Freezer Humidity

I was called to look at a recently installed walk in freezer the other day due to it regularly breaking down.

The interior light was wired on the same circuit as the compressor and every time the bulb popped it threw the rccd breaker with a loss of stock. Any way, the door has been built without any threshold just a rubber draught strip on the bottom of the door and frost is everywhere on the inside as you can imagine.

A couple of data loggers were put in to try and see what was happening to temps and humidity but I put the wrong temp logger in there - I used an insertion probe logger without the probe - doh!! anyway I have now put both loggers back in for a set of new data.

From the results of the humidity data it would appear that lowest %rh was about 80 and the highest was about 95%!!

What would you expect the RH to be in a decent -18C freezer?

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Originally posted by frank

A couple of data loggers were put in to try and see what was happening to temps and humidity but I put the wrong temp logger in there - I used an insertion probe logger without the probe - doh!! anyway I have now put both loggers back in for a set of new data.

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Another of those "I can't believe I did that" moments? The worrisome thing is that as I get older and wiser I seem to be making more stupid mistakes rather than fewer.

The humidity may be quite high. Anybody got a low-temperature psychrometric chart? Mine's loaned out. Anyway, the dewpoint of the air will be approximately equal to the surface temperature of the coil (the saturated suction temperature of the refrigerant). Knowing that, you can find the RH of the -18°C air. The colder the coil with respect to the air (greater TD), the drier the air.

Moisture migrates in response to differentials in vapor pressure, hence will always move from warmer to colder. The usual problem with freezers is that the coil dries the air, actually dehydrating exposed product over time ("freezer burn") through sublimation (evaporating directly from the solid phase, ice, to vapor).

So if frost is on the walls, it is entering the box faster than it is deposited on the coils or is being expelled from the coil after defrost. It is a given that the gaskets should be checked, but if this door is opened frequently, is a fork-lift door without an air curtain (get one!) and opens onto a high humidity ambient (kitchen or bakery?), increasing the number of defrosts per day will help.

But I would check first for a defective or miswired evaporator fan delay control. (Probably the latter, since it's a new installation.) If the fans come on immediately after defrost, warm, moist air will be driven off the coil and create the conditions you describe. Look at the ceiling around the discharge side of the coil and if it is heavily covered with frozen water drops, bingo.

The light bulb popping may be entirely unrelated. If it is protected by a glass or plastic globe that gradually fills with water, the point of entry of that moisture is from the electrical conduit penetrating the box wall. In the interior of the box all electrical switches and flex may be moisture tight, but if the inside of the electrical conduit has not been sealed on the warm side of the penetration, moisture will enter and condense on the interior surface of the fixture.

The RH in active freezers is always quite high. Some good observations, Rog.

The fact that this was a recently installed box tells me that we need to seal all the electrical penetrations. Heck, all the penetrations. But the one most likely missed is the conduit entering the box.

Of late, I've come across quite a few new supermarket installations, where the conduit penetrations were left open to the box interior, causing problems such as blown bulbs, or worse.

Worse is the freezers utilizing multiple (Usually 4) lamp, 8' flourescent VHO fixtures. The conduit enters the fixture from a J-box on top of the freezer, usually 3/4" trade size, three wires go through it. The wires are generally 12 ga.

The excess air space allows enormous amounts of humidity into these boxes, resulting in numerous cases of destroyed lampguards. They fill with ice, and come crashing down.

The humidity inside a freezer is normally high. The humidity in the store is usually considered "low to normal", unfortunately, when dewpoint inside the freezer is -20ºF, 74ºF air at 50% RH can cause an incredible ice dam rapidly!

Seal those conduits!!!!!

Typically, freezers are rated for a maximum of a 10 degf T.D. in order to prevent dehydration of stored (food). Inherently, the RH would be rather high like the 90% that you indicate, unless you are servicing a freeze-drier.

You can determine what the design TD is, by taking the condensing unit capacity (btuh @ specified SST), and dividing that capacity by the capacity of the unit cooler (btuh) per deg(F) T.D., then using the information regarding the SST and design box temperature (or an industry cheat sheet) to determine what the design RH should be. Of course if it is not in line with your readings that would indicate some system malfunction which is likely better determined through standard refrigeration service techniques.

I would suggest that you are dealing with sensible infiltration however, and should concentrate on those "sealing" or defrost control jobs. :)

I agree with the thoughts about infiltration on the light fitting and I will look at this.

I really consider that the problem with the excess moisture and frosting is caused by the door not having a threshold but it is nice to have some feed back about RH values

Everything in nature goes from higher to lower. Temperature, pressure or humidty, in an attempt to balance. The floor may not have been sealed? Concrete has plenty of water in it especially if new. A missing gasket on the threshold? Why? The defective fan delay is a likely culprit, adding a trap to the condensate drain may even help. Seal any and all penetrations. I would think the frost would tell you where it is comming in, where it builds up the most like the threshold. Is there a block there being constantly broken away?

You are right about the threshold - all of the frost accumulates around and on the door opening. We have also found that the installer has only put down a 40mm insulating raft on top of the concrete slab - no way near enough insulation thickness for the floor - a world full of cowboys!!

One other thing, as we get more into winter, the atmospheric RH will go down, and alot of the frosting will subside, giving the illusion that the problem if fixed, till humidity returns in the summer next year. Unless the relative humidity stays high at that location year round of course.