To Control the Humidity by Duct Heater 5

[nattapong.s]

Hi,

I need some advice regarding the humidity control.
...Outdoor Condition 95 F and 60%RH and Indoor 74 F 50%RH.

Our existing substation & Control Room is located in Tropical Zone (Thailand). My Client is facing the problem the exceeding in Relative Humidity in room cause the panel surface wet.

The system itself is Centralized Single Zone Constant Flow AHU and OA 40% are introduced to mix with RA. From my first investigation, the A/C was designed and selected with a massive margin over the design load in which it merely seem 30 -40% Part Load

I offer many solutions to customer however they accepted only the adding the Duct Heater (preheating coil) to simulate the artificial load in order to let compressor continuously operate. My queries are

1. The Sensor to drive the Duct Heater could be Humidistat or Temp sensor.
2. Is it ok to preheating coil only do we need reheat coil?

Thanks in advance.
Best Regards,

 

[moideen]

are you sure that the system is sized correctly? over sized system will increase the rh.

 

[nattapong.s]

Moideen,

The existing system is not sized correctly (a huge margin is added). my Client need my service. then, they okay with only one adding Duct heater to simulate the load and let the compressor continue its doing.

 

[willard3]

There are more variable you will have to track down, ie, how many rows is the cooling coil, what are the leaving air conditions now and etc.

 

[MintJulep]

It's there to control compressor load.

So control it from something that is indicative of compressor load.

Maybe compressor current.

Maybe suction pressure.

 

[lukaiENG]

OP, If your goal is to limit the RH in the room then I would think that a reheat coil would be better than a preheat coil.
[ul]
[li]A preheat coil will increase the entering condition onto the coil which will give the coil a load to run against but in all likelihood the supply point will have a relatively high RH - because that is how cooling coils work. This may not be an issue for you if you ahve enough load in the space to bring the temperature up/RH down enough.[/li]
[li]A reheat coil will take a near saturated supply condition and move it fairly dramatically away from the saturation curve. It will of course also increase the space supply condition but I imagine this is not really a major factor if your system is as over-sized as you say. If this is an issue then default back to your pre-heat coil, or perhaps both.[/li]
[/ul]

In terms of how to size/control:
[ul]
[li]Preheat coil:[/li]
[/ul]
I would think you work out what temperature you need to have entering the DX coil to have stable/continuous operation and size accordingly. I would control off the downstream air temperature (before the cooling coil) because this is simple and reliable.
[ul]
[li]Reheat coil.[/li]
[/ul]
Size it to bring the supply condition down from 90/100% RH to ~70/80% RH (or whatever you feel appropriate based on system) and control based on downstream air temperature.

 

[BronYrAur]

Why not add hot gas bypass to the refrigeration circuit instead of falsely-loading the incoming air?

 

[berkshire]

Bron Yr Aurs solution sounds better, run hot gas bypass then get the coil down as close to freezing (With the bypass stopping freezing.) as you can to maximize water removal then with re heat get the air back up to a comfortable room temp.
B.E.

You are judged not by what you know, but by what you can do.

 

[GT-EGR]

The duct heater should be a reheat coil, not a preheat coil.

You have to adjust your control strategy so that your current unit just maintains a constant discharge air temperature leaving the cooling coil, and your new duct heater controls the space setpoint by reheating the air as much as needed to keep the space from being overcooled.

Your reheat coil needs to be sized to heat the quantity of your current AC unit air from 55ish (or whatever you set your discharge air temp off cooling coil depending on what RH you want indoors) to whatever temperature you need to heat the space on a design heating day (if that exists). It also then needs to be able to modulate in between so it accurately matches your space needs on any given day.

Preheat coil won't really work, if you do it that way you are really just trying to do a reheat coil, but on the reverse side of the coil, but then your cooling coil will have such a high entering sensible temp that it won't reach a low enough temp to pull the moisture out of the air.

You also may want to question why it's 40% outside air, that's your biggest source of humidity gains, and dialing that in to the actual number you need will have a significant impact.

 

[Drazen]

First you need to analyse how coil control works vs. usual conditions, like doctor needs examination of patient before prescribing a therapy.

All else is sort of walking in dark.

 

[nattapong.s]

Thanks you all guys. I am weighing your all suggestion and integrating the finding the optimum one. Cause i have not much choice from client buy convincing them at best.