is the room AC not properlly design?

[MKimagin]

For some reason I can't believe that the room (portable) AC are not properly design.
Since I come to US 17 years ego , I always question the way how the air stream in AC is directed (always or most of the time) to the ceiling instead of to the floor.
The natural thing for cold air is to flow down. I find that more logical to direct it down to the floor because because we all living on the floor not the ceiling.
If we direct the air flow to the ceiling (where the worm air is) at best it give you average temperature and humidity for the whole volume of the room. The question is why do we need to cool the part of the room (ceiling) that we do not occupy?
I also find out that the air conditioner is most efficient when the hottest air is feed in to the cold side radiator. The wormiest air is close to the ceiling not a floor.
So can somebody explain why we using the AC air flow in revers?

 

[IRstuff]

You need to mix the air. Typical air stream from an A/C is actually quite cold. If you don't mix the air, you'll wind up with 55ºF on the floor and 90ºF at head level, which is quite uncomfortable.

TTFN

 

[MKimagin]

That approach is good to remove humidity , but no efficient when the humidity is low.
Can the AC be design to provide output air temperature let say 78F .
I see the new models that have digital thermostat, that can be adjust very precisely (from like 67F to 90 something). Does this temperature apply to the input air or to the out put? I know that all room AC have one thermocouple on inlet , do they have another on outlet?

This also apply to the central AC, I remember in summer i check the temperature with Fluck remote thermometer and the temperature from the outlets show like 73F (at list in central AC provide exact set temperature) but the room was like 86F (we run some environmental tests in lab and we have difficulty to balance the AC power - the load from the test was like 20kW). They increase the power of cooling, but I believe just removing the fixtures of the outlets so the air will not be aloud to mix with hot air just fall on the floor and vent the hot air could solve the problem with out increasing the cooling power.

 

[pennpoint]

MKimagin
Not sure where you are located here in the US. But, here in the West most homes are built with supply ducts near the ceiling, because we do not build basements or crawlspaces. So it's air supply from the ceiling to the floor. Another thing, studies have shown that if a persons head or upper body feels cool then they are satisfied to being comfortable, regardless of the stratified temperature differences, head to feet. Its a matter of perception.
I think air conditioning design (heating & cooling) for average home takes a very low priority in the scheme of the overall design of the home. Unfortunately, homeowners are more concerned with the type of counter tops their getting.

Regards

pennpoint


Best regards
pennpoint

 

[IRstuff]

The outlet temperature is supposed to be constant, being set by the refrigerant's Joule-Thompson characteristics and the environment around the expander.

I would suggest that you read up on A/C cooling cycles, though. The standard A/C shuts off after its cycle and the expander and the ductwork, being mostly in the roof, will heat up well above the setpoint until the next cycle begins, which is started by a cooldown in the expander. After a time delay, the blower turns on, which results in hotter air being blown in until the the air passing through the expander actually reaches a register.

The digital thermostat sets the temperature AT the thermostat, nowhere else. Generally, the thermostats are located near the intake, simply because the wire run is shorter. This usually means that the upstairs is generally warmer than the downstairs, as there is a thermal gradient between the thermostat and the roof.

I have no idea what you think is "proper" design. There is no perfect A/C system. I have yet to be in a large building where every room had an acceptable temperature. Short of having a thermostatically controlled A/C or register in every room, a perfect balance can never be achieved.

TTFN

 

[quark]

MKimagin,

Your question is partly answered by yourself and fully by other members. Like IRStuff commented, no AC system is perfect (no any other system, per se) but present AC system is not imperfect either. I will try to break up the issues one by one.

1. Why supply at the ceiling level?

Like you said, the density of cold air is higher and even you supply it at the top, cold air comes down and hot air goes up. Hot air enters the AC unit, gets cooler and comes down etc. There are stratification issues if your room height is too high other wise mixing of the air and convection are just enough to make even room temperature.

