New HVAC/Chiller Arrangement Q.

[shanemac9]

Hi,

In college studying energy engineering but i just started placement.

I'm looking into a HVAC system for a 3 story office building at the moment. There is currently a 60kW chiller on the roof that is only able to run at half its output power. My job is to look into repacing it as it is beyond repair.

Here's the twist: The chiller not only sullpies cold water to the AHU coiling coil, but also supplies cold water to coiling bars in the roof of the ground floor for localised control.

But this office space on the ground floor was recently converted into an open plan office. There are 8 cooling bars and only 4 are above someone desk. Its not that large either. i'm guessing 100m^2.

How do i figure out if it is worth while to remove the cooling bars and do all the cooling on the roof rather than piping down the cold water to the ground floor? Am i right in saying this might allow for a purchase of a smaller chiller?

I'm thinking it involves finding the highest possible air temperature in my region (Cork, Ireland) and Q = mC(T.high-T.low). But if someone can shed some light on it that'd be great.

Cheers
Shane.

 

[willard3]

Find someone where you work to help you with this. This a comp-licated design problem that requires a lot of experience.

 

[shanemac9]

k, i'll ask some more on this side. I should call in the company who handle the control instruments so i'll ask them too.

I'm probably off because of a lack of physical experience but:

-Chiller need to cool air only
-why cool air twice, involving circulating water to ground floor, when room isn't segregated.

So sesign condition:

-Outide = 30 deg c
-inside = 18 deg c
-V = 1.2m/s (current flow rate on drawings)
-row.cp = 1.2 assumption
-minimum 20% fresh air
-RETURN AIR TEMP = 21

Q = (density of air)(Cp of air)(Flow rate)(Temp difference)
Q = (.2)(1.2)(1.2)(-12) +(.8)(1.2)(1.2)(-3)
Q = -6.9 kW

I could be missing the big picture!

 

[317069]

you are not cooiling air twice.
- the air handler is for the internal zones, and prvide fresh air for building
- tha water bars or radiators are for perimeter zones and usually habdle the building envelop load.
- 1.2 m/sec is the water velocity in pipes not air duct velocity, air duct velocity should be around 4 to 6 m/s.

 

[KiwiMace]

I would recommend the following:

You will get nowhere trying to design this replacement yourself, and no employer would expect you to do it well. You should then focus on achieving a good design brief and engaging someone else to do the design. Collate information and drawings of the existing systems or sketch out a schematic, get nameplate details from equipment.

Where you can add value is to estimate the energy efficiency of the current system, research and decide on a realistic goal for what you want to achieve and determine the annual savings. This will help your employer decide what they are prepared to spend.

If you have some prelim budget, talk to a consultant up front and you'll save a lot of time and get more options.

 

[shanemac9]

ok thanks Kiwi. i'll switch my focus a bit. I'll be interested to talk to who ever does end up designing it as i'm getting the impression that there's something i'm missing.

- 317069: the supply of the Air Handeling unit is 1198 l/s according to the AHU schematic, thats where i got 1.2m^3/s (forgot to type in the ^3). The cooling bars are in the roof where the supply air passes over them before entering the room. They are only used to the cool air under large cooling loads i'm told.

Reading back on the forum i realised that all i'm trying to do is size the chiller. So my question should have been "How to I size a Chiller". But I just googled it there and there's a lot i can be reading up on.

Cheers for the feedback, it's hady to have a soundboard like a forum to hear yourself think.