DOAS and chilled beam/radiator

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Hi, all?

I?m simulating a project with chilled beam/radiator plus dedicated fresh air system, I search from archive, it seems most of you using DOAS (a dummy zone) and IU/FCU to simulate, DOAS provide the fresh air and IU/FCU use the ?Outdoor air from? DOAS, the radiator use baseboard instead with zero fan energy power.

But the DOAS in my project is 1) variable speed to reduce the fresh air energy base on occupancy schedule; 2) wheel heat recovery with exhaust air; 3) during the transition period, the DOAS will bypass the wheel to provide maximum fresh air to offset all the heating and cooling demand and chilled beam and radiator will shut off.

Now, my method is :
1) using VAV system to represent DOAS , enter the ? Design supply/return ? flow to show the DOAS fresh air and exhaust air volume;
2) Define the wheel heat recovery with fresh air and exhaust air, and operation control is ?OA exhaust DT?
3) Define the dummy with the same maximum fresh air volume, and also control the cool and heat temperature (otherwise the heat recovery will not work at all).
It seems works very well. But when I reading hourly report, I found the fresh air volume keeps the maximum all the time, so the fan running at the rated power the whole year, which results in the large fan energy consumpation.
How can I make the DOAS?s fresh air volume variable and fan variable according to the acutal IU/FC?s fresh air volume ??
Any comments and suggestions will be appreciated.

Eleanor Shen

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If you are using an active chilled beam, then you don't need to model the doas - just use 100% OA for your ahu.

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All chilled beam systems that I have worked on have 100% OA so there is no recirculation. Not sure I understand the point about needing to recirculate air back to balance the system and control RH. With the DOAS - you control dew point so there shouldn't be a problem. As far as I know the only condensation issues occur with passive beams (as opposed to the active beams that are really induction units).

Since the chilled water coils don't really see the supply air, but see room air (its only the induced air that flows over the coils) I would recommend checking the room functions before going to chilled beams. If there is a high latent load in the space, it might not be the best option.

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Hi, Neeraj and Vikram:

Thanks for your promote advice. I'm reading DOE22Vol3-Topics.these days and found that IU's one application is just ceiling radiator .

Now, I have changed my system type to IU system with no separate MAU systems, but I still have two puzzles :
1) The fan control is gray which means you can't choose variable, so is it says that the fan is running the full power all the time?
2) How to define the induction ratio and make it reasonable? I'm not sure whether the recirculating air volume will affect the fan power. it seems not.

And another question is about water economizer? Can we simulate water economizer now?
For eQUEST 3.64, there are two methods, one is the water economizer in airside system, but the weakness of this water economizer is it will add the AHU fan power.
The other one is the water economizer chiller in waterside system, but it seems the function is not matural, I tried , but the hourly report shows the chillers still running in the winter for very low cooling load.

Thanks in advance.
Eleanor Shen

Date: Sat, 13 Aug 2011 02:11:07 -0700

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Eleanor,

1. In my experience - active chilled beams typically operate at constant volume (set to the ventilation rate in the room) and you vary the cooling rate with a 3-way valve on the cooling coil to vary the cooling capacity of the beam.

2. Different beam designs have different induction ratios ?C I would look up the manufacturers data. The typical range is around 3-6.

3. Yes ?C you can simulate a WSE now ?C you need to add a chiller and select waterside economizer as chiller type. Not sure why your example is not working ?C if you attach an inp file I??m sure someone can shed some light no that.

Vikram Sami, LEED AP BD+C

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Eleanor,

I??ll add to the conversation. I would agree with Vikram in that most chilled beam projects tend to be designed with constant volume primary air. It simplifies the design and helps to reduce first cost. If you do want to pursue VAV primary air then you (or the project engineer) should speak with the vendors you??re considering to see how the beams perform at lower airflows (especially if you??re expecting to get capacity from the beam when doing so).

Regarding 100% OA or RA, this decision has to consider a few things. First, one has to acknowledge that the capacity of the beam per its given length is, among other things, dependant on the amount of primary being supplied to it. Often this primary air cfm is greater than the minimum ventilation air needed, especially pronounced in perimeter spaces if loads are not minimized well.

Second, as was discussed in this string, the primary air must be capable of addressing the latent loads in the space. Depending on your method of dehumidification (i.e. typical cooling coil, desiccant wheel, etc.), this may be a challenge with minimum ventilation airflow. You??ll need to consider how much primary air is needed. If you find that more than the Min OA is needed, then you??ll need to increase accordingly.

