Dedicated Outside Air w/ VAV Air Handlers 1

[swoosh172]

Can I have a constant speed DOAS system serving 10 air handlers that have variable frequency drives? Will the outdoor airflow rate remain the same throughout all operating points of the air handler fan? I'm assuming it won't since the pressure in the mixing box will change and I would need some sort of VFD on the DOAS unit. If I do need a VFD, what is the sequence of operation and what controls the DOAS fan speed. Thanks in advance.

 

[GMcD]

If your DOAS system is sized to provide the minimum ventilation air to satisfy ASHRAE 62, then leave it at constant volume. The variable volume AHU's will just use less return air in the mixing section and you'll still be delivering the minumum required ventilation air to the spaces, as well as make-up air for constant exhaust systems (washrooms, general exhaust, etc.). That's assuming the VAV AHU's are sized and varying the supply air flow for cooling (or heating) demand to their spaces/zones during a normally occupied period.

 

[SAK9]

There are two ways to go about this:

1.Constant volume DOAS delivering constant volume to each AHU irrespective of what speed they are running at.Obviously when the AHUs ramp down on the VSD,the mixing box static pressure will drop resulting in less fresh air being drawn.You need to open the fresh air air damper on the mixing box more to keep the air flow constant.One way of doing this is to hold the differential pressure across the damper constant no matter what the static pressure in the mixing box is.

2.A variable volume DOAS responding to change in fresh air requirement from each AHU.The fresh air damper in each AHU will modulate in response to a CO2 monitor in each of the zones.The DOAS AHU will have a duct static pressure sensor tuned for the maximum air flow.Any DOAS flow reduction will cause the static pressure to rise and will slow the unit down.

Option 2 is more complex and is required only if the occupant load is likely to vary signicficantly ie lecture theatres,auditorium,restaurant etc

 

[swoosh172]

These are pretty small air handlers, no more than 2,000 cfm each. I am not sure that the outside air will drop much when the ahu speed ramps down, but I do have to submit for LEED Silver and I want to make sure it is correct. What do you think about having a pressure independent VAV box on the OA duct to each air handler?

 

[cry22]

Smoosh
You have to give all data when asking a question. This data of 2000 CFM max for AHU is important in your problem and should have been stated.
second, you need to state amount of OA and total DOAS unit size for people to make a sound judgment
third, you need to mention the LEED aspect and what part of LEED are you pursuing? are you implementing a DCV control strategy? using CO2 sensors? if you do, you need means to control the reduction of OA into the building.

From what you state, you system is no more than a FCU with DOAS, you just call them AHU. we're talking about 10% OA? (200 CFM for each unit? total DOAS=2000 CFM, or 2500 CFM?) if so, then your system is fairly small, and both your DOAS and AHU have fans HP are so small that VFD's do not make sense any way.

It appears that CV DOAS, CV AHU's with manual balancing is the way to go.
If you get tangled into that air flow measuremnt, DCV control etc, you'll be spending a fortune for minimal gain. No way in hell you can have a pay back on such small systems.

I'd go CV all the way, AHU's and DOAS unit - as for VFD's for such small systems, it is as they say in Brooklyn "Foggetaboutit"

 

[SAK9]

I fully second cry22 's views.

 

[swoosh172]

Thanks Cry and SAK. I ran the energy model and I was only getting a 2% savings with the VFDs so it is not worth it. I was surprised to see that I was also breaking even on the energy wheel tonnage savings vs what it costs to run the erw motor. The OA is only 1800 cfm as cry guessed, so I guess energy recovery is also not worth it.

 

[cry22]

Smoosh
There is no Silver Bullet in sustainability.

You have to throw a lot of darts at a prroject to save energy. They do have to make sense though.

I'd say ERW total enthalpy wheel is worth looking at (again another data you did not indicate in your problem statement - your Locality weather data) - the harsher the climate, the more opportunities to save.
It is not just tonnage, it is also Boiler size (then again, are you in a heated climnate?)
I would pay attention to that ERW load, if the load comes down say from 55 to 50 ton for example, then it may make sense because you'd be going from a larger RTU or air-cooled chiller to a low end (verify actual KW of equipment) - from 60 to 50 ton, your 50-ton will be rightly sized w/ERW, while your 60-ton will be oversized w/o ERW.

The following are some of the major things you should consider upfront when doing a Sustainable design
1. ERW total Enthalpy wheels
2. Low-E Glass
3. Lighting - Min ASHRAE 90.1 1 W/SF or even better LED lighting
4. DCV using CO2 sensors for large occupancy spaces to provide diversity.
5. "Right-Sizing" of equipment, actually UNDER-Sizing is GOOD, especially for residential. this can't be stressed enough, we've seen people provide a 20-ton AC unit for an elevator machine room when a 5-ton will do. Check tose calcs.
6. Air-side econimizer when climate permits.
7. VFD's on Fans & pumps etc of 10 HP motor and larger.
8. Good control sequence of operation - TURN-OFF equipment when you can, nothing beats OFF.
9. System choice for the right occupancy.
10. some rules of thumbs: using points 1 thru 4 above, you should get around 500SF/ton for cooling and 18-20 BTUH/SF of heating in a moderate climate such as the US mid-atlantic.

Last but not least, watch out that sensible load going down so much that you have under 0.5 CFM/SF of cooling, you need to move some air, I like to see no less than 0.7 CFM/SF (especially for VAV systems), then again watch that dehumidification problem resulting from high SA temp. (58-59) when moving more air purposely.

You have to find a balance of things that make sense and apply them, that's why we are hired as engineers, to find all the solutions for each specific problem.