View Full Version : Measuring capacity of air diffuser

31-03-2017, 07:29 AM
Hi all,

im trying to get my head around how to measure the capacity in kw of what could be delivered out of an air diffuser.
im used to measuring the capacity of what coils deliver based on the following formula
kw = l/s x 1 X 1.2 x temp difference.
i understand I can measure the return air and get a temp difference.

what I'm battling with is say I have 2 rooms each with a diffuser that's delivering the same temp air and same air quantities but the rooms are different temps then my td will change and then the capacity changes ??

why if each diffuser is delivering the same temp and l/ s isn't the capacity the same of the air being introduced . Why would the room temp influence the capacity.
( I have assume the td to be based on the supply air temp and the room temp.)
if the one room is colder and we introduce the same warm air into both the formula dictates that we are introducing more capacity into the colder room.

therefore how can the colder room be getting more capacity than the warmer room if the flow rates and temps are the same out of the diffuser??

i have bought the the flir tg165 Infra red thermometer and have being thinking of how to measure the capacity of airdiffusers once I've measured flow rates and temps but I've stumbled on how to measure the temp difference . Obviously I've got the supply temp and the return air temp so it is easy enough but have got stuck on how the room temp , which is the temp I was going to use as the return air temp influences the capacity.
Is there a way to measure the capacity of air being introduced without a temp difference based on only the supply air temp and flow rate or do you need a temp difference???

my head aches.

31-03-2017, 09:25 AM
Would it help disregard the notion of "room temperature" and hypothetical two rooms

Instead, think of coil air on air off temperature (I know as "Split")

Now think this coil with zero split (Compressor is stuffed, no split air on air off), apply zero to your formula, Capacity Kw is obviously zero.

Now, apply 1k split, 2k split, 3k split etc. each k split increase offers corresponding increase kw given same volume air flow.

Unfortunately, real world calls for additional considerations in terms of "room" (solar radiation, infiltration, thermal radiation, internal loads, outside air etc etc, remember, factors could be making the room "cold")

If room is unexpectedly cold, check to see if my ex wife is sitting in the corner

Happy tinkers

31-03-2017, 10:00 AM
I can understand the capacity gained through a coil, but I'm trying to measure a quantity of heat capacity just being delivered out of a ceiling diffuser , therefore only one temp ( supply air only )
all the formulas require a temp difference ( ra and sa) , I'm looking for one that only requires supply air and l/s.?

therefore how many kw of heating or cooling are we getting out of that diffuser? I can always measure the return air temp at the return air grill, but I was wondering if I can get a rating just by measuring the supply diffuser.?

31-03-2017, 10:10 AM
I guess it could still work..

"Lots" of "cold" air entering a "warm" room probably is more Kw than "some" "coolish" air entering a "moderate" room, stuffed if I can put numbers to it.

I just refer to it as a two-chihuahua-power (has the power of two chihuahuas) for a small unit or your choice for a big unit - not too many arguments there. Not many people will argue about 1/2 chihuahua!

31-03-2017, 10:54 PM
Drew, what about this link.
See sensible heat load compared to air volume.


01-04-2017, 12:07 AM
Hypothetically, If you have 2 rooms and let's say the room 1 is 20'C and the supply air is 30'C so room 1 has a DeltaT of 10'C.
Room 2 on the other hand might have a return air of 15'C but supplies air at 30'C so room 2 has a DeltaT of 15'C.
Even though the air volume may be exactly the same, more heat is added to the air that goes to room 1, to heat it from 15'C to 30'C so that's where the extra capacity comes from. If the extra heat wasn't added to the room then the supply air would be 25 and the capacity would be exactly the same as room 1.

If the TD were both the same but the air volume to only one of the rooms changed then this would also change the capacity to that room, even though the DeltaT's were the same.

With the sensible heat formula that you've been using, the capacity (Kw) will always change if either the air volume/mass flow rate changes or the temperature difference changes.

To use the formula to work out the air volume you do need the temp difference and the Duty (Kw)
Equally to work out the temp difference, you need the duty(KW) and the air volume.
Finally to work out the duty, you need the air volume and the temp difference.

02-04-2017, 09:14 AM
Thanks guys.
I think I need to understand enthalpy and kw.
A watt is a j/s . Therefore I think we require something to do work ie the coil.
Therefore I need the proportional l/s through the diffuser and the temp difference between the supply and return air to see how much work in kw is done.
I cannot use just the supply temp and apply to formula as I'm working in work done ie kw??!
easy enough to measure both I suppose. Just being lazy and curious.

02-04-2017, 10:02 AM
By taking into account enthalpy, you're measuring the latent heat content (due to the moisture in the air) as well as the sensible heat. This would be the total work done by the coil.

Substitute the delta T for the humidity ratio difference (kg water/kg dry air).
You would still need to use the return air and the supply air enthalpy values to calculate this.

The formula needs 2 temperatures across the coil to work, before and after.