Relative humidity in a cooling room

[Lars Ekström]

Hi!

I have a question regarding relative humidity in a cooling room. We want to maintain 8 degrees Celsius witch is no problem. The problem is relative humidity, during July I had min 65% Rh, max 99.9% Rh and an average of 95 % Rh. We have lately had some problems with mold. We store archeological test object’s and samples in the cooling room.
This week I tested to put a dehumidifier in the cooling room and the Rh went down to around 50 % Rh. I then tock out the dehumidifier and when I check my data log I can see that the Rh slowly creeps up to above 95 %Rh in a couple of hours. The cooling room has no ventilation; the door is opened and rapidly closed maybe four five times a day.

What I don’t get is how/why the Rh increases after I have been running my dehumidifier down to 50 % RH when I don’t have any air exchange to the room??

Is it normal to have so high Rh in a cooling room and no ventilation?

Regards

Lars Ekström

[r.bartlett]

Yes perfectly normal you need to look at a psychometric chart . We have exactly the same issue and are about to install a desiccant dehumidifier to bring Rh back to 50-60%

[Brian_UK]

Every time the door is opened outside air is admitted.

Also any personnel in the room are breathing beings and as such generate moisture, so short of fitting a controlled atmosphere air lock and breathing apparatus your RH will always rise.

[alex1081]

Hi Lars,
the humidity cntrol in the room with low temperature is very complicated.
You should know these parameters before to start for troubleshooting:
- Do you have CW unit or compressors. If compressors how many steps ?
- the setpoint must be in the room 9°C and 50%RH ? Which tollerance +- °C and RH .
- what is the temp and RH in the room where you are opening the door ?
- Have you fresh air taking from outside ?
- could you make a simple drawing of the room with hight an wide with position of the air cond, door and where they are testing the archeological things.
- What is the thermal load inside the room and the minimum cooling capacity of the air conditioning ?
- Do you have no ventilation inside the room? The air conditioning is not flowing all the time the air from suction to delivery ?
- Do you have heaters inside the air conditioning ? If yes what is the capacity in kw ?

The deumidifier that you installed inside is increasing the thermal load because normally they are compact and they have the evaporator and condenser in the same box.
Did you try to put only a simple heater with air flow , at least 2kw, and check how is changing the RH in the room.
You should have air flow inside the room or you are going to have different C and RH around the room.
Is very complicate to decrease the relative humidity with 8°C room temperature, you can see in the psy chart what is the water quantity at this temp and humidity.

At you disposal for any more info.


Ciao
Alessio O.

[Tesla]

Hi Lars
Humidity control in an archive/archaeological storage requires a larger refrigeration capacity than just temp control along with a lot more complexity. along with this humidity is very hard to measure accurately and expensive to control. You require temporary fix (like your dehumidifier) to reduce mould then a redesign from a company experienced in this type of application. Some sort of dehumidification is required such as a heater or desiccant wheel inline with the refrigeration process. This can be viewed graphically on a psychometric chart. For memory isopropal alcohole mixed with water kills mould best. Light, airflow rate, temp & humdity need to be controlled to minimise the mould. There is a good reference on this in a book Principles of Refrigeration by Roy J.Dossat.

[Lars Ekström]

Hi again!

Thanks for the answers.


So it looks like my options is to make sure that very little warm air gets in the cooling room, maybe see if I can raise the temperature a little to get some margins or install a dehumidifier.

The size of the cooling room is 73 cubic meters (m3).

alex1081, tried to answer you questions below

- Do you have CW unit or compressors. If compressors how many steps ?
We have a LU-VE S3HC with R134A coolant fluid..

- the setpoint must be in the room 9°C and 50%RH ? Which tollerance +- °C and RH .
8 degrees C +-1. Rh so low so we don’t get any problem with mold.

- what is the temp and RH in the room where you are opening the door ?
23 degrees C and Rh around 60% this time of year.

