Help with Defrosting Zero Degree Evaporators - Ice Biuld Up

[Warpig]

We are struggling with ice buildup on a couple of our units in our blast freezer which is held at 0 DEG F. It doesn't happen on all the units in this freezer, just the same two which share the same valve bank. Our current defrost schedule is every 4 hours, 5 min PO, 15 min HGD, 3 min EQ, 3 min Fan Delay. The units are KRACK cooling units and we run a liquid overfeed system. Any help or suggestions would be greatly appreciated.

[Grizzly]

Hi Warpig.
A couple of questions.
Where are these 2 units in relation to the rest IE.

Are they nearest the door / entrance? Are the drain lines clear?
Do you know if the loading of the chamber is relevant?
Are the heater elements for defrost pulling the correct amperage?
Is the airflow through evap correct IE Is fan pulling correct amperage.

Just a few questions to start.

Grizzly

[Warpig]

1) Where are these 2 units in relation to the rest IE. They are in the same vicinity, same air flow, heat load, etc.

2) Are they nearest the door / entrance? No, they are back in the corner of the room farthest away from an open door where moist warm air could be an issue. The product that is put under these units is very cold to begin with so there is no major heat load directly under them.

3) Are the drain lines clear? Yes, drain lines are clear but the ice build up starts around the frame of the unit and the drip pan. Once this layer of ice forms then the subsequent defrost drips over this ice layer and not into the drip pan.

4) Do you know if the loading of the chamber is relevant? I'm not sure what you mean by the "chamber"?

5) Are the heater elements for defrost pulling the correct amperage? These are hot gas defrost units with hot gas defrost running though the drip pans also. They are bottome fed with hot gas going through the drip pan first.

6) Is the airflow through evap correct IE Is fan pulling correct amperage. Yes, air flow is good and the coils do not have ice build up between them. The ice buildup starts at the bottom and if left untreated will eventually create a huge ball of ice on the unit.

[Zero_bar_gage]

Hi Warpig,
few questions from me too, just to have a clear idea and hopefully ... to give you a suggestion.
0 def F are in the evaporator, isn't it? So which temp approx is required for the chamber (I mean the cavity, the place where you put the food to be cooled down). May it be something like 20 F?
Or maybe I misunderstood, do you mean the chamber is held at 0 F? If so, what is the evaporating temp?

Then I'd want to be sure about the defrost schedule:
defrost each 4 hours, clear
5 min PO, what do you mean by PO? I guess you run the compressor for 5 min to get the right discharge pressure?
15 min HGD, hot gas defrost for 15 min! It is quite long. Which size of evaporator are we dealing with?
3 min EQ, after hot gas you cut compressor off and wait for a pressure equilizing (should be)?
3 min Fan Delay, you cut the compressor back in and wait for 3 min befor running the evaporator fan?makes sense

Then you told that the units share the same valve bank. I know just a little about over feed systems, anyway do you mean both the evaporator are supplied through 1 single expansion valve?

Or there is a common valve to bring liquid to the evaporators and then this liquid expands through two separate valves?

The hot gas pipe is common to both the evaporators?

Just wondering, is this an ammonia system?



Hope to receive your kind clarifications before giving you any useless advise.

Zero

[RANGER1]

Warpig,

Are these new units, or have they been in awhile?

Have they worked OK in the past?

Have you checked units are clear after defrost, as 15 min is pretty short.

What is hot gas defrost pressure/temp? Would have thought 75-80 psi (515 kpa 10 deg C).


Could there be any way ambient moist air moist could enter room ie damaged panels etc?

What product are you freezing & is it in cartons etc?

[Warpig]

Are these new units, or have they been in awhile? These are old units and as far as I can tell (I've only worked here 3 years) they have always had issues. Or at least the problem has come and gone but it is back again.

Have they worked OK in the past?

Have you checked units are clear after defrost, as 15 min is pretty short. Yes, the other units in the area defrost completely after 15 minutes. I'm wondering if defrost is too long causing excess steam in the area and thats what starts freezing back up on the frame work.

What is hot gas defrost pressure/temp? Would have thought 75-80 psi (515 kpa 10 deg C). You are correct. I'm not exactly sure but most of our defrost pressures are approximately 75-80 psig.

Could there be any way ambient moist air moist could enter room ie damaged panels etc? No, this has been ruled out with infrared thermography to try an determine hot or moist areas.

What product are you freezing & is it in cartons etc? Meat products and they are all in covered totes, vats, containers.

[Warpig]

0 def F are in the evaporator, isn't it? So which temp approx is required for the chamber (I mean the cavity, the place where you put the food to be cooled down). May it be something like 20 F?
Or maybe I misunderstood, do you mean the chamber is held at 0 F? If so, what is the evaporating temp? The freezer (chamber/cavity) is being held at 0 deg F and our suction is at 8 PSIG giving us a -12 deg F ammonia temp in the evaporator.

Then I'd want to be sure about the defrost schedule:
defrost each 4 hours, clear
5 min PO, what do you mean by PO? I guess you run the compressor for 5 min to get the right discharge pressure? PO = Pump Out. Our compressors run 24/7. We shut the liquid valve and continue to pump out the remaining liquid in the evap. in order to put hot gas through the coils.

15 min HGD, hot gas defrost for 15 min! It is quite long. Which size of evaporator are we dealing with? I'm not sure of the capacity but the size is approximately 3 FT x 3 FT X 10 FT LONG. It has (3) 3/4 HP fans.

