DOAS Chilled Beam no way to dehumidify 3

[scha0786]

Doing a design review of a 90,000ft^2 two story building with DOAS serving chilled beams in classrooms. Currently there is an ERW that has a configuration of: first a dissicant wheel then the heating coil and finally the chilled water coil. The rooms typically have one wall exposed with 70% glass, 30 students and about 8 2'x2' active chilled beams. The outer bank ACBs have a dual coil that can always use hot water. The inner ACBs have only a single coil that is either receiving chilled water or hot water (depending on what the plant is doing).

My concern with the design: There is no way to dehumidify the air going to the chilled beams, cooling coil is after heating coil and the only reheat available is the 3 outer bank ACB in each classroom. Is this a valid concern or am I over thinking this?

My concerns:

-I fear that there is no way to dry out the building during a Monday morning cool down after a long muggy weekend.
-Return air humidity will slowly increase, which in turn will increase the return air dew point and cause the high temp chilled water set point to climb in the mid to lower 60s causing over heating of rooms.
-Rooms will slowly sub cool over time, "cold and clammy feel"

Oh and they have a new Gym and this unit is configured the same way, heating coil and then cooling coil. The dehumidification sequence I got from the engineer is to slow the fan down to 50%. My concern here is with the new wood floor and maintaining a consistent 55% humidity in the gym to prevent wood floor problems.

 

[scha0786]

I agree that they need some sort of reheat after the cooling coil. Since the unit is chilled water it would have to be a sensible wheel or a run around loop. Another option that they could have done was used a desuperheater on the air cooled chiller for reheat of the two DOAS units.

 

[scha0786]

Further movement on this. The design firm clearly wants this sequence to happen.

When return air humidity rises above set point (55% RH), the chilled water valve modulates open as a function of deviation from set point.

I my mind, won't this make the situation even worse because now you could be sending 46F air into the spaces, lol.

Testing is going to start in the next month so I'll let everyone know how it goes.

 

[RandomUserName]

Thanks for following up! This has been a fun one.

"When return air humidity [...]"

What return air? I thought this was a DOAS + ACB system? If so there is no return air, only supply air from the DOAS and exhaust air.

Even assuming they meant exhaust air humidity, they way that is worded sounds like the sensor will be in the exhaust air duct? I hope all rooms are similarly loaded. You are taking a common average humidity of combined rooms at this point; could be poor control.

I would hope the RH sensor is in each room. Even then, if the RH sensor is modulating the ACB water valve, the only way we will take moisture out of the air is by condensing it out and draining it via the ACB. That is not a typical design for an ACB; they usually provide sensible heat rejection only. Sounds like it's gonna rain baby!

The ONLY way to control humidity is via the DOAS. And with no reheat downstream of the final CC on the DOAS, you will over cool your rooms as we have discussed before.


Curious,

Have you ever called the mechanical engineer just to discuss what you believe will be an issue with him? That is usually the best way to approach these things.


Here is what I see happening in the very near future with your system:

The addition of RH sensors in each space and temperature sensors in the supply air duct work controlling a reheat coil on each branch supply air duct serving said space. If the RH sensor calls for dehumidification, a signal is sent back to the DOAS to run the CC valve wide open. All reheat coils will be tasked with maintaining a supply air temp setpoint via the duct mounted supply air temp sensor and a hot water valve on the reheat coil.

 

[lilliput1]

The heating coil in the DOAS is a preheat coil to protect the downstream chilled water coil from freezing if there is not enough heat from the ERW or if the ERW is down. The space heating is by the reheat coil in the perimeter ACB. The indoor ACBs will have hot water in their coil when heating is required. For morning cool down or warm up, turn on units earlier.
Adequate dehumidified air must be supplied to the ACBs to do sensible and latent cooling without condensation. Ask engineer for list of schools with this type system and check them out.

 

[scha0786]

Randomusername -

you are correct that the humidity sensor is in the exhaust air stream at the DOAS unit.

you are not correct, no space humidity sensors are being installed.

Yes I have contacted. THe response I received is exactly what lilluput1 typed, lol. Maybe this is a common design now, maybe in northern climates this will work. Although it seems lately that we are seeing higher than normal wet bulb temperatures for longer periods of time.