Dew point and humidity dilema, can you solve it?

[Nick Phoenix]

Greetings learned person!

 

I bring to you a problem of condensation. We have a 5000 sq ft basement with a tiled concrete floor. Next we introduce 25-34 people and have them enjoy the sport of table tennis for 4-5 hours. It is Spring and the relative humidity is high. The players perspire. Humidity increases in the basement. There are two bucket collecting dehumidifiers running non stop, and one small airconditioner running.

After 2-3 hours the floor begins to draw moisture out of the air. The condensation makes the floor slick, and players cannot play. The condensation starts where the floor meets the exterior walls and slowly moves toward the center of the room.

 

How do we stop the condensation? The answer must be valid for sunny and rainy days.

 

You may use any text book, or mathematical formula to come to your conclusion. The most effective solution presented will have the eternal gratitude of the Phoenixville Table Tennis Club!

 

Thank you, you may begin.

[John Cuthber]

Get lots of ventilation. With enough air flowing through the place you won't get condensation.

Alternatively, get more dehumidifiers, but they will be more expensive to purchase and run.

[insane_alien]

john said it, either have lots of air flow or get more dehumidifiers.

 

if you have windows on both sides of the base ment then you should set up fans blowing inwards on one side and fans blowing outwards on the other to create a good cross flow of air. if the humidity outside is 100% then there isn't much you can really do appart from heating up the floor but that is unlikely to be a popular move with the players.

 

although, an altogether easier way of dealing with it is to have a mat on the floor where the players are so they can have some traction.

[Mr Skeptic]

The problem is that you have a cold surface that reduces the temperature of the air; when the temperature of the air drops the amount of water it can hold (but not the amount of water it does hold) drops. If the humidity rises to above 100% then condensation results. The two solutions are to warm up the basement surfaces (such as by sufficient airflow) which will prevent the air from being cooled, or to reduce the humidity of the air (using a dehumidifier) to prevent it from passing 100% humidity when it does get cooled.

[Tacobell]

This is a basic thermodynamic problem.

 

H20(g)-H20(l)

 

 

if deltaG<1 then condensation happens.

 

In condensation, delta S is negative and delta H is negative.

 

You want to make deltaG>1, so you have to increase T.

[insane_alien]

or you can take away the humidity by having adequate ventilation.

[Nick Phoenix]

This is a basic thermodynamic problem.

 

H20(g)-H20(l)

 

 

if deltaG<1 then condensation happens.

 

In condensation, delta S is negative and delta H is negative.

 

You want to make deltaG>1, so you have to increase T.

 

Where can I find the definition of the variables?

[insane_alien]

delta g is the change in gibbs free energy

delta h is the change in enthalpy

delta s is the change in entropy

T is the absolute temperature.

 

the equation won't actually help you as it says nothing of external conditions, internal conditions or the amount of sweat evapourating from 30 odd table tennis players.

[Nick Phoenix]

So as I see it, I will ventilate with fans at ceiling level, since that is where the windows are.

 

But, what if it is raining or there is high relative humidity? If the floor is colder than the dew point, and I ventilate when it is raining, will the floor get slippery sooner?

 

Are there markers in between relative humidity, air temperature, dew point, and floor temperature that I can watch for to determine if I should or should not ventilate?

 

For argument's sake:

Air Temp is 78 degrees F,

dew point is 62 degrees F,

floor temp is 54 degrees F,

If relative humidity is < X ventilate, if it is > X, do not ventilate.

Is this a correct way of looking at the situation?

 

I also assume that if the floor temperature > dew point, then I will not get condensation. With air temperature remaining constant, regardless of relative humidity, uless we reach 100%. Is this also correct?

[insane_alien]

the dewpoint will get closer to the air temperature as the humidity rises, these are not independant variables as you seem to have assumed.

 

due to the fact that this basement is full of hot sweaty people jumping around the humidity inside will nearly always(except in the most extreme of conditions) be greater than that outside so ventilation should always help.

 

if you are in humid environment, then an investment in some mats for the players to play on would be your best bet. its cheaper than the air conditioning you'd need.

