HVAC Load Calculations 1

[RobsVette]

Hey guys,

At a job we are doing here in NY, I have come across something that is unusual to me. Here is the situation.

We are constructing a new building here and the loads for the cooling system dont seem to add up in the following two ways. I personally have never done a cooling calculation for an entire building but using these simple common sense methods, things arent adding up.

- The building is approximately 700,000 square feet. If you do 10 watts per square foot, I came out to just under 2000 tons of cooling. Which makes sense because the building has three 1000 ton trane chillers (with one as a spare)

from here is where it gets funny,

each chiller needs 1700 gpm of evaporator flow. However they have not speced 3 chilled water pumps, but 4 chilled water pumps. Each with a flow rate of 1700 gpm. You would think that if 2 chillers are required you would provide 1 pump for each chiller, yet they call for 4.

- If you add up the total amount of chilled water required for the air handlers in the building you get about 8500 gpm. If you use an 80% diversity, you get about 6650 gpm required. All of the piping in the building is designed for about 6650 gpm.

I would think that the amount of water flow for the air handlers should add up to the approximate load for the square footage of the building. Am I incorrect here?

Either 1 of the two following scenarions

They are either short 1 chiller

or

They have hugely oversized the piping and air handlers.

I have never heard of a building have a load like that is so high before.

I dont think it should make any difference but the building is set up for variable flow primary cooling.

Please let me know what you guys thing. Again I am a third party casual observer, but to me something doesnt look right in the design.

Any help is appreciated.

 

[cry22]

“I personally have never done a cooling calculation for an entire building but using these simple common sense methods, things aren't adding up”. So, you’re not an engineer but you play one on TV, heinh?

No, if you use rules of thumb: for an older building, you’d use 250 SF/Ton, and you come up with 2800-ton, not 2000-ton as you say. Your rule of thumb is wrong, although we should not be talking rules of thumb at all, because you do not mention key factors such as occupancy and building type (a lot of fresh air requirement increase the load substantially)
1. Designers oversize for a variety of reasons when they do: fear being one of them, they were told to do so is a common reason, especially for a government building.
2. Government buildings typically call for future expansion, new computers, new server rooms, etc.
3. So, let’s give your engineer the benefit of the doubt, and say he was told to provide additional spare capacity. Ok, 3000-ton looks good as a plant size. It does not look oversized at all actually.
4. Now you take 4 pumps at 1700 GPM each (with 10F delta T) that gives you 2.3 GPM/ton, meaning each pumps handles 739-ton of cooling
5. 739 GPMx4 pumps= 2956-ton (or 3000-ton), which matches the total chiller plant installed.
6. Looks like the Engineer has figured a control sequence to reduce energy consumption in his pumping by specifying multiple pumps for a VPF system (and have a diversity in case one pump fails).
7. From here my friend, I would go into the control sequence of operation, I am sure you will find some light in there, But by all means, the plant size and the pumps size are good.
8. Don’t look at total air handler GPM in the building, coils are sized for peak load all the time, pumps and chillers are sized for the building block load. Again, good approach by the Engineer, Air handlers sized for peak, and building for Block.
Looked at from here, you’ve had a damn good engineer in my book, he did a hell of a job.

See how nice we are here at Engineering tips? and you didn't even buy me a sandwich. Can you imagine what kind of answer you would've gotten had you bought me a steak?

 

[Ryendube]

Last 8 buildings I have worked at (new construction in GTA) all use HRV as main with Fan Coil as aux in larger suites.

 

[Ryendube]

^^^^^^^ weird posted in wrong thread sorry....