Flow rate for AHU 1

[preed]

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?

 

[SAK9]

 

[dbill74]

Deep subject this is.

Read here you must

Find master for padawan.

Paid for this, I am not.

hehe

 

[willard3]

Hire an Engineer familiar with HVAC.

 

[BronYrAur]

Sounds like you have already calculated it. Somehow you know that 10-14 air changes per hour are needed. Simple volume calculation from there. You need help determining the actual load. 1500m2 of concrete wall construction in the artic or 1500m2 of glass windows in the dessert? The load comes from a host of factors, not just the size of the building. From there you calculate air flow ... and a lot of other things such as temp and humidity. As Willard3 said, you need to hire and HVAC engineer to design something.

 

[preed]

I am asking because I'd like to able to understand how to workout these things.

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-room-d_867.html

 

[dbill74]

There is no simple answer or rule-of-thumb.

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.

 

[preed]

A variable like air changes per hour would already has variables like building use, occupant type, occupant density, mechanical, electrical equipment etc. built into it. It seems like a good variable to use to find the fan capacity.

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.

 

[dbill74]

There are rules of thumb available, but a rule of thumb for building A may not be applicable to building B. They are not necessarily the same for different rooms in a building. For example, an office buiding with individual offices along the perimeter; offices facing the equator will need more air than offices facing east/west or the poles.

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.

 

[preed]

No engineer is using rule of thumb for 100% design that's pretty obvious! Good experienced engineers will have feel and estimation techniques for all these, it's very important before design for decision making.

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!

 

[lilliput1]

preed the (1.5L/s)/m^2 are for residential type buildings and the tabulated airchange rates in the engineering toolbox website is for outdoor air for ventilation, not for conditioned supply air which is a mix of return air plus outdoor air. In the US typical supply air for offices would be about 1 to 1.5 CFM/SF which is 7.5 to 11.25 ACPH based on 8 ft ceiling height. Cooling load would be about 300 to 400 Sq Ft per ton.

 

[ChasBean1]

I didn’t see preed's question as being so complicated. The only assumption here is ceiling height. Let’s say it’s a 9-foot ceiling. If a 10-14 air change per hour (ACH) is needed, and we use a ceiling height of 9’ and assume the ACH is 12 (halfway between 10 and 14), we have a 29,000 cfm airflow.

Did I miss something here?

 

[LittleInch]

chas Bean,

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.

 

[dbill74]

The two posters above have given some good ROT for office buildings.

That's assusming Preed is talking about an ofice building, which I've not seen him mention what type of building this is, office, gym, auditorium, school, etc.

I've also not seen AHU's sized based on air flow, it's always been cooling and heating capacity.

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.

It's never too early to get the HVAC enginer involved with in the design process.

 

[ChasBean1]

I find this typical of trying to find truth in anything. It's so elusive, and takes a genius to identify what is true versus something that is funded by some entity.

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.

 

[willard3]

So, we can conclude that you are looking for free Engineering.

You will have a better design and system if you hire an Engineer familiar with HVAC.

 

[preed]

I am assuming its a standard operations building with a few office and some CPU rooms. The building height should be 3m to 4m. I was looking at the following website:

http://www.contractingbusiness.com/service/use-air-changes-calculation-determine-room-cfm

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!

 

[BronYrAur]

It all comes back to the load. the rules of thumb are just ballpark orders of magnitude. The problem is that too many "designers" only use the rules of thumb. My house is a perfect example. I only have 1,900 square feet of living space, so how many tons of cooling do I need? If you use the rules of thumb, you would probably say 4 or 5. I have 5 tons of cooling, but I can't keep the house below 80 degrees once the OAT gets much above 80. The equipment works properly; it is just undersized. I have vaulted ceilings with a large amount of East- and West-facing glass. The idiot who "designed" the HVAC in my house no doubt used some square foot per ton rule of thumb. The large solar load was never figured into his calculations. So to say you need 10 to 14 air changes per hour is a dangerous thing if you don't really know why. As I said in my first reply to this post, it is a simple volume calculation if you already know how many air changes you need. But you don't know until you do the load calculation.