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lana
16-01-2006, 02:33 PM
Hi there,
Please have a look at the drawing. Everything is explained there. Any Ideas?
Cheers:)

US Iceman
16-01-2006, 04:01 PM
When the suction line temperature is below 0C you can have frost. This has nothing to do with superheat or liquid returning from the compressor.

Stop and think about this. If the evaporator temperature is -42C, the suction line will be colder than 0C (where frost starts to form). Even if you have 30C of superheat, the line can still be frosted since the suction line is still below 0C.

At low temperatures frost does not indicate liquid flood-back or any particular problem. Frost simply shows up when the surface temperature of the pipe is below 0C.

lana
16-01-2006, 04:13 PM
When the suction line temperature is below 0C you can have frost. This has nothing to do with superheat or liquid returning from the compressor.


Hi US Iceman,
If there is a cold vapour flowing, I think there will be only droplets of condensed moisture on the pipes not frost. I have seen many -40°C systems where there is no frost on the accumulator and the suction line (parts without insulation). Again I think that, only liquid has high latent heat therefore has the capacity to turn the water into frost.
Correct me if I am wrong.
Thanks.
:)

lana
16-01-2006, 04:33 PM
No, not true at all. And in hot climates where you're typically condensing above 50°C, adjusting superheat to clear the suction line frost can quickly lead to a compressor replacement (overheating). Talking -40°C systems here.

Hi Marc,
The question I have is why accumulators always frost where there is liquid present?
I have built an ice machine with accumulator (te=-15°C). At the end of the cycle there is liquid flooding into the accumulator. At this point frost can be seen at the bottom of the accumulator. But during whole cycle it is completely free of ice.
Appreciate your responce.
Cheers:)

lana
16-01-2006, 04:42 PM
I just downloaded you pdf and now understand your question...

Ambient dew point variations?

Suction line retriction?

You mean there might be a restriction in the suction line which causes this problem (frost)?
Cheers:)

US Iceman
16-01-2006, 06:22 PM
I have seen many -40°C systems where there is no frost on the accumulator and the suction line (parts without insulation).

Frost can form at 0C. If warm air is circulating past (or a warm ambient with no air circulating past) the component, the frost will melt at different rates. If the suction line temperature is below 0C and liquid is present, the frost forms during this condition since the liquid refrigerant is boiling and removing more heat than that being exchanged to the air.

I think your next comment answers the question.


I have built an ice machine with accumulator (te=-15°C). At the end of the cycle there is liquid flooding into the accumulator. At this point frost can be seen at the bottom of the accumulator. But during whole cycle it is completely free of ice.

During normal operation the accumulator has higher superheat and since the accumulator temperature is probably close to 0C, the frost melts due to ambient conditions.

However, when the end of cycle is approached some liquid is flowing back into the accumulator. When the evaporating temperature is -15C and liquid is leaving the evaporator, the liquid is boiling and keeps the accumulator cold enough to freeze the frost.

In your drawing, you say the coil has a lot of superheat. Must be if the outlet header is not frosted. However, the trap at the outlet can hold liquid. If the pressure drop through the suction riser is too high, any liquid in the trap will flash and the returning two-phase mixture could produce the localized frosting on the lines.

There should also be an inverted trap on the top of the suction riser. The drawing does not show one.

It seems like you have more than one problem here.

lana
17-01-2006, 04:32 AM
Hi US Iceman,
You are right. There are more than one problem here and there is no inverted trap. OK thanks for the frosting theory but what about the restriction and the high pressure drop? If there is high pressure drop then the compressore will run at lower te and gives less capacity which is the case here. Will this make the compressor wear and tear and overheating worse?
Cheers.:)

US Iceman
17-01-2006, 05:32 AM
If there is high pressure drop then the compressor will run at lower te and gives less capacity which is the case here.

That is correct. If you know the design evaporating temperature, determine the evaporating pressure. Now, what is the suction pressure? The difference is the dP of the suction line. Have you calculated the dP for the installed piping lengths, and does this agree with the measured value?

I seem to remember from your drawing that you had a suction line filter. How many cores does it contain and what are the pipe connection sizes? Depending on the number of cores and connection sizes, the suction filter can greatly contribute to an excessive pressure drop.

I have attached a file with excerpts from the Sporlan bulletin for your review on this matter.

As you are probably aware of, any increase in suction pressure can significantly increase the refrigerating capacity of the system. Therefore, it is important that you verify the cause of the high losses.

The suction riser may be too small for the required load also.


Will this make the compressor wear and tear and overheating worse?

Absolutely... The suction superheat at the compressor has a direct impact on the capacity of the compressor, the discharge temperature of the gas, and can contribute to abnormally higher oil temperatures. With the addition of the high suction line pressure losses, the increase in pressure ratio also reduces the volumetric efficiency and further increases the discharge temperature. In essence, everything is going the wrong direction!

All of which can significantly reduce the life expectancy of the compressors due to a decrease in oil viscosity, potential copper plating and acid generation.

If the suction superheat at the compressor can be adjusted to a reasonable value, you should see noticeable results.

One additional question about the evaporator... Are the coil circuits properly designed and is a refrigerant distributor being used? The high evaporator superheat and occasional flooding sounds like an imbalance in the liquid feed to the evaporator coil???

lana
17-01-2006, 11:06 AM
Thanks US Iceman,
Very good explanation as always.

