Redesign for Latent Load

Question

Redesign for Latent Load

sendinc (Mechanical)

(OP)

24 Dec 23 08:48

Hello all, I recently started working in an area with high sensible and latent loads. I am trying to provide a solution for a previously installed system in a recently commissioned building where they are having condensation issues. The building was designed with FCUs to provide cooling for the room loads and four AHUs to provide 11% ventilation (100% OA, no return air at the AHU). The building was designed with a return plenum. The designers claim the problem is lack of filter maintenance causing negative pressure and infiltration. Our team thinks there is an issue with the design (and latent load calculation). The measured LAT at the AHU was 57F rather than designed 55F. I recently checked and the building is positively pressured with dirty filters.

My questions are:
-What are the most common designs to handle high latent loads? Does it sounds like the system is undersized (either chillers or AHUS) or is there a specific coil better suited to handle the latent load? Is it possible to install a different coil or something to provide precooling with the original coil and a reheat?
-Is it common to install a DOAS without a return duct? What are your thoughts on return plenums?

It appears they designed using for a sensible design day (114F DB/86F WB). I calculated the load using the 0.4% dehumidification day (93.3F DB/ 88F WB) and the total cooling load doesn't seem that different (2 tons more for latent design values for a 56,000 sqft building). However, if I draw a line between the latent design conditions and the actual measured LAT (57F, assume 90% RH), there is no ADP that would make this process physically possible. Am I missing something?

The load modeling completed from the designer was done in HAP for the FCUs only. They assumed an SHR of 0.9, but the modeling doesn't include any outdoor air. They calculated the cooling load for each individual room. I summed these values and came up with 196 tons. They installed two 113 ton air cooled chillers for a total capacity of 228 tons with a designed leaving chilled water temperature of 43F.

MintJulep (Mechanical) · Answer 1 · 2023-12-27 00:16:58

Quote (sendinc)
if I draw a line between the latent design conditions and the actual measured LAT (57F, assume 90% RH), there is no ADP that would make this process physically possible. Am I missing something?

I think you have a good approach. However, you need to do some of these psychometric analysis with actual measured air in, air out, water in, water out, not assumptions.

Quote (sendinc)
The measured LAT at the AHU was 57F rather than designed 55F.

You have a humidity problem. Maybe you should measure the humidity too.

Quote (sendic)
They assumed an SHR of 0.9

Where? for the FCUs or for the DOAS? It's really heard to follow your post, because you throw everything in together in your descriptions.

In either case, SHR = 0.9 and "area with high latent loads" don't go together well.

Is it common to install a DOAS without a return duct? Sure, all DOAS units that are 100% outside air have no need for a return duct.

What are your thoughts on return plenums? They are entirely common.

sendinc (Mechanical) · Answer 2 · 2023-12-27 12:03:11

Thanks for the feedback, I appreciate it!

Yes, SHR at the FCUs was modeled as 0.9. Their modeling didn't consider any latent load from outdoor air (I guess they figured they had it covered by the AHUs).

I calculated the load at the AHUs using the sensible design day and came up with a coil SHR of 0.54 and a coil SHR of 0.32 if I use the 0.4% dehumidification design day. I’m pretty sure their air handler wasn’t designed for that kind of latent load. My intent was to 1) identify missteps in the design 2) propose potential solutions.

Knowing the coil SHR, is there some kind of a retrofit we can do with the existing air handlers to replace the coil with something better suited to handle the latent load?

It seems inefficient to not utilize mixing at the air handlers considering the latent load. No?

HVAC-Novice (Mechanical) · Answer 3 · 2023-12-30 00:22:34

What type of construction? Are the FCU drawing air from that return plenum?
Are the FCU designed to dehumidify and have a condensate pan etc.? what are their DAT?
What climate are you in, what is the use of the spaces?

57°F DAT indeed is a bit high. Not sweating, but not great. Depends on what the FCU are doing here....

what are the humidity conditions in the space? I assume humidity is too high, but what is too high?

sendinc (Mechanical) · Answer 4 · 2023-12-31 09:36:59

It's a dormitory. There are essentially three wings separated by two atriums. Hot/humid climate up to 114F DB and 88 WB in the summer.

I don't have any data on the FCUs yet, but I've requested the tech specs. The measured temperatures in the rooms during the summer was between 73-77F with RH between 55 and 66%. They were designed for 73F, 55% RH. I don't think they were designed to handle latent load, but they are plumbed with a condensate drain. They are designed with a EAT of 75F DB/62 WB and a LAT 55F DB / saturated (100% RH).

I was hoping they would let me study the system next summer so I can take my own readings, but they are asking for redesign options ASAP.

