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Greenhouse Heat Sink / evaporative cooler / humidifer

Came up with this design, anybody using anything similar? Did it work?

No apologies for kiddy drawing, best i can do in the jungle on an IPad :(


6222331317_be08507ef6_m.jpg



Edit:

Forget the pc fan, it will have to be a ducted fan, speed controlled, as used in hydroponics
 
I don't use anything of the sort, but counter current heat exchange is more efficient. If you could reverse the circuit somehow so the cold air was coming from the main tank and the cool water ran anti-parallel to the hot air, you might see better exchange.

All that being said, evaporative cooling in the regular misting sense would probably be just as efficient and less complex.
 
It is countercurrent and the main purpose is a heat sink to heat the greehouse at night.
 
If the arrows on the above diagram are correct, the air and water travel in the same direction. A countercurrent system would have them travelling in opposite directions so there was always a strong temperature differential between the two.
 
Hi all

@ Physalaemus
I think what manders is saying is that the air is blowing upwards from tthe bottom of the tank from a 'dip pipe' duct or similar.

@ Manders
I think that Physalaemus is right in his supposition that a simple evaporative coling system (either misting or wet media type) would be easier to construct and so long as you have sufficient water circulation give you a heat resevoir aswell. Watch out for Legionaires desiese if you get atomisation or small droplets.

Cheers
Steve
 
Ok back out of the jungle so can respond in a clearer manner, Steve you right i was intending to put a pipe down the center, although more recently i was thinking to just feed it in directly at the bottom to save space in vertical vessel.

Good point about legionaires and droplets.

Not sure a standard evaporative cooler would either transfer or store enough heat to be usefull but i'll look into it. Seems to me you have the same number of moving parts and the complexity is rather similar but less efficient at heat storage.

Of course the tradional heat sink idea is to use gravel, but you need an awfull lot of gravel to have a significant effect, using water would be much more effective, 3 X by volume, if you can get good enough heat transfer.

Anyway still open to ideas until i've finalised the design.
 
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Hi Manders
Back out of the jungle? sounds exotic!

For the heat sink you could use a gravel and water base I suppose. The gravel (from memory) would be about 1250 kg/metre cubed and when mixed with water (1000kg/metre cubed) to eradicate air spaces would give you a higher thermal mass than with water alone.

Evaporative cooling may work well depends on what your 'on' conditions, temperature and humidity are as to what you can get out of it, a quick look at a psychrometric chart should tell you, but if you are in a jungle with high humidity you are right, you wont get a lot, but it will be no less effective than that of your layout as the cooling effect can only follow a wet bulb temperature line.

You may well need a method of topping up the water level automatically,depending on teh water usage rate, unless you can do it manually as required.

Not sure what your primary purpose is, cooling or a heat sink, or both, or just to minimise the temperature swings in a temperate or tropical climate.

Hope that helps a bit.
Cheers
Steve
 
Steve, i'm based in the uk, just been out hiking in thailand for a while.

Original purpose was to save nightime heating as a normal heat sink, later i realised it could also usefully reduce the peak summer temp and maintain some humdity (ideally id like to do away with the autovents as they cause the humidity to plummet when they open).

So yeah if it can reduce the temperature swings and maybe save some heating in spring and autumn then it be worth constructing.

Sorry dont see the point of mixing gravel and water, water stores 5.2 X more heat than gravel on a mass basis. When i did the calcs **** strawbridges 1m3 (1.8 te) of gravel effort could barely store 2kWhrs of heat which is useless. Can get the same effect from only 300 kgs of water.
 
Hi Manders

See what you are doing now, in which case what you propose will work, i've never used a system like it but there are people (I think even in this forum, but cant find the link) who have created evaporative coolers with pumps and wetted media but not gone the extra stage and used a large reservoir of water as a heat sink. The exchanger design and air volumes will need thinking about. I presume you intending to speed control the fan on humidity rather than temperature, just allowing the heat sink to dissipate its heat by conduction to the surroundings.

Take your point on the gravel mix, I was just thinking of the mass rather than the thermal properties, apologies.

cheers
Steve
 
  • #10
Came up with this design, anybody using anything similar? Did it work?

Put warm water into the lower tank and get cool water out? How that?

If the lower tank is placed in a refrigerator you can put warm water into it and take cool water out.

Otherwise you put lukewarm water into the lower tank and take lukewarm water out. I cannot detect a big cooling effect in the outlined design.
 
  • #11
Jesse, the cooling effect is by transfer of sensible heat from the air to the cold water and further by evaporation of the water into the air. Of course if the air is cooled enough it may be 100% saturated anyway and hence the air will be cooled close to the water temperature entering the vertical heat exchanger.

The lower tank will have 'warm' water entering, mixing with a much larger volume of cold water and hence the water exiting will be cooler than the feed water. At night the conditions are reversed.

I think the trick will be to balance the water and air flows to get it working in the right way and hence both pumps need to be variable flow.

Hot air actually contains very little energy compared to water / deg C.

So hopefully instead of having say a 20 C swing in temperature over a 24 hour period, it can be controlled to a 10 C swing in temperature (at very low cost).

---------- Post added at 10:03 AM ---------- Previous post was at 09:48 AM ----------

Hi Manders

See what you are doing now, in which case what you propose will work, i've never used a system like it but there are people (I think even in this forum, but cant find the link) who have created evaporative coolers with pumps and wetted media but not gone the extra stage and used a large reservoir of water as a heat sink. The exchanger design and air volumes will need thinking about. I presume you intending to speed control the fan on humidity rather than temperature, just allowing the heat sink to dissipate its heat by conduction to the surroundings.

Take your point on the gravel mix, I was just thinking of the mass rather than the thermal properties, apologies.

cheers
Steve

Plan is to just adjust both the water and air rates manually and see what happens over 24 hours, a few dataloggers will come in handy... Probably will adjust air rate to change the rate the heat is released back into the greenhouse at night but until i try and see what happens its hard to say exactly.

Agreed, Water/air distribution in the exchanger could be tricky and have a huge impact on how well it works...
 
  • #12
High Manders

Without making a fine spray which would be best, the air will need to pass over wetted medium for maximum cooling effect, the more medium the better obviously, but this may impose too great a resistance on your fan, depending obviously on what it is and how much of it there is.

Taking an arbitrary greenhouse size of say 3m x 2.4m x say 2.3 high and say 10 air changes per hour that gives an airflow of 0.04 m3/s (100mm duct will do), or mass flow of about 0.056 kg/s. Original air temp (within greenhouse, full recirculation on the air side?) say 30C and 38-40% saturation, then passing this through your kit (assuming 100% efficiency) would give an off plant (pardon the pun) condition of 20C saturated absorbing 0.766kg/hr of water. Excess water pumped will obviously return to the reservoir.
 
  • #13
Thanks Steve, thats an interesting drop in air temp, I can allmost envisage chucking out the autovents and living with 30-35C daytime maximum and high(er) humidity than what i get now. Even if the evaporative cooling effect is reduced, should still be able to absorb much of the air temp into the water resevoirs.

started building a spreadsheet model of the heat balance over an 'any time of year' 24 hour period ( with lots of simplifying assumptions) But had so far neglected the evaporative cooling effect, have to fix that one when I get time...
 
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