Formula for HVAC

[sid.fontenot550]

We are looking at the filter requirements for our substation control house units. My question is this:

What is the formula to calculate the size of the return air filter for HVAC units? Supposedly it is ? sq inches per ton.
The number we have is 172 sq inches per ton but that seems high.
Anyone have any ideas?
Thanks.

[Wanderer]

We are looking at the filter requirements for our substation control house units. My question is this:

What is the formula to calculate the size of the return air filter for HVAC units? Supposedly it is ? sq inches per ton.
The number we have is 172 sq inches per ton but that seems high.
Anyone have any ideas?
Thanks.C'daddy... I've shot off an email to a friend who is a mechanical engineer, hopefully he'll get back to me with something useful.

[sid.fontenot550]

C'daddy... I've shot off an email to a friend who is a mechanical engineer, hopefully he'll get back to me with something useful.

Thanks Pony ;-)
I am going to call the guy I work for after hours also.

[Wanderer]

Thanks Pony ;-)
I am going to call the guy I work for after hours also.~sigh~ great, now my brains are leaking out of my ears...
got some questions and comments on how many tons, space for ductwork and filters... quality of filters... size, I don't know... here's his answer below, most of it is Greek to me. Hope it helps.


Filters are rated based on air flow. Therefore the biggest question is what is the air flow in cfm ?

Best guess.
Depending on the type of filter selected the design air flow through typical filters ranges from 250 to 500 fpm. Therefore if you are assuming 400 cfm/ton then the sq.in./ton would range from 115 to 230 (172 falls nicely in the middle). On the other hand if you have a lot of outside air (not real likely) the cfm/ton could be as low as 200 and this would mean the range would be 58 to 115 sq.in./ton.

Without knowing the air flow I would be on the more conservative side and try get close to 230 sq.in./ton which is about 2.5 tons for each 2'X2' filter.

[sid.fontenot550]

~sigh~ great, now my brains are leaking out of my ears...
got some questions and comments on how many tons, space for ductwork and filters... quality of filters... size, I don't know... here's his answer below, most of it is Greek to me. Hope it helps.

I know the feeling ;-)

Thanks for the info. That is a great start. Thank your friend for me also!

[Wanderer]

Thanks for the info. That is a great start. Thank your friend for me also!:shock: NO!!! cause then he'd speculate for another 20 minutes on the tonnage of units and the differences between them and the effect on CFM... ~shudders, curls up in a ball~

[revacservice]

We are looking at the filter requirements for our substation control house units. My question is this:

What is the formula to calculate the size of the return air filter for HVAC units? Supposedly it is ? sq inches per ton.
The number we have is 172 sq inches per ton but that seems high.
Anyone have any ideas?
Thanks.

The unit ton in this instance is a unit of cooling capacity and as such not related to the size of the filters for an air conditioning unit and it would be a dangerous rule of thumb to use.

The size of the filters are Dependant upon the air flow, the grade of filtration required, the pressure available to over come the resistance of the filter among others.

Other respondents suggest a air velocity of 500 ft/min (2.5 m/s) which is a reasonable maximum value, higher velocities can be used with special consideration. if the fan is a has a small impeller diameter it will advisable to use a much lower velocity of 300 ft/min (1.5 m/s).

Unfortunately as we are considering a cooling process there is no simple formula to calculate the air flow as there will be some dehumidification (latent cooling), the simple flow x specific heat x temperature difference = heat flow doses not apply in this instance.

My best advice is to consult the air conditioning units technical data, determine the actual air flow rates and the fan pressure available and then select an appropriate filter using manufacturers data and considering the grade of filtration required.

From your description the usage a reasonable level of filtration will be required, probably a commercial disposable would be sufficient, giving a higher grade than a washable but not to the level of HEPA filters used in clean rooms and operating theaters and a lot better than the standard filters provided by some unit manufactures which only take out sticks bricks.

After making your selection compare the dimensions of the filters with the unit connection size, and use the larger. if the filters are a bit bigger than necessary its no real problem it just means you don't have to change them so often, also when using multiple filters in one bank, try and keep them the same size as it makes maintenance easier.

Hope this is of some help

regards

Don Smith