Humidification and Dew point control

[friartuck]

I have been asked to look at conditioning a number of large rooms and also to humidify tham.

The rooms need 100% fresh air (Smoking areas--night club/casino etc)

The airflow rates are varied but I have four plants in excess of 12m3/sec.(one is nearer 17m3/sec)

What is the best way to humidy these areas. I guess spray washer (Electric hum will be massive)

If spray washer, is dew point control easy to implement and do i need a special 'dew point' sensor or is this simply a standard dry bulb sensor?



Friar Tuck of Sherwood

 

[MintJulep]

These folks seem to be the biggest player in dew point and humidity sensors:

http://www.vaisala.com/

Spray wash is probably a good choice. It would have the possible side benefit of removing some of the smoke and allowing you to use less that 100% OA.

 

[DUMechEng]

friar,

what are your oudoor air design conditions?

 

[friartuck]

Typical Uk Weather conditions. Winter -8C 100% sat and summer 28C 50% sat

PS Dew point control...is it easy to achieve or am i complicating things. What about gas fired humidifiers?



Friar Tuck of Sherwood

 

[MintJulep]

What are you hoping to achieve with dew point control? Seems like overkill for a smoking lounge to me.

Anything that adds humidity via steam also adds sensible load which needs to be countered by the refrigeration system. Kind of an energy pig.

 

[Whoops]

For that flow-rate air, a BMS system would be very advisable. You'd use standard temp and Rh sensors and the BMS outstation would be configured to calculate the dew point from the Rh input. Vaisala do dew-point sensors, I believ but they're very expensive; I've never had cause to use them. The standard Sontay Rh sensors are capable of doing what you require.

I recommend that you obtain some input from a trusted BMS contractor at the design stage; the control system could make or break such a project.

 

[quark]

The humidification process is isenthalpic and this infact reduces the air temperature. Even with steam, as the sensible heat of steam is much lower than its latent heat, there will always be a temperature drop.

I would advise plotting the process on a psychrometric chart and see how much extra moisture addition is required. I don't have a Psychrometric chart at hand now but I feel the summer conditions can be maintained by just cooling down the air and thus increasing RH. Air, in winter condition, to be heated up first and then moisture added for efficient process.

I feel a humidistat is just fine and can control within an accuracy of +/-2%.

What are the required room conditions?

Regards,

 

[lilliput1]

Check humidification with natural gas heating by dristeem. Size for minimum humidification requirement since latent load from people will reduce humidification load.

 

[friartuck]

Conditions are 22C 50% summer-21C 50% winter

I was thinking if I used a spray type washer that the process follows the wet bulb line and would therefore reduce the chiller load.

If you consider air at -8C outside, this could be heated to say 21C, then spray washed which would cool it and humidify it (Saving energy, since I would need compressed air to atomise the water and not steam from an electric source or stean boiler plant)

When you use a system like this, you have to try and get the right spray washer 'off coil' conditions by varying the pre-heat and washer spray rate (I think thats how you do it anyway).
Its HOW you actually measure the off coil which gives it it's name 'dew point control'. I always thought a standard dry bulb sensor was used in the ahu for this purpose, but a humidity sensor was used as a limit device to stop over humidification (in both the supply duct (and one in the room for final control??)



Friar Tuck of Sherwood

 

[quark]

A correction to my earlier post. There won't be any temperature drop if we use 100% saturated steam for humidification. But if it is wet steam, sensible heat of steam as well as that of air assist the vaporization process and there are likely chances of reduction in air temperature.

With 28C and 50% inlet conditions, 22C and 50% can't be acheived with humidifaction alone. You can land up near 22C and 60 to 65%RH, which seems to be good. What you plan for winter condition is perfect. Just preheat it and then air wash it.

When we deal with AHUs, DP at coil is always maintained constant and so you can control the required parameters with only DBT. I would rather go with a DBT for reheat and then an RH sensor for humidity control.

http://www.armstrong-intl.com

provides you an excellent and free handbook + selection software for humidifiers if you register.

Regards,

 

[lilliput1]

For this non critical aplication, it is not recomended to humidify above 30% RH. Otherwise you can get condensation at windows. Actually we don't even bother humidifying bars & dining rooms because the people give off enough latent load to humidify the space.

 

[imok2]

Frier, This may be of interest
Humidifiers using the adiabatic process exchange sensible heat of air with the latent heat of water to accomplish evaporation. The result is a drop in air temperature while the total heat (enthalpy) remains the same. In other words, moisture is added to the air at the expense (or benefit) of a drop in air temperature.

Representing the adiabatic group in this comparison is the ultrasonic atomizing humidifier. This system generates a fine mist through high-frequency vibration of an electrically powered transducer. It was selected because of its very high operating efficiency. Recent models have demonstrated atomization of water droplets down to 1 micron, which improves absorption capability. The U.S. Department of Energy published a Federal Technology Alert on these systems, which can be obtained at (

http://www.eren.doe.gov/femp/prodtech).

The total read is at

http://www.esmagazine.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2503,74868,00.html

 

[HVAC68]

Do you really need a humidifier ? Check the resultant RH by doing a cooling load calculation. You may get the answer there. My experience is - Have never provided humidifers for a bar application - agreed that my outdoor design conditions are much humid than yours.

HVAC68

 

[lilliput1]

Humidifiers other than direct steam injection are high maintenance items. On non critical spaces, the maintenance staff usually don't bother turning them on to avoid maintenance work.

 

[cme]

I would use a air atomization unit like Carel/DGH ..... you need a screw compressor, tank and waterline

 

[lilliput1]

With air atomization you would need to use RO water or else the calcium carbonate in water will be released like dust powder on the airstream.

 

[ChasBean1]

Friartuck,

Issue 1 is that I'm skeptical of the need for a 50% RH during winter. In summer, it's okay to ask for such a dehumdification value from your air handlers if the duty calls, but a 50% RH in winter in the UK and many northern sectors feels like Pea Soup. You might not realize this until you walk into the space from the non-humidified, surrouding 69°F, 10% RH space. To me, it's ludicrous to ask this of a high volume make-up air systems serving cocktail lounges...

Let's re-check the needs first, hopefully from an authority who understands the psych chart. If you're asked to do something nuts from the client, talk them in off the balcony before obliging.

-CB