Question Regarding High Balance Loss Coefficient (Cb) Values in Balanced Terminals

[Sávio Almonfrey]

Dear CONTAM Support Team,

I am currently performing a stair pressurization simulation in CONTAM using a duct network with balanced terminals.

The objective of the simulation is to maintain approximately constant airflow at the terminals in order to satisfy the minimum velocity criteria at the stair doors. For this purpose, I used the “Balance Terminal” option and defined a design airflow rate of approximately 0.7 m³/s for each terminal.

However, after running the balancing process, the “Balance Loss Coefficient (Cb)” values calculated by CONTAM became extremely high for all terminals. The values are ranging approximately from 74 to 107.

My question is related to the physical interpretation of these results and how to reduce these values to something more compatible with real balancing or airflow control devices.

What I expected to observe was:

• higher Cb values for terminals closer to the fan;
• lower Cb values for terminals farther from the fan;
• and, most importantly, lower absolute values overall, compatible with real balancing dampers or airflow control devices.

However, CONTAM is generating high Cb values for almost all terminals, including the upper floors located farther from the fan and duct network.

From my understanding, this indicates that the network has excessive available pressure and that the software is artificially creating losses at the terminals in order to achieve the desired airflow rates.

I have already tried:

• modifying the fan curve;
• reducing the available pressure;
• adjusting the operating curve;
• modifying network parameters;

but the Cb values still remain excessively high.

I would like to better understand:

1. How does CONTAM physically interpret the “Cb” parameter during the “Balance Terminal” process?
2. Is there any recommended methodology to reduce these values?
3. How can I obtain a more “natural” network balancing, avoiding excessive artificial losses at the terminals?
4. Would it be more appropriate to increase distributed losses in the duct network (branches/ducts) instead of concentrating losses at the terminals?
5. Is there any practical recommendation regarding acceptable Cb ranges to represent real devices?
6. Is there any recommended modeling strategy or configuration to prevent all terminals from receiving excessively high Cb values?
7. How should I interpret the fact that even the farthest terminal still presents a high Cb value?

I am attaching the model file for reference and analysis.

Thank you very much for your support and assistance.

Best regards,

Sávio Almonfrey - Firefighter

[Sávio Almonfrey]

Dear CONTAM Support Team,

I hope you are doing well.

I would like to kindly follow up regarding the email I previously sent about the stair pressurization simulation and the high “Balance Loss Coefficient (Cb)” values generated during the “Balance Terminal” process in CONTAM.

I understand your team may be busy, but I would greatly appreciate any support or guidance you could provide when possible. Your assistance would be extremely valuable for helping me better understand the physical interpretation of these results and the most appropriate modeling strategy.

Please let me know if you need any additional information or files from my side.

Thank you very much again for your time and support.

Best regards,

Sávio Almonfrey
Firefighter – Brazil

[Dols, William Stuart (Fed)]

Duct design incorporates friction losses within the ducts and dynamic losses at duct fittings.

This is a complex topic that requires specific information for various duct geometries.
You can review this information in various design handbooks, including the ASHRAE Handbook of Fundamentals, chapter on Duct Design.

 

You can also read about the Cb values in the Theoretical Background -> Airflow Analysis -> Ducts section of the User Guide.

It is essentially the dynamic loss coefficient for a zero-length duct (i.e., a duct terminal).

 

I’m attaching some work I did quite some time ago related to an example in the ASHRAE Handbook of Fundamentals.
I didn’t finalize the work, but it may help you understand the duct design process.
The example is from the Duct Design chapter of the 2005 and 2013 versions (they are the same example, but numbered 6 and 7, respectively).

 

- Stuart

 

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[Sávio Almonfrey]

Dear Stuart,

Thank you for your response and for sharing the duct design example. I have reviewed the material and also read the theoretical section of the CONTAM User Guide regarding duct flow analysis and the definition of the Cb coefficient.

However, I am still struggling to understand why CONTAM generates such high Cb values during the balancing process.

Based on the attached ASHRAE duct design example, I understand the concepts of duct sizing, balancing dampers, equal friction, and static regain methods. My question is specifically related to the way CONTAM performs terminal balancing.

In my model, the airflow rates are successfully equalized among the terminals, but the resulting Cb values are extremely high (approximately 70 to 110). What I do not understand is why CONTAM requires such large loss coefficients to achieve the desired balancing.

From an engineering perspective, it seems that the same airflow equalization could potentially be achieved with lower balancing losses, resulting in lower system pressure requirements and, consequently, a fan with lower performance specifications and lower energy consumption.

Therefore, I would like to better understand:

1. How does CONTAM determine the required Cb value during the balancing process?
2. Is the balancing algorithm searching only for a mathematical solution that matches the target flow, regardless of whether the resulting Cb values are realistic?

My main doubt is not about the duct design methodology itself, but rather about the internal logic used by CONTAM to calculate these balancing coefficients and why the resulting values become so high.

Any additional explanation would be greatly appreciated.

Best regards,

Domingos Sávio Almonfrey

[William Dols]

If you followed my original advice, you should have obtained fairly reasonable balance coefficients, as shown in the BAL file below, i.e., a maximum value of about 23 for the terminal closest to the fan.

0 ! Flow balance iterations converged
!fan duct # flow [kg/s] Prise [Pa] RPMratio
!trm jctn # rel-flow [-] Cb [-]
fan 20 6.11638 258.164 7.627449e-01
trm 1 9.999047e-01 0
trm 4 1.000052e+00 0.144494
trm 7 1.000019e+00 1.19158
trm 10 1.000010e+00 3.56458
trm 13 1.000006e+00 7.82924
trm 16 1.000004e+00 14.5876
trm 19 1.000003e+00 23.4892
end

Once the fan ratio is set (0.7627449), the design flows are close but not exact due to the nature of your building when the Vas_Paredes leakage is reset to 1 cm2/m2.

You could increase the ratio to achieve higher flows.

The balance process is a fairly simple iterative process that has no "understanding" of what meaningful coefficients are.

I've attached the models I used for balancing: wsd-01/-Balance/-PostBalance/-OriginalVasParedes.PRJ.

When you run your original model, before balancing, there are some fairly strange flow patterns due to the nature of your building, but I did not get into the details.

That is why I recommend balancing the duct system with as little influence from the building as possible, and with all temperatures set to CONTAM's standard of 20 C.

- Stuart

[Sávio Almonfrey]

Dear Stuart,

Thank you very much for your detailed explanation and for providing the balancing models.

I reviewed the files and reproduced the procedure you used. After analyzing each modification, I now fully understand the balancing methodology adopted in CONTAM.

In particular, I verified that, for the purpose of determining the balancing coefficients (Cb), it is indeed necessary to minimize the influence of the building on the duct network. Increasing the wall leakage area to a very large value effectively makes the zones behave almost like outdoor conditions, allowing the duct system to be balanced without the pressure interactions generated by the building.

After performing the simulations, it became clear that the objective is to isolate the duct network from the effects of zone pressures, temperature differences, stack effect, and other building-related influences, so that the balancing process reflects only the characteristics of the duct system itself.

I also now understand why the balancing algorithm can generate large Cb values when the building effects are present, and that the algorithm simply iterates toward the target flow rates without evaluating whether the resulting coefficients are physically significant.

Your explanation and the example models clarified the entire procedure for me, and I now have a much better understanding of how the balancing process should be carried out in CONTAM.

Thank you again for your time, support, and for helping me understand this topic in greater depth.

Best regards,

Domingos Sávio Almonfrey