Storage Facility Condensation

[leapark97]

Hi there

I am a non technical guy, new to this site and in need of any and all advice on dehumidification. I run an underground storage facility that at certain times of the year (April to September) is very prone to condensation build up and need a solution to this. I am happy to try and answer any questions that you technical experts throw at me and hopefully this will lead me to a solution. I am not expecting and "off the shelf" answer and fully realise that my problems may require a bespoke design. Hopefully over to you guys!

[NoNickName]

You may want to start with the details of the nature and value of the goods stored, the required temperature and humidity, the size and volume of the storage area, and the amount of ventilation required.
Did you try anything yet to alleviate the problem? If yes, with what results?
Your budget for solving the problem would also help us.

[leapark97]

The storres are ex Royal Navy spares contained in a variety of package types from cardboard to purpose built wooden/steel crates. We do not heat the facility and the ambient (depending on time of year) ranges from 7 to 12 degrees celcius. RH ranges from upper 90s% to 70% depending where in tha facility you are. The overall area is 280,000 cubic metres split roughly equal into 3 zones. Each done is ventillated by its own bank of fans that distributes air in under-road ducts and up through periscopes. During the winter months we "vent" air down an airshaft from the surface - which dries the area reasonably well, it's during the non venting months that we get the problem - as the temperature rises due to non venting due to worsening surface conditions. We currently have 6 Calorex DH600 units but these provide only very localised drying. The price is important - but the correct solution more so.

[NoNickName]

Mhmmmhhh.. How frequently are the stores accessed during the day?
How much air is changed when forklifts enter or leave the stores?

Is it possible to completely shut down the air change?
How would you rate the wall and ceiling insulation?

The volume is huge. Any solution provided would be quite expensive, both as a capital cost and as running cost.

[tmm]

At the risk of stating the obvious condensation takes place where hot and cold air meet. If you can eliminate one or the other conditions can be stable.
For example many caves/tunnels are used for the production of cheddar cheese since the temp and humidity vary little irrespective of external conditions.
The figures you quote show that air voulmes are meeting within the store and producing the problems.
In short the problem is a matter of air circulation although matters can be improved with better storage arrangements. For example are the storage items placed on pallets with adequate distance from the structure to promote good airflows producing a more even temperature and hence less moisture? Do you have the entrance protected by say plastic slats to limit the ingress of uncontrolled outside air?
If you PM me I will provide more details if you require.

tmm

[NoNickName]

That's my concern too. leapark, could you please keep it public, so that everyone could learn from your experience.

[lowcool]

you can buy dehumidifiers off the shelf.china make heaps

[NoNickName]

you can buy dehumidifiers off the shelf.china make heaps

Be serious. 280.000 m3.

[leapark97]

I have attached a document that shows some history, the air intake strategy we use now and some more information on the facilities. The stores are randomly stored around the mine and we do have 6 localised dehumidifiers - but they simply are not man enough for the job. The caverns are around a figure of 8 roadway system and the roads act as main air arteries when the air enters the stores although the flow of air does pass over the caverns before discharge to the surface (on venting). The previous owners (UKMOD) used to heat the area for control of the problem but that IS prohibitively expensive to anyone but the taxpayer! Hope this helps?

[NoNickName]

Yes, it helps a lot. But it needs some brainstorming. Surely a AHU (air handling unit is necessary), but these days heat can be reclaimed and recovered to dehumidify with efficiency. I'll be back in few days.

[Peter_1]

We've installed some years ago some Munters dryers in large storage buildings (3) for the Belgium Army.

[Gary]

Seems like insulation/vapor barrier is in order. Perhaps spray foam?

[Peter_1]

Read your word file. It's 10°C in the mine, so not that good conditions for a condensation dryer.
Those you installed now have a capacity of 1.800 l/day but a which conditions is this?
How much do they extract actually?
At these cold ambients, you better go for an absorption dryer. You could feed them with electricity or gas.

You only have problems in summer situation which is obviously, so what could help is to preheat the intake air with heat freely available in summer (heatpipes, flat plates) In that case, running costs would be less but this warmer air can hold more water.

