When dust/mist/fume systems vent through a roof stack, there are several options to let the air out without allowing rain in. Here’s a quick rundown on the options to help decide which is right for your application.
Rain Caps
Rain Caps (AKA “rain hats” or “China caps”) have been around for over a century. The concept is simple. A “cap” is elevated above the stack to keep out the weather. Air blowing up the stack bounces off the underside of the cap and is vented out the sides.
- PROS: This is a tried-and-true method for keeping rain and snow from falling down the duct stack.
- CONS: When the air hits the cap, it must bounce back against the airflow, in order to escape. This causes the fan to work harder to continue to push air up the pipe, resulting in higher energy consumption and cost.
No Loss Stacks
No Loss Stacks (AKA “stack head” or “capless vents”) allow air to escape without any obstruction as the stack opens directly to the sky. The stack above the roofline has a larger diameter than the duct going into the building. Rain impacts the inside of the pipe and runs down and out without pouring into the smaller diameter duct leading into the building. This is due to two phenomena: 1) Rain falls at an angle, not straight down, and 2) The updraft of air moving through the pipe pushes the air outwards towards the pipe walls.
- PROS: Almost zero air resistance means no extra work for the fan resulting in lower energy costs.
- CONS: Increased chance of water getting into the duct stack on systems with limited up-flow or where there is significant downtime. During very heavy rainfall, small amounts of water may leak into the vent stack.
Flap Stacks
Flap Stacks (AKA “flapper vents”) have a larger diameter outside pipe above the roofline (similar to a no-loss stack head), but they offer the added rainwater protection of flaps that are pushed open by the updraft of air. When the system is off, the flaps close, eliminating the opportunity for water to get into the duct stack.
- PROS: Virtually no chance of rainwater getting back into the building or machinery.
- CONS: The flaps add a small amount of pressure, causing the fan to work slightly harder. Also, snow or sleet can settle on the closed flaps preventing them from opening properly.
Jet Caps
Jet Caps (AKA “jet cowls”) use a funnel to catch all rainwater before it can enter the stack. Because the funnel cone points downward, air coming up from the duct stack is easily diverted out of the top, without causing extra work for the fan. Water caught in the funnel drains onto the roof through a metal tube.
- PROS: Virtually no chance of rainwater getting back into the stack leading into the building and machinery and very little resistance for the fan to overcome.
- CONS: The jet cap is the most expensive of the stack-top options.
