> In alt.energy.homepower Jeff Watson wrote: > : Hi, Anyone have any info on the use of a thermal chimney to increase > : natural ventilation in a space. <> Not much to be said actually. You want a flow drawn up by the heat convection. That implies a large crossection tube as tall as you can get it. The outside can be black, to make it hotter to help the convection, but you don't want the room to see the hot surface if you can arrange it so. You want the design to draw well in a wind. If the wind can force a back flow, then you get roasted when the wind blows. Not what we want, eh? And the other side of the equation is the intake. You would prefer the house be filled by the coolest air you can get ahold of. Large underground chambers that have been chilled by the night air, perfect, unless they have too many spider webs and dust and mold. <> If you had read the post you would see that the water was separated from the air before it entered the house. Also, the heat is carried away from the air by the water droplets(mist). You are confusing humidity with mist. Go spend a couple of hours on IBM's patent server researching cooling tower designs pertinent to small buildings and homes. <> The water spray will rapidly raise the humidity of the air to around 100%, at the same time lowering the temperature by quite a bit. If you had chosen a fluid with a low vapor pressure, you could expect the droplets to act as heat exchanger without humidifying the air. Actually, I do not recall if humid air is denser than dry air at the same temperature. Maybe it just feels that way when you have to sweat through it. But with water this chimney describes a form of a swamp cooler and the air should fall because it is cooler. This would be optimized if the chimney lets air out at the bottom of the room and there is another to vent the hot air from the top. <> Air is mostly nitrogen, Fred. A mole of dry air weighs about 29 grams. A mole of water vapor weighs about 18 grams... <> Time to whip out your psychrometric charts! Assuming a constant pressure, moist air at one dry bulb temperature (i.e. 80F) is actually less dense at high relative humidity (i.e. 90%) than low (i.e. 10%)! But an evaporative cooler lowers the drybulb temperature (the heat required to evaporate the water comes from the air), so the resulting cooled air's density is likely higher than the inlet air's density. <> You'd have to cool the air a lot more than that to get the density up to that of dry air. The moist air will still be less dense than the dry air. <> Drier air is denser, given the same temperature, since water vapor only weighs 18 grams per mole (22.4 liters) and dry air weighs about 29 grams for the same volume, right? And cooler air is denser, given the same water content, because of the ideal gas law, PV = nRT. So it's patently false to say that humid air is denser than dry air when temperature differences are no MORE than 10-15 F <> Humid air is less dense. Warm air is less dense. Ask a pilot, it is easier to take off on a cold dry day. <> > Cold air is denser than warm air. This occurs because > heat agitates molecules making them push farther > apart. This is why hot air balloons rise. > Air can can carry water. By the same principle, cold > air can carry more water than warm air. Nope. The warmer the air, the higher the percentage (by weight, volume or molecule count) of it can be water without moisture condensing out as liquid. Water molecules attract each other a lot more than air molecules do, because each water molecule is like a charged capacitor, with a positive end and a negative end. The negative ends attract the positive ends of other molecules. This holds water molecules together so that water boils at 100 degrees C while air boils at a much lower temperature. Air molecules are balanced (no charge concentrated anywhere on the outside), don't attract each other, and so fly away with a lot less encouragement. > But this doesn't mean you can't have warm humid > air that is heavier than cold dry air. Yes it does. When air "carries water" the moisture displaces air molecules, so that at any given temeprature and pressure, and volume, the total number of molecules is constant. And water molecules (H2O, molecular weight around 18) are lighter than the average air molecule (mostly N2, molecular weight about 28 and O2, molecular weight about 32). So moisture makes air (all other thinga being equal) less dense. Batters can hit a baseball further on a humid day than a dry day, because the ball can push humid air out of the way easier. I argued long and hard with a science teacher in the eigth grade on this, and it turns out he was right, though he couldn't expalin it to me as simply as this, for some reason. Took me till physics and chemistry to understand why his memorized answer was correct. <> I think that if you check a psychrometric chart you will find that cold air is capable of holding less water than warm air. You may be mistakenly considering the relative humidity which will tend to increase as a sample batch of air is cooled but this, by definition, gives you the percentage of moisture in the air per the maximum amount that it can hold. Ie, take some air at 80Deg F and 50% RH then lower the temperature on that same air and your RH reading will rise. If you keep