operators to deal with the chemicals. 

The sash in the fume hood is a physical barrier between the work area and the operator. It is a transparent glass that moves vertically or horizontally, depending on the equipment design. The operator can adjust its position following safety while providing a clear view of the occurring process.

Some advantages of fume hood, include but are not limited

Prevents inhalation of chemicals 

Spilling of chemicals in the laboratory or area of installation

Face velocity in pharmaceutical fume hoods

The main factor of the fume head's performance depends on the face velocity, which is the air drawn into the fume hood. Its velocity must be optimally selected for adequate ventilation and hood operation.

The recommended face velocity of the fume hood must lie in the range of 80 fpm and 120 fpm. However, these values can also differ from application to application. If face velocities of fume hoods deviate from these recommended values, it will disturb the airflow pattern and ventilation capability of hood, and must be rectified for optimum hood performance.

If the face velocity is too much, it can cause turbulence around personnel in the working area, drawing contaminants through the sash.

Meanwhile, lower face velocity cannot capture the required contaminants from the working area, and result in compromised performance. Operator movement and door opening/closing ( in the hood's area) will further worsen the fume hood's performance under too low face velocity. 

To prevent effects from the outside world, face velocity must not deviate from standard values, by

·        Carefully selecting the fume hood’s location

·        It must be installed as far as possible from the source of airflow and disturbances, such as windows and doors. 

Components of pharmaceutical fume hoods

Some standard components of fume hoods are mentioned below

Main Body

It is the main structure on which all the components are installed. Examples of some components, include

·        Ventilation and exhaust systems

·        Blower

·        Sash

·        Exhaust ducts and ducts

Baffles

Baffles are small mechanical structures installed in the hood. Typically installed in the back of the equipment, they direct air toward the exhaust and prevent air trapping or blockage in the hood. 

For efficient performance, these baffles must not be blocked with any material, as it will affect the airflow inside the hood. 

Slots

Slots in the fume hoods are also used to maintain uniform airflow in the working area. Like baffles, they must also be kept clear. 

Sash

The sash is the movable door in front of the working area that acts as a physical barrier. The sash movement can move horizontally or vertically as per the equipment design. 

Sash is used to adjust opening, which in turn affects the face velocity of the fume hood.  The sash, in addition to the airflow system, is adjusted to provide optimum airflow conditions for optimum contamination control.

Before starting the operation, the sash position is adjusted. After, the sash position must remain in that particular position during the entire process. Otherwise, it will disturb the airflow pattern and the ability to retain contamination control during the whole process. 

Exhaust Plenum

The exhaust plenum is a mechanical structure that distributes the exhaust air toward the exhaust blower. Its design enables a constant flow through the blower without causing turbulence at the exhaust side.

Airflow system

As mentioned earlier, it is necessary to maintain the face velocity in the recommended range. Otherwise, it will affect the overall performance of the fume hood, and the main factor in maintaining the face velocity is the position of the sash. During operation, personnel constantly changes the sash position, due to many reasons, such as placing and removing items inside fume hood, which effects the airflow parameters.

To counter this effect, fume hoods are installed with variable air velocity that constantly motor the air velocity. If velocity changes, the blower is automatically adjusted to compensate for the change in air velocity.

The airflow system consists of

·        (blower) motor driver to control the speed of the blower

·        Appropriate sensors to monitor the airflow in the hood and increase or decrease the blower speed in response to the airflow. 

Air Blower

Air blowers generate the required airflow and pressure in the fume hood. They are connected to the fume hood through appropriate-sized ducting. For an efficient ventilation function, it is necessary to select appropriate blower. If not, it will affect the airflow in the fume hood.

Blowers for fume hoods are selected based on the following criteria.

Fume hood design requirements, i.e., hood size and application requirements

Duct design, i.e., duct length

Pressure drop in duct, if any

There are two configuration in which blower is used with the fume hood - built-in and external.

In the built-in blower design, the blower is attached and comes with the fume hood. They require short duct lengths, are noisier, and are suitable for less harmful applications. 

In the external configuration, the blower is not attached to the fume hood and is connected externally to extended ductwork. They are designed for positive pressure and are quieter than the in-built blower.

Common challenges related to pharma fume hoods

Let's discuss some common challenges related to fume hood that degrade its performance. 

Blower Problem

The blower is the main component of the fume hood that defines the satisfactory operation of the fume hood. It is responsible for maintaining the required airflow and ventilation. 

The blower can become problematic for many reasons, including

Problems with variable flow systems, such as sensor systems, motor problems, drive systems, and associated auxiliaries. 

Mechanical faults in the blower, such as misalignment

Vibration in the blower

These faults can be prevented by thorough inspection at pre-defined intervals. Especially when the fume hood is not in use for an extended period ( in times of no process requirement), where the problem can remain unnoticed. 

Duct Leakage and blockage

Ducts can often leak if proper material is not used in the duct construction, similarly, if exposed to harsh environments. 

Leakage in the duct can cause airflow to drop, and more blower power would be required to cover the velocity loss. Since the duct has been leaked, more velocity can also increase the magnitude of the leakage.

Another disadvantage of leakage is that contaminants can be leaked into the environment.

To prevent problems related to ducts, standard materials are always used for their construction. Similarly, tests should be continuously conducted to inspect for leaks. 

Additionally, any blockage in the ducting will also result in the blower not being able to achieve the required airflow inside the fume hood. 

Air Velocity sensors

An air velocity sensor monitor the air velocity in the fume hood, which the controller then uses to adjust the set airflow. If the sensor becomes problematic, it will be difficult to monitor the exact value and consequently adjust for change in the required airflow.

To prevent problems with the air velocity sensor, make sure it is regularly calibrated to ensure its optimum performance. If detected faulty, it must be immediately rectified or replaced.