low temperature VAV systems

[marcoh]

I am looking at an office air conditioning system utilising low temperature VAV systems. The client is not interested in chilled beams, displacement AC, but does want the system to be as energy efficient as possible.

Pros include significantly lower air quantities, ducts, risers, energy efficient and cheaper! Cons include additional duct insulation, careful selection of diffusers, more potential for condensation, and lower space RH.

Nominally the supply temperature into the space is to be approximately 10 oC. What is the lowest temperature that air could be supplied to into a space?

 

[joejoedancer]

Marcoh, don't forget another "con" in the form of decreased chiller efficiency if you are lowering your chws temp to achieve the colder discharge air temp. If you are staying with the same chws temp, your fan horsepower savings recognized from lower airflows will take a small offsetting hit from the increased pressure drop across coils that now need to have a larger area (more rows potentially) to effect the same heat transfer through a smaller LMTD. Also, your "con" of lower RH is mostly often times going to be a "pro" for you. Regarding your question about lowest temp, I have read (have no real experience with these systems myself)that typicall 46 to 48F is typically used. Visit the Trane website where they have an "engineer's newsletter" specifically addessing this application.
Cheers

 

[marcoh]

I have considered decrease in chiller efficiency. System parameters I am looking at are:

Chilled water: 5 to 12 oC (slightly low but using CHW temperature reset)
Supply air off coil: 10.5 oC (11 oC into space)
Coil pressure loss: 100Pa (4 row coil and low air velocities across coil)

I've got equipment selections on everything and at the moment this would seem like the best solution.

Agree that lower RH is good but lowering room RH is a 'con' relative to energy consumption. Design ambient RH here is quite high (26oCwb)so all outside air has to be dehumidified to the lower room RH. Space RH is relative to the supply air temp and lowering the supply air further would lower the room RH too far.

Now just need to convince the architect to provide a well performing facade with shading that works and decent glazing!!!

 

[lilliput1]

It may be better to go with Underfloor Air Distribution. Less air would be required without having to lower the supply air temperature. Instead it would be raised to 60 - 63°F.

 

[joejoedancer]

anecdotally, you hear different versions of what route (conventional VAV supply temps, low VAV supply temp, UFAD) results in the lowest life cycle cost, but there must be some energy modeling studies done that have determined this for various climates and facility types. Not seen any yet though. If your project budget allows (who's does though these days), you could try modeling the systems with a program like Trane Trace 700 or similar to get a feel for what system may result in the lowest LCC. There are of course other factors than LCC to consider, ease of maintaining the system, occupant comfort, IAQ, etc. Not an easy thing to evaluate, but as the industry improves, these kinds of analyses will be at our fingertips.

 

[remp]

Marcoh,

I was involved in the design build a few years ago of an energy efficient building.
I had a VAV system with low supply air temps of about 10 deg C. This brought the supply air flow rate right down to about 2 to 3 l/s msq for the internal zones (if I remember rightly. I was a bit worried about air distribution but I got good air diffusers, be carful there when selecting them.
Energy efficiency of the chiller was down because of the low water temperature but 5deg C is not too bad, we did this.
I must say it work grand, duct sizes were away down and all the risers and VAV boxes were smaller then normal.
Building got 4.5 out of 5 star rating.

Yea make sure the glazing is goood U-vlaue and shading coefficient, this gives a major saving on HVAC loads. There is little point in the mechanical engineer design a really efficient system if the bld envelope is not designed smart b y the architect.

I dont know what country you are in but low RH can be solved with an elelctric steam humidifier ( you could make allowiance in ahu and switichbords for future addition but for an office block we did not do this at all.

Also good idea to take the fresh air to a minimum, what code are you design to.? This is another major energy saving and method of controling the minimum fresh air shoudl be well defined.


Good luck

Remp

 

[122653]

add screw chillers and ice storage..and burn ice during the day.w/low temp vav 44 deg

 

[marcoh]

I am in Australia, therefore building is being build to Australian Standards (outside air requirements are defined in AS1668.2).

There will be CO2 sensors to control outside air but during normal operation most air will be required for toilet exhaust makeup and pressurisation so offers minimal benefits.

Total cooling requirements will be around 1800kWr and chillers will probably be water cooled screw chillers but am also looking at magnetic bearing centrifugal chillers. Need to do the simulations to look at payback and energy efficiency.