MintJulep (Mechanical) · Answer 5 · 2023-12-31 11:02:11

This paper looks interesting.

https://cdn.intechopen.com/pdfs/30773/intechtherma...

HVAC-Novice (Mechanical) · Answer 6 · 2023-12-31 20:36:02

By type of construction I meant construction data that are related to possible infiltration. Do they have operable windows in those dorms? a blower door test can confirm. is HVAC designed to have neutral to positive pressure?

Cooling coil leaving temp always will be 100% when dehumidifying.

Is your AHU DAT measured at cooling coil discharge, or after the fan (if it is pull-through setup)?

What are actual FCU DAT and is condensation observed? It sounds like they are designed to dehumidify (to some extent).

I would use consistent humidity-related units. Rh (%) is pretty useless since it changes so much while being the same water content). Use dew point.

i don't doubt it is humid since that likely is based on actual complaints. but make sure all measured values are from calibrated instruments and have proper sampling points. Cheap/uncalibrated instruments easily are a few % off. This is important, especially here at the edge of the barely acceptable.

trashcanman (Mechanical) · Answer 7 · 2024-01-02 20:29:10

You must start out with a VALID test and balance report. Most of the T & B reports I saw over about 50 years of design, few were correct. Get a report that makes sense. Then, do your analysis of the system. Off the top of my head, 57 Deg.WB will NOT DO MUCH DEHUMIDIFATION.

sendinc (Mechanical) · Answer 8 · 2024-01-09 08:22:57

Thank you all for your feedback! I think the AHUs were designed using extreme temperature outdoor air conditions (instead of extreme moisture) and too high of a dew point of the leaving air. These resources were helpful too:

-TRANE Engineers Newsletter Volume 49-1
-ASHRAE dehumidification design guide

I think a redesign will be required, so we will probably get our wish to study the system again this summer. I'll take measurements with everyone's recommendations. I'm waiting on the T&B.

I'm worried the leaving water temperature off the chiller won't be able to provide the required dew point. I came up with a dewpoint of 44.6F of the leaving air temperature (sound reasonable?). The water temperature off the chiller is designed for 42.8F.

We need to provide ideas to manage the problem in the meantime. Any dehumidifier recommendations out there?

SAK9 (Mechanical) · Answer 9 · 2024-01-09 20:06:50

You can solve the problem by supplying ventilation air at a neutral temperature say 21C.The ventilation AHU needs to cool down to at least 13C( or lower as allowed by the chilled water flow temp) to wring out all moisture from the air and then reheat it to 21C.This is how it is done in hotels located in hot and humid weather. I have also seen desiccant wheels being used in some tropical hotels for treating the ventilation air( they have plenty of steam available for reactivating the desiccant)

FacEngrPE (Mechanical) · Answer 10 · 2024-01-20 11:01:50

A run around coil loop can provide reheat to your outside air unit and may lower the apparatus dew point if the current leaving air temperature is significantly higher than the chilled water temperature.

The suggestion to obtain a current / known valid TAB report is a really good one as even if the design is good, incorrect adjustment could make it fail to function as designed.

Look at the control arrangement on the OA AHU, what happens to the dewpoint and leaving temperature at part load. Some arrangements can lose humidity control at part load. If this unit supplies air at a dewpoint higher than needed the, local FCU's are unlikely to be able to finish the de-humidification.

chicopee (Mechanical) · Answer 11 · 2024-01-27 19:27:12

To minimize condensation, reduce the air flow from the AC so that it runs for a longer period of time. Frequent cycling the AC "on" and "off" does not allow enough time to reduce condensation. Obviously there are other factors such as having too much outside air and too much water vapor from showers in the dorms.

MTES93 (Mechanical) · Answer 12 · 2024-04-12 06:17:33

I just glanced thru this. You have a tad bit more chiller capacity than your estimated load? Can we set the chilled water temp lower? I know how load calculations go, I've done them for years. On a 'bad' day what is the loading on the chillers?

BronYrAur (Mechanical) · Answer 13 · 2024-04-24 19:01:17

What plenum? Are these fan coils in individual rooms? How is the OA introduced - just into common spaces, or actually ducted into each fan coil?

nuuvox000 (Mechanical) · Answer 14 · 2024-04-26 16:40:57

It sounds like the AHU's were designed to cool down the air but weren't necessarily designed to dehumidify to certain requirements. If that's the case, then you can either cool the air down further (people will likely get uncomfortable) or install a system designed to dehumidify the air. This can be a dehumidifier unit or you can cool the air further in your AHU and then reheat it (add reheat section).

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Redesign for Latent Load