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Old 07-31-14, 07:29 AM   #51
AC_Hacker
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Quote:
Originally Posted by stef110 View Post
Now is the question i need to calculate super heat to know is there is enough refrigerant or there is to less. When i use the readings on the picture. Witch values do i need to take the pressure of the low side? and the temperature of that? or do need to take this measurement on cooling function?
I think I will leave the Super Heat issues to someone with more experience like jeff5may, or other who have more experience.

I don't have a very good idea of exactly how your system is structured, but I assume that you are using the system for heating and you are using your minisplit to gather heat from the air outside (evaporator HX) and sending the heat to a refrigerant-to-water HX (condenser HX), where circulating water will pick up the heat and deliver it inside the house. Am I correct?

If my understanding is correct, your condenser is your refrigerant-to-water brazed plate HX.
Temp Cond In: 12.69
Temp Cond Out: 25.00
I am guessing that "Temp Cond In" is measuring the water temperature that is entering your brazed plate HX, and that "Temp Cond Out" is measuring the temperature of the water that is leaving your brazed plate HX. If this is the case everything looks just great.

Temp Gas Line: 68.94
Temp Liquid Line: 32.56
I would expect to see the liquid line temp to be higher, which would reflect the work of the compressor. This seems counter-intuitive.

Temp Outside (ambient) looks reasonable
Temp Outside (ambient): 26.37

I assume that these values are from sensors that are attached to your mini-split, and that:
Temp Evap In: 32.06
...is measuring the temperature of the hot refrigerant liquid just before it passes through your metering device.

And that:
Temp Evap Out: 36.44
...is measuring the gas line temperature where the refrigerant gas is leaving your outside condenser coil... correct?

You didn't specify, but was this photograph of your pressure readings taken when the compressor is not running??
Pressure Blue (Lo Side?): 375
Pressure Red (Hi Side?): 350
Is my understanding correct??

I looked through your NL site and I did not see what kind of refrigerant you are using??

-AC

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Last edited by AC_Hacker; 07-31-14 at 07:33 AM..
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Old 07-31-14, 08:57 AM   #52
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Default Wait a moment with defrosting! :eek:

Do you have any metering device (capillary tube, txv, whatever) towards the BPHX for reverse operation during defrosting? If not, in defrost operation you you will flood your evaporator (BPHX) and liquid will run into your compressor!

Another weak point of your setup , The Split AC in first place is designed for cooling. (as I recognise from your previous pictures)The small diameter pipe takes now the hot gas from compressor to BPHX (condenser). And the bigger diameter pipe takes the liquid back to evaporator. Because the vapor has 1000x the volume of liquid, there is big pressure drop. Causing your compressor has to work harder. I would swap the lines for better efficiency.

About superheat, subcool. Take a look on your pressure gauge. There is a pressure and (more) temperature scale(s). What we see on the picture, your R410 has the pressure roughly 350psi and on the temperature (for R410) scale 45°C. Your superheat is the difference between temperature derived from pressure, and the actual measured temperature.

But I see some problem on your last attached picture. The blue gauge does not shows the evaporator pressure! It shows the pressure front of your metering device. From that we can't get any more info, just your subcool is arround 12°C (45-33). It is simply on the wrong side of capillary tube or txv!

Please make a sketch about your system, and mark where you measure what. That would help us to understand what's going there.

T.
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Old 07-31-14, 09:01 AM   #53
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thanks ac, my software is a little bit confusing the names are based on cooling so the normal state a conditioner will work.

So the temperature readings are when we look at HEATING mode:

Condenser (hx inside) water temperature in going = 32.06 degrees
Condenser (hx inside) water temperature out going = 36.44 degrees

evaporator (radiator outside) in going refrigerant (right after expantion valve) = 12.69 degrees
evaporator (radiator outside) out going refrigerant = 25.00 degrees

the outside air temperature is indeed 26.37 degrees

The gas temperature 68.94 degrees measured at the point it is leaving the outdoor unit.
The liquid temperature is 32.56 degrees this is measured at point in enters the outdoor unit just before it goes into the expansion valve.

used Refrigerant is R410A

The pressures on the gauges are a little bit confusing the low side gauge has a little offset. The red one reflects the right pressure but on my case high side and low side are always the same. This should also not be right?

just did an other test, let the unit run for 10 minutes and looked the ingoing temperature was 31 degrees (on the liquid line) and the temperature on the gauge (according to the pressure) was 40 degrees. witch give me a super heat of 9 degrees?

