How can I calculate the heat transfer in an air to air heat exchanger?

[mess]

TL;DR Summary

I would like to consider building a heat exchanger for my campervan. In Canada it can get as cold as -40 where I will be travelling to, I would like to see how much energy/fuel ill be saving with my heater by doing so, to determine if its worth building.

I am considering building this type of heat exchanger, as I have seen several youtube videos on the build. The inside tube is aluminum,

I am planning on building one from the top of my roof to nearly the floor, so ill say 5 feet. I am thinking of using a 1.5" inch aluminum tube, and bringing in 15CFM of fresh cold air per person, and the out pipe surrounding the in pipe will consequently be removing 15CFM. Assuming its just 20CFM for me and my puppy at the moment.

I calculated the surface area is 580 in^3 for that inside aluminum pipe.

I see that aluminum has a heat conductivity of 205.0 w/mK.

I think the pipe will be about 1mm in thickness if I was to guess.

The inside temperature should be 21C, and the outside ide like to calculate for -5 for now.

Thats all the data I feel is needed, though I am not sure what formula to use, and it gets complicated to me because the in-pipe will be flowing cold at 15CFM, while the out-pipe will be flowing hot at 15CFM. Also there is some equilibrium across that surface area between the in and out temperatures at some point, to some degree, which makes it even more complicated (calculus?).

Any suggestions on how I can go about calculating this?

This is all just for fun and exploring science and the world :D

 

[phinds]

the in-pipe will be flowing cold at 15CFM, while the out-pipe will be flowing hot at 15CFM.

Yep, that's what will happen all right, although "hot" is a gross exaggeration. I don't get how removing heat from the camper is going to make you more comfortable.

 

[anorlunda]

ping @russ_watters

 

[mess]

Yep, that's what will happen all right, although "hot" is a gross exaggeration. I don't get how removing heat from the camper is going to make you more comfortable.

Removing heat is not the objective, but a consequence of removing co2, which I am trying to mitigate through an exchanger.

 

[phinds]

Removing heat is not the objective, but a consequence of removing co2, which I am trying to mitigate through an exchanger.

Uh ... well the subject line "Air to Air heat exchanger" plus the fact that you made NO mention of removing CO2 threw me off

 

[russ_watters]

Summary:: I would like to consider building a heat exchanger for my campervan. In Canada it can get as cold as -40 where I will be traveling to, I would like to see how much energy/fuel ill be saving with my heater by doing so, to determine if its worth building...

Any suggestions on how I can go about calculating this?

It's nigh-on impossible to calculate the performance of a heat exchanger from scratch. Fortunately, I don't think it is even necessary. That's a really big heat exchanger and will likely be highly effective. Let's just pull a number out of the air and assume you recover 75%. So how much energy does that save you? Well, you were told in your other thread that 15 CFM takes 1,500 BTU/hr to heat (440W). So the most you could save is around 330W or 1100 BTU/hr.

But it gets worse. All of your exercises so far have assumed your van to be hermetically sealed. It's not. I'm not convinced the ventilation system is even necessary (heating, yes), but even if it is, I'd probably want to keep that 15 CFM as pressurization/to reduce infiltration. Whatever the amount of infiltration is, it's lost energy for your heat exchanger. And if your infiltration is greater than 15CFM, then your heat recovery is zero.

 

[mess]

It's nigh-on impossible to calculate the performance of a heat exchanger from scratch. Fortunately, I don't think it is even necessary. That's a really big heat exchanger and will likely be highly effective. Let's just pull a number out of the air and assume you recover 75%. So how much energy does that save you? Well, you were told in your other thread that 15 CFM takes 1,500 BTU/hr to heat (440W). So the most you could save is around 330W or 1100 BTU/hr.

But it gets worse. All of your exercises so far have assumed your van to be hermetically sealed. It's not. I'm not convinced the ventilation system is even necessary (heating, yes), but even if it is, I'd probably want to keep that 15 CFM as pressurization/to reduce infiltration. Whatever the amount of infiltration is, it's lost energy for your heat exchanger. And if your infiltration is greater than 15CFM, then your heat recovery is zero.

75% would be great, but is there any hint that it would be around that? is there a way to do some educated guess at that? And i did seal it as best i could with sprayfoam and foam panels. I have a CO2 sensor in there. It can maintain the CO2 level for days if i don't open a vent.

