Encyclopedia of Environmental Science and Engineering, Volume I and II

496 INDOOR AIR POLLUTION

operation carefully since not all hoods require

same face velocities.

3) In cooperation with laboratory management

determine the minimum number of hoods requir-

ing continuous operation. Determine if a hood or

hoods can operate intermittently or a minimum

and estimate if its exhaust flow can be eliminated

insofar as its effect on air conditioning load is

concerned.

4) Avoid the use of hoods to store material and

merely provide local exhaust.

5) Determine the acceptability of face screens or

shields or horizontal sliding panels.

6) Locate hoods so that they are set clear of door-

ways and frequently traveled aisles.

7) Determine if laboratory management is willing to

take a “slip” in room conditions when more air is

exhausted than is originally planned.

8) Consider use of perforated ceiling supply hood

arrangement with conditioned air supply through

ceiling diffusers for spot cooling effect.

General

Exhaust stack should be vertical and straight and discharge

up; no weather caps should be used. Brief^1 suggests when

open-face velocities exceed 125 fpm, install an atmospheric

damper downstream of the hood just before the exhauster to

prevent excessive indraft velocities when conventional hoods

are used. At high face velocities, laboratory equipment placed

within the hood should be set so that points of release of con-

taminant are at least 6 inches back of the hood face. This can

be ensured by placing a ¼ inch thick edging 6 inches wide on

the bench top near the hood entrance face. Brief^1 found that

concentrated head loads within the hood proper, exceeding

1000 watts per foot of hood width created thermal vectors

that require higher face velocities for control. Obstruction of

hood face by large objects is discouraged; blockage causes

control problems.

PART 2

Factors to Be Considered in Fume Hood Selection

In the selection of a fume hood the following factors should

always be considered:

Space

• What actual space requirements will be required?


• What are the future requirements?


• What physical space is available?

Function

• What chemicals and procedures will be involved
  in this application? (Highly corrosive, TLV, etc.)


• High heat procedures?


• Extremely volatile?

Location

• Are your present ventilation capabilities ade-
  quate and will they be taxed by the new hood
  installation?


• Is the area where the hood will be installed
  adequately suited to the new installation? For
  instance, high traffic areas give rise to undesirable
  crosscurrents and cause materials to be drawn
  from hoods. Hoods should not be installed next to
  doors but preferably in corners.


• Is the operation such that the use of an auxiliary
  air system might compromise the safety of the oper-
  ator? Safety is paramount in any hood application.

Hood Construction Materials

Although basic hood design has changed very little, many

advances have been made in the materials from which hoods

are constructed. Here are some of the basic materials and

their more distinctive features.

Wood

• Generally poor chemical resistance.


• Inexpensive to fabricate and modify in the field.


• Can present a fire hazard in applications involving
  heat and flame.


• Poor light reflectivity causes a dark hood interior.

Sheet Metal (Cold rolled steel or aluminum)

• Requires secondary treatment for chemical
  resistance.


• Demands extreme care to avoid damaging the coat-
  ing since corrosion can occur in damaged areas.


• “Oil canning” due to thin-gauge metal causes
  noise in operation.


• Relatively inexpensive.


• Usually heavy and cumbersome to install.

Fiberglass

• Excellent chemical resistance.


• Lightweight for ease of installation or relocation.


• Easily modified in field with readily available
  tools.


• Sound-dampening because of physical construction.


• Some inexpensive grades can cause fire hazards
  and are not chemically resistant.


• Available with good light reflective properties for
  a light and bright work space.


• Shapes are limited to tooled mold configurations,
  and can be moulded with covered interiors.

Cement/Asbestos (Transite)

• Excellent chemical resistance.


• Has inherent sound dampening qualities.


• Excellent fire resistance.


• Heavy and difficult to install.

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