Heat Exchanger Question

[wilsonudevil]

Hi all,

Just got a quick question about heat exchangers in general.

I'm using a copper pipe, finned water-air heat exchanger. Water flows through the pipes perpendicular to the air flow.

Will the efficiency of the HE remain the same if I put cold water through it (to cool the air) instead of hot water to warm the air?

Typical temperatures are around 20C - 30C (I'm using the heat exchanger as part of a ventilation device).

Thanks chaps!

[Yuri B.]

Hello Narkom.
Depending on climate, let's say in India, at winter, an AHU will typically draw in, I believe, 10C air through 45C coil, at summer, 45C air through the 10C coil. The same ratio.

[Yuri B.]

Even more - in the above example - hot summer air being more humid, thus more heat conductive, the effiiciency of the cold coil should increase.

[nike123]

I agree with Yuri. I think that only difference is in sensible and latent heat.
When heating, SHR is 100%.
When cooling, SHR is from 100% to 0%.

[Yuri B.]

Good evening Nike.
One thing I forgot - heat transfer direction.
Abiding to that my example, at winter, 45C coil will heat 10C air, at summer, 45C air will heat 10C coil.
Water's conductivity is higher even no matter how humid is air. Thus, winter's 45C water in the coil will be heating up the 10C air better than summer's 45C air heating up the 10C water in the coil. Thus heating is more efficient - probably, in any case.
So, comparing heating/cooling, efficiency of a HE does not depend so much on temp differencies, but on the heat transfer direction.

[Yuri B.]

Or am I wrong again?

[nike123]

Or am I wrong again?

I am electrician, you are electrician.
We need some thermodynamic engineer to clarify that case. Sometimes common sense is not good guide.

[desA]

Hi all,

Just got a quick question about heat exchangers in general.
I'm using a copper pipe, finned water-air heat exchanger. Water flows through the pipes perpendicular to the air flow.
Will the efficiency of the HE remain the same if I put cold water through it (to cool the air) instead of hot water to warm the air?
Typical temperatures are around 20C - 30C (I'm using the heat exchanger as part of a ventilation device).
Thanks chaps!

If the following assumptions are made:
1. Flows are sensible, not phase change;
2. Flows are pure counter-flow;
3. Terminal temp differences are the same (Tw,i-Ta,o), (Tw,o-Ta,i).
4. (UA) constant

Then both systems are equivalent.

The problem lies in assumptions (2) & (4).

(2) Flows are actually 'cross-flow' & the effect on the cross-flow correction factor (F) needs to be evaluated. I estimate this difference to be small, though.

(4) The area 'A' remains constant, but the term 'U' may actually vary, since it is a function of the tube-side heat-transfer coefficient. Here, the hot & cold water properties will vary, but not enough to alter the heat-transfer coefficient inside the tubes too much. Since the water-side heat-transfer coefficient is far higher than the airside coefficient - it implies that the airside dominates the HX performance. So, in reality, the difference in hot & cold water properties wont influence the overall HX performance much.

So, in summary, as long as you comply with items (1) & (3), then the two options would give similar results - within the bounds of your experimental uncertainty, I'd estimate.

I hope this helps.

[wilsonudevil]

Thanks guys- very helpful!

Maybe it'd help if I was more specific regarding my temperatures. If I were to conduct two experiments on the same day (to ensure constant(ish) humidity) with the following conditions:



Then I could expect the efficiencies to be similar?

[Yuri B.]

I am now sure the HE will give up the same amount of heat in heating mode as it will take in in cooling mode (provided the water's and the air's specific heats are the same).

[icecube51]

be sure to mount a condensation drain underneed. yore system will work,but the cop will be a bit low. for that you need to make a hole lot of calculations. if it is for home comfort and on a free given base its word it.

Ice

[wilsonudevil]

Much obliged for all the help guys...