After you consider these two aspects, the primary airflow needed for achieving desired cooling capacity and the latent capacity of the primary air, then you??ll need to evaluate the energy implications of at 100% OA vs. RA system. You may find that you need a volume of primary air 2-3 times the OA needed, thus doubling or tripling the amount of OA that is being conditioned if you choose the 100% OA approach. Depending on climate and any airside energy recovery device, this can have a dramatic impact on energy consumption.

You asked some good questions. I??d encourage you to work closely with the engineers doing the design so that you can talk through some of these items.

Good luck.

Paul Erickson LEED(r) AP BD+C

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One VAV approach that I've seen used successfully for larger chilled beam
spaces (open offices, conference rooms for instance) that have DVC is to
have VAV controls on the air handler supplying the chilled beams, and have
air valves that shut off primary air to a portion of the chilled beams in
the space when CO2, cooling, and heating needs for the space are all being
met, and throttle back the supply and return fans to match the reduction in
air volume. Modeling to account for the reduction in air volume was done
thru post-processing hourly reports.

Steven

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Colleagues

I wanted to mention another difference between ACBs and any other ??conventional?? system.

Specific fan power (W/L/s) can be significantly higher that that for traditional systems as the process to create induction ratios depends on static pressure in the beam??s chamber.

This requires extra energy as the ACB does not just deliver OA to the room; it is supposed to entrain another 3-5 air volumes through heat exchanger....

Unfortunately, manufacturers of the ACBs do no like to mention this fact when list ACB??s advantages.

Cheers

Sergey Zhukovskiy

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This is true - there is often an increase in static pressure with a chilled beam system.

However

in some cases - you more than make up for that by the reduced quantities of supply air. If you can drop your air change rate from 10-15 ACH down to 6ACH you come out ahead.

Like everything else - you have to do the analysis to make sure it makes sense. It often does.

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But LEED requires all the supply flow and HVAC equipment capacity strictly base on the actual design. So most of my projects I will simulate using the design parameters as possible as I can.
According to DOE document, uisng UI system to simulate cooling beam, the suggested pressure drop is only 1.0~1.5 in. water. In fact the fresh air unit with wheel heat recovery will at least have a press drop about 1000Pa.
If I use the actual parameter, the fan energy very be huge. How do you deal with this situation?

Best regards!
Eleanor

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Dear Sami

Absolutely agree with you regarding some apparent advantages of the ACB technology. Just wanted to mention several issues that are extremely important.

Data regarding ACBs static pressure that is usually provided by the manufacturers and fan capacity needed are absolutely different things.

Main difference between the full-air and ACB systems is that in the full-air system the dynamic pressure is mainly not converted (partially) to static pressure on the whole path up to the discharge grille (diffuser). This process is well understood by the designers and pressure drop in the ducts or on filters (index run), etc is accounted for to match capacity of the fan.
ACB's technology needs the air being capable of doing ??entrainment?? job. This requires application of some extra energy to this air (pressurising this air).

Manufacturers of the ACB do not bother to consider how the whole system including ducts and plenums behave, assuming that the duct is ??working?? as a regular duct and that the dynamic pressure is converted to static pressure in the plenum (and nowhere else). Usually ACB??s ??plenums?? are not working properly as plenums and the conversion of the static pressure does not happen in the ??plenums?? (this is a desirable, but not achievable process). Being blocked by the ACB units the whole duct system starts working as a plenum (pressure vessel) itself.

Probably customers who have installed the ACBs experienced increased static pressure in their duct system and some issues to achieve the required by pressure specified by the ACB's manufacture.
This pressure increase results in the increased leaks through the tiny wholes and cracks in the ducts, ACB units, etc ?C and this is extra noise, lost capacity and a number of other issues. The customers are trying to achieve the static pressure in the plenums and jump on the AHU??s fan that is working on the full capacity and can not achieve the desired flow....

Just to summarise the provided information I would say that the ACB??s technology is living technology, nothing here is with ??magic?? as some of the manufacturers wish to present. It has a number of advantages, though many disadvantages must be also recognised when designing the ACB system. The main attention should be paid to the fan and the duct system and reasonable questions addressed to the manufacturers of the ACBs.

Cheers

Sergey

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