Split Face or Intertwined 1

[AbbyNormal]

Which type of DX coil do you prefer and why?

Application 2 circuit coil (2 compressors) constant volume cooling an office space handling 15 to 20% ventilation air with 80F dewpoint.

One stage will always be calling, second stage will cycle on/off in occupied mode.

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

 

[NCDesign]

I just went back and looked on a psych chart and I think the answer really depends on your load. My applications haven't been that critical so I haven't worried too much about it.

A face split coil will remove more moisture but will have a higher leaving air temp. An intertwined coil will do just the opposite and remove less moisture but supply you with a lower leaving dry bulb temp. So if your partial load has a lot of moisture I would say use a face split. Otherwise I wouldn't worry to much about it.

A couple other things to keep in mind. How well is the air mixed once it leaves the coil? If the coil is in a blow through arrangement you could have stratification in your duct because the air isn't turbulent enough to mix. This could be a major problem depending on your duct routing, takeoffs, and configuration as well as whether the coil is a horizontal or vertical split. If it is a draw through configuration the fan should do a pretty good job of mixing the air off of the coil. You also need to make sure the 1st active section is on the bottom (if it is a horizontal split coil). If the top section is active and the bottom isn't you could have problems with condensate blowing off the lower section.

I didn't really answer your question, but hopefully this gives you some good information to make your decision.

 

[AbbyNormal]

Split face is easy to look at on the chart, it is basically mixing leaving air and entrering air. Trying to visualize the intertwined on a chart right now.

These are two draw through air handlers, the space load is highly sensible given that it is an office, a lot of humidity in the ventilation air being mixed in. The mixed air would need coils working at SHR=0.69 & 0.71.

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

 

[NCDesign]

If you want to see the intertwined plot your mixed air temp and draw a coil line from it to 55F. Then take half of your total capacity and calculate your enthalpy change. Draw your enthlapy line across the chart, when you cross your coil line you have your leaving air temperature from an intertwined coil at half capacity.

 

[AbbyNormal]

I need to think about this.

With split face mixing 50% leaving air with 50% entering air. Without plotting it I would most likely be right in the middle of the coil process line.

With intertwined, it sounds like I would be in the middle of the process line as well.

These are draw through air hndlers so would get some mixing from the blower.

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

 

[NCDesign]

The intertwined will definitely be on your coil line. With an intertwined your full coil face is active so you are bringing all of the air down by half of the total capacity. It's design means you are on the coil line.

On the split face coil, part load leaving air temp will not be on your coil line. Just for the reason you stated above. If you chart the split face on the psych chart you would draw a straight line from the coil leaving temperature to your entering air temp. You would then use the 50% air mixture to calculate your dry bulb temperature and plot that point on the line you drew from the coil leaving temperature and the entering air temp. This is a bit of a oversimplification because it would take some assumptions/calculations of your expected part load and several iterations to get your exact condition

Getting back to your original question. I hadn't paid close enough attention. If you are using 20% OA at 80F Dewpoint (assumed 93/83 for calculations) you will need that extra moisture removal at part load. Assuming 8,000 CFM total air, 1,600 CFM of outside air, and your desired conditions leaving the coil are 55/54 you would have ~174MBH just in outside air load on a design day. That is a SHR of 0.38! I would want the split face coil for part load to try and keep the humidity from spiraling out of control. Which would be especially prone to happen on a very humid non design day. If I remember correctly you are in a tropical area so I would think this would be a very likely occurrence.

Finally, if you can I would use a deeper coil than the standard 3 row commercial coil. I would try and get at least a 6 row coil for better moisture removal. If this isn't a built up system you may want to look at a unit designed especially for large amounts of outside air. If you don't you may end up dealing with the same problem as MAragorn except your equipment won't be oversized.

 

[AbbyNormal]

with split face, one circuit on, the air leaving the bottom coil leaves at the intended dry bulb/wet bulb, the air leaving the top half of the coil is the same as it enters.

So to mix air connect the points with a staight line. I think this mixing line would be the exact same as the coil line.

It is a six row coil, the weather here is pretty steady for the rainy season. This year has been hot and dry, dewpoints hovering around 77. Usually it is 78 to 81 for six months.

part load will occur in the winter, which will be typically sunny, low 80s dry bulb, low 70s dewpoint.

The worst situation here is a tropical storm hovering in the area, can get a week or two of 82F, 80 dewpoint and no sunshine.



