1. Enclosed / Semi Enclosed Hood

The most important part of designing a laboratory ventilation is to identify the airborne hazard by listing the chemicals used ranging from reagents and analytical chemicals which can become airborne and cause exposure risk to worker’s health. Once the chemical and the process has been identified snd analysed, the choice of hood must be decided based on the dispersing energy profile and the toxicity of the suspended hazard. There are two type of hoods or two form of geographical shapes can be used to remove contaminant from the laboratory settings, which are enclosed hood and exterior hoods. In the laboratory settings fume hood or the right way to name it is laboratory hood (as not all hood control fumes, in fact laboratory settings hardly generates fumes but emits more of vapour and gaseous). The list of enclosed hoods apart from laboratory hoods are :-

1. Laboratory Hood (4ft, 5ft, 6ft, 8ft, customised)

2. Biological Safety Cabinet

3. Dry Box or Glove Hood

4. Horizontal and Vertical Laminar Flow (It is for sample protection and not to be sued for workers protection).

5. Oven Exhaust

Apart from the enclosed hood, exterior hoods are also used in the laboratory settings which are as follows :-

1. Extension Arm

2. Canopy

3. Specialised Laboratory Hood Design (customised usage of face area for removal of contaminant at storage and test areas).

1.1 Laboratory Hood features 

The laboratory hood which is the common choice of hood used mostly must have aerodynamic air entry characteristics. To minimise turbulence at the entry and to have high efficiency in removal, the air entry must be fluid and smooth. Airfoil sill and beveled jambs are profile on the hood which can enhance a smooth air flow into the enclosure. These features are important not only to reduce the hood entry coefficient but also to ensure the turbulence at the entrance of the face areas are also minimised. 

Baffle plate / slots at the plenum is very crucial to generate homogenous air entry at the face of the hood. A plenum is a pressure equalisation chamber. The equal pressure enclosure is fixed with a baffle plate to create a slot kind of gap in the deep end of the laboratory hood to force uniformity of flow along the length of this gap. This uniform velocity which is negative pressure will create homogenous air displacement in the laboratory hood enclosure extending the pattern till the face opening at the sash segment. This makes the face opening and the cross section of hood to be effective in removing the airborne emission at the point of generation, conveying it homogeneously into the then ducting towards the air cleaning device. 

1.2 Make up / Supply Air Distribution. 

One of the important criterion in designing the LEV hood and defining the capture velocity will be the room draft which is governed by the overall volumetric balance in room. As such, the design of any LEV system in the laboratory must take into consideration of the General Ventilation features which are as such :-

1. As apart of preliminary works in gathering design info, measure the room draft and determine the room pressurisation.

2. Add general ventilation with 10~15% variance in room pressurisation to determine the amount of air supply / make up to be delivered in the room. A LEV system without a supply air system will have reduced efficiency due to the lack of air caused by the vacuum state the dominating exhaust air flow has created.

3. Design the placement of the exhaust grilles closer to the floor arranged through out all angles of an enclosure to ensure the room air currents covers the entire cross section of the room. The supply air diffusers are to be dropped at balance location feeding the air to the spread out exhaust grills. Proper usage of K factors in the design numbers to further enhance proper balance of air movement in support the ventilation system.

4. If the laboratory hood is used as the sole mean of exhaust air, any placement of the terminal throw velocity at the front of the laboratory hood must have diffusers with throw velocity 50% lesser than the face velocity of the hood. For example if the face velocity is set to be 80 fpm, the throw velocity must be lesser 40 fpm. A perforated air supply panel is much effective for the low throw velocity compared to grilles and diffusers. This value is far more lower than the common 300 to 500 from range of supply air means. However this is required to ensure the air movement into the laboratory hood is not disrupted by high throw velocity. A design following this basis will have high efficiency of air movement from the diffuser into the hood face entry while enhancing the capture efficiency. American Society of Heating, Refrigerating and Air Conditioning (ASHRAE) Research project have obtained low concentration test results at the breathing zone concentration deploying 50 acfm/ft2 face velocity which have equipped the LEV system with a good General Ventilation (GV) system. This value is low but efficient compared to a LEV system needing 150 acfm/ft2 for the same occupational setting without the provision of any GV System. 

1.3 Capture Velocity (Face Velocity) Ranges for Laboratory Hood

As the segment above describes the elements of good General Ventilation, this segment will serve as the guidance in selecting the suitable capture velocity for the laboratory hood. The term capture velocity is not absolute to only exterior type. Laboratory capture velocity parameter is also known as face velocity due to the encapsulation factor of the hood making the face opening as the capture velocity plane, however the usage of capture velocity will be much appropriate due to the functional objective of the laboratory hood which is to capture airborne hazard. The ranges of the capture velocity recommended based on American Conference of Governmental Industrial Hygiene (ACGIH) 30th Edition are as such :-

a. Laboratory hood with supply air throw velocity below 40 fpm, horizontal sash, equipment placed 12 inches inside the face opening, hood located away form movement or door. RECOMMENDED RANGE : 60 fpm