Here is a pic: (WIP and needs wall covers still)

 

[mess]

Came across this: https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html

would "18.5.2 General Counterflow Heat Exchanger" give me what i need?

 

[256bits]

Came across this: https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html

would "18.5.2 General Counterflow Heat Exchanger" give me what i need?

Yeah, sure.
Here's another one, same thing, with LMTD.
Your 'picture' is a single pass, single tube in shell counter flow heat exchanger.

What's your velocity of air through the pipe>
And the pressure drop?
Will you still get 15cfm?
With calculations, you will find out if you need more tubes single pass, or a stronger blower.

You need insulation on the exterior shell.
You need the exit temperature of the outflow to be greater than freezing for no ice buildup, or assume at least non condensing ( what's your humidity level interior ) which means a suitable temperature differential between hot and cold for that to not occur ( meaning less heat extraction and less savings ).
Perhaps a parallel flow design could be investigated in that regard.

Since you don't know the exit temperatures, for the calculations you pick them and do some iterations until everything comes out nicely.

 

[Lnewqban]

Let me see if I understand the problem:
Does the diagram show an economizer to re-capture some of the thermal energy of the air that is exhausted from the living space?
The reason for that mass of exhausted warm air is to make room for fresh intake cold air?

What source of heat does the living space have?
Why do you believe that you need so much ventilation?

What is going to move all that air?
Is the air around the external surface still?

 

[Keith_McClary]

Here is a DIY thread for "cross-flow heat exchanger".
I have this type in my house. (This is the 50% efficient type mentioned in the thread, not the 92% Euro type!)
Note, there will be condensation, and you will want filters so they don't get plugged up with cottonwood fluff, etc.

 

[mess]

Let me see if I understand the problem:
Does the diagram show an economizer to re-capture some of the thermal energy of the air that is exhausted from the living space?
The reason for that mass of exhausted warm air is to make room for fresh intake cold air?

What source of heat does the living space have?
Why do you believe that you need so much ventilation?

What is going to move all that air?
Is the air around the external surface still?

not sure what an economizer is. Yes the reason for exhausting the warm air is to make room for fresh outside air (co2 being the main concern).

the source of heat is a 5kw diesel heater. it has a powerful fan (for the space), and i have some aux fans (high static pressure) ill be using.

which external surface? the van, heat exchanger, or the surface of the inner tube in the heat exchanger?

 

[Lnewqban]

Thank you.
An economizer uses some of the energy of the exhaust to change the temperature of the make up air.
I previously referred to the exterior surface of the heat exchanger.

 

[mess]

I built a basic counter flow prototype. It was a warmer day today so I didn't get to test it in negative temperatures ( I will over the next few days).

It is a 3"x8' flexible aluminum tube, inside a 4"x8' pvc pipe. using two fans. one fan sucking from out to in, and another fan sucking from into out. I measured a higher CFM using the anemometer when it was sucking vs blowing air ( i don't know if that's just these fans).

performance:

16C inside
10C fresh air out (going inside)
6C outside

~20CFM in and out

How can I calculate its efficiency, or total watts saved in heat? to see if its worth it for the space it will take up in my campervan.

 

[jrmichler]

First, a diagram to make sure that I understand your description:

The efficiency is the heat added from the heat exchanger divided by the total heat needed, while the energy saved is the heat added from the heat exchanger.

Energy saved is calculated as follows:
Total heat needed is $(16 C - 6 C) * 1.8 deg F / deg C * 20 CFM * 0.018 BTU / ft^3 / deg F * 60 min/hr = 390 BTUH$

Heat added from heat exchanger is $(10 C - 6 C) * 1.8 deg F / deg C * 20 CFM * 0.018 BTU / ft^3 / deg F * 60 min/hr = 160 BTUH$. This is the energy saved by using the heat exchanger. Then divide by 3.412 to get watts: $160 BTUH / 3.412 = 47 watts$. This heat exchanger was saving 47 watts under the conditions listed above.

The efficiency is $160 / 390 = 40$%.

The thermal conductivity of the inner pipe has only a small effect on the total heat transfer. The largest effect is the thermal conductivity of the air film on the inside and outside of the inner pipe. You would probably find that changing the inner pipe to plastic would have only a small effect on heat transfer.

It's a good idea to also measure the temperature of the air discharged to the outside, and compare the heat removed from the inside air to the heat added to the outside air. That gives you a check on the accuracy of your measurements.