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

 

[NCDesign]

It shouldn't unless your full load condition is sensible cooling only. Your psych chart should have some example coil lines already on it. (I have a bunch of Trane psych charts, they come with some black coil lines spaced out about every 8F~9F Dewpoint from 74F Dewpoint on down.) They go straight to the left (sensible only cooling) until you hit about 77-80% RH and then they start to curve down and to the left slowly. Moving closer and closer to 100% RH but never quite reaching it. I'm gonna try to illustrate with some good ole' ascii art.

x=coil entering air temp (78 DB/ 65 WB)
o=coil leaving air temp (55/54)
______ & / =coil line


___________________x
_/
_/
/
o

Now draw a straight line from o to x and then calculate your new dry bulb temp (0.5*o DB)+(0.5*x DB) (using the numbers above that would be 66.5F DB) . Plot the mixed dry bulb temp (66.5F DB in my example) on your connecting line and you will have your part load, mixed air, split face coil leaving air condition (man that is a mouth full). Which in my example is ~66.5/58.8 and will not be on your coil line. For my example I have oversimplified just a bit and assumed that everything connects and the system is perfectly balanced at exactly half capacity. I can explain this further but it is really hard for me to do without having a psych chart to reference while talking in person.

 

[ChrisConley]

part load will occur in the winter, which will be typically sunny, low 80s dry bulb, low 70s dewpoint.

I wish that described my summer conditions.

 

[AbbyNormal]

you would not want this weekend's design conditions, hurricane Dean

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

 

[AbbyNormal]

I never used the coil lines that you have been talking about NCDESIGN, but they appear to model the cooling dehumidification process as initially a sensible cooling process and then once the air is cooled so that it is about 85% RH, the process line starts to curve as moisture is removed.

I agree that this type of behaiour would occur inside of a brand new refrigerator when it was first plugged in. As the walls begin to gradually drop in temperature the air inside the fridge begins to cool, finally getting to the point where moisture has to condense out of the air.

But for how cooling coil works, I always seem to like Willis Carrier's explanation of air contacting a surface of the coil, and air that bypasses the surface of the coil.

If the surface of a coil is at or below the dewpoint of the entering air, mositure begins to condense out.

For a behaiviour like these coil lines suggest, then the majority of the heat transfer has to be taking place where the coil surface is above the dewpoint temperature of the entering air.

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

 

[AbbyNormal]

Been looking at a building an assembly occupancy, lightly occupied 90% of the time. Present ventialtion rate over ventilates the low occupancy but less than 60% of what is needed with full occupancy.

4 stages of cooling.

Coil is intertwined horizontally split, top half is second and fourth stage, bottom half is first and third stage. More like a split face and then each half is intertwined if you will.

The air is poorly mixed and most of the warm, humid ventialtion air sails through the top half of the coil. System is oversized and third stage will not come on until a full space load. I do not believe fourth stage has every come on so in a full load situation, as a best case, half of the humid ventilation air passes through a cold coil section.

In my part load situation especially with only first stage on or cycling on, 90% of the ventilation air is untreated.

In this situation I think split face would be better. Repipe the unit to have the bottom half of the coil be first and second stage and direct the humid air to the bottom of the coil.

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

 

[leftyp11n]

I've been told by the president of a large HVAC contractor that when using two split face coils stacked vertically, the upper coil should be cycled on first. The reason he gave was that the majority of the air will pass thru the dry coil (less resistance). It's supposedly better if both coils are wet. If there is a drain pan under each coil this is probably a mute point. What do you think folks? Is this hogwash or a good tip?

 

[marcoh]

More air will pass though a dry coil, but doubt the difference would that great.

Trane has some info on comparisons the split/intertwined coils. Refer to the following link for a more info:

http://www.trane.com/commercial/library/vol17_1/enews17_1.pdf

 

[AbbyNormal]

That was a good read marcoh, thanks, it makes a case for intertwining the entire face for DX VAV.

The lastest one I was looking at was again a constant volume one, a Trane MCC35 actually.

The way Trane intertwined this air handler is to basically split the coil in half into a top section and a bottom section, and then to intertwine each half with two stages of cooling. So as I mentioned it is really a combination of split face and intertwined. I guess they just want to cut the coil freezing potential in half.

No intermediate drain pan between the two coil sections on this one, so you have to make the bottom coil cold first.

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

 

[leftyp11n]

I remember now that the problem we had with stacked coils was condensate being blown into the supply duct when the upper coil condensate reached the more "open" dry coil below. We had an average flow of 500 fpm but more velocity throw the lower coil. Separate drain pans under each coil would have taken care of this.
Marcoh thanks for the link!