Stack Draft Calculator

Example Data Table

Professional Guide: Stack Draft in Construction

Stack draft is the natural pressure difference that moves hot gases upward through a vertical stack. On construction and industrial sites, it supports safer ventilation, helps maintain stable combustion, and reduces the risk of backflow at inlets, doors, or equipment hoods. Draft depends mainly on height and the density difference between the warmer gas inside the stack and the cooler outside air. When the inside gas is hotter, it is usually less dense, which increases uplift and produces positive draft.

This calculator estimates draft using air and gas density from temperature, barometric pressure, and gas molecular weight. It also offers an optional loss model that subtracts friction and fitting losses from the theoretical draft. The loss model is useful when you know the flow rate and want a more realistic net draft, especially for long stacks, smaller diameters, caps, dampers, or multiple elbows. If the net draft is small or negative, the stack may struggle to vent reliably during start-up or cold conditions.

A practical workflow is to begin with theoretical draft to understand the “best case” potential from height and temperature. Then, enable losses to test how diameter, flow rate, and fittings reduce available draft. Keep inputs realistic: use measured flue temperatures when possible, site ambient temperature near the outlet, and local barometric pressure. If gas composition differs from air, adjust molecular weight to match the expected mixture. Small changes in temperature and height can shift draft noticeably, while losses rise quickly with velocity.

Example scenario (metric): a 18 m stack, 600 mm diameter, flue temperature 180 C, ambient 25 C, and flow 7200 m3/h can produce a positive draft in the tens of Pascals, as shown in the example table. If you increase flow without increasing diameter, velocity increases and losses grow, reducing net draft. If you increase stack height or flue temperature, draft generally increases. Use the exported CSV or PDF report to document assumptions and share results with supervisors, safety staff, or commissioning teams.

Important note: field conditions can vary due to wind, stack termination, heat losses through walls, and transient operation. Treat this tool as an estimating aid and verify critical designs with applicable standards and manufacturer guidance.

FAQs

1) What does “stack draft” represent?

It is the pressure difference created by density differences between hot gas inside the stack and cooler outside air, driving upward flow through the stack.

2) Why must flue temperature be higher than ambient?

When flue gas is not warmer, the density difference may disappear or reverse, which can reduce draft or cause downdraft and backflow.

3) When should I enable flow losses?

Enable losses when you know flow rate and fittings. It helps estimate net draft by subtracting friction and minor losses from the theoretical draft.

4) What is the friction factor and how do I choose it?

It represents wall friction effects. For smooth ducts and clean stacks, values around 0.015–0.03 are common. Use conservative values when surface roughness is uncertain.

5) What does the minor loss coefficient K include?

K groups losses from entries, exits, bends, dampers, caps, and transitions. Add higher K for multiple fittings or restrictive terminations.

6) Why does diameter affect net draft so much?

Diameter controls flow area. Smaller diameters increase velocity for the same flow rate, increasing dynamic pressure and losses, which reduces net draft.

7) How should I interpret negative net draft?

Negative net draft suggests losses exceed buoyancy under the given conditions. Consider increasing height, reducing fittings, increasing diameter, or raising flue temperature.