Freezing suction line

[Reefgeorge]

I have a 2002 16k btu Cruisair remote dx unit that has the following problem. The charge levels are running well below the chart guidlines at 48 psi suction and 175-200 psi liquid. The evaporator delta t is 18-23 degrees. The filters are clean and the airflow seems reasonable. The unit cools well starting at high load and superheat is running high at 30-40 degrees but as soon as you hit a normal evap load (78 degrees and less than 50% humidity), the superheat crashes towards zero and the suction line starts freezing at the suction service valve. The low suction pressure and initial high SH suggest a low charge but thefinal cooling stage at low load act like an overcharged system or a system with low evap airflow. Where do i go from here? Thanks,

George

 

[krush]

You go to a pressure/temp chart and see that 48psi on r22 is about 25 deg F...which would mean you are freezing the condensation. The superheat is so high because you are undercharged.

When the evaporator turns into a block of ice, your superheat goes to 0 and your suction line freezes. And it acts like low airflow over the evap because your evap is a block of ice!

You need more charge. I'm not the local cruiseair expert, but I think your unit is fixed orifice, not TXV....so measuring superheat is appropriate.

This is why we tell people you can't just charge by pressure and say "Oh, just get XX psi on the low side and you are good". There is lots of stuff that has to be interpreted on them gauges--it ain't rocket science, but most people don't understand it.

PS: My simple redneck way of charging a fixed orifice unit if I don't feel like measuring super heat and using a sliderule or HP calculator...charge till I get some cold coming back to the compressor. If your compressor is sweating, you am be a little overcharged.

 

[Reefgeorge]

I take it then that you are never done charging if the saturation temp on the suction side is less than 32 degrees (58 degrees) because once the evaporator runs out of something to do (near set point, fan on one speed segment, humidity low, low temp load), the below freezing sat temp will translate into a freezing evap/suction line?

As an experiment, i lowered the charge to the minimum that would cool the cabin and meet the evap delta t min of 15 degrees. That turned out to be 44 psi/20 degrees. The cabin was cooled, the superheat was 35 so the suction line was cold to the touch and sweating and the superheat did not crash to zero at low evap load (end of the cabin pulldown). I know that if i increase gas past this point, evap delta t will go to 20-23 degrees and the system will freeze at low load. This would suggest that i need to push past all of this to a sat temp above freezing and a starting high load superheat around say 10-20 and have the faith that after the cabin pulldown and low load condition, the superheat will stay put because the evap cannot freeze and i will not slug the compressor?

So freezing occurs in a middle band of low charge below which there just is not enough gas to freeze the coil despite the low theoretical sat temp and above which, freezing is not possible, just floodback/slugging if you go to high?

BTW, what line temp range equates to "cold can of beer" cold becuase a can of bear comes out of the refrigerator at 37 degrees and, depending on the humudity, will still be sweating up to a temp in the 60's?

Thanks.

 

[krush]

Slugging: I've done a lot of dumb things and still do dumb things, but I've never slugged a compressor--and this includes charging with the 30lb can upside down so I can get to drinking beer faster. Now that I said that, I'll probably have it happen. The way the compressor are designed is that the suction goes in the bottom of the "can" the inlet to the compressor is pretty high from inside that can. In other words, you'd have to overcharge pretty damn high to slug the thing. There is time for the left over liquid to evaporate in the bottom of the compressor...plus there is an accumulator....but I digress.

Anyway, to give a simple answer to your thought process. Unless the ambient cabin temp is really cold (which I presume it isn't) your coil temp should not be below freezing...you have an A/C system, not an icebox.

Are you positive you have good flow and your evap coils aren't dirty? Your latest experiment kinda points to that. With good airflow you shouldn't be able to freeze the coil under normal conditions. With a low charge, coil freezing may happen if the ambient temp gets low enough and the cooling load on the unit is low enough.

A very high superheat means that your compressor isn't getting good cooling. This is the problem with low charges--the compressor will overheat due to lack of cooling.

 

[REBrueckner]

I've never known as much as I'd like about air conditioning but the advice above is right on...a freezing coil for me means to check air flow (is the fan running and air moving) and if that's normal, likely I am undercharged....

