Discharge temp needed to maintain room temp
Discharge temp needed to maintain room temp
(OP)
We have a project where the required temperature in the room is 70 deg F +/- 3. Someone told me that in this situation, you have to plan on 67 degree F supply air an d73 degree F return air. Someone else said to provide the required cooling we should discharge 57 degree F air. Will 57 degree air mean we are out of spec on the temp requirement?
Thanks for any insight you can give me!!
Thanks for any insight you can give me!!





RE: Discharge temp needed to maintain room temp
What is your heating load and when or is the room interior?
What is your cooling load?
No matter what anyone says, first you have to do the above to know delivery air temps.
RE: Discharge temp needed to maintain room temp
The narrower the temperature range, the larger the volume of air that must be supplied.
TTFN
RE: Discharge temp needed to maintain room temp
RE: Discharge temp needed to maintain room temp
The systems we use are min 14 degrees delta T geothermal abd 15 Delta T Heatpump/AC. I would be leary of a 6 degree differential requirement.
Bubz
RE: Discharge temp needed to maintain room temp
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
Bubblehead is right that 6F is rather small temperature difference. Good suggestion from AbbyNormal, as usual.
Read the FAQ403-1255 for all your fundamental doubts.
RE: Discharge temp needed to maintain room temp
RE: Discharge temp needed to maintain room temp
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
That is a silly mistake. Read it as 70F instead of 73F
RE: Discharge temp needed to maintain room temp
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
If you want to use very cold air to air condition you can use less, but you need to ensure the air is well mixed (diffuser selection) or it will 'dump' into the space.
What kind of space is it?
RE: Discharge temp needed to maintain room temp
We are also trying to maintain 70 Degrees DB with 50 Degrees F Dewpoint (it's about 50% RH).
I will be reading the FAQ as pointed out by quark. They look very helpful.
If I understand shrisconely correctly, as long as we have good mixing from the diffusers, we can have a colder temperature and still remain in spec.
RE: Discharge temp needed to maintain room temp
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
If it is mostly sensible you may want to think about blowing the air through the cooling coil.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
thatsa biga unit!
hehehe.... ok ok... outta my ballpark now!
Ima Noob!
Bubblehead
RE: Discharge temp needed to maintain room temp
Supply air CFM required = 1200000/(1.1x16.5) = 66116 say 70,000 CFM. Note if most of the ductwork is out of the conditioned space, then allow for duct leakage loss.
Latent heat capacity =66100 x .69143 x (.0084-.0074)x 7000 =319,925 Btu/hr Confirm if ok and adjust C.coil LAT accordingly
supply fan bhp = 70000 x 4.5 /(6344 x .65) = 76.4 bhp
return fan bhp = (70000 - OA CFM)x1.5 /(6344 x .65) =
at 15% OA RA bhp = 21.6
Check supply air temperature rise = (76.4+21.6)x2545/(1.1x70000) = 3.24
Refine & redo calcs
RE: Discharge temp needed to maintain room temp
He will have a bit of air moving for sure, and a draw through fan is going to reheat that air, stealing the cooling. Could lose a couple degrees or 10%. Try to over cool you are forced to dehumidify even more or move even more air. Almost like a spiral.
So, sometimes in that situation it is easier to blow the air through than to over cool to compensate for fan reheat.
Easier for the system to remove the fan heat ahead of time than to pull extra Btus out of air that is already on the cool side.
With a DX system for example, not that they would be using one here, maintaining 70F probaly knock close to 25% off of the nominal capacity of the equipment if they were maintaining 75F for example.
So blowing air through sometimes is the answer.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
RE: Discharge temp needed to maintain room temp
You mention down stream filters, I hate them but could be some half loaded HEPAs if it is any kind of clean space environment.
But besides air flow, it is just easier for the coil to take the blower heat out directly. More heat going into the coil, more heat it can grab.
If that space load is all sensible, then you end up being forced to dehumidify in order to get the supply air cold enough to compensate for the draw through reheat.
I did a main telephone switch and I took a lot of steps to avoid condensation, including deliberately winding up the air flow so that the supply dry bulb was 5 degrees above the space dewpoint. In my project, blow through was less airflow and less total cooling.
A real humid load and the fan reheat helps you out, but it is at least worth looking at blow through on the big sensible SPACE loads, like SHR>=0.90
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
RE: Discharge temp needed to maintain room temp
If it was critical computers then, it could also be rendundant muliple systems and main cooling plant
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Discharge temp needed to maintain room temp
The system does have multiple units as our client always wants redundancy and safety factors etc etc.
Thanks again.