[Mr Skeptic]

So long as the humidity is anywhere below 100% and you ventilate enough, there will be no overall condensation. The ventilation would raise the temperature of the walls to that of the outside, and at that point there will be no more condensation (since the walls are not colder) and any water on the walls will evaporate (since humidity is less than 100%). The closer to 100% humidity the more ventilation will be necessary.

 

There will be no condensation if the outdoor air is cooler than the basement walls. Which brings me to a third option I forgot about: air conditioning the basement. So long as the influx air is cooled more than the walls, it cannot cool and cause condensation at the walls.

 

As for the dew point, it depends on the humidity in the air. If you measure humidity in relative humidity, then the dew point depends on both the temperature and humidity in the air. The reason the dew point matters is that if the walls are above the dew point, there will be no condensation on them so neither ventilation nor dehumidifier would be necessary. (as insane_alien said, however, sweaty athletes would raise the local humidity significantly).

 

I think the best choice is dehumidifier. It would probably be more expensive than ventilation, but cool dry air is fun for athletes.

[J.C.MacSwell]

You need to heat the room considerably before the event to raise the temperature of the walls and floor then ventilate the room during the event.

 

If the surface temperature of the walls/floor are below the temperature of the dew point they will "sweat".

 

Adequate insulation can help as well, and of course lowering the dew point or RH of the ventilation air.

[Nick Phoenix]

Here are the changes we can make.

-We can add one more dehumidifier (we have 2 already)

-We can place fans on the floors and fans in the windows

 

That's about it.

 

Last night we tried fans along the walls only, facing opposite directions. Condensation did not occur till about 9:15, we opened at 7.

 

I would like to try venting using a fan in one window pointing out and another pointing in. I am fearful that this could cause the condensation to occur right off the bat, I was hoping to find some kind of measurable marker to figure out if that would happen. If it did, the entire night is lost.

The one dehumidifier has a meter on it. It said that when we started, humidity is about 80%, it brought it down to 63% before anyone arrived, and it read 69% when the condensation started to occur. (this unit is in a corner however, if that affects measurements)

[Mr Skeptic]

Last night we tried fans along the walls only, facing opposite directions. Condensation did not occur till about 9:15, we opened at 7.

 

Do you open in the morning or evening?

[Nick Phoenix]

We open in the evenings, however, it is also a borrowed facility. We do not have any control over it during the day.

[Nick Phoenix]

Greetings learned person!

 

 

I bring to you a problem of condensation. We have a 5000 sq ft basement with a tiled concrete floor. Next we introduce 25-34 people and have them enjoy the sport of table tennis for 4-5 hours. It is Spring and the relative humidity is high. The players perspire. Humidity increases in the basement. There are two bucket collecting dehumidifiers running non stop, and one small airconditioner running.

 

After 2-3 hours the floor begins to draw moisture out of the air. The condensation makes the floor slick, and players cannot play. The condensation starts where the floor meets the exterior walls and slowly moves toward the center of the room.

 

 

How do we stop the condensation? The answer must be valid for sunny and rainy days.

 

 

You may use any text book, or mathematical formula to come to your conclusion. The most effective solution presented will have the eternal gratitude of the Phoenixville Table Tennis Club!

 

 

Thank you, you may begin.

 

It's that time a year again. I thought I'd get a jump on the condensation problem.

Here's what I've got:

On rainy days, just run 2 dehumidifiers.

But which do you think would work better on clear days? Run dehumidifiers or ventilate with 2 fans and open windows?

[lemur]

It's that time a year again. I thought I'd get a jump on the condensation problem.

Here's what I've got:

On rainy days, just run 2 dehumidifiers.

But which do you think would work better on clear days? Run dehumidifiers or ventilate with 2 fans and open windows?

Fans simplify everything. They cool and dehumidify purely by air-movement.

[John Cuthber]

Fans simplify everything. They cool and dehumidify purely by air-movement.

Except that they heat the air slightly and don't remove humidity.

[Schrödinger's hat]

One option that no-one has mentioned so far is heat. Get the room (especially the surfaces, but hot air will do this indirectly) warm enough and the water will not condense. This is generally the approach used in cold areas in which I've lived for things like stopping condensation on car windows in the absence of air conditioning.

A combination of hot air coming in and ventilation will also carry water out of the area more efficiently, although this is hardly energy efficient and if your country does not have an excess of hydroelectric/other renewable power then it is hard to justify. Fan heaters pulling air into the room and fans blowing the air out works very well.