2. Why not supply at the bottom?

The key is to diffuse the air before it hits you to avoid cold draughts and subsequent uncomfort (as suggested by IRStuff).

3. Why to go for lower temepratures of air in the age of precision controls?

Like you questioned in your second post, your set temperature is the return air temperature for the AC. The capacity is designed in such a way that the highest temperature existing in the system is the set room temperature. You supply the air at lower temperature than required, then it picks up heat from the room and reaches to the required temperature then goes for further cooling. Say, you want your room to be maintained at 22⁰C and if you maintain the same at AC outlet, the air heats up due to heat load from the room and ultimately you end up with higher temperature than required.

During the design of HVAC system, the sensible and latent heat loads are calculated and line is plotted with the ration of sensible to total heat load as slope on psychro chart. The extension of this line onto saturation curve gives you the idea about the supply air temperature. Once you fix the constant supply temperature, the load variation due to seasons, day hours and occupation etc. will be taken care by the capacity control of the AC system.

 

[abeltio]

perhaps you could install an A/C pointing downwards and check the effect for yourself... instead of going thru these theories.


saludos.
a.

 

[MKimagin]

Right now I live in NYC. The humidity and temperature some time is really bad ,so AC is necessity.

I do not suggest to build the outlet on the bottom of the room, it should be on the ceiling , however, instead direct the air horizontally close to ceiling and wait for air to slowly fall down and mix with warm air, direct the air straight down to the floor.
The air temperature should not be lower that then desirable.
The air stream should not be directed on any person for long period of time.
The idea is to displays the warm air with cooler not mix it.
If you apply that idea you do not need to cool down the whole volume of room, just the level up to (let say) 6' or 180cm or so (for living quarters - bedroom, living room - that might be lower , probably not higher then 150cm). Normal room hight is 2.5m to 3m. That mean you will need to cool air volume that is 30-50% lower. The displace air have temperature and humidity is higher. That mean cooling and removing the humidity should be much more efficient then in conventional approach (if i am correct the smaller the temperature differential between cold and hot coil in AC the higher the efficiency).
The analogy for my approach can be glass of water, with boil water. If the water is boil ,that mean some molecules of water touch the hotter parts of glass wall and move up and evaporate. That not mean that the water temperature have significantly higher temperature then 100C. The air is not exactly a water, however some of the properties can be apply. The air warmed by the walls, utilities, people and so on, will move up and slowly feel up the room from top down. Once the thermostat detect the limit temperature at certain hight it will turn on the AC and pull out the warm air and push cooled air in to room.
I think that the comfort level of cold air should be secure, with at list 30% lower volume provided, and the AC will got air temperature that guaranty higher efficiency of operation.

One of the problem that I am concern is the CO2 that is produced by people. When it warm it will be close to the ceiling , and then once cooled it will stay most likely close to the floor (it heavier then air). I might accumulate there , not good.

pennpoint ,
You are right that applying the colder air from head down give most comfortable feeling (that because 30% of human heat is dissipate thru head), but that on the beginning, but if you blow on your head even warm air you will very quickly find out that it is very uncomfortable - try to open window in your car in summer - next day you will have problem with your neck for sure. The natural way for human is to have slightly higher temperature apply to your head not your feet.
You see the idea with displacing (not mixing) warm air with cold is nothing new, any body from northern countries can tell you that to cool the room they just open the widows and the warm air will be displays very efficiently by cold air and the action go from bottom up.

 

[MKimagin]

abeltio
I do planing some experimentation in this summer

 

[imagitec]

The air temperature should not be lower that then desirable.

If the air supplied by the AC is cooler than the air in the room, it will pick up heat. Therefore, if it is not initially lower than the desired temperature, it will be higher by the time the occupant senses it.

The idea is to [displace] the warm air with cooler not mix it.

Unfortunately, the hot and cold air do not behave like oil and water.

