Data Sheet for Recoheat stove heat recovery
- Will Burrows
- Oct 30, 2020
- 1 min read
Updated: Nov 4, 2020
Here are the calculations for heat output of the Recoheat unit, based on the readings taken in the little demo I did that you can see in the video at https://youtu.be/zlEMADUz0Ak
Essentially, that shows - what is completely consistent with the Recoheat's output over the last two years - that air pumped in at about 15°C is pumped out at about 515°C (I'm understating it a bit).
So how does that translate into output in kilowatts, such as your stove is rated with? The stove I was demonstrating it with is a 5kw Ecosys.
Here are the calculations:
Calculating the heat transfer value
Area
Surface area is
@100mm length of 8mm internal diameter s
tainless steel tube = 0.00251 m2
@ 1830mm length of 8mm internal diameter stainless steel = .045933 m2
Properties of Air
1 litre per second = 0.001 m3/sec = 3.6m3/hour
Temperature in approx. 15 °C
Temperature out approx. 515°C
ΔΤ = Change in temperature ≈1.205 kg/m3
Flue gas temperature ≈ 500°C - 600°C
U = for air to air heat exchanger 10-40 W/m2 °C
Heat Transfer
Q = UAΔT
Q = Heat transfer (watts)
U = Overall Heat Transfer Co-efficient (w/m2k) = 40 w/m2k
A = Surface area =.045933 m2
ΔΤ = Change in temperature = 500°C
Therefore:
Q = 40 x .045933 x 500 = .91866w
Q = .91866kw
Yes, that's right. A small fire in a 5 kw capacity stove is producing an ADDITIONAL .9kw of heat within 4 minutes of lighting.
Basic problem with your heat transfer calculation. The formula is correct for a steady-state condition where both surfaces are at a constant temperature. In this case, the output is at 500C, so that at that end there is virtually no temperature difference. That means the average temperature gradient is half, so the heat transfer is half - c 450W.
(That assumes that the inside remains at ~ 500C, and is not cooled by the transfer. Unlikely...)
That is, of course, just the heat exchanger's capability to transfer the heat- it says nothing about the capacity of the donor fluid to supply it, or the working fluid to take it. The latter I have calculated elsewhere to be 580W max at…
Output calculation. Max output at 1l/S-
1l = 0.0012Kg, with heat capacity of 1012 J Kg-1 K-1 = 1.21 W K-1
Output @ 500C, change 480C gives 580W. That is the maximum that can be transferred, assuming 100% heat exchanger efficiency. 8mm bore gives ~50 mm2 cross section. At 1l/S that is 20m/S, and a time inside the coil of < 0.1 S At 500C, the pressure inside the coil will be ~ 3 Bar. Pumping 1l/S against that pressure would take ~270W with a 100% efficient pump. Not 37. The numbers do not add up. If the exhaust truly is 500C, then the flow is lower. Much lower.
An output of 500C air, at 3 Bar, with a pressurized…