URGENT CFM REQUIREMENTS

[bosse1950]

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

 

[ChrisConley]

Well, if we assume that the room has no external heat gains or losses the simple calculation is:

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.

 

[imok2]

I believe the sensible heat factor (SHF) should be taken into consideration when taking the dry bulb DT over an evaporator to determine airflow. The dry bulb DT could vary from about 15oF, very humid return air, to 25oF, very dry return air. The 20oF DT is assuming about 50% relative humidity return air.

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

 

[HVAC68]

Q = 1.08 x cfm Delta T is true if Q is the sensible load.

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

 

[imok2]

An application such as a computer room would have a high sensible heat load and a 0.9 SHF would probably be a better choice.If you have the instrument and time to take wet bulb readings and determine relative humidity, do it. Your judgement of proper airflow will be more accurate. If you do not, you’ll have to add to your judgement of airflow your judgement of the return air’s humidity. Remember high humidity = lower DT and low humidity = higher DT.

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.

 

[walkes]

Imok2, HVAC68

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.

 

[HVAC68]

No evaporator coil ? You mean take 100% outside air directly into the room ? without cooling or heating or dehumidfying ?

HVAC68

 

[walkes]

Yes, I mean that the source air temperature for cooling is 50°F as per item D above. I don't know if it is outside air or conditioned air, I am assuming outside air since the ambient temperature is 50°F

 

[HVAC68]

In that case, only heating and maybe dehumidification is required. Will it be 50degF throughout the year ?

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

 

[GMcD]

Hold on a sec. Why are you all providing free engineering services here? I'm sure all our codes of ethics, no matter we are dialing in from say something about being fairly compensated for engineering design. Who will take the liability if all your design advice doesn't work? All I'm saying is let's be a little more professional about how we approach some of the responses here.

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.

 

[walkes]

I guess if we go back to the initial post, my answer would be to use something like Carrier HAP to input the required information. Set up the space, and HVAC system to suit your requirements.



GMcD, no war stories lately?