Exactly what I've been going on. Thanks for confirming.

Do you agree that using enthalpy per lb dry air is the right basis? Outdoor air changes density as you condition it to indoor conditions. So like I said, I've been able to confuse myself trying to factor in that volume change. I'm pretty sure that's why we use the lb-dry-air basis, to avoid worrying about density changes.

I must admit that's one I never got my teeth into. WTF does enthalpy per pound of dry air have to do with anything when the definition of enthalpy is that it includes latent heat??? The only thing that matters to an air conditioner is enthalpy per cubic foot of real air.

Then you convert that to the target conditions inside the building, and all the oddball variables drop out of the equations. The air always ends up at the target density and enthalpy. If the air is denser (or less dense) when it enters, it will be the same density as the inside air in about 4 seconds.

 

WTF does enthalpy per pound of dry air have to do with anything when the definition of enthalpy is that it includes latent heat???

Haha, well, it's just a scale factor. I'm virtually certain it eliminates worrying about the problem with density changes. If you specified enthalpy content on any other basis, like volume, you'd have to constantly recalculate everything.

 

Of course, once you've lowered the humidity of the inside air (and reduced its enthalpy), and there's not a lot of air coming in from the outside, then most of the air conditioning energy is just used to maintain the inside air temperature.

Correct. In a sealed, stagnant, building with no bio-creatures in it, the calculations are about R values of the walls and ceiling, temperature differences from inside to outside, and solar radiation. However, no building (for humans) is hermetically sealed. Air infiltration is measured is air changes per hour. Consider...if there was absolutely no air infiltration, you would suffocate in your sleep!

Air infiltration brings outside air, humidity, and therefore enthalpy. That is figured into the air conditioner size, as are the number of humans and their expected activity level (for both sensible heat and latent heat).

 

If you specified enthalpy content on any other basis, like volume, you'd have to constantly recalculate everything.

Actually, you can find enthalpy and density for any combination of temperature and humidity on a psychrometric chart. When you consider that there is no such thing as 0% humidity for any naturally occurring place on Earth, I still don't know why enthalpy per pound of dry air has any usefulness.

 

I still don't know why enthalpy per pound of dry air has any usefulness.

Hang on...If you know the enthalpy for dry air, the rest of the enthalpy must be latent heat, so you can do a Sensible Heat Ratio from those numbers. Unfortunately, the practical approach doesn't do it that way, so enthalpy per pound of dry air is not a useful tool in calculating air conditioners. That's why I never got a good grip on it.

 

Is there ANY situation where you would choose to open your windows to higher enthalpy outside air? For instance, could really hot dry air with 30 BTU/lb dry air somehow be better to cool down than the cooler, moister air already in your house at 29 BTU/lb?

Well yea...

https://en.wikipedia.org/wiki/Heat_recovery_ventilation

In autumn and spring there may be no thermal benefit from the EAHX—it may heat/cool the air too much and it will be better to use external air directly. In this case it is helpful to have a bypass such that the EAHX is disconnected and air taken directly from outside. A differential temperature sensor with a motorized valve can control the bypass function.

The problem with windows, is that it lets in pollen. Autum is great fro ragweed and spring tree and other pollens. Bringing that air in filtered into the HVAC system, therefore makes a lot more sense.

 

Well yea...

I think you have room to elaborate on this.

The question was, "Is there any reason using higher enthalpy outdoor air is better than recycling indoor air in an air conditioner."
When you bring a heat exchanger into the equation, that isn't the same question, but it does have reasonable answers. If the outside air was warm and raining, the enthalpy would be high, but the sensible heat would be low. That is a reason to use a sensible heat exchanger.

The other use is for heating a house. Using a heat exchanger to move indoor heat into incoming outside air allows energy savings on heating the building, but it's not because better enthalpy, it's because fresh air is needed and you're just mitigating the cold outside air temperature.

 

The question was purely about the thermodynamics, but @KeepItSimpleStupid has raised a good point. A smart window controller should allow for closing in response to high pollen counts just as it would close for rain in the forecast. Most weather services offer pollen/allergen information, so it wouldn't be that hard to include this capability.

 

Actually, you can find enthalpy and density for any combination of temperature and humidity on a psychrometric chart. When you consider that there is no such thing as 0% humidity for any naturally occurring place on Earth, I still don't know why enthalpy per pound of dry air has any usefulness.

