[Greenbuilding] Dessiccant Potential
Gennaro Brooks-Church - Eco Brooklyn
info at ecobrooklyn.com
Mon Sep 24 11:52:52 CDT 2012
That is an expensive bit of machinery - $4,000....
Gennaro Brooks-Church
Director, Eco Brooklyn Inc.
Cell: 1 347 244 3016 USA
www.EcoBrooklyn.com
22 2nd St; Brooklyn, NY 11231
On Mon, Sep 24, 2012 at 12:49 PM, Eli Talking <elitalking at rockbridge.net>wrote:
> Since becoming aware of equipment that uses heat to recharge
> (dehumidify) desiccant, I have been intrigued by the idea of using heat to
> recharge (dry out) desiccant to be used to adsorb humidity within a space.
>
> http://www.iaqsource.com/product.php?p=novel-aire_comfortdry-250&product=173848shows a unit that uses hot water to recharge the desiccant that could
> potentially be produced from solar energy.
>
> I have been contemplating the possibility of removable desiccant that can
> be recharged by any heat source outside the thermal zone. I have been
> incrementally upgrading my home thermal envelope to be tight. I night
> flush the house during the summer and generally achieve 70F with a high
> relative humidity of 80% in the morning. I am wanting the identify how
> much desiccant would be required to dry that air to 50%RH at 75F. Since I
> live in a wooded setting, the active solar opportunities are limited for
> recharging the desiccant. However, if the desiccant can be removed, it can
> be carried to an outdoor heater or drier for recharging. In my situation,
> I could take it to a solar dryer located in my field with full sunlight.
> One could also put it on a wood stove located outside when dehumidification
> is needed during rainy weather. Once a mass of desiccant has been
> recharged(dried) it can be stored in a vapor tight container such as a zip
> lock plastic bag until it is needed. In this way, one could stock up on
> dry desiccant.
>
> Below I attempt to quantify the issues. I am hoping some of the smart and
> knowledgeable people on this list might review the numbers to see if I have
> I understand the issues correctly. For example, the delta volume at 50%RH
> and 80%RH at 75F divided by delta humidity ratio should give me the volume
> of a #humid air, at least at that temperature. If I am right, 1# of vapor
> (1 pint in liquid volume) occupies more than 21cf. That is a huge volume
> expansion which give me some intuitive understanding of vapor pressure.
>
> In locations with better solar access than I do, I would propose the
> conversion of a thermal syphon solar air heater located on the South side
> of the building be tasked with recharging desiccant in the Summer. Many
> technical issues to be resolved to make that work. For now, I am focusing
> on my situation using the removable desiccant. The question is how much
> desiccant is needed.
>
> I have become re-familiarized with the Psychometric chart to determine
> humidity ratio and volume, enthalpy at each combination of RH and
> Temperature. From this information I attempt to determine the weight of
> the humid air at both 50% and 80% RH at 75F. From these two conditions, I
> determine of the change in humidity ratio and volume. Dividing the
> cf/#da(dry air) into the zone volume to determine the #da in the zone. I
> multiply that times the humidity ratio in #ha(humid air)/#da(dry air) to
> come up with the actual weight of the vapor in the space. Since we only
> want to reduce humidity to 50% and not bone dry, the humidity to be removed
> is only the difference.
>
> I probed into a local university library to access the ASHRAE references
> to read their 2 main articles regarding desiccants. I learned that the
> solid desiccants physically a*d*sorb humidity through capillary action as
> opposed to liguid desiccants a*b*sorbing humidity with a chemical
> reaction. Page F32.4 was a chart that showed the adsorption capacities of
> 3 different properties. The Gel 8 which is described in comparison with
> other desiccants has the capacity to adsorb .21 x weight of desiccant in
> vapor at 70F. I think this is a Silica Gel. I am looking for a good
> source of these desiccants. At any rate, the article also says that left
> in contact with the humid air, the desiccant will adsorb vapor until it
> matches the RH of the air. Therefore, the size of the mass of desiccant
> needs to have the capacity to adsorb 100%RH vapor so that when it adsorbs
> 50% of that, the air and the desiccant will be 50%RH, the target condition
> for comfort. I imagine a fairly simple box in the thermal zone with a fan
> that circulates room air through the desiccant until a humidity sensor
> switches off the fan when the target humidity say 50% is achieved and cut
> on again when the humidity goes above say 55%RH.
>
> The zone I show in my numbers is 24’x16’x8’=3072cf. If my logic is
> correct, than I need 2x weight of vapor at 50%RH or weight of air at
> 100%RH divided by adsorption capacity of dessicant of .21 to find required
> weight of desiccant of 34#. I think this is somewhat oversized because we
> are starting out at 80%RH and not 100%RH.
>
> I installed a Heat Recovery Ventilator in my house instead of Energy
> Recovery Ventilator that allows latent heat recovery because I do not have
> active dehumidification such as compressor driven refrigerant systems.
> However, with this active desiccant removal, an ERV would be a key part of
> reducing the vapor removal needed by drying out the incoming fresh air when
> it is more humid than the outgoing stale air. A tight building with latent
> heat recovery of ERV, allows for the possibility of using heat the recharge
> desiccant that can be used to dehumidify indoor air to achieve comfort and
> healthy conditions without refrigerant driven systems with their energy
> cost and incremental environmental degradation that is inevitable with
> leaking refrigerant.
>
> I do not have a person to review these issues with locally. I hope this
> list will wade through the logic to identify my errors and add to their own
> experiences on using desiccants.
>
> Eli
>
> HUMID AIR - DRY AIR RELATIONSHIPS Tdb(F) 75.00 75
> change RH(%) 50.0 80.0
> Vol(cf/#da) 13.68 13.80 0.12
> Vol(#da/cf) 0.073 0.072 0.001
> Zone Volume(cf) 3,072 3,072
> 3,072 Weight Dry Air (#da) 224.56 222.61
> 1.95 Humidity Ratio (#ha/#da) 0.0095 0.015
> 0.01 21.82 cf/#ha Weight Humid Air (#) 2.133
> 3.339 1.21 Vol Water Vapor as Liquid(cf) 0.03441
> 0.05386 0.02 Ratio Volume Liquid/vapor
> 0.000011 0.000018 Adsorption Capacity Req (2x # at 50%RH)(#)
> 4.27 6.68 2.41 Adsorption
> Capacity of Desiccant relative to desiccant weight 0.21 0.21 0.21 Weight
> of desiccant req to dehumidify to 50%RH(#s desiccant)
> 34.04
> Sensible Heat Added/latent heat removed(btu/#ha) 970 Latent Heat
> Removed-Sensible Heat Added(btu/#da) 1,170 Sensible
> Heat Added/#da(btu/#da) 5.21 Change in
> Temp(F)=sensible heat added(cf) *{1/zone vol (cf)}*{1/Air Specific Heat
> 1.08(btu/cf*F)}= 0.35
>
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