Evaporative air conditioning

[pmagowan]

I am just looking for people's experiences and ideas on a concept I have. In short:

I am designing a ventilation system for a boat. The plan is that it will be a heat revovery ventilation system. In essence cool air from outside is drawn into the boat and distributed via ducting and vents. Similar vents take stale warm air from inside and exhaust to the outside. The two streams pass through a heat exchanger which transfers the heat from the exhausted air to the incoming air. This reduces the relative humidity of the incoming air and so acts as a dehumidifier while at the same time preventing loss of heat. Additional heat can be added to the ducting via an inline heater (eberspacher or the like).

Now this works for cool climates but in a hot climate I was wondering about the addition of an evaporative air cooler. This would work on the basis of passing air over a wet surface which causes evaporation and thus cooling. A heat exchanger would transfer this 'coolth' to the incoming air ducting (a direct system would add too much humidity to the boat).

Energy consuption would be a few small fans and a small water pump.

I was thinking about using salt water from the sea on the cooling plates because it would already be cooler and it would conserve drinking water. Problem is you would make salt.

Is this a crazy system? What are peoples thoughts/ideas ?

 

[maby]

Certainly not crazy - I have a similar concept in a commercial domestic air cooler unit that is stored somewhere up in our loft. We bought it many years ago when our son was young and suffering from significant asthma during a hot summer. It certainly worked, but I have to say that the degree of cooling was disappointing and we bought a couple of conventional portable compressor air con units a few years later.

 

[pmagowan]

Certainly not crazy - I have a similar concept in a commercial domestic air cooler unit that is stored somewhere up in our loft. We bought it many years ago when our son was young and suffering from significant asthma during a hot summer. It certainly worked, but I have to say that the degree of cooling was disappointing and we bought a couple of conventional portable compressor air con units a few years later.

The degree of cooling is dependent on a number of factors. I think having it indirect, through an exchanger, will improve the effective cooling in that the air will have a lower relative humidity. Also a boat is a relatively small space so should be easier to cool. It should hopefully be able to knock 10 or 15 degrees off the temperature of the incoming air (assuming hot dry external air etc),

 

[maby]

The degree of cooling is dependent on a number of factors. I think having it indirect, through an exchanger, will improve the effective cooling in that the air will have a lower relative humidity. Also a boat is a relatively small space so should be easier to cool. It should hopefully be able to knock 10 or 15 degrees off the temperature of the incoming air (assuming hot dry external air etc),

Hmmm, I will follow this with interest - my experience would suggest that 10 degrees is rather optimistic. But, hey, the calculations are not particularly difficult - the specific heat of air and specific heat of evaporation of water are published numbers, so it should not be too hard to work out approximately how much water you need to evaporate to get the effect you are looking for.

 

[pmagowan]

Yes, I have done rough numbers and it basically comes down to heat exchanger size, surface areas etc. A compressor type is obviously more effective but uses a massive amount more energy and is much more expensive. I am hoping to make this into a smallish unit that simply fits in place. Currently my sailing has no need for air con hence the initial focus on the MVHR unit (mechanical ventilation heat recovery). This would have the most advantages in that it could be designed with relative humidity detectors within the boat and thus automatically remove stale and wet air from kitchens, heads and wet lockers etc. It converts the ventilation system for the boat into one large dehumidifier. The air con side of things might need to be an add on since it is more complex and would require some maintainance.

 

[maby]

OK...

so 1 cubic metre of air weighs around 1kg at around 30 degrees and has a specific heat of around 1000 J/kg.

The latent heat of vaporisation of water is around 2000 kJ/kg...

so, if my mental arithmetic is still working at this late hour of the afternoon, to cool 1 m3 of air by 10 degrees, you will need to extract about 10kJ of heat which means evaporating 1/200 kg of water = 5g - does that make sense?

 

[Jabs]

I think it will rather depend on the rh of the incoming air. Dry air will encourage evaporation hence cooling effect. Very humid air won't be cooled much at all.

Evaporative coolers are used in domestic installations in the US in dryer climes, new mexico, for example. ive never seen one in the north, ny or nj for example.

Good luck!

Tony

 

[maby]

oh, and a moderate ventilation fan shifts rather more than 1m3 of air per minute, so you need to be evaporating about 5cc of water per minute more or less...

