Flow rate for AHU
Flow rate for AHU
(OP)
Hi does anyone know how to go about calculating the flow rate for a 1500m2 single storey building with 10 to 14 air change per hour? What are the standard guidelines for selecting the size of an AHU fan?





RE: Flow rate for AHU
RE: Flow rate for AHU
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RE: Flow rate for AHU
RE: Flow rate for AHU
RE: Flow rate for AHU
I've noticed from previous projects that AHUs are sized anywhere between (1.5L/s)/m^2 to (2L/s)/m^2 for typical buildings. When I use the suggested air change volumes. The number seems too high.
Choose the air change rate from the following website:
http://www.engineeringtoolbox.com/air-change-rate-...
RE: Flow rate for AHU
The design of an HVAC system is comlpex and takes into account a large number of variables including:
Building location on Earth
Building shape (including roof) and orientation (which direction is north.
Building construction, how are roof and walls constructed, what materials are used
Windows - number and shapes
Door - number and sizes
Building use
Occupant density
Occupant activity, what are the occupants doing in the building; sitting at desks or a gym with people working out
Equipment/lighting density
Outside design conditions, winter & summer
Indoor design conditions, winter & summer
Project budget
All these factors and probably a few I haven't mentioned have an impact on the size and capacity of a building's HVAC system.
On smaller buildings it is not economical to custom build AHU's to meet the exact design loads so off-the-shelf units get used a lot which will contibute to the large variances in L/s/m^2.
So when you ask "how to go about calculating the flow rate for a 1500m2 single storey building with 10 to 14 air change per hour?" We are going to direct you to books and experienced professionals who can walk you through the design process. Something that these forums are really not good at doing.
RE: Flow rate for AHU
Temperature conditions and things like that would be major variables for calculating heating/cooling loads. Although they will need to be considered for fan sizing not sure if they are significant.
A lot of preliminary design briefs will estimate fan capacity, heating loads, cooling loads etc. Its possible to know the exact capacity of an HVAC system when design is at 100% but its' a little strange that there would be no rule of thumb or a quick approximation technique.
This means that an experienced HVAC designer would have no clue about what range the capacity for the fan should be around.
RE: Flow rate for AHU
Also, rules of thumb are only good for providing a rough estimate of the HVAC equipment size which is "good enough" for space planning purposes and as a figure check of calculations. Even then, if the final calculaiton is outside the rule-of-thumb range, it is not necessarily the end of the world or a bad calculation. That's where good engineering judgement and experience comes in.
The ACHs on the engineeringtoolbox.com page you posted are rule-of-thumb figures. You still need to do actual calculations. In AHU selection, ACH is only gonig to give you a required air flow, which is only part of what an AHU does.
Also affecting your designs are codes and standards. Which codes and standards you need to adhere to varies county to country, state to state, even city to city; I have seen cities with unique building ordinaces complementing the building codes. You need to contact the local AHJ to confirm and verify which codes and standards to use for a particular project. Although many codes and standards may appear to be the same, there are subtle differences.
If you have your mind set on a rule of thumb, consider that for every ton of cooling, the typical off-the-shelf AHU will deliver 350-400 CFM. A critical assumption for this rule of thumb is that you have about 15% humid outside air mixed with return air.
To try to use rules of thumb to get to 100% design is poor engineering practice.
RE: Flow rate for AHU
400cfm/RT is standard in my area according to a local supplier as well! I've ordered the manuals from carrier that was suggested so I am looking forward to reading them.
It's not necessary to hire an HVAC engineer right away especially for preliminary discussions about design. I'd want to be able get these rough estimates on my own.
I am asking for advice purely for my knowledge do not worry I am not going to design an HVAC system based on thumb rules. I am not fit to design HVAC systems I am new to the field.
Thanks for the help! :)
RE: Flow rate for AHU
RE: Flow rate for AHU
Did I miss something here?
RE: Flow rate for AHU
It's not clear, but I think preed is looking at two different things.
An AHU typically recirculates the air already in the room for cooling / heating purposes. Sometimes it adds in some fresh air (5-15%), sometimes it does that separately.
The air changes per hour relates to FRESH air entering the space to provide oxygen / reduce CO2 content etc.
The AHU cooling/heating fan load is much higher than the air changes per hour load.
The two posters above have given some good ROT for office buildings.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow rate for AHU
I've also not seen AHU's sized based on air flow, it's always been cooling and heating capacity.
It's never too early to get the HVAC enginer involved with in the design process.
RE: Flow rate for AHU
Anyway, let's go back to this:
preed (Mechanical)(OP)14 Aug 17 14:13
Hi does anyone know how to go about calculating the flow rate for a 1500m2 single storey building with 10 to 14 air change per hour? What are the standard guidelines for selecting the size of an AHU fan?
The only missing variable is the ceiling height. I'd assumed 9'. The rest is simple math.
Sorry if I come across as noodgy.
RE: Flow rate for AHU
You will have a better design and system if you hire an Engineer familiar with HVAC.
RE: Flow rate for AHU
So my issue is that using the above values the calculated CFM value seems high compared to what I am used to seeing!
Doing some more research and as suggested by dbill74 & LittleInch the primary duty of the airflow rate of supply air is to remove some kinds of loads from the occupied zones. I am going to look into doing rough heat and humidity load calculations.
ChasBean:29,000cfm
lillieput1/LittleInch/dbill74: (400cfm/RT)*(1500m^2)*(10.7639sqf/m^2)/(300sqf/RT)=21,530cfm
Solid answers! Thanks for the help Gents!
RE: Flow rate for AHU