Additionally this does not make the area as unpleasant for sport/etc as one would think. The lower relative humidity levels make sweating more effective as well as making it less muggy.

[lemur]

Fan heaters pulling air into the room and fans blowing the air out works very well.

Additionally this does not make the area as unpleasant for sport/etc as one would think. The lower relative humidity levels make sweating more effective as well as making it less muggy.

It sounds so counter-intuitive to use heat when it's hot to lower the relative humidity, but it does make sense that sweating would be more effect and your skin would probably have a higher "net dryness" than without the heating and air movement. Air condensation and sensitization to heat are two aspects of air conditioning I don't like. If indoor areas were slightly warmer than outdoor areas when it's warm, it would actually feel nice to go outside relative to the indoor temperature. I wonder if the hot air blown in would absorb humidity and then carry it out if you had high ceilings with good ventilation at the top? Does damp warm air rise as easily/fast as dry warm air?

[John Cuthber]

"It sounds so counter-intuitive to use heat when it's hot to lower the relative humidity"

Heating air will always lower the RH.

What's counter intuitive about it?

 

"It sounds so counter-intuitive to use heat when it's hot to lower the relative humidity"

Heating air will always lower the RH.

What's counter intuitive about it?

[Zorchalate]

I think that what's counter-intuitive about it is this:

Increasing the temperature DOES lower the "Relative" Humidity but. not the "Absolute Humidity". The "absolute humidity" is the amount of moisture in the air (usually measured in grams per cubic meter or something like that).

The "relative humidity" (which is what they mean when they talk about "humidity") equals: "(amount of moisture in the air)/(amount of moisture that the air could hold at the current temperature)". The higher the temperature, the more moisture a cubic meter of air can hold (it doubles for about every 20 degree farenheit increase). Hence "raising the temperature" will NOT change the "absolute moisture content" (the numerator in the equation for relative humidity) but it WILL increase the "amount of moisture content that there CAN be without saturation or condensation" (the denominator in the equation). As with ANY fraction, a bigger denominator means a smaller fraction (assuming the numerator to be the same). Therefore, the "relative humidity" upon increasing air that is already "hot" WILL go down. The "dilemma" is that you would think that if the relative humidity goes down, it will FEEL cooler. That is true IF you keep the temperature the same.

If you "raise the temperature" thereby "lowering the relative humidity", you will get two factors working against each other. The "increased temperature" will make it feel hotter while the "decreased relative humidity" will make it feel "cooler". I think that YOUR question is: Which will win out?? Will the increased feeling of hotness from the temperature outweigh the decreased feeling of hotness from the lower relative humidity (thereby making an overall effect of feeling hotter)?? Or will the decreased feeling of hotness from "lower relative humidity" be greater (thereby making an overall effect of feeling cooler)??

I have looked at charts where the absolute moisture content in the air is held constant and seen what the relative humidity is at various temperatures. Then I've looked at the Heat Index given by certain temperatures and certain relative humidity. Whenever the same amount of water vapor is in the air and thereby the relative humidity is strictly decreased by raising the temperature, the "hotter feeling" from the increased temperature has ALWAYS WON OUT OVER the "cooler feeling" from the decreased relative humidity.

And now that I'm thinking about it, one could look at it like this: In the course of a day, the "actual

Moisture content" usually does not change much if at all. What usually changes is the temperature. This being the case, the relative humidity (RH) is at it's LOWEST when the temperature is HIGHEST (usually about 2 or 3 in the afternoon) and at it's highest when the temperature is LOWEST. Yet we always feel hotter at the time of the highest temperature/lowest relative humidity than we do at the time of the lowest temperature/highest relative humidity.

This, of course, is assuming that the only thing that's changing is the temperature. When the "moisture content changes", that's another can of worms.

But I think that YOUR "dilemma" or confusion was that you wondered if increasing the temperature might make it feel cooler because it would decrease the realitive humidity and from my experience, I believe that I can say an emphatic NO.

 

Matt

[Nick Phoenix]

I think that what's counter-intuitive about it is this:

Increasing the temperature DOES lower the "Relative" Humidity but. not the "Absolute Humidity". The "absolute humidity" is the amount of moisture in the air (usually measured in grams per cubic meter or something like that).

