Heat Sources for Drying Shed
Answered by: Richard Alan Miller
Question from: Jean-David Derreumaux
Posted on: March 13, 2007

My question has to do with sizing the heat source: Following your answer to my question on dryers a few weeks ago, I got your books and found your basic drying shed design, which is close to what I thought of doing initially. For this project I have now, I will build a shed 10’ by 16’, 10’ high. The trays will be spaces 12" as I don’t anticipate loading them too high. There will be 4 stacks of 8 (3’x6’) trays. For mobility reason, I will use a propane heater in case the shed needs to be moved.

What kind of heater would you suggest: a simple convection heater or a garage heater? And what power (BTU or sqft) would be necessary for the quantity I am thinking about? Also what size exhaust fan would you recommend? I guess one speaks in term of "CFM". Do you think recycling the warm air and drying it before reintroducing it in the shed would be relevant? This information would be very helpful for this project as I am not too versed in the realm of mechanics... more in the realm of plants really.

I have the thought to ultimately have the air warmed up initially by the sun, collecting the heat from a collector under a metal roof.

I have actually written a booklet on drying technology, as part of the series listed at

www.herbfarminfo.com

I am also available as an outside consultant, t0o help you custom build something specific for your needs (now and in the future). I’ve done so many of these, one person set up a company to make "ginseng Dryers" for the world at large (Eugene, OR).

The design in my book is basically a fundamental fruit dryer. The 17% grade is critical for laminar flow of heat and saturated air out of the system. Your design would like around 180,000 BTU, or something you might use for a mobile home (electrical or natural gas).

The exhaust fan must be large enough to pull a partial pressure inside the system. These means you are trying to exhaust more air than is pumped into the chamber (including heat). This is noted by having plastic covering the various joints and edges. The plastic wants to be concave inward, trying to go inside the chamber -- about 1 atmosphere.

Recycling the hot air is useless. It is so saturated with water as to want to be given back to The Lord. In fact, what really drys most of the product is the flow of air, not heat. Heat is used in the second and last stages of drying only. That is to speed up the process, rather than allow it to stagnate.

This means that morning sun is often good, like in a windrow. But the real drying process occurs with air flow, not heat, ultimately. Freeze-drying is when you sucj as much air out of the system as possible in a small amount of time. This means your exit fan is most critical, not the heat fan into the system. Got the concept?

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