How does reverse cycle air-con work?

[henryf]

and those Scandinavians would appreciate the extra heating in the winter!

Given boats are hardly what you would call energy efficient it should be possible to work out a great solution to piping hot air in the winter.

Henry

 

[John100156]

Interesting perspective - should we accept it or ignore it?

 

[jfm]

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!

Imho that's a seriously good and cost effective idea John. I mean, use the fuel not the sea to steal heat from, for the r/cylce airco when working in heating mode. Quite easy to plumb, £1000 of hardware, with diverter valves so that if you run it so long that you make your fuel colder than the sea then the system reverts to using the sea again. Yup, top idea for boats in northern climes imho.

No use for folks in the med who need cold fluid for airco - your fuel will never be colder than the sea

 

[John100156]

Imho that's a seriously good and cost effective idea John. I mean, use the fuel not the sea to steal heat from, for the r/cylce airco when working in heating mode. Quite easy to plumb, £1000 of hardware, with diverter valves so that if you run it so long that you make your fuel colder than the sea then the system reverts to using the sea again. Yup, top idea for boats in northern climes imho.

No use for folks in the med who need cold fluid for airco - your fuel will never be colder than the sea

Top marks JFM, and with the engines running, heating the fuel, the transfer is even better! When engines are not in use though, it would not take too long to cool the fuel/water tanks (Mmmm not sure about your ones though ) better to use a flowing source of energy in reality, to transfer the energy from, hence the normal use of air or water/SW.

I was thinking more of the problem with using SW in colder climates. Clearly the process involves what is in effect very cold SW, that we extract energy from, this cools the SW down even further (thank goodness its salt water, hence its freezing point is lower than 0C), it's this energy, plus that so well described by ...6K above, that through the reverse cycle HP process provides the total energy transferred to the internal spaces and that is what heats the boat. The warmer the SW entering this system, within limits of course in winter, the better the heat transfer and hence efficiency. The reverse is true in summer when cooling (OK fridge boys don't bash me for over-condensing, we know how we can sort that).

So, if the SW entering in winter runs through other HX's, I quite like the battery chargers which might benefit from cooling even in winter, or any other waste/extract systems (and that's the fun bit to play with) on its way through to the AC outdoor HX, it will heat the SW up slightly, using 'free heat' (not electric) which makes the AC unit overall more energy efficient for nothing.

Now, we all know with ideas there are many variables that will inevitably need to be considered, losses to offset gains, benefits to offset drawbacks, always is, not least of which would be extra pump energy to push water through the system, but it's fun to play around with them. Someone may come along and suggest why don't we just pump the heat energy from the battery chargers into the space, but that's the beauty of a refrigeration HP system, it is an extremely efficient system for transferring heat from much lower temps to higher and vice versa hence their excellent COP!

All of the above is easily calculable/quatifiable, what fun, if only I had the time!

 

[kcrane]

SWMBO came up with one option. The hot water tank sits above the AC condenser and quite a lot of the time we have the water heater switched on when moored up. So there is a source of heated water that the majority of the time just gently cools, is reheated, then cools etc. using existing electric element.

Could that be piped through a HX (I'm learning the lingo) with the SW when a) the AC is running reverse and b) when SW temp is below a threshold?

 

[superheat6k]

Apologies but I am afraid I leave my developments for industrial sized 415vac systems typically around 500 - 1000 kW !

The problem here is that even with these significant volumes of seemingly free waste heat energy, there is very little we can do with it. That's why Cities like London end up with a micro climate several degrees higher than the normal ambient in the Summer. All these buildings ultimately have to reject this heat into the outside atmosphere.

The waste heat from A/C systems is a low level heat - lots of it but at barely elevated temperature. This is fine if we simply want to heat the cabin air through one stage of heat transfer, but at each stage the heat energy has to transfer there has to be a significant temperature difference between the two medias.

A typical running reverse cycle system will be condensing the refrigerant at no more than 40oC, to achieve this the cooling air passing over will at most warm to 28 - 30oC, but it will be too hot at that level so needs to mix with its surroundings to achieve a comfortable space temp of around 22oC.

