How much energy is in air @ 70 degrees F? Enthalpy? Entropy?

RayWohlfarth

I am working on my comparison of steam to forced air and need your help. I am trying to estimate how much energy is in a cubic foot of air at 70 degrees F. I was hoping to use that as my starting point and then calculate the temperature drop as the room loses heat to the outside. I am not sure if that is enthalpy or entropy. In any event, I can find all sorts of calculations of how much energy is required to change the temperature but nothing to tell me how much energy is in that cubic foot at air at certain temperatures. I found something from RSES which says the following:
Btu/Lb of air @ 70 degrees F
Sensible 16.9
Latent 16.61
Total 33.51
I am thinking I should use the sensible calculation but I am so confused
Thank you in advance
Ray

Jamie Hall

Can of worms. What you are looking for, @RayWohlfarth , is the heat capacity of the air -- which will be expressed in BTU per pound per degree Fahrenheit, and is 0.24028 Btu(IT)/(lbm °F). Then you need the density: 0.0797 lb/ft and you can figure the heat capacity per cubic foot. That's all at 32 F, but it doesn't change that much at 70 to get ballpark figures.

It's not much. Air (and most other gasses) are miserable heat transfer media for that reason. Your major heat capacity in the space will be any objects in that space.

HVACNUT

You guys and your math.
A/H with a steam coil should do it.

RayWohlfarth

@Jamie Hall Thanks I did this study for hydronics and used the btus in water and calculated the btus in a hydronic loop. I used it a kind of heat sink. I was trying to do the same until my brain fried. I was thinking about how much heat would be inside a 2,000 square foot home with 12 foot high ceilings at 70 degrees. I would then subtract the heat loss through the walls. I think I used up all my grey matter.
@HVACNUT LOL thanks you cut through the crap

Larry Weingarten

Hi @RayWohlfarth , See if you can get a little time with Allison Bailes, here... https://www.energyvanguard.com/blog If anybody can make sense of it, he can. And he's a gentleman!

Yours, Larry

RayWohlfarth

Thanks @Larry Weingarten I appreciate it

hot_rod

Wouldn't you need to apply a unit of work to the question, or a time frame, like BTU's vs BTU/hr?

Jamie Hall

hot rod said:

Wouldn't you need to apply a unit of work to the question, or a time frame, like BTU's vs BTU/hr?

Yes -- to get any sort of remotely useful information, you have to measure a delta T over a period of time -- and the answer will come out (if you've flipped everything around properly and used the right units and all that stuff!) in BTU/hr -- a measure of power.

EBEBRATT-Ed

I agree with @Jamie Hall .25btu/lb/degreeF. But air being miserable as Jamie said it's really the heat loss of the space that your concerned with. The stored heat energy of air doesn't amount to much. The stored capacity would be in the structure, walls floors furniture that has been heated to 70 deg

Theoretically if you started up a boiler at 0 deg outdoor temp and the building was 0 deg and the boiler and radiation size was perfect for the heat loss you would never gain temp. We all know this never happens but stored heat is ignored as far as I know

hot_rod

Say for instance 10 cubic feet @ 70F of air changes temperature to 60F, now you have energy transfer. Putting a time to that gives you BTU/hr rate? Or KW/h whatever energy unit you chose.

Leonard

Interesting , what are you calculating, response time?
IF have cold sheet rock, warming that up will likley take more BTUs than air.

Used to set back thermostat on the weekends when weren't home, seemed to take 4-6 hours to get house back up from ~ 65 to 70 or 75. Even though hot water baseboards got hot right away.

Add into your calculations long term electrical energy cost of forced air blower, think they are ~1 hp ~ 740 watt. Assuming it runs 50% of time in winter, at $.19/KWH that alone is ~$ 51/month

RayWohlfarth

Thank you everyone. My working theory is for instance a 10' x 10' 10' room with a temperature of 70 degrees F would have more internal energy or heat than the same room at 68 degrees F. I am trying to use the energy inside the room as a starting point. For example, if the room has 50,000 Btus and the heat loss is 6,000 Btus per hour. How long would it take for the room to lose enough btus to drop the temperature to 68 Degrees and start the furnace.

mars_6

One way to do this would be to do a manual J heat load calculation with the room in question, keep all the parameters the same except the indoor air temp and see what the diffrence is. Then take the cubic foot of air divided by btuh and that may give you what your looking for. Just a thought.

mars_6

I think the termonalagy is latent heat, not sensable heat.

RayWohlfarth

Thanks @mars

ch4man

you need dry bulb temp, wet bulb temp and a decent psycrometric app. that should give to BTU per pound of air. then figure how many cu/ft it takes to weigh a pound

Harvey Ramer

One could say that energy does not exist unless it is compared to a lack of energy. It only becomes apparent when it transfers from a source to a destination.
On an atomic level, it is measured by the activity of atoms. The hotter the media, the greater the atomic activity.
Btu's are always a measurement of a specific amount of energy transferred in a specific amount of time.

