Using a heat exchanger to increase latent heat extraction
This is a follow-up to a discussion from an article that was recently posted about dehumidifiers — https://www.greenbuildingadvisor.com/article/dehumidifying-with-portable-unit-vs-ducted-system? *– and a discussion in the comments. I wanted to break that discussion into a new thread because it’s really a separate topic, and posts in the Q&A seem to get more and better comments.
I feel greatly indebted to Charlie Sullivan, who turned me on to the idea of using a heat exchanger in combination with a regular air conditioner to make a dehumidifier with very high latent heat removal. If you look at posts #8 through #11 in the comments, I expound on the idea in great detail, and I came to the conclusion that doing so would use about 75% less electricity for the same level of dehumidification as a standalone dehumidifier, or even a conventional whole-house dehumidifier.
The idea is that the output of an air conditioner is run through a heat exchanger, where air entering the air conditioner is cooled and air leaving is warmed. If the heat exchanger were 100% efficient the air would leave at exactly the same temperature as it entered at, there would be no sensible cooling. When the air was cooled, moisture in it would condense out, so there would be the same level of latent cooling as a similar-sized air conditioner would develop.
This is such dramatic improvement over conventional dehumidifiers that I’m wondering if there is a catch that I’m not seeing.
Thoughts?
*(Ironically the article was a “Q&A Spotlight,” where questions here get expanded into full articles. So we’ve gone full circle!)
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DC, there are a couple extra characters at the end of your link. Here it is for anyone trying to find the discussion:
https://www.greenbuildingadvisor.com/article/dehumidifying-with-portable-unit-vs-ducted-system
Thanks, not sure why it's doing that. I think I've fixed it now.
Interesting.
With a water coil, this is not too bad of a DIY add on. Get a 2nd coil put it after the cooling coil.
Set the water flow rate and air flow rate through the cooling coil so the water outlet temperature gets to pretty close to room temperature. Run this water into the 2nd added in coil as a reheat. If you want cooling and dehumidification, bypass the water around this 2nd coil.
A more universal setup would be to use a counterflow HRV core. One side gets the air flowing to the AC coil, the other side, the air out of the AC coil. If you want cooling, bypass the HRV core.
The reheat is basically what I lay out in post #9, "Scenario two," in the original article. It is more efficient than a stand-alone dehumidifier, but not nearly as good as using a heat exchanger.
Using a counterflow HRV core is exactly what I have in mind, all of the sensible heat you took out of the air gets returned and reused, so it's super-efficient.
In theory it could be used on any air conditioner, so it's more of a configuration option to an existing system than a new piece of equipment. The one thing I'm not sure about is that your system has to be able to modulate pretty low. In the scenarios I lay out removing the sensible load and just having the latent load reduces the total load to about 40%. So the compressor has to be capable of running at 40%. The less humid it is, the lower that has to be.
This is a reason that I think this approach is particularly well-suited to hydronic cooling, because with a buffer tank a chiller is essentially infinitely capable of modulation. The other reason is that people with hydronic systems often want to do 100% sensible cooling -- cooled floors or panels -- and this is a way of complementing that with 100% latent cooling with no energy penalty.
The HRV route would not be too bad to try. You would need a pretty big core though, part to increase heat transfer with the small delta T but part to reduce friction losses.
Big resi HRVs are around 200CFM and those need almost 0.3"WG pressure for rated flow on each side. So that would be 0.6" total, doesn't leave much for house ducting. If you can run at 100CFM it will probably work but definitely not using that with any modulating AC. It would work with a small hydro coil.
If they would still make those dual core HRVs, you could re-plumb it with a small hydro coil and get fresh air and dehumidification in one box.
Thanks, that's the kind of detail I'm looking for.
The HRV's I'm looking at have a built-in fan, like this one:
https://www.supplyhouse.com/Honeywell-Home-VNT5200H1000-TrueFRESH-Heat-Recovery-Ventilator-200-CFM
So they shouldn't affect house ducting. What's more of an issue is just finding a HRV that has enough capacity to go on even a small ducted system.
