If you were Registered and logged in, you could reply and use other advanced thread options
|
Posted by on March 9, 2007, 7:32 am
Scottish "warm rooms" dry clothes. If 1 lb of 70 F air at 50% RH with vapor
pressure Pa = 0.5exp(17.863-9621/(460+70)) = 0.374 "Hg (using a Clausius-
Clapeyron approximation) and humidity ratio wa = 0.62198/(29.921/Pa-1)
= 0.00788 enters a well-insulated 130 F closet with 90% RH, ie Pc = 4.74 "Hg
with wc = 0.117, and wc-wa = 0.109 pounds of water per pound of air flows
out, we need 10/0.109 = 92 pounds of air to evaporate 10 pounds of water.
Raising the RH of 92 pounds of 70 F air from 50% to 100% as it leaves
the heat exchanger requires evaporating 92x0.00788 = 0.72 pounds of water,
ie about 720 vs 10K Btu, for a COP of 10K/720 = 14, vs 3 for a heat pump.
If we start and end with 40 F outdoor air at 40%/100% RH, wa = 0.0021, and
we only need 10/(0.117-0.0021) = 87 pounds of air. At 100% RH, 40 F air has
ws = wa/0.4 = 0.00525, so we only need to evaporate 87(ws-wa) = 0.27 pounds
of water with 270 Btu.
Longer drying times and higher closet temps lower the airflow rate and
the size of the heat exchanger... 92 pounds of air in 8 hours at 11.5 lb/h
makes cfm = 11.5/(60x0.075) = 2.55 = Cmin, approximately, and E = 0.95
= NTU/(NTU+1) makes NTU = 19 = AU/Cmin = 1.5A/2.55 = 0.588A, with heat
transfer area A = 32 ft^2, eg 2 4'x8' layers of plastic film, for 95% heat
recovery. Vertical films could have thermosyphoning airflow. We might turn
on the closet heater (a 30 watt bulb :-) with a humidistat whenever the RH
is more than 50% to make the "dryer" operate automatically, with minimal
clothes handling.
Some European dryers seem very efficient, with long drying times.
I wonder if they work like this.
Nick
|
|
Posted by Jimi on March 9, 2007, 9:26 am
> Scottish "warm rooms" dry clothes. If 1 lb of 70 F air at 50% RH with
> vapor
> pressure Pa = 0.5exp(17.863-9621/(460+70)) = 0.374 "Hg (using a Clausius-
> Clapeyron approximation) and humidity ratio wa = 0.62198/(29.921/Pa-1)
> = 0.00788 enters a well-insulated 130 F closet with 90% RH, ie Pc = 4.74
> "Hg
> with wc = 0.117, and wc-wa = 0.109 pounds of water per pound of air flows
> out, we need 10/0.109 = 92 pounds of air to evaporate 10 pounds of water.
>
> Raising the RH of 92 pounds of 70 F air from 50% to 100% as it leaves
> the heat exchanger requires evaporating 92x0.00788 = 0.72 pounds of water,
> ie about 720 vs 10K Btu, for a COP of 10K/720 = 14, vs 3 for a heat pump.
>
> If we start and end with 40 F outdoor air at 40%/100% RH, wa = 0.0021, and
> we only need 10/(0.117-0.0021) = 87 pounds of air. At 100% RH, 40 F air
> has
> ws = wa/0.4 = 0.00525, so we only need to evaporate 87(ws-wa) = 0.27
> pounds
> of water with 270 Btu.
>
> Longer drying times and higher closet temps lower the airflow rate and
> the size of the heat exchanger... 92 pounds of air in 8 hours at 11.5 lb/h
> makes cfm = 11.5/(60x0.075) = 2.55 = Cmin, approximately, and E = 0.95
> = NTU/(NTU+1) makes NTU = 19 = AU/Cmin = 1.5A/2.55 = 0.588A, with heat
> transfer area A = 32 ft^2, eg 2 4'x8' layers of plastic film, for 95% heat
> recovery. Vertical films could have thermosyphoning airflow. We might turn
> on the closet heater (a 30 watt bulb :-) with a humidistat whenever the RH
> is more than 50% to make the "dryer" operate automatically, with minimal
> clothes handling.
>
> Some European dryers seem very efficient, with long drying times.
> I wonder if they work like this.
>
> Nick
>
yup, they do Jim
|
|
Posted by on March 9, 2007, 1:25 pm
>> Raising the RH of 92 pounds of 70 F air from 50% to 100% as it leaves
>> the heat exchanger requires evaporating 92x0.00788 = 0.72 pounds of water,
>> ie about 720 vs 10K Btu, for a COP of 10K/720 = 14, vs 3 for a heat pump...
>>
>> Some European dryers seem very efficient, with long drying times.
>> I wonder if they work like this.
>
>yup, they do Jim
I've looked and haven't found one. Got a brand name with performance specs?
Nick
|
|
Posted by on March 10, 2007, 2:28 am
>
>> Raising the RH of 92 pounds of 70 F air from 50% to 100% as it leaves
>> the heat exchanger requires evaporating 92x0.00788 = 0.72 pounds of water,
>> ie about 720 vs 10K Btu, for a COP of 10K/720 = 14, vs 3 for a heat pump...
>>
>> Some European dryers seem very efficient, with long drying times.
>> I wonder if they work like this.
