Cfm airflow altitude8/22/2023 ![]() 4 x 3 x 125, or 1500 CFM.Īs you can see, this calculation does not take into account the type or size of the torch you may be using because it is not necessary, if you use the ACGIH recommendations. Per the Recommended Practices, the CFM requirement for this hood would be 4 x 2 x 125, or 1000 CFM.Įxample: Wall or bench workstation mounted hood, 4 feet wide by 3 feet deep (or high – if you use a bench mounted workstation hood with sides that extend down to the table top, measure the height of the opening). For Island type (or ceiling hung) hoods, the recommended air flow is also 125 CFM per square foot of hood area.Įxample: Ceiling mounted hood, 4 feet wide by 2 feet deep. The Practice states that for Wall Mounted hoods, the recommended air flow is 125 CFM per square foot of hood area (length times width). Among those, the nearest to our specific process is Restaurant Hoods (over cook tops). In the 22nd edition, 1995, it states a variety of processes that require ventilation. The American Conference on Governmental Industrial Hygiene (ACGIH) published its Recommended Practices for Ventilation. The first item that we need to know is what size fan (in CFM) we need for our design. I will present several different designs to show how each affects the total design. Įach one of these numbers or calculations factors into the design of an exhaust system. The chart we will be using in all these calculations can be found in here. This is number is a constant for specific duct types and is usually presented in a look up chart form. Loss Factor: A multiplier, usually fractional, that is the amount of friction induced by ducts. As SP increases, the efficiency of the fan to move air goes down, or, to state it differently, the higher the SP, the lower the CFM from design. SP increases as the size of the duct decreases, with the addition of bends, and with any amount of turbulence. The total pressure against which the fan moves air. Velocity Pressure: The pressure created by trying to force air at a given Velocity through a given duct size. Velocity: The speed the air moves inside the duct. It is based on how much air a given fan can move against a given amount of pressure. The amount of air that a ventilation system can move. So, where do we start? Well, let’s talk first about a couple of important numbers and calculations that have to be made first.ĬFM: Cubic Feet per Minute. We’ve talked about the basics, now let’s take a look at some basic design issues. These will not change the speed with altitude change and will therefore produce less pressure at high altitudes.Ok. The exception is for AC motors that are designed to run very close to synchronous speed at sea level. This RPM change generally will compensate for altitude changes. ![]() An air moving device powered by an electric motor will change RPM with a change in altitude due to the change in density (less load at higher altitude). The pressure developed by an air moving device will change proportionally with the air density if the RPM is kept constant. How does fan and blower performance change at 1,000 and 2,000 meters altitude? Searching about this I found the following in an well-known german fan manufacturer FAQ:Ĥ7. We expected to get similar results and about 84% of pressure drop. We have been a little surprised by the fact that the flow was even higher than at our place (300m above sea level). We have been doing some airflow measurements in a ventilation installation on site, at 1500m above sea level (0.84 density factor). ![]()
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