here is a little schematic but with out the 4-way valve once i made a drawing but i trow it a way if it is really necessary i will look how everything is hooked up!


http://gallery.svitterzon.nl/cache/a...%20waardes.png

Did a quick look at the unit and draw on paper but it is identical to this picture:
excepts that my HX is a bit further away.



http://gallery.svitterzon.nl/cache/a...2017.56.32.jpg

My expansion valve is not a real expansion valve. I don't have it on picture and you cant even see it because it is well packed with tape. But from what I feel and see it is a thin tube and on begin and end an small "container" I draw it on the paper.




stef

Last edited by stef110; 07-31-14 at 11:01 AM.. Reason: adding schematic
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Old 07-31-14, 02:19 PM   #54
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You should connect your blue gauge to the compressor suction line (where you indicated 25° on your drawing). For superheat calculation you need the pressure from there. The pressure at the current measurement point (where the yellow hose connected) is irrelevant.

Your metering device is capillary tube. Most probably it's built together with an antiparalel check valve wich let the refrigerant flow free in reverse direction. You need similar thing front of BPHX for the planned defrost operation.

T.
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Old 07-31-14, 02:26 PM   #55
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I cant get my blue meter over there is there an other way to check?
I amuse harmless to use it this way on heating mode? because all the split heating units work this way? Some one on this topic told once that if i want to know the suction pressure i run cooling mode instead of heating so gas line will be liquid line is this correct?

stef
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Old 07-31-14, 02:50 PM   #56
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As long as you don't reverse the cycle it looks ok.
Because of the small difference between outdoor temp and evaporated refrigerant temp I have the assumption you have nice big superheat. It's not efficient but safe.

If you reverse your valve, you can easely damage your compressor by flooding. If you want to reverse the cycle, you need to install capillary tube+check walve for BPHX too.
Then you can read suction line pressure from the point your red gauge is connected currently. But that suction pressure would not be the same like in HP mode.

By the way. I have the impression, you have too much refigerant in your system. The Red gauge and the temperature prior capillary tube shows you have 10° subcool after condenser. It's causing higher than necessary condenser pressure and less efficient heat transfer in condenser. By reducing the charge, you can reduce subcool. But before anithing, you should be able to measure suction line pressure in reality.

I don't know what is your purpose with this system. You want to tune it to have the most efficient setup, Or you are satisfied if it is a bit more economical than electric heating.
If the first is the case, I recommend you to take a deep breath and break into your outdoor unit. First I recommend you to throw away capillary tube (not so far, you can use it as defrost metering device), install TXV or (preferred) EEV. Then add liquid tank and sight glass after condenser. Don't forget to add some service ports to the lines. With these modifications you would have a system where you have control over subcool and superheat almost independently.

T.

Last edited by takyka; 07-31-14 at 03:07 PM..
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Old 07-31-14, 03:36 PM   #57
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I really appreciate all the help, It is my first project. What i have in mind is to heat our home in the Forrest and spring with this unit if possible. It doesn't have to be the most efficient unit around at this moment but i need to run it safe! not that after one day of running the compressor is dead then this hole project will be very disappointing. If i can let it run for the up coming winter, to test what is does I am happy.
I really liked the build of this one and i already want to build an other one. With the experience from this one.
At the moment i don't have equipment to discharge the hole unit so i can do modifications on the inside. So is it possible to make it work with this setup safely?
For the defrost i can shut the unit of wait a few minutes switch the valve and then turn it back on? would this be a safe approach?

stef
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Old 07-31-14, 04:25 PM   #58
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"For the defrost i can shut the unit of wait a few minutes switch the valve and then turn it back on? would this be a safe approach?"

No! The problem is not with timing. In reverse flow there is no restriction in the loop as the check valve paralel to the capillary tube, opens. You let to run so much liquid to the BPHX (in this mode evaporator) on the high pressure it can't evaporate all the juice. Liquid reach the compressor and as liquid is not compressible it destroys the the internals.

But the solution is simple. You can take the other capillary from the indoor unit and install it on the bottom pipe of your BPHX. Watch for orientation!

T.
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Old 07-31-14, 07:56 PM   #59
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OMG loook here... a capillary tube. What a problem! OH NO!!!!!

Not.

These mini-split units don't need a metering device outside the box. You aren't going to kill your unit running in reverse. Let me explain...

These units are made to withstand lots of torture from the outside world. All the working parts are inside the box. The indoor heat exchanger can be whatever you like. As long as there's not much pressure drop (from your readings I can tell there's not) and the indoor unit will actually exchange some heat (it had better), it will work just fine. The capillary tube doesn't care at all, it will work in both directions and do its job just fine.