Psychrometrics and gas thermodynamics are used for a lot more than just HVAC, and in most applications it's hugely simpler to use the mass-of-air basis for all the other properties. That's why psychrometric charts rely on it.

 

The question was purely about the thermodynamics, but @KeepItSimpleStupid has raised a good point. A smart window controller should allow for closing in response to high pollen counts just as it would close for rain in the forecast. Most weather services offer pollen/allergen information, so it wouldn't be that hard to include this capability.

It's simpler than that. Just put a filter on the outdoor air intake and deliver all outdoor air to the cooling coils before it joins the space full of humans.

As for stopping on account of rain? I thought we were talking about enthalpy. Imagine a cold front approaching. The outside air drops 20F in 2 hours and a freezing rain starts. The enthalpy is low so it's good air to help an air conditioner with. The fact that the humidity is 99% is irrelevant to the enthalpy number.

"But why would the air conditioner be on if it's already 50F outside and dropping?"
It depends on the shape of the building and the occupancy. Large buildings have enough internal heat sources to need air conditioning when you wouldn't expect it.

 

I'm only talking about a window (or whatever) that one would close to keep rain out. Nothing fancier than that. Most residential homes don't have dedicated fresh air intakes? I would be interested in controlling that as well but I've never seen one in a home.

 

Most residential homes don't have dedicated fresh air intakes?

True. I would say it's dead easy to do that, but it's only dead easy if you're in the business. Cut a hole, add a grille, pipe to a filter that is accessible for cleaning and repair, then to a motorized damper and into the intake side of the air handler. Easy to say. Not so easy to do.
And rarely done because outdoor enthalpy is almost never lower than indoor enthalpy during air conditioning season. Then there's all that sensing and controlling stuff.

 

Nothing fancier than that. Most residential homes don't have dedicated fresh air intakes? I would be interested in controlling that as well but I've never seen one in a home.

Carrier has the ability to handle one in their Infinity systems.

"But why would the air conditioner be on if it's already 50F outside and dropping?"
It depends on the shape of the building and the occupancy. Large buildings have enough internal heat sources to need air conditioning when you wouldn't expect it.

It's interesting to note that with an all brick home with almost no insulation (wall insulation is foil backed drywall), late evening, the house radiates the heat in the bricks back into the house. This usually happens after the sun goes down - No suprise here.

With the Psycometric chart, there is also a "comfort chart" or a set of humidity temperature conditions that most humans are comfortable at.

With that said, it's very likely that we have to raise the temperature by usually a degree when it's raining outside.
Active De-humidification and/or humidification is not provided in my system. The thermostat tries to futz with humidity to some extent through say cycle times and fan speed or whatever. It can and probably does use the outside air temperature. Programming is is the time you want the measured value (temperature) to be are where the programmed time, not when to change it. The thermostat knows the BTU/hr capacity of the AC and the furnace.

 

Drapes, for instance should really be open a lot of the time at night time in the fall, spring and summer especially when the nights are cooler than the days. In some houses that's never gonna happen.

 

With that said, it's very likely that we have to raise the temperature by usually a degree when it's raining outside.

Now you're talking about radiant heat from the walls being in the comfort equation, and you're correct. The setting on my thermostat at noon is too cold for my comfort at 4 AM because the walls are not radiating heat like they were at noon.

 

Similar thing up here. We need a higher thermostat setting in super cold weather, because the walls get cold. Skin radiates heat to those cold walls and you need a warmer air temperature to make up for the extra radiation loss. In the summer our walls may get warm but humidity seems to be a bigger problem than the wall temperature. I mean, the A/C set point doesn't go down as the outdoor temp goes up from, say, 80 to 95°F.

 

Sizing HVAC is a real pain. I did do it in a class a few times. Besides that, I did put my AC unit in a 1982 car from nothing but a box including charging etc. Did get an EPA small appliance, automotive license. Just applied similar techniques when working with Vacuum systems up to UHV range.

 

Sizing HVAC is a real pain.

You ain't just whistlin' Dixie!
One time I showed the neighbor kid a printout of 18 pages of my calculations and he said, "What program did you use for that?"
ROFL! It all came out of my head!

That's modern children. They can't imagine I would (or could) create 18 pages of math with a pencil.
Then I did the duct drawings.

 

And I bet an awful lot of A/C units get bought under the assumption that more (capacity) is better. Not true!

 

Thanks everyone for the share valuable information about air conditioner.