With the installed pipe size, I calculated the suction pressure drop as 3.5°C drop in saturation temp.?!!
I always size the suction line for 1°C drop in the saturation temp. The design te is -40°C but according to the suction pressure I have -44°C therefore there is a large pressure drop. There is no doubt there is an unacceptable pressure drop here.
About the evaporator I have to say that it has a distributor and the size is also correct (I checked it) and I had the same evaporator installed in another double-stage system with perfect results. I don't see any problem in the evaporator section.
Anyway, I have given my suggestions to the owner about the piping lets see what happens.
Thanks again.
:)

afeef
18-01-2006, 12:52 PM
hi us iceman
from your post i understend that : in -42 evaporator temprature we not should be worried if we see ice in sution line,,,,is that true?

afeef
18-01-2006, 01:11 PM
to us iceman
from your explanation ,,,i understand that if there ice on suction line for -42 evaporator is it normal condition ,,is that right?

afeef
19-01-2006, 08:39 AM
hi us iceman
according to your replay , i understand that it is not harmfull if we see ice arround the suction pipe in -42 evapotator temp.
is this right?

US Iceman
20-01-2006, 03:02 AM
afeef,

Sorry for not replying sooner. I have been out of the office for several days and may have missed your question.

To answer your question... Yes.

One thing we have to remember is that ice formation by itself does not indicate liquid flooding back to the compressor.

If the suction superheat is present at the compressor suction valve then the suction line is dry. Even if the suction line temperature is below 0C.

Frost is not a true indication of liquid flooding. It only means the suction line temperature is below 0C.

If the suction superheat is zero and you have frost on the suction line you could experience liquid flooding.

The superheat determines if liquid flooding is occurring, not the fact that frost/ice is present.

Another way to check for liquid flooding on low temperature systems is to check the discharge superheat at the compressor discharge service valve.

If the compressor has some small quantity of liquid entering the compressor, the discharge temperature will be much lower than normal.

The discharge superheat is determined by: Discharge temperature minus the saturation temperature of the gas at discharge pressure.

For a quick check, the actual discharge temperature can be used.

Renato RR
20-01-2006, 08:27 AM
If we have 50 C condesation temperature how smal can be discharge superheat so we can conclude that liquid enters the compressor.

Renato

US Iceman
20-01-2006, 09:24 PM
Renato RR,

If you are condensing the refrigerant at 50C, the actual discharge gas temperature (at the compressor discharge valve) should be greater than 50C.

The way to compare this is using the data from a properly functioning system. If the suction superheat is normal, measure the discharge temperature. The discharge temperature could be at 76C or higher. The difference of 76C-50C is the discharge superheat.

If the discharge temperature is much lower, or you can see it decreasing rapidly, the suction superheat is too low or the compressor may be experiencing a little liquid flood back.

This is very important especially for screw compressors. If the discharge temperature is not high enough, refrigerant can be in solution with the oil. This reduces the oil viscosity.

Dan
21-01-2006, 03:02 AM
The evaporator super-heat is very
high therefore 4 rows of the coil and also the evaporator outlet header are free of any frost. Also
the vertical part after the oil trap is free of frost. But after this point (the orange lines indicated
above) including the suction filter are frosted to the compressor inlet ??!!! Any idea?

Hi, Lana. We really need to discuss the pipe temperatures instead of whether they collect frost or not. Are the early piping runs in the evaporator actually -40 deg? What is the temperature of the pipe where the TEV bulb is clamped?

Does the riser not frost because it is in the conditioned space and then the horizontal run to the suction filter exist in a non-conditioned space?

Just tell us the temperatures of the piping and it will become evident what you have. :)

lana
21-01-2006, 04:27 AM
Hi, Lana. We really need to discuss the pipe temperatures instead of whether they collect frost or not. Are the early piping runs in the evaporator actually -40 deg? What is the temperature of the pipe where the TEV bulb is clamped?

Does the riser not frost because it is in the conditioned space and then the horizontal run to the suction filter exist in a non-conditioned space?

Just tell us the temperatures of the piping and it will become evident what you have. :)

Hi Dan,
The pipe temperature was measured in the horizontal section and it shows -23°C which in turn gives 17°C superheat. The temperature of the bulb can not be measured because it goes behinde the evaporator and there is no access. And yes the horizontal section goes to the machine room. I think what we have here is high suction pressure drop and not adjusted TEV.
Cheers:)

afeef
21-01-2006, 07:40 AM
thanks us iceman
very nice explanation ,,,on the counter way, if the suction pipe have no frost on it (ubove 0 evapotator temp.) this is not means that flooding is not exist,,,
am i righr?

US Iceman
21-01-2006, 11:20 PM
afeef,

If you are above 0C for the saturation temperature of the refrigerant, the line will not frost. You can still have flooding back to the compressor however, without frost being present on the suction line.

This is why it is important to check the evaporator superheat and the compressor suction superheat. The evaporator superheat shows how the TXV is adjusted and operating.

The compressor suction superheat shows how much additional heat is picked up in the suction line.

Does this help you?

afeef
22-01-2006, 07:04 AM
thanks a lot us iceman
its clear now
afeef:)