 

[mess]

Thank you for that jrmichler! 40% is not that great :(

The thermal conductivity of the inner pipe has only a small effect on the total heat transfer. The largest effect is the thermal conductivity of the air film on the inside and outside of the inner pipe.

What is the air film (didn't find much on it online), and how could I improve it?

 

[jrmichler]

What is the air film (didn't find much on it online), and how could I improve it?

That's a topic that takes up a significant portion of a semester course in heat transfer. You can search heat transfer film coefficient to find many good hits. The best approach is a good textbook on heat transfer. An Amazon search for books on heat transfer found several. Look for a book that's at least in the second edition, and has a title including the words heat, transfer, and fundamentals.

Alternate, and easier, approach is to make a larger heat exchanger.

 

[mess]

That's a topic that takes up a significant portion of a semester course in heat transfer. You can search heat transfer film coefficient to find many good hits. The best approach is a good textbook on heat transfer. An Amazon search for books on heat transfer found several. Look for a book that's at least in the second edition, and has a title including the words heat, transfer, and fundamentals.

Alternate, and easier, approach is to make a larger heat exchanger.

Is it related to the Thermal boundary layer? So would some aluminum fins attached radially around the outside and inside of the inner pipe, which would cause the air to vortex, improve this, by breaking up this film/layer? Or am I misunderstanding this.

 

[mess]

You would probably find that changing the inner pipe to plastic would have only a small effect on heat transfer.

Also if material doesn't matter, then would 3d printing an exchanger with a much larger surface area and "nuances" to break up the air film be a much better approach?I also found there are thermal conductive filaments e.g. GRAFYLON 3D and TCPOLYI think this is the way to go, now I just need a design.

 

[Keith_McClary]

Alternate, and easier, approach is to make a larger heat exchanger.

Or have many layers with thin air passages, as I https://www.physicsforums.com/posts/6431854/bookmark. The one I have is has ~150 one foot square layers and is sized for a three bedroom house (with basement). You should be able to make or buy something smaller and cheaper, sized for a van (although it also depends on the number of occupants).

As I understand it, the high efficiency Euro models are like this, but with the top and bottom corners cut off, so you get more counterflow (instead of crossflow), but less total airflow.

 

[mess]

Or have many layers with thin air passages, as I https://www.physicsforums.com/posts/6431854/bookmark. The one I have is has ~150 one foot square layers and is sized for a three bedroom house (with basement). You should be able to make or buy something smaller and cheaper, sized for a van (although it also depends on the number of occupants).

As I understand it, the high efficiency Euro models are like this, but with the top and bottom corners cut off, so you get more counterflow (instead of crossflow), but less total airflow.

Yes i just looked into adopting an already made HRV like that, and its a really good option. I am trying to see how efficient they are but I haven't had much luck trying to find data on it.

I then also came across ERV's which would actually even be better for the campervan since humidity is big factor in such a small space.

The performance of the ERV shown in the last part of the video appears really good, but I can't calculate the efficiency since I don't know the CFM of those fans though

 

[mess]

Assuming the above ERV has 50 CFM, I calculated 95% efficiency.

inside: 77.5
outside: 105
supply: 79
exhaust: 96

x = 97.2 = 1.8 deg F / deg C * 50 CFM * 0.018 BTU / ft^3 / deg F * 60 min/hr

77.5-105 * x = 2,673 BTU/H
(supply - outside)
79-105 * x = 2527 BTU/H

2527/2,673 = .95

I think I could have done it with the difference between the outside and exhaust @ 50 CFM, but wasn't sure how.

wait a sec, so then can't i just cancel out x? and its simply

105-79 / 105-77.5 = 26/27.5 = .95? no need for cfm and those other conversions, in this case.

 

[Tom.G]

... I don't know the CFM of those fans though

I found these in the comments for the video:

How much CFM was flowing through the demo unit?

And 5 months later the response:

Sorry for the delay... we haven't been over to that particular lab much since COVID started. We were there today and took measurements. OA Intake = 71 CFM, Supply from ERV = 51 CFM, Return to ERV = 37 CFM, Exhaust = 56 CFM. The plexiglas is not caulked to the side panels of the demo unit or to the middle panel that holds the wheel. So there is leakage, which explains why the airflows don't all match exactly.

Also, if you look at the video around the 7:20 time, you can see the name of the company that supplied the Demo unit (with lots of magnification, it shows as AIRXHANGE) ; a Google search readily finds them.

Cheers,
Tom