I�m trying to make sense of it all. This seems like an obvious answer, and I have read through the archives - but I�m still not clear on how this works.

Regarding selection of A/C capacity, I understand that the �total� BTU�s is the combined total of �sensible� and �latent� capacities. My question is that if the actual latent load is very low (such that most of the latent capacity for water vapor removal is NOT being utilized), does the �leftover� amount of latent capacity count towards the overall BTU�s needed for the actual/overall cooling capacity? (In other words, does the remainder of latent capacity act as �sensible� capacity?)

A theoretical example (I still don�t have HVAC-Calc yet) would be if a house has a total gain of say, 22,000 � of which only 1000 is latent. A 2 ton unit would theoretically have enough �total� capacity to remove the 22,000 BTU�s, but it may only be rated for 17,000 sensible & 7,000 latent. In a case like this, is it necessary to go up to a 2.5 ton system?

Big G

I use Right-J load calculation program. It recommends the equipment based on the sensible load. Usually the latent load on all that I have sized is rarely over 15% of the total load. So like you say there should be some "left over sensible capacity", but I would like to know the answer to that question too. I have had good luck sizing them using sensible only, but I may actually be oversizing some. Most systems I install are heat pump systems, so being slightly oversized will take your balance point down a few degrees and prolong those strips from energizing. Good question? I just found this site and can't seem to get away from it. We all have questions and being self-employed, nobody to ask. This is a great place. It usually would not be a good idea to ask your competition in the same town would it? Ha. Take it easy.

[Edited by AmanaMan on 03-22-2002 at 08:19 PM]

The ratio of sensible to latent capacity is a function of the coil temperature, controled by the airflow. Most manufacturers rate their units with several different airflows and coils, allowing you to "fine tune" your equipment selection to handle exactly what you need for a particular application.

Now, let's discuss the question of "extra" capacity. If you had some theoretical "dry air" with 0.00 moisture content. No matter how cold your coil was, no moisture can condense. So, you have 24K in latent capacity and 24K in sensible assuming 70% relative humidity, how does that work out in the dry air situation? Well, "some" of the latent capacity would be consumed in sensible heat capacity, but most of it would be wasted by flooding liquid refrigerant back to the compressor. I'll try to figure out a way to calculate this exactly, but I could more likely find some test data to back it up.

Hope this helps some, but my recommendation would be to use the manufacturer's tables and charts to size the equipment for exactly what you need.
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There are no stupid questions. . .just a lot of inquisitive idiots.

I see what you wrote about - the variations in sensible & latent capacities detailed in the manufacturers specifications when differing coils are used. I also understand what you have written, up until the following - I am either dumber than dirt, or I am just too tired to grasp what you wrote here:

quote:

---

So, you have 24K in latent capacity and 24K in sensible assuming 70% relative humidity, how does that work out in the dry air situation? Well, "some" of the latent capacity would be consumed in sensible heat capacity, but most of it would be wasted by flooding liquid refrigerant back to the compressor.

---

I think I was too tired to write any more, and abbreviated too much! Most of the latent capacity can be reallocated for sensible cooling, but not without modifying the system.

The project I am working on now has a table detailing sensible vs latent capacity as a function of air volume per unit of time. This system can vary from 47% latent to less than 5% latent, by increasing air volume.

So, in a residential application: combine your 2 ton condensor with a three ton indoor coil, set the fan to produce 450-500 cfm per ton of cooling. Now, look that up in the manufacturer's tables and see what you have. The old GE tables I have here show (for a BWR724 with a BWE736) a sensible capacity of 22,750 and a latent capacity of 2,375.

But, my point in the post I made last night was this: if you combine a 2 ton A/C with a 2 ton coil, with 400 cfm per ton, you can not assume the unneeded latent capacity will just automatically move over and produce sensible cooling.
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There are no stupid questions. . .just a lot of inquisitive idiots.

BamaCracker:

Is it possible to play with the coil size in the single package unit?

If yes, if I want a different coil, is it installed without changing the price of a new unit, or I will have to pay extra?