One issue I see right away is that unit draws 180 Watts for 200 CFM. In my scenarios, I was calculating 183 Watts for 100 CFM. So having the HRV powered ups the power consumption by about 50%.
Not sure that is what you want to start with. That doesn't look like ECM blowers, so pretty high power use. It is also a cross flow core, so efficiency is not that great.
I think taking say two of these in parallel:
https://gasexperts.ca/product/lifebreath-hrv-hexagon-heat-core-68-221r/
And using the blower of the HVAC to push through both sides might work. Might be able to get 150CFM or so through it at reasonable power.
Still far cry of 400-600 cfm needed for a ducted AC setup.
Akos, here's my mental image:
There's an air handler with a coil in it which can provide either heat or cooling. When you're cooling, at the flip of a switch the air path changes, and instead of just going through the coil air comes in at the return, goes to the inlet of the heat exchanger, to the intake of the air handler, back to the other side of the heat exchanger and from there to be distributed. This is done with motorized dampers in the ductwork. So now you've got three distinct modes: heat, cool, and dehumidify.
I think -- but I'm not sure -- that it would be simplest to have a booster fan for dehumidify mode, to make up for the additional resistance from the circuitous route. The alternative would be to have one fan that is big enough for dehumidify but can be turned down for heating or cooling.
Another option is this:
https://www.lifebreath.com/product/rnc-200/
A 200 CFM HRV that uses the core Akos pointed out in #7. Comes with a fan. I happen to have a 6000 BTU hydronic cooling coil in my workshop right now, the thought would be to insert that coil and make a super dehumidifier.
Thoughts?
Besides the cost, that is a good option.
I would guess you would have around 60% efficiency at full flow and maybe 70% at 100CFM. The blowers are not the greatest, you are dumping probably another 100W of heat from them into the air stream.
For a quick test, I think the dual water coil setup is easier and cheaper. You'll probably need thicker multi pass coils to get the delta T down on both chiller and recovery coil. This setup is much more sensitive to flow rates( you need the water out of the cooling coil to get close to room temp before sending it to the recovery coil), so might be harder to set up.
So you've hit on the things that I think would make or break this idea: efficiency of the heat exchanger, air flow rates, and power consumption. Plus of course cost. The idea is to build a better humidifier, if it really uses 75% less electricity then some cost is justified, but if it ends up being really no better than what's out there then no cost becomes competitive.
I don't quite get what the recovery coil is. I assume that's heating the air? Where's it getting its heat from?
DC,
If this was an easy thing to do, you would see it everywhere. So my guess, it is harder than it sounds.
If you use a thick enough chiller coil with low enough water flow rate, the RWT on it will eventually approach supply air temp. You can use this "warm" water and pass it through the next coil do heat the 40F air back up to near room temperature.
The air going across the coil is going to be coming off of the heat exchanger at roughly 51F. So the return water is never going to be warmer than that. You could however pump it somewhere else where you just need sensible cooling, like a panel radiator or a heated floor.
Agree that if it was easy it would be widespread. On the other hand, it does take time for ideas to get accepted. Dehumidification is becoming a bigger issue as houses get better insulated and have lower sensible loads. And HRV's are still kind of on the fringes of the mainstream.
You have to look at a multipass coil (for reference, front is air in side). The 40F chilled water enters along the passage set on the back, this is where the 50F air leaves. Along each pass of the coil, the air temp is higher and so is the water temp. At the final pass on the front, assuming the flow rate is low enough, the water temp would be close to the air temp.
To the "if this was easy to do" critique, there are in fact people doing it:
1. It's the key technology that makes Thermastor's Santa Fe dehumidifiers substantially more efficient than most (and also makes their boxes large, which has been controversial in energy star specs).
2. Thermastor offered exactly this product as a "split dehumidifier", model SD12 https://www.bakerdist.com/therma-stor-4033170-ultra-aire-sd12-dehumidifier--deaae13 . No longer for sale. It's an evaporator wrapped in a heat exchanger with an outdoor condenser.
3. A company called Dewair offers these with both evaporator coils or hydronic coils, at commercial scale and one for residential applications. https://dewaircorp.com/products/dehumidifiers/
As far as the dual coil setup, that is a way that some large commercial HVAC systems do heat exchange, where it's called a run-round coil. If that were more practical than a typical HRV core, why wouldn't those be used for typical residential HRV units?
