Ventilation Using Stack Effect Principals 1

[Matador]

We manufacture pulp for papermaking. The first step in the process involves chipping logs and storing the chips in a massive A-Frame building. I would estimate the building footprint is 100 ft by 300 ft long. The A-Frame height is about 100 ft.

Wood chips kept in storage tend to give off heat which creates causes massive amounts of humidity in the building. Last week I was called over to view the problem. The walls were dripping condensation and I couldn't see 1/2 way down the building when standing on a platform at the top.

A couple of years ago they added 5 louvers on each side at bottom to get some air flow and a exhaust fan at the top. While investigating the extent of the problem I turned off the exhaust fan and I couldn't see where the problem worsened. The exhaust fan capacity is at its maximum and it is functioning properly. I thinking about opening up the top of the building along the ridge cap to create stack affect ventilation. The air would enter the bottom louvers & exit the top.

It was very humid outside the day I witnessed the condensation problem.

Could someone provide a formula to help me size the ridge cap openings? I would like to use stack affect principals to create the required air flow. There is usually a pretty brisk wind blowing outside and the air is stagnant inside the A-Frame.

 

[butelja]

This formula comes from some old vendor literature from Western Engineering & Mfg. Co. They manufacture gravity ventilator type ridge vents. They give:

A = Q / (26.4*V + 9.4*sqrt(H*dT)), where

A = Throat free area, ft^2 of ridge vent
Q = Quantity of air exhausted, ft^3/min
H = Vertical height, ft, between ventilator and air inlets
dT = Temperature differential, °F, between inside air and
makeup air
V = Normal wind velocity, mph

 

[Matador]

Thanks for the info.

I'll review the drawings and do a few calculations to determine the expected air flow.

 

im trying to track down western engineering and mfg.
thanks

 

[butelja]

I haven't had luck tracking down this manufacturer. The information I had is a 25 year old xerox copy of original literature. I don't even know if they are still in business.

 

[ChasBean1]

Matador, would you be confident that the ridge venting would solve the problem? Stack flow might still be limited by the open area of the five low vents on each side. Just a thought. My first thought is, is this a building that should be enclosed? In other words, what if the A frame were kept for rain and weather, but the end walls were eliminated or at least opened significantly? Or how about a piston-flow effect, where one end could be one or two very large exhaust fans and the other end could be one or two large dampers?

I just wanted to offer these up as potential alternatives. I think the ridge vent idea certainly wouldn't hurt. Good luck with this project. -Chas

 

[Matador]

Good point ChasBbean1.

I have been recording the difference between the outside & inside temperatures and noticed that there is not always a positive delta T. (hotter on the inside)

The material in the building appears to act like a huge heat sink. For instance its often warmer inside the building in the morning and colder in the afternoon.

The other day there was a substantial wind and it was blowing in through the bottom vents on one side (which made me happy) but when I checked the vents on the opposite side it air was exhausting. Perhaps very little gained.

My concern is that without a delta T based on a hotter temperature inside the building, ridge vents may do nothing at all. Its also possible that they could make the problem worse. For example the one exhaust fan inside the building is pulling some air from the vents at the bottom. This flow could be lost if the exhaust fan would pull air from the new ridge vents, thus short circuiting the present air flow path.

Feedback appreciated.

 

[drfox]

This may be a silly question,,, Heat of course rises, but this air is ladden with humidity. Will the principal of gravity venting perform well regardless of how many openings there are in the roof?

 

[Matador]

I don't think you can say it will work.

I have experienced a situation trying to start a fire in a wood stove where the air flow is down the chimney instead of up. Once the chimney heats up the flow reverses.

For a chimney or roof vent to exhaust it needs a driving force (delts T or wind velocity), and the amount of flow depends on the resistance to make up air flow. In a new home there is less infiltration so the make up air flow can be restricted. In my experience with the wood stove I opened the window until the fire got going.

As far as the ridge vent is concerned without a delta T the wind velocity has to do all the work. In my case it would cost ~$10,000 to install ridge vent and another ~$10,000 to fix the mess if they don't work.