“Why Can’t We Put It In the Hood?”

In my 40 years of pilot plant and laboratory experience I must have heard variants of this phrase a thousand times. Most were delivered in tones of high anguish to mild aggression. And my answers rarely made the individual happy although I like to think they often made them safer.

Hoods or fume hoods are designed to capture emissions or vapors generated by the operations taking place inside. They have a long history of reliable operation if properly used. These last three works should be engraved on the face of every hood ever made.

Hoods work best when they are empty. In fact, tests have shown that empty hoods often operate very reliably down as low as 60 FPM face velocity even if they were never designed for low flow operation.  When empty, hoods are very efficient at capturing vapors generated inside even at low exhaust (and hence face velocity) rates. Crowded hoods, on the other hand, often have significant problems. They block the proper exhaust patterns and make the hood increasingly less efficient. And the more you put in a hood and/or the more you do in a hood, the more exhaust you need to ensure it always works reliably. Hence most research hoods must operate at 80-120 FPM unless specifically designed for lower flow operation. But no hood works well if it is too overloaded. In this case the equipment in the hood essentially blocks the exhaust path and prevents containment.

What are the more common problems?

·        Storage. Hoods are not supposed to be used as chemical storage locations. NFPA 45 Fire Protection for Laboratories using Chemicals clearly prohibits this as do almost every other laboratory safety reference. As laboratories become more crowded, hoods also sadly tend to become storage locations for spare equipment, spare parts, and anything else looking for a home. Chemicals are often stored in the hood using the argument they are safer there due to the high exhaust. Most building and fire codes allow more flammable liquid and gas storage if the additional materials are in a “ventilated enclosure”. All too often this is considered to be a laboratory hood. Worse, many laboratories are so short of bench space that hoods become the repository for electronic equipment, spare parts, and even consumable supplies.

·        Large solid items like ovens, solvent storage cabinets, and refrigerators. These are often placed in hoods to eliminate odors, capture fugitive emissions, or leakage. Unless the hood is very large compared to the item this rarely does anything except cause the hood to work poorly if at all. Hoods cannot draw exhaust through solid objects so the exhaust stream must bend around the object and then needs to try and reform in front of it. This needs a lot of depth to work; more than is usually available.

Ovens are also occasionally placed in hoods due to the “heat load”. As most laboratories have air change rates of over 12 per hour this is ludicrous. Yes, the area next to the oven may be hotter than further away but rarely does this affect a normal laboratory.

·        Items that have no need to be placed in a hood like electronics equipment, instruments, analyzers, computers, monitors, etc. (I can assure you the list is endless.) These block the hood exhaust and have no rationale purpose for being inside except convenience. Their blocking the air flow, however, brings this convenience at the cost of reduced safety.

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Shelves and supports. Most hoods draw exhaust from 3 places in the rear. Almost any larger shelf or support unequally divides the hood so that part (above or below) the shelf gets more exhaust while the other gets less. This often creates a problem. Large supports, particularly those with significant solid components such as panels or oversize gussets can create severe disturbance along their length. Partitioning a hood, a common trick to separate a flammable source from ignition sources and allow one hood to be used for both is almost always likely to create some problems unless the partition is properly designed (which it usually isn’t). At best, these require a higher exhaust rate to work properly.

·        Panelboards, signs, and similar large solid planar pieces (in one memorable case a white board!). Again, the hood cannot effectively draw exhaust around them unless they are deep in the hood. And again, their need to be in the hood is incredibly questionable.

·        Operations and equipment too close to the front of the hood for it to work properly. You need to be back from the front sash at least 3 inches; 6 inches is better and more reliable. This again allows the exhaust to reform adequately in front of the hood and keep all the vapors inside the hood.

·        High velocity sources such as fans and blowers often found on instruments and equipment. These disturb the hood exhaust and can totally disrupt its proper operation. This is a problem no matter which way they are directed. Outward, they can overwhelm the intake. Sideways, they create tremendous flow disruptions. Even backwards into the hood can create low pressure zones that disturb the proper hood exhaust.

·        High thermal loads from furnaces and heaters. These create thermal drafts that also can disrupt proper hood operation, particularly as they exceed 1KW/foot.

