URGENT CFM REQUIREMENTS
URGENT CFM REQUIREMENTS
(OP)
I need a calculation program for actual cfm of cooling air flow requirements for the following senario:
A) desired final equipment room temperature: 75 deg.f
B) overall equipment room size = 60.5ft. X 12.66ft. x 25ft. high
C) overall equipment room btuh heat load = 190,683
D) external source air temperature utilized for cooling of equipment room = 50deg.f
E) equipment room wall construction: polyurethane panels 4" thick
F) equipment room floor = non insulated concrete
F) external ambient temp.= 50 deg.f
your input on this matter will be greatly appreciated
Ray Bosse
A) desired final equipment room temperature: 75 deg.f
B) overall equipment room size = 60.5ft. X 12.66ft. x 25ft. high
C) overall equipment room btuh heat load = 190,683
D) external source air temperature utilized for cooling of equipment room = 50deg.f
E) equipment room wall construction: polyurethane panels 4" thick
F) equipment room floor = non insulated concrete
F) external ambient temp.= 50 deg.f
your input on this matter will be greatly appreciated
Ray Bosse





RE: URGENT CFM REQUIREMENTS
Q = 1.08 X delta T X cfm
In your case
190,683 = 1.08 X (75-50) X cfm
CFM = 7065
If there are external loads you can calculate them, but from your external ambient temperature I'm assuming your summers are even colder than mine.
RE: URGENT CFM REQUIREMENTS
The air handler formula BTU/HR=CFM x DT x 1.08 won’t work with an air conditioning coil unless a SHF is considered.
BTU/HR x SHF = CFM x DT x 1.08
For example: If we try to determine the CFM’s using the formula for a three ton air conditioner on which we’ve taken a 20oF DT without using a SHF, we will get an erroneous answer.
36,000 = CFM x 20 x 1.08
Solving for CFM we’d get 1666.67 CFM. This indicates too much air.
Inserting a SHF of .75
36,000 x .75 = CFM x 20 x 1.08
Solving for CFM we get 1250 CFM, about the right amount of air for a three-ton system.
So with that in mind, I would say 5,300 CFM would be a better choice
RE: URGENT CFM REQUIREMENTS
With 190683 Btu/hr (15.9 tons) for a 766 sqft room (60.5' x 12.66') - works out to about 48 sqft/ton (check figure) - which is very very high - What is the application ? Is this a very high sensible load application ? If yes, the cfm would work out to be very high. Also, the SHF would hover around 0.9.
With the same calculation done by imok2 and substituting 0.9 for SHF instead of 0.75, the cfm works out to 6360 cfm. With that the cfm/sqft (another check figure) works out to 8.3 - Again very high.
Yes, there are applications with high sensible load - is yours one like that ? Just check out for errors, if any in your total heat load calculation.
We use check figures to locate errors, if any.
HVAC68
RE: URGENT CFM REQUIREMENTS
If the DT is greater than required across the evaporator, we are dealing with an airflow problem. Too low an airflow makes the air stay in contact with the coil too long.
RE: URGENT CFM REQUIREMENTS
from Bosse1950's description I would assume that he is using 100% outside air for cooling the space. Therefore no evaporator coil. 7065 CFM as per ChrisConley should be correct for this application.
RE: URGENT CFM REQUIREMENTS
HVAC68
RE: URGENT CFM REQUIREMENTS
RE: URGENT CFM REQUIREMENTS
With such a low outside ambient temperature - effectively meaning no outside heat gain - rather, heat loss only - the heat generation inside seems to be too high. See the check figures I had indicated in my earlier post. Seems very very high to me - What kind of application is this to generate such a huge sensible load ? Steam turbine room ?
HVAC68
RE: URGENT CFM REQUIREMENTS
My advice to the inital post would be:
Calculate the heat gains/losses from the room envelope that the equipment is in, get a total heating/cooling load, then calculate the CFM required based on the air source being used. Is the air source a constant temperature? How is the room being controlled? Is the heat output of the equipment constant? What is the ambient humidity required? What are the source air conditions? Too many un-answered questions before plunging in with free engineering here folks.
RE: URGENT CFM REQUIREMENTS
GMcD, no war stories lately?