evap pan freezing

[mark houser]

have problems with huge freezer with multiple evaperators. drain lines clear but evap pans freeze up bad causing distortion of pans. my ? is what is the best way to keep pan from freezing? thanks for input. mark

[Dan]

Hi Mark, I think you should tell us more about the size of the freezer, type of coil, defrost type, etc.

If you have an electric defrost, I would take a good look at the placement and performance of the heaters in the drain pan. It is rare for a drain pan to ice up when the drain is free flowing, so I would look at the drainage of the pan. You might have previous damage that doesn't permit free drainage to the now open drain.

With gas defrost, the things to consider are similar, but you may or may not have electric drain pan heaters. If it is piped drain pan heat, make sure the pipes are positioned and getting good refrigerant flow.

Another consideration is whether you need a "drip" or "drain" time after defrost. If refrigeration resumes immediately after defrost termination, it is possible that the accumulated water in the pan has not had time to be eliminated from the pan through the drain. Any water remaining in the drain pan will refreeze and you can have a situation where this builds up slowly, cycle after cycle, until the drain pan ice covers the drain outlet.

Simply stated, you either have a drain pan damaged from a previous event that needs to be replaced, a faulty drain pan heater either because of performance or placement, or too short a defrost, and/or drain period between the defrost termination and resumption of refrigeration.

[herefishy]

Hmmmmmmm.... My first thought..... Are drain P-traps installed that might be required in this application?

Only a thought.....

[Dan]

Oops. Forgot something pretty basic there, Herefishy.

[Dan]

I rethink why I left out the traps in the drain pan as a worry. My recollections are that lack of drain traps cause frosting or icing problems on the coils, not so much in the drain pan.

The dynamic is that moisture is driven through the evaporator and through the coil when a trap is missing. Am I missing another dynamic that causes moisture to collect on the drain pan when a trap is missing?

[Brian_UK]

One thing with traps in drain lines Dan, at least with fans/ducted systems, is that the water may not always drain away against the difference in pressure between inside and outside.

The trap allows the pressure to be overcome.

[That's my twocents worth anyway]

[herefishy]

My recollections are that lack of drain traps cause frosting or icing problems on the coils, not so much in the drain pan.

Hi Dan

I agree with you as that would be my normal (only) train of thought regarding the issue. But when mark says "drain lines are clear", I wondered why the water was staying in the pan. I assume that markhouser is qualified, and addressed the obvious (heaters, etc.).

As Brian_UK indicated, the PD from inside the store, to the outside (or other) location of the drain termination may be preventing proper drainage. there may be some other dynamics at work here, causing a negative pressure scenario in the freezer.

With the drain pan cleared, or drain open into the freezer, I would like to do a smoke (candle) test at the drain outlet to see if it is "sucking", so to speak.

I envision an intermittent occurence of the phenomenon. Being a "Large" freezer with multiple evaporators, I futher envision a processing plant with a loading dock. I get the indication that mark cannot identify the cause of the failure at the time he is servicing the problem (regular working hours). My "crystal ball" also indicates to me, that the drains terminate somewhere in the plant, not outside the building, .......perhaps....... in the loading dock area.

[Dan]

One thing with traps in drain lines Dan, at least with fans/ducted systems, is that the water may not always drain away against the difference in pressure between inside and outside.

I agree, Brian. And I should caution that I am not familiar with many industrial blower coils...especially ducted and penthouse designs.

I still would think that the primary indication of an untrapped drainline would show up as ice or frost on the coil surface if we had large entrainment of moist air via the drain line during refrigeration. That it would not build up within the drain pan. But I can picture a drain pan not draining because of negative pressure. I picture it easily with air conditioning, but get lost picturing it in a freezer.

[Dan]

As Brian_UK indicated, the PD from inside the store, to the outside (or other) location of the drain termination may be preventing proper drainage. there may be some other dynamics at work here, causing a negative pressure scenario in the freezer.

Some good thinking. Gosh I hate it when somebody poses a question and then I just make a fool of myself rambling and then fine thinkers remind me of the stuff I didn't think about and then the original poster doesn't ever get back with the solution.

Actually, I only hate the part when the person with the problem never gets back.

I still wonder, though. If we have a drain line acting as a conduit for moist air entrainment, wouldn't we see the effect during refrigeration on the fins of the coil sooner than we see the effects in the drain pan?

[herefishy]

I still wonder, though. If we have a drain line acting as a conduit for moist air entrainment

Dan....... stop beating yourself up !!!!

The drain would act as a conduit, for perhaps one day or the number of cycles before the drain pan "plugged the conduit".

If you were dealing with medium temp scenario, the water would never freeze into a plug...... therefore the entrainment would commence indefinitely, possibly.

But if a trap is required and there is no mechanical trap, the water level in the pan will rise to equilibrium with the pressure difference, and result in stopping the entrainment.

[Dan]

Dan....... stop beating yourself up !!!!

LOL, Herefishy! My wife says the same thing.

The drain would act as a conduit, for perhaps one day or the number of cycles before the drain pan "plugged the conduit".

I am not sure what you are saying here. Are you saying that the plug appears on the drain pan? That the plug forms in the adjacent part of the drain where it connects to the pan?

If you were dealing with medium temp scenario, the water would never freeze into a plug...... therefore the entrainment would commence indefinitely, possibly.

You have sparked a thought, here. Let me define entrainment:

Unconditioned air exposed to negative pressure that delivers through a unit cooler through an untrapped drain pipe.

So now, methinks... why do we assume that we have a negative pressure in the drain pan area?

Most blower coils I am familiar with will have a negative pressure, because they are "draw-through" coils...... the fans exert a negative pressure throughout the entire unit cooler assembly because they "suck" the air through the evaporator. But there are designs that are "blow" through." I have to wonder how their behaviors would apply. Deeper traps is one idea that pops into my head.

But if a trap is required and there is no mechanical trap, the water level in the pan will rise to equilibrium with the pressure difference, and result in stopping the entrainment.

That observation I need help with. Are you differentiating a gravity trap compared to a check valve arrangement? Are you saying that the drain pan will flood until it becomes the trap? But, alas, the topic is a freezer, and my questions are a digression.

Going back to the original question, I think it would be easy to observe and repair frozen drain pan problems. I think electric defrost is probably best, if that would answer mark houser's original question.

[FreezerGeezer]

Back when I did commercial installation, we always trapped coldroom evaporators. The reasons given were to avoid cold air being drawn through the pipe, causing it to freeze, and negative pressure problems causing condensate to stay in the drip tray & freeze. Many / most(?) drip trays have electric heaters as well, to combat exactly this problem. Freezer coldrooms should also have a trace heater wrapped around the drain, for the same reason. All my installations, as were all those done by that company (as I recall) also had armaflex on the drains. Most blocked drains I came across were due either to the drain trace heater, or the drip tray heater failing, and allowing the condensate to freeze.