Humidifier Controls

[remp]

A steam humidifier for an AHU on a job Im reviewing shows its served by medium temperature hot water, so its not electric. The design drawing shows a control valve on the MTHW line as it enters the humidifier.

What exactly control the units operation? Is it just the space humidity sensor that controls the operation of the control valve. Is it that simple, or is there more to it.??

 

[ChrisConley]

Well, it could indeed be that simple. In addition to a space humidity sensor there could also be a high level humidistat and a variety of other safety controls: low water cutoff, etc.

Humidity control does tend to be pretty simple because a swing of 15% RH isn't that noticeable, so on-off with a large deadband is quite common as well. So unless you are in an area that requires tight humidity control I wouldn't expect too many bells and whistles.

 

[imok2]

A relative humidity of around 40-60% at a temp of around 75*f =-3 would be considered comfortable for most people assuming proper ventilation and no stratifacation and yes humidistat can be in the room.
Roger

 

[quark]

It is simple in the sense that a humidity sensor is sufficient to control the humidity. However, the location of the humidistat is critical particularly if you have more than one room being controlled with the same AHU.

As for humidification, I suggest to place the room humidity sensor near to the supply point (for dehumidification it, obviously, is near the return point)

What is the medium of humidifier? Hot water or steam?

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[remp]

quark

it is hot water at about 140deg C

regards

 

[quark]

billy!

It's not that it makes any difference by the medium but just wanted to know which is the exact medium as you stated both steam and hotwater in your first post. Generally you will get better control with hot water.

My two suggestions are,

1. Two use a proportionate control valve for hot water.
2. As the humidification process is isenthalpic just check for any temperature drop of the humidified air stream (plot it on a pshchro chart). If it is substantial you may have to go for reheating.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[remp]

Thanks for the tips everybody

 

[ChasBean1]

Controls I've seen are a humidistat in the room or in a return or exhaust duct from the room proportionally controlling the valve, a duct mounted high limit humidistat at the humidifier discharge to prevent a steam bath, and an air flow or air pressure proving switch that kills the humidifier if there's no flow.

As far as the throttling (proportional control of the valve), I'm not sure if that's desired w/hot water being injected and being relied on to flash to steam. There may be just an on/off control to ensure the throttling point is at an orifice. I'd look into this some more.

Quark, how would we see a temp drop injecting 140°C steam into the airstream?

Regards to all, -CB

 

[quark]

CB!

With steam it may not be a problem and that is why I enquired about the humdifying fluid. If it is hot water, even at 140[sup]0[/sup]C, then there will be a problem.

Ther required additional heat(minimum 410kCal/kg) for evaporation will be pulled from the air stream.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[str8]

Billy,
the humidity sensor, as CB and Quark explained, controls the steam amount on the supply duct with regards on the second sensor installed on the return, for proper humidification adjustable setpoint and limits.
If you are talking about a humidifier based on applying an electrical current to electrodes submerged in a container of water, then the valve, that you are talking about, maybe has a second purpose (beside controlling the amount of water from the feed line and of course, the amount of steam produced), which might be a drain from the tank dictated by the min/max level transducers and the water refreshment rate regarding the water conductivity.
Luck.