If you look at your handwritten diagram, The expansion device is a cap tube, a meter or two length section of 1-1.5 mm ID tubing. It acts just as a resistor does with electricity. The more luxurious models have a check valve that shunts a small portion of this resistance in cooling mode. With a shorter cap tube, there is less resistance, so you have more mass flow (current) at a higher evap temperature. In heating mode, the extra resistance gives you a lower ultimate evaporator temperature (for better heat gathering in winter) at the cost of less heat transfer. This is irrelevant at low temperatures, since flow is naturally pinched off by low suction pressure at the compressor intake.

At freezing temp of water, the r410 evaporates around 100 psig and gains nothing more in the evaporator due to low ambient temp. At 25 degC ambient, evaporator pressure is roughly double that at 200+ psig. Being a constant displacement device, the compressor can only pump what it is sourced by the evaporator pressure. The cap tube is a relatively linear device, and it doesn't care whether it has gas or liquid flowing through it. If your system is undercharged, the condenser will run out of liquid at some point and feed the cap tube gaseous refrigerant. The cap tube will pass much less overall mass through it, and evaporator pressure will drop, limiting flow through the compressor. The main idea is that cap tube systems follow evaporator discharge temperature.

As for charging the system, these units are critically charged. A very few test conditions are chosen to be godly and everything in the unit is designed around these few conditions. Weeks and months of trials are run, and an optimum refrigerant charge is determined. This value is printed boldly on the ID plate of the unit. Any and all reputable service techs and engineers will tell you that all mini-split units can only be charged by weight of refrigerant, period. I will tell you that unless you really modify your outdoor unit, you aren't going to get much better performance than what you get by charging by weight. Your optimum charge will be within a few grams of the charge on the faceplate.

There is a way to measure superheat with your system the way it is. In cooling mode, you take a pressure reading on the big line (evaporator discharge-->reversing valve-->compressor suction) and compare this pressure to its equilibrium temp on a PT chart for r410a. If you're lucky, your gauge will have a scale built in. You then take temperature readings along the evaporator discharge line and subtract the PT chart value from these readings. Unless your tubing is super-insulated, the cold refrigerant will draw heat through the tube walls as it travels towards the compressor. This temperature gain (sensible) above saturated suction temperature (from PT chart) is the superheat.
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Old 07-31-14, 09:13 PM   #60
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Looking at your diagrams and readings, I can tell your system is doing well. Since it is in heating mode, we can only get pressure readings from the high pressure side of the system. The valves you have installed are on the same side of the system... the 3/8 line is compressor discharge (gas) line and the 1/4 line is the liquid line. The 3/8 line is at 375 psig, which corresponds to 60 degC saturated condenser temp, at 68.9 degC you have 8.9 degrees of superheat (not the superheat that matters...). The 1/4 line is at 350 psig, which translates to 55 degC saturation temp, and at 32.6 degC the liquid refrigerant has subcooled 22.4 degC below its condensing temperature!

What this means your indoor heat exchanger is doing quite an awesome job at extracting heat. The approach temperature is only .5 degrees, so you could definitely slow your flow on the heating water and not lose hardly any heat transfer. This slower flow would yield you up to 15 degrees hotter water if you need or want it.

If you want to check superheat on the indoor side of your system, you will need to run the unit in cooling mode. In this case, the 1/4 line will have saturated liquid/vapor mixture running through it and the 3/8 line will have superheated vapor flowing through it. You would take your superheat reading from the 3/8 line, but from the effectiveness your heat exchanger is showing during heating mode, you will most likely have another close hx approach and gobs of superheat present. The bottleneck in the system is definitely not your indoor hx.

To measure superheat of the outdoor side of your system, you would need to install a service valve in the line between the outdoor hx and the reversing valve. If your unit was charged by weight, it will do its job just fine. If you haven't messed with the control board inside the outdoor unit, it might even have its own defrost control built in. It would be foolish for the manufacturer to include a reversing valve if the unit could not defrost itself if it needed to.

Also, you can measure the heat transfer of your unit the way it is. If you know the flow rate of the water through your hx, you can calculate instantaneous heat transfer. If not, you will need to know the volume of your heat store tank and estimate the volume of the plumbing running between it and your hx. Heat transfer of the water side is all sensible, so it is easy to set up a time trial of your loop as long as you don't use any of the heat generated during the trial.

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