KEES slotted backshelf hoods are ventilation devices that exhaust fumes and particulates from welding, manufacturing, and other heavy-duty industrial processes. They play a key role in creating a safe and compliant workspace, protecting workers and products alike.
Engineers frequently ask KEES how to correctly specify a slotted backshelf hood. They want to understand the factors that go into optimal hood performance, chiefly:
How much air do I need to pull through my hood?
What is the pressure drop for pulling that air through my hood?
What kind of fan should I purchase to ensure the right airflow against the pressure drop?
KEES has helped many engineers answer these questions, determining the airflow, pressure drop, and optimal fan type for their specific installation. In this article, we’ll show how we do these calculations step-by-step through a hypothetical scenario of an engineer specifying a KEES slotted backshelf hood.
Step 0: Constants
Before we start performing any calculations, we need to define our constants.
KEES Constants
For slotted backshelf hood calculations, KEES uses the following constants:
Slotted backshelf hoods have four ½” slots evenly spaced across the face of the hood.
The hood face velocity (Vface) is 2000 feet per minute (FPM).
The cubic feet per minute per foot of air entering the slots is ~350 CFM per foot.
Engineer Constants
For our hypothetical slotted backshelf hood calculations, we’ll assume the engineer comes to KEES knowing these things:
The desired size of the hood, based on the size of the work piece. In our scenario, the engineer wants a 3 ft wide hood.
The pressure drop of the exhaust ductwork the hood will use. In our scenario, the engineer knows the duct pressure drop (Fduct) is .5 inches w.g.
Step 1: Determining Airflow
The first calculation performed for a slotted backshelf hood concerns airflow, or how fast the air needs to move into the slot to capture particulates. Air that moves too slowly will result in less powerful suction, whereas air that moves too quickly will waste energy.
The calculation for determining airflow is:
CFM per ft x width of hood = Airflow
In our hypothetical scenario, our engineer has told KEES they want to specify a 3 ft hood. We multiply that against our constant of 350 CFM per foot to get our optimal airflow:
350 CFM per ft x 3 ft = 1050 CFM
For our hypothetical engineer’s 3 ft slotted backshelf hood, air needs to move at 1050 CFM.
Step 2: Calculating Pressure Drop
Pressure drop is the resistance of air to moving, calculated by the difference in pressure loss between two points. In our scenario, the two points are the hood and the fan. The fan needs to overcome the pressure drop in the hood plus the pressure drop in the connected ducts to move air at the desired airflow.
First, we’ll calculate the static pressure loss at the hood entry using this equation:
Hood entry loss coefficient x Velocity pressure at hood face opening (VPface) = Hood static pressure loss (Pentry)
The hood entry loss coefficient is a constant determined through testing done by the American Conference of Governmental Industrial Hygienists (ACGIH). For a slot hood without a flange, the ACGIH coefficient is 1.78.
VPface is calculated through this equation:
VPface = (Vface/4005)2
As defined in Step 0, our constant for Vface is 2000 FPM. The equation for hood static pressure loss for our hypothetical slotted barrel hood looks like this:
1.78 x (2000/4005)2 = 1.78 x .25 inches w.g. = .45 inches w.g.
Next, we calculate the velocity pressure of the ductwork (VPduct) using the same equation to determine VPface. For our hypothetical scenario, we’ll set the duct velocity at the commonly used value of 3500 FPM.
(3500/4005)2 = .76 inches w.g.
Finally, we can determine the pressure drop across the system by combining our measurements from the first two equations with the Fduct value provided by the engineer in Step 0.
Pentry + Fduct + VPduct = System pressure drop
The final calculation looks like this:
.45 inches w.g. + .5 inches w.g. + .76 inches w.g. = 1.71 inches w.g.
The pressure drop from duct to hood in our hypothetical scenario is 1.71 inches w.g.
Step 3: Picking a Fan
Once an engineer understands the airflow requirement at the hood and pressure drop of their system, they have all the data needed to choose a fan strong enough to overcome said pressure drop while delivering air at the desired airflow.
In our hypothetical scenario, we determined the engineer needs a fan that outputs 1050 CFM against 1.71 inches w.g. of resistance. They can use that data to review fan selection tables and find the fan model that will work for their system. Now they can rest easy knowing their slotted backshelf hood works as intended, removing the air contaminants from the work piece and keeping the environment clean and safe.
Work with KEES to Determine Specs for Slotted Backshelf Hoods
KEES knows the questions engineers have about industrial hoods – and we’re prepared to answer them. For over 50 years, we’ve helped engineers specify slotted backshelf hoods to meet the needs of their toughest projects. We work one-on-one with clients to individually design slotted backshelf hoods to their exact specs for optimal performance and coverage.