cooling the air, the RH will hit 100% and the moisture will start to fall out in the form of condensation as you continue to cool the air. Thus the dew on the grass in the morning. <> Fascinating - from the to-and-fro-ing I'm guessing that there will be a cross-over point where one becomes heavier than the other. I suspect most agree that air gets lighter as it gets warmer. I also suspect that air gets heavier as it gets more humid. If it got lighter as it got more humid, then <> > >Time to whip out your psychrometric charts! Assuming a constant pressure, > >moist air at one dry bulb temperature (i.e. 80F) is actually less dense at > >high relative humidity (i.e. 90%) than low (i.e. 10%)! > > > >But an evaporative cooler lowers the drybulb temperature (the heat > >required to evaporate the water comes from the air), so the resulting > >cooled air's density is likely higher than the inlet air's density. > > > >Brian A. Rock > > > You'd have to cool the air a lot more than that to get the density up > to that of dry air. The moist air will still be less dense than the > dry air. Again, time to get out your psychrometric charts. Assuming sea level pressure, here's an example: Let's assume 90F, 25%RH outside air. We then pass it through a "passive" (no fan) evaporative cooler which produces 90% RH air. This is a constant enthalpy process, but is also commonly approximated as following a constant wet bulb line. On our handy psychrometric chart {Thanks to Willis Carrier}, we plot the starting point (outside air) and read that the specific volume is about 14.05 ft3/lbmda or a density of 0.0712 lbmda/ft3. (lbmda = "pound-mass dry air") Now following the constant enthalpy or wet bulb line up & to the left to 90% RH, we find the outlet condition. It's roughly at 65F dry bulb. The specific volume at the outlet is about 13.6 ft3/lbmda or a density of 0.0735 lbmda/ft3. The more humid outlet air is denser than the inlet, so barring other greater driving forces, the less bouyant humidified air will fall through the passive "thermal chimney". But inside/outside pressure differences, wind, actual DBT and RH, cooling water temperature, tower design, and other factors will affect the flowrate and direction. If the outside air is already very humid, you probably won't be happy with evaporative cooling! But I used a fan-driven "swamp cooler" with good success in high & dry Colorado. Be sure to maintain the water condition, however, since green stuff grows very quickly. A few locations even bar evaporative coolers for some purposes (i.e. schools) just because of this IAQ concern. <> I've seen some interesting ones in backwoods, portable toilets. 1. One I saw had a solar photovoltaic cells running a DC fan like one in your computer. It sucked the smelly air out of the composting toilet below the toilet seat. Works good if people leave the toilet seat lid down. 2. Another one had a 10 inch round pipe about 9 feet tall. This black pipe was on the south side of the toilet. The sun heats up the black pipe and hot air rises from the toilet out the top 9 feet above the ground. This also works well. The pipe could be metal or plastic, but I would perfer metal since it is a fireproof material and resistant to the sun UV damage. Thermal mass touching the chimney would tend to keep it from heating up quickly in the day and would cut down on the updraft. 3. You can also get one of those spinning round ball vents at Home Depot for $18 to $24. When the wind hits it from any direction, the sheet metal ball spins and sucks air out of the attic. I suppose it would work if you hooked it to the top of the solar chimney in 2 above. I've always wondered why home builders don't use one of the methods above instead of bathroom fans. <> > Hi, Anyone have any info on the use of a thermal chimney to increase > natural ventilation in a space. Nick's rule of thumb: CFM = 16.6 Av sqrt(H dT) where Av=vent area, H=height in feet, dT=temperature difference in degrees Farenheit. CFM being Cubic Feet per Minute. People have gotten confused and talked about cooling towers (used in arid regions to move air down) with solar chimneys (used to move air up, arid or not). While black stovepipe has a certain degree of solar chimney action, a more typical approach is a semi-glazed box. The interior of the box is black-painted or has collector plates, and the unglazed sides might be insulated. Sun shines, air heats, hot air rises. > What materials should be used for the thermal mass? I guess you could try to do something with mass to keep air moving after the sun went down, but the usual approach is to minimize mass, so the thing will draw air in a reasonable period of time when the sun hits it. > What shape should it take, wide bottom, small top or vice > versa? Neither. A very slight increase in size towards the top might make sense, but generally, restriction of either end will not be good, .vs. keeping it the same size. >What materials could absorb heat and take it up and out? Air. That's what a solar chimney moves, so you'd best use it. To heat the air, you might use black paint on wood, blackened aluminum (recycled printing plates, etc), or greenhouse shade cloth to absorb the incoming solar radiation. But the air that those heat will be what "takes it up and out". Wood is perfectly safe in a solar