[BLDG-SIM] need a reality check for modeling fan energy of a 100% CV reheat system

Jason Glazer jglazer at gard.com
Thu Jun 8 11:56:15 PDT 2006


Tom,

I really appreciate you summarizing the emails you directly 
received after posting your question to BLDG-SIM.  It 
helped me understand something new about simulation and 
benefit from many experts in the field all in a concise 
email.  A great help.  I hope others will follow your 
example.

Thanks again,

Jason

On 7 Jun 2006 at 14:00, Thomas White wrote:

> Greetings everyone,
> 
> It is a blessing to get everyone's ideas on how to model a
> Constant Volume, 100% outside air system that exhausts all
> return air. BLDG-SIM is a great virtual community. Thanks to
> everyone who contributed suggestions, including John
> Aulbach, Mike Rosenberg, Jeff Hirsch, Bill Talbert, Kevin
> Warren, Peter Simmonds, Kendra Tupper, Rohini, Gerald Pde,
> Kate, Andy Lau, Rohini Brahme, Fred Porter, Kate Turpin, and
> Jeff Haberl.
> 
> Below is a synthesis of the key ideas you all have offered.
> Although you might be able to get all these details from the
> DOE-2 manuals (maybe not?), it's a boon to have all of your
> modeling strategies in one place. Thanks.
> 
> ...Tom White
> 
> 
> ***** MODELING RETURN AIR and EXHAUST AIR in 100% OSA (no
> return) APPLICATIONS
> ****************************************************
> 
> 
> 1. You have to model exhaust air as return air if you want
> the exhaust air stream to account for energy (heat) recovery
> (an ERV system).
> 
> 2. This principal (1) applies even if all return air
> exhausted, that is 100% outside air requirement, no RA
> recirculation.
> 
> 3. In DOE-2.2/eQUEST, you can input a return fan with a
> lower airflow rate than the supply fan. This is simpler and
> more accurate than declaring exhaust fans in all the zones. 
> 
> 4. To examine heat recovery, you must use the central fan
> method. But be aware that the "return" fan heat will be
> added upstream of the HR exhaust coil or wheel, so it's best
> to input as fan kW and set the fan delta-T to some tiny
> virtual setting. 
> 
> 5. Even if "the priority is infection control," VAV is
> possible in many zones. It's worth examining "Can Health
> Care Ventilation Standards Be Maintained without Breaking
> the Operating Budget?," which was part of the ASHRAE IAQ
> 2004 conference and available as a single article at
> www.ASHRAE.org. 
> 
> 6. While most zones in hospitals have minimum ventilation
> airflows in excess of heat gain-based airflows, the converse
> does occur, unoccupied periods also occur, and the
> AIA/ASHRAE guidelines allow reduced flow during unoccupied
> periods. Exhaust VAV boxes/valves may be required to
> maintain flow balance, but the savings are large.
> 
> 7. If exhaust air is not ducted through an energy recovery
> (ERV) system, the fan energy can be accounted for directly
> in the Outdoor Air tab at the zone level.
> 
> 8. You can specify return air as a CFM (under the Flow
> Parameters for a system). If the supply air equals the
> return air you can use kW/cfm. If the supply air is greater
> than the return air for a system, you have adjust the kW/cfm
> accordingly.
> 
> 9. If you specify exhaust air (instead of return air), you
> have to also specify a fan schedule, which may be found
> under Fan Control and Placement of the system Fans tab.
> 
> 10. If you want to isolate exhaust fan energy, you can
> assign a separate meter, just as you can for any individual
> load.
> 
> 11. Specifying kW for exhaust fans (to account for the
> energy) also allows you to use VFDs and, therefore, you can
> see energy savings that would otherwise be unaccounted for.
> 
> 12. If you specify exhaust fan CFM at the zone level in
> Outside Air under the Outdoor Air tab, the air is added to
> the rest of the OSA cfm you specify.
> 
> 13. You can eliminate additional cooling load to a space or
> system from exhaust fans by adjusting the sensible and
> latent heat ratio, setting both to  zero.
> 
> 14. To account for exhaust fan energy, you can add the
> static pressure of EA fans to the SA fans.
> 
> 15. A couple of additional "tricks" you can use to
> account for return air fan energy are these: (a) Use a VAV
> system with return fans and set the economizer to 100% OA.
> (b) Set the specifics of the exhaust system as a dedicated
> exhaust, but be mindful of the fan power. (c) Increase
> supply fan static pressure to emulate two fans (both SA and
> RA).
> 
> 16.	You can cross-check with ASHRAE's 865-RP accuracy test,
> which includes system and zonal fan energies at varying
> delta-P. Spreadsheets used for the 865-RP systems, including
> CV Reheat systems are available at the Energy Systems
> Laboratory, Texas A&M University http://www-esl.tamu.edu/
> 
> ************************************************************
> ****************
> 
> -----Original Message-----
> From: bldg-sim at gard.com
> To: bldg-sim at gard.com
> Sent: Sat Jun 03 22:10:17 2006
> Subject: [bldg-sim] need a reality check for modeling fan
> energy of a 100% CV reheat system
> 
> Hello everyone:
> 
> I'm modeling a 400,000 sf hospital with 100% outside air and
> constant volume reheat system. The priority is infection
> control and not energy savings, so the total ventilation
> load is HUGE. 
> 
> Here's my question. I've input all the CFM values for each
> supply air handler (about 500,000 cfm combined), but --
> strictly speaking -- there is no return air since all air is
> exhausted. So, how do I account for the kWh of all the
> exhaust air handlers? The exhaust air handlers run about 2/3
> the CFM of the supply air, with the difference made up by
> exfiltration and a multitude of special-purpose exhaust fans
> (in the labs and so on).
> 
> I figure my only real option is to treat the exhaust air
> handlers as electric loads in the building. They don't
> contribute to the heating and cooling loads, but they are a
> significant energy use.
> 
> Can you think of an alternate way to model exhaust air fans
> to account for their energy use while eliminating any energy
> use for return fans?
> 
> ...Tom
> 
> 
> Thomas J. White, P.E.
> Associate
> Lead Engineer, Energy Services
> LEED® Accredited Professional
> 
>  <http://www.glumac.com/>  
> ________________________________
> 
> 320 SW Washington, Suite 200
> Portland, OR 97204-2640
> T.  503.227.5280  F. 503.274.7674  D. 503-345-6246
> 
> Thinking. Inside the building.
> http://www.glumac.com <http://www.glumac.com/> 
> 
> 
> 
> 
> 
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=========================================================
Jason Glazer, P.E.  jglazer AT gard DOT com  847 698 5686
GARD Analytics - http://www.gard.com/
1028 Busse Highway, Park Ridge, IL 60068
Building Energy Simulation and Analysis
Admin of BLDG-SIM list for building simulation users




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