High Rise chimney effect? 1

[buildingconsultants]

We are proposing to design a 12 storey executive condo high rise building with a four pipe fan coil system. If we introduce fresh air into each suite. How will this affect the building make-up air system if at all? It has been suggested the the building will not work do to the 'chimney effect". What would be the remedy to this situation?

Thanks,

buildingconsultants

 

[stevenw]

If you are pretreating the make-up air, adding it at the elevator lobby on every floor, you should not have any problem with "chimney effect".
You could have "chimney effect" in the the toilet exhaust riser but the air is on its way out.

 

[ChasBean1]

Your question isn't too clear to me - isn't the introduction of fresh air into each condo the same as building makeup air? Are you saying there's a separate system that serves core areas? There is generally a chimney effect in high rises that promotes upward flow in a building, but mechanical systems can usually overcome such affects. Can you clarify the problem? tx, -Chas

 

[235zzzo]

Hello (again) Buildingconsultants!

Following the last reply from Chas, with which I agree, it´s clear that the mentioned chimney effect can plays with, instead againts us! Let's see how!

Indeed, some data is missing in your question. To give an accurate answer, we need to know how the make-up air is provided to the building. Is it in a natural way, or by a mechanic system? In which flowrates relation?

Any way, concerning to your system you just have to make a flowrates balance to that building, as a whole and per each flow, that is: supplying a measure of 100% of fresh-air (at the recomended ASHRAE rates, for instance) at each returning side of each local unity (for instance) and exausting 100 ~ 105% of used air, from the toilets, smoking/lodge/bar areas.

If you apply these criteriums you can "live" with any others existing ventilation systems, whatever they are, (if they are dimensioned correctly! you have to ask/be sure!), being of a natural or forced (mechanic) kind.

Lastly, the chimney effect as matter of fact, can be of some help, to let you consider an exaust-ventilator with bit-less power, (in relation to the case in which that effect wouldn't exist!)

Hoping this can help you! Good luck!
zzzo

 

[buildingconsultants]

Thank you all for your response. As per your comments to clarify the situation, here I go...

Ventilation air must be heated and can be introduced into the room through a corridor make-up air system or directly into the fan coil. Corridor make-up air systems require indirect gas fired blowers, providing air to the corridors and then ducted directly into the suite. These systems are also required to be air-conditioned which requires a large cooling system. This air is then individually exhausted from each suite. By providing the fresh air to fan coils, the required corridor ventilation system can be dramatically down sized to a hall pressurization system and therefore cost savings can be realized.

Our investigation of this type of building has indicated that the “stack effect” needs to be addressed in the ventilation design. The stack effect in buildings is the same a stack effect in a chimney. The draft produced in a chimney depends on the difference between the temperatures of the flue gas and the outside air as well as on the chimney height. During cold weather similar action occurs in buildings, regardless of height. The outside cold air is denser than that inside, so that the reduction in pressure with height is more rapid outside. This produces infiltration in the lower floors and exfiltration in the upper floors. Depending on heights, the stack effect can cause significant pressure differences within the building.

Due to the above mentioned stack effect, it is our recommendation that a small in-line supply fan be installed on the fresh air intake to the fan coils. This will ensure that we get positive ventilation to each suite. As well, the hall pressurization system will not be consistent through the structure but will vary so that more air is provided at the lower levels.

Now with this system, I am curious as to whether or not I will increase or decrease the 'stack effect', based on all of your industry experience. Thanks for reading my mini-series. Hope this clears a few items up.

building consultants

 

[235zzzo]

Hi Building Consultants

Trying to be more conclusive:

1. If I understood, there's a high building (12 floors) and it has a vertical interior hall along its high and the phenomenon we are talking about, is the "stack/chimney effect", along that hall.

2. This mentioned effect, as we know, it depends mainly on:
- The high of the interior hall,
- The air temperature difference between the first floor and the 12 floor, it is its "driving thermal potential";
- And, this is important, the quantity of air involved. This last parameter depends itself, on the volume of the hall and flowrates, you will admit/impose to the system (the interior hall).
This has to be evaluated in a very carefully way!

3. From the air conditioning process of the suites, there will be a positive ventilation in these spaces, which supply additional air flowrates to the hall. These quantities, must be evaluated along the twelve floors.

4. Additionlly, you are thinking to supply, at the hall,
more air at the lower floors. If you do this, no doubts, you will increase the stack effect, more air momentum along the vertical hall. In my opinion, at the hall the criterium must be, just replace the air at convenient rates at the top levels of the hall, but in a controled (variable flowrates system).

5. In order to balance all the hall space, you are correct in considering a small variable flowrate AHU as well, at the lower floors, in order to supply some make-up air to the hall itself, but keeping as low as possible the quantity of air involved.

