Opening a door between a vacuum and atmosphere

[DanielG1]

TL;DR

How do I determine the air flow between a vacuum and atmosphere when a door is opened?

I work as part of the engineering team running an incinerator. When the ash is removed from our furnace, it creates an open system between the furnace (which is close to being a vacuum) and the atmosphere. For many reasons, this is not ideal. We are looking to add a second door to this ash discharge, so that the system can remain fully closed. In order to justify the expenditure, I am required to give a value for the heat lost every time the ash discharge door is opened, and then scale this for the amount of energy lost annually, and then the financial impact. I am not sure where to start with these calculations as I never covered something like this during my degree, nor have I encountered this problem during my career.

Any help or advice would be greatly appreciated.

 

[Baluncore]

Welcome to PF.

What mechanism is employed to remove the ash?

Now, with one door, the furnace will rise to atmospheric pressure for the entire period that the door is opened. What effect does that have on heat loss ?

Opening a door between atmospheric pressure and near vacuum will result in an explosive decompression of the furnace, a bit like an aircraft skin failure.

With two doors, the furnace will draw in only a fixed volume of air, from between the two doors, when the inner door is opened. If the volume between the doors is small compared with the furnace volume, then the furnace will not rise all the way to atmospheric pressure when the inner door is opened.

 

[DanielG1]

The ash collects on top of a simple drop door which is released to allow the ash to enter a skip, before the door is closed using a set of hydraulic rams.

The heat loss issue is connected to the ingress of air at atmospheric temperature that would take place with the door open, which would in turn lower the temperature in the furnace.

Your comment on the proposed two-door design is exactly what we are hoping to achieve. With a second door the furnace temperature and pressure should not vary too much during a de-ash, which would be both safer and more efficient.

 

[Baluncore]

I think you should consider a butterfly valve as the top door. That would be balanced and so would not need a great hydraulic pressure to remain closed during operation. The door would invert quickly, only once per cycle. The ash side could be brushed after inversion, with the ash falling to, or through, the bottom door.

The pressure of the chamber between the doors could be gently equilibrated before operation of either door. That would prevent a sudden rush of air that could blow ash back into the furnace.

The bottom door could be a simple flap, in the shape of half a cylinder that, when closed, comes close to the upper butterfly. That door would be a self-sealed plug, held by the pressure difference, while the butterfly was inverting. That would minimise the volume of the intermediate chamber, and minimise the energy needed to equilibrate the pressure.

Is the base of the furnace circular or rectangular? That will decide the shape of the butterfly, circular or rectangular, and the shape of the lower door, hemispherical or semi-cylindrical.

If the ash removal process occurred more often, a heat exchanger could minimise energy loss during the two pressure equilibration phases of each cycle.

Maybe it is time to search the literature on current technology, time to stop thinking outside the firebox.