Maintaining humidity at low level (10-20%) at two different temperatures (32 F – 86F)
Zubaidi
| Posted in General Questions on
Hi ,
I am conducting a lab experiment in which I need the Relative humidity to stay as low as (15%) but without changing or interfering in the atmospheric temperature of the lab room.
The conditions of the experiment as follows:
Lab room Temperature Corresponding Relative Humidity
32 F 15%
86 F 15%
What kind of Dehumidifier I need to get ? (Lab Area : 1100 ft sqr)
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Replies
Any dehumidifier will raise the temperature in the room. Most will also not operate a temperatures near freezing. I think you’ll need something closer to an environmental chamber here and not an off the shelf dehumidifier.
Bill
What if I used a dehumidifier at the beggingng to reach a 15 % relative humidity, then I start cooling the environment usin AC to reach to Zero ? An AC does not interfere in changing the moisture content in the lab room, does it ?
Deleted
Look for some kind of desiccant dryer with post cooling.
So Does a desiccant dryer with post cooling will keep the relative humidity as low as 15% when changinging the lab room temperature ?
Yes, it can.
Since air conditioners produce air at close to 100% humidity to reach your goals, I am guessing it will be necessary to cool the air down to about 0°F and then reheat the air to your desired temp.
I think commercial test chambers cost about 10,000 a cubic foot. So if your 1100 sqr lab has a 8 foot ceiling 1100x8x10,000=$88,000,000.00
My guess is you will end up paying an engineering firm to design and build your custom system at a cost million dollars and cost thousands of dollars a month to operate.
Walta
What about a test chamber intended for this purpose? Something like https://www.associatedenvironmentalsystems.com/chambers/walk-in
Thank you guys, The test chamber is an impossible alternative ( no way can be used due to insufficient funds).
Like , does it work if i use the dehumidifier to reach to a Relative humidity of 15%, then change the lab atmospheric temperature, without the need to add moisture in the air. Like is there any apparatus that is capable of changing the temperature in the room withouth 1- adding moisture in the air and 2- without pumping outside air into lab room ?
Zubaidi,
Any electric heater can change the temperature of a room without adding or reducing the amount of moisture in the air, but by definition it will be changing the relative humidity, because the humidity is measured relative to the temperature of the room.
The only way you can keep the same relative humidity as you change the temperature is by adding or removing moisture.
Malcolm,
So Do you suggest trial and error calibration by using both (Electric heater and a dehumidifier ) in order to maintain the 15% Relative humidity when changing temperature to the desired values ?
Thank you for your help
32F at 15% RH is EXTREMLY dry air. About the only way to get there is with desiccant dryer. The dew point there is -10F, which you can't easily make with any refrigeration device since it will freeze up.
If you can reduce the volume, you can get pretty dry air by using an air compressor (compressing air removes moisture) followed by a dessicant dryer.
Poor man's temperature chamber is a chest freezer+space heater with an external thermostat.
Two air feeds to the chest freezer, one after the dryer, one before the dryer. Mix the two to maintain the humidity you want. PIA, but doable with manual control.
Moving the experiment to some place that is naturally cold and dry goes a long way toward making this possible. Consider the the Desert Mountains of Nevada. Most anyplace near one of the poles likely to give you a few weeks were you could heat the air and end up at 32F at 15% RH
Walta
I live in Iowa, we have the worst winters ever lol , but avg humidity around 60 %
Probably the easiest thing to do is this:
1- completely seal the room. Go against all the usual recommendations here and completely line the room with poly sheeting, sealing all the gaps with caulk between the sheets. Be especially carefully to seal the door with good weatherstripping, maybe even doubling up on both sides for a double seal.
2- put some electric resistance heating in the room. This can just be a few space heaters. More heaters means you can raise the temperature in less time (higher delta T, faster rise time), and you can overcome more thermal losses so you can achieve higher temperatures. Adding some insulation to the room will allow higher temperatures with less input energy, if needed.
3- use a dehumidifier to dry the space as much as you can.
4- use a desiccant like silca gel and a fan to get down to the final humidity level
5- I’m not sure you can get cold enough with a normal air conditioner to get to the final low temperature setpoint you want. This one is a maybe.
Now use the resistance heat and the air conditioner to move the temperature up or down. Relative humidity is the water carrying capacity of the air AT A GIVEN TEMPERATURE so it will go up and down as you change the temperature. As you raise the temperate, RH will drop in a sealed space. Lower the temperature and RH will rise. Since the room is complete air AND vapor sealed (remember all that poly sheeting you just installed), the amount of water vapor in the enclosed air volume is fixed.
The bigger the space, the more complex the system becomes. I’ve worked in some places with drive-in environment chambers (they tested entire vehicles), and they often had multistage refrigeration systems. These were special labs. Note that these labs rent out their facilities, so you might just see if you can rent a real environmental chamber for the duration of your experiment.
Bill
Thank you Bill, I will try follow your guidlines.
Is there a lower limit to the RH, or is sub-10% OK?
I’d say 10% is probably a low limit in a practical sense. Most humidity sensors are unreliable below 5-10%, and they also tend to get very non-linear down that low.
