The required heat in this case is = 5 L/min x (700-30)/837 = 4 kW. For multi-coil heater, it is better to know the watt density (W/cm2) of each coil and then multiply with the total area available to obtain the available heat duty. Since air has poor thermal conductivity,  you need to increase the Nusselt number of the system by increasing local velocity (Reynolds number). Increasing residence time/pathway is not much beneficial since the heating area does not change if you are using the same heating coils. Choose higher watt density coils and ensure higher Reynolds number by putting some baffles/higher inlet velocity to ensure the desired outlet temperature.

@Subhasish Mitra

Dear Mitra,

Why did you divide by 837?  and Plz, take a look over the last draw, the extra metal path could increase the time of conduction between the gas and wall surface, as a result the over all heat transfer increase!

Regards!

837 is used as a conversion factor to take care of  L/s to m3/s (0.001) and avg. density (~ 1.19 kg/m3) and then heat capacity (1.005 kJ/kg/K) - [1/(0.001*1.19*1.005)] ~ 837. Heater manufacturers use this simple equation to determine the heat duty.

Please see the below link:

http://www.indeeco.com/c10_20-4catalog.pdf

If you take into consideration of temperature change effect on the physical properties, the conversion factor would definitely change.

From the point of view of "increase in time of conduction would enhance heat transfer",  you can also reduce the flow rate of gas to have more "conduction time" in a given volume.

Would that help you? Certainly not since conductivity of air/gas is low and the only way you can enhance heat transfer is

- by providing more surface area (m2) like fins on the coil surface

- higher watt density coil (w/m2)

- higher velocity (Reynolds number) or turbulence (create vortex by boundary layer separation)  in the system

The new draw gives couple of flow path changes, so it may help by providing some turbulence but would be good if you provide some baffles inside the box to enhance the contribution of turbulence and definitely use finned coils for more heat transfer area.

Trust me on this. Have been a design engineer for number of years before joining academics!

Hope it helps,

SM

By using Newton's law of convection, q=hA (Dfference of temp). h value is known to us, whether it is forced circuation (h=130 units) bor free and natural circulation (h= 100 units, q can be suotably assumed. ( 500 Watts or 1000 W). Hence area of the coil can be estimated from which diameter of the coil can be determined. This is some what simple calculation, rather than assuming dimensionless numbers from which coil diameter can be esitmated.

If pressure drop is not a major concern, then any measure that diminishes the volume of air that flows far away from the coils will produce an improvement in heating the air. In your first picture, the air that flows in the periphery of the tube (and it is the majority of the air) is too far from the central coil. You must force this air to come near the coil.

To achieve this, there are a few things you can do:

- You could use a tube of smaller diameter. The annular passage would become smaller and velocities would increase. This would promote heat transfer for two reasons: higher convection coefficient, and less fluid flowing far way from the coil. I have used successfully used this rationale in the past, when designing a air-cooled combustion chamber (I got success where others failed).

- You can add baffles welded to the duct so that the air moving in the periphery of the duct is forced to deviate towards the axis, where the coil is. The baffles could occupy sectors of, say, 120 deg or 90 deg, and alternate along the length of the duct, for as long as the coil is present. The baffles need not to be too high, I'd say 1/3 of the annulus "height" would do the job. 

- Your suggested idea (last picture) should also work, for the same reason I pointed above: it also promotes less volume of air to flow far away from the coil (think about it).

In my opinion, you may have to rearrange the orientation of the heaters.  Coiled helical coiled wires may be better.  

To increase the heat transfer, think of adding twisted tapes in the annular space between the heater and the outer tube.  

For such a high temperature applications, the heater may become red hot, so avoid certain material, which may tend to smoke or burn.  

Check on the insulation of the duct.  

Consider reduction of air flow rate. 

Regards,

Dr.S.Ravindran

Dear  Ravindran,

Already I used Coiled helical coiled wires, More over there is no space between the pipe wall and heater other wise the velocity will decrease so Re(turbulent) decrease.!