I do not suggest to build the outlet on the bottom of the room, it should be on the ceiling , however, instead direct the air horizontally close to ceiling and wait for air to slowly fall down and mix with warm air, direct the air straight down to the floor.

Unless the outlets are uniform across most of the ceiling, this will create extreme and uncomfortable temperature variations. You will simply have rising and falling columns of air with very little lateral mixing. I work on a large floor with only cube partitions. The AC outlets are in the ceiling, spaced about every fifteen feet. The grills on these direct the air laterally. Even then, the people near the outlets often shiver, while people further away are comfortable.

Improving the effective efficiency of an AC system is a worthy goal. Unfortunately, your system may have the opposite effect by creating larger temperature gradients. One of the main contributors to the inefficiency of heating and cooling systems must be an uneven distribution of the effects. At one extreme, the whole system needs to work harder to make sure everyone is comfortable. At the opposite extreme, someone near an AC outlet opens a window on a warm day to keep from freezing.


Rob Campbell, PE
Finite Monkeys -

http://www.livejournal.com/users/robcampbell

 

[imok2]

Conventional system supplies air from the ceiling level while under floor air conditioning systems(UAS) supplies air from floor level and returns to the air-handling unit from the ceiling.
UAS by virtue of its design has the advantage of moving air in the same direction as the thermal lift in the room. Heat generated from the occupied zone normally moves upwards. This upward movement helps the UAS in removing heat from the occupied zone. It is important to note that the conventional system supplies air from the top and continuously fights against the thermal lift. The cool supply air mixes with the rising heat from the occupied zone and it will require a colder supply air to provide the same space condition. So you see, we try to do ying and yang
:>)

 

[IRstuff]

If the object is to keep the air near your upper body and head cool, then a UAS must be capable of projecting cold air up to 5 ft upward from the register AND mix horizontally at that height. Otherwise, you'd get cooler, but not colder.

On a hot day, going from 95ºF to 85ºF around your head is clearly an improvement, but insignificant compared to getting 65ºF air around your head. Having the flow come from the ceiling reduces noise and velocity, since the sinking cold air automatically mixes on the way down.

Part of the heat flow constraint is to satisfy and constrain the heat influx from the ceiling, by cooling the ceiling and getting rid of some of the infrared flux as well. A ceiling register does this as well as forcing a full mixing of air as the cold air sinks.

Obviously, this chews up more energy, but it's not about efficiency, is it? After all, efficiency considerations would have you simply insulate better, which alters the thermal flows altogether.

TTFN

 

[MKimagin]

IRstuff
"5 ft upward from the register AND mix horizontally at that height"
The question is why do you like to provided air that have temperature lower then desirable and mix it with the warmer air that accupy the uppert portion of the room.
My be I shouldn't ask why, just: is this still necessary?
If I am correct the todays AC can provide you air that have exact temperature that you desire so that approach may not necessary any more.
You may got that 78F on the floor with out danger that any body will freeze there.
I think it should have uniform temperature up to 5ft do to fact that warmer air will be continuously pull out so there is no access pressure. When you remove one mass of air it will be displace bay other.
The analogy could be pipe, you pushing the air from one side and puling the air from other side.

You have very good point about the noise, I never think about this.
I am electrical engineer, and I know that any redirection of RF signal in transmission line have a consequence in terms of propagation. Same is with air or gases on general. When air is redirected, the particular distances in that fixture may create standing wave so noise is produced.

You have another good point.
" infrared flux"
I thought about that in term of passing heating for the winter, that anoter subjest that I probably start new discussion.
The thing with radiation of warm object it depend of color (ceiling in out case). In most cases it is white, so the radiation should minimal (or at list net difference between floor radiation and ceiling radiation should be minimal). The air it self do not radiate much, however humid air might have higher value , but I do not think is significant.

imok2
“So you see, we try to do ying and yang”
Instead of doing ying and yang, you should do just ying I think that more natural and more efficient.

