Room Humidity

Yes. Basically, the water vapor content in the air varies with temperature. The higher the temperature, the greater the water vapor may be present. It does not mean however that humidity is going to appear in the air when you heat it.

Think about car anti-fogging. Turn on the heated air, and the heater air will warm the windows, arresting the water with it. When you use air conditioning as anti-fogging, what you do is to remove air humidity by condensation in the heat exchanger, drying the air, and allowing it to absorb water from the internal vehicle surfaces, when its gonna be warmer.

dear All

Thank for your explanation.Actually i'm having problem to maintainn the room temperaure with the required humidity ( 24 +/-1 celcius and 40%-50% RH)

I have checked and clean cooling coil, primary /secondary and HEPA filter as well but it wont work. The chilled water supply is 6- 7 degree celcius and i think this is good enough for my room.We can maintain the room temperaure but not for the humidity.

Using heater will increase the room tempearuture.The crossing air also not happen to the room.

can anybody tell me that i have missed out something to check with this system?

I will add to the previous answer another question. Just how big is this room?

Knowing the volume of space will allow you to calculate how much water you need to add to the system. This information will help you to decide on the base capacity of the humidity generation system you will be adding.

Depending on the size and how much humidity you need to add at any one time. Steam injection is an option but not the only one or the most practical in most cases. Outside of a direct injection of clean steam (never use plant steam for breathable air) or a dedicated boiler or hot plate steam generator system, you also have the option of water atomizer spray nozzles and ultrasonic fog generators. Some of these systems come as pre-assembled units that you can add to your air system with little fuss or muss.

Also remember to use a half way decent controller and add a mechanical humidistat as a safety cutoff if things run away humidity wise.

Hi,

The RH of our small computer room is 35%. The mechanic told me it's difficult and expensive to repalce or adjust the Fan Coil Unit. He suggested we put some water in a contain and leave it in the computer room. However, it seems safer to me to put a humidifer there. COuld you recommed a brand and corresponding models?

Thanks and regards,

Jeff

http://en.wikipedia.org/wiki/Relative_humidity

Excellent comment !

open one valve of hot water in your batchroom and observe your mirror, the that you see in your mirror is humidity for hot temperature and water steam

Relative humidity is the amount of water the air can absorb before condensation,or rain occurs, and this can vary with temperature and atmospheric pressure.Absolute humidity is the amount of water in a measured weight of air, a ratio that is not affected by temperature. What you may have is simply an over sized unit that is short cycling,not allowing the unit to run long enough to drop the humidity in the room.The setpoint is reached, and the unit turns off before the humidity is lowered.If The humidity is getting too low, as in a computer room, then steam injection may be required.

Am I missing something?? I am not sure what the ambient environment is. Are we talking 100 degree F desert? or equatorial jungle. It seems this might be two seperate problems.

And I will make my usual bitch LOCATION LOCATION LOCATION!!! That at least gives us a hint of the problem.

Sincerely

Bill

Dear

THanks for your information above.its help me so much.Now let me do the summary based on my situation here.Actually my place is tropical country heavy rain is normal every year.

My working enviroment are having high humidity 75-85 % in all GMP rooms.Using the aircondition are helping me to reduce the humdity 50% average.My water cooled liquid chiller can supply the chilled with the temperature 5-6 degree celcius.This chilled water will absorb the water vapor from my return air in the AHU.The cooling coil in AHU condense the water vapor become water.Please correct me if im wrong.

is this the way cooling coil absorb the water vapour?if yes the chilled water temperature inside the cooling coil is the main factor to reduce the humidity?The cleanlines of cooling coil also will affect the effectiveness of system to absorb the water vapour?

I agree that to install the preheater at the return air duct before crossing the cooling coils will help to reduce the humidity.can i try and proceed with heater installation?please advise me!

Best regards

syachah

I agree with you up to the last point. If you are getting the temperature down to where you want it, but the humidity is still too high, you need to lower temperature on the coils if you can. The heat the air after the coils to bring it back up to the desired temperature. Heating the air before the coils won't help. Warm air will carry more moisture, but after the air is in the return duct, there is no way to absorb more moisture from the room at that point.

It might help to understand what happens to the humidity by imagining the progress of the moist air as it passes through the chiller.