Bill
This is not a simple design problem. You really need to understand how to read a psychrometric chart and what it means to understand the design issues. You have not mentioned whether or how often you need to cycle between the two temperatures. This will make a huge difference in the equipment required to meet the conditions. Also, whether any airflow is required - do you have any processes, equipment, etc that generates any sorts of gases or toxins (or heat) that have to be removed? Airflow increases the power demands of all of the equipment.
As a simple summary, "Relative humidity" is a measure of how much moisture the air holds compared to how much moisture it can hold without condensation. Air at 32F cannot hold as much moisture as air at 86F. If you take a sealed container with a mixture of water vapor and air at one temperature and then raise its temperature, the relative humidity will go down, but the absolute humidity remains the same.
Psychrometric charts show you both the relative and absolute humidity. Absolute humidity is the actual percentage of water by weight in the air (generally grains of moisture per pound of air), and it remains constant as the temperature changes. As the temperature in your lab air changes, you will have to add or remove measured amounts of moisture to maintain the same relative humidity. FWIW, the dewpoint of 86F air at 15% RH is very close to 32F. This means that, if you start with your "warm" lab conditions, and cool the air to 32F, the relative humidity will go up to 100% and your cooling coils will ice up. So, you will have to dehumidify faster than you cool.
The psychrometric chart also gives the enthalpy of the air at each condition of temperature and humidity. That tells you how much energy it is going to take to move from one condition to the other and it will allow you to size your equipment.
You might find this article from the folks at Desert Air to be helpful:
https://www.desert-aire.com/resources/application-notes/dehumidification-and-psychrometric-chart
In fact, you might want to give them a call to see if they have any equipment that could work in these conditions, or can point you in the right direction. There are definitely industrial chillers and dehumidification equipment that can do the trick, but not on a shoestring budget.
Are you able to share anything about what you are testing? It will likely make more sense to control this experiment in dew point or parts per million h2o. Make sure you fully understand the psychometric chart: https://www.engineeringtoolbox.com/docs/documents/816/psychrometric_chart_29inHg.pdf
What is the minimum amount of space you need?
What sort of control accuracy and what measurement accuracy do you need?
How long do you need to maintain the specified conditions? How much variation is allowed during that time period? How quickly do you need to transition from one condition to the other?
What is your budget?
We would need the answers to these questions to suggest an appropriate solution.
No standard dehumidifier is going to to do 32f 15%rh. Mid 30’sf is as low as you can go under careful operation before it plugs with ice.
A freezer coil will in combination with an electric heater will get you there as long as there isn’t excessive initial moisture in the room. Desiccant dryers will also work.
I think what Dana is getting at is something like you pull the water content of the air down super low with a bunch of desiccant or molecular seive. Work inside a chest freezer with a space heater inside. There are plenty of industrial humidity sensors that will work. Vaisala is a good brand to look for on eBay. You won’t have good control over the humidity, but it will be low and you will know what it it is.
Hi Matt ,
I am testing the impact of significant variation in atmospheric temperature on the hydrologic performance of multiple types of permeable pavements (drainage, detention, evaporation ) within lab controlled environment, I am simulating rainfall at different return periods and evaluate the performance of these permeable pavements under different temperature conditions. The reason why I need the relative humidity to be as low as possible is to encourage evaporation. this is why I am fixing RH at two different temperatures
Where are you located, and how long do you need to operate the facility at these conditions? As suggested above, you can let Mother Nature help you out. If your outdoor wintertime temperatures are cold enough, you will only have to warm the air to your 32F test conditions, and the humidity will probably already be close to your requirements. A small amount of dessicant drying may be all that is necessary. Then you would have to add heat and moisture to reach your high temperature conditions. Going back down would be more a process of ventilation than cooling/dehumidifying.
I am testing the impact of significant variation in atmospheric temperature on the hydrologic performance of multiple types of permeable pavements (drainage, detention, evaporation ) within lab controlled environment, I am simulating rainfall at different return periods and evaluate the performance of these permeable pavements under different temperature conditions. The reason why I need the relative humidity to be as low as possible is to encourage evaporation. this is why I am fixing RH at two different temperatures. I will be running the experiment at different rainfall intensities. experiments will be conducted at two stages : 32 F stage and 86 F stage, ( simulating rainfall events once a week for 3 months for each stage )
I think you’ll find your simulated rainfall is likely to overwhelm your dehumidification system. You’re going to need a pretty large environmental chamber to keep the conditions you want stable. You really are probably best off renting a large environmental chamber for your experiment.
Bill
Is the goal to just remove all the water from the pavement prior to next rain event?
You likely won’t be able to achieve your RH specs until all the water is gone. Your choice of dehumidification equipment will just determine how long it takes to dry out. “Wind” will significantly improve drying rates.
Ya exactly, I will be using exhaust fans to and indoor fans to accelerate the process of drying
Remember that all that drying is doing is evaporating the water off the surface and into the air, which will raise the humidity level in the room. Your dehumidification equipment then needs to remove that moisture from the room. Every drop of water you put in as simulated rain has to be removed by your dehumidification equipment.
Desiccant dryers have a finite capacity to remove moisture, so they’ll reach saturation and stop working. You can usually regenerate desiccants in an oven though — silica gel is especially easy to work with in that regard.
Bill
You might find that you can rent space from a local manufacturer that has an environmental test chamber. Auto manufacturers have these, as do companies in a variety of other industries.