Using portable A/C for DHW Heater

[nexsuperne]

With all the electronics removed, remember that there will be no restart delay onthe compressor. Make sure you allow 2 minutes between restarts to avoid compressor lock up. Eventually, max temp will be limited by the compressor motor thermal protection shutting it down.

[xotet]

Originally Posted by AC_Hacker

Looks like your unit is working!

Is it possible for you to get R410a in your area?

That might be a simple path to success, allowing you to keep the cap tube, etc.

-AC_Hacker

Yes, I have acces to R410A, But I´m evaluating to use R407C since it gets hotter at lower pressures, this gone be interesting to test.

According R410 P/T table:

Cd 57ºC=36 Bar (522psi)
Ev 7ºC=10 Bar (145psi)

I hace an Schedule to do some tests:

-First remove all the electronics and see the max discharge temp.
-pull away the condenser and put in a water container.
-Measure the actual condenser lenght and make a new condenser (coaxial, like acuario´s made)
-Test with Multiplate HX
-Install TXV

Also I have a doubt regarding the COP,

COP=Energy Out/Energy In

In my case COP=2000/800=2.5

Should this the best can I expect?

[AC_Hacker]

Originally Posted by xotet

I have a doubt regarding the COP,

COP=Energy Out/Energy In

In my case COP=2000/800=2.5

Should this the best can I expect?

xotet,

I have not tried submerging the condenser in a tank of water. I went straight to a multiplate HX because I was trying to get really good efficiency.

My initial COP results, not counting the power loss from the pumps, was variable, but depending on the temperature of the source, I typically got about COP = 4.5 and higher.

I was using the refrigeration system for heating, so the heat from the compressor is added to the useful work, and increases the COP.

If I'm not mistaken, you intend to use your system for cooling, and the heat from the compressor will be subtracted from your useful work so the COP will be smaller.

There was another person in the Geothermal forum who was doing an experiment where heat output was the useful work. His test was similar to yours, in that he was submerging the condenser in a tank of water and his COP was around two, maybe a bit less.

Good HXs, whether they are manufactured or home made will really improve the performance.

-AC

[xotet]

Originally Posted by AC_Hacker

xotet,

If I'm not mistaken, you intend to use your system for cooling, and the heat from the compressor will be subtracted from your useful work so the COP will be smaller.

-AC

No I´m going to heat the water, the use of the compressor heat to increase the performance it´s a great idea, and also the heat radiation of the HX to the ambient may also used to increase system perfomance. This can be done positioning this components closer to the evaporator, the airflow from the evaporator´s fan can get this heat through the evaporator increasing the superheating when the ambient temp is low. The negative part comes when the ambient temp is higher and the heat from this componentes is added to the ambient temp through the evap.

Also I´m worried about evaporator capacity at higher ambient temps, theorically COP should be higher but, If the refrigerant not cools down the compressor inlet temp (and also the pressure) should high damaging the compressor. I don´t know if only adjusting Fan speed to increase as much as possible the airflow through Evaporator may be sufficient to get the system working.




Thanks Again

[xotet]

Hi Folks,

Today I finally removed electronics from the A/C unit, So I run First test to see what´s the Max temp can be reached on the condenser inlet. The results have been disappointing, just a few minutes past 1/3 of the evaporator is frost. While temp on compressor discharge increasing the frost too, after about 15 min the evaporator is fully frosted, and the Max temp=45ºC.

Ambient temp= 15ºC R/H=76%

(Tin= Condenser inlet Tout= Condenser Outlet)

Tin= 26 Tout= 23
Tin= 30 Tout= 23.8
Tin= 34 Tout= 24
Tin= 38 Tout= 24
Tin= 40 Tout= 23.8
Tin= 42 Tout= 23.8
Tin= 45 Tout= 23.8

I´m confused with this data, maybe the problem is the lack of a device can control evaporator temp (TXV) or maybe the problem is the high subcooling.

[xotet]

Ok, just run third test, just to know why I´m having evoporator frost. For this test I´m covered a half of air condenser in order to get high temperature on condenser outlet.

