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

Thomas White twhite at glumac.com
Wed Jun 7 14:00:16 PDT 2006

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?
Thomas J. White, P.E.
Lead Engineer, Energy Services
LEED® Accredited Professional


320 SW Washington, Suite 200
Portland, OR 97204-2640
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