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..

 

[Super65]

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

 

[Ziggypump]

Just up your airflow and recalculate.

 

[mauricestoker]

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.

 

[Super65]

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.

 

[Super65]

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.

 

[Super65]

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.

 

[stern]

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

 

[Ziggypump]

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.

 

[mtngreen]

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.

 

[mauricestoker]

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.

 

[Super65]

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?

 

[Super65]

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.

 

[Super65]

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.

 

[walkes]

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.

 

[stern]

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.

 

[Super65]

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

 

[mauricestoker]

Super65,

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