So, what can you do to avoid the problem? Many things.

·        Storage. Store solvents in approved solvent storage cabinets. Store non-hazardous chemicals in cabinets, benches, or bench shelves. Store cylinders and lecture bottles in approved gas cabinets, exterior locations, or specially designed ventilated enclosures. If you want to make use of the additional flammable storage allowed by building and fire codes, purchase a ventilated enclosure designed for this purpose. (Why does it work better? Because you should not be doing anything in it except taking chemicals in and out. Even if it is less effective during these movements, their duration should be so minor as to not be a hazard.) Take all inappropriate items like gloves, boxes, spare parts, empty unused containers, and others out of the hood and store them in a bench or shelf. For those who have no other room go through and clean out their laboratories of the items that are taking up space and have not been used for years. If you have not used something in the last 1-2 years the odds are excellent you will never use it (some spare parts excepted). That equipment once used for an old program should be trashed. If the program ever arises again you can deal with it at that time. Sadly room is finite and you have to get rid of things.

·        Large solid items like ovens, solvent storage cabinets, and refrigerators. Put them outside the hood. If the contents are particularly odorous or hazardous provide local ventilation ducted directly to the inside of the equipment so that they are continuously ventilated. In this case, the required exhaust rate is usually small enough not to interfere with normal operation and almost always will not overload the heating or cooling system. The saving in energy will probably pay for the installation costs in a year or less. (A larger refrigerator, say 30” x 30” x 60” that has 6 internal air changes an hour will only require 3.1 CFM vs 800-1,200 CFM for a hood of similar size opening.) Be careful, however, not to vent a solvent storage cabinet as discussed in my post on this subject. (See https://www.dhirubhai.net/pulse/should-you-vent-flammable-storage-cabinet-richard-palluzi for details.) In many cases, simply placing the item near (but outside) a hood may be sufficient for the hood to capture much of the odor or emission. In other cases, providing a local exhaust near the front opening is also very effective.

·        Items that have no place in a hood like electronics equipment, instruments, analyzers, computers, monitors, etc. These should be mounted outside the hood on benches or hung off the top of the hood, the side of the hood or on a stand or table outside the hood. Yes, it takes some extra work but it is incredibly more space efficient and much safer. You may need to extend a core or build a shelf or buy a table but this is better than causing your hood not to work properly

·        Shelves and supports. Avoid their use whenever possible. Support equipment on open strut without a plate blocking the airflow. (in most cases you can set the strut to just capture the device’s “feet”.)  If this is not possible use perforated plate to at least allow airflow through the floor of the support. Try and keep at least 2” between any equipment and a shelf for better airflow. Keep the shelf or support as small as possible. Avoid large side panels if possible. If not, make sure they end at least 3-6 inches from the front of the hood and be prepared to have to increase the hood exhaust to pass a face velocity test.

·        Panelboards, signs, and similar large solid planar pieces. These do not belong in a hood. Put them outside.

·        Operations and equipment too close to the front of the hood for it to work properly. Ideally you would work no closer than 6” to the front of a hood. As most hoods are only 26-30” deep this is often difficult. Squeezing the 6” is possible but with attendant risks of problems. A single beaker 2” in from the front will probably be fine. A 3 foot long clam shell type furnace 1” back will almost certainly create problems.

·        High velocity sources need to go outside a hood. Most are due to fans in instruments which, as noted above, don’t belong in hoods. Some, are part of experiments. These generally require a detailed individual hazard analysis and risk assessment by someone very knowledgeable about proper hood operation. Sadly, many times these result in the need for a very special enclosure versus a standard hood.

·        High thermal loads are a problem. While ovens and similar sealed sources do not belong in laboratory hoods as discussed above, many other types of heaters are required. Catalyst tubes are heated in clam shell type furnaces, micro units have heat traced lines, small hood based pilot plants have heated reactors, etc. If properly insulated, then the thermal effects on the hood should be manageable. Placing the unit more towards the rear (even at the risk of less accessibility) and properly insulating all exterior surfaces can address most of the issues.

A hood is a wonderful device for keeping laboratory personnel safe but it is not foolproof. You need to make sure that you don’t put things in the hood that prevent it from protecting you. Understanding what you should and should not put in is the first step.