Helpline offers Process Heating's readers a chance to solve problems that crop up in their heat processing applications.

I need to dry large quantities of different medicinal herbs from about 60 to 75% moisture to preferably about 2 to 3%. Hot air drying at temperatures exceeding about 220°F (104°C) is ruled out. The dryer capacity required is 500 to 1,500 lb/hr.

It has been suggested that either a radio frequency or microwave dryer might fit the bill. What do you suggest? - C.J.

I just read the question and answer regarding whether or not to use microwaves or radio frequency (known collectively as dielectric heating) for drying herbs (January/February 2000, p. 53). The answer shown is misleading because it doesn't address the crucial issue of cost. Some simple calculations can help determine whether dielectric heating is economically sensible. The questioner wants to dry 500 to 1,500 lb of herbs per hour, so I'll use 1,000 lb. Also, I'll calculate reducing the moisture content from 75% to 3%:

1,000 lb of herbs at 75% moisture would be approximately 250 lb solids and 750 lb water. After drying to 3% moisture, the herbs would consist of 250 lb solids and 7.7 lb water. So, you have to evaporate 750 lb minus 7.7 lb, which equals 742.3 lb water/hour.

Dielectric heating will evaporate 2.5 to 3 lb water per hour, so in this case, you need 742.3 divided by 3, or about 250 kW of dielectric heating. This would amount to a capital cost of $ 500,000 to $ 1,000,000 at least! What is probably better is a hybrid system employing hot air or infrared drying to about 10 or 12% moisture, followed by microwaves or radio frequency. This approach will dramatically shorten the time at much lower capital cost.

By the way, the suggested microwave/vacuum dryer to accomplish what the questioner wants would be even more expensive than what I've calculated. - Bob Schiffmann, R.F.Schiffmann Associates Inc., (212) 362-7021 or e-mail microwaves@juno.com.