- Have you fresh air taking from outside ?
W don’t have any ventilation at all to the cooling room.

- could you make a simple drawing of the room with hight an wide with position of the air cond, door and where they are testing the archeological things.
Se attached photos.
IMG_0337[1].jpgIMG_0336[1].jpgIMG_0335[1].jpg

- What is the thermal load inside the room and the minimum cooling capacity of the air conditioning ?


- Do you have no ventilation inside the room? The air conditioning is not flowing all the time the air from suction to delivery ?
The fan to the LU-VE is running all the time and circulating the air in the room. And no ventilation.

- Do you have heaters inside the air conditioning ? If yes what is the capacity in kw ?
No heaters. The dehumidifier that I have been testing with is a small movable one.

Iam thankful for all the help i can get.

Regards

Lars Ekström

[chemi-cool]

There are a few important things that will help you keep humidity low.
Slow air flow.
Fins on the evaporator should be about 12 to 16 fins per inch.
Install heating elements in front of the evaporator to keep the unit running, controlled by humidistat.
Do as Richard suggested and add to the room desiccant dehumidifier.

[goshen]

Hi
as chemi advised ,use a special controller for this purpose as dixell offers(xh260) ,this will keep the comp running to dry the air ,and use heating elements to keep your temp at the needed setpoint,i would use proportional heating elements with a cv.
good luck

[alex1081]

Hi Lars,
looking your installation I suggest you, in order to reach the targhet, install a small compact deumidifier with pipe and tubbe for the water , or you need to remove the water from the tank periodically. I suppose you are not removing a llot of water due to room temperature and you can check before in order to understand also the best position for the compact humidifier.
You cooling unit are starting periodically to maintain the room temperature and in this small period, I suppose you have very, very small thermal load, and you cannot remove the condensated drops from the evaporator coil but they are returning in the room when the compressor stops ( sponge effect ).
You should also speak with you customer in order to share the situation and understand exactly wher they would like to have the measuring point of the room temp and humidity.
In this way you can check wich position is the best for the compact deumidifier.
The RH shoud be controlled by the humidostat ( humidity control in the compact dehum ) that is not so precise but could be enought for your installation.
In my opinion in this way you are saving energy , not installing heaters , because the your cooling unit have to work in order to maintain the low temperature in the room and the dehumidification will be the result if the compressor is working at least 10 minutes or you are going to have the sponge effect with very high and low peak , that you can see if you have a paper recorder pf T and RH parameters.
This problem is standard in the metering rooms where you don't have thermal load and you need to have a very accurate humidity also .
Temperature 23 +- 2°C and RH 50 +- 3 %.
In this case they are producing special units with post reating hot gas coil and in this way the compressor is working all the time and the control of the humidity is taken by the Humidifier that are feeding the water that you are removing by the compressor.
But this is anoter story with anoter more complicated setting during the start up that must be done with real and final thermal room condition.

I hope to be done helpful for your problem.
Please let me know if you need some more info.


Best Regards and Ciao
Alessio O.

[Lars Ekström]

Thanks for all the answers.

alex1081
This sponge effect sounds interesting and I think that is our problem. Are there any other solutions then to put a dehumidifier in the cooling room to solve sponge effect.

And again thanks for all the answers.

I have many more interesting question this autumn ex problems with a freeze dryer.


Regards

Lars

[r.bartlett]

Air lock entry systems will help to keep ingress but are an expensive option

[alex1081]

Hi Lars,

my main question is :

How many time in an hour the compressor start and for how long time is running ?
If is starting at least 3 times and is running for 10 minutes each you are able to remove some water physically from the fins of the evaporator coil due to gravity of the water drops or you are only removing some temporary humidity and release it when the compressor stops.

If the cooling capacity of your Lu Ve S3HC , from 4 to 27 Kw looking in the catalog, is too big for the room condition you cannot remove water from the air.