b. Laboratory hood with supply air throw velocity below 40 fpm, equipment placed 6 inches inside the face opening, hood located away form movement or door but with minimal movement near the face opening. RECOMMENDED RANGE : 80 fpm

c. Laboratory hood with supply air throw velocity below 60 fpm but not placed at or near the face opening, equipment placed 6 inches inside the face opening, hood located away form movement or door. RECOMMENDED RANGE : 80 fpm

d. Laboratory hood with supply air throw velocity below 60 fpm but not placed at or near the face opening, equipment placed 6 inches inside the face opening, there are some movement near the hood opening. RECOMMENDED RANGE : 100 fpm

e. High velocity will cause unnecessary energy losses due to the removal conditioned air. High velocity will also defeat the actual purpose of airborne hazard removal. Eddy currents are formed due to the need for user to be located blocked the direct passage of air into the face opening. This situation will create a vacuum hot spot in front of the workers causing disruption of homogeneous air movement. The vacuum will force the air movement to divert towards the vacuum forcing a cyclonic eddy in-between the user and the face opening. This intensity of the eddy get much larger with the increase of the face velocity forcing out the captured airborne contaminant towards the workers instead of the baffle plate and the plenum slots.

f. A stand alone or high capture velocity is not a blanket specification. Ideally any laboratory ventilation system must be based on and integration of GV and LEV System. The incorporation of Mechanical Ventilation Air Conditioning (MVAC) System should come right after the finalisation of the GV-LEV System as the final figure of Exhaust Air is required to calculate the appropriate amount of cooling load in the overall MVAC designing System.

g. Laboratory Hoods can be placed near the doors If there are other doors within the same enclosure or if the traffics are low with the doors normally being kept low. However, a stand alone LEV system with door of the enclosure being closed will extreme air tightness resulting difficulties in opening doors.

h. All apparatus must be kept minimal 6 inches inside the face opening.  This is because the front of the areas is still regarded as hot spots and the plane at 6 inches inwards will be the an optimum emission carry over plane towards the plenum. Emission of the air borne hazard at this plane will synergy will with the homogenous air flow and will the conveyed along. As for a setting of face velocity at 60 fpm, this ideal plane should be set at 12 inches inside the face opening. 

2.0 Biological Safety Cabinet Features

Biological Safety Cabinets are enclosure hoods which has roles more than safe guarding the workers. Biological Safety Cabinet is to product the environment (room) and the product being worked on. Unlike the laboratory hood which deals with airborne hazard in the form of vapor, gas and aerosols (mist, fog, particulate matter, smoke, fibres and fumes), the biological safety cabinets control the combined hazards of chemical, biohazards and infectious agents. The airflow and the air cleaning device are integrated to perform its roles in mitigating the hazard. Biological Safety Cabinets are divided into three classes varying in its integrity of protection.

2.1 Class I Biological Safety Cabinet and How Velocity Is Used For Functionality and Control

BSC Class I is designed to safe guard personnel and environments (either direct room if it is ductless or the ambient where the vertical discharges are done via chimney). It does not protect the product (sample) as unfiltered air from the room enters the enclosure. The incoming air flow removes suspended hazard generated inside the enclosure, passes through the high efficiency particulate air filter (HEPA) before being recirculated or exhausted. BSC provides a sterile work environments for handling biological agents, cell cultures and other materials. It is commonly used for procedures which are assessed to generate risk level from low to moderate. The work opening inflow ranges from 75 fpm to 100 fpm. The hood can be ducted or ductless. Transport velocity can be a range of choice best on economical approach. The coefficient of hood entry loses is similar to standard laboratory hood as BSC Class I also comes with the plenum design for homogenous air movement across the enclosure. 

2.2 Biological Safety Cabinet Class II and Its Design Specifications

Class II Biological Safety Cabinets can be classified as engineering control equipment which provides protection to the product, personnel and the environment. The BSC in this class can be furthered differentiated based on the fraction of air being recirculated into the cabinet in comparison to what is being discharge, incoming work air inflow velocity (Similar like face velocity) to the enclosed work surface, ducted or ductless, plenum pressurisation (negative or positive pressure plenum). The air is drawn into the front opening grille, encapsulating the airborne hazards generated in the enclosure and prevent the escape via the work incoming air which serves like pushing air inwards. The permeated air is from HEPA for both the ductless and ducted system. The encapsulated work area is classified as sterile work space meeting the needs of Biosafety Level 1~3 for any microbiological laboratory.

The class II Biological Safety Cabinet can be divided into several groups which are as follows :-

Type A1 The exhaust air is permeated from HEPA Exhaust at the percentage of 30% while 70% will be recirculated as laminar down flow permeating through HEPA Supply. The exhaust can be in the form of ductless or ducted approach. The incoming work opening inflow is at the minimum velocity of 75 fpm.

Type A2 The exhaust air is permeated from HEPA Exhaust at the percentage of 30% while 70% will be recirculated as laminar down flow permeating through HEPA Supply. The incoming work opening inflow is at the minimum velocity of 100 fpm.

