CFMS vs Tons 2

[stern]

Hello there,

I have a space that requires 2000 cfms according to room volume and air changes per hour but when I calculate the sensible plus latent loads 10 TR are required.

The rule of thumb is 400 cfm per ton so how does this work? what is the logic of this? 10 tons would be 4000 cfm or could there be the 10 TR with 2000 cfms.

Thank you..

 

[IRstuff]

The standard AC rule of thumb refers to square footage per ton, not air flow, and that particular value is on the low side, which is what your typical AC salesperson might use to sell you an AC system.

The air changes are different:

http://www.engineeringtoolbox.com/air-change-rate-room-d_867.html

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[stern]

Thanks IRstuff but the rule of thumb is 400 cfm per ton, I know this could vary with the application but I see everyone in A/C using this as general rule.

Im using psychrometrics software to calculate tons required and the 2000 cfm is an input value.

What I obtain is 10 tons required for total heat removal (sensible and latent), my doubt is if I recommend the 10TR whats with the cfms?

 

[cdxx139]

Rule of Thumbs are for contractors.
Psychrometric charts are for engineers.

400 cfm/ton is for typical office application. I remember some threads on this that people went into moe detail.

Find your delta enthalpy (Mixed air enthalpy - supply air enthalpy) using a psych chart. Plug into:
Total load = 4.5 * CFM * deltaEnthalpy.

That is your total load (whether its close to 400 cfm/ton or not)

Good luck

knowledge is power

 

[mauricestoker]

As a general rule, people have different size thumbs.

I don't think people in Alaska would use the same tonnage as someone in Texas.

A better rule of thumb is if someone tries to sell you AC based solely on square footage, use your thumb to gesture toward the door.

 

[stern]

Yeah right guys,

but the answer I was looking for I found it in another thread from Yeldud (Mechanical) and I think hes an engineer not a contractor

"The A/C unit only need to circulate enough air for the tons of sensible load ocurring in the spaces, because the latent load removal occurs at the evaporator".

Make sense?

 

[sspeare]

stern,

You say you "found your answer in another thread." If so, why are you still asking the forum if it "makes sense"?

I mean no offense, I really don't, but I have to ask: do you know what a psychrometric chart is?

Rules of thumb can be used by contractors, engineers, even architects (maybe I'm going too far with architects!!), but --> they can get you into big trouble if you don't know the fundamentals. Take it from somebody who knows. I'm a P.E., mechanical engineering, and I have been burned big-time by short-cutting the design process with "rules of thumb".

Calculate the load, and develop more information! Then post it on this forum. Then we'll see if the experts on the forum believe you have done YOUR part!

 

[stern]

sspeare,

Thanks, yes I know what a psychrometric chart is, I have used it for years and psychrometric software as well.

I just was directing to the guys that answered me, not all the forum!

I kept looking because their answers didnt satisfy me and I was sharing another guys thread that seems to hit the nail, but wanted their opinion.

I understand your point regarding rules of thumb and I agree.

I already calculated the load and it was included since my first post.

Perhaps I need more to learn on how to use this forum...

Regards,

 

[sprinkler1000]

"Im using psychrometrics software to calculate tons required and the 2000 cfm is an input value"

Stern: I do not know how your software works, but I assume you know that when you are estimating cooling loads, the air flow rate is an output based on your load calculation. More precisely it depends on your sensible load as someone said.

Air flow rates per ton depends on the equipment, e.g. vrf handles low flow rates per ton, but I do not think that as low as 200 cfm/ton.

 

[stern]

Yes sprinkler I agree with you,

But now if the sensible load requires the 2000 cfms and the total load is for 10 tons, what air flow would the 10 tons AHU be handling?

This was more or less the doubt since the beginning.

 

[imok2]

To repeat a previous answer: If 400 cfm of air and a delta T of 20 degrees is achieved, this is equal to only 400X20X1.08=8640 btu/h. However, this neglects the latent heat portion of the cooling load.

 

[mauricestoker]

The number of air changes per hour is not the same as the thermal load requirements. If outside air is below design temperature, you don't need any air conditioning, though your fan would still run to meet the ACH.

What is your plug load, personal loading and envelope load? Are you going to dehumidify with the coil? Round here that would mean about a 54* F discharge to stay within ASHRAE 55 range. What is the outside air set for?

Is there an electrical rule of thumb for estimating how much it costs to air condition based on square footage? Would this change if the building is insulated? Would this change if someone was running a heater inside the building?

 

[317069]

You said "I have a space that requires 2000 cfms according to room volume and air changes per hour"
- At the calculation stage, space cfm requirement is not based on ACH; it is based on sensible load, supply air temperature and space design temperature.
- after you select your equipment, the final cfm is what in the manufacturer product data.

Then you asked:” if the sensible load requires the 2000 cfms and the total load is for 10 tons, what air flow would the 10 tons AHU be handling?"
- if you have sensible load that needs 2000 cfm( again not based on ACH) and your total load is 10 ton that means: at 2000 cfm your equipment should be able to remove the sensible load and latent load(don’t depend only on the total number)
- The latent load has nothing to do with the cfm, you could have two equipments, both of them work on the same cfm but their nominal capacity is different, in other word you could have two units, one of them has a total capacity of 5 tons at 2000 cfm, the other has a total capacity of 6 tons at 2000 cfm too.

 

[MechEngNCPE]

cdxx139 has it right. That will give you your cfm at the dh required to satisfy your load. Now, if you need more air to satisfy your ac/h such as in a cleanroom, increase cfm accordingly.

 

[willard3]

Rules of thumb frequently precede arbitration where the arbitrator asks: "did you do an actual heat gain/loss or just use the rule of thumb?"

Rules of thumb are for preliminary estimates only and, when applied to final designs are the basis of lots of lawsuits.

 

[317069]

if you have two units work on the same cfm, same entering air condition, and same leaving air condition, that mean same enthalpy on both side of the coils, but physically the coils themself are different(one is bigger than other)
will these two units have the same latent capacity?

 

[cdxx139]

Same enthalpy tells me same total energy. Not necessarily same latent energy, unless the grains are the same, but this feels like a trick question. Is it? 317

knowledge is power

 

[317069]

it is not, it is just a question to learn.
- if you have two zones having the same sensible load and different latent load, will you provide them with a same or with a different cfm ?
- if you have two zones having the same zone loads but different coil load, will you provide them with a same or with a different cfm ?
- equation Q=4.5cfm(delta enthalpy) is used to determine the coil capacity not the cfm required for a zone.
- cfm for a zone could have any amount as long as the supply air condition is on the zone SHR line
- final selection of cfm is what in the equipment manufucturer product data which tells you this equipemnt in this condition will remove this amount of sensible heat and this amount of latent heat.

try this peoblem by using your equation:
space condition: db=75F, RH=50%
outside air dw/wb: 93/75 F
outside air ratio is 20%, return air is 80%
supply air db/wb: 55/54.51 F
zone sensible load is 21600 Btu/hr
what are the zone cfm, zone latent load, coil sensible load, and coil latent laod.

 

[stern]

317069 thank you,

you mentioned something very interesting " cfm for a zone could have any amount as long as the supply air condition is on the zone SHR line"

Ive been searching some sites and the ACH method taking into account space volumes to determine cfms is much used, besides in my psychrometric software the cfms are an input, not an output.

In real practice ive found cases where i dont have any load balances, just delta Ts required (OSA and desired room temp) and the space volumes so the use of ACH.

Id really want to see some answers to your questions.

 

[mauricestoker]

317069,

Most of the t-stats I've seen read dry bulb only, so even if envelope conditions or return temperature was given, I'd say the latent temp does not mean squat once you've gone past the cooling coil (unless you're talking about a walk-in, in which case the TD would be selected for latent loading).