Paging Mr. Brad White

Brad White

There is no discernible difference between a constant volume and variable volume system as far as determining the air flow volume required to condition a space.

With constant volume systems, your control variable is temperature, either cycling a DX system off (and losing humidity control), adding reheat ("driving with the brakes on"), or if using a dual-duct or multi-zone systems you can back off the cold-deck in favor of the hot-deck.

With VAV systems, the calculation sets the maximum required air volume (at a more or less fixed temperature which is great for dehumidifying), and to control the space you just use less of it when cooling is satisfied, more when not.

Mark_46

CFM vs Outlet Temp

Brad,

In early March you provided me the formula below:

(Sensible heat BTUH / (Delta-T x 1.085) at sea level

Three questions I hope you will help with:

1. Does the term 'sensible heat' refer to heat loss or heat gain of a given structure/space?

2. Is the formula valid for Hi-V systems where CFM is about 60% of conventional systems?

3. The formula yields a CFM rate, correct?

mitchb

I am not Brad

but I use that formula all the time.

1-Sensible heat is the output of the unit.
2- Actually do not know
3- Yes, CFM

Brad White

Not to step on toes but

1. Sensible heat if you are talking about a cooling system is a portion of the total capacity. In a typical system this "Sensible Heat Ratio" would be about 75%. In other words, a 12,000 BTU air conditioner would remove 9,000 BTUs of sensible heat per hour.

For most cooling applications, this represents your cooling load or heat gain calculation from envelope transmission, solar gains through glass and radiated from people, lights and equipment. Select a system using this number and you normally would have enough residual latent capacity to handle the dehumidification part.

Good practice says to take those things into account though- an area of high population such as an assembly hall or theatre will have a tipping point where latent (moisture-based) heat removal over-rules sensible heat gains. Just be mindful.

2. The formula is essentially valid for Hi-V systems because you have a delta-T taken into account. Your LAT of a Unico system would be in the high 30's/low 40's so that dictates in general terms, your airflow. Because the air is cooled well below the space dewpoint however, it is really, really good at dehumidifying. Your Sensible Heat Ratio may be lower because of this but moving less air at a colder temperature works out nicely.

3. Yes, it gives you CFM.

mitchb

OUCH MY TOES!!! just kidding

No problem..I didn't look at it from a cooling point of view for sensible..I looked at it from heating which is 100% sensible.

The 70-75% figure is what I use for cooling.

Mitch

Mark_46

Yee haa

Brad, thanks a million.

Now that I know how to handle the sensible heat portion of the formula, when I run the numbers the results are very close to what Unico specifies and the CFM values I have measured using a Magnehelic gauge and Pitot tube. The stars have aligned ).

One other question if I may:
1. The 'sensible heat ratio' approximation of 75% applies to heating as well?

Brad White

Sorry, Mitch!

I had meant to acknowledge the "heating as sensible only" angle and give you the benefit of the doubt -and an "out".

I figured that was what you meant and should have said so.

Brad White

Mark- you are a sensible fellow...

Nothing latent about you that I can tell

SHR is a cooling-only term. All heating is sensible unless you are adding humidity of course. Another subject.

Isn't it the best feeling when your measurements corroborate your predictions? When the stars align, as you said.

Cheers!

Brad

mitchb

s'alright

I am not sweating it..I just read the question one way, and you another.

More complete answer..no offense ever taken.

Brad White

Thanks, Mitch

Well said on your part.
Always want to be on the good side of everyone

Mark_46

Definitely

Yes. It is very rewarding when, as you say, 'your measurements corroborate your predictions'. It's days like these I feel like an evil genius in a secret lair - bu-wa-haha.

So, cooling SHR is apprx. 75% and heating is 100%, correct?

mitchb

you are

too good a source of info to be on your bad side...I may need someting from you...lol

Honestly see no reason to get offended on the www anywhere as most people I only know via the medium, there is never anything personal, just a different view.

The nice thing is the OP got lots info, and questions answered, and forces me to keep an open mind.

Lord knows I dont want to create fights (although I did with the inconvenient truth thread).

I learn tremendous amounts from your posts and that makes me a better person. They say you should learn something new everyday..well the wall is where I do that.

Keep smilin..;-}

mtfallsmikey

And, another question

Is this formula valid for use with VAV systems?