 

[pvb]

In essence cool air from outside is drawn into the boat and distributed via ducting and vents. Similar vents take stale warm air from inside and exhaust to the outside. The two streams pass through a heat exchanger which transfers the heat from the exhausted air to the incoming air. This reduces the relative humidity of the incoming air and so acts as a dehumidifier while at the same time preventing loss of heat.

I'd agree that it would reduce loss of heat, but how can it dehumidify anything? The amount of water vapour entering from outside is just the same.

 

[whiteoaks7]

One of life's little co-incidences - I was going to ask if anyone had experience of evap air con since Transcool seem to be making a big marketing drive in UK at present. The physics I looked at suggests that unless you have dry air to start with then evaporative cooling is only going to drop the temperature a few degrees - certainly not enough to make me part with £350 Transcool want - but I'm hoping I'm wrong.

Isn't this why they're called desert coolers tho? Great in Arizona and Alice Springs, but UK or Med? I dug out a psychrometric chart and followed a constant enthalpy line from a starting RH to approx 90% RH and looked at the temperature difference. In Greece and UK average RH seems to be about 50% (100% here this week ...)

Have you thought about a two-stage evap system? Personally I think you'll struggle to get the efficiencies you need but I hope you succeed and please publish if you do.

 

[charles_reed]

No relation to cooling a boat, but I was presented with a chilled beer in Sigri, Lesbos, cooled in an evaporative chiller.
RH was only about 45% at the time - and outside temps 25-28C.
Perhaps Vyv Cox would like to comment?

 

[pmagowan]

I'd agree that it would reduce loss of heat, but how can it dehumidify anything? The amount of water vapour entering from outside is just the same.

The air from outside is heated and thus the relative humidity decreases. It enters the boat and its relative humidity increases as it absorbs water. It is then exhausted where it is cooled and its relative humidity increased until or unless it reaches 100% when it will condense.

 

[pvb]

The air from outside is heated and thus the relative humidity decreases. It enters the boat and its relative humidity increases as it absorbs water. It is then exhausted where it is cooled and its relative humidity increased until or unless it reaches 100% when it will condense.

So the inside of your boat is wetter than the outside?

 

[pmagowan]

So the inside of your boat is wetter than the outside?

Yes! Adding heat to air makes it 'relatively dryer'. Adding wet clothes, humans, cooking etc makes moisture.

 

[pvb]

Yes! Adding heat to air makes it 'relatively dryer'. Adding wet clothes, humans, cooking etc makes moisture.

But you said in the OP you'd have a heater too. Why not just let the heater do the dehumidifying?

 

[ghostlymoron]

All this sounds very complicated for a boat environment. I usually sail with the main hatch open (unless it's pi33ing down) so any type of aircon is going to be struggling.

 

[pcatterall]

Desert Coolers using the water evaporative principle used to be very common in my years spent in hot dry regions especially where masses of power for compressor type coolers was not available.
We found them very effective and even used them in ( double skinned) tents.

 

[pvb]

All this sounds very complicated for a boat environment. I usually sail with the main hatch open (unless it's pi33ing down) so any type of aircon is going to be struggling.

Agreed. It seems the OP is planning to have all the windows/hatches/washboards closed and then use battery power to run fans to pump air through his heat exchanger, for what I can only imagine will be minimal benefit. "Crazy", as he suggested, might be appropriate.

 

[charles_reed]

Agreed. It seems the OP is planning to have all the windows/hatches/washboards closed and then use battery power to run fans to pump air through his heat exchanger, for what I can only imagine will be minimal benefit. "Crazy", as he suggested, might be appropriate.

anchored, with an open forehatch, is the most practical answer - of course if it's raining you should have chosen your cruising area more carefully.
Crazy, no... ill advised, probably.

 

[duncan99210]

Sitting here in the Med just now the air temp in the boat is 31C and the rh is 69%. Using the air flow from the wind, it feels cool enough to be comfortable. If it gets to feel to hot, then a simple fan for an hour or two in bed is enough. Think the projected system is over complex. Anchor, open the hatches and don't spend much time below.... Better spend money on a decent cockpit bimini with additonal side shade pieces to generate the required shade.