[response to #30]
"No longer for sale" is a huge red flag for me.
I assume that the reason it's off the market is that it didn't sell well. I think the performance was fine, and met the specs listed. Consumers couldn't buy a split unit and install it themselves, so you'd need HVAC company buy-in. And they might not be ready to think through the advantages of putting the condenser outside.
But it is true that you can get to the same comfort results with a self-contained dehumidifier and run extra cooling to make up for the heat it puts in the space. Since the other Santa Fe units use the same tech, they have the same L/kWh performance, so it couldn't sell just on that metric--it needed people to think through whether they want the heating minimized, and conclude that they do.
DC, can you draw a picture? I'm not quite following the whole air path on the system.
I'm going to try and do some pictures. My Sketchup is down so I'm reduced to using paint. Since the forum software is pretty attachment-unfriendly, I'm going to do each picture in its own post. First picture is just a heat exchanger as it is normally used, to bring in ventilation air from outside while recapturing the heat in the exhaust air. I just got this from an image search. I'm adding this because my sketch of a heat exchanger is going to suck, refer to this to see what I mean.
Second picture is just a conventional air handler doing air conditioning. I put in some stats about the air temperature and humidity.
Since pictures often display at low resolution here, the intake air is at 75F, 55% RH, dewpoint of 58F; the exhaust air is at 40F, 100% RH, dewpoint of 40F.
OK, third picture, this where I put them together. The input to the air handler comes off the heat exchanger, the output of the air handler goes back into the heat exchanger.
The stats:
Heat exchanger room air primary input (yellow arrow): 75F, 55% RH, dewpoint 58F.
Heat exchanger output and air handler input (orange arrow): 51F, 100% RH, dewpoint 51F.
Air handler output and heat exchanger secondary input (blue arrow): 40F, 100% RH, dewpoint 40F.
Heat exchanger final output (yellow arrow): 75F, 28% RH, dewpoint 40F.
This assumes 100% efficiency on the heat exchanger. One of the things I'm trying to get a handle on is what are realistic efficiencies and flow rates for a heat exchanger. The idea falls apart if I can't get reasonable efficiency at a flow that meets at least the minimum flow requirement of the air handler.
As drawn, this functions solely as a dehumidifier. The next step in the design would be to include dampers to bypass the heat exchanger and allow the air handler to function as an air handler and provide straight heating and cooling when needed.
Ok, now I understand. The air fed back to the A/C unit would be fairly cold after the heat exchange process I guess, would it not risk freezing the coil in all this?
Yes, you have to have some sort of modulation on the cooling coil to match the heat it removes to the airflow.
To quantify, in the scenario I'm describing, where air comes in at 75F and 55% RH and leaves at 41F, the cooling load is reduced by about 60% using the heat exchanger as opposed to just running the air handler as an air conditioner with the same airflow. So it has to be capable of modulating to 40% or lower. Or alternately, it has to be undersized as an air conditioner.
Undersizing isn't terribly attractive, because it means in air conditioning mode it's going to do less dehumidifying, and presumably you're going through this trouble because you want to maximize dehumidification.
Yes, that all makes sense now. It's an interesting proposal, a bit expensive to set up I think though. In a high humidity area, I can see it really helping. It's often high humidity but not super hot out in the midwest here.
Hi DC,
Did you consider a rotary heat exchanger? These are in small ERV/HRV systems and also can be rather big.
like: https://www.hoval-energyrecovery.com/en/products-and-solutions/#product-anker-2
have look here: https://www.hoval-energyrecovery.com/en/applications/ This is kind of what you plan.
One rotary exchanger beside a ducted system - that you come from the return through a first half of the RHE to the coil to the other half and then leave into the duct to the house again. These units do not have to have the big pressure loss and can be made with a nice cross section.
We have the small units from Hoval in Germany also as a residential ERV which do not need a winter bypass.
regards
I will look into it. Thanks.
It seems that the big selling point of the rotary exchanger is that it is vapor permeable and allows humidity to move between the input stream and the output stream. In heat-recovery ventilation this is desirable, but in what I'm doing it isn't. I want all the humidity to be present when the air goes through the air handler.
DC,
Yes - I agree with you that a moisture transport by the heat exchanger is not wanted at all. A rotary system without the molecular sieve - just the metal vanes - would do the trick.
edit - on second thought:
how does any ERV work? Is the vapor driven by the absolute humidity differential (by the partial pressure of the vapor) or by the relative humidity across the membran? (or rotary vane) The cold air leaving the coil is at 100% rel. humidity and lower partial pressure, if that humidity would be passed through the ERV to the incoming warm air it would not go back into the room but back into the dehumidifier for another condensation cycle - boosting . But - I guess that does not happen??
My understanding is that that fundamental physics is that some ERV membrane materials work based on partial pressure (when it's behaving just as a membrane) and some work more on humidity (when a material, such as paper, absorbs the moisture from one stream and releases it in the other). But the heat exchange is good enough that the temperature difference across the membrane at any given point is small, and the way you expect it to behave is the same whether you figure based on % humidity or based on partial pressure.
But for this you'd use an HRV core and there's none of that going on, and you don't want there to be.
Does this picture sum up your idea?
The way refrigeration and heat exchanger work the following 5 things must be true.
1 Point C must never be allowed to fall below 40°F or the evap coil will freeze over and stop the air flow.
2 The evap coil will lower the point B about 20°F at best to point C.
3 Point D will always about 10°F cooler than point A.
4 Point E will be 20°F warmer than D.
5 Point C will have a 100% relative humidity.
If you optimized the air and refrigeration flows you could make the temps, be
Point A starts at 75° 55%
Point C is 40° at 100%
Point B is 60°
Point D is 65°
Point E is 85°
Walta
Thanks Walta.
That looks correct. I agree with #1 and #5 without qualification.
#2, the amount of cooling depends upon the flow of refrigerant and the coil temperature you can achieve without freezing up. It's not really going to depend on the point B temperature.
#3, if the heat exchanger is 100% efficient then Point D will be the same temperature as point A. The extent to which Point D is colder than Point A is entirely determined by the efficiency of the heat exchanger. Which is kind of a tautology, as the efficiency is defined as how much of the temperature difference is transferred over.
To #4, I'm not sure what the point of the "cond coil" is or where it gets its heat from.
#2 continued -- but the cooler it is at Point B, the more dehumidification you get.
Which is just another way of saying again that the more efficient the heat exchanger, the better the whole idea works. With a 100% efficient heat exchanger you get 100% latent heat removal, 100% of the cooling going to dehumidification, zero sensible cooling.
If this is a self contained unit, with the "cond coil" is the condenser of the same refrigerant loop cooling the evaporator coil, this is a diagram of a Santa Fe high-performance dehumidifier. I suppose one could have a refrigerant valve that would use that coil as the condenser in cool shoulder seasons when you want a little heat with your dehumidification, but send the refrigerant to an outdoor condenser when you don't want that heat.
I agree that the temperature drop over the evaporator coil can be much less than 20 F--the idea is to make most of the enthalpy change be associated with latent heat removal, so the ΔT is less than it would be if it were mostly sensible--as well as setting up the system for a lower refrigerant flow so you aren't removing as much enthalpy and you aren't making the compressor work as hard.
“#2, the amount of cooling depends upon the flow of refrigerant and the coil temperature you can achieve without freezing up.”
20° is one of the things that happen in real life with refrigeration systems. Yes you can get larger deferential across a cooling coil but to make it happen gets very risky in that you may fail to boil all the liquid and damage the compressor and the total number of BTUs being moved fall off quickly while the COP drops way off.
“It's not really going to depend on the point B temperature.”
Point B is assumed to be 20° warmer than the fixed temp of point C
“To #3, if the heat exchanger is 100% efficient then Point D will be the same temperature as point A."
If we lived in a perfect world perpetual motion machines would be everywhere. It is just not possible to get the heat energy to flow without a potential difference and 10° is generously close to perfect. Note if the perfect heat exchanger somehow connected points B&C there would never be any heat for the evap coil to remove.
