Basic Capture Hoods
Local exhaust which results
in capture effect is such that the hood utilized captures, contains or receives
contaminates generated by the local source. The hood generates the capture effect by
converting duct static pressure to velocity pressure and hood losses (e.g., slot and duct
entry losses). The concept is described by the following formula:
(See NRC Publication's #29,
#31 and #84)
Hood loss (HL) is equal to:
EQ (1) HL = K x VP = {SP(h)} = VP
Where
K = loss factor
VP = Velocity pressure in exhaust duct
{SP(h)} = Absolute static pressure approximately 4 to 6 duct diameters upstream from the hood entrance.
The capture hood’s ability to convert this static pressure to velocity pressure is given by the hood’s coefficient of entry C(e). This is further defined as follows:
EQ (2) C(e) = Q(ideal)/Q(actual) = divide VP/SP(h) = divide 1/(1+K)
Any time you enclose the material giving off the contaminate emissions, you will be able to greatly reduce the amount of air required to produce the required capture velocity. You must always keep the source of contaminate emissions as close to the local hood as possible. The hood must also be designed to allow a smooth entrance of air into the hood so that all of the air entering the hood will be capable of capturing the contaminates.
The idea of the local exhaust system is to prevent worker inhalation of contaminates. For this reason, the hood has to be located so that it does not cause the contaminates to move through the occupant’s and/or worker’s breathing zone in order to make its way to the hood entrance. This is especially true if the hood is to provide protection to workers leaning over an operation which involves utilizing an open surface tank or welding bench.
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