How does reverse cycle air-con work?

[kcrane]

My Cruisair air-con packed up last week while doing reverse cycle duty as a heater, giving a "Lo PS" error. Turned out to be a fairly common problem - a slow leak and it had run out of gas. Fixed today by Seacraft.

Turned my mind to how it works and why it doesn't work well once the water temperature drops.

To practise thinking rather than Googling I tried to work out how it operates.

Have I got it right that the air-con uses the cooling effect of compressed gas evaporating to chill fresh water which is then pumped to air handlers in the cabins, which blow air over the chilled water to transfer cold air to the cabin?

When the gas is re-compressed presumably it gives off the heat it absorbed, which is transferred into raw seawater, which is then pumped over the side. Thus a stream of water above ambient is being pumped into the sea when the air-con is in cool mode.

If that's right, then on 'reverse cycle' I'm assuming the heat from compressing the gas is transferred to the fresh water, which is pumped to the cabins and the air handlers now blow warmed air into the cabin.

When the gas is allowed to evaporate the cooling effect is transferred to seawater, which is pumped over the side, now below ambient temperature, exiting the boat colder than when it was sucked in.

I can see this working until the chilling of the seawater pushes it towards zero, when it would ice up and stop the process.

That's my amateur guess as to why, at a certain seawater temperature, you can't get anything from reverse cycle.

Stands by to be shot down...

 

[Nick_H]

That's exactly as I understand it, but we could both be wrong!

 

[kcrane]

That's exactly as I understand it, but we could both be wrong!

Not so bad being dumb if you are in good company

 

[BartW]

That's exactly as I understand it, but we could both be wrong!

+1

 

[GSL]

Don't know the exact technicalities of how it works, but can tell you that once the water is too cold, ie Southampton in February, it does tend to ice up the water return lines and stops working, takes a long time to thaw out as well in the middle of winter.

Have never had that problem in Sant Carles!

Graham

 

[henryf]

But do you actually need the sea water when using the unit in heating mode?

I can see that excess heat is a bad thing and so when chilling the sea water does a job. But when running reverse the air handlers are dissipating the heat away from the unit.

If the worry is that things would freeze up without sea water when you are asking for heat in the winter then some for of closed loop to add a bit of heat to the chill side of the evaporator is all that's needed.

The boats are sufficiently thermally inefficient that you will always have sufficient heat drawn away from the evaporator so it doesn't overheat.

Henry

 

[BartW]

But do you actually need the sea water when using the unit in heating mode?

yes you do,
you do take energy from the seawater to be used in the chillers,
the seawater that comes out is colder then that which goes in.
principle of a heat pump (is that the right word you use ?)

 

[henryf]

yes you do,
you do take energy from the seawater to be used in the chillers,
the seawater that comes out is colder then that which goes in.
principle of a heat pump (is that the right word you use ?)

But surely a heat pump and and A/C compressor are two different things?

With the A/C compressor you have two sides. The hot side where the refrigerant is compressed and the cold side where it expands. So in the summer you need to cool down the compression side so it doesn't overheat.

But in the winter you could simply take the heat to use round the boat and give away the cold providing it doesn't freeze and stop the cycle.

I'm not saying because I know but because I assume. And yes, assumption is the mother of all............

Henry

 

[jfm]

I would say that's exactly correct Kevin

Thus, to have great aircon in warm weather you want cold sea water to take as much heat as possible out of the now-condensed gas. You dont get that in the med, you get warm 29 deg seawater, which is your enemy (in this regard), so you need to spec super-strength airco hardware "tropical spec"

And to get great r/cycle heating, you want warm seawater to get as much warmth as possble into the chilled vapourised gas. But you don't get that in cold UK weahter, hence r/cycle doesn't work well in deep Uk winter (as Nick and i were warning henryf about a couple of months back when he said it was fab, in er warm seawater!)

So this is all a bit of an awkward wrong-way-roundness. When you want airco cos of hot weahter you want a cold sea and when you want heating on your boat you want a warm sea. But you get neither of those things. Only solution is to fit bigger equipment

 

[jfm]

But surely a heat pump and and A/C compressor are two different things?
Henry

Nope they are the same hardware. In winter you take heat out of the seawater to warm up the uber-cold evaporator side, so putting heat into the airco refrigerant fluid, and the warmer the seawater the more heat you can get out of it per second as it passes thru the heat exchanger

Incidentally, and all different, I believe dometic offer the option of 230v heating elements (little kettle elements) inserted into the aircon chilled water circuit, at the air handler end of tyhings, to heat the water some more and improve the effectiveness of the system as a heating system. Can be retrofitted easily Kevin?.

 

[BartW]

But in the winter you could simply take the heat to use round the boat and give away the cold providing it doesn't freeze and stop the cycle.

thats exactly what a heat pump does,
it gives the cold to the seawater
but if the incoming seawater is too cold, you won't get much heat

 

[henryf]

But doesn't the heat from compressing the refrigerant come into the equation ?

Henry

 

[stuey_two]

Reversible aircon quite common in France, excesss heat disspated to external air in summer, required heat extracted from external air in winter.
Cheap systems wor.k as heaters down to about -7, better systems (twice the price) work down to about -15.
DIY kits available off the shelf from Brico Depot - Calais
Don't know how they would cope with sea water splashes.

 

[BartW]

But doesn't the heat from compressing the refrigerant come into the equation ?

