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Keeping Cool: Newsletter Challenge (June 2012)

Posted June 01, 2012 12:00 AM
Pathfinder Tags: challenge questions humidity

This month's Challenge Question: Specs & Techs from GlobalSpec:

To be comfortable in an air conditioned room, it is imperative to dehumidify the room. Why?

And the answer is:

Warm air contains water vapor. When this air is cooled the water vapor condenses into tiny water drops, because at a lower temperature the water molecules move slower so they combine easily to form tiny droplets. This process makes the air inside the room to be humid and low temperature humid air is not comfortable to the skin.

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#1

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 2:43 AM

High humidity in the heat will decrease the effectiveness of losing heat through sweating, Cooling a room decreases the amount of moisture the air can hold, so it will condense into water.

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#2

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 8:08 AM

As the air is cooled, the molecules become more compact, raising the relative humidity, the ideal recommended comfort level is ~50%, so as the air is cooled, moisture must be removed to keep the humidity level in a comfortable range usually between 40-60%...

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#3

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 9:26 AM

Evaporative Cooling used in many places in fact increases the room's humidity. Okay so this type air conditioning is used where the humidity is all ready pretty low. But as you see you do not have to dehumidify a room to be comfortable.

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#4

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 12:14 PM

I see the question, but where's the challenge?

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#5
In reply to #4

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 1:10 PM

Maybe the challenge is to determine what the challenge is? I too thought about evaporative cooling, and how the statement could remain true in this instance....

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#7
In reply to #5

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 3:10 PM

STP. Standard Temperature & Pressure. That is all you need (besides the correct mix of oxygen) to be comfortable.

Human beings primarily rely on sensible perspiration (eccrine sweat glands) to regulate the body core temperature in conjunction with blood vessels near the skin surface.

Previous posts have nailed it with the fact that relative humidity is directly related to air temperature (higher temperatures can hold more water vapor).

At about 70°F (20°C) a humidity of 50% allows for just enough cooling by evaporation to take place to keep the body temperature stable without being too cool and dry or too hot and clammy.

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#6
In reply to #4

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 2:15 PM

Maybe the challenge is poking holes in the logic of the statement.

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#8

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/01/2012 11:41 PM

With lower humidity, it takes less cooling to be comfortable than with high humidity. It's not imperative, though. Swamp coolers add humidity but work well in areas that have low humidity outside. Comfort is a matter of preference. My son has complained the humidity is too low, but I prefer central air conditioning which dehumidifies.

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#9

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/02/2012 7:30 AM

Yes, it is correct that we need drier air to feel comfortable and not have pools of sweat forming on our skin.

I live in a tropical climate and know all about humidity and feeling uncomfortable.

This subject raises an interesting secondary issue about sizing of an air conditioner. A lot of people around here just think that more BTU´s are better however this is wrong. I installed a UPS at an office and the air conditioner they installed was too big. The room was really cold (uncomfortably so) but HUMID and I showed them this with a relative humidity meter which recorded 86%. Since the air conditioner was oversized the compressor did not run long enough to dry the air. The unit was changed out for a smaller one and now the room is at 45% with 25°C which is perfect for the UPS and its batteries.

There are good, on line guides for sizing an air conditioner and it will surprise you when you discover you dont need as many BTU´s as you might have imagined.

Regards

John

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#10

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/02/2012 12:41 PM

The key word here is comfortable. Come on down to South Texas with an air temperature of 98 F and a dew point of 75 F. Sit in front of a "swamp cooler" like we used to have, and keep wringing the water out of your shirt that comes from that saturated 75 F air coming from the "swamp cooler". If the dew point were 20 F, it would feel like Arizona. Note that a "swamp cooler" will only chill down to the dew point, unless you are chilling the water.

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#11

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/02/2012 2:59 PM

No the feed supply /air in to room will be lower in humidity by the cooling process. If you do run a de-hum it will or can frezee up the coil and hurt or damage compressor.

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#13
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Re: Keeping Cool: Newsletter Challenge (June 2012)

06/02/2012 5:52 PM

No the feed supply /air in to room will be lower in humidity by the cooling process.

Let me be sure I understand this. If I take a mass of air at a given temperature (say 90 F) and a given humidity (say 45%) and chill it by 20 degrees, the resulting Relative Humidity will be less than 45%? "Fascinating", he says with cocked eyebrow and distinct Vulcan accent.

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#14
In reply to #13

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/02/2012 7:14 PM

I think he is thinking of the room having an external supply of conditioned air, and then running a dehumidifier as well. With insufficient humidity to warm the dehumidifier coils, it can freeze up.

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#12

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/02/2012 5:01 PM

The human body cools itself by the evaporation of sweat. If the relative humidity is high, our sweat doesn't evaporate and we become wringing wet. This is true regardless of temperature at which we need to cool ourselves. This has been implied in the previous posts but not quite stated.

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#15

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/04/2012 12:43 PM

Comfort RH is a subjective variable. recommended level ( ~50% or 40 - 60%) is a statistical value based on surveys. RH is very high where I live, both summer and winter. Some people feel it's too dry at the recommended level. Those are also the ones complaining the most about temperature and humidity "schocks" between AC and non-AC ambients. Sometimes they try to get a higher RH with sprays or evaporative gadgets.

There is an economical factor in favor of smaller evaporators operating at lower temperatures. I think having little or no condensation could improve comfort and avoid problems like mold in our climate (never tested it!).

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#17
In reply to #15

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/04/2012 6:35 PM

That's not the whole truth....Humidity below 35% tends to dry out the mucous membranes, lowering the body's ability to fight off bacterial infection....and humidity levels above 65% are conducive to mold and mildew growth....So these levels are maintained for health reasons, the fact that most people are more comfortable at these levels, is just an added bonus....

