Evaporator heating up

[PeteD]

I think I am having trouble optimizing the charge in this boat freezer system. Perhaps the problem is something else that I have not considered. Take a look below at the temperature rise I am seeing at the temperature sensor at the bottom of the second expansion plate. (lots of context and details provided below). Could anyone clue me in on what is going on here? (over-charged, under-charged, something else, etc). Thank you very much for helping me with this problem.

You can see the temperature at the expansion plate-mounted thermostat sensor (see illustration below). As you can see there is a very large and long rise in temperature at the sensing point on the second of the two in-series expansion plates, shown in the red-shaded box on the graph. There is an identical system for the refrigerator that uses only one expansion plate but does not show any significant rise in plate temperature once the compressor turns on. (There is a plot of part of the refrigerator cycle below also and you can see that after only a very slight temperature rise, the expansion plate temperature drops rapidly - since the compressor only runs a 13% duty cycle, I did not plot the long slow curve of the temperature rising back to the compressor cut-in point). If I could eliminate this rise in temperature in the freezer expansion plate, the freezer system would be much more efficient (like the refrigerator is) and the duty cycle would be much lower, saving all sorts of power, which on a boat is a commodity second only to drinking water). Unfortunately, I do not have on board a digital thermometer for directly measuring the suction line to determine the effective superheat setting of the expansion valve but I suspect that would vary wildly over the cycle since there is no compensating bulb on the expansion valve. Both of the graphs below start at "0 minutes" when the compressor cuts in. The red-shaded box shows the portion of the curve that really should not exist. I could not be any more happy with the refrigerator. Short of a total system failure, I could not be any less happy with the freezer.

http://www.dublerfamily.com/freezer.jpg

http://www.dublerfamily.com/refrigerator.jpg

Here are some details on the system. It is a small system with a Danfoss BD80 12 volt compressor. The expansion valve does not have an external compensation bulb (it is just adjustable to a pressure which is currently about 0.25 psi). (That's just how this system was designed). It has no accumulator. The refrigerant is R134a. The condenser is both forced air (aka fan) and sea water cooled. The water input temperature is about 85 deg F. There are two expansion plates in series. The thermostat temperature sensor is mounted on the side of the second expansion plate near the bottom of the plate. The first plate frosts from the bottom up. The second plate frosts from the top down. There is no frost on the lines exiting the ice box. The high-side pressure reads about 125 psi.

Again, many thanks for any help you can provide!
Pete

[Magoo]

Hi PeteD.
a sketch of pipe schematics is worth a thousand words , can you post a sketch plz.

[PeteD]

Magoo, I'm glad to provide any additional information that I might. The schematic is so simple... it is just like the textbook refrigeration cycle. The expansion valve does not have an external compensating bulb and there is no accumulator. Here also is a picture of the business end of the system and an illustration of the plate arrangement.

[PeteD]

Perhaps I should move this thread to "Troubleshooting". Can anyone tell me how to move the thread there?

Thanks!

[Brian_UK]

Perhaps I should move this thread to "Troubleshooting". Can anyone tell me how to move the thread there?

Thanks!Thread moved.

Just a thought, are those eutetic plates or simple evaporator plates?

[PeteD]

Great question Brian. (Sorry, I just also copied the post over to Trouble Shooting. You can certainly delete one if you like). I had a eutectic system in the past (aka "holding plates") so I am familiar with them. These plates are supposed to just be evaporators. They have a solid stainless shell on them to make them more rugged that open coils/fins but as far as I am aware, there is no eutectic fluid in the plates (there is no port for filling or draining and they are comparatively lighter than they would be if they had fluid in them).

I am suspecting that there is just not enough refrigerant in the system but the pressures seem correct and go up if I add any more refrigerant. I do not want to keep adding refrigerant without being sure that is the likely problem.

Thanks!
Pete

[nike123]

Sketch of actual evaporator setup is not clear to me!
I think that you need to connect evaporators in such way (if they are not) that both refrigerant entry is at bottom of plate and both exits are at top of plates. That mean that exit from first plate is at top and connected to entry at bottom of second plate.
Than you will be able to charge correctly, since superheated refrigerant will exist only at exit of second plate!

[PeteD]

Sketch of actual evaporator setup is not clear to me!
I think that you need to connect evaporators in such way (if they are not) that both refrigerant entry is at bottom of plate and both exits are at top of plates. That mean that exit from first plate is at top and connected to entry at bottom of second plate.
Than you will be able to charge correctly, since superheated refrigerant will exist only at exit of second plate!

Thanks for the input. Let me first say that the system did work well earlier and the plate configuration has not changed. The points of entry/exit of the tubing from the plates do not directly correspond to the routing of the tube inside the plates. On the first plate, the flow is from the bottom to the top (although both tubes exit at the top and the expansion valve is at the top). On the second plate, the flow is from the top to the bottom. The tube between the plates runs across the top of one of the sides of the box that does not have a plate on it. All that said, there is no easy way for me to reconfigure the plates. The manufacturer built the plates and tubing custom for my box with gas-tight quick connects that are arranged in a one-way-to-install configuration.

[RANGER1]

PeteD,
Can you give us more information on TX valve & how it is set.

[PeteD]

PeteD,
Can you give us more information on TX valve & how it is set.

Sure, thanks for trying to help out here.

The expansion valve has no external temperature compensation bulb. It early just functions like a pressure regulator. The is a knob that has a removable threaded plastic cap. I measure the low side pressure back at the port at the compressor. A turn of about 1/6 of a rotation will change the set pressure by about 1/2 to one psi. I have it set to slightly over zero psi so the pressure is still a bit positive and if there were to be a leak, air would not be sucked into the system. At this pressure, the temp should be just over negative 15 deg F. So if the gas is at that temperature, how does it get the plate so hot at the other end of the plates? That would be a huge amount of superheat right? How would the amount of refrigerant in the system play into this?

Does this help any? Incidently, the refrigerator system has the valve set at about 3psi, negative 7.5 deg F.

[Brian_UK]

Try setting the regulator to give you 4psi, just wondering if the system Is getting starved by the valve.

[Magoo]

The auto expansion valve is not feeding enough refrigerant for load, a thermostatic would regulate better.

[nike123]

I think, as Magoo said, AXV is not well suited for this application. It is better suited for flake icemakers and similar applications with low pressure drop at evaporator and steady load.
Since you have 2 evaporators in series, with (possibly) fairly big pressure drop, you should use TXV with external equalization port.
And, if both plates are identical, I will use distributor and connection in parallel.

[PeteD]

I have tried pressures as high as 3psi and see similar warming but I can't then keep the box as cold as I need to. 4psi would be about neg 5 deg F. Not sure if that would be cold enough but I can try. WOULD a short charge cause the same sort of starving and would that starving cause the heating we are seeing?

[PeteD]

I definitely agree with you and Magoo, a TVX with external sensor would be better as would parallel plates but this is how the system was designed and it has worked fairly well in the past. I would, at least for now, since I am in Curacao on the boat sitting out hurricane season and don't have the ability to redesign/rebuild the system here, like to get what I have working better. SO, if the AXV is starving the end of the expansion plate chain, could that simply indicate a short charge? (By the way, I have taken the adjuster out of the valve and warmed the valve to chase out any moisture that might be in there above the diaphragm, but I don't think there was any because if there were I don't think it would 1) be easily adjustable and 2) maintain the pressure as well as it does).

[PeteD]

AGAIN, THANKS for the dialog. I hope we can come up with a theory on why this system is misbehaving and I can get it running better for now. I think the duty cycle should be closer to 25-30% instead of the 57-59% I am seeing.

[Magoo]

I would really like to have your problem in Curacao, what an idiotic place to be. But not too flash with hurricanes or similar
Best advise would be to crank open the auto TX and monitor from there. In creasing refrigerant flow will saturate coils and the achieve desired plate temps, but that will mean that most of system charge is in the plates. Best to have a solenoid vav before auto TX so as system does not equalize in off cycle and create flood backs on start up.

[seanf]

What pressures do you get on the off cycle, and what temps do you get in the box?

[Brian_UK]

As the consenus seems to be poor refrigerant flow then perhaps there has been a leak and it is short of gas (SOG).

Remember of course the pressures/temperatures will not be ldeal until the load has been pulled down and the box nearly at design temperature.