Henry

for good efficiency, this heat has to come from somewhere (not only electric)
this comes from warming up the other side of the system.

you can best compare with the fridge at home,
on the back you have a radiator that heats;

- for airco, you "use" the inside element (and the outside element will become warmer= waste)
- for heating you use the outside element (and the other side will become colder= waste)

sorry for sounding like a teacher

 

[lynall]

Airco/heat in my house not bought for the heat only for the airco, which since i fitted them we havent had!
Point being the fan runs whether its in heat mode or cooling mode, so guess the same for a water cooled machine?


Lynall

 

[jfm]

But doesn't the heat from compressing the refrigerant come into the equation ?

Henry

Yep, but the source of that heat is mostly the sea. It has to come from somewhere; doesn't grow on trees. So before you compress it, and get it hot so you can get heat out of it, you "preload" it with (free) heat from the sea. The only way you can do that (becuase heat only flows one way: hot to cold) is to make it uber cold, colder than the sea, which is what the evaporator does. It then sucks up heat from the sea. THEN you compress it, and its temperature rises, and you can suck out the heat that the sea gave it (as distinct from heat caused by the compression alone, which isn't there for the free taking; you have to pay for that, which is what the COP factor that John100156 explained not long ago is all about. )

(I have been a bit slapdash becuase the trick is condensing and vapourising rather than pure compression, but its broadly the same thing so let's not worry about that)

1. Compress/condense the refrigerant, so it heats up, and you use that heat (ie you take the heat out of the refrigerant) to warm the loop of water that goes around the boat to the air handlers.
2. Then of course it's gone a bit colder so you need to heat it up again. To do that, you have to revapourise it (which makes it uber cold, given that it was already depleted of heat to start with as stated in 1)
3. The uber cold refrigerant goes thru a seawater heat exchanger and gets warmed up by the sea so now it is a bit cold but not uber cold.
4 Then you compress it agian and it gets hot, much of the heat being £free heat taken from the seawater in step 3, as opposed to heat imparted by the paid-for electricity in the compressor motor. Then you're back to step 1

COP (coeff of performance) is often numbers like 2 or 3. In other words for every joule of electricity you buy to operate the compressor in the a/c chiller unit, you get 2 or 3 joules of heat energy in your hotwater loop that warms your cabins. The extra free bit is stolen from the sea. Though of course in the summer you put it all back so you're not really a thief. Indeed, you're only ever moving the free heat to a place a few metres away from where it was to begin with

 

[superheat6k]

My specific expertise this one. I fix and build very large air conditioning systems for my living. My handle 'Superheat 6K' refers to the level of superheat a fridge system should have as the refrigerant enters the compressor to operate safely (sad isn't it but I have to spend the profits on something ! )

Firstly all Fridges and all Air-conditioiung systems are 'heat pumps'. We simply refer to cooling process machines as a 'fridge' or an 'air-conditioner', when in reality we are using a mechanical system to 'pump' the heat out of the space to be cooled.

So where does this heat go ?

Well it plus the energy we use to drive the system (at the compressor) goes out at the condenser - the bit at the back of your fridge that gets hot, or on a water cooled marine system to the sea water.

A Heat pump / Air-conditioner simply reverses this process using a four way solenoid valve - the evaporator and the condenser change roles, so instead we refrigerate the sea, and use the heat drawn from it to heat the space we would normally cool in A/C mode.

The refrigerant fluid we pump around this system with the compressor conveniently changes state Gas to Liquid & vice versa at pressures and temperatures desirable for our chosen process, and as it does it absorbs (on evaporating) or gives up (on condensing) latent heat, and this transfer of a relatively large volume of heat energy is an almost free transfer and is the reason why Fridge systems are so efficient.

I am afraid I can go on about this for hours. If you want to know more about how fridges, Air-conditioners and heat pumps work please simply ask.

Before I close we are the only industry operating with a positive COP we get more energy out of our process than we put in - this is why Heat pumnps are so popular e.g. 1 kW electrical power in will result in up to 5 kW useful heat energy out.

Hope this helps explain what happens in the reverse cycle heat pump / air-conditioner.

 

[John100156]

Just to support .....6K who has explained the process well, some people wonder where the additional energy comes from in this process, that is why you can get 3+kW heat out from only 1kW of electrical energy in.

Without going too techy, we can do the calcs if you wish: If the AC system is a sea-water one, then in heating mode, the system absorbs the balance of energy from the SW itself, even if it's at a low temp, because it's temp is significantly above the boiling point temp of the refrigerant that is circulating through that part of the system. In effect it evaporates or boils the refrigerant and cools the SW!

 

[kcrane]

Incidentally, and all different, I believe dometic offer the option of 230v heating elements (little kettle elements) inserted into the aircon chilled water circuit, at the air handler end of tyhings, to heat the water some more and improve the effectiveness of the system as a heating system. Can be retrofitted easily Kevin?.

That is interesting... I'll check it out.

 

[John100156]

Now here is something to ponder: I designed a HP system many years ago for 200+ 200 seater meeting halls in the UK, it used a similar sort of principle. I diverted air that was usually extracted from the hall (RA) to mix with air entering the outdoor condenser unit (OA), this preheated the air-on the outdoor section of the HP in winter (-1C OA mixed with 23C RA) increasing the units COP, but also in summer (23C RA mixed with 28C OA). The first systems installed in the 80's. Still operating very successfully today.

To use an electrical heater in the way suggested to preheat the SW, would work but reduce the systems overall COP of course, but if you could use waste heat (perhaps that rejected in close proximity to battery chargers or the like) it would be fun. How about running the SW through a SS coil in you CW tank (unlikely to work for long unless a huge tank) or even fuel tank, it would cool your drinking water picking up heat. That sort of thing, not given it any thought, just for fun. Come on ....6K we need innovation remember JFM is building his next boat!