If we wanted to use the waste heat to warm our domestic water (to 60oC) we would have to drive the condensing temp up to 70oC (this requires a revised compressor design). Firstly the volume of water to warm, even on a small system would be massive, and as the condensing temperature & consequent pressure is raised a substantial rise in the compressor load arises, and the COP tails off. For most heat pumps operating at this level for heat recovery, the difference in operating condition to that of the cooling process is too wide, so these will be designated at heat pumps only, not reverse cycle.

A/C systems on a boat will be quite heavy on the available power supply already - a 12 kW system will require a 4 kW compressor - this will draw 12 amps, excluding its start peak load, and most marinas only put out 16amps so not much left for anything else.

There are several other problems with trying to develop a fancy heat recovery system.

1 If the seawater is reasonably warm (above 8 - 9oC) then there is plenty of heat energy available (minimum outlet temp is 3oC to avoid freezing the water, even seawater - get this bit wrong and expect a very very expensive repair if the evaporator freezes and splits).

A flow rate of 1 Litre of water / sec will transfer approx 4 kW heat energy per oC change, so if we assume a 3oC delta T across the evaporator a flow rate of 2 litres / sec will transfer 24 kW. For a typical 12 kW system the heat rejection will be around 16 kW assuming a Co-efficient of performance of around 3. so either a delta T of 2-3 oC or slightly less flow on the cold side.

2 This means a sensible minimum seawater temp of 5-6oC to avoid having the water coming out too cold. On an air-cooled system the minimum is -5oC, as below this the available heat energy left in the sir tails off, plus any airborne moisture will freeze on the cold coil so then the system requries complex defrost systems.

3 If we try to use the waste heat also in a liquid cooled cooler, at a flow rate of 2 lts/sec we will warm up any tank to the maximum we can sensibly achieve of around 35oC within short order, and 35oC is barely lukewarm. Yes you can have it hotter, but there will be a significant energy penalty at the compressor and for a reverse cycle system the upper limit for heat pump mode would be about 40oC.

4 Such a system with intricate valves and pipework would be very expensive and would take up valuable space in the confines of a tight engine room.

5 Refrigeration systems are designed with a careful balance between the major heat assemblies - Evaporator - Compressor - Condenser. Trying to make one of these assemblies in two separate parts and with differing operating temperatures, and then expecting them also to function in reverse cycle is just not practical.

The net gain for all this even if it could be made to work would be very limited. If it did work the commercial makers of these systems would already be offering their systems with such whistles and bells.

For this reason my advice would be don't even go there. Accept the basic fact that Reverse cycle A/C units will cool fine and heat fine within their design limits and capacity, but will do little else. A Marine system will run out of sensibly available heat from the seawater at around 6oC.

Even complex buildings with vast qtys of low level waste heat can do little with it, and on a boat, even a 70' one, the complexity and cost, let alone the reliability factor would outweigh the energy saved in this manner, and with low Return on Investment such follies are avoided.

 

[John100156]

Blast, I just typed a long and detailed reply to 6K but it logged me out, so I lost it - perhaps later as I am enjoying this debate.

Just a quick one to ponder: We sometimes want to use our HP's in winter when at anchor (I'm out to Sant Carles before Xmas) and when we do, we use our generators.

It would be so simple to mix the warm SW-off the generator with the SW-entering the HP to increase the refrigerated effect and COP. We can then use the cooler water off the HP and pipe it back to the generator exhaust to cool it before it leaves the boat. It can revert back in summer.

Anyway, I must go and fit the new towel holder in the en-suite or SWMBO will kill me..... Domestic life, it has a habbit of bringing you back to Earth....

 

[John100156]

OK towel rail fitted (twice) how on earth could SWMBO detect it was not level beats me....

The following is mainly for 6K really:

You will understand the pressure enthalpy relationship I am sure, but some fridge engineers (not suggesting you) don’t fully comprehend how much more efficient a system is when you simply raise the evaporating temp in a system. In winter, in heating mode, the following benefits will result using R123A Refrigerant - sorry for the crude sketch but I have rather quickly checked all enthalpies.

It assumes a condensing temp of 40C (Ideal for heating the air in the boat). I seek to show in principle the benefits of raising the refrigerant evaporating temp from say -5C (black lines) compared to +5C (red lines):

SOrry if the image is upside down it looked OK on Photobucket..