If my calculations are right, I need 35,470 sensible and 2,300 latent - about the same picture...
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--vt

Just got a flasback. This months RSES mag. shows the manual J work sheet with the different HTM's and all. We used to get a sensible gain and multiply it by 1.3 for total. I think there was a multiplier for temperature swing also. It has been a long time since I used that Format.
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Dumb sounding questions are better than Dumb mistakes

VT, package units do not really have the option of changing the coil, but they do have tables showing a very wide range of airflow , gained by utilizing the "high or low static kit."

On belt drive systems, the static kits involve changing pulleys (sheaves) and belts. In direct drive units, you adjust the speed taps to the limit of the unit's ability, then upgrade the motor. When the unit is purchased, a high static kit can be requested for a small additional charge.
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There are no stupid questions. . .just a lot of inquisitive idiots.

Bama it is fun to discuss issues with you.
I would say without water on the evap, your TD will increase. I have seen 55 degree TD charging systems on warm dry spring days. It is harder to get the heat out of the air if there is no water. When evap TD drops so does compressor ratio and amount of freon pumped. I would say you could lose up to a third of your capacity (just a quess). Superheat would drop, but it does not seem that the compresser would flood. Even if the superheat dropped to zero, the quality of the freon would be nearly gas. The return gas would have low density and not cool the compressor as well. Hotter compressor could handle a little more liquid.
I would like to see some of that data. I quess the deciding factor on the flooded compressor would be outside temp.
I agree you would not be able to convert all of the unused latent heat to sensable.
Living in the humid south, I like to use 30% latent capacity in my load calculation.

I looked at some old tests today, to see why my unit is not getting the EER I need. We ran a 20 ton unit in the psychrometric rooms about 2 years ago, and it had a sensible ratio of 82%. Now, a year later, with the same compressors, but lower airflow, we are getting a 63% sensible ratio. That means 37% was going for humidity removal. On a 20 ton unit, that's 7.4 tons of latent capacity. Tomorrow, I'll see if we have time to drive up the indoor wetbulb and see how the capacity shakes out, and what happens to the superheat.
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There are no stupid questions. . .just a lot of inquisitive idiots.

Higher indoor humidity (higher wet bulb temp) would increase the load on the unit due to the heat of vaporization of the additional water. In other words, the unit must remove some btu's from the water vapor in order for it to condense. Higher load= higher superheat
Higher load = higher actual SEER (that's what oversizing the airhandler does, bigger coil, higher CFM)
Let me know if this is what you are seeing Bama.

What I'm wondering is, is there any effect on varying the ambient humidity on the outdoor unit? In the cooling mode, since there is no phase change I would guess there would be no effect? Right? In the heating mode (where the outdoor unit becomes the evaporator) I would guess you would get more heat out of the air as the humidity increases (due to the water giving up heat as it condenses) until the coil starts building up ice which would effectively begin removing the load or heat source. Lee

Well, I have been shown in an irrefutable way that outdoor humidity does not affect the unit at all in cooling. In heating, it should affect it the same as indoor humidity does in cooling, except for the icing up issue. Maybe this reduced load due to lower wet bulb temperatures is the reason heatpumps need charge compensators?
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There are no stupid questions. . .just a lot of inquisitive idiots.

The water vapor in the air outside has a higher heat capacity than dry air, but there is so little of it compared to air. The engineer in me says: sure the miosture in the air has an effect, it is just too small to measure.
SEER and capacity depend on evap pressure and condenser pressure. Higher dew point means water can condense out on the evap at a higher temp. The evap temp is a little below the dew piont temp. Higher evap temp means higher compression ratio and more capacity and higher SEER. It would work the other way with low dew piont.
Air flow CFM is very tricky. Slower air give lower TD and more condensation up to the piont that there is not enough air. Then the TD starts to rise and condensation, SEER, and capacity start to fall.
Evaporators are funny. It is all about the balance between heat in the air outside the coil and heat needed to boil freon inside the coil. It was very frustrating in school trying to predict the results of air/evap changes. Then we go to the lab and find out that we assumed some values that had little effect the results in one situation controled the results in another situation.

Bama I would like to see some details on your ultra oversized coils tests. I am assuming the tubing size does not change enough to affect freon flow through the evap very much. The freon path is a very large difference between refrigeration and AC as well as the relative size of the evap. Refrigeration coils drain from the bottom. AC coils drain from the top. Large evap with large tubing size would eventually drop the freon velocity to the piont that oil return becomes a problem. Where does that problem start? Another question: when you are using a bleed off TEV, how does the larger evap size/freon charge effect freon migration? There would be a piont where a larger coil would cause more harm than good. I did a paper on that in school. Teacher really baked my ass over that paper. I still think he was an idiot.

[Edited by kim on 03-20-2002 at 09:08 AM]

If the candidate system is a little short on sensible capacity but has excess latent capacity it may still be adequate if its only a few thousand btu's. In this case an increase in fan speed will result in more sensible capacity and less latent capacity. The reverse is also possible by a reduction in fan speed.

In SC's climate, it is recommended that at least 25% of the total cooling load be dedicated to moisture removal. This means the Sensible Heat Ratio should be 75% or less. The lower the number in the Sensible Heat Ratio, the more of your load is used to remove moisture. There are high SEER units with high Sensible Heat Ratios, meaning they are more efficient but little of the load is dedicated to removing moisture. And there are lower SEER units with lower Sensible Heat Ratios, meaning they are less efficient but more of the load is used for moisture removal.
So when you're concerned about moisture removal, ask your contractor about the Sensible Heat Ratio rather than the SEER.

What is the physical difference in two coils of the same tonnage, one with a high latent load rating, and one with a low rating.

I'm trying to understand the physics here.
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TB


Everyone knows something I don't.

There might be lots of differences, there may be none. For instance, I can increase static pressure (slowing the air) and increase heat absorption by increasinf fin area. (more fins per inch, wider fins, rippled fins) which would give me more latent capacity, but the exact same tubing arrangement with narrower fins, fewer fins per inch, smooth fins, will have more sensible capacity.

Or, take the exact same coil, but one has a piston, and one has a TXV. Or, both have TXVs, but one is set for 6� superheat, and one is set for 15� superheat. (lower superheat means colder coil, menaing more latenet capacity.

Here are the results of the test I promised:

A 25 ton RTU, in a psychrometric room, running 8000 indoor CFMs and 10,000 outdoor CFMs. The indoor dry bulb is 80�, the outdoor drybulb is 95�. The only variable is the indoor wetbulb (noted on chart).

(Wet Bulb Temp)(Total Capacity)(Sensible) (Latent) (EER)
(67) 310474.9 BTUH 207707.7 102767) (10.23)
(65) 297247.7 225611 71636.7) (9.83)
(63) 287816.5 242629.31 45187.2) (9.53)
(61) 283486.5 261374.6 22111.9) (9.5)
(59) 282045.5 269071.41 12974.1) (9.46)

As you see, like we first assumed, some of the latent capacity moves into the sensible column, but some of the capacity just disappears. These are TXV systems, so the superheat remained unchanged. But, in theory, if you tried this with a fixed orifice metering device, the superheat would drop off and the system would start to flood back.

Hopefully, this will help illuminate some of the discussion that has taken place here.

Man! Tables don't do too well ni here do they? Sorry for the smilie faces, but they are to divide the data columns.

[Edited by BamaCracker on 03-21-2002 at 09:24 PM]
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There are no stupid questions. . .just a lot of inquisitive idiots.

Good work Bama! Your results are just as I expected. As you put more heat load on the unit both the total cap. and eff. increase. The way I think of a heat pump is similar to a water pump. The heat source for the heat pump is like the suction (water source) side of a well pump. Anything you can do to make it easier for the pump to suck in water will increase the capacity and efficiency of your system. Pumps are better at pushing than they are at pulling so improvements on the input side usually help more than on the output side. Of course on a heat pump both indoor and outdoor units need to have max. contact between air and refrigerant.

Next question: What happens to the capacity and EER of a heat pump in the cooling mode when you remove some of the load by: (1) slowing down a variable speed blower to increase latent heat ratio (or is it reduce sensible heat ratio) like so many recommend in the humid south or (2) increasing static pressure due to undersized ducts, too small return, dirty pleated microparticle filter etc? I am assuming the compressor is not a variable speed unit.

