Process Planning

At first, this seems like a straight forward process, but there are several things to consider:

• How long will it take to cool down the system?
• How long will it take to draw a hard vacuum on the system?
• How long will it take to freeze the product?
• How long will it take to dry the product?

But as the planning goes on, we have more things to consider:

How many cycles can we run in a given amount of time?  

Can we speed up parts of the process?

When can we place the product in the freeze dryer?

How much energy will it cost to chill the appliance down?

Many of these things can be determined and tracked by the RPi.  We won't know the answers until we run some tests. But we have to start somewhere.  So from this standpoint, I will build a highly flexible process that I can tweak as I collect more data.

We know we need to chill the appliance down.  We also know we need to process the product to be dried.  So we can save some time by doing both of these at once.  The first stage is to Hit the start button, so to speak, and start processing the product.

STEP 1.  PRE-CHILLING/WARM LOAD:

Purpose

Chill the appliance, pans etc down to freezing temperatures and load the appliance with warm, processed product so it can be frozen.  

Starts

 When the operator clicks the START button

Ends

When the warm processed product is all placed in the appliance AND the appliance has reached the target freezing temperature.

Note

During this stage, the trap will also be brought down to freezing.  The main goal is the product chamber, but the trap will take significantly longer to reach its target temperature, so it will be chilling through all stages until its target is reached.

Procedure

Hit the start button.

Place all pans and parts etc. that will be needed in the freeze drying process into the appliance so they can be chilled down.

While the pre-chilling is happening, you can process product that is not already frozen and place it in the appliance.   You should slice the product, then remove a pan and place the product on the pan quickly. Then place the pan back in the appliance.  Process the next quantity of product, then remove the pan, place product, replace the pan.  Repeat this until all product is placed in the appliance.

STEP 2. COLD LOAD

Purpose

Load processed and frozen product into the appliance. 

Starts 

When the appliance and all internal parts have reached the freezing temperature after the Warm Load stage is complete.

Ends

When pre-frozen product is loaded into the appliance.

Note

The frozen product cannot be loaded into the appliance earlier since the warm internals of the appliance would partially thaw the frozen product.

STEP 3. FREEZE

Purpose

Make sure all product is frozen solid.  

Starts

As soon as all frozen product is placed in the appliance and the appliance has reached the target freeze temperature

Ends

When we are confident all product is frozen.

Note

At this time, I have no way to determine this, so this stage will likely be a timed stage. For example, If you load cold product ONLY, this stage may be 15 minutes.  But if you load warm product, it may take 2 hours.  Experimentation will be needed to determine these times. 

STEP 4. TRAP

Purpose

Bring the trap down to target temperature, if needed.  During all previous steps, the trap will be dropping in temperature.  

Start

When the user clicks the START button.  The trap will be chilling during all of the previous steps.

Ends

When the trap reaches its target temperature.

STEP 5. DRAW DOWN

Purpose

Bring the appliance down to pressures where sublimation occurs.  

Start

When the product is all frozen AND the trap reaches its target temperature

Ends

When the internal pressure reaches 600 Pa.  

STEP 6. DRY

Purpose

Sublimate all of the ice from the product and trap it all in the cold-trap.  

Start

When the Trap reaches its target temperature AND pressure

Ends

When the chamber temperature and pressure cross the sublimation line.  In reality, it may take a bit longer than that, but that crossing is what indicates the water sublimation is complete. 

Note

During the drying stage, we are not concerned with the product temperature since we are holding the pressure in a region that water cannot exist as a liquid.  So, during drying, we do no need to continue to cool the product, unless the pressure starts rising due to excessive sublimation.  I also expect a significant amount of self-cooling to occur during sublimation since sublimation is an endothermic event.  The trap temperature WILL need to be maintained so as to keep it in the solid-ice region of the state-transition-diagram.