[Gasification] Mk2 Chip Guillotine

[Mark Ludlow]

Dear Kevin,
I agree but as a practical matter in a counter-flow dryer, the incoming
"cold" material is a condensing surface for the saturated air soon to leave
the dryer. This may sound counter-intuitive, but only the surface of the
feed material gains moisture in this scenario. The most efficient dryer
makes use of both sensible heat as well as latent heat. The enthalpy of
condensation provides a substantial amount of heat to the incoming feed.
Depending on the temperature of the in-feed, it is quite possible that the
exhaust can be well below saturation and may in some instances present
opportunities for partial recycling (mixing). The goal is to create as high
a heat transfer coefficient as possible. This usually implies high velocity
of the drying airstream.
Best regards,
Mark

-----Original Message-----
From: Gasification [mailto:gasification-bounces at lists.bioenergylists.org] On
Behalf Of Kevin
Sent: Sunday, January 26, 2014 4:04 PM
To: Discussion of biomass pyrolysis and gasification
Subject: Re: [Gasification] Mk2 Chip Guillotine

Dear Andrew
----- Original Message ----- 
From: <ajheggie at gmail.com>
To: "Discussion of biomass pyrolysis and gasification" 
<gasification at lists.bioenergylists.org>
Sent: Saturday, January 25, 2014 7:18 AM
Subject: Re: [Gasification] Mk2 Chip Guillotine


> On Fri, 24 Jan 2014 07:25:06 -0400, you wrote:
>
>>Both temperature and relative humidity are important when drying wood.
>>
>>With the same air flow, you can dry wood just as quickly with 80 Degree F 
>>air and 80% RH, as you can with 60 Degree F air and 60% RH.
>
> I think this needs qualifying, whilst the same amount of flow can hold
> similar amounts of water from the chip surface at these two
> temperatures the moisture movement to the surface will be faster at
> the higher temperature. So depending on chip geometry the 60 Degree
> air may end up less saturated than the 80 Degree air.

# In a practical dryer, it is probably best to adjust air flow rate such 
that the RH of the air leaving the dryer is close to, but less than, 100% 
relative humidity. At lower temperatures, with the same relative humidity at

entering, the rate of pickup of moisture would probably be lower, and 
accordingly, the air flow rate would have to be reduced, so that the 
"leaving air" was adequately saturated with moisture.

# You are quite right that if 80 F and 60 F air were blown through a bed of 
chips at the same flow rate, then the 60 degree air would probably be less 
saturated on exit.

Best wishes,

Kevin
>
> AJH
>
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