This air will have a pressure (P), temperature (T) and a dewpoint (DP). The DP is the temperature at which water vapour condenses.

The pressure P is atmospheric and is constant, except for a very slight increase due to fan pressure (that can be ignored here).

Now decide on your desired room temperature and RH. Lets us assume 20C and 50%RH. The maximum water vapour that the air can hold at 20C is 17.3 grams per cubic metre. If the RH is to be 50% this means the actual amount of vapour allowed must be 8.6 grams. This has to be the dewpoint (From vapour pressure tables) this occurs at 9C.

That means your chiller must cool the warm air to get the vapour to condense on the coils down to 9C to achieve the desired vapour pressure. You must also drain the water from the room (into a sealed bucket or pipe).

However the air leaving the chiller at 9C will be 100%RH - so it must be heated up to 25C for the RH to drop to 50%. Which is what you want.

In other words, the warm air enters the chiller, cools to 9C, the vapour condenses and drains away, and the air reheats to 25C.

The inlet conditions only affect the chiller/heater size.

Dear syachah,

No! Do not place your pre-heater on the intake side of your return air duct, before the evaporator/cooling coil. This just reduces the total production potential of your cooling coils' primary job of removing humidity from the air. Remember that the most effective way to cause the cooling effect is to lower the Latent Heat, not the themperature. Adding heat up stream from the cooling coild raises the Latent Heat. Much of the energy to reduce the air to the dew point will have been wasted by the fact that many warmer air molecules and water vapor molecules will go through the coil and be down stream from the cooling coil before reaching the dew point for the water vapor passing through it; this is just a waste of dew point production capacity, without an increase in water vapor removal.

Two things you could do if the coil is producing air too cold or in greater volume than your comfort needs. First, since it is a chilled water system - there is no reason that you could not slow down the blower to some degree or put in a choke damper in the return duct. (Note: don't do this for a direct expansion mechanical air conditioner system because volume of air is a function of full refrigerant change of state from liquid to vapour and insurance against compressor slugging.).

A "guillotine" choke damper or piece of cardboard over a portion of the intake louvers can have that effect for a chilled water cooler system, albeit they generate appreciable noise. This will increase water vapor removal production markedly! Second - if the volume of air is still too great and the space satisfies the set point of the thermostat too early - put your "pre-heater" after the cooling coil and then you can call it a return duct re-heater, ta day! Make sure it has over-heat protection should the blower fail and the re-heater has no other control(s). Actually, it should be powered up with a holding coil in the same circuit that feeds the blower - at a minimum - with at least an on/off switch and over-heat thermostat protection in series with the normally open holding coil. Turn off or loose power to the holding coil - it opens, simple.

Very well thought out comment. Since holding coils can jam/ fuse closed, I would put the over- temperature protection in series with the heat source control.

Fair enough

Hi..Tinker65

great knowledge u hv share...

may i know when u said abt the choke damper having the effect of a chill water cooler; i dun quite get what u mean. How does addding a damper will improve the vapor removal process?

Another industrial question; my factory HVAC also having some problem...currently 2 HVAC are in series ; HVAC 'A' is used to pre-cool several downstream AHU units... one problematic downstream AHU 'B' is having high RH problem, although we can achieved the supply temp setpoint.

We have try to investigate the problem both in terms of mechanically failure and design failure. Still no result.

Pre-cool unit HVAC 'A' supply temp setpoint is 12degC; however we only managed to get 14degC but cooling valve already 100% opening. The chilled water supply / return shown no big differential pressure; or we could have suspect cooling coil choke..(it is a new 3 years old AHU)... Although i did hear hissing sound inside the AHU.

One particular downstream AHU 'B' is having a problem of high RH (abt 70% ) with return air temp at 21degC and supply temp 14degC. The cooling coil valve is only open at around 6% cos we are already getting the set point temp. The problem is that we can't get the set point RH of 55%. Also there is no heater coil installed after the cooling coil; and we are not going to installed it.

For AHU 'B' i also noticed there the differential pressure of the chilled water supply & chilled water return line is quite high of 1.4bar. Also i noticed visually the condensation on the drain pan is not much. The drain pipe is not choke.

