[Greenbuilding] Dessiccant Potential

christian corson chris at ecocor.us
Mon Sep 24 10:05:53 PDT 2012


Buy a single ASHP and call it good. 3/4 ton will get it done. Install it
yourself and save a G. $1600.00 bucks and a lifetime of heating/cooling and
de-humidification.
Done and done.
80% RH indoors........................yuk.  Night flushing in humid
climates..............double yuk. Dont trade T for RH.

c

PS. dont get lost in the math, let common sense prevail.
*Christian Corson*
*EcoCor Design/Build*
ecocor.us
chris at ecocor.us
207 930-5088



On Mon, Sep 24, 2012 at 12:52 PM, Gennaro Brooks-Church - Eco Brooklyn <
info at ecobrooklyn.com> wrote:

> 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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