A little schematic with flows on it could help more

[r.bartlett]

A project like this is unlikely to be 'solved' via an internet forum. Sure Idea's can be bandied about but judging by the word doc someone has already been involved.
I suggest you contact someone like Munters who will assist on a project of this size

http://www.munters.co.uk/en/uk/

http://www.munters.co.uk/en/uk/products--services/dehumidification/Dehumidification-system/MXT-series/?Product=EA699853-A3F3-4C86-9078-A0D48161304F

[NoNickName]

I think a crossflow heat exchanger with total air recirculation would do the trick. Basicly, you would need to keep the air tempreature above the dew point all the year long, without dehumifying, which is very expensive process.
I will post a schematic asap.

[Peter_1]

I contacted this morning the service engineer on the storage facility of the Army and one of them with MXT's from Munters was used only for 8 months and then shut down. They are still in place since then (+/- 1998) but never ran again, just started up a few times per month. Perhaps they will sell those. I don't know if they're in good condition. But as NNN said,drying energy wise an expensive process.

[lma]

Why was the lower temperature limit for FFA cut off lowered to –7 °C from +2°C ?

[leapark97]

I have gone the route of Munters et al and those that responded simply want to want take an easy/expensive route (with no guarantees) of selling off the shelf units.

I wrote the word doc from experience gained and input from our BMS engineers. We concluded that a bespoke system of a unit/units across the front of the fans - treating the air before distribution into the mine is what's needed but am open to alternative solutions so will ltry Munters again. Some useful ideas here to discuss with them.

Application of foam or other insulation to oolithic limestone pillars/roof and walls in a 15 acre underground facility is simply not feasible.

The reason that the air intake temperature was changed in winter was to allow more "good" dry air to be drawn in for longer. We found that sometimes we had great conditions but were not taking advantage of them because the temperature was too high.

[lma]

During the winter months .... what is the temperature of your walls ceiling and floors ... both on surface and depth of say about a metre .. if you can obtain that data... also during summer check the same. My suspicion is your moisture problem is occurring from evaporation from within the ground, if the temperatures are maintaining constant within the grounds. I suspect The lower temperatures during the winter months would cause the walls to ice up at that low temperature (the ice would act as the insulator barrier ). During summer .. this barrier is not there leaving your walls open both to the thermal warmth of the ground and the moisture stored within it.

[NoNickName]

No, the problem is very simple. They have got this mine which is at approx constant temperature of 7°C to 12°C all year long. They also have some sort of air change.
During winter time, the dew point of the ambient air is below the air temperature, therefore no condensation occurs.
In summer time (between April and September) the dew point of the ambient air rises above the dry bulb temperature of the cave and condensation happens.

I've got the solution for you.

You need to install a air to air heat exchanger so that heat is gained from the ambient air during summer time, but keeping ambient air isolated from cave air.
Basicly you need to heat (without the heating expenditure) by getting free heating from ambient air.
The cave must be aeraulically separated from ambient, though. In summer time you will need to recirculated cave air and don't exchange it with ambient.
Gates must be as much as possible closed to avoid entrainment.

The only running cost of this system are the fans. No actual mechanical or electrical heating or cooling or dehumidification happens, just heat exchange.

I have a schematic I can send you. Please contact me via PM.

[Gary]

Basically there are only two choices, heat the air and/or dehumidify the air.

[Gary]

If there is some minimum fresh air requirement, then this should be the first problem addressed. The fresh air needs to be dehumidified before it enters the space. Possibly the existing dehumidifiers could be used for this purpose.

[chemi-cool]

it looks as most of the moisture is coming in from the walls, you should check with building experts if there is a way to seal them with any polymer based paint.
That can cut the size of the dehumidifiers.

[leapark97]

The fresh air venting is not the problem. We only take in air to the facility from the surfcace when it is better (has lest moisture) than the underground. Doing this dries the mine out by expelling the wetter air. There is no limit on the amount of fresh air. When conditions are better we vent until the lower set point moisture limit is reached - wee previous attachment.

I agree that the fabric of the mine (walls and roof) are part of the problem. They are a breathing stone, cold and damp. But as with application of foam (see previous post) - sealing the walls/roof is an impossible task - its a mine and 15 acres with pillars! It simply won't work.

We accept that we have to either treat the air by heating/cooling, dehumidification or a combination thereof. That seems self evident - it's finding how to do that efficiently.

[NoNickName]

leapark, please have a look at my last post, please. One page back.