chimney, since it should never "stagnate", as an active collector can when the heat collection is turned off while the sun shines; if the solar chimney heats up, air moves and it cools down. <> > > Hi, Anyone have any info on the use of a thermal chimney to > > increase natural ventilation in a space. > People have gotten confused and talked about cooling towers (used in > arid regions to move air down) with solar chimneys (used to move air > up, My 12' by 58' by 7' (inner dimensions), strawbale solar chimney has both. It has 2 solar chimneys (at each end) which draw air up and out of the greenhouse and which in turn draw it down through the 1 cooling tower (in the middle). The topview looks something like this: --------------------------------- |solar | |Cooling| |solar | |chimney| |Tower | |chimney| --------------------------------- Letting CFM1 be the airflow through Solar CHimney #1, and cfm2 in Solar Chimney #2, then the airflow through the cooling tower is: cfm1 + cfm2. I have not measured my airflow, but cigarette smoke in my chimneys get sucked right into it's input pretty fast. Gregg Nelson in Aspen Colorado, wrote this about his 2' by 2' by 5' solar chimney (glazing on 1 side: "As far as airflow measurement, it was taken with a FE1000 Primary Flow Measuring Element by Paragon Controls of Forestville, CA. The unit was slightly modified by a friend of mine who owns it, but combined with common sence I think the 700 cfm measurement holds water (693 cfm was the actual average measurement over 54 cooling days in July/Aug. 1998)." The "solar chimneys" work by solar radiation... i.e. the sun shines through the plastic south, east, and west walls. This is different than a "thermal chimney" (i.e. a stovepipe or masonite chimney), which is heated by conduction (the sun heats up the metal, the metal heats up the air). The inside surface of the South wall of my solar chimney is painted black and is made of hardboard (like masonite and which lasts years in the sun --- as opposed to plywood). 3.5 inches from the south wall, I stapled shadeclothe to the studs. This is used to absorb the solar radiation. In 1 of my "solar chimneys", I copied Steve Baar's method, and screwed metal lathe every 3 inches at a 45 degree angle. The metal, in this case, is INSIDE the solar chimney, and thus doesn't lose heat directly to the outside; whereas for a thermal chimney, the metal is on the outside and so alot of heat is directly lost to the outside. Putting a stovepipe (thermal chimney) inside a sunspace would be better than not in a sunspace IMO. My cooling tower has misters at the top. The mist drops straight down and some hits the walls of the cooling tower and the floor and condenses and gets the floor wet. Some of it gets sucked towards each solar chimney. I need to build a couple of zigzags with rain gutters to collect this stuff. If I had to do it over again, I would put the cooling tower on the outside of the greenhouse with an opening in the wall near the ceiling, or have the bottom of the cooling tower enter a underground pipe which has another opening into the greenhouse.... > > What materials should be used for the thermal mass? > > I guess you could try to do something with mass to keep air moving > after the sun went down, aye. there is a power plant which has water in polypipe which absorbs heat during the day, and then gives it off at nite to the air in the thermal chimney so that the air still rises at nite and still turns the turbine which makes electricity. >but the usual approach is to minimize mass, so the thing will draw > air in a reasonable period of time when the sun hits it. Aye, but I suppose, 2 sets of polypipe could be used. 1 set would be located in the thermal chimney, would be drained during the day, and would be filled with hot water at nite. The other set would not be in the chimney, but would be located in a sunspace and filled with water during the day, absorbing the sun's heat and at dusk, would be pumped up to the pipes in the chimney. Valves would be used to control drainage... <> Ok, so I'm looking at a chimney shaped mass rising out of the building. The top and bottom will be about the same size. The chimney will be covered (or constructed) from wood with black paint. 1) Should it have a large opening into the space or small vents. 2) Should the opening be at the floor or at the ceiling. 3) Should it be centrally located or a one end of the space. <> You may consider putting some inexpensive 4" ads pipe in every trench bottom you run for power, water, etc for a source of ground cooled air into your house. <> If you do this, you should install some kind of filtering at both ends to keep spiders, dust and water out. You may want to plan for some forced circulation on a regular basis, to dry these tubes out. When they are cool, and the air coming in is hot and humid, you will get a lot of condensation. Where condensation, mold. Other than a regular drying session, and some method of rapidly cleaning and disinfecting them, I can't think of any way to make this maintenance free. Anybody have practical experience to offer? <> NO i would not run an underground trench as a source of fresh air, moist places cause moulds to grow and legionnaire's disease bacteria grow in moist places, and summer earthcooling is dangerous in