Rule of thumb 2

[ChaChaMan]

I know already as a rule of thumb you need 450 cfm per ton of cooling.

How many cfm per 1KW (3412Btu) of heating ?

Thx in advance !

 

[MintJulep]

You can't use cfm with kW. CFM are inch-pound and kW are metric. The same unit won't be able to use both.

 

[ChaChaMan]

sure you can .....just factor the conversion, wich I state in parentheses in my post.

 

[ione]

1 ton of cooling = 12000 BTU/h = 0.00029*12000 kW = 3.48 kW

So 450/3.48 = 129.3 cfm per kW

 

[ChaChaMan]

My point was, don't you need less air flow when heating, as apposed to cooling ?

 

[ione]

Thermal power is thermal power

Air mass flow rate * specific heat * deltaT

It doesn’t matter whether cooling or heating, just use consistent units

 

[IRstuff]

He's looking for differences in ACPH between heating and cooling.

TTFN

FAQ731-376

 

[willard3]

The difference will be proportional to delta T for heating/cooling.

 

[ChaChaMan]

I thought given that usually deltaT is bounded, (ex.Return Air at 60F and Supply air at 85F ) and air is usually at 35 % rh during heating, and specific heat is constant, that maybe the outcome as to cfm/Btu is generally within typical range.

 

[DRWeig]

Decide what temperature supply air you need, and what temperature you will control the space you're heating. I guess you've already computed a heat load since KW of heater is a given...

Example: 1 kW heat load, room temp desired 70°F, supply air temp desired 120°F. That gives you a delta-T of 50°F. It's all sensible heat, so the air flow will be:

CFM = (1 kW * 3413 btu/kW)/ 50°F * 1.08 btu/°F/cfm)
CFM = 63 cfm per kW

So a good rule of thumb for those conditions might be 65 or 70 cfm/kW?

Good on ya,

Goober Dave

 

[DRWeig]

Oh, and that's roughly half the cfm per btu of cooling, due to: 1) higher delta-t with heating, and, 2) heating is all sensible, no latent.

Good on ya,

Goober Dave

 

[ChaChaMan]

DrWeig...that's what my intuition was telling me ....«half the cfm/Btu of cooling»

Also, you say 100% sensible, however the air does contain 35%rh. Is it really 100% sensible ?

 

[DRWeig]

Yes, the heat you're adding only raises the dry bulb temperature (with corresponding drop in RH -- but dew point remains unchanged). Walk youself through the process on a psychrometric chart, it's a big help in understanding.

Heating adds no moisture to the air, nor does it remove any moisture from the air. It only changes temperature.

Good on ya,

Goober Dave

 

[DRWeig]

One more quick note -- the 1.08 btu/°F/cfm will change with both humidity and pressure changes. It's valid at sea level and 50% RH.

However, it doesn't change much. For purposes of this discussion, it's best left alone. After all, we're talking about cfm of air, which nobody can measure to 3-digit accuracy in most building applications.

As pressure (and thus density of the air) rises, the 1.08 factor will go up. As humidity (and thus specific heat of the air) rises, the 1.08 factor will go up.

Good on ya again,

Goober Dave

 

[ChaChaMan]

My question has been answered. Thank you to all who chimed-in for the very valuable insight !!!

 

[ChaChaMan]

Post-script :
If you are installing duct heaters, you need a minimum of 50 cfm per KW of heat. Other wise your ductheaters will overheat. So there is a minimum, or lower limit, you could work off of !

 

[mauricestoker]

How does that work for you with economizers?

 

[ChaChaMan]

....sorry Maurice, not sure what you mean ?

 

[mauricestoker]

I mean when you go into free cooling on a standard configuration, would your rule of thumb be applicable? If you go to a dual duct/dual economizer, what is the relevance of the rule of thumb for heating? It goes to the idea of the energy use being dependent on the delta T and the outside air portion. If a rule of thumb worked, then the economizer and relief dampers would not need to modulate. As a result, area served per ton can vary over 200%.