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:[email protected]] 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: <[email protected]> To: "Discussion of biomass pyrolysis and gasification" <[email protected]> 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 > > _______________________________________________ > Gasification mailing list > > to Send a Message to the list, use the email address > [email protected] > > to UNSUBSCRIBE or Change your List Settings use the web page > http://lists.bioenergylists.org/mailman/listinfo/gasification_lists.bioenerg ylists.org > > for more Gasifiers, News and Information see our web site: > http://gasifiers.bioenergylists.org/ _______________________________________________ Gasification mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/gasification_lists.bioenerg ylists.org for more Gasifiers, News and Information see our web site: http://gasifiers.bioenergylists.org/ _______________________________________________ Gasification mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/gasification_lists.bioenergylists.org for more Gasifiers, News and Information see our web site: http://gasifiers.bioenergylists.org/