humic climates, and for all other climates, well if it is used for preheating in winter there is no such danger.just be very careful about sloped drainage, watertable, and soil type. I have used natural cooling from my irrigation water by running the water through a radiator, any size or type, and have a large dia air ducts with thermosyphoning cold air down, the ceiling can house a cooling surface, but catch the drip! <> > NO i would not run an underground trench as a source of fresh air, > moist places cause moulds to grow and legionnaire's disease bacteria > grow in moist places, and summer earthcooling is dangerous in humic > climates, I don't disagree, based on the smell of cool damp covered earth trenches and such, but I thought that some folk are actually doing this with some success? Maybe they "did it right"? I would offer at least two ways to resolve the problem. The active way, force circulation, a regular soap spray up and down the cool storage surface, maybe small children with scrub rags (Just kidding). A more lossy but less dangerous way. An intermediate heat exchanger to cool the house without exposing it to the cool moldy damp. I think I would opt for a large water tank under the house, and a second one perhaps above grade. Then your big flat plate solar collectors put hot water in the top, and cold water in the bottom, by convection, no controls necessary. You could easily use the hot water to produce cooling, and the cool water is just what you want anyway. This forces a separation of house air from the perhaps moldy environments underground and makes the storage media movable, which is kind of nice. The house will still need fresh air, and should have an air exchange heat transfer to cool incoming air with effluent air, and then cool it additionally with the cool water. Say. I wonder if anybody has a nice relatively low tek version of a chiller using hot water, say around 140F? I think that is easy enough to get with a flat plate low tek solar water heater. The only ones I know about use ammonia and water or even more exotic compounds. Seems to me one of the alcohol's and water will do that without that nasty danger of a leak. <> > NO i would not run an underground trench as a source > of fresh air, moist places cause moulds to grow and > legionnaire's disease bacteria grow in moist places, I agree about the biological problems. Except, that I believe that Legionnaire's Disease only grows in hot humid places. The cooling towers where it's a problem is quite warm. The earth tube never get to the temperatures where Legionnaire's Disease thrives. Other problems to be sure but not Legionnaire's Disease. > and summer earthcooling is dangerous in humic > climates, and for all other climates, well if it is > used for preheating in winter there is no such danger. > Just be very careful about sloped drainage, > watertable, and soil type. I have used natural cooling > from my irrigation water by running the water through > a radiator, any size or type, and have a large dia air > ducts with thermosyphoning cold air down, the ceiling > can house a cooling surface, but catch the drip! > > -- > natural dynamics home comfort by design. > Silent off the grid ventilation, for summer cool, > and wood stove heat to supplement south side glass. > Photography, computer graphics, web pages. > > Sent via Deja.com http://www.deja.com/ > Before you buy. Have fun! -- CUL8ER \ \ \ \ \ \\ \ \ Receiver Powered by\ \ \ \ \ \\ \ \ [*] Thermonuclear \ \Solar\Energy\from the Sun \ /////| Energy(the Sun) \ \ \ \ \\ \ / / /\/ / /| \ \ \ \ \ /\ / \/ / / / | WA0VBE \ \ \ \ / /\ \/ / / \/ /| Ziggy \ \ \/ / / \ \/ \/ /\ | \ / \ \/ / /\ \\ / \ / / | "Red Rock Energy" === ===\ / \ / \ === \ / === Duane C. Johnson, Designer=== === \ \ === / | 1825 Florence St Mirrors,Heliostats,Controls & Mounts| White Bear Lake, Minnesota \ \ / | USA 55110-3364 \ \ | (612)635-5O65 work \ \ / | (651)426-4766 home copyright \ \ | (612)583-2O62 Red Rock Energy Site (C)980907 \ / | redrok@redrok.com (my primary email: address)===\ | \ | duane.johnson@unisys.com (Unisys address) \ | http://www.redrok.com/index.htm (My New Web site) \ | <> > Ok, so I'm looking at a chimney shaped mass rising out of the building. > The top and bottom will be about the same size. The chimney will be > covered (or constructed) from wood with black paint. Plus glazing, I would hope. If you won't use glazing, metal pipe would be better, since the wood doesn't conduct heat well. If you use glazing, with the black pain on the inside, that's a good thing - if not, it would be bad. The outside can match the rest of the house finishes with litte effect on performance. > > 1) Should it have a large opening into the space or small vents. > 2) Should the opening be at the floor or at the ceiling. > 3) Should it be centrally located or a one end of the space. 