 

[Reefgeorge]

Yes the filter right on the evap is clean as is the rough metal filter in the grill. The fins are shiny like new (my wife has asthma so we have to keep all ac filters squeeky clean and run HEPA filters in the boat and at home). The airflow to both ceiling grills seems to be normal (ceiling intake/evap coil/fan(s)).

Here are the results of yesterday's experiment at the 3 hour point(cabin started at 93 degrees, external temp 90+) and your probably right about the hot compressor:

Sucton P/T 44psi/19F
Head P/T 200psi/102F
Evap delta T 15F
Indoor dry bulb 78F
Indoor humidity 45%
Cooling water 84F
Engine room temp 84
Ending Superheat 38F
Subcooling 11F
Compressor temp 212F
Compressor type Tecumseh AKA8515EXD

I do notice that in the previous case where the charge was a little higher and the suction line freezes, as the superheat tanks to zero the compressor starts making more noise so I thought that this was the start of the slugging process? Given that these systems have an accumulator, it makes sense that these things should be pretty immune to extreme variations in conditions.

BTW, the other unit has a frozen compressor. 2 questions:

Does increasing the start capacitor value to have a "harder start" capacitor ever get these things going for some additional life without overloading something? All the windings ohm out OK, no shorts, no opens. The compressor cycles on overload shutdown and high amp draw. I replaced both capacitors, the relay and the triac. Pretty much everything except the logic board and the compressor. No luck. If this is dead-dead, what is your take on a new unit with 417A, evacuate the 417A and start over with R22? Are there any drawback to replacing the 417A?

Thanks.

 

[krush]

Also, where are you getting your temp measurement for superheat?

Edit: On the locked compressor...try banging on it--yes I'm serious. You should be able to find a locked rotor current spec as well. Maybe steve, the A/C guy can help with that.

 

[Reefgeorge]

A thermocouple/clamp on the suction line just before the service valve/port.

 

[krush]

Is that near the inlet to the compressor or is it closer to the evap? My reason for asking is that the superheat will only go up as you get closer to the compressor. 212degrees is pretty dang hot too!

If you are sure the coils are clean, the blowers are working right, and no clogged filters...I'd say you are showing symptoms of a low charge.

Here's some theory and charts if you are interested: http://www.achrnews.com/Articles/Feature_Article/e0b126071885a010VgnVCM100000f932a8c0

 

[Reefgeorge]

Its at the inlet to the condensor unit, at the end of the copper run from the evap coil. That puts it 1 foot away from the compressor and 20 feet from the evap coil. Given that superheat is going up by the time it gets to the compressor, if I read 0 here, it is definitely zero all the way back to the evap coil.

Have you ever tried a "harder" start capacitor (read bigger than spec) to revive a frozen compressor? I'll get out the rubber mallet and give it a whack while it's cycling on no start/thermal shutdown. If it starts, won't it just freeze again, or can the hammer act like using a defib unit and give me a few more years (assuming that I stop eating cheeseburgers)?

Thanks.

 

[Boatsb]

These compressors are cooled with the suction line freon. If it is 200+ degrees that's just another sign of low freon.

Why not charge it to the proper levels and see if it works? It seems all this discussion is going no where unless you get the pressures where they should be.

 

[Reefgeorge]

Well I went back down to the boat after 18 hours with the "minimum" charge scenario and after the system was able to pull out the rest of the humidity and get to minimum load, it froze up again. So besides the hot compressor problem, the unit still freezes eventually. This charge level can cool the cabin but has these other performance problems, but you already knew that. I was not able to try the higher charge scenario, I'll post back when I get to it this weekend.

In the meantime, with the frozen compressor on the other unit, I'll bang on it to see if that helps. Can increasing the start capacitor value help without other problems? Tecumseh says that you can go up one capacitor value from 72-88 to 88-108. Any higher? If that unit cannot be salvaged, how do you decide on compressor vs entire condensing unit? If its whole condenser, are there any problems reclaiming the 417A and replacing with R22?

Thanks.

 

[krush]

Any updates? Just curious to see if the problem got solved.

