High rise VAV - multi zone

[remp]

Ona 30 storey building the client want floor by floor air handling units. He wants a VAV system
However the space allocation for them is pretty small.
Can I design the air handling unit such taht it can serve 5 zones, East, west, north, south and centre zone. In other words, one fan or perhaps 2 at 50% each, delivering air into a common plenum and off the plenum, five cooling coils one for each zone. A pressure sensor in the plenum can control the fan speed for when the VAv boxes open/close?
Any body see this before?

 

[marcoh]

What about a multi-zone AHU? Single cooling coil, face/bypass dampers for each zone?

For a 30 storey building it will be more space efficient to use central plant AHU's and dedicate a central floor and roof to plant. Even though the client will loose a whole floor there will be more lettable area (ie income!) as there is no plant on each floor.

 

[NCDesign]

marcoh the cost of the duct for a single multizone unit to serve a 30 story building would be outrageous. It also would leave the owner with very little flexibility in current tenant fit-ups or future renovations. You would also have some monstrous chases to get supply up and return down.

remp two common ways of handling a high rise like that in the states is either with self-contained units and a condenser water loop to cooling tower(s) on the roof, or use a central chiller plant and have air handler(s) on each floor. The tonnage and available floor space of each floor will determine the number of units. If a single unit can handle the tonnage (AHU or Self-Contained) there is no need for multiple cooling coils to meet the different zone loads. Your VAV boxes would be doing that on a zone by zone basis. Your VFD would modulate the system fan as necessary and your control system would maintain the required leaving air temp or each self-contained or AHU.

 

[remp]

NCDesign
I agree you can have one AHU per floor with one coil but this is not really efficient for perimiter zone and internal zone off the same unit. if one AHU serves both you will need reheat for the centre zone becuase perimitre VAV boxes will loopk for cooler air and internal zones will look for warmer air. I want to avoid this and give each zone its own cooling coil. What you reckon.?

 

[GMcD]

Remp: is there any opportunity to do some real engineering and do a cost-benefit on improving the envelope so you DON'T have such a severe perimeter vs interior zone HVAC and comfort control issue? That being said, if the envelope has at least some amount of solar control designed into it, it is quite common to design a single AHU per floor with VAV reheat for both interior and perimeter zones and have it function well. Other flavours include a plain VAV system (no reheat) for both perimeter and interior zones, with four-pipe fan-coils for the perimeter zones. Another common spec office building approach is a DOAS fresh air unit serving either ceiling mounted four-pipe fan-coils, or water source packaged water to air heat pump fan-coils.

Based on my experience and reviewing energy modeling of typical office buildings, a very efficient system consisting of a floor by floor AHU with 100% free cooling capability, serving a straight VAV (no reheat) air distribution system, with perimeter four-pipe fan coils, can provide the best bang for the buck. Going to an underfloor air distribution system can also be another step for energy saving and better local comfort control.

However- I still think that you should be beating up the Architect to get the best envelope design you can to minimize solar gains and the usual fast-acting peak thermal transient loads around the perimeter, to make the perimeter system as minimalist as possible.

 

[NCDesign]

remp there are a lot of exceptions and what ifs to what I am about to say, and a lot depends on the design of the building interior and its location but here's my two cents.

We do a lot of these buildings in the Southeastern US and we don't do any reheat in the interior zones and only reheat the exterior zones, usually using parallel fan powered VAV boxes. The keys are zoning properly and sizing the VAV boxes appropriately. For the most part your internal load will be fairly constant as the lights and computer equipment won't really change.

If you are having issues with over cooling an interior space the VAV box was sized improperly or the zoning isn't right. Your minimum position on the VAV box should be what is required to maintain the code required outside air. If you do this you shouldn't have any issues with over cooling a space without reheat.

An added note, GMcD is right on the money regarding keeping the load out of the space in the first place. Work with the architect and the owner to get the best envelope that you can. That means smaller mechanical systems and less load variation from interior to exterior zones. Be prepared to translate less load into $$'s saved.

 

[marcoh]

NCDesign, I never meant a single unit for the whole building! You are right and this would be impossible.

With central AHU plant located on say levels 2,15, and roof you would probably require 12 large AHU's for a traditional VAV system compared to probably 60 (assuming 2 AHU's per floor depending on floorplate area). In either case large riser will be required for smoke exhaust which can double up as return air risers.

There are some additional costs in ductwork, but there are also signifcant saving in reduced number of AHU's (controls, electrical, piping, commissioning, maintenance, economy cycle damper)and acoustics is much less of an issue. And also want to get as much plant that requires regular maintenance away from tenanted spaces.

Depends on the client, but most large project I have worked on, the rule has always been to maximise NLA as the payback from additional rent will easily cover any additional construction costs.

 

[NCDesign]

marcoh in doing that do you still utilize VAV boxes for the zones or are you running individual trunk lines for each zone?

 

[marcoh]

NCDesign, would still use VAV boxes for zoning.

Always interesting how you get 10 HVAC engineers always come up with 10 different designs, and most would would work! I've never been to the US and my only exposure to US HVAC design is in Die Hard movies, so I alway assumed all duct and grilles there were oversized (ie design code requirements for for man/terrorist/hero access) and all fans did 1 rpm?

 

[NCDesign]

We also have to spend a lot of money to make sure the inside of the ducts are really, really clean. I mean you wouldn't want anyone to get dirty.

 

[SAK9]

Remp,


To answer your question, there are no issues with using a common AHU for perimeter and interior zone. For winter heating you will need reheaters in the perimeter VAV boxes. You do not need any reheaters in the interior zone as the interior load is constant irrespective of the season.

The downside of one AHU per floor concept is that it is difficult to get the system to operate on an economy cycle as you need large supply/exhaust louvers on every floor. You would also need to provide a dedicated smoke extract system .But such a system has certain pluses like flexible after hours operation, better reliability etc

 

[atlas06]

A mechanical room at level 15 with louvers with not liekly fly with the architect. Those louvers at mid-level of a high rise are sooo! 60's, aren't they?
Requesting a decent mechanical room at each floor can actually make money sense.
Mechanical rooms are rentable floor space while shafts are not rentable, creating two large shafts will lose money for the building owner.

VAV at each floor (single duct or fan-powered), with 100% OA unit and a relief shaft is the way we design in the US nowadays.

Now you have more than one engineer thinking the same (or is it 3 engineers saying the same thing?) how often does that happen?

 

[marcoh]

In Australia plantrooms for base building services, ie the AHU plantroom if floor by floor, is not included in the legal definition of lettable area, hence the drive to increase the lettable area on each floor.

Seems we have leasing agents and lawyers stipulating the HVAC design concept!!

 

[atlas06]

I just learned an Aussie word "lettable".
In the US, we call our drawings and specifications: "Contract documents".

lettable!!!!