Regards!

Check on the insulation of the duct.

Consider reduction of air flow rate.

Regards,

Dr.S.Ravindran

Dear Azd,

since your thermal boundary condition is a fixed heat flux, the area and the pass length should have no impact on the mean temperature. The mean temperature should only depend on the energy transferred to the system.

All geometry, Reynolds number changes would increase the temperature for a fixed temperature of the heating coils as it would be the case for a water based radiator.

A bad design of the heater should result in higher surface temperatures of the coils and thus the same air temperatures.

Could it be that there is a temperature profile with the 200 degree at a location with lower than average temperature? Or could the massflow have been increased by entrainment i.e. by air sucked in from the ambient?

Best regards

Bastian

Thanks for all of you,

Now, It is the time to act!

Best Regards!

Azd.

Dear Azd,

is it necessary that you have pure heated airas a testing gas?

If you can tolerate a small percentage impurities, such as exausts gases, you can burn a small quantity (few percent by weight) of fuel (such as propane) to heat up your air without major problems simply by combustion.

A burner is much easier to design and make than a heat exchanger.

Good luck for your experiment

@Giovanni Maria Carlomagno

Buongiorno,

It is necessary for the experiment  to have pure heated gas.

Thanks.

Buonasera Azd,

Anyway, remember that air is not a pure gas but a gas mixture:

Then, as far as the apparatus you built is concerned, I have the following comments.

There is too much space between the heathers so a lot of air that passes between them is not going to be heated. This means that the air passing over the heathers should be heated up to a very high temperature because, after, it has to mix with the unheated air so as to reach 700°C.

The heaters you are using do not efficiently work at the back face of the coil supporting rod (opposite to that where the air is impinging) because flow separation occurs there and the support is made of poor thermal conductivity material for electrical insulation reasons.

The conclusion is that it is better to have your heating metallic coil directly exposed to the flow with the minimum support presence.

You could do that by wrapping the coil between two insulating, small iameter, distant rods, adjacent to two opposite walls of a rectangular duct, and, by going back and forth and with a narrow pitch between coils, developing your exchanging surface.

With this geometry, you will have also the advantage of having the second row of coils somehow staggered with respect to the first one, so to increase heat transfer.

Be careful not to leave zones without the coil presence. The unheated air that goes through them will later mix with the heated one and lowers the gas temperature.

If you want to save money, you could also built your heat exchanger in two stages: the first one with the more economical nicrome alloy (preheating the air at an intermediate temperature) and the second one with the more expensive kanthal (heating your air up to 700°C).

Again, good luck and Buona notte.

Giovanni

I see that it not practical to think to heat the air electrically up to 700 C. You need very high heat transfer surface area with very low bypass factor which drametically increase the piping power.

"I see that it not practical to think to heat the air electrically up to 700 C. You need very high heat transfer surface area with very low bypass factor which drametically increase the piping power."

I cannot believe, that this is true, since any  hand held heat gun can actually do this job.  However, it might be a challenge to install the wires into a piping system without generating major heat losses.

@Bastian Schmandt

Not "any hand held heat gun can actually do this job" with 5-10liter/sec, with  700 C Degree increment in temperature.

Regards!

@ Azd

I did not received any comment to what I suggested.

Regards

@ Azd

My comment  referred to the enormous piping power and was not meant as a devaluation of your work. ;-)

By the way... Some stock heat guns claim to heat air up to at least 600°C with airflows from the aforementioned range.

Cheers

Bastian

@Giovanni Maria Carlomagno

Buongiorno,

I was in another state, Just came back. And as you mentioned I used an exposed coil to the air. More over, I am going to enhance the exchanging surface by adding additional fins.

Regarding your suggestion of two stages heater, It will be the next step if one heater does not work.

Grazie Giovanni .

@Bastian Schmandt

Dear,

Using the heater gun is not sufficient for this experiment, Since the flow rate and temperature are not controllable. I am using control panel with electric circuