I
in the Chemical laboraiory Edifed by NORMAN V. STEERE, School o f Public Health, University of Minnesoto, Minneapolis, Minn., 55455
II. Ventilation of Laboratory Operations.
Part Two
The prohlo~nsof 1:~borntoryventilation involve more than the capture of potentially injurious materials. The function of a hood, as discussed in Part 1 of this article, is to perform this capture. However, the ellicieney of any hood will depend on the adequacy of the system for removing the toxic s u b s t a m captured. Also the find disposal of such materials obviously demands considerations of safety. This article will discuss the topics of transport and disposal. The Literat,ure Citat,ions were included with the previous installment (see THE JOURNAL, 41, A108 and A l l 0 (1964)). Duct design, material, and velocities, well as fan location and design are the factors governing transport of vapors, gases, and pertieuletes from the laboratory to the point of disposal. Fan and duet desims should provide convenient access ports for inspection and cleaning (IS).
Good
Bod
Elhow Itesistanee Equivalent
Branch entry design.
Provision for control of inhalation hazards is one of the most important functions of s laborstory building. Design of the building should allow streamlined ductwork, and shaftways large enough far future additions of ductwork. If fire spread could be controlled, flexible plastic duct,= might provide streamlining with economies of space and installation costs.
Duct elbow design.
1 1
FAN
Duct Material Material for hood exhaust ducts should resist corrosion by chemicals and moisture to which the ducts will he exposed. Fire should not be able to spread from one duct to another (4). Careful consideration is recommended before selecting plastic duets, in order that duct use is limited as little as possible. "Industrial Ventilation" (IS) gives properties and limitations. Bituminous coatings can create fire control problems, and can be hazardous if used with perrhloric acid vapors.
Figure 16. Left: Bad location for fon. Erhovrt system under positive pressure; exhaurt leaks out. Right: Good locoton for fan. Exhaust system under negative pressure.
Transport Velocities
Disporol of Removed Contaminants
A minimum duct velocity of 2000 ft/ min is recommended ( I 3 4 ) for vapors and gases; B velocity of 2500-3000 it/ min is suggested to scavenge condensed moisture (4). Transport velocities for particulate material will range from 3500-4500 ft/min; good practice would be to determine the appropriate velocity from "Industrial Ventilation" (IS).
Consideration should be given to contamination or air pollution problem which could result from direct discharge of hood exhaust to the atmosphere (1, 4, 6, 16). There is an increasing need to provide filters, collectors, condensers, scrubben, or other air cleaning equipment.
Fan Design Figure 14.
corrosion-resistant impellers. The size and type of equipment should be based on hood needs and duet design and selected by competent ventilation engineers. For long life and good fan performance, we recommend rentrifugal fans that have straight or radial blades.
Bad Figure 15.
Poor
feature
The best location for hood exhaust fans is on the roof of the building-and noise is not the -in reason. Locating the fan on the roof places the duct system under negative pressure and assures that any leaks whirh may develop will not allow contaminants t o escape into the building (1, 4, 15). Figure 16 shows the results of leaks in the ducts of two similar systems, one with the fan on the hood and one with the fan on the roof. With the fan on the hood, any leaks are into the laboratory. Fans on the roof can be screened srchitecturally to preserve visual esthetics. If necessary, fans can be located in a. mechsnical space just below the roof.
8s
Efficient duetwork seems to be the forgotten stepchild in huilding design. The most horrible example we know is the stainless steel hood, with air-foil and sash, strangled by duetwork with five right, angled bends between hood and roof. Such a duct system seems to have been built rather than designed, with the expected results that excessive fan horsepower is required to keep the hood from being inadequate. Figure 14 shows good, poor, and had elbows and gives an indication of the greater resistance of shorh-adius elbows (IS). Figure 15 shows good and bad branch entries (1.9). Both figures illustrate the principle that streamlined flow is more efficient,.
am
Fan Location
Transport of Noxious Materials
Duct Design
...a
Fans for laborstory exhaust systems should be heavy duty equipment with
Dispersion A laboratory ventilation system with the best hoods and best transport system is a failure if the exhaust returns to the (Continued on page A184)
Volume 41, Number 3, March 1964
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A1 83
Safety
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