Question:
My goal is to achieve a full list of specifications for the design of a continuous furnace so as to achieve one-piece (at a time) manufacturing flow. The process I am investigating is the de-embrittlement of maraging 250 steel after a manganese phosphate conversion in the aerospace industry.  

I’m trying to understand if it’s possible to achieve a continuous flow whilse maintaining temperature uniformity. To some degree, temperature loss when inserting new pieces needs to be minimal because soak time is not counted when temperature is not within specified boundaries.  

I will give a very quick sum-up of some of the requirements:

a.) Product is currently heat treated in large batch furnaces, but a new lean-manufacturing production line is being set up.
b.) The furnaces are electric (no gas available)
c.) Temperature needs to be uniform in the furnace and between given limits (AMS 2750D and client specifications)
d.) Soak time is counted as long as temperature is within these limits. When temperature is outside these limits, timing stops and is resumed only when the temperature returns within these limits. Since the measuring thermocouple will be placed at entrance and exit of furnace (i.e. coldest zones), temperature loss when opening and closing must be minimized.

One piece is inserted into the furnace every 90 minutes with a maximum of nine pieces. The pieces have a length x width x height dimension of 1,100 mm (43”) x 80 mm (3.125”) x 80 mm (3.125”). What measurements can/have to be taken in order to prevent temperature loss when inserting or removing a piece?  

I estimate the total working volume of the furnace to be 1,400 mm (55”) x 1,800 mm (72”) x 500 mm (20”). I am considering the following options and would like your input.

1. Given the small height of the workpieces, is quick opening and closing of the furnace doors a good idea?
2. How about using a preheated zone before and a heated zone after the working zone of the furnace with internal furnace doors (this is an expensive option, I presume)?
3. What about using a hot-air curtain (of course, only when the furnace door is open)?
4. What about using a curtain made of high-temperature-resistant material in front of the furnace doors?
5. Should we program the furnace so that the fan stops when the doors are open and more heat is supplied?
6. Are there any other options?
7. Do you have an opinion on what would be better–a horizontal conveyor belt or vertical chain system (the pieces have holes that permit hanging them)?
8. What measurements can/have to be taken in order to prevent temperature loss when inserting or removing a piece?
9. There will be a minimum distance between the furnace door and the working volume of the furnace. If I wish to keep the dimensions of the furnace as small as possible, this distance is the limiting factor. I thought of solving this by using three separate conveyors. The conveyor in the working volume moves backward and forward to put the pieces in the optimal position (with a minimum safety distance). Is this a possible option? 
10. Do you know of any other ways this is solved? Any estimates on this distance?

 The answer to these questions is coming up in Part 2.