6. Finally, you must pay attention, in order to minimize as much as you can, the in-and-ex filtrations along the building.

zzzo

 

[ChasBean1]

buildingconsultants, sorry it's been a while. It sounds like you may be trying to offset a problem that may not be as big an issue as you've been led to believe. I can't see any reason to add a fan at the inlet to a fancoil unit. This would effectively be three fans in series (the makeup air unit fan, the inline fan, then the fancoil unit fan) which doesn't make much sense from a power consumption perspective. I think with this logic all the horsepower you trim off the central system will be made up by a slew of 1/4 HP motors (or power approx. equal to that of each exhaust fan) spread around lower floors.

Play around with specified air balance numbers (such as a greater volume low in the building and lesser volume high, but don't lose sight of the main purpose of the system). If I read it right, the design you describe has each suite negative anyway, as the suite only contains a recirculating FCU and an exhaust fan, and you're concerned about a higher magnitude of negave pressure in the suites on lower floors (? - correct me if I'm wrong). If the FCU is an above-ceiling unit, you could run a duct from the corridor to the inlet of the FCU (without another inline fan) and balance the makeup/return volumes so the makeup air just exceeds the exhausted volume for each suite.

Buildings also tend to have a lot of exhaust sources on the ground floor (laundry, kitchens, janitor closets, etc.). Ensure first floor makeup air exceeds the exhausted volume. My opinion is that having this barrier on the ground floor is a key to reducing any drastic issues with stack affects. Providing appropriate floor-to-floor firestopping as required for high rise buildings will also reduce the stack effect.

Lastly, don't lose sight of humidity control. If you're dumping high volumes of outside air into interior corridors with low heat generation, there may be temperature problems. Occupants may complain of corridors being too cold in the summer, prompting building owners to raise discharge temps of makeup air unit(s). Result could be lack of moisture removal by the coil, funky summer fungi, and a sick building... Unfortunately from an energy perspective, some re-heat via coils or fin tubes will be necessary.

The answer is sort of generalized as I'm still unclear on a few issues (is makeup via a single, large rooftop system or units at each floor? Are there operable windows? Are the FCU's ceiling hung or floor mounted on the exterior walls, etc.) so forgive me if I described some things you're already well aware of. Hopefully some of these thoughts help. -Chas

 

[buildingconsultants]

Thank you Chas for your response. To answer your questions:

Yes makeup is via a single, smaller rooftop system.
Yes there operable windows.
The FCU's are horizontal concealed mounting in either storage or closet areas, with supply air outlet and return air inlet with fresh air knockout.

Am I getting any closer to a complete description yet. I hope my words are not getting too confusing.

Thanks
B

 

[traildogdw]

We are designing a very similar bldg, 14 storys, 10 suites per floor, suite fan coils with fresh air into the return plenum, roof top make up air to pressurize the corridors. From reading this thread, it seams the only thing to do to counter act stack effect is to supply more via the Make Up Air unit to the lower floors than the upper floors. Is there a simple way to size the air flow to each floor?

 

[nilford]

Go to the ASHRAE Fundamentals, 1997, page 25.8 for the equation that applies. Their example shows 0.16 inches diff S.P. for a 20 storey building, with 45 degrees delta T. I tend to use a bit more and just take it up on good dampers. With openable windows it sounds like you probably will have more than enough fresh air most of the time unless you use a substantial positive pressure in the suites. You should not need floor to floor dampers if your shaft is fireproof, just fire dampers and balancing dampers for each penetration.

 

[235zzzo]

Hi Everybody!!!, Hello Buildingconsultants are you still there ???
It has been quite a while, since the last year!

Shall we warm-up this discussion? Why not? It has been also a few years, I don't read any ASHRAE handbook, but I will do it very soon. I'm just waiting for the new ones. Anyway, about this subject, let´s having some approach, in these terms:

A] You have an enormous vertical space, like a chimney as it is indeed!

B] Once you add air to that space, it will rise, no doubts!

C] But, what you want to prevent, mainly at the upper floors are that strong air-draft, which turns to be very disturbing and unconvenient, however you have to supply fresh air to that space.

D] So, you have minimize the quantity of air, considering the air you add to bottom floors will rise and it will converge to the top as the sum of all flow-rates added to every floor.

E] Stategy: - The make-up air has to be the very minimal specially at the bottom floors, the minimal at middle floors (and I don´t Know how to process technically that situation in the States), since all the added air will go up, the uppers floors will have more than enough "almost" fresh air, so you don't need to add make-up air (at all !?) at these last floors.

F] To do that, try to forsee one more role to the fire dampers, forcing them to work all the time for the balance referred above. It's racional, practical and correct!

G] If the regulations of yours (I presume in the States) are very restrict, you have to mobilize active ways to deal with the problem. That could include parcial air extractions at the middle floors or, very expensive, to consider active reverse stack effects. The architect has already to understant this phenomena and to introduce some technical spaces (courettes) and openings along the building.... but that is another story, another project!

Finally I call your attention to the all above replies, as well.
Kind regards
zzzo