The "relative humidity" (which is what they mean when they talk about "humidity") equals: "(amount of moisture in the air)/(amount of moisture that the air could hold at the current temperature)". The higher the temperature, the more moisture a cubic meter of air can hold (it doubles for about every 20 degree farenheit increase). Hence "raising the temperature" will NOT change the "absolute moisture content" (the numerator in the equation for relative humidity) but it WILL increase the "amount of moisture content that there CAN be without saturation or condensation" (the denominator in the equation). As with ANY fraction, a bigger denominator means a smaller fraction (assuming the numerator to be the same). Therefore, the "relative humidity" upon increasing air that is already "hot" WILL go down. The "dilemma" is that you would think that if the relative humidity goes down, it will FEEL cooler. That is true IF you keep the temperature the same.

If you "raise the temperature" thereby "lowering the relative humidity", you will get two factors working against each other. The "increased temperature" will make it feel hotter while the "decreased relative humidity" will make it feel "cooler". I think that YOUR question is: Which will win out?? Will the increased feeling of hotness from the temperature outweigh the decreased feeling of hotness from the lower relative humidity (thereby making an overall effect of feeling hotter)?? Or will the decreased feeling of hotness from "lower relative humidity" be greater (thereby making an overall effect of feeling cooler)??

I have looked at charts where the absolute moisture content in the air is held constant and seen what the relative humidity is at various temperatures. Then I've looked at the Heat Index given by certain temperatures and certain relative humidity. Whenever the same amount of water vapor is in the air and thereby the relative humidity is strictly decreased by raising the temperature, the "hotter feeling" from the increased temperature has ALWAYS WON OUT OVER the "cooler feeling" from the decreased relative humidity.

And now that I'm thinking about it, one could look at it like this: In the course of a day, the "actual

Moisture content" usually does not change much if at all. What usually changes is the temperature. This being the case, the relative humidity (RH) is at it's LOWEST when the temperature is HIGHEST (usually about 2 or 3 in the afternoon) and at it's highest when the temperature is LOWEST. Yet we always feel hotter at the time of the highest temperature/lowest relative humidity than we do at the time of the lowest temperature/highest relative humidity.

This, of course, is assuming that the only thing that's changing is the temperature. When the "moisture content changes", that's another can of worms.

But I think that YOUR "dilemma" or confusion was that you wondered if increasing the temperature might make it feel cooler because it would decrease the realitive humidity and from my experience, I believe that I can say an emphatic NO.

 

Matt

 

MY "Dilemma" is I just want the water to stop condensing on the floor after 2 hours of play.

Which happened yesterday, May 24th. We started at 6pm in Phoenixville, PA 19460. 8:30pm the floors started getting visibly slick. Someone did fall. Room temperature and humidity did increase during that time. We did not open windows, just used 2 dehumidifiers and an AC. The dehumidifiers were pretty full by the end of the night, 10pm.

Next week we use the fans and ventolate again. (do I do that even if it is raining?)

[lemur]

Yet we always feel hotter at the time of the highest temperature/lowest relative humidity than we do at the time of the lowest temperature/highest relative humidity.

Isn't that because lower temperature water in the air absorbs more heat from your skin than dry air at the same temperature? Moist hot air just makes it harder for sweat to evaporate from your skin, which makes you feel hotter because you seem to sweat more because the sweat isn't drying.

 

 

MY "Dilemma" is I just want the water to stop condensing on the floor after 2 hours of play.

If it is raining and I'm in a car, I usually use the heater to dry the inside of the windshield and use windshieldwipers to remove condensation from the outside. If you would run the heater, the floor might stay dryer but you might not be happy with the air temperature, idk. Just a thought.

[Nick Phoenix]

THE FLOOD!

 

May 31st, 2011.

93 F

 

We got to the club at 3pm, opened 3 windows. Two fans were exhaust, one fan was intake.

5pm, condensation begins in the corners.

6pm, condensation is spreading

7pm, visible condensation all over the floor

8pm, this is the most ridiculous scene ever! Water everywhere.

 

So to all those interested. Ventilation is NOT the answer. Maybe if you had a windtunnel sized fan.

 

Next week we try 1 AC unit and 3 dehumidifiers.