The following can be clearly seen:

A decrease in Compressor Work (hence power consumed) as much as 23%
(hD1 – hC1 compared to hD - hC)

An increase in refrigeration effect of 4.3%
(hC – hB) to (hC1 – hB1).

A very marginal decrease in heating effect of 0.43%
(hD – hA) to (hD1 – hA)

An increase in COP, quite staggering at 35%
((hC – hB) / (hD - hC) = 4.81 compared to ((hC1 – hB1)/(hD1- hC1) = 6.51
therefore (6.51 – 4.81)/4.81 = 35.3%

Now if you run the calcs for an increase in condensing temp from say 40C to 45C the COP actually decreases from 4.81 to 4.03, which represents a 16.2% decrease.

You and I could go on to debate the effect of liquid sub-cooling and suction vapour superheating but better that discussion is held outside of this forum on the back of a boat consuming large quantities of beer…..

Don't you just love air conditioning and refrigeration?

I have been in it for well over 30 years and just love it. Mind you, most of my hands-on fridge work was done many years ago with Amonia, R22 and R12 on some very large HVAC systems. In those days you needed to design not only the coils and TEV's but also the distributors/roses.....

 

[superheat6k]

Nice to see some proper calcs. I am sure you mean R134a. The response is also a bit techy.

The limiting factor here will be the optimum evap temp that can be achieved, and that solely depends on the seawater temperature where the boat is.

Also I would expect the system originally discussed will be on R22 or R407C, as R134a is not energetic enough for most split type A/C systems.

However, to look more closely at your suggestion, I agree that raising the evap temp wil make the system more efficient, and more heat ebnergy will be available for the heat recovery process, but I would be reluctant to operate at -5C evaporating, even in seawater, hence my suggestion of a minimum sea temp of 5 - 6oC, allowing a 4oC Delta T to the Saturated Suction Temperature, the coldest the evaporator would see would be 1-2oC, and that is as close as I would go, otherwise there is a risk of ice formation within the evaporator from skin freezing.

If the refrigerant is R407C this effect will be worse as the glide effect will mean the lower boiling components will be flashing off below freezing.

It will work well in the Med with sea water I expect above 12oC even in the depths of Winter, but in the Solent area rivers the sea temp could fall to a few degrees, and I routinely observe ice forming on the river bank where I keep my boat. This is especially risky in brackish waters where the salt dilution is less.

However the idea of pre-warming the seawater by using the run off from the generator would be worthwhile, as long as there is more flow out from the generator than the evaporator would need.

 

[John100156]

Yep now we're getting there, R134A as on my notes not as incorrectly stated in the text. It was only used as an example to show the theory, it would be fun to play around with real temps/pressures, if only we had the time, it's good to know that we have experts like you on the forum - sorry to OP for the diversion into tech-talk, but I thoroughly enjoyed the debate, all in good spirit.

 

[stearman65]

Reverse Cycle

Is the correct term. In refrigeration mode the condenser/radiator dissipates the heat & the evaporator provides the cooling. Then through a change over or reverse cycle valve the process is reversed.
Stearman65

 

[kcrane]

Yep now we're getting there, R134A as on my notes not as incorrectly stated in the text. It was only used as an example to show the theory, it would be fun to play around with real temps/pressures, if only we had the time, it's good to know that we have experts like you on the forum - sorry to OP for the diversion into tech-talk, but I thoroughly enjoyed the debate, all in good spirit.

Apology accepted as it is always nice to see enthusiasts at play

 

[John100156]

Is the correct term. In refrigeration mode the condenser/radiator dissipates the heat & the evaporator provides the cooling. Then through a change over or reverse cycle valve the process is reversed.
Stearman65

Generally, before reverse-cycle heat pumps became so popular, we just had mainly cooling only units, so we identified the indoor unit which did the cooling as the evaporator because the cooling effect was due to refrigerant evaporating in that coil, we called the outdoor unit the condenser, because, you guessed it, the refrigerant condensed in it.

Then someone went and fitted a reversing valve into the system so in essence it sent hot gas to the indoor unit to condense and refrigerant to the outdoor unit to evaporate.

So even though we still sometimes call the outdoor section the condenser regardless, it is also an evaporator. Probably best to call them indoor section and outdoor section.

Hope that explains it for you.