About the efficiency of heat pump and temperature.
I read sower that increasing the temperature in the room by 1 degree (not sure F or C) reduce power consumption by 10%.
Can some body confirm that.
I also wonder how to calculate the efficient of heat transfer for heat pump (AC).
I see the EER rate on the room air conditioners , and I find out that the test is perform with temperature on hot side of 95F and the cold side of i thing 78F.
I wonder how the efficiency vary when the temperature condition are very up and down for both sides.

 

[IRstuff]

Because, air mixes on its own. If you supply only 68ºF to a 95ºF ambient, your A/C will run continuously, because you will NEVER get to your target temperature. And, you'll simply waste energy, since your A/C is built around a Freon cycle and the only way to supply warmer air is to mix it with warm air to begin with

Unless you supply A/C to an insulated bag over your body, natural convection will cause automatic mixing and warming of the air.

Even then, with a typical body area of 1.8 m^2 the bag will get about a 200 W heat load with a 27ºF delta and a HTC of 8 W/m^2/ºC. Note that this will be from both sides, since your body is hotter than the air around it.

The same general concept applies for a typical 10ftx20ft room with a ceiling at 95ºF and air at 68ºF. With an HTC of 4 W/m^2/ºC, the heat load is about 1 kW.

As for emissivity, please do some research before you make erroneous conclusions:

http://www.electro-optical.com/bb_rad/emissivity/matlemisivty.htm

Just about every white paint listed has an emissivity greater than 0.85. However, since the floor is usually at a cooler temperature, the net difference may not be negligible. A 95ºF ceiling and an 80ºF floor will have a net radiant influx of about 185 W for a 10 ft x 20 ft room. If the room is at equilibrium at 95ºF and your body surface at 80ºF will receive a radiant load of 18 W.






TTFN

 

[MKimagin]

Emissivity of warm ceiling is not a problem at all, you can install ceiling made of polish aluminum, or other metal and you will got something like 0.04-0.02 emissivity.
That mean radiant load receive by body,at maximum can be reduced by 50% (or 9W) if polish metal could be installed on the ceiling (I say 50% because floor and walls also emit radiation).
I see that approach in many commercial buildings. I never realized how important it is.

"If you supply only 68ºF to a 95ºF ambient,your A/C will run continuously"
Don't you realize this is exactly what AC doing right now?
You still stock on the notion that you have to supply air with much lower temperature then desirable.
That not true when you exchange the air volume, not mix.
The only way to prevent mixing is to separate the location of intake and outtake (let say one side of the room to another, or locate the outtake lover then intake). Then you locate 2 thermocouple, one on outtake and one on level that you wanna have the air with particular temperature. Both of them you connect to control unit and that it.
Today AC have variable speed and pressure (scroll compressor)

http://www.copeland-corp.com/cp_rf/prod_sol/cp_rf_products_scroll_dig.htm

The whole idea of mixing cold and warm air come when there were no means to regulate temperature, the compressor work virtual on constant speed and pressure. It ingenuous it that time, but I think we can do better then that.

 

[imagitec]

The instant I read the first line in MKigamin's last post, I laughed out loud. Sounds like a typical electrical engineer at a mechanical design review.

Rob Campbell, PE
Finite Monkeys -

http://www.livejournal.com/users/robcampbell

 

[IRstuff]

Hey, I resemble that remark!

TTFN

 

[DuctPipeMan]

MKimagin,

I hope that I can help...

- Remember that the HVAC system is a SYSTEM and even the space is part of it.

- The main purpose of the system (in cooling mode) is to remove the heat from the space in a way that keeps the occupants comfortable.

- Your theory that displacing the air would be better than mixing has merits... in some cases. First, you need to have enough height to maintain stratification. In the cooling mode, air stratification would seem to be natural since we are trying to keep the occupied space cool. But how do you do that when the occupied zone is only 12-18" from the ceiling? I would suggest that you need 12' or higher ceilings before this is even possible. Second, you have to keep the air flow through the occupied zone from disturbing the hotter stratified layer. One way is to use exposed duct systems with the grilles located @~ 8' off of the floor, but this causes the coanda effect which induces air currents through-out the room. If you use sidewall grilles, how do you keep furniture and partitions from creating distrubnces.

