AHU general questions

[paduk]

Hi everybody,
I need to get a rough idea about how many AHUs I will need to condition a building.
This building will be a museum with three main exhibition areas plus a number of coffee shops, resturant and offices.
Do you know if there is a guide or something like that to figure out how many AHUs and how big they need to be considering some parameters such as use of the room, floor area, volume ecc ecc.
Moreover, would you say it is a good idea to consider a AHU associated to each exhibition area??
I m sorry for the poor formulation of the question but actually I dont have any idea where to start from.
Also any further questions will be appreciate so I can look at the problem in a more complete way.
Thank you for your help.

 

[willard3]

1. For the first pass you can group rooms by occupancy schedule and put all rooms with one schedule on one AHU.

2. You can also group by use, ie, are the environmental requirements different for different display areas? Separate AHU's by environmental requirements.

3. Hire a Mechanical Engineer as what you are proposing is a lot less simple than it appears.

 

[paduk]

Hi Willard3,
I know this is going a lot harder than I though.
For environmental requirements, do you mean temperature, RH inside the rooms?

 

[Guancho34]

What I like to do is take out a marker and floor plans and, as said above, break down the floors into sections that have similar requirements, ie the display areas v restaurants v office space. For a museum I assume that humidity control will be critical in the display areas, so each might be a zone. The restaurants will each probably have differing requirements so each would get a zone (each would most likely want their own unit anyways for utility and control purposes). The office space would likely be a single zone. Once I have the first cut done I like to then go back though each zone and identify any critical areas that would need special consideration and break these area out into separate AHU’s. For example, do you need the kitchen section to be on a separate AHU than the dining area? Are there any high occupancy areas that should be separate? LAN rooms? Extremely sensitive displays? And so on… Some areas may require redundancy in case a unit goes down. Additionally, some zones may need to be broken into parts due to unit (and don’t forget duct/shaft) size (space limitations), control or redundancy requirements set by the Client. Here are some ‘back of the envelope’ guidelines that I use that might help in unit selection/sizing (these are very broad and I only use then to give a knee jerk response, proper heat load calculation should be preformed for unit sizing):
- Office space is around 0.75 to 1 cfm/sf
- Restaurants, to play it safe will be 1.0 - 1.5 cfm/sf
- About 350-400 cfm per ton.
- To give you an idea 20,000 cfm (‘high pressure’ VAV supply) will require about a 42x30 duct.
- 50k is 80x40 (these duct sizes are just for an idea, you really need to break out the Ductulator to get all the size choices. I assumed 0.25 in wg loss per 100 ft or 2500 ft/min whichever causes the bigger duct).
- What your space requirements because these units will get very large. For modular type units the cross section is maintained at a maximum of 500 ft/min.

Hope this gets you moving in the right direction.

Ps 'Environmental requirements' encompasses everything. Humidity, temperature, occupancy, hours of operation, space usage, etc...

 

[marcoh]

Paduk, depending on where you are in the world, I'd be happy to give you a fee proposal for engineering consultancy services to design all building services you may require in a museum!

Seriously though note that the earlier you get serious proffesional advice on plant layouts, space planning, ducting etc, the easier it will be for you in the long run.

 

[paduk]

Thank you so much everybody.
Marcoh thank you for your offer but I work in engineering company so I think they gave this job to me just as a sort of training process given that I have just started, thank you very much anyway.

 

[marcoh]

Paduk, Break the building down into multiple small elements, eg area by area, cooling system, heating system, pumping systems, and systematically design a solution for each element. A large project is really just numerous smaller project all combined into one.

 

[imok2]

Most design of building envelopes, including museum building envelopes, is based on the American Society of Heating Refrigeration and Air Conditioning Engineers profiling method of envelope analysis (ASHRAE 1989). Using this technique, a designer applies the following steps: