How Can Increasing the Heat Transfer Area Improve Air Heating Efficiency?

[Unrest]

It seems more and more like you're losing too much heat through the walls.

How about this:
- Measure the flow rate
- Measure the temperature increase of the air
- Calculate the power that would ideally be required to achieve that (like I did)
- Measure the electrical power used by the heaters
- The difference between the electrical power used, and the power needed to heat the air should be equal to the rate of heat loss.
- Assume heat loss is constant with temperature (terrible assumption, but it's easy!)
- Add another heater whose power is equal to the heat loss rate.Another idea:
Disconnect the compressed air and let it flow by natural convection. If that gives >350C air temperature then you can gradually increase the flow rate till it's just right. If it doesn't work then there's no hope without replacing the heaters or insulating it more.

I don't think water will help. It'll carry heat out the walls faster as well as to the reactor faster.

 

[ruzfactor]

I think you are right. Too much heat is lost through the walls. Most of it is in the wall surrounding the coil heater. So at moderate flow, air looses the heat through the walls. At high flow the air looses less heat but the heater is unable to cope with high flow, that's why the heater temperature decreases from the set temp. Please correct me if I am wrong.

I am now planning to use the inline heater to heat the air an directly send to the reactor vessel part by removing the coil heater part.

Btw, will it be possible to approximate the heat loss through the walls? Although I am using some insulation around the walls but still heat is lost.

 

[Unrest]

Sounds like it. But it sounds like you're just guessing. Why not calculate what should happen, then you won't have to spend ages modifying it just to find it still doesn't work.

Can you measure or estimate the mass flow rate?

I guess you can safely assume the heater is using full power because it can't reach it's set temperature.

 

[ruzfactor]

Sounds like it. But it sounds like you're just guessing. Why not calculate what should happen, then you won't have to spend ages modifying it just to find it still doesn't work.

Can you measure or estimate the mass flow rate?

I guess you can safely assume the heater is using full power because it can't reach it's set temperature.

Unfortunately I don't have any flow measuring device attached to my system. I really want to calculate what should happen, but don't know where to start from.

 

[Unrest]

Did you try my "other idea" of using a very low flow rate (eg natural convection) to see if it does work slowly.

If so then it's all fine, just increase the flow rate till it just barely works and you're done. Unless that's too slow.

Other than that, I think knowing the flow rate is kind of crucial, and you won't be able to design anything without that. Your air's coming from a tank, right? How about weigh the tank before and after to see how much mass was lost during the operating time.

 

[ruzfactor]

I have tried with low flow rate. The heat transfer is much less. My air is coming out from a compressed air cylinder. Could you please explain why the heater is not able to heat the air? could it be because of heat loss through the walls?

 

[Unrest]

I have tried with low flow rate. The heat transfer is much less. My air is coming out from a compressed air cylinder. Could you please explain why the heater is not able to heat the air? could it be because of heat loss through the walls?

Which heat transfer is much less? Heat flux into the reactor? Does that matter if it heats more slowly? Did you see the air temperature (or heater temperature) increase with a low flow rate?

I'd really like some idea of the flow rate to quantify it. However it's hard to imagine anything else besides loss through the walls.

 

[ruzfactor]

Heat transfer is much less in case of low flow rate. I need the reactor to be heated within a shorter period. But it all failed. A new design is the only solution I guess. Is the heat capacity of damp air higher than the dry air? How can I prove (theoretically) that damp air could improve heat transfer? Need some help on that.

 

[ruzfactor]

Is there any effect on temperature, when a flow experience sudden enlargement?

 

[Unrest]

It's a pity you can't know the mass flow rate. With that and your existing measurements you can confidently estimate the losses and predict what wattage heater is required.

I probably know no more than you about using wet air. My guess is it would have a higher heat capacity, but I don't think that's important, just using a higher flow rate should do the same job.

I really think you should work out what went wrong with this design before trying something fancy. Otherwise the fancy thing will fail too. You can probably do it just by replacing the heating element if you have some data to make predictions with.

Is there a reason you don't have the heater wound around the reactor? Why transfer it so far through air? And why is the air flowing?

When gas expands it cools. That will be happening to the gas coming out of the tank. It could easily be below 0C, but probably not enough to make a difference to the overall design.