Is there anything wrong with air temp not reaching the dewpoint; thereby not condensing on the cooling coil as much??? Do we really need to reach near the dewpoint to aid in condensation of the cooling coil thereby reducing RH.

For AHU 'B' RH setpt required=55% ; supply temp setpt require=14.5degC.

Dewpt should be =11.6degC?? Since we only managed to get actual supply temp 14degC not near the dewpoint???

Can anyone advise which mechanical parts could have contribute to the problems or is there any the problems relating the cooling temp not low enough thereby RH still high at 70%??

Many thanks for any kind soul to share knowledge...

God Bless...

One other thing which could be done to remove more humidity, as long as the cooling unit has sufficient capacity: slow down the speed of the air going past the cooling coil. This will allow the coil to be more efficient at reducing the temperature of the air going past. This in turn will reduce the amount of moisture the air can contain at 100 % relative humidity.

This will mean less cold air at a lower speed, but at a colder temperature, and at a lower humidity.

(Just make sure not to slow the air down so much the coil starts to frost over.)

Yes, higher temperatures decrease the humidity, see a psychometric chart. Higher temperatures raise the dew point.

You say higher temperature raises the dewpoint!!

For the dewpoint to rise at higher temperature there will need to be source of water that can evaporate into the air

Without a source of water, by example, if room air has a temperature of 25C with a dewpoint of 15C; the dewpoint will still be 15C whether the temperature is doubled or trebled or whatever.

Or do you mean higher pressure?? That will raise the dewpoint - or pressure-dewpoint should I say.

And therefore, the point is, raising the temperature before running highly humid air through a cooling coil or evaporator coil BEFORE it reaches these devices is counter productive to the primary purpose of "chilling air," i.e., which is to lower its latent heat and thus remove the moisture from it; such that once the air is recirculated into the cooled space, the overall humidity is lower...

.

Your point is well taken

There is one situation where raising the temperature of a room will decrease the humidity: when is is below freezing outside for an extended period of time. About a week will do.

Once the water vapor outside is locked up as ice the water vapor inside will be driven out. We've measured %RH as low as 7% in the northern USA during winter.

You need to have a good humidification system and a well sealed building to maintain humidity. Forced air heat seems to exacerbate the problem.

Other than this situation humidity responds as the other posters have noted.

Correctimundo!

The most cost effective way to create human comfort (as well as extend the life of upholstery, fabrics, wall paint, etc.). in dry winter months is to increase the humidity of the heated space. Efficient humidification systems, downstream in the return ducts of the heater are the preferred method. Stand alone humidification systems in a room are next best especially for those suffering from respiratory discomfort, but do not help much in other, multi-room heated spaces.

Any of the above are much more cost effective than direct heating without humidification because they raise the latent heat in the air while not appreciably increasing the sensible temperature of the air except for the infrared from small fan and pump motors (the latent heat needed to evaporate the H2O is absorbed from the ambient air and without a heater to make up the loss of sensible heat, humidified air would start to feel or have the affect of clammy cold air.). The energy expenditure to raise humidity is negligible in comparison to raising the temperature a dozen or more degrees to compensate for the rapid moisture loss from skin, fabrics, etc. in very dry winter-time air... Which is why one feels so chilly in a "hot" room. The static charges caused from friction between dry sufaces and skin can become quite powerful and painful, as well.

I reiterate what I said in comment #6, controlling latent heat is ever so much better than focusing on controlling the sensible heat. Humidity control is the secrete to controlling latent heat... However, in terms of equipment outlay or efficiency of operation - humidity control is more expensive in Summer months since one must re-heat de-humidified air in most instances or install slug protection for compressors if slowing the air flow over mechanical cooling system coils is used to increase de-humidification. And for heating systems, one must install Humidistats, humidifiers, water sources, etc. However, it costs a small percentage less for heating one's air in the winter as the indoor/outdoor temperature differential can be upwards of 12 degrees less than one is used to if the air is properly humidified.

That's a well thought out explanation; I like it ! Way to go Fonzie !

Oh, p shaw... Elementary my dear Watson.

Humidity is an exprssion relating to how much water is contained in any given mass of air.

