Oversized DX Equipment causing High Humidity

[MAragorn]

I am looking at a project that was not my design but is now mine to resolve.

Without having access to the early design decisions, it appears that the rooftop units are about 2 x as big as necessary. When the thermostat calls for cooling, it is satisfied pretty quickly, thus the compressor does not run all that long, so there is not a whole lot of dehumidification going on. Additionally, this is on the coast, so all the time the compressors are NOT running, the system is dumping a fairly humid mixture of outside air and un-cooled return air.

As I said, these are DX units, with 2 stage electric heat. To my advantage, they are 2 compressor units, so 2 stages of cooling.

Now in the good old days, I would have used a chilled water system with hot water reheat, cooled all the air down to 55F then reheated for comfort, thus yielding the sweetest, dry-est system in town. But not in these energy conscious days.

To obtain a dry space, I COULD replicate the good old days, driving the air cold then using those honking big EHCs to reheat. Not a good plan.

I COULD re-sheave, driving the cfm low, and the air cold to have the compressor run much more of the time...except...I believe I am going to be told by the manufacturer that the minimum acceptable cfm for the unit won't be low enough to match load/cfm/humidity to give a comfortable space and not damage the compressor. It will be better, but better probably won't be good enough.

So do you long-time DX guys have a magic bullet to shoot and put this high humidity problem down?

 

[AbbyNormal]

Without the outside air, constant fan is an evaporative cooler in the off cycle, probably to get a net moisture removal you would need the compressor running for 20 minutes straight, other wise the water on the coil just re-evaporates.

With both stages cycled off, and that steady fan drawing in outside air, I think you are doomed.

Slowing the fan removes more moisture, but not solving the real problem, humidity from outside air getting pumped into the place when the compressors are off.

Add a dehumidifier if there is no way of ventilating more intermittently, or put in smaller equipment so at least one stage runs steady.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[MintJulep]

desiccant wheel.

Replace evaporator coils with ones with low fpi.

Hot-gas injection at evap entry.

Hot-gas bypass at compressor.

 

[DrRTU]

Abbynormal is correct. I was in your shoes a few years ago with an injection molder. When the molding machines were not at high loads the 75 ton RTUs were cycling and space temp was at design but RH was 68%. It’s the re-evaporation and not running at 100% would drive the moisture back into the space. I tried to go to intermittent fan for a time period to get the evap to drain but it did not improve much. Most mfg. do not publish part load latent rating. A desiccant unit and reheat sequence solved the problems. Can you re-zone so one unit can carry the complete space. I redid a tool room AHU with

http://www.rawal.com

and it helped keep the molds from rusting.

 

[chilled]

Set up the system to use the electric reheat in conjunction with the mechanical cooling? An option if the increase in operating costs aren't prohibitive.

 

[imok2]

It takes time to build up to useful moisture removal — it takes 10 to 20 minutes of operation, for enough moisture to build up on the coil so that it falls off. “If things do stay off for a while, that mass of moisture will return to the airstream and not enough moisture will build up and fall off, so the moisture becomes entrained back in the supply airstream. When the compressor cycles on and off and the fan stays on, the unit starts up and reaches steady state; then it shuts off, but the sensible effect continues. The coil becomes an evaporative cooler, even though no refrigerant is flowing through it. In essence that off-cycle evaporation is an adiabatic process, with virtually no heat exchange. Then The original latent load of the ambient air is degraded by the added moisture off the coils.
I would do two things: 1) disconnect the second stage compressor and 2)Put the fan on on auto ..to come on when the compressor comes on.

 

[imok2]

If all else fails you might consider replacing the lead compressor with a digital scrool with unloading down to 50%
from Copeland

 

[williamsGA]

Is it possible to use a hot gas reheat coil? You can let the compressors run making 55F leaving air, and then reheat the air with the hot gas. Since the hot gas is recovered energy you don't violate the energy code. We use this system quite often in Georgia.

 

[BronYrAur]

I agree with williamGA. Go with hot gas reheat. Energy wasting aside, you probably don't have a large enough electrical feed to simutaneously run the the compressor and electric reheat.

Does anyone have a slick way to pipe up the hot gas reheat?

 

[imok2]

If wasting energy is not an issue then hot gas by pass is an option go here:

http://www.trane.com/commercial/library/vol32_2/ADM_APN007_EN_0503.pdf

 

[DrRTU]

For HGRH, the better solution is to pipe to the evaporator.

http://www.sporlan.com/90-40.pdf

 

[AbbyNormal]

probably be a lot cheaper to just add a dehumidifier than to try and convert to hot gas reheat.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

I like the York DR RTUs with the 'alternate reheat' mode

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[DrRTU]

Correction on my last post, for HG bypass, use

http://www.sporlan.com/90-40.pdf

For HG reheat you would need an ORD, ORI, check valve, 3-way valve and condenser coil in leaving air and a receiver. You should install an active drain. Sporlan used to have a guide on heat reclaim, but all I could find was the ORD / ORI booklet

http://www.sporlan.com/90-30.pdf

. You would spend tons of hours reverse engineering a HGBP or HGRH into an existing box. I have used the Carrier and York RTUs with the factory mounted hot gas reheat coil without problems.

 

[NCDesign]

Are the compressors the same size? Not likely with a light commercial unit, but sometimes their compressors are not the same size. If they are not identical you might be able to adjust the logic to make sure that the smaller compressor is always the first to come on. Just something else to take a look at.

 

[marcoh]

Depends on the how much oversized you are but I have seen systems where cold supply has been mixed with return to address humidity issues. Lowers the coil air on conditions, lower supply air conditions, therefore more dehumidification but you need to run a larger fan (space supply air plus bypass air)

 

[MedicineEng]

If it is so oversized, is it an option just to work with one compressor?

If you have free/cheap steam, you can also try to dehumidify the air with a dessicant system.
I've used a system from a dutch company, and it works quite nice. Before they were called "Kathabar" but 2 years ago they were bought. I don't remember the new name now. Try to Google it.

 

[MAragorn]

Here is an update:

I got from the manufscturer that if I "lock out" the 2nd compressor I can cut airflow to 200 CFM per ton on the operating compressor.

So I am thinking: Use the T-stat to enable the compressor, and the leaving air sensor to drive the air to 55F or colder. Using a VFD run down to this minimum of 200 CFM per ton. If the temperature gets too far out of setpoint, ramp up the air and turn on the 2nd compressor. But based on historical data, I don't think that will happen...maybe ever.

Additionally, carefully reviewing the OA schedule, I am able to reduce OA by maybe 60% and still comfortable meet code.

 

[AbbyNormal]

Reduce the outside air by 60% would eliminate half of the latent load, easy experiment

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[MAragorn]

Too late!

Right now all the OA is temporarily blocked off until we get this resolved.