To your question about the room full of air...
Lets assume the drybulb temp was 70 degrees F, relative humidity was 50% and you are sea level.
The Enthalpy (btu per lb of air) would be 25.29905.
The density would be 0.07454 lbs per cubic foot.

If you have a "sensible only" heat loss from the above, to 68 degrees dry bulb, your relative humidity will rise to 53.5%.
The Enthalpy will be 24.80462 btu/lb
The Density will be 0.07482 lbs/cubic ft.

Hope that helps.

mars_6

You have to love the the colective brain box that the Wall is.

RayWohlfarth

thanks @ch4man and @Harvey Ramer looks like I have my homework for this weekend.
@mars I am often amazed at the brains here especially this late in the day LOL

Harvey Ramer

To give more context on Enthalpy. Apparently, at sea level, 0 degrees F, and 0% humidity, equals 0 Enthalpy. I suppose that's as good a place for enthalpy to start as any. They had to pick a number.

Jamie Hall

The three laws of Thermodynamics: First Law -- you can't win. Second Law -- you can't even break even. Third Law -- your initial stake is zero.

On picking a number for entropy and enthalpy -- if you are serious about playing with such things, it is best to start at absolute zero for temperature and a perfect vacuum for pressure, and start adding things up from there.

Harvey Ramer

Jamie Hall said:

The three laws of Thermodynamics: First Law -- you can't win. Second Law -- you can't even break even. Third Law -- your initial stake is zero.

On picking a number for entropy and enthalpy -- if you are serious about playing with such things, it is best to start at absolute zero for temperature and a perfect vacuum for pressure, and start adding things up from there.

It would be important to start from absolute zero for scientific purpose.
In HVAC enthalpy is primarily used as a comparison tool. Such as using the delta enthalpy across an evaporator coil to measure it's performance. As such, I guess where they started didn't matter.

I agree, when measuring specific energy content in a media, for it's own sake, one would have to start with a complete absence of energy.

jumper

What Jamie says: >>Your major heat capacity in the space will be any objects in that space<<. That determines how long it takes to bring room to temperature. To maintain temperature delta T is difference between room air temperature and supply temperature. Multiply that by CFM and by specific heat on a volume basis.

RayWohlfarth

thanks @Harvey Ramer @Jamie Hall and @jumper I have lots to figure here. I appreciate all the expertise

Leonard

Basically what your talking about is energy to force a transient change in room temp VS a ~ steady state heat loss thru outer house walls.

An engineering thermodynamics book will likely have air enthalpy tables/graphs in the back ( been a while since I took thermo courses). Or can just run the equations.

The tables/graphs typically reference some arbitrary point as zero. So typically you want to know the CHANGE in enthalpy between 2 cases ( ie different temps). One warning: I remember the Proff said for some parameter there were 2 arbitrary zero points in general use, so don't use different tables/graphs together unless you check them.

If you want to get real accurate likely will have to separately calculate enthalpy change of "dry air" and "water vapor in the air" then add them together to get total change . Books for heating guys may have this done for you, I don't know.
Interesting random link on this: https://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html

But as already said, I suspect energy to heat air will be almost nothing compared to energy to heat solids in the room , (sheetrock, floor, furniture, and to lesser degree baseboards and water in them). Air is basically a heat transfer fluid to warm solids in the room. For a fixed room thermostat setting, likely mostly the outer walls/ceiling/floor as they cool between furnace cycles.

Not going to need to deal with entropy. That's for more complicated thermo work like working fluid cycle design: pumps, compressors, turbines, utility power plants , pre-engineered AC packages (ready to install).......

RayWohlfarth

Thanks @Leonard This has grown legs and turned into a monster project. I feel like Rick Moranis in Little Shop of Horrors. "Feed Me"

Leonard

Your question was if room has X BTUs in it how long it takes for thermostat to cool and restart heating.

If you want to do it engineering accurate that can become a very complicated analysis. Invokes natural convection heat transfer. Between air to cold walls, also between air and solids in the room including walls/floor/ceiling. And then between air and wall thermostat is on, and thermal mass of that wall. Can be done in FEA (finite element analysis) thermal software, but not worth it. Easier to get a handle on the numbers by test.

I Haven't run the numbers , However my gut feel is ignore energy content of air as a thermal mass, it's likely ~ negligible compared to energy content of sheetrock/floor/solids. That will greatly simplify the analysis. ( As a check on this assumption you should calculate energy it takes to change air temp and wall temp to confirm air energy is negligible)

Sheet rock and plaster on my old house is 1.1 inch thick, some newer houses only have 1/2 inch sheetrock