“To #4, I'm not sure what the point of the "cond coil" is or where it gets its heat from.”
Any heat the evap coil extracts from the air flow has to go to a condenser coil somewhere in a complete system.
Walta
"Any heat the evap coil extracts from the air flow has to go to a condenser coil somewhere in a complete system."
I'm envisioning it's going to an outside unit. Dumping the heat inside is something I talked about in scenario #3 in my original post, this is more similar to a conventional dehumidifier. Using the heat exchanger gets you about a 45% savings in energy usage over a conventional dehumidifier. Which is good but not as good as dumping the heat outside.
But that energy usage assumes in both cases that the heat needs to be removed by air conditioning. It's kind of unknowable whether surplus heat is an asset or a liability. I could see wanting that heat, or some of it, if you have a shoulder season where you want dehumidification and heating. And since the heat exchanger isn't going to be 100% efficient, you're going to get some incidental sensible cooling, you may want to counteract that with heating.
And any cooling you do to remove unwanted heat is necessarily going to remove humidity as well, which needs to be accounted for somehow.
Cooling you do to remove unwanted heat might not remove humidity--if you don't need it to, you can run the sensible cooling evaporator warmer, leading to less lift between it and the condenser, and higher COP. The dehumid coil is run colder and requires more lift, so the COP is lower, but you are only doing the dehumidification so you are consuming less energy with that lower COP.
But I think the main advantage efficiency advantage is being able to get pleasantly low humidity without needing to over-cool the space. And if you can leave the temperature higher, that actually means less moisture removal is necessary for a given RH target, since 55% humidity is higher moisture content at 78 F than it is at 70 F.
"20° is one of the things that happen in real life with refrigeration systems. Yes you can get larger deferential across a cooling coil but to make it happen gets very risky in that you may fail to boil all the liquid and damage the compressor and the total number of BTUs being moved fall off quickly while the COP drops way off."
Akos, what I'm trying to do is spitball some numbers, and the more I can do to make them realistic the better. So this is helpful information. In my original post I started with the assumption of air entering at 75F and leaving at 40F. I got those numbers from looking at the BTU output and CFM for a ducted Mitsubishi minisplit. I then evolved the design to include the heat exchanger, but I wanted to keep the latent heat removal the same so I could do an apples to apples comparison of the different scenarios.
I will rework my calculations to a) assume a 20F drop over the cooling coil and b) assume less than 100% efficiency of the heat exchanger. I don't know what is reasonable, some sources say up to 95% efficiency, some say less. I think I'll start with 85%.
The minisplits seem to give more BTU per cfm than traditional units. The rule of thumb I was taught is 400 cfm per ton. If you have incoming air at 75F and 55% RH that gives exhaust air at 54F and 100% RH -- 21F drop.
OK, running the numbers with 85% efficiency and a 20F delta over the coil I get a coil exit temperature of 29F. That's not going to work.
If I drop the efficiency to 50% I get:
Stage 1: (Point A to Point B, the warm side of the heat exchanger)
Entering air: 75F, 55% RH
Leaving air: 57F, 100% RH
Sensible cooling at 100 CFM: 1944 BTU/hr
Latent cooling at 100 CFM: 200 BTU/hr
Stage 2: (Point B to Point C, the cooling coil)
Entering air: 57F, 100% RH
Leaving air: 37F, 100% RH
Sensible cooling at 100 CFM: 2160 BTU/hr
Latent cooling at 100 CFM: 2523 BTU/hr
Stage 3: (Point C to Point D, the cold side of the heat exchanger)
Entering air: 37F, 100% RH
Leaving air: 56F, 48% RH
Sensible warming at 100 CFM: 2052 BTU/hr
Overall at 100 CFM there is 2723 BTU/hr of latent cooling. The air enters at 75F and leaves at 56F so there's 2052 BTU/hr of sensible cooling. The sensible heat ratio is 43%, which is lower than what you could achieve with an air handler without freezing the coil.