 

[Reefgeorge]

Actually, further complications developed - the cooling sea water pump failed and is very difficult to access and replace. I'll be down there tomorrow continuing to R/R the pump and then it's back to charge levels and frozen compressors.

 

[Reefgeorge]

I finally got the cooling water pump replaced and fired up the system. I set the charge at:

81 degree sea water
78 degree indoor dry bulb
40 percent indoor humidity
23 degree evap delta T
11 degree sub cooling
58 psi suction
220 psi liquid

As the cabin gets close to set point, once again the superheat goes to zero. BTW, superheat started at 25 degrees at full cabin load this time, in line with the chart. I tweaked the charge at the end to keep the suction saturation temp just above 32 degrees.

 

[krush]

How many pounds did you put in? I always try to use a scale to at least get an idea how much I'm putting in a system. It's just another tell-tale tool to use.

 

[spcoolin]

George~

With your water & air temps as described...I would expect to be running 73-74 psi suction & around 235 psi head...

As I said over at my site...Compressor temp is important...Not enough refrigerant return will cause it to run hotter than nominal, and damage the compressor.

When you try again...It's not likely that the seawater temp has changed much (however the cabin temp will)
Forget about sub cooling & superheat for now...You are not even in the neighborhood of being fully charged yet if all is well in the system...

Try charging to 235 head and see where your suction runs...Depending on cabin temp above 70 degrees...I would expect the suction to be running anywhere between 62-75 psi...
Then watch compressor temp...If it starts getting cold...Remove a bit of freon...
If the compressor temp starts climbing to say 120 degrees...Add a bit more....

The most important thing to do is charge for compressor temp...Then we can talk about TD's & such...

BTW, your TD should be no more than 18 degrees & no less than 15 degrees...

Steve~

 

[krush]

I'll gladly defer to Steve...listen to him!

Not that it matters, but I agree with him 100%...still undercharged. If you are scared of overcharging, get a scale to keep track. I'm sure Steve has a ball park figure to use.

 

[Reefgeorge]

OK, I'll ignore SH and charge to the Cruisair charts while looking for a maximum compressor temp of 120 degrees and a minimum head pressure of 235 and report back.

Is the compressor temp measured on the top of the can where it is hottest or somewhere on the side?

I forgot to include it but the top of the compressor case was running 180 degrees, but you already knew that it was still too high.

Thanks.

 

[spcoolin]

On the top George~

You can start to tell you are getting overcharged if the compressor can around the suction tube on the compressor starts to sweat...The more you overcharge...The bigger the circle of sweat gets around that tube...Ideally you want the line cold, but no circle of sweat on the can...However a small circle at lower cabin temps is ok...And at this point...The pressures are what they are...We can then interpret what is happening in the system by reading them, but not until we know the system has enough refrigerant to properly cool the compressor.

Sub cooling & superheat are great tools for charging air cooled systems where you may not have access to the compressor due to the fan blades spinning...But they are really not needed in this case with a R-22 system that you can access the compressor...

These Cruisair systems (as most R-22 A/C systems) are designed (cap tube & air flow sized) so that they should run above 32 degree evap (58 psi) down to around a 68-66 degree cabin....Even when properly charged, they will ice in ambient temps below that, and of course reducing air flow such as undersized duct, or slow fan speeds (reducing the amount of heat picked up) can raise that point of icing into the 70's...

This is why the SMX control has a de-icing algorithm that shuts off the compressor for progressively longer periods (before setpoint is reached) the lower the cabin temp is below 70 degrees...It's described in the SMX manual...

But what I'm getting at here is...The lower the cabin temp...The lower the suction pressure (and thus evap coil temp) will go...Therefore with a warmer cabin you must start out with higher suction pressure to allow for the decrease in pressure & temp as the cabin cools...

The 15 to 18 degree Temp Differential (TD) between the air entering the air handler, to the air exiting the discharge grill will always be maintained...And this is where most people get confused in thinking their A/C is not doing as well as it should...

The warmer the room temp...The warmer it comes out grill....
If the cabin is 90 degrees...Don't expect lower than about 72-75 degree air out of the grill....But if the cabin is 70 degrees you might have 52-55 degree air out...

Steve~