Type B1 The exhaust air is permeated from HEPA Exhaust at the percentage of 70% while 30% will be recirculated as laminar down flow permeating through HEPA Supply. The exhaust air will be ducted out. The incoming work opening inflow is at the minimum velocity of 100 fpm.

Type B2 The exhaust air is permeated out 100% from HEPA Exhaust. The air supply will be from a standalone supply fan permeating from a HEPA for particulate matter free air diffusion. This class of BSC are used for radionuclides and hazardous airborne chemicals.

2.3 Biological Safety Cabinet Class III and its features

Class III Biological Safety Cabinets provides protection to the product, personnel and the environment similar to Class II. The BSC in this class provides highest level of control integrity. The booth is totally enclosed using view glass as the means to conduct analytical works using gloves attached on outer segment of the enclosure.

3.0 Laboratory Equipment

There are certain type of laboratory equipment such as oven, ICP and AAS requires local exhaust ventilation systems. These vents are to be designed based on the individual functionality introducing optimum amount of capture velocity for airborne hazard removal from the equipment generation while ensuring the excessive amount of air conditioned air are not removed from the working area unnecessarily. As to ensure OPTIMUM design flow rate are chosen as design basis these measures must be followed by any designers or system suppliers :-

• Usage of right hood or suction point.

  Use of plain opening duct for oven vent is must energy efficient compared to a big face opening canopy.

• Usage of acceptable design standard in deriving the system flow rate

  Apart from selecting equation based on exterior and enclosure hood, capture distance and flange fitting are very crucial in determining the optimum flow rate standard formula from reference such as ACGIH Design Manual.

• Maintain effective capture distance

  Capture distance must always be made narrow befitting the process suitability while overcoming cross draft.

• Adequate provision of make up

  Once crucial parameters which often gets overlooked is ensuring the make up air are adequate. Extreme negative pressurisation will not have adequate air going into the fan impellers for its to churn the rated flow rate. Optimum rate of pressurisation either negative or positive must be within the region of 10~15%.

3.1 Canopy Hood

As the booth type of hood which is the laboratory hoods are used to enable analytical activities to be performed in an enclosure, the equipments in the laboratory can have several other form of exterior hoods options such as canopy and plain opening hoods.

Canopy hoods use to be the most common choice of hood as it is easy to install. However canopy hood is one of the worst selection of hood unless it is really required due to the volume of air it will exhaust out from the space. Unlike enclosed laboratory hood which has a sash to control the volume of air entering its enclosure, the canopy hood is wide open and will remove big volume of air continuously from the room making the air conditioner to work harder while increasing the room pressurisation.  As situation as such, the selection of the canopy hood must be done carefully subjective to the emission cross section of the contaminant, its dispersion pattern and its buoyancy. 

Canopy hood is to be selected when there is a huge plane of emission. If the emission is just a simple small oven vent, a better option is to use a plain opening hood with opening slightly wider than the vent collar. The selection of a canopy for a bigger plane to contaminant generation must adhere to these specifications :-

• Selection of type between open, two walls and three walls enclosure. Enclosures should be the first choice to reduce cross draft from the room.

• The height from the emission plane must be kept as short as possible to enhance the energy saving design option. High capture distance canopy will not be a feasible design as the cross draft will diminish the conveyance of airborne contaminant into the suspended hood.

• The canopy hood hood entry must be tapered at 45 degree minimum. If this is not done, the uniformity or air entering the canopy face opening can never be met amidst the increase of the unnecessary static pressure losses due to vena contracta (turbulence) at the mouth of the duct connection.

3.2 Plain Opening Extraction Arm

Other options of applying ventilation in removing airborne contaminant at the point of generation will be plain opening hood being used in static form or moveable hoods. Direct vent mounting can be a better choice if the equipment specification clearly spells out the required venting flow rate. If this crucial information is not available, making a direct venting can be affecting the process especially if it involves heating as over exhaustion may require the combustion or heating process to work harder causing unnecessary overload of power consumption. If the direct mounting needs to be done, have the fan speed to be controlled by a frequency inverter. By having this option, in the event the ventilation flow rate needs to balanced, the fan speed can be manipulated to drop the flow rate rather than using a damper. A damper will create additional resistance in the system but the fan will be running at a higher consumption which is completely waste of energy. A manipulated speed equates to lower work which needs lesser electricity to run.

The selection of the plain opening need to take into consideration the capture distance and the plane of the emission. Every analytical works is best to be done inside the laboratory hood. Plain opening is best to tackle smaller vent from machineries. If it is required to be added for analytical works, the designer MUST ensure the capture distance is to the bare minimum. The required flow rate varies with the square of the distance from the source.

Adding flanges or baffles are greater options to optimised the flow rate and enhancing the capture efficiencies. A baffles blocks unwanted air in the front or sides of the hood while the flange blocks unwanted air at the back of the hood. Usages of flanges and baffle reduced the flow area optimising the usage of flow rate in the capture velocity generation. The square root of the hood area should be used as the basis to size up the width of any flange.