The heat removed goes to a condenser coil. If the condenser coil was in the exhaust air path (at Point D) it would net you 2722 BTU/hr and the exhaust air would be at 100F. Alternately you could have an outside coil and an indoor coil and split the heat 43%/57% and be sensible neutral. This is not accounting for the heat from any electricity used to run fans in the heat exchanger.
This doesn't sound terrible. Total dehumidification running 100% would be 50 pints per day, which doesn't sound like a lot for an expensive piece of equipment. But it would consume much less electricity than a standard dehumidifier.
I don't think we take 1960s rules of thumb like 20 degree temperature differences across coils as necessarily constraints for systems with vastly better controls than were available then. And we especially shouldn't apply an air-conditioning rule of thumb to a high-performance dehumidifier.
It does depend on whether you are thinking of something hacked together on site vs. engineered equipment. For hacking something together, you can simply use chilled water (or glycol) that is kept no colder than 32 F, and you are guaranteed not to have problems freezing up the coil.
“ I don't know what is reasonable, some sources say up to 95% efficiency, some say less. I think I'll start with 85%.”
I did not take the engineering classes but my guess is 5% at a 5° differential and 95% at 120°.
Walta
[reply to #30]
"and also makes their boxes large, which has been controversial in energy star specs"
Can you explain this further? I didn't realize that the physical size of a unit was a factor at all in Energy Star ratings.
The physical size is used to put dehumidifiers in different categories, and the size of the ThermaStor products puts them in a category where the standard is much higher--jumping up from 2.09 L/kWh to 3.3. Yet the database lists none better than 2.47. ThermaStor does have four models that beat the 3.3 target, so I'm not sure why those aren't in the database. They also have one that's compact enough to qualify for the 2.09 target. The net result is that their one energy star model is one of their lowest efficiency models. The rumors are that a cartel of other manufacturers pushed for this weird rule, but I don't know why Energy Star went along.
OK, I found the Energy Star specification, for ducted dehumidifiers there is a lower requirement for case size under eight cubic feet. Which seems tiny. And irrational.
I'm going to go off on a bit of a tangent and look at the Energy Star specs for ducted dehumidifiers. Weirdly, they divide them into categories by physical size. The requirement for small units, under 8 cubic feet, is 2.22liters per kWh or better. For large units it's 3.81.
This quantity is basically dimensionless, like COP. It's COP times latent heat ratio. Water has a heat of vaporization of 683 Wh per kg, so 2.2 liters represents 1.5 kWh. So for small units the ratio has to be 1.5 or better, for large units it has to be 2.6 or better.
The test is done at 65F, 60% RH. The highest possible latent heat ratio for a single stage under those conditions is 36%, that represents cooling the air to 32F. Any colder and the coil would freeze. Even 32F is probably unattainable.
At 32F, the heat pump would have to have a COP of 4.17 to achieve a ratio of 1.5, and 7.22 to achieve a ratio of 2.6. The first number is probably achievable with careful design. I don't think the second number is achievable with a single stage.
With a two-stage (IE, a heat exchanger) you can raise the latent heat ratio. How high you can raise it depends upon the efficiency of the heat exchanger, at 100% efficiency you get 100% latent heat ratio.
If you had the same COP 4.17 heat pump that is considered Energy Star in a small, single stage dehumidifier, you'd have to have a latent heat ratio of 62% to be considered Energy Star in a two-stage. I'll have to check my math but my initial calculation is that requires a 50% efficiency in the heat exchanger to get 62% latent heat ratio.
So basically they have two standards, one for units with heat exchangers and one without. Which doesn't seem to make sense from a policy perspective, it's not like there's some reason people should be discouraged from seeking out more efficient equipment.
OK, I checked my math and my new answer is the heat exchanger has to be 66% efficient. Here's my work:
At 100 CFM, if air at 65F and 60% is cooled to 32F, the latent cooling is 1984 BTU/hr. If the latent heat ratio is no less than 62%, the total cooling has to be no more than 3200 BTU/hr, which means the sensible cooling can be no more than 1216 BTU/hr. When the air is cooled to 32F, the sensible cooling is 3564 BTU/hr. In order for the sensible cooling to be no more than 1216 BTU/hr, at least 2348 of 3564 BTU/hr needs to be returned to the air, or 66%.