# Question

• Any guide for Natural draft heater ducy sizing and stack effect calculation?

Aug-2021

• Agshin Bakhtiari, TNA, bakhtiari_af@yahoo.com

The term "Draft" is commonly used to determine static pressure in any point of heater. In a Natural Draft heater, Stack should be high enough to overcome all pressure losses of flue gas through convection section, stack (including entrance & exit losses), damper, ducts, etc to get the negative pressure of 0.1 in WC (at least) in top of radiant section or point of minimum draft location at 120% of normal heat release with design excess air as API STD 560 recommends.

Stack effect could be estimated using following equation:

SE = 0.52 * L  * P  * (1/Ta - 1/Tg)

Where:

- SE: Stack effect (in WC)
- L: Height of stack (ft)
- P: Atmospheric pressure (psia)
- Ta & Tg: Summer air temperature & average flue gas temperature, respectively (R)

For calculating stack diameter and off-takes from breeching to stack, flue gas velocity of 7.6 m/s is a common-practice value.

Sep-2021

• Wilfredo Ditan, JGC Philippines, Inc., ditan.wilfredo@jgc.com

Pls refer to API 560 for some guidelines in duct sizing and stack effect calculation.

Basically for hydraulic duct sizing, given the design flue gas flowrate, we perform draft, press drop and velocity calculations. Velocity guidelines are available in API 560 code, it should be calculated whether there is enough draft to overcome the pressure drop across the duct. This is also the principle applied for stack effect or stack height calculation. In addition to the design flowrate case, the min velocity at heater turndown condition should be checked in ordsr to prevent downwash. Sometimes reducing cone is applied on the top to increase the exit velocity but the draft has to be rechecked if it is enough against the additional pressure drop from the cone.

Sep-2021

• Sanjay Bhargava, Bharat Petroleum Corporation Limited, sbhargava2160@gmail.com

Suggested answer: Stack effect is due to density of cold air outside and warm flue gas inside. An example if water with a density of 1,000 kg/m3 is there externally and kerosene with a density of 800 kg/m3 inside, each 10m of stack will produce 2m of stack effect.

Similarly one can calculate based on temperature of external air (warmest day) and that of flue gas – allow for some heat loss to decide average stack flue gas temperature. Google for a typical formula. Remember to deduct skin friction loss due to flue gas flow inside the stack and account for heat loss via stack surface to determine stack flue gas temperature.

In ducts, as in pipes, bends/ expansion/ contraction cause pressure drop.

Google ASHRAE Duct Design to know more.

Sep-2021

• Sanjay Bhargava, Bharat Petroleum Corporation Limited, sbhargava2160@gmail.com

Please read inch of H2O column in my previous answer as mm of H2O column. Sorry for the inconvenience caused.

Sep-2021

• Sanjay Bhargava, Bharat Petroleum Corporation Limited, sbhargava2160@gmail.com

In natural draft the draft in combustion chamber about -25 inch of H20 column, increasing to -5 inch of water column in arch zone. The stack height should be minimum 60 meter from ground for proper dispersant of flue gas plume.

Aug-2021