Thermal load in the room are lamps , lu ve fans and some humans .
Am I right ?

In the specific of the Lu Ve unit, for cold rooms, is written that they have a specific plus " reduced dehumidification " that is good for cold room but not for your application.

From the other friends of the web site you have received also good suggestions:
- Reduce air flow
- change evaporator coil with another one with more fins per inch
- install air look entry system
- install heaters in front, on the exit of the air, of the evaporator with or without proportional heating
- install a compact dehumidifier

They are correct , but you have to think that you are going to modify the units" loosing the warranty ", increasing the final cost of the solution and energy consumption.

From my point of view, if you are installing , in the same position of the evaporator, and using the same condensed water drain collector, a compact and very low power consumption dehumidifier you are solving the problem.

Starting from the concept that they are not requesting very good control of the humidity but accepting very large range but not the mold.

Ciao and Best Regards
Alessio O.

[Lars Ekström]

Hi again and thanks for the answers.

I have been looking at some charts and the compressor starts 9-10 times every 24 hours and runs for about 35-40 minutes. So it seams that the LU-VE are a little to good. Last week I did some tests and every time the compressor starts the rh drops to 65% and when the compressor stops the Rh slowly climbs upp to 99%.

As you say a small dehumidifier will be my best option.

Thanks for the help.

Regards

Lars

[Lars Ekström]

Thermal load in the room are lamps , lu ve fans and some humans .
Am I right ?

Yes you are right.

Did you mean 3 times every hour?

[alex1081]

Hi Lars, if your compressor is starting 9-10 times a day and totally is working for 35 40 minutes, the thermal loas is not enought in order to dehumidifies with Lu Ve unit.
You can solve your problem only with compact dehumidifier.

Ciao and Best Regards
Alessio

[r.bartlett]

This is what we are about to install in a clients cold room

http://www.eipl.co.uk/desiccant/DD400.html

[Psychro]

This is what we are about to install in a clients cold room

http://www.eipl.co.uk/desiccant/DD400.html

Whatever desiccant unit is used you need to make sure the model is not designed to work on a total loss system (i.e. no recirculation). With many small desiccant units there is only one fan in the unit which brings in both the process and react air. The wet air leaves the system, which in a small archive room, is likely to create a negative pressure (additional moisture load). You require a desiccant unit with two fans so the process (recirculated room air) and react air/wet air (fresh air) are separated (a balanced system). - Regards Psychro

[r.bartlett]

Whatever desiccant unit is used you need to make sure the model is not designed to work on a total loss system (i.e. no recirculation). With many small desiccant units there is only one fan in the unit which brings in both the process and react air. The wet air leaves the system, which in a small archive room, is likely to create a negative pressure (additional moisture load). You require a desiccant unit with two fans so the process (recirculated room air) and react air/wet air (fresh air) are separated (a balanced system). - Regards Psychro

The unit we have selected will be sited outside the coldroom.

It'sa two fan unit

It will draw air from the cold room dehum it and return it back into the room.

the process air is to be drawn from the warehouse and warm/wet vented outside..

[Psychro]

The unit we have selected will be sited outside the coldroom.

It'sa two fan unit

It will draw air from the cold room dehum it and return it back into the room.

the process air is to be drawn from the warehouse and warm/wet vented outside..

Sounds perfect.

You'd be surprised how often consultants/contractors design/install desiccant units in the cold store (at -25°C) and wonder why the rotor reactivation sector freezes up.

[Lars Ekström]

Hi Lars, if your compressor is starting 9-10 times a day and totally is working for 35 40 minutes, the thermal loas is not enought in order to dehumidifies with Lu Ve unit.
You can solve your problem only with compact dehumidifier.

Ciao and Best Regards
Alessio

What i can se it runs 9-10 times ever 24 h and 35-40 minutes each time. so in total 6-7 hours every 24 hours.

I have been trying to add a diagram but cant get the file small enough.

Regards

Lars