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#16

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/04/2012 12:49 PM

comfortable, imperative, dehumidify.....

Pulling apart the question shows that it is not an accurate statement. It is a suggestion.

If the statement was "While air conditioning a space (assumes mechanical cooling, not an evaporator) it is imperative for dehumidification to occur. We all know the "Coke Can Effect". Take a Coke can out of the fridge, put it on the counter it attracts the heat and moisture in the air while transfering its its temperature to the spcae around it. This is the same as air conditioning, removing heat from the space. Moisture is removed with that heat durring evaporation and the temperature it occurs at. Now you could keep that moisture in the air by moving the air faster over the evaporator but then you will reduce the efficenicy of the system. Or slow the fan down to pull out more moisture and raising some efficency in the system.

In order to be "comfortable" in a conditioned space one must first consider the location of that space in our world. In a room in Arizona, Nevada, Palm Springs you will more than likely have a humidifier in that space to add moisture to the air for comfort (and so the materials in the room don't die from dry rot)or it will suck the moisture out of you. Texas, Carolina's, Florida you willl want to remove the moisture and the latenet heat that it holds, so that your body can naturaly asperate as needed.

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#18

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/04/2012 9:06 PM

Sorry to hijack the thread, would high humidity have an adverse effect on paint drying (i notice always that the pigment starts to clot together during the summer using tamiya paint)

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#19
In reply to #18

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 6:31 AM

Well, it is a water soluble paint and there is going to be an ideal range of humidity/temperature for that paint to dry.

If it is too high or too low it will impact the surface finish. You could try keeping track of the humidity and temperature when you paint in a logbook and that will tell you what works best. You will need to experiment, but if you do not keep records you will never find the best combination.

Also, some paints have "dryers" you can add to them to counteract the effects of temperature and humidity.

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#25
In reply to #19

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 9:30 PM

thanks for the info, i have no airconditioner so the temp. and humidity in my room, changes with the weather

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#20

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 10:16 AM

Dehumidifying air is a natural occurance when cooling. Air can only hold a certain amount of moisture, and this amount is temperature related. Warm air is able to hold more moisture than cooler air. When the air at room temperature passes over the cooling portion of the air conditioning unit, it tries to cool down, but it has to release some of its' moisture in order to do so. The energy used to cool the air is used up in condensing this water vapor, therefore in the initial stages of cooling, the only thing that happens is the air gives up its' moisture. Once this new equalibrium is met then the cooling energy can now remove heat from the air. That is why humidity is less in the winter than in the summer.

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#21

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 11:30 AM

Cooling alone does not provide comfort,that's why we don't blow air over a block of ice to cool the space.

As some have mentioned, our body's cooling system,is evaporation . If the humidity is high we will evaporate less through the body, and will not be as comfortable as we would be at the same temperature with lower humidity.

The Trane Manual quotes a study done years ago,where people were moved from room to room and asked about comfort level. Don't quote me, but it showed something like, 80% of the people felt the same comfort at 78 degrees with 50% RH as they did at 74 degrees with 60% RH.

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#22

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 1:08 PM

Humidity is relative to temperature. The same amount of moisture will have lower relative humidy at higher tempoeratures. As the air cools, if the moisture is not removed the humidity will increase causing the air to feel damp. As the humidity increases, the dew point also increases. When the temperature reaches the dew point things will become wet.

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#23

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 1:51 PM

The humidity in the air acts as a "wetting agent" allowing heat to dissipate from a person's body. They will then feel "colder" or "hotter", so they will continuously adjust the air conditioning system.

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#24

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 3:21 PM

Assume that the room is at the desired comfortable temperature and humidity. People in the room add latent heat to the room air through breathing and sweating as well as sensible heat from their bodies. Therefore, all other things being equal, it is imperative to dehumidify as well as cool the room in order to maintain the desired conditions.

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#26

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/05/2012 11:07 PM

To be comfortable in an air conditioned room, it is imperative to dehumidify the room. Why?

I am assuming that the question refers to Direct Expansion " Air Conditioning" rather than Evaporative "Air Conditioning".

To achieve a reduction in air temperature the Latent heat stored in the air in the form of water vapor must first be removed to allow the actual air temperature, Sensible heat to be reduced. As noted in another answer the Humidity in the air is what is most important in that if the air is dry, perspiration on your skin can evaporate and cool your body. As the level of Humidity goes up the less evaporation occurs and the less cooling you get.The dryer the air the better you cool.

To get proper "comfort cooling" DX cooling needs run time to keep the cooling coil below the dew point as long as possible to remove the moisture frm the air and provide dry air to allow easy evaporation of perspiration. Oversized systems satisfy the system temperature control to quickly and do not allow any run time for the cooling coil to do much de-humidification of the air.

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#27

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/06/2012 7:29 AM

air condition usually re-breathes the air from the room back into the room - every humidity will so add to the humidity was in the air before re-breathed.

it adds up till it begins to rain.

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#28

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/14/2012 9:28 AM

If the room temperature is high (it's hot) we sweat. We sweat in order to decrease the temperature of our body.

Sweat is, basically, water. Water evaporates easier if the the air around our body is drier. Evaporation means the water took heat from our body and became vapor.

More evaporation, more heat exchange between our body and the surrounding air. Therefore we feel "cooler", more comfortable, because our body's temperature decreased, when the air is dehumidified.

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#29

Re: Keeping Cool: Newsletter Challenge (June 2012)

06/19/2012 9:42 AM

Having just received the Techs and Specs email on which this question was posed, I thought I'd pop in and say "It will rain", but the answer's already been posted.

C'mon CR4 Admin! Keep up at the back!

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