Discharge temp needed to maintain room temp

[hvacnerd]

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!!

 

[willard3]

To start with, have you done a heat gain/loss on this room?

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.

 

[IRstuff]

Another way of looking at it is that the heating load must be offset by the amount of cooling provided by the A/C.

The narrower the temperature range, the larger the volume of air that must be supplied.

TTFN

 

[imok2]

hvacnerd, What RH are you looking to hold?

 

[Bubbleheadski]

6 degrees delta T wont cut it. any system I have worked on says to much air flow or undercharged at 6 degrees delta T.
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

 

[AbbyNormal]

discharge air control with room reset

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[quark]

If you have to maintain 70F and your return temperature is 73F then you have some portion of the controlled space beyond 73F. This is negating your requirement. So, your return condition is obviously your control condition.

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.

 

[imok2]

Well I can't seem to get an answer so I will try this> A proper delta T for air conditionig is usually about 18*F at 400 cfm/ton However, depending on the wet bulb, it can vary anywhere from 15 to 20 *F depending on what wet bulb the return air temperature is. So Is 57*f OK? depends on what the dry bulb and wet bulb temperature is and what kind of RH you are trying to maintain.

 

[AbbyNormal]

I like the sensible heat gain of the space approach the best for setting cooling airflow.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[quark]

If you have to maintain 70F and your return temperature is 73F then you have some portion of the controlled space beyond 73F

That is a silly mistake. Read it as 70F instead of 73F

 

[AbbyNormal]

could also have ducts in unconditioned space or heat from lights etc in a plenum ceiling

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

The amount of conditioning you can supply to a room is related to the amount of air that you are suppling and the difference in temperature/enthalpy between the supply air stream and the room.

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?

 

[hvacnerd]

Sorry, it was a very busy morning. We are assuming 1,200,000 BTUH heat being added to the space from equipment, lights, etc that we need to cool. The room has no exterior walls to worry about.

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.

 

[AbbyNormal]

This a data centre, cooling off racks?



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

How much latent load do you have?

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.

 

[Bubbleheadski]

1 million what? Ow my head.... cant think that high! LOL
thatsa biga unit!
hehehe.... ok ok... outta my ballpark now!
Ima Noob!

Bubblehead

 

[lilliput1]

Looking at the pschrometric chart you see that 70°F db anf 50% RH correspond to 50°F dewpoint. This is your minimum supply air temperature assuming saturated condition off the cooling coil. However if there is latent load in the space, you need to supply cooler air to provide dehumidification. Try 49°F supply air. Temperature difference = 70-49=21°F. Now say fan temperature rise is 3.5°F and supply duct temperature rise is 1°F. This leaves you 21-3.5-1=16.5°F to handle room loads because supply air comming off the diffuser is estimated to be 49+3.5+1=53.5°F.
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

 

[AbbyNormal]

yes liliput, he will hit a wall just before 50F. Especially if it is a flat sensible cooling process line.

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.

 

[lilliput1]

Yes AbbyNormal there would be CFM reduction if a blow through rather than a draw thru was used. I did make a mistake in my example becayse the return fan heat would be upstream of the cooling coil so it would not figure in the discharge air temperature rise foa a draw thru. Disadvantage of blow through is that the saturated discharge would be problematic if you have final filters downstream or if the room served has lots of electronic or electrical gear.

 

[AbbyNormal]

Well I just pulled a couple degrees of fanheat off of the top of my head, but it is not unrealistic at all to be higher if it was just a supply fan and there was no return fan.

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.