[mad fridgie]

Your problem is your control scenerio,
In winter you are bringing in dry air, which does a first clance, seem good, BUT, what you are actually doing is bring the thermal mass of the caves, down in temperature, which means when the more humid months arise, you are increasing the chance of condensation.
Without know all the details, heres the cheap fix, Recirc in winter (not 100% fresh air), move exsting dehumidifiers, close to air intake duct/recirc fans duct.
When ambient is below lets say 12C, recirc, with dehumidifiers in this air stream, when above 12C, introduce some fresh air (not total freshair), (keep the recirc running,) forced through the dehumidfier, mix with the recirc air, control fresh air intake on the " moisture content of air out" of the dehumidifier, making the most of the occassinally warm, dry ambient conditions. I would suggest you have VSDs on fresh air intake fans. (It is a bit more complicated than this but i hope you get the idea)

[leapark97]

Nonickname
Have tried a pm but no response and your email fails?

Mad fridgie
Thanks for the advice. I will discuss with our BMS emgineers. It is true that since taking over the facility we have experience these problems at lower moisture levels. You will see in my document from a previous post that we feared we had made the mine act like a sponge by drying the mine out too much. The ambient is around 10 deg c so will look at using that as the control temperature.

[mad fridgie]

Nonickname
Have tried a pm but no response and your email fails?

Mad fridgie
Thanks for the advice. I will discuss with our BMS emgineers. It is true that since taking over the facility we have experience these problems at lower moisture levels. You will see in my document from a previous post that we feared we had made the mine act like a sponge by drying the mine out too much. The ambient is around 10 deg c so will look at using that as the control temperature.
The higher the temp the better, your are not drying out the mine to much, you are cooling down to cold.
Without see a graph, i supect that around feb you reach your lowest temp 7C, gradually rising to september. by now your temp will just start to fall, do not bring in lots of cold fresh air. Just recirc run de-humidifiers, you will have to go through alittle pain first (it may not seem to be drying), it is next year you are aiming for. Little or no reduction in mine temp. What else is required needs a more complex set of calculations

[TRASH101]

Hello Leapark97,

Correct me if i'm wrong, from what I recall about oolitic limestone your fears of "sponge" are well founded, especially in our green and pleasant and wet land. Have you commissioned a geological survey to ascertain the porosity and permeability of the limestone and will there be consequential structural problems caused by drying (if indeed you can get anywhere near dry)

[leapark97]

Hi Trash101

The stone is a breathing stone and accepts the ingress of water through the water table naturally ie it gets damp. In places there are natural fissures that let water through and this is collected in a sump and pumped to the surface. The mine was constructed using the proper method of pillar retention and we have a surveyor who tells us that it is safe. The previous owners used to heat the mine with no apparent deterioration to the structure - and we won't ever achieve the temperature they raised it to.

[r.bartlett]

Hi Trash101

The stone is a breathing stone and accepts the ingress of water through the water table naturally ie it gets damp. In places there are natural fissures that let water through and this is collected in a sump and pumped to the surface. The mine was constructed using the proper method of pillar retention and we have a surveyor who tells us that it is safe. The previous owners used to heat the mine with no apparent deterioration to the structure - and we won't ever achieve the temperature they raised it to.

A good friend of mine is a freelance geologist who may be of assistance if you need any technical help of this level..

[Gary]

I suspect the Calorex DH600 has a control strategy which limits the evap coil temp to avoid freezing the coil and is designed for higher temp operation.

The closer this can be tweaked to 0C, without freezing the coil, the more moisture it will remove.

The air flows through the evap coil and then through the cond coil. Ideally the temp of the air leaving the evap coil (between the coils) would be 11C below the warehouse temp. If the warehouse temp is 10C, the evap air off temp would be -1C, which would cause the coil to freeze.

It may be a good idea to check the temp between the coils and tweak the controls as close to freezing the coil as possible. If the best that can be achieved is spot condition of 12C@70%RH, then I suspect the between coil temp is only getting down to about 6-7C.

Also, raising the warehouse temp a few degrees would result in less danger of freezing the coil.

[Gary]

I think NNN is on to something here. He has offered the best solution so far:



I've got the solution for you.

You need to install a air to air heat exchanger so that heat is gained from the ambient air during summer time, but keeping ambient air isolated from cave air.

[NoNickName]

I sent a sketch of the system to the OP. Later I will also post it here.

[leapark97]

Thanks for all the input and advice guys. I have taken all on board and am looking into the outline solution proposed by NNN. The general consensus seems to be that a refrigerant dehumidification solution is no good at the ambient temperatures we have. So am also looking at dessicant dehumidifiers as a possible solution. Will keep you posted. Thanks again.