So, if we fitted a reversing valve and some bits to your fridge, when you energise it, the coil at the back would get cold and your ice-box hot!

 

[rafiki_]

Apologies for my ignorance, but I am really only familiar with automobile MAC, and we are transferring to 1234YF gas, well, if the Germans stop playing silly beggars. Is the boat use not considered MAC, and subject to the same regs?

 

[Piers]

How does reverse cycle air-con work?

Not all that well, in my experiene. Having said that, let me quantify that I have only been exposed to reverse air on our Fleming 55, which has two Cruisair units which are checked every other year for pressure etc.

It could, of course, just be installation issues. For example, the (SMX II Control Systems (DX)) control units do not function despite a cruisiar specialist staring at them and giving them a first written warning. All they will do is blast full speed cold or hot air, and what a noise that blast creates!

Apart form the main saloon, there are individual sub-units for the three cabins and pilot house which have the same probelm plus the added nuisance of being at the end of the runs and therefore most inefficient.

Having said all this, we have hardly had cause to use them so I haven't really tried to find someone who might be able to fix them. Instead we use fan heaters and are now toying at fitting a Webasto central heating system.

Cry for help....Does anyone know a really tip top Cruisair engineer?

 

[superheat6k]

Now for a complete deviation from Motor Boating ...

Refrigerant 1234YF is part of a newer refrigerant group known as HFO's, and this particular gas is to replace R134a an HFC (as distinct to R12 which was a CFC -now banned, and R22 an HCFC in final stages of the banning process presently).

Use of HFO's in vehicles is for New design cars only presently. I think this was from the beginning of either this year or last, however as there is only one production plant in China making the stuff for the time being car manufacturers have been given more time to change from R134a to 1234YF.

The primary reason for phasing out HFC's is the GWP factor (Global Warming Potential) now the environmental issue for getting rid of CFC's & HCFC's conerning Ozone depletion has now been resolved, however as far as GWP is concerned, HFC's may be worse pollutants, so the environmental lobby want HFC's banned to.

A key advantage of CFCs, HCFCs and most HFCs was / is their non or low flammability. HFOs are increasingly flammable, and this presents a major problem for a machine with an embedded electrical motor, especially when the motor burns out (as is remarkably common !).

The problem for the fridge industry using not just R134a, but also derived blends of this gas - R407C, R404A, R410A etc, do not yet have available replacement refrigerant fluids for the cooling systems that are mandatory under other parts of the law, especially those concerning food temperature control, or where required to achieve high volume cooling - Building A/C, data centres, etc.

I am not fully conversant with the law as far as use of R134a in vehicles is concerned, but as a proportion of total Refrigerant use by volume, car use is a small %. However, any vessel already designed or built would not come under the rules for non use of R134a. Any system operating on any other refrigerant cannot be changed to a fluid that does not exist (and it is possible such ideal fluids may never exist !)

I know all this is a bit boring, but this is my day job as my business is directly affected by these changes to refrigerant rules, indeed long may it continue.

For the time being if your systems are running on R134a, R410A or R407C then this gas will be available for the foreseeable future. If you are on R22 you need to act very soon to change this to a Direct Replacement, such as R417a, R422D or R438A, but note do not expect this to be a painless nor cheap process.

If you are still using R12 or R502 then you are beyond help !!!
(not breaking the law to be using such equipment and gas, but if it breaks or leaks forget it).

 

[John100156]

?........Cry for help....Does anyone know a really tip top Cruisair engineer?......

Unfortunately, my vac pump, gauges and scales were sold many years ago but any good fridge engineer should be able to analyse and find/rectify the faults on your system, only problem is that for every 10 engineers IMO you may only find 3 or 4 that really know their stuff, and boy-O-boy, because of the steep learning curve and lack of understanding, it's the client that pays the price (OK, I would expect 6K to batter me for stating that, but hey-ho ).

As a consultant, I am amazed when inspecting installations nowadays. I would have killed my engineers for leaving ends of fridge pipes un-capped in very dusty damp environments overnight as I observed on one occasion on a multiple VRV installation last year. I made them pull an entire run out because of it. Only a few months ago I pulled up an engineer on another site in Lincoln who had left his gauges with hoses and vac pump line flying loose on dusty ground in the rain, boy did he get a telling off when he tried to bullsh*t me. But then again I have seen some excellent engineers at work at Unilever, GSK and Imperial College, so they are out there.