3 min EQ, after hot gas you cut compressor off and wait for a pressure equilizing (should be)? Yes, we equalize the coils slowly to eliminate any liquid hammer.

3 min Fan Delay, you cut the compressor back in and wait for 3 min befor running the evaporator fan?makes sense. Yes

Then you told that the units share the same valve bank. I know just a little about over feed systems, anyway do you mean both the evaporator are supplied through 1 single expansion valve? We do not use expansion valves as the system is a flooded.

Or there is a common valve to bring liquid to the evaporators and then this liquid expands through two separate valves? There is only on valve for both units. After the valve nest the piping "TEES" and supplys both units.

The hot gas pipe is common to both the evaporators? Yes. They would both defrost at the same time.

Just wondering, is this an ammonia system? Yes. 2 Stage Liquid Overfeed (Flooded) sytem running 35 PSIG suction on the high side and 8 PSIG suction on the low side.

[nh3mannnnn]

For one, its sounds like your lacking on your defrost time and pump down. Have you checked the obviouse problems fan rotation, blocked coil, back pressure regulator for the defrost ( is it set at 75 to 85 pounds)?

[nh3mannnnn]

Also what is your hand expansion set at? Do all the evaporators share a same common suction header? If so how far down is the suction header connected?

[servicefiter562]

Have you checked your check valve in the drain pan . I agree with nh3mannnnn on your defrost time and a 5 min pump out time seems really low for a recirclated system. Take a look at the tubes of your evaporaters and see if they have dimples in them,if they do you are stessing you coil due to hot gas hitting liquid. Do you here a banging noise from coil when it defrosts

[Nh34life]

Sounds to me that you need to confirm that the defrost pressure reg is set right! 10 C might be ok but follow the T d to pick where the best flow on HG cycle it may be that you obtain a better flow of HG from 5 C ?
Also an in crease in pump down will ensure you are not wasting all your HG time on blowing out Liquid NH3

[RANGER1]

Warpig,
Can you please explain exactly where ice is building up?
Are you 100% sure the 2 units in question are free of ice after defrost?

If it is the fins icing over, what is fins per inch (usually 4 I think for freezer).

I would find it hard to believe 15 minutes is to long, but stranger things have happened!

If you could post a picture of it , that wold be good ( with ice on it of course).

[Magoo]

If fitted ,check the suction stop vav and bypass relief to ensure you get effect pressure increase for defrost, 15 minutes seems a fairly short time frame. The plates in the bypass releif do go out of wack with age and releif at lower pressures. Perhaps strip and re build with a service kit.

[Zero_bar_gage]

In my poor opinion, if the other units have the same parameters and are running under similar conditions, there's nothing to change on the defrost schedule. The fact that both the evaporators are de-iced by one single HGD system and are both iced up says a lot. There's something wrong with it! Maybe not a relevant conclusion... but a good point to start.
Are we really sure that HG is flowing through the coil?
I agree with RANGER. You should check that after defrost the evaporators are actually free of ice. I'm not really familiar with this kind of systems, but I guess there's a solenoid valve somewhere enabling HG, isnt'it? Is it really opening during HG? And the HG piping, could it be blocked?
Hope to hear from you good news

P.S.
More than giving you suggestions, I'm learning some new things.. just wondering, why don't end the defrost by temperature instead of time?

[Magoo]

With a pump overfeed supply check that the coils are pumped out before HG defrost, otherwise the coil is a big heat sink for HG and coil will fill with more liquid, possible supply s/v is passing. Again another service kit re-build required.
Hot gas defrost is a bit weird terminology, due to the fact that gas is quite cool by the time it gets to evaps., effective ness is the pressure build in evap that does the defrost, one non-action in control can screw the whole thing up

[Warpig]

Thanks everyone for your extremely knowledgable comments and suggestions. We have been investigating everything you guys suggested and we found multiple issue that we may have to address but we are going to approach the troubleshooting process 1 step at a time...and for good reason. After looking at piping configuration, defrost schedules, room air flow issues, liquid check valve, suction stop valve, etc. we went tback to the simplest checks first before making major modifications. Much to our surprise this is what we found. We wanted to verfiy our defrost sytems first and this started with our HGD pressure setting. Sure enough we found that is was set to 95 PSIG! Wait a minute, we just checked this not too long ago, what happened. Well I can't answer that but what we do know is that the suction regulator is not working properly as we tried to adjust it and it never changed from 95 PSIG. Now when the unit came out of defrost the solenoid actuated and the suction came down to 10 PSIG. Our first step will be to change the regulator and adjsut our HGD pressure to 65 PSIG. I'll let you know what happens. Thanks again!

[sterl]

Warpig, if you found your problem, thats great but: You might want to find the Krack diagram for connecting hot gas defrost to your specific unit. While 3 by 3 by 10 is not a huge unit: many of Krack's were designed to be connected such that the HG flows through the pan heating loop and the main coil in Parallel, not in series. This was characteristic of the Krack units that employed a dimple wall pan instead of a header set of U-Bends for pan heating; and at least for many of their bigger, low temperature coils, occurred back in the mid-1980s. Those that were not connected like this would never defrost properly and would make a lot of iciicles around the perimeter of the pan; as well as a lot of icing on the fan rings and similar. In short: There was simply not enough total HG flow and in a cold space the main coil would not encountere enough flow to lift condensate in the suction header. As a result after a few cycles, the unit would ice up in the bottom 4-6 rows and often this would be "near clear" ice.

[Zero_bar_gage]

Hi Warpig,
did you finally solve?
Rgds

Zbg