Generally, it is related to temperature. The higher the air temperature the more water that air can hold until it becomes saturated (i.e. it can't hold any more water).

If the air tempurature were to fall, the amount of water the air can hold on to would fall and the excess the water it contained when it was warmer would condense out.

There are two measurement concepts to keep in mind.

"Relative humidity"and "Absolute humidity"

Relative humidity is the most commonly encountered mesurement where water content is expressed as a percentage "relative" to the maximum water content the air could be holding on to if it were at a saturated condition. This percentage can change depending on air temperature. (say the air is contains one cup of water at 40 degrees F and it is almost saturated (99 % relative humidity) if the air tempurature were to rise, its ability to hold more water increases and its "relative humidity" percentage falls because it can hold more than the one cup of water at the thigher temperature than it could originally)

"Absolute humidity" is just that. Its just a straight measuement of how much water is contained in a set mass of air. (i.e. "this mass of air contains one cup of water period.") It is not related to temperature (except when the air becomes so cold it can't hold the water in the first place)

For example: Say our air contains one cup of water at 40 degrees F and we raise the temperature to 60 degrees F, the air still contains one cup of water. (even though the air can now hold more water, we didn't add any water to it) If the air temperature drops below 40 degrees, it loses ability to hold its one cup of water, excess water condenses out and the air then contains less than one cup of water "absolute humidity"

Sorry...a repost...appreciate any commenst..

Another industrial question; my factory HVAC also having some problem...currently 2 HVAC are in series ; HVAC 'A' is used to pre-cool several downstream AHU units... one problematic downstream AHU 'B' is having high RH problem, although we can achieved the supply temp setpoint.

We have try to investigate the problem both in terms of mechanically failure and design failure. Still no result.

Pre-cool unit HVAC 'A' supply temp setpoint is 12degC; however we only managed to get 14degC but cooling valve already 100% opening. The chilled water supply / return shown no big differential pressure; or we could have suspect cooling coil choke..(it is a new 3 years old AHU)... Although i did hear hissing sound inside the AHU.

One particular downstream AHU 'B' is having a problem of high RH (abt 70% ) with return air temp at 21degC and supply temp 14degC. The cooling coil valve is only open at around 6% cos we are already getting the set point temp. The problem is that we can't get the set point RH of 55%. Also there is no heater coil installed after the cooling coil; and we are not going to installed it.

For AHU 'B' i also noticed there the differential pressure of the chilled water supply & chilled water return line is quite high of 1.4bar. Also i noticed visually the condensation on the drain pan is not much. The drain pipe is not choke.

Is there anything wrong with air temp not reaching the dewpoint; thereby not condensing on the cooling coil as much??? Do we really need to reach near the dewpoint to aid in condensation of the cooling coil thereby reducing RH.

For AHU 'B' RH setpt required=55% ; supply temp setpt require=14.5degC.

Dewpt should be =11.6degC?? Since we only managed to get actual supply temp 14degC not near the dewpoint???

Can anyone advise which mechanical parts could have contribute to the problems or is there any the problems relating the cooling temp not low enough thereby RH still high at 70%??

Many thanks for any kind soul to share knowledge...

God Bless...

Waterlucky

I have some experienced in control the humidity.First you have to check cooling capacity required in your production room , the heat load , air change, what type of equipment in the room installed.You have to make sure you room is free from the source of water or any condensation from the equipment.

If your chilled water in/out volume , pressure and temperature is in good condition, then you have to check your fin coil is clean or not?.check the pre-cooler fin coil and the AHU itself , the blockage of fin also can obsrtuct the water vapour condensation and carry the moist into your room.

If all above is in good condition, and the humidity still high , you have to check the room pressure , AHU filter inclusive the HEPA filter.make sure your room pressure is positive in order to prevent the cross air contimination from outside.if your room is in negative pressure , the fresh air from outside which carry the high humidity will flow into you room and finally you will get the high humidity constanly.

you also have to check the air intake flowrate.Install the inverter for the AHU motor in order to control the air flowrate.if high volume of air goes into the AHU with less chilled water volume then will cause the high in humidity.

lastly you can install the dehumifier after your pre cooler.

you can try...i have tried all above and it works. my room design is 35-40 RH with 23 degree celsius.