# Air flow calculation



## theconsult (May 8, 2011)

When I have to do electrical calculation, the first thing I will think about is the load capacity. But this time I need to do calculation for ventilation of a room (an electrical room). The maximum temperature is about 45degC. The room dimension, as I guess, it covers in 5 meter wide, 10 meters long and 3 meter high. So, we will install a ventilation fan for this room. Yes, this is a requirement.
So, what is the factor I should think of to do the calculation? What I need to do? What is the method applied for this? If I use an electrical ventilation fan for this room, is that so-call mechanical ventilation? Then what to be called if I use an air conditioner instead? 
Thank you,


----------



## beenthere (Mar 5, 2009)

Fan= Mechanical ventilation
A/C = mechanical Cooling.

You need to know the temp of the air you will be drawing in to know how much air you need to move to cool the room/equipment. along with how much heat the equipment will generate.


----------



## theconsult (May 8, 2011)

Thank you, the room temperature should be remained around 35degC. It means that I need to take out the temperarute mass of 10degC. Correct? I will use the normal fan installed on the wall.


----------



## beenthere (Mar 5, 2009)

BTU/(temp deltaX1.08)=CFM

The above is for Fahrenheit.


----------



## GettingBy (Aug 19, 2010)

beenthere said:


> BTU/(temp deltaX1.08)=CFM
> 
> The above is for Fahrenheit.


So for Celsius it should be 
BTU/({temp delta} x 9/5 x 1.08)=CFM
with {temp delta} in degrees C.

This will help
http://www.onlineconversion.com/
with the other stuff.


----------



## theconsult (May 8, 2011)

Sorry if I have the other stupid question. If I use this formula, how I can get the correct BTU? And what is CFM? Temperature delta is the temperature different before and after we install the fan? I have several books on HVAC so if you can suggest me the way how to do. I can do it.

Thank you for your time taken to answer my question,


----------



## GettingBy (Aug 19, 2010)

Cut and paste the line below into the Google search bar.

define: cfm


----------



## beenthere (Mar 5, 2009)

The BTU would be the heat that is generated in the room your going to cool.

CFM is Cubic foot a minute.

So if the equipment in that room is generating 50,000 BTUs of heat. And the desired max air temp in the room is 95°F, and your intake air is going to be 70°F.

50,000BTUs/(25°F temp delta*1.08)=1852CFM


----------



## flashheatingand (May 3, 2008)

So, where does one obtain the heat load of a network tower, and/or several monitors? Back in school, I do recall some sheet of paper with that info, but, really do not remember the source


----------



## flashheatingand (May 3, 2008)

Another valid question is what is the formula in metrics. The world is getting smaller.


----------



## theconsult (May 8, 2011)

What would be different from GPM and CFM?


----------



## flashheatingand (May 3, 2008)

I suspect there may be a language barrier. GPM is for a fluid such as water (Gallon per minute). CFM deals with a quantity of air (cubic feet per minute). Another challenge to this situation is that you are dealing with metrics which is foreign to most of us, including myself. 

First off, can you bring the room down to the desired temp simply by mechanical ventilation? or do you need some form of mechanical cooling. (fan only versus air conditioner). If you can bring in cooler air from an adjacent area and vent out the hot air, you would use mechanical ventilation. If you need an air conditioner to bring down the temp to what you want, you use mechanical cooling.


----------



## beenthere (Mar 5, 2009)

As above. Generally GPM is Gallons Per Minute, referring to a liquid. And CFM is Cubic Foot Per Minute, referring to a vapor.


----------



## beenthere (Mar 5, 2009)

flashheatingand said:


> I suspect there may be a language barrier. GPM is for a fluid such as water (Gallon per minute). CFM deals with a quantity of air (cubic feet per minute). Another challenge to this situation is that you are dealing with metrics which is foreign to most of us, including myself.
> 
> First off, can you bring the room down to the desired temp simply by mechanical ventilation? or do you need some form of mechanical cooling. (fan only versus air conditioner). If you can bring in cooler air from an adjacent area and vent out the hot air, you would use mechanical ventilation. If you need an air conditioner to bring down the temp to what you want, you use mechanical cooling.



Elevator and electrical equipment rooms are often cooled with fresh air only. As they are generally allowed to be kept at some temp under 100°F. But not say required to be kept at 75°F. So X CFM of 90°F air will often be enough to keep the room and equipment cool enough. Depending of course on the amount of heat the equipment generates.


----------



## theconsult (May 8, 2011)

How do I know the exact BTU of the electrical equipments are generating inside that room? I just can take temperature measurement.


----------



## GettingBy (Aug 19, 2010)

theconsult said:


> How do I know the exact BTU of the electrical equipments are generating inside that room? I just can take temperature measurement.


Assuming all the elec. power in is converted to heat, 100,000 BTU = 29.3 kwh, so 29.3 kw for one hour generates 100,000 BTU. 

A cubic meter is 35 cu. ft., so
BTU/({temp delta} x 9/5 x 1.08)=CFM
becomes 
BTU/[({temp delta} x 9/5 x 1.08) x 35] =Cu. meters per minute


----------



## MechAcc (Feb 27, 2005)

Ventilation will require bring outside air into the room to replace the air being exhausted. This air will contain dust and dirt which accumulate on the surfaces of the electrical equipment. 

Air conditioning the room would lower the dirt accumulation and may be a better solution. If air conditioning is the method you choose price in a head pressure control for low ambient conditions.


----------

