CFM vs Capacity
CFM vs Capacity
(OP)
Realistic hypothetical: Lets say you are designing an office with DX split systems and the load calc comes to:
35,700 sensible btu/h
3,500 latent btu/h
39,200 Total.
This could probably be covered nicely by a nominal 3.5 ton system pretty nicely. Now lets say the cooling cfm required is 1595 based on a 20 degree TD. Would you size your unit based on the capacities, or select a unit that can be the CFM.
I struggle with this a lot with offices because of the high sensible heat ratios where I will have a 0.95 + sensible heat ratio. Any opinions?
35,700 sensible btu/h
3,500 latent btu/h
39,200 Total.
This could probably be covered nicely by a nominal 3.5 ton system pretty nicely. Now lets say the cooling cfm required is 1595 based on a 20 degree TD. Would you size your unit based on the capacities, or select a unit that can be the CFM.
I struggle with this a lot with offices because of the high sensible heat ratios where I will have a 0.95 + sensible heat ratio. Any opinions?





RE: CFM vs Capacity
RE: CFM vs Capacity
RE: CFM vs Capacity
RE: CFM vs Capacity
I understand what you are saying, but if I selected a compressor based on the load, and a fan based on the CFM, the unit would have to produce well over 400 cfm/ton which isn't possible with DX equipment usually.
In my case, lets say I selected a 3.5 ton compressor to meet the capacity. Then I selected a fan that could provide the 1595 CFM. That would equate to 456 CFM/ton. The unit probably would not be able to do any de-humidification at all. It doesen't really have any latent involved here but even so.
In my opinion, its better just to go to a 4-ton system.
RE: CFM vs Capacity
Not sure how dehumidication is an issue if you have no latent load.
The CFM/TON (TR-tons refrigeration) depended greatly upon the outside air portion and internal load. As you seem to have neglible internal and outside air latent loading-otherwise you woud not have 95% sensible load.
At roughly tons and 1600 CFM, won't you have 400 CFM/TN? No outside air, minimum internal load, what difference does a rule-of-thumb make? For the climate where I'm at, high humidity, applications for 100% OA often come out to 190 CFM/TN.
RE: CFM vs Capacity
I know some people like to try and make the static exactly equal to the CFM they are trying to achieve based on the fan curve but this usually isn't reasonable in my experience. I always figured the higher the static, the more money it costs to run.
RE: CFM vs Capacity
I would say using the rule of thumb 400 CFM/ ton is just that, a rule of thumb but I wouldn't want to exceed that by too much.
RE: CFM vs Capacity
RE: CFM vs Capacity
For packaged equipment (roof tops) we will size on the sensible capacity based on our loads. Look closely at the sensible capacity of equipment you are looking at. Coil bypass factors and related come into play in addition to the zone SHR. A 4 ton roof top unit will most likely not give you 4-tons of sensible cooling as they are designed approximately around the 400 CFM/ton rule (meaning coil bypass factors and some assumed zone SHR?? Not sure what they do... I used to). Thus/ with packaged equipment you almost always end up w/ an over-sized jobber.
On split systems I have successfully designed larger indoor coils (for the sensible load) and smaller ACCU's (condensing units). However check closely w/ the mfgr when doing this.
So on your hypothetical system I would be looking at a 4-ton coil (1,600 CFM) and a 3.5 ton ACCU.
RE: CFM vs Capacity
RE: CFM vs Capacity
There is virtually no infiltration in this building (2nd floor of a almost new 3 story building)and the outside air requirement is almost negligible so I don't really think the climate affects me too much with respect to dry vs humid.
RE: CFM vs Capacity
RE: CFM vs Capacity
RE: CFM vs Capacity
RE: CFM vs Capacity
RE: CFM vs Capacity
RE: CFM vs Capacity
35,700 btu/h = CFM*1.085* delta T
delta T = Tmix-Tsupply
You know your delta T because you know your mixed air temperature and your discharge air temperature. Solving it for a twenty degree delta T you get 1645 CFM.
Use your psych chart to determine what supply air discharge temp you need to achieve a certain relative humidity in the space. Remember to use the slope at the 95 sensible ratio line.