1, 2, & 3 all depend on what you are trying to do. If you wish to exhaust hot air, draw from the ceiling. For maximum draft, a big opening is better, but if you want a distributed airflow, several small openings do that better. The source of incoming air usually dictates logical placement of center or end, to get good airflow patterns in the space. <> >Hi, Anyone have any info on the use of a thermal chimney to increase >natural ventilation in a space. What materials should be used for the >thermal mass? What shape should it take, wide bottom, small top or vice >versa? What materials could absorb heat and take it up and out? Thanks >in advance for any help. Jeff Watson Years ago Scientific American published an article on cooling towers in the Middle East. The ones they described were towers beside/plumbed into large domed cisterns and plumbed into buildings (multistoried) on the opposite side of the towers. The tower had a small window near the top with a black-painted metal plate offset from the tower wall and suspended right behind the window. As I remember the ground floor opened to the cistern whereas the upper floor opened to the tower; the stairwell conducted cool air throuout the building. <> greetings, this is my first time in your discussion group; i was referred here by somebody in a permaculture newsgroup. my question is about sizing requirements for solar chimneys. i want to ventilate a living space approx. 25 by 35 feet and 9 feet high. i was considering 3 metal chimneys painted black; an appropriate diameter and length of these is my question. ie. what should the ratio of surface area to volume for these pipes be? another option would be to have a chimney with a metal plate behind a glass window. this would seem to be a better design but would require higher building costs... <> > my question is about sizing requirements for solar chimneys. i want > to ventilate a living space approx. 25 by 35 feet and 9 feet high. Where do you live? > i was considering 3 metal chimneys painted black; WHere did you get this idea from? I'm not sure this would work as well as some other designs. I guess it'ld do ok as long as the outdoor temperature is hotter than the indoor temperature... but you propbably want to vent the house to draw in outside air which is cooler... Since your metal is exposed directly to the outside, it loses heat to the outside fairly easily... Now, metal pipes enclosed in a sunspace might work better. I've seen an article which used a black stovepipe in a sunspace to ventilate a small building designed as a latrine. They wanted ventilation even if it was cold outside. Which leads to your next point: > another option would be to have a chimney with a metal plate behind a > glass window. this would seem to be a better design but would require > higher building costs... hmmm.... Without any glass enclosure, you might need to make your chimney's 50' tall (more on this below). With glass, you might only have to have them 8' tall. Some powerplants have masonite chimney's which absorb the sun's heat better because they have a better thermal capacitance. > an appropriate diameter and length of these is my question. ie. what > should the ratio of surface area to volume for these pipes be? Steve Baar, in his book called "Sunspots" has some recommendations on this... but for a solar chimney of a different design than yours. One equation you can use for estimation is this: cfm = 16.6*Av*sqrt(h*deltaT) where: cfm = cubic feet per minute Av is the cross-sectional area of your chimney h is the height of your chimney deltaT is the difference in temperature of the air in your chimney and your room. deltaT can be estimated by looking up solar radiation data which requires knowing where you live and what month it is, and using ohm's law for heat flow on your solar chimney. 25 by 35 feet (875 sqft) and 9 feet tall = 7875 cuft. 2 Swamp Coolers on singlewide 12 by60 = 720sqft mobile homes each supply about 2500 cfm for a total of 5000 cfm. You probably don't want that much. If you can get deltaT = 20F, and you want CFM to be 2100cfm, and you have 3 solar chimneys... each supplying 700cfm, and you are using 8" diameter stovepipe as chimney's, so that A = pi*(8"*1'/12")^2 = ~1.3 sqft. 700 = 16.6*1.3*sqrt(h*20) 32.4 = sqrt(h*20) h = 32.4^2/20 = 53' tall But suppose you have 1 solar chimney 8' by 2' by 8' tall. h=8. .. And instead of using stovepipes, why not use metal lathe positioned horizontally but at a 45 degree angle. .. and use them every 3 inches. The lathe takes up some of the vent space... so Av = ~ 8'*2'*5/8 = 10sqft. make the northwall out of hardboard or masonite, and the south, east, and west walls out of glass, or greenhouse solid "plastic-like" glazings like "sunlite" cfm = 16.6*10*sqrt(8*20) = 2100cuft Toby <> Please excuse my ignorance, but I'm just wondering where the replacement air comes from if as I guess the solar chimney causes warmed air to go up and out of the dwelling ? You did say ventilate which I guess means not just circulating the air inside a closed space...? If this is a hot arid environment, is it just drawing in more hot air from outside ? I had seen ideas for a rock-filled basement or large water tank to provide thermal mass for cooling the intake air for such systems. My other reading on traditional desert (?Morroccan) dwellings was that the solar chimney actually represented a thermal mass in combination with the thermal mass of the adobe-style house, which cooled overnight, allowing cool air to fall into the house during the day until the chimney heated up later in the day and drew the excess hot air out. I am not sure how this fits with the thermal mass of the house releasing warmth during the cold desert night (I forget - it's been a while...)? <>