Started the compressor and after 3 min a little frost came on the evaporator, the condenser out temp was 25ºC, subcooling with 3ºC. In this conditions I tought the problem would be repeated, so I covered all of the condenser surface, then my theory was confirmed the condenser inlet temperature was increased until 60-64ºC the condenser outlet was about 35-40ºC, the subcooling was 20-24ºC, power consumption= 2,5A max ( compressor max rated current = 3.2A), I think the compressor discharge temp must be high but don´t want to damage the compressor, prefered to be careful.

You could think that this is basic , but this is a real demostration on how all temp/pressures on the refrigerant circuit are related, and helps a lot to understand the system operation.

Now I have a lot of questions to get answer:

1º How to control the HP when the subcooling it´s higher, for example water in tank at 10ºC , Ambient temp is low 10ºC.

I think in two ways, the first and most simple it´s control the water recirculating pump, to avoid pump starting before getting a minium temperature on HX, this will prevent low evaporation temp. The second is to install a Hot gas (Discharge line) / cold gas (suction line) HX, this will prevent low refrigerant temperatures on compressor inlet, allowing better HP startup, once the evaporation temp is higher then the solenoid will close.


Here is a basic idea:







A pic of the test

[AC_Hacker]

Originally Posted by xotet

Ok, just run third test, just to know why I´m having evoporator frost. For this test I´m covered a half of air condenser in order to get high temperature on condenser outlet.

exotet,

I think the way you are going about this is very good, changing aspects of your system and measuring the differences these changes make, in order to better understand the system's behavior.

But, I'm afraid that I don't understand the bottom part of your system? What is the part that is enclosed inside the green ellipse in the picture?

Also, frost forming is not necessarily a bad thing, It just means that your evaporator is operating significantly below the dew-point of the air. If you could reduce the temperature of your condenser so that it did not frost up any more, your difference in temperature (delta-T) between the air temp and the temperature of the surface of the evaporator would be lower, so your efficiency would suffer.

Most units that have a frost issue, have a frost detector and a de-frost cycle to allow the frost to melt, and then they begin again.

It would be interesting to see some pics of your setup and the frost forming, etc. so we can get a clearer idea of what you are doing.

-AC_Hacker

[xotet]

Originally Posted by AC_Hacker

xotet,
But, I'm afraid that I don't understand the bottom part of your system? What is the part that is enclosed inside the green ellipse in the picture?
-AC_Hacker

Hi AC, sorry for the pic, I made it superfast and isn´t the best.

I´m talking about this:



With Heat exchange between the Hi press (Hot) Compressor Discharge and (Cold) Low press Compressor suction frost will be reduced and te desired condensing temp will be reached fast. The solenoid valve wich is badly drawed will bypass this HX when the evaporating temp is higher, this also will prevent damage to the compressor due to liquid ingestion when the evaporating temp is low.

On another hand Frost it´s not god because of the evaporator efficiency will reduced, in fact when I have this conditions I can see the compressor load going down About 1.3 Amps only, this means low performance.

[AC_Hacker]

Originally Posted by xotet

On another hand Frost it´s not god...

I agree that frost is not god, however delta-T is very compelling.

I am very interested to see how your system turns out!

Keep up the good work.

-AC

[xotet]

Hi Folks, still waiting some parts, but working on the test rig. At this time I removed all the parts from the A/C unit, only used the compressor, evaporator and condenser. Put it on a Test rig. I´m going step by step, it´s slower but it´s a good way to get knowledgements.

Today I run the first test, unafortunatelly isn´t very good, started the compressor and evaporator fan, checked the temperatures and Compressor current, after few minutes the compressor was overloaded, at this time stopped the system, I tought that the problem comes from the evaporator fan because it´s different from the original.

After datailed inspection I found the problem, it´s related with the Termostat NTC probes. I´m using standard plastic NTC probes (comes with termometer and termostat), this kind of probes are not good to sense temp on pipes, measuring 10-12ºc less, this caused the condenser air not starting.

So becareful selecting types of probes, I will by some metallic NTC probes and run the test again.

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