Subject: Re: Plastic flap dampers

From: [email-masked] (Nick Pine)
Date: 12 Oct 1995 04:58:25 -0400

Marge Wood writes from the AE list:

Nick Pine writes:

.>How much can a plastic flap damper weigh, and still be extended to 45
.>degrees from the vertical by convection airflow into a solar closet?

.>Say the damper is 1' tall x 1' wide, hinged at the top, and rising hot air
.>with velocity V is pushing it sideways, thereby lifting it. Assuming it
.>doesn't bend, at 45 degrees, the force of the moving hot air will have to be
.>the same as the weight of the square foot of plastic. According to the
.>calculation below, the plastic can weigh up to 9.2 grams, if the closet
.>is 8' high and the passive air heater has an average air temperature of
.>120 F and the closet has an average air temperature of 100 F, and the
.>plastic and two U-turns for the airflow don't slow down the air very much. 

.>10 H=8'closet height (feet)
.>20 TI=100'interior closet temperature (F)
.>30 DT=20'temperature rise of closet air (F)
.>40 V=468*SQR(H*DT/(TI+460))'natural convection airspeed (feet/min)
.>50 VPW=(V/4005)^2'velocity pressure (inches of water)
.>60 VPG=454*VPW*62.4/12'velocity pressure (grams/ft^2)
.>70 PRINT VPG

.>   9.210386

.>Cheap saran wrap weighs about 1 gram per square foot. Solar Components
.>in New Hampshire (the Kalwall people) used to sell 1 mil Teflon, weighing 
.>about 5 grams per square foot. I think 1/2 mil Tedlar is lighter, non-clingy
.>and strong at high temperatures. It weighs about 400 ft^2/pound or less,
.>ie just over one gram per square foot, but Dupont has a minimum order of
.>something like $3,000... Perhaps Real Goods will distribute this product,
.>along with thicker, UV-transparent, 10-year Tedlar film for sunspaces,
.>heat-sealed into large sheets, like greenhouse poly film.

>I never realized the hole had to be so big.

The 1' x 1' hole was just an example, Marge. The size of the vent holes
should be proportional to the area of the sunspace glazing.

One rule of thumb for Trombe walls (yuck), from page 159 of Mazria's
_Passive Solar Energy Book_ is to make the total area of the vents
approximately 1 square foot for every 100 square feet of glazing.
But then Mazria goes on to say in Table IV-14b on page 167 that vents
in Trombe walls don't increase their average efficiency much anyhow,
just a percent or two (more in cloudy climates), perhaps because the
thermal mass of the wall absorbs most of the solar heat, so not much
gets into the house via the flow of warm air.

Low-thermal-mass sunspaces (dark colored sunspaces with, say, insulated
frame walls vs. masonry walls, with NO masonry floors or containers of water,
etc.) and air heaters (eg poly film lean-to greenhouses with shadecloth
absorbers) are more efficient house heaters, since they can get cold inside
at night, and don't lose much heat to the outside world at night. They
should have larger vents, perhaps 2% or 4% of the glazed area (bigger vents
allos more air to flow, which lowers the operating temperature inside the
sunspace when the sun is shining, which lowers the heat loss from the sunspace
or airheater glazing to the outside world.) We could also use a fan, with a
sunspace attic fan thermostat in series with a house heating thermostat,
and a motorized damper. 

>And what would happen if you just sort of cut a square of something like
>the side of a gallon milk jug and hinged it over the hole somehow?

That plastic would be way too heavy. We need things that can be lifted by
warm air currents, like dust, or smoke, or milkweed, or dry cleaner bags.

>Seems to me like anything like Saran wrap would get wrinkled or something.

Yeah. And Saran Wrap likes to stick to itself. Altho it might not if we
rubbed it with a little talcum powder or soap, or sprayed it with an
anti-static spray. And Saran Wrap is somewhat heat sensitive: the directions
on the package say to avoid contact with hot food in a microwave. Perhaps
this means that the film would stick to the metal mesh ("hardware cloth")
backing part of the damper, at high temperatures. Sunspaces don't get this
hot, but solar closets might. We need to try this...

>Do you put anything around the edge to stabilize it?

I wouldn't, because that would make it heavier...

>and can you use a window opened a little bit for the bottom air source,

Sure. Theoretically, you only need one damper for the hole at the top OR
the hole at the bottom of the sunspace, not both, because a damper in either
place will prevent air from circulating through the sunspace. But people
often use two of these dampers, in case one sticks open. It also seems to me
that we can lose some house heat to the sunspace through even one open hole,
especially the bottom one, if warm house air flows into the sunspace through
the top part of the bottom hole and rises up into the cold sunspace, and
cold sunspace air flows back into the house through the bottom part of the
bottom vent hole, eg at night. 

>assuming of course that the heater has a duct of some sort between
>the window and the heat source?

You lost me there. It seems to me that these sunspaces don't need ducts,
just some kind of glazing spaced away from a dark wall, and a vent at
the top and a vent at the bottom, for instance, partially-opened first
and second floor windows, with plastic film dampers. 

Nick