>>
>>yup, they do Jim
Apparently not...
http://en.wikipedia.org/wiki/Clothes_dryer
Condensation dryers
Just as in a normal dryer, condensation dryers pass heated air through
the load. However, instead of exhausting this air, the dryer uses a heat
exchanger to cool the air and condense the water vapor into either a
drain pipe or a collection tank. Afterwards, this air is run through the
loop again. The heat exchanger uses ambient air as its coolant,
therefore the heat produced by the dryer will go into the immediate
surroundings instead of the outside.
Because the heat exchange process simply cools the internal air using
ambient air, it will not dry the air in the internal loop to as low a
level of humidity as the fresh, ambient air. As a consequence of the
increased humidity of the air used to dry the load as well as the
increased complexity of the design, this type of dryer requires more
time and energy than a traditional dryer. However, it is a valid option
where long, intricate ducting would be required to vent a traditional dryer.
http://www.greenpeace.org/international/campaigns/climate-change/take_action/12_steps
Clothes dryers
Traditional clothes dryers are very energy intensive. So-called
'condensation' models – without an exhaust tube – use even more energy.
http://www.lib.niu.edu/ipo/1998/ic980618.html
Instead of drawing heated air through the clothes and venting it
outdoors, a no-vent dryer uses a simple condensing process to dry the
tumbling clothes. Air inside the dryer is heated and circulated through
the damp clothes. Since it is warm air, it absorbs moisture from the
clothes as it dries them.
This warm damp air now circulates through a small heat exchanger built
into the base of the dryer cabinet. Another small quiet fan draws room
air in through separate passages in the heat exchanger. The two air
flows do not mix and no conditioned indoor air is lost outdoors.
This room air cools the warm damp dryer air causing the moisture to
condense into a tray. This dryer air is reheated and circulated through
the tumbling clothes again to absorb more moisture. When the clothes are
dry, you slide out the water tray and empty it. It is basically
distilled water that you can use on your plants. For greater
convenience, install a tube kit to run the condensed water to a drain.
Bob Irving writes:
> In our bit of Yoorup, we use lines over the bath to dry our clothes
> initially, removing the final dampness in an airing cupboard (closet)
> which is warmed slightly by heat from our hot water tank. It takes a
> bit longer than a drier, but it uses much less energy....
This still requires space house heating energy to evaporate the water,
about 1000 Btu/lb, vs 330 for a heat pump and 72 for a closet with
an air-air heat exchanger. It looks like typical condensing dryers
also use lots of energy...
http://ths.gardenweb.com/faq/lists/laundry/2004120958010854.html says:
In a condenser dryer, there are two separate "loops". The inside "loop"
of air is sealed from the outside environment - air from within the drum
is heated, then blown through the tumbling clothes, then the
moisture-laden air is passed through a heat exchanger, where the water
recondenses. The same dry air is then reheated, where it is again blown
through the drum and clothes, and the cycle begins again (this is a
more-or-less continuous process).
The outside "loop" in a condenser dryer consists of either air or water.
Some condenser dryer models are air-cooled, and use the ambient room air
as a heat sink, by blowing it across the outside of the heat exchanger.
These dryers will tend to heat the indoor air in one's laundry room
significantly. Note however that ONLY heat is released - all MOISTURE is
contained within the unit. The condensed water can be either pumped away
to a drain line (e.g. into a standpipe shared with the clothes washer)
or stored in a container within the dryer to be emptied later (not all
models offer both options). All standalone Euro condenser dryers are of
this type, i.e. units from Miele, AEG, Bosch, Asko, Malber, and
Eurotech.
In "combo washer/dryers" (i.e. machines that can BOTH wash and dry the
clothes), the ventless condenser system is also widely used, but in
these cases the condensers are water-cooled. During a dry cycle, several
gallons of cold water are used to condense the moisture evaporated from
the clothes, which again is pumped away through the drain line. Most of
the "combos" currently available in North America use this method - i.e.
units from Equator, Splendide, Malber, Haier, Quietline, Thor, LG, and
Eurotech. Note that unlike the air-cooled design, these models do NOT
significantly heat the indoor air in one's laundry room - but on the
other hand, the fact that they use extra water during the dry cycle must
be taken into consideration, especially for anyone on a very limited (or
expensive) water supply.
Nick
|
|
Posted by Beloved Leader on March 9, 2007, 2:52 pm
nicksanspam@ece.villanova.edu wrote:
> If....
[snip]
It's a lot easier just to hang them up.
|
Page 1 of 2 1 2 > last >>
| Similar Threads | Posted | | Electric Dryer not heating up/drying the clothes | November 13, 2005, 12:39 pm |
| Oil spots after washing/drying clothes/laundry | August 28, 2006, 8:31 pm |
| Oil spots after washing/drying clothes/laundry | August 28, 2006, 8:31 pm |
| Oil spots after washing/drying clothes/laundry | August 28, 2006, 8:31 pm |
| clothes dryer - noises and increased drying time | December 31, 2005, 5:21 pm |
| Caulk isn't drying?! | September 23, 2005, 3:24 pm |
| Dryer not drying | February 4, 2007, 10:42 pm |
| Bit by the non-drying GE Silicone | July 29, 2008, 11:26 pm |
| Dryer not drying completely | February 13, 2005, 12:42 am |
| ABS Cement Drying Time | May 25, 2006, 1:55 pm |
|
|
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap