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Air Handlers: 18k btu w/600cfm or 24k btu w/480cfm?

Air Handlers: 18k btu w/600cfm or 24k btu w/480cfm?
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ThirdHatt

Madisonville, LA

posts 18

12:44 pm January 7, 2012
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I have a Marine Air Chilled Water system which consists of two 60k btu compressors (new '08) and 11 original Lunaire air handlers (old and aging). The salon has two 18k btu air handlers under the sofas and one 9k btu behind the bar. One of the 18k btu low profile units under the salon sofa has very weak airflow even though the evaporator is very clean. I think it's time to replace the unit but the "low profile" units are significantly more expensive than the regular units. The new 18k btu low profile unit is rated at 600cfm because it has two blowers. I am considering replacing my old low profile unit with a regular 24k btu unit that is rated at 480cfm with the High Velocity blower. I will eventually replace the other 18k btu low profile and 9k btu regular unit with the same 24k btu unit so there will be no significant change in capacity.

 

How would you compare the cooling capacity/airflow of an 18k btu unit with 600cfm to a 24k btu unit with 480cfm?.
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Steve Pooler

posts 1127

1:17 pm January 7, 2012
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Where are you getting this CFM info ?

Typically…One Ton (12K) of A/C requires 400 CFM…So a 24K unit would be 800 CFM

 

Are you sure these are Luneair units instead of Aqua Air ?

Not that it's not possible, but I don't remember Luneair making chilled water systems…

Aqua Air however was Big in them & had most of the OEM chiller boat accounts back in the day…

One important thing about chiller systems is loop water flow & maintaining Air handler total BTU that the loop flow rate was designed to carry…

Air flow thru the coil/s is no where near as important in chilled water systems as it is in direct expansion (refrigerant) systems

Just food for thought Confused

 

Steve~
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ThirdHatt

Madisonville, LA

posts 18

2:11 pm January 7, 2012
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Post edited 2:14 pm – January 7, 2012 by ThirdHatt

Steve,

 

I am getting the CFM data directly from Marine Air for each unit. I am positive these old ones are Lunaire units and the old chillers were Lunaire as well. I st

ill have the paperwork and manuals from the when the original system was installed in 1992. They say Lunaire on each unit. This is a custom boat (Trident) built in your neck of the woods, Tampa! 

 

I am going to get the flow controlled option so that the air handler will take exactly what flow it needs. 

 

So, back to the cooling question, how would you compare a 18k btu air handler with two blowers flowing 600cfm to a 24k btu air handler with one blower flowing 480cfm?

 

THANKS!
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Steve Pooler

posts 1127

5:31 pm January 7, 2012
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Third Hatt~

The info I have shows you must have your CFM specs reversed:

http://www.dometic.com/8cb8131…..1195.fodoc

http://www.dometic.com/218273c…..ae81.fodoc

http://www.dometic.com/4725167…..5885.fodoc

 

Depending on model…The 24K units move approx 200 CFM more air than the 18K's

To answer your question practically…Never ran into Luneair air handlers and did work on some Tridents here but…

In my experience anything you buy today, Seems to move more air than any unit I have experience with from back then.

 

Steve~
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ThirdHatt

Madisonville, LA

posts 18

6:12 pm January 7, 2012
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The ATL18DC is the low profile unit I was referring to that flows 600cfm.  The AT24HV is the regular one I was referring to that flows 508cfm (sorry, not 480). Either way, the 24k btu unit is flowing roughly 100cfm LESS than the low profile 18k btu unit. This is why I am confused as to which unit will cool better if the 18k has less capacity but more airflow.

Your thoughts now that you know the two exact units I am referring to and the exact cfm ratings?

 

Thank you Steve!
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Steve Pooler

posts 1127

6:52 pm January 7, 2012
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Post edited 11:33 am – January 8, 2012 by Steve Pooler

I see your point…The spec for the AT-24 does seem a bit low & could be a typo since the similar DX 24K air handler is 800 CFM…Please See: http://www.dometic.com/953ad6c…..510e.fodoc …I believe that they use the same blower on both the DX & CW 24K units with single HV blower, but could find out for you during the week when the factory is open…However, like I said it's not as critical with CW since the loop or air coils never get down to below 32 degrees like a direct expansion system could…

I can say that I have installed one low profile dual DC blower unit & it moved a TON of air…In fact it was very loud with wind noise & we had to restrict the high speed in the control to keep it from being so noisy in the installation application it was in.

What will happen is…With a given loop temp & room temp…The one that moves more air will be picking up more heat, but likely have higher discharge air temp…Lowering fan speed lowers the amount of heat it picks up, thus lowering outlet air temp. But add a larger coil and moving the same amount of air (same CFM)…The larger (coil) unit should be removing more heat.

Knowing all that makes it hard to definitively answer your question… Confused "Cool Better" is a subjective term that some incorrectly quantify as colder air discharge…Removing heat is the objective & that's the reason for BTU ratings & Dometic is usually pretty straight up with their ratings…

I think to further try to answer the question…You really only want to run High speed when the cabin is many degrees away from your setpoint temp to bring the temp down as fast as possible…Once the cabin is cool you want the lower CFM to reduce air noise…

BTW…You do know that you need special control board for the DC blower motors correct ?

Hope all that helps…Let me know if you want me to check on the possible typo…

 

Steve~ 
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ThirdHatt

Madisonville, LA

posts 18

12:46 pm January 8, 2012
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Wow Steve, that would make total sense if it is a typo. I do see that all the other 24k btu models are arounf 800cfm which is what i would expect and what I am looking for, more flow. This is why I have been wondering why the 24k was quoted so low.

The unit that I am replacing splits the air into three vents. The old unit barely flows now and I would like to to move more air. The vents are at the heat sources, two windows and a door so they are installed where they should be but more flow would help that side of the room. It turns out the 24k btu unit will fit under the sofa where the current low profile Lunaire is, so I see no real need to get the much more expensive low profile unit.

 

I did not know that I would need a special control board for the DC blower. Bummer, more expense!

 

Steve, thank you so much for clarifying for me. I greatly appreciate it!
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Steve Pooler

posts 1127

1:10 pm January 8, 2012
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Post edited 1:28 pm – January 8, 2012 by Steve Pooler

Just out of curiosity…What temp is your loop set to maintain ?

One of the biggest advantages to CW is the quieter/lower blower speeds you can run…It's all about volume of air not velosity & with larger or more duct it may not feel like you are moving much because of the lower exit speed…

But if your loop temp is too high you may not be getting the Temp Differential (TD) that would help your situation…

You mention new chillers & I know Marine Air Factory settings for loop are above 50 degrees return…

If you can tell me what the display inside each Chiller electrical box reads for both inlet & outlet I may be able to help you out there…Note: Both compressors need to be running so I can tell loop TD before I can tell you if you can lower the setpoints… 

But…Don't get me wrong…I'd love to sell you an air handler Wink

 

Steve~
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ThirdHatt

Madisonville, LA

posts 18

2:40 pm January 8, 2012
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Post edited 2:41 pm – January 8, 2012 by ThirdHatt

That's a good question Steve. I actually do not know what temps they are set at. I'll have to get back to you on that. I almost never run both chillers because one really cools (or heats) the boat extremely well. When the boat is running I will use both to load up the generator but at the dock I use one because it saves alot on the electricity bill. Also at some marinas the dock breaker will trip if I run both.

 

I really have no complains about the system. It has ample cooling and airflow throughout the entire boat without hotspots, etc except for this one weak air handler under the port salon sofa. The salon stays cool in summer but I can't get it really cold like I can most other parts of the boat.
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Steve Pooler

posts 1127

3:08 pm January 8, 2012
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Well there really is a lot more to CW systems than may be evident to you (no offence) and your issue could be as simple as that AH being air bound…
I'm on phone now so I am going to make this short but do please ck what I asked & we can start there…Also be sure you have 15 to 18 psi of water pressure in the loop…
Then get back to me….

Just as a heads up…The chillers are supposed/should be setup to run staged…The only time both run is if the second is needed to keep the loop within our set parameters which will help dehumidify by keeping the loop at the lower temp….

If you are thinking of spending money on an air handler…That money may be better invested in SmartStarts for those 5 ton compressors first…They really do reduce your starting amperage unless you already have 3 phase chillers & VFD's

Steve~
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