Reverse Cycle Air Con / Heating... how does it work?

[z1ppy]

Sorry for the dumb question but how does it work?

Appreciate it will require shore power connection or generator?

Understand it can heat as well as cool?

thanks in advance.
Z1ppy

 

[superheat6k]

An AC unit, your boat fridge, kitchen fridge, etc are all heat pumps. They pump heat from the place required to be cooled to a warmer place that can accept the heat. Because they use less external energy than is 'pumped' they can be used as a very efficient means of heating.

On the standard system the direct cooling transfer of the heat energy from the region to be cooled is where the liquid refrigerant is evaporated into a gas, absorbing latent heat of evaporation in the process, thus the vessel that achieves this is termed the 'Evaporator'. Conversely the secondary transfer where the heat energy is rejected at higher temperature (and gas pressure) is where the hot high pressure gas is cooled down and condenses back into a liquid - this vessel is termed the 'Condenser'.

On a fridge the evaporator is the grid inside that gets cold. The Condenser is the grid on the back that gets hot.

So on a conventional AC system the evaporator is the part inside and the condenser the part outside (or where the seawater gets circulated on a water cooled system).

Now in a 'Heat Pump', where use of the waste heat is desirable, the function of the AC mode evaporator and condenser are swapped over using a simple changeover valve, known as a 'Reversing valve'. Once this has changed its position the gas flows from the vessels are swapped, so the evaporator inside the cooler becomes a condenser and vice versa.

Now the warm gas condensing is emitting heat into the space around it. The heat is effectively taken from the normal rejection region, so the air or water normally used to cool the condenser is now providing the heat energy, in effect the cooling air or water is now being cooled (refrigerated).

Another key benefit of any refrigeration system 'heat pump' is its efficiency - because latent heat of evaporation / condensation of the refrigerant fluid provides a massive amount of 'free' energy transfer refrigeration is the only area of engineering where more effect is achieved than is used, e.g. a co-efficient of performance above 1 is achieved, indeed for modern AC units a COP of ~ 3 or 4 (or sometimes better) can be achieved.

But this gets better when using a heat pump for heating. This is because all the energy input from the outside through the compressor motor is transferred into the gas being pumped. In cooling mode this is just waste heat energy that is rejected in the condenser, but in heat mode this waste heat is a valuable addition, so for any heat pump system it will always achieved a COP of +1 to that achieved in cooling mode, which means a decent efficient cooling system with a COP of 4 will achieve 5 in heat pump mode.

COP is the same as EER (energy efficiency ratio).

The above is easier to explain at first hand with a system in front of us and pen and paper in hand. However, this Youtube video explains it quite well https://www.youtube.com/watch?v=14MmsNPtn6U

 

[Robin]

Just had to switch our home one from cool to heat for 36 whole hours in Florida, first time ever, but when outside temps rose a tad today to 50F had to go back on cool. ON our water cooled boat ones it worked fine on heat with water temps above 40F, below that used a fan heater, cooling was fine with water at up to 80F

 

[Portofino]

Sorry for the dumb question but how does it work?

Appreciate it will require shore power connection or generator?

Understand it can heat as well as cool?

thanks in advance.
Z1ppy

In simple terms
An electric motor powers a compressor to compress a gas ,which intern evaporates .To evapoourate it draws heat .
Think spraying alcohol on your skin .It feels cold cos it’s drawing heat away .The liquid boils off leaving no trace .

The “ Reverse “ bit is at described.

There’s a reversing valve in the boat Aircon , this allows the water to flow the oppersite way ( you don’t have this on your car AC btw ) .Here the gas is compressed into a liquid .This generates heat which pumped the reverse way round heats the water in the heat exchanger.
The warm / hot water then goes through the air handler - looks like a mini car radiator and a fan then fans air across the fins and warms up the boat .

If it’s on cold - ie A/c mode in the summer the colder water (cooled from later heat of evaporation - liquid to gas refrigerant in the evaporator ) is pumped round the air handlers with fanned warm air in - cooler air out - it chills the boat .


There are two circuits ,the refrigerant circuit to heat when compressed or when evaporated cool ,and the water to the air handlers circuit ( mini rads with fans ) ,Which can be different temps depending on wether the unit is on chill - cool or “ reverse “ warm
The reversing valve either sends the water to the pump for heat or to the evaporator for cool .

Traditionally these compressors tend to be old fashioned 220/240 v units like your fridge/ freezer at home .
When the desired temp is arrived at a thermostat turns them off and when the temp moves outside of the set range a thermostat turns on the ac motor .ac being alternating current btw .
Problem is ac motors require a spike or cause a spike in current draw ( something to do with the position of the commutator?)
You can hear it when your fridge next literally bumps into action .
On a boat on Med marina shore power limeted to typical 32 A on a 50 ftr or 16 A on a 34 ftr ,depending on what else is on say a charger , water heater , wife’s hairdryer etc etc the ac start up spike from the AC compressor could trip out something .
Obviously the more units ie 3 / 4 the greater the risk .
Sometimes these days ac AC motors have a soft start ,but even so there’s still a risk on dodgy shore power of somthing tripping out .
Frigomar make a D.C. brush less motor .DC is like 12 v in a car .With a potentiometer this motor s speed can be slowed or speeded up like a car fan motor .Crucially it does not turn off ,it runs all the time thus taking the risk of start up spikes tripping out - out .
These D.C. motors also draw less current ,so you have more headroom on your shorepower or geny .
You could buy a bigger geny ,but you can,t up the shore power A on the dock - er safely and without a raised eyebrow from the marina authorities.

At home you can run ,within reason —- As many ac motor devices as you like , you not limeted to 32A , 16 A or what ever A coming into the house .

 

[z1ppy]

thanks all... i have read superheat6Ks post more times than i care to admit and it still means sod all to me!! sometime its easy being a simpleton!

will do some more work on this over the next week or so.

 

[superheat6k]

thanks all... i have read superheat6Ks post more times than i care to admit and it still means sod all to me!! sometime its easy being a simpleton!

will do some more work on this over the next week or so.

I suggest we meet and discuss this over a beer, with a pen and paper too.