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CFMS vs Tons
2

CFMS vs Tons

CFMS vs Tons

(OP)
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..

 

RE: CFMS vs Tons

(OP)
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?
 

RE: CFMS vs Tons

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

RE: CFMS vs Tons

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.

RE: CFMS vs Tons

(OP)
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 smile

"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?
 

RE: CFMS vs Tons

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!

RE: CFMS vs Tons

(OP)
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,
 

RE: CFMS vs Tons

"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.

RE: CFMS vs Tons

(OP)
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.

RE: CFMS vs Tons

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.
 

RE: CFMS vs Tons

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?
 

RE: CFMS vs Tons

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.
 

RE: CFMS vs Tons

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.

RE: CFMS vs Tons

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.

RE: CFMS vs Tons

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?
 

RE: CFMS vs Tons

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

RE: CFMS vs Tons

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.
 

RE: CFMS vs Tons

(OP)
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.

RE: CFMS vs Tons

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).

RE: CFMS vs Tons

Sorry Maurice I had to ad this, Wouldn't you use a humidistat for that in conjunction with a tstat?

RE: CFMS vs Tons

Just up your airflow and recalculate.

RE: CFMS vs Tons

Super65,

I wouldn't use a humidistat with it becuase I would never trust it to stay in calibration for calculating the dew point for controlling the cooling coil valve. If I were going to go to dew point control, I'd probably be looking at a chilled beam first. In most applications, it suffices to have a cooling coil temperature below the dew point 99% of the time. Where I'm at, that's about 54* F. I would include a humidistat for humidification.

I'm not sure what the connection is between calculating minimum air flows for indoor environmental quality and latent loading, and it has no connection to thermal loading calcultions. For example, if limiting discharge temperature as per ASHRAE 55, using max 95* F supply air, and 4 ACH will not provide enough heating, the only thing to do is increase your supply air. If you already ordered and installed your AHU based on 4 ACH, you are SOL. Or, you might reset the discharge temperature to about 135* F, and hope no one notices until you're out of town.

Answering the question in the OP, there is no logic in the rule of thumb.  Sort of like "I need to hook up 480V from the MCC to the fan motor, why can't I use thuis spare lighting cable?" You will find more logic in watching old episodes of Star Trek with lots of Vulcans. Repeating what is often said in the threads, recommend you get a mechanical engineer to do mechanical work. If this is some sort of cross training thing, maybe you could hire a mechanical to do the electrical design, then he/she can pose their questions on an electrical forum.

RE: CFMS vs Tons

Mauricestoker,

Sorry I realized that was an ignorant response after I posted it. I knew better.

I teach a night class at the local tech center and latent heat is probably the most complicated subject to explain to people who just want to service acs or install them. When you get into the psychometric charts and talking about grains etc and the change of state and heat absorbed beyond..I tend to lose them, when that's in the first chapter. Enthalpy is not a good subject for most people.  

RE: CFMS vs Tons

I did want to add this, if you're cooling a home or building with basic construction grade materials, and you run a manual J calculation it will come within 400-1000 btu's of 400 square feet per ton. Then the equipment you purchase is only in increments of 6,000btus per ton. I am not saying this is correct to do, but I have done homes using both methods to compare and you can get away with it, but you are doing a lot of assuming and if you are wrong it will be on you.

If you do not size for the latent load, humidity will be a large factor. Sick building syndrome was created this way. Pushing too much air with to little humidity control, or vice versa. Air quality can be linked too humidity control most of the time. Too much or too little contact with your evaporator in the summer.

Now that being said. 400 CFM per ton is what most units were based on when they were 10 seer or less, with PSC single speed or non variable speed fan motors.

ECM or other variable speed technology can allow a unit to absorb a varying amount of Btus through the cooling and condensing coils. Allowing you to have varied air speed and achieve the same capacity. Air flow will vary with any motor based on static pressure, duct size. You just want to match air flow across the coil to absorb and displace the amount of BTUs required to absorb the load.

RE: CFMS vs Tons

I meant 500 square feet per ton and increments of 6000btus between tons. (1/2tons) and Rules are thumb are like Maurice said..vulcans and faeries and bullcrap.

RE: CFMS vs Tons

(OP)
Forget about rules of thumbs, why manufacturers catalogs have cfm ranges about 400 cfm/ton ?

 

RE: CFMS vs Tons

Here, you have 2000 cfm with 10 tons of cooling.  Size a rooftop or air handler and see if the 10 ton unit can handle 2000 cfm.  If not, up the cfm to 3000-3500 cfm and recalculate your OA and psych chart.  Take into account air changes and all of that, but if you need 10 tons of cooling, you need 10 tons of cooling.  If the space is fairly large, the excess air should not be a problem.

RE: CFMS vs Tons

If you are using standard AC units forget the ACH calculation and base the CFM on the load calc.  The ACH calc has nothing to due with the heat gain into and within the room.

RE: CFMS vs Tons

Super65,

I didn't think it was a stupid question, but rather a segue into the subject. A stupid question would be asking your wife if her sister is hot. I think mtngreen answered the issue in a lot less words than I did.

RE: CFMS vs Tons

Stern,

Nominal cfm at .1 static for most manufacturers was the equivalent of 400cfm per 12,000 btus. It was in blower performance data, is that the question or did I miss the point?

RE: CFMS vs Tons

Here familiarize yourself with one of these. http://icpindexing.toddsit.com/documents/086477/50911400101.pdf

Go to page 25 this is a pretty good example of a 15 ton unit, bigger than what you're talking about and it can if installed this way use 3900 cfm. Where as if you use you're rule of thumb you'd expect it to move 6000cfm whereas the range of this particular unit can be anywhere between 3900 to 6000cfm.

This one page should answer your question, or the equipment manufacturer of your choice.

RE: CFMS vs Tons

http://icpindexing.toddsit.com/documents/086477/50811160201.pdf page 30 shows a 120kbtu (10 ton) that could move 2120cfm which is probably close to what you want. Just showing you example of how airflow could very and that the rule of thumb as you now are aware I'm sure is crap. As we collectively provided many confusing answers to justify such.

RE: CFMS vs Tons

Wasn't the 400 CFM rule of thumb based on an average sensible heat ratio of 0.72, Such that it is a nominal value.

12,000 btuh of which 8640 btuh sensible.  Then with a 20 dT  (75F room temp and 55F supply) the airflow is approximately 400 CFM.  Q = 1.08 CFM * dT

The nominal 400 CFM/ton would be close enough in preliminary stages, without knowing details about space loads.  The sensible heat ratio would vary once details are brought in to the discussion and therefore the airflow would vary with it.  The manufacturer's equipment should have some capacity to adjust airflow from the nominal amount for actual design conditions.

 

RE: CFMS vs Tons

(OP)
Thank you walkes,

I agree with your simple explanation, in the field there are cases where loads are unknown and you have to give an estimate in A/C requirements so this rule could be a good start.



 

RE: CFMS vs Tons

Very nice. These are the reasons I need to take engineering classes.

RE: CFMS vs Tons

Super65,

I would recommend looking at ASHRAE 90.1 Appendix G. That would be a starting point for calculating loads.

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