I am designing a Microwave based vacuum dehydration system for waste management sector and food dehydration. Currently I'm using a 3kw output 2.45GHz magnetron as heating element. My vacuum pump is an oil rotary vane pump with 500lpm capacity, it is able to achieve a vacuum of 0.013atm inside the dehydration chamber.

I have incorporated an agitator to mix the material inside the dehydration chamber.

Currently I'm testing raw carrots and it's moisture content is around 80%. The input weight of the material in dehydration chamber is 50Kg, so amount of water to be removed is 40Kg.

How do I calculate the time required for complete drying of this amount of material. How do I calculate how much steam is being generated every minute. Or is the power out of the heat source too low

Thanks in advance

  • $\begingroup$ Start by figuring out how much power you need to turn 40kg liquid water to steam. Add how much you need to heat 50kg carrots to boiling. You can approximate 50kg carrots as 50 kg water. That should give an approximation for a minimum energy requirement. Since you know your microwave power output, divide energy by power to get a minimum time. Actual can likely be approximated as some factor times the minimum. In reality, entropy may prevent you from truly removing 100% of the water, but you can get probably as close to 100% as you need to for a price... $\endgroup$
    – Abel
    Dec 14, 2023 at 20:06
  • $\begingroup$ calculate time required for complete drying ... measure the time $\endgroup$
    – jsotola
    Dec 15, 2023 at 0:25

1 Answer 1


The point of microwave assisted freeze drying is to decrease the process time for obtaining quality results. Currently, freeze drying 50 kg of carrots might take four days in the chamber. With microwave assist, this can be reduced by a factor of ten once the process is dialed in. As the process proceeds, the surface of the carrots become dried but the interior is still moist, and the diffusion time is large. You can't run too low a vacuum or the surface of the product gets too dry and it won't cook right or have the right mouth feel. So you have to reduce the vaccuum pump power to maintain the optimal temps without over drying the surface of the carrots. What the microwave assist does is target the moister center of the carrots and make that moisture more mobile. This reduces the moisture gradient in the product and greatly increases the rate that moisture can be removed from the system. The trick is all in the control system and experimenting until you get it right. The FDA, USBS, and military have been developing these for about ten years, and they are still in the tinkering phase. You need either really intelligent AI type sensors that can monitor the moisture gradient inside the product, or you need really, really consistent product infeed to get that 10:1 time savings. Otherwise, you just have to dial everything back and gain expertise gradually. Use flash frozen infeed and preprocess to a consistent starting moisture if at all possible.

In other words, there isn't anything to calculate until you run 100 batches and have some bounds to work around.

Some good references cited in here - https://www.mdpi.com/2227-9717/11/2/327#:~:text=The%20energy%20efficiency%20varies%20considerably,for%20the%20100%25%20drying%20state.


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