Encyclopedia of Environmental Science and Engineering, Volume I and II

INDOOR AIR POLLUTION 499

the Add-Air feature supplying 50% untempered air, $1,500

would be saved in capital equipment and $75 to $90 to annual

operating costs. (Figures are in 1992 $.)

MAINTENANCE AND TESTING

Since the hood performance may be affected by the cleanli-

ness of the exhaust system and the direction of rotation of

the exhaust fan, it is important to provide a maintenance

schedule of inspections and performance testing throughout

the year to make certain that the fume hoods are operating

safely and effi ciently.

If fi lters are used to remove dust and other particulates

from the exhaust air, they must be periodically inspected and

replaced if necessary. Corrosion of ductwork and damper

mechanisms should be watched and debris should be removed

from inside the ducts, especially at startup time. Excessive cor-

rosion of ducts may cause leakage of air into the system or the

failure of balancing dampers that will affect capture velocities

well below their design fi gures. Remember to check fan rota-

tion since this most often causes poor exhaust performance.

PERFORMANCE TESTING

Two performance tests should be conducted periodically

on all hoods. One for fume leakage and the other for face

velocity. The test for fume leakage consists of releasing

odorous fumes such as ammonia or hydrogen sulfi de within

the hood. If fumes are detected outside the hood, especially

around the face opening, the capture velocity at the sash

opening may be inadequate, or there may be an interfering

air disturbance. Cleaning the exhaust system, adjusting the

air fl ow damper, or increasing the fan speed may improve

the performance if low face velocity seems to be the prob-

lem. If, on the other hand, leakage seems to be caused by

interference from an auxiliary air supply stream or other

velocity near the sash, the nature of the interference may be

investigated as follows: placing liquid titanium tetrachloride

on masking tape around the periphery of the sash opening.

Observations can then be made of the path of visible fumes

to determine where there is spillage into the room. Smoke

bombs have also been used to determine fl ow patterns at

sash openings and to identify interference.

A hot wire anemometer is usually used to measure

actual face velocity. This is done as a traverse over the

entire sash opening, including especially all edges and

corners. The overall face velocity average is obtained by

averaging the velocity readings at prescribed positions of

the traverse.

These testing procedures are diffi cult to standardize and

are dependent on subjective observations. Thus, they are

considered to be unadaptable and inadequate. The American

Society of Heating, Refrigeration, and Air Conditioning

Engineers (ASHRAE) has set up a research project for

developing fume hood performance criteria and new test

procedures for such laboratory equipment.

SAFETY FEATURES

Interconnection of Hoods

If two or more hoods independently serve a single room or

an interconnecting suite of rooms, all of the hoods in these

rooms should be interconnected so that the operation of one

will require the operation of all. If this is not done, there is a

strong possibility that fumes will be drawn from a hood that is

not operating to makeup air demands of those in operation.

Alarm for Hood Malfunction

All hoods should be equipped with safety devices such as

sail switches to warn personnel that the air volume exhausted

from the hood has dropped to a point where it will not pro-

vide suffi cient capture velocity for safe operation.

Fire Dampers

Most building codes require fi re dampers in all ducts that

pass through fi re walls and fl oors. However, it is important

not to install them in fume exhaust systems. Should a fi re

occur in a hood, or if heat from a fi re nearby such a damper

should cause the damper to close, the fume backup into the

facility would prove disastrous.

EXHAUST FROM LABORATORIES

A laboratory should exhaust 100% of the air fed to it. If the

materials that are being handled or tested in the laboratory are

hazardous enough to need a hood, the presence of these materi-

als in itself should dictate 100% exhaust. An accidental spill or

accidental release of materials at a bench or hood can result in

DRAIN TYPE STACK

Support

overlap

6 in. min.

1/2 in.

drain

4 + D, min.

D + 1 in.

D

FIGURE 7

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