[Bldg-sim] ASHRAE App G VAV Fan Curve

[Michael Wetter]

Fred,

The reason for "forcing" zero power at zero mass flow rate is because the
temperature raise across the fan is dT = P/m_flow/cp, where P is the power
of the fan, m_flow is the mass flow rate and cp is the specific heat
capacity. If P were not zero at m_flow=0, one would get an infinite large
temperature raise. Forcing P=0 if m_flow=0 is for numerical reasons, which
is important as continuous time simulators cross the region of very small
flow rates when equipment is switched on or off. However, proper
implementation of controls will not simulate the fan at such low speed for
extended periods of time, as this region is outside of the capability of
the real equipment. Therefore, this treatment does not affect your energy
saving calculations.

For a more in-depth discussion about this topic, see Section 4 in
http://www.tandfonline.com/doi/pdf/10.1080/19401493.2013.765506

All the best,

Michael


On Wed, Feb 26, 2014 at 11:08 AM, Porter, Fred AEC
<FPorter at archenergy.com>wrote:

>  Thank you Nathan,
>
> I think a reasonable sp reset fan curve is better than the ASHRAE baseline
> curve at most values of interest.
>
>
>
> I did just notice that the ASHRAE curve appears to have been "forced"
> through 30% power at 50% flow which is the minimum performance listed in
> 6.5.3.2.1. For some reason, it was also "forced" through zero power at zero
> flow.
>
>
>
> Fred
>
>
>
> *From:* Nathan Miller [mailto:nathanm at rushingco.com]
> *Sent:* Wednesday, February 26, 2014 11:42 AM
> *To:* Porter, Fred AEC; bldg-sim at lists. onebuilding. org (
> bldg-sim at lists.onebuilding.org)
> *Subject:* [External] RE: ASHRAE App G VAV Fan Curve
>
>
>
> I think it is pretty clear the way 90.1 App. G is written that you SHALL
> use that specific curve for the baseline VAV system fan power. Now it is a
> different discussion about whether that curve properly accounts for any
> ASHRAE required controls strategies, fan performance requirements, etc, but
> that doesn't really matter, here is the curve to use.
>
>
>
> Proposed, as always, should accurately reflect how the system will
> operate. If you think a more advantageous fan curve is appropriate, such as
> to account for static pressure reset, go ahead and use it, just as you
> could create a custom curve for boiler or chiller part-load efficiencies.
> Don't hide the fact, make it clear to the reviewer that you are using
> different curves, state the source for your proposed curve and see if they
> want to comment or challenge the results.
>
>
>
> For what it is worth (keeping in mind that past rulings do not set
> precedent), on multiple projects I have worked on I have used "static
> pressure reset" fan curves in the proposed, the mandatory baseline fan
> curve in the baseline, and it has never even been questioned by the
> reviewer.
>
>
>
> *Nathan Miller **-* *PE, LEED(R)AP BD+C, CEM*
>
> *Mechanical Engineer/Senior Energy Analyst *
>
> *RUSHING* | *D* 206-788-4577 | *O* 206-285-7100
>
> *www.rushingco.com <http://www.rushingco.com/>*
>
>
>
> *From:* bldg-sim-bounces at lists.onebuilding.org [
> mailto:bldg-sim-bounces at lists.onebuilding.org<bldg-sim-bounces at lists.onebuilding.org>]
> *On Behalf Of *Porter, Fred AEC
> *Sent:* Wednesday, February 26, 2014 9:40 AM
> *To:* bldg-sim at lists. onebuilding. org (bldg-sim at lists.onebuilding.org)
> *Subject:* [Bldg-sim] ASHRAE App G VAV Fan Curve
>
>
>
> All,
>
> Does anyone know, particularly through having claims accepted by GBCI,
> whether the ASHRAE App G VAV fan curve is supposed to represent a fan
> system with or without static pressure reset. I always thought it was
> supposed to represent a system with sp reset, conforming to 6.5.3.2.3.
> However, most of the curve power values are even higher than curves
> representing VAV/VFD systems without sp reset. A confusing aspect is that
> compared to other VFD fan curves, the power values are high at high flows,
> but low at low flows, even going to zero at zero flow, though that is
> really meaningless.
>
>
>
> As an aside, the 2013 Title 24 ACM now defines fan curves for central VAV
> systems with and without sp reset, taken from Taylor's VAV design guide. SP
> reset is the "baseline" or "standard."A
>
>
>
> Thanks for any experience or thoughts on this.
>
>
>
> Fred
>
> *Fred Porter, BEMP, LEED(c) AP*
>
> Senior Energy Analyst
>
> *Architectural Energy Corporation*
>
> *A Member of NORESCO Energy and Sustainability Services*
>
> 2540 Frontier Ave, Suite 100, Boulder, CO  80301
>
> Phone 303.459.7425 |  fporter at archenergy.com
>
> www.archenergy.com
>
>
>
>
>
>
>
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