[Equest-users] Crack method of infiltration not set up correctly?

Ng, Lisa lisa.ng at nist.gov
Fri Dec 17 07:06:15 PST 2010


James,

Out of curiosity, how did you output the "pressure difference due to wind & stack effect"?

The Crack method doesn't include an explicit temperature correction term, ΔT, but it does account for the stack effect (which is the pressure difference caused by air density/temperature gradient). But I couldn't find how this stack effect is calculated in the documentation, unless it uses the same equations as for the S-G (Sherman-Grimsrud) or ASHRAE-ENHANCED methods.

Table 32 in the documentation reports that for the Crack method, wind-speed correction depends also on the following SITE-PARAMETER keywords: TERRAIN-PAR1, TERRAIN-PAR2, WS-TERRAIN-PAR1, WS-TERRAIN-PAR2 and WS-HEIGHT. The method that references these parameters is the S-G infiltration method (see http://gundog.lbl.gov/dirpubs/10852_ShermanGrimsrud.pdf). Perhaps your site parameters are specified such that wind is not affecting the infiltration rate.

Nevertheless, if cfm = INF-COEF * dP0.8 * Area for a wall (help file under Envelope Components > Exterior wall), then for your case cfm = 7.5*.109004 (inches of water)^.8*(45*8) = 458 cfm, not 703.896 as calculated by eQuest below.

Maybe someone with more knowledge of the documentation can shed light on the calculations done by eQuest for infiltration. It would be interest to me as well. I am reviewing infiltration calculations/assumptions in several energy software.

Lisa




From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Thursday, December 16, 2010 3:16 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Crack method of infiltration not set up correctly?

I have another question about whether people have gotten wind speed to have an effect on crack infiltration.

Below is an hourly report for a few hours of January 1st.  I have a building set up with each space's infiltration based on the crack method, with an infiltration schedule of all 1.0s.  Exterior walls are set up with crack coefficients pretty high (leaky building) - the coefficient is about 7.5, which corresponds to a 0.6" crack around the entire perimeter of the wall (it's a leaky corrugated metal wall construction).

This report is for a typical west (270) facing wall, about 45 feet by 8 ft tall.  70 feet below the neutral level.  As expected, there is CFM infiltration, which is great.  The CFM values decrease as you go up the building (12-story), which is perfect.  Means the whole crack method vs neutral height is working.  However, there appears to be NO impact of the infiltration based on wind speed.  The CFM for this given wall is entirely dependent on the OA DB temp, and does not change at all with wind speed.  See below.  Highlighted areas show similar temp hours but with varying wind speed.  CFM for this 1st floor exterior wall doesn't change at all with differing wind speeds.

I thought that the crack method was supposed to alter CFM based on wind speed, but NOT alter based on difference in indoor and outdoor temp.

What am I doing wrong here?  For a leaky building, wind speed SHOULD have a greater impact on CFM than stack effect.


Var 4

Var 17

Var 19

Var 3

Var 4

Var 7

OA DB

Wind Spd

Wind direction

Outside air film U-value

Pressure diff due to wind velocity & stack effect

Crack method air flow for wall (cfm)

19

6

270

1.81016

0.109004

703.896

19

6

270

1.80501

0.109004

703.896

19

8

270

2.10433

0.109004

703.896

19

4

270

1.50129

0.109004

703.896

18

5

315

1.66397

0.111218

711.009

16

3

248

1.37812

0.115674

725.112

12

0

0

1.13297

0.1247

752.869

9

4

248

1.53612

0.13157

773.33

9

0

0

1.10755

0.13157

773.33

12

0

0

1.10909

0.1247

752.869

18

3

180

1.35996

0.111218

711.009

21

10

180

2.40378

0.104603

689.541

21

3

180

1.34907

0.104603

689.541



GHT Limited
James Hansen, PE, LEED AP
Senior Associate
1010 N. Glebe Rd, Suite 200
Arlington, VA  22201-4749
703-338-5754 (Cell)
703-243-1200 (Office)
703-276-1376 (Fax)
www.ghtltd.com<http://www.ghtltd.com/>


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