Surface computing a exchanger Double Feed

[lesolognot41]

Hello everybody
I am asking for your help to size a double flow CMV exchanger.
I went through forums (futura and econology) but cannot find the answer.
Some formulas, no example of use.

I will develop the calculation I made so that you can correct me if necessary, this will be useful for me but certainly also for others. In attachment a spreadsheet (free office) to facilitate the calculation to others, if my calculations are good of course ...
https://www.econologie.info/share/partag ... BW4WtD.xls
I did little thermo at school, and I don't remember ... the following calculations use formulas and data gleaned from the internet, touched and mixed by me ...

Here are the data:
A house in the countryside, 60m² distributed in living room / kitchen 30m², bedroom 20m² and bathroom 10m², Solognote house with insulation in short-term project (100mm of wadding in the walls facing the outside, not on the walls overlooking a barn and an unheated cellar planned for extension, and 300mm in the attic, nothing on the ground because impossible, single glazed window, waterproof frame at least the first year). The main purpose of the DF is to distribute the heat from the stove to the other rooms, since the cross walls and the layout do not allow it. Reducing losses is not the main goal, other work would be more effective ...

Data and formulas found:
Ventilation standard:

Kitchen: 30-90 M3h-1
Bathrooms: 15-30 M3h-1

So in my case Total need: 45-120 M3h-1

Air:
Density: r = 1.2 Kg.m3
Special heat: Cs = 1000 J.Kg-1.K-1

If flow Q = 150 M3h-1 = 180Kg.h-1 = 0.05Kg.s-1
If temperature difference DT = 20 ° K (20 ° C inside; 0 ° C outside)

Power P to be transferred = Q * Cs * DT = 1000W
 
Therm flux j = P / (S.dT) = 25 Wm-2.K-1 if V = 6m / s and space between plate = 2mm (arbitrary)
S: exchange surface, V flow speed
Flow determined by this graph



 
So heat exchanger surface S = P / d. dT = 1000 / 25.dT
 
Either exchanger: stale air (temperature T1 mass flow rate Q1) becomes extracted air (T2 Q1) by transfer to fresh air (T3 Q2) which becomes blown air (T4 Q2);
Q1 and Q2 are supposed to be equal if good sealing and balanced ventilation ...

Efficiency h = (T4-T3) / T1-T3)

T4 = h (T1-T3) T3 +
 
IF h = 0.95
T4 = 17 ° C
dT = T1-T4 = 3 ° C (difference between extract and insufflated)
T2 = T3 + dT (difference between extract air and fresh air)
 
exchanger surface S = P / d. dT = 1000/25 * dT = 13m

My calculations seem to you they coherent, or am I near the plate (heat exchanger) ??

Sorry to have been so long, but hey, it's not easy, huh !!
Thanks in advance

Greg

[lesolognot41]

Well, my subject fascinates the crowds, 31 views but no answers ...
Good night

[chatelot16]

20 ° inside 0 ° outside delta T20 ° ???? no

if we aim for an efficiency of 80%, a delta t lower than 4 ° and a counter-current exchanger are required

at the hot end of the exchanger the interior air enters at 20 ° and the outside air leaves at 16 ° .... the delta T is constant over the entire surface, up to the cold end where the indoor air comes out at 4 ° and outside air comes in at 0 °

if the surface is increased the same heat flow can pass with a lower delta t and the efficiency increases

you do not need a plate of material that is too good a conductor of heat because it short circuits the hot and cold end, and it reduces the efficiency if the exchanger is not long enough

[lesolognot41]

Hello Chatelot
I explained it badly then.

DT is between outside and inside air, to calculate the power to be transferred. DT * Q * Cs

dT differential between stale air leaving the exchanger and fresh incoming air = 3 ° C for 85% efficiency.

I did not talk about the planned exchanger:
Crossflow made of 0.3mm aluminum offset plate, in a wooden box covered inside with offset plate, polystyrene outside.

Are you suddenly too conductive material? I would tend to say that if there is not much conduction, it is that the air flow is not sufficient, but I do not see how to assess this. one more difficulty ...

So, if the beginning of the calculation is good, it will then be necessary to calculate the passage section, the pressure drops, which will give me the geometry of the bazaar, then it would be necessary to calculate the delta temperature between the two ends according to the conductivity of the materials? already that I take the te to assess the pressure losses ...

well, I haven't taken out the clamberries key.

[lesolognot41]

I'm wrong, not corssflow the exchanger, opposite flow ...
Otherwise limited return ...

[chatelot16]

the thermal conductivity of the plates does not need to be high: even with simple plastic, it is the exchange with the air which limits the flow, not the thermal resistance to cross the thickness

for the conduction between the 2 ends, calculate the thermal resistance as if the plate pack constitutes a thermal bridge between inside and outside

passage section = thickness x width x number of plates
length = length of the exchanger

in the case of fairly short commercial heat exchangers they are plastic ... I imagine a large thermal bridge if we made aluminum with the same proportion

I like the exchanger in galvanized sheet, less conductive than aluminum, and more solid than plastic, but I change the proportion, longer and less wide: thanks to the length the thermal bridge of the sheet pack is less serious

[Obamot]

@lesolognot41 yet someone gave the answer there.

@chatelot yes and we can say hat. Not everyone knows that galvanized steel is less conductive than aluminum. We would rather tend to believe the opposite, since the aluminum is much lighter!

If we want the minimum conduction for a material insensitive to humidity / condensation, the best would be glass, ridiculous thermal conductivity for this type of material, which is as mass as concrete! Since a conductivity between 40 and 50x less than galvanized sheet (But why didn't they think about it ...? And a long or spiral glass tube is very easy to do!)

[chatelot16]

@chatelot yes and we can say hat. Not everyone knows that galvanized steel is less conductive than aluminum. We would rather tend to believe the opposite, since the aluminum is much lighter!

why do you think thermal conductivity depends on density? thermal conductivity resembles electrical conductivity ... the best conductors are gold, copper and aluminum

lead may be heavy but it has a rather shabby electrical and thermal conductivity

iron is medium and its conductivity depends a lot on its purity, medium steel is much less conductive than ultra pure iron

the conductivity of the plates can produce a thermal loss between the hot end and the cold end, but this is not a reason to seek the lowest thermal conductivity ... it is still necessary that the heat passes through the plates!

[Obamot]

Very just! But it is a generality, not a rule.

Diamond (carbon compound) has a high thermal conductivity but a low electrical conductivity. Graphene (same family as diamond, also composed of carbon) is a much better thermal conductor and also one of the best electrical conductors.

About aluminum, that's why I hate the sandwich bases of modern pans, with an aluminum plate sandwiched between two thicknesses of stainless steel ... With successive expansions / retractions, it always ends up release (for example when there is overheating). The bottom bomb and the pan is fucked up!

[dede2002]

Hello Obamot,

It's funny I was thinking about it that night, stacking the 3mm tiles. that I disassembled to make an exchanger in my laundry room. To make 10 m2 of exchange with 40 tiles it will weigh 80 kg!

Is the glass likely to split with a hot end and a cold end?