Also look at the concept of using supply air closer to the room temperature. A typical office space needs ~ 1 CFM/SF of air @ ~50 defF to maintain 72 defF space temperature. If all we do is raise the supply air to 68 degF, the airflow needed will be ~ 5.5 CFM/SF. A 200 SF office will need 1100 CFM of supply air. We have increased the duct system and AHU cost fivefold and made it almost impossible to remove any moisture from the space.

I don't advocate taking everything for granted, but I also don't like reinventing the wheel all of the time either. You asked for advice here. Listen and use it.

In church sanctuaries, gymnasiums, and atriums, using stratification makes sense, but displacemnt air using higher discharge temperatures does not.

I try to design for my clients, systems which deliver as cold an air stream that the equipment will handle and deliver comfortably to the space.

Remember, we as engineers are to take technology, and apply it to a practical end. Don't spend $10.00 trying to save a penny.



Earl M Keith, PE
Tennessee

 

[MKimagin]

CkvlPE
I will take your remark to consideration (you definitely know what you talking about), however, I am not ready to give up...yet
If you read my first post I try to apply this idea to the single room or just apartment with let say 2 3 people living in there. How it is in that case? The demand for the air exchange should be significantly lower, so definitely the portable AC can do that (it design for single room any way)
In order to push the air down with out significant disturbance, you need eider slow down the stream (it will probably produced not enough volume for air to exchange) or increase the area of outtake. That might be a challenge (as you said what to do with the furniture).
The stratification always exist, you see right now in my room on the floor level is like 21C or 69.9F, at 7FT it show 24C or 75.2F
the room is 9FT high (out side is 0C or 32F). The humidity is 30%.
In summer, of course, the numbers significantly increase.
I did some experiment with just running the AC in one room, then turn off the AC and wait until the air come down.
Even in so small room the stratification start showing up, not only that, the humid air that lick thru windows, and that produce by people in the room, accumulate close to ceiling.
Also the convection of heat from the external walls in the upper portion of the room slowing down do to smaller temperature differential.
TV set in the room make significant difference due to 150-200W of heat load. Even better if you have a plasma, that babe can take 500-600W or more.
What interesting about TV set, the heat convection filing up the room with warm air from top down. It also create chimney effect.

 

[DuctPipeMan]

MKimagin....

Does your apartment have occupied space above? How much exterior wall and window area do you have and what direction does it it face.

I digressed too far I suppose.... In that small portable window type A/C units are NOT designed to provide the comfort that most people expect.

Most people judge their own comfort level in the zone around their head and shoulders.... Which is where window units are designed to dischare air. (The cooling effect of the high velocity air movement is part of the comfort equation) Mfg are only trying to keep you cool....

As you stated the air will naturally stratify to some extent. Lower discharge air temperatures (DAT) will increase that. As I pointed out in my last comment, increasing the DAT (although reducing the effect of bouancy on the stratification), it will increase - the equipment size, cost, fan power, moisture in the space.

If changing the method to a displacemnt model for window type A/C units is considered. I would believe that added cost of equipment and fan power cost will not be offset by and reduction of space load obtained by allowing the stratification to occur.

Benjamin Franklin siad "a penny saved is a penny earned" but also consider.......you and I have probably spent more money in the time used for this discussion than can be saved in your apartment.

From your original post:
So can somebody explain why we using the AC air flow in revers?

I do not think that I can offer any more to answer this than to say that Window units are designed to deliver the most cooling potential in the smallest, cheapest, and transportable package with little regard to total user comfort.

If someone wanted to do what you are asking, they would not use that kind of system.

Need I say more.


earl keith

Earl M Keith, PE
Tennessee