AC/R is an excellent profession and good engineers like most industries are worth their weight in gold. A good fridge engineer often needs to be an electrician, plumber, controls engineer, a little electronics, drainage engineer, mechanical engineer for pumps and the like, heating engineer, understand good HVAC design (trust me psychrometrics is a science in itself) as well as his fridge. I could probably go on.

I reckon you need to sweet talk 6K to see if he would take a look at it for you! We have Volvo Paul for our Diesel work - and perhaps 6K for our fridge, if he's good, I am sure he won't be that busy........... Sorry couldn't resist that

 

[rafiki_]

Now for a complete deviation from Motor Boating ...

Refrigerant 1234YF is part of a newer refrigerant group known as HFO's, and this particular gas is to replace R134a an HFC (as distinct to R12 which was a CFC -now banned, and R22 an HCFC in final stages of the banning process presently).




A key advantage of CFCs, HCFCs and most HFCs was / is their non or low flammability. HFOs are increasingly flammable, and this presents a major problem for a machine with an embedded electrical motor, especially when the motor burns out (as is remarkably common !).

I am not fully conversant with the law as far as use of R134a in vehicles is concerned, but as a proportion of total Refrigerant use by volume, car use is a small %. However, any vessel already designed or built would not come under the rules for non use of R134a. Any system operating on any other refrigerant cannot be changed to a fluid that does not exist (and it is possible such ideal fluids may never exist !)

I know all this is a bit boring, but this is my day job as my business is directly affected by these changes to refrigerant rules, indeed long may it continue.

If you are still using R12 or R502 then you are beyond help !!!
(not breaking the law to be using such equipment and gas, but if it breaks or leaks forget it).

For cars, 1234 yf should have been in for new types from Jan this year, however DuPont in China was not ready to supply, so a derogation was implemented to Jan 2013. Supply is now OK but the Germans, led by Daimler Benz are saying that the flammability issue means that vehicles will not be safe with the new gas. No one has found a flammability problem, and in reality, compare with gasoline, this is a no brainer. It looks like 134a is interchangeable with the new gas, so some manufacturers are changing the gas in new cars, and getting Type Approval extensions where possible.

Sorry to be anorakky with this, but interested to hear that the gas is the same as domestic, despite a boat having a mobile a/c.

 

[Nick_H]

.. the (SMX II Control Systems (DX)) control units do not function despite a cruisiar specialist staring at them and giving them a first written warning. All they will do is blast full speed cold or hot air, and what a noise that blast creates!

I've been trying to get hold of a new blower unit for the same system. People keep telling me they can get them from America, but when I ask them to, everything goes quiet.

Anyway, during my enquiries, I was told there are lube points for the blower bearings, but I haven't been back to the boat since to check that out. If so, that may help to quieten the fans, although I guess most of the noise is just turbulent air. I don't find the compressors too bad (noise wise), maybe it's just where they're positioned.

 

[kcrane]

Not all that well, in my experiene. Having said that, let me quantify that I have only been exposed to reverse air on our Fleming 55, which has two Cruisair units which are checked every other year for pressure etc.

It could, of course, just be installation issues. For example, the (SMX II Control Systems (DX)) control units do not function despite a cruisiar specialist staring at them and giving them a first written warning. All they will do is blast full speed cold or hot air, and what a noise that blast creates!

Apart form the main saloon, there are individual sub-units for the three cabins and pilot house which have the same probelm plus the added nuisance of being at the end of the runs and therefore most inefficient.

Having said all this, we have hardly had cause to use them so I haven't really tried to find someone who might be able to fix them. Instead we use fan heaters and are now toying at fitting a Webasto central heating system.

Cry for help....Does anyone know a really tip top Cruisair engineer?

I have good news for you.

Seacraft Marine (Mark Porter and Mark Spink) are based in Poole.
www.seacraftmarine.co.uk - PM me if you want a mobile number.

I asked them to fix my aircon last Tuesday and it was done by Thursday lunchtime. I thought the cost was reasonable.

Mark noticed that our aircon also blasted air out (noisily) almost regardless of the fan setting on the control and he adjusted it, so that now we can alter the fan from reasonably quiet to a strong blast.