[PeteD]

Box is measuring 15.7 to 17deg F. The heating of the plate is causing this variance. On the refrigerator we get no variance in the box temperature, a veteran steady 35 deg F.

I have not measured the pressures without the cc compressor running. I imagine they drift towards equalizing. Next time I put gauges on the system I will record that through a full on/off/on cycle

[PeteD]

"...not be ldeal until the load has been pulled down and the box nearly at design temperature."

Right...so the high side pressure may be higher than ideal early in the cycle and then settle down to the ideal?

[Brian_UK]

"...not be ldeal until the load has been pulled down and the box nearly at design temperature."

Right...so the high side pressure may be higher than ideal early in the cycle and then settle down to the ideal?Yes, that is correct.

[Gary]

The condenser is both forced air (aka fan) and sea water cooled.

At the same time? How does that work?

[Gary]

"...not be ldeal until the load has been pulled down and the box nearly at design temperature."

Right...so the high side pressure may be higher than ideal early in the cycle and then settle down to the ideal?

What is the ideal high side pressure?

[RANGER1]

PeteD,
What if you change position of thermostat to higher position on the plate.
Assuming this system is critically charged & second plate is acting as a sort of slop pot on start ups etc.
If you changed position it should run longer if there is enough refrigerant in it or alternatively lower set point.

Expansion valve would try to hold set pressure also if enough refrigerant in system.
Expansion valve may be noisy as well if low on refrigerant.

[PeteD]

At the same time? How does that work?

The condenser is a tube-in-tube coil where the refrigerant runs through the inner tube and the sea water runs though the outer tube. This coil then has fins and air runs over the fins (electric fan). The advantage of this dual system is that when at sea, water cooling is more efficient than air cooling, and when in dirty water or when the boat us out of the water, just air cooling can be set and the refrigeration still can be run.

[PeteD]

The refrigerant should be 15degF warmer than the cooling water. (The manufacturer tells me). So with the incoming cooling water at 85degF, the refrigerant should be at 109 deg F. With R134a that's 125 psi.

[Gary]

Did the manufacturer tell you what the AXV setting should be?

[nike123]

Maybe this article can help:
http://www.achrnews.com/articles/89450-the-dream-of-a-constant-pressure-refrigeration-system

[PeteD]

Did the manufacturer tell you what the AXV setting should be?

First they said 3-4psi. Last they said zero psi. I am not convinced they know... Sigh...

[Gary]

I would go with about 3 psi.

During the run cycle, does the low side pressure hold steady or does it fluctuate?

[PeteD]

I would go with about 3 psi.

During the run cycle, does the low side pressure hold steady or does it fluctuate?

I set it at about 2.5 psi today and watched it through the whole cycle. The pressure is of course higher after the system has equalized a bit during the compressor off portion of the cycle. It quickly goes to the set pressure after the compressor kicks in and only varies slightly if at all until the compressor turns off. The high side is around 123 psi with a little bit of running a few psi higher early in the cycle. This pressure corresponds to the 85 deg F cooling water plus 15 deg F = 100 deg F. I also added just a little refrigerant and am seeing some improvement in that the temp increase at the plate after the compressor kicks in is now much less. I am now experimenting with various set points and hysteresis settings. I have a duty cycle meter that I made with an Arduino board monitoring the system and it was that meter that gave me the first clue that there was a problem. Now I am tuning to optimize ( minimize) duty cycle while maintaining the right box temperature. I'll report the results in a few days.

[Gary]

You might want to monitor the condenser outlet subcooling also. Especially if you are adding refrigerant.

[PeteD]

Here's an update... Much improved... Thanks again for all the thinking on this and all the advice!

Added refrigerant to get end of cycle pressure up to 135psi which has the beginning of cycle pressure up to about 155 psi. The suction side is set to 3 psi. Set point at 2.5 deg F with a 3.0 deg F hysteresis. Cycle time is way down and duty cycle is now running 48%. Yet to see if the overall box temperature will drop to where I want it - still a little too warm near the top. Will try re-activating the circulating fan in a day or two if it does not improve but last I had that on I found it increased thermal loss from the cabinet. See graph below... much more like the theoretical curve I hypothesized. Just hoping there is not a slow leak going on...
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