I have 3 standard 12 SEER systems (fixed orifice) with a sensible heat ratio of about .72 to handle the sensible load. I also have independent dehumidifiers to handle the latent load and keep relative humidity at 50% during the warm season. The dehumidifiers are designed specifically to remove the latent heat and are much more efficient than heat pumps. In other words, I believe having one system to control sensible heat (heat pump with T-stat) and another to control latent heat (dehumidifier with humidistat) is a better solution than trying to get the heat pump to control both. Any comments? Thanks, Lee

Ok, lets see if I got my thinking cap on straight. The way I see it, heat is heat. Sensable heat + latent heat = total enthalpy in the air. If we got a 2 ton system, installed and running right, it will remove 24,000 btu/h enthalpy from the air, whatever combination that comes in. So that means to me, the system will cool the air untill it reaches its dew point as it passes through the coil, at which point the enthalpy being removed is from latent heat, not sensable heat. The sensable heat will stop dropping untill enough latent heat has been removed from the air that a new dew point is established. So the closer the temperature of the air entering the coil is to its dew point, the greater the percentage of latent heat being removed.

If we increase the time the air is in contact with the coil/liquid refrigerant, the system could remove more latent heat, since the sensable heat would be limited, and a smaller percentage of the 24,000btu/h enthalpy gain of the coil would be sensable heat.

Am I ok so far?
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TB


Everyone knows something I don't.

Hey Bama:

Some very interesting stuff, having a controlled environment test chamber must be awesome!

If I interpret your charts correctly you are operating the system with constant condensing conditions and varying the humidity of the evaporator conditions. I interpret your data as having a range of RH from 50.8% to 26.6% and SHR from .669 to .954 and you are recording roughly a 10% loss of total evaporator BTU's over this range.

Now Paul Harvey wants to know "the rest of the story". How much of that 10% loss is being redistributed to provide additional cooling of the refrigerant in the metering device as delta P/T across it increases when the evaporator begins to run colder? How much loss is attributable to a change in volumetrically efficiency of the compressor as the pumping ratio increases under these same conditions? Is it a Piston compressor? If the lower evaporator temperatures are causing the vapor density to change and the volumetric efficiency is decreasing at the same time was the net effect that mass flow of refrigerant increased or decreased in your tests?

Thank you for sharing your insights and for any additional light that you can shed on this subject. Tom R

For what it's worth, I think I read in ACCA Manual S a few weeks ago that much (but not all) of the excess latent capacity DOES convert to sensible. Therefore it's OK to select higher latent capacity than what you need, but not vice versa.

I really didn�t mean to open Pandora�s box with this, but I want to thank everyone for your helpful replies. There is some very interesting theoretical information, and some conflicting opinions on what is the 'right' thing to do. It is apparent by BamaCracker�s experiments that not all of the excess potential latent can be used for sensible.

But I�m still not quite clear on how to properly calculate the system size now that I have done an accurate load calculation. It seems that my speculation was pretty close � I need about 21,000 Sensible & 840 Latent. Do I need a 2-ton condenser with an oversized coil (more surface area) and a reduced airflow, or do I need a 2.5-ton condenser with ??? coil?

Thanks Again!

Big G

Did you ever get a response back?

As far as I know every manufacturer creates and publishes "expanded" ratings. They may go by different names, and contain varying amounts of data, but they comprise all the different sensible and latent capacities at different air flows, different coil and condenser combinations, and different super heat settings (for TXV systems)

These data will be the best source of the answer for your question. I can tell you that a larger coil, with higher airflow, will get you more sensible and less latent, but I can't tell you how much for each different combination. Only the manufacturer can do that; by way of their published data.
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There are no stupid questions. . .just a lot of inquisitive idiots.

Unfortunately, the local Goodman distributor just kind of has a deer-in-the-headlights look when asked about extended or expanded ratings. That is why I ended up starting a new thread asking for expert recommendations from Robin or AllTemp on my specific situation. ( http://hvac-talk.com/vbb/showthread.php?threadid=12504 )

Big G