Q: Using Carrier's Method... 2

[CuriouslyGeorge]

When accounting for sensible heat in outside air, Carrier shows the following formula: OASH = 1.08*cfm*(Toa - Trm). Where OASH = outside air sensible heat, Toa = outdoor air temp, and Trm = room (conditioned space) temp.

But, why isn't it: OASH = 1.08*cfm*(Toa-Tsa)? Where Tsa = supply air temp, which is what the air has to be cooled down to?

I need a good argument for why the latter formula is too conservative to convince a colleague as we're now getting a larger answer using this variation from the Carrier formula!

Personally, I believe the answer is something like you just have cool down the outside air to match room conditions. By doing this, you end up with a smaller duty and the coil will only cool so far thus limiting your SA to conditions closer to the room design temp. (Maybe I'm applying some kind of short-cut???)

Thanks,
- CuriousGeorge

 

[KenRad]

Imagine a case where your outdoor air conditions exactly matched your room conditions. What contribution would the outdoor air make to your sensible cooling load? Of course the answer is none. No matter how much of this "neutral" air we push into the room, the sensible load is unaffected.

In the same way, we establish the sensible load contribution of outdoor air by what it would take to bring it to room conditions. Now we introduce the air in the return plenum and cool it to supply air conditions, but this is a result of the loads in the room, not the outdoor air itself. Consider a case where this room had no cooling load at all. The outdoor air would then have to only be brought to room conditions before entering the room.

---KenRad

 

[HVAC68]

George,

KenRad is right. I suggest that you put actual conditions in a psychrometric chart. You will get a much better picture.

HVAC68

 

[pegboy]

I had the same question a ways back and called Carrier to explain. After you think about it for a while, it does make sense.

I know when I calculate the sensible load for outside air for wintertime, I always use 70F(Trm)-10F(local Toa). I don't worry about the actual leaving air temperature from the heating unit. Any heating of the outside air above 70F covers the sensible loss from the room.

This works the same for cooling.

 

[lilliput1]

Because you get a credit in cooling the air below the indoor room design temperature. It does cooling to offset other sensible loads. So the cooling from room temperature down to ccoling coil leaving condition is not included as a penalty.

 

[NNACons]

But, why isn't it: OASH = 1.08*cfm*(Toa-Tsa)? Where Tsa = supply air temp, which is what the air has to be cooled down to?

No you are confusing two important things.

1) OASH = 1.2x(l/s)x(Toa-Tra) where rm = room conditions.
2) RSH = 1.2 x (l/s)x (Tma-Tsa) where ma = mixed air on conditions and sa=off coil supply conditions.

OASH + RSH = TSH (total sensible heat of space).

note formula 2 is the complete supply air amount including the outside air component which has had its sensible load component already accounted for as part of formula 1).

hence the remaining sensible load must be the cooling of this o/a plus return air (S/A = O/A + R/A) from ma conditions onto the coil to required off coil temperatures.

your formula is really only valid for a 100% outside air system where there is no r/a and hence the o/a introduced must be sensibly cooled from o/a conditions to standard room conditions... and then again from this condition (mixed on coil condition - although no mixing as no return air) to required off coil condition.

For any other situation your formula is incorrect.

proof of the above.

OASH for 100% outside air TSH = 2) + 1)
= [1.2x(l/s)x(Toa-Tra)] + [1.2 x (l/s)x (Tma-Tsa)]
= [1.2x(l/s)x(toa -Tra)] + [1.2x(l/s)x(Toa- Tsa)]
where Tma is the 100% o/a introduced.

Hope this helps

 

[quark]

Excellent post by Lilliput1 and a star for him.

Presuming cooling application and pure sensible loads everywhere,

If you are using the formula(in your terms 'the carrier formula') for a recirculated system, it can work provided your total room sensible load is taken care volume flow rates excluding the fresh air flowrate.

If Q1 is the recirculating flowrate in cfm and Q2 is fresh ari flowrate in cfm,Tm is temperature of mixed air in deg.F, Ts is supply air temperature in deg.F and Tr is temperature to be maintained in the room in deg.F, if 1.08xQ1x(Tm-Ts) will take care of the room sensible load then 1.08xQ2x(Tm-Tr) will take care not to add any extra sensible load to the AHU. 1.08xQ2x(Ta-Tr) will give you extra benefit redundant.

If this is 100% OA application then it should be 1.08xQ2x(Ta-Ts) where Ta is ambient temperature. If the inside conditions are exactly equal to outside conditions then Ta = Ts and there won't be any load on the system. But if you use 1.08xQ2x(Ta-Ts) there will be temperature increase in the room because you are not taking enough care of the room sensible load. Ta is ambient temperature is deg.F

NNACons,

Your first equation can be modified as OASH = 1.2x(l/s)x(Tma-Tra) where l/s is flowrate of fresh air.

Secondly, in your proof, if l/s is fresh air flowrate then the term 1.2x(l/s)x(Toa-Tra) is redundant.

Regards,

 

[NNACons]

Just noted an error in my previous post\

100% o/a
[1.2x(l/s)x(toa -Tra)] + [1.2x(l/s)x(Tra- Tsa)]
which becomes

[1.2x(l/s)x(Toa-Tsa)] = Total sensible heat of 100% outside air

Quark
Your first equation can be modified as OASH = 1.2x(l/s)x(Tma-Tra) where l/s is flowrate of fresh air.

Quark yes it can be modified to that where ma is taken as oa ambient condition and ra is the room design condition

but i think
OASH = 1.2x(l/s)x(Toa-Tra) where rm = room conditions.
is more universal for non 100% outside air systems.

 

[quark]

My point is that when outside air mixes with recirculating air, part of OA sensible heat is transfered to the recirculating air(that is why mixed condition temperature, for a cooling application, is higher than return air condition). So 1.2x(l/s)x(Toa-Tra) can be replaced by 1.2x(l/s)x(Tma-Tra) where Tma is the temperature of the mixed air.

Regards,