Airflow and system capacity

[splus]

So if i have a 2 ton split system r410 with 700 cfm across the coil system charged correct with a 18 degree temp drop across the coil and i adjust airflow to 900 cfm adjust charge and have a 16 degree temp drop across the coil which is the better system as far as total capacity?

[kls-ccc]

At 18/700 cfm your getting 13,600, at 16/900cfm you get 15,500 ~ Sensible. This does not say anything about what is happening to your latient. YOu need to get a wet bulb measurement to get the total capacity.

Formula'a are CFM x 1.08 x temp drop = sensible. CFM x 4.5 x enthalpy drop = total heat.

[gonnagetcha]

To figure capacity the formula is - enthalpy difference x 4.5 x cfm

to get the enthalpy difference you need the webulb readings then you can convert that to enthalpy using a calculator

with air conditioners most people adjust airflow to 350-400 cfm and heat pumps around 400-450 cfm

with the lower range of airflow you gain better humidity control

[gonnagetcha]

At 18/700 cfm your getting 13,600, at 16/900cfm you get 15,500 ~ Sensible. This does not say anything about what is happening to your latient. YOu need to get a wet bulb measurement to get the total capacity.

Formula'a are CFM x 1.08 x temp drop = sensible. CFM x 4.5 x enthalpy drop = total heat.

you beat me to the punch

[Six]

Delta T is not an indicator of a proper charge

[ferd1942]

Designwise, 350 CFM/Ton is used in moist climates (less sensible, more latent), 450 is used in dry climates (more sensible, less latent). 450 provides higher total cooling.

[splus]

Thanks for all the good info,i am running into alot of systems that the airflow is either way to high or set on low speed and with days of 90 degrees an above a system not set up right on installation is having issues.i had one 2.5 ton system running at 800 cfm with a 18 degree temp drop that ran almost all day,i got the airflow right and the temp drop was 16 and it would cycle at 74 degrees on a 90 degree day and it never did that before.

[motoguy128]

Ideally, you use dehumidification on demand and slow the blower when more latent capacity is needed. Otherwise you have to find a best compromise between sensible and latent capacity.

On the systme above, the perfect point might be somewhere inbetween. Maybe 900-950CFM on a 2.5 ton system so it has enough sensible for desing conditions, but enough latent for cooler weather.

[Ed Janowiak]

Total capacity will be higher with a higher sensible heat ratio. Higher air flow will give you a higher SHR, lower airflow produces a lower SHR. You should match the SHR of your equipment to the SHR of the space you are trying to condition.

[hewitt]

What method did you use to get your cfm

[ch4man]

Total capacity will be higher with a higher sensible heat ratio. Higher air flow will give you a higher SHR, lower airflow produces a lower SHR. You should match the SHR of your equipment to the SHR of the space you are trying to condition.

Ed, what do you mean by" the SHR of the space" ??

are you talking design conditions?

ive been living and breathing capacity calculations this year to learn and the more i do the more i find myself not trusting rated airflow by dipswitchs or blower curves/TESP

[Ed Janowiak]

Your total gain is the sensible gain plus the latent gain. If you divide the total gain into the sensible gain it will give the SHR of the space (sensible to latent ratio).

A "perfect" match is a system with the equpitments SHR matching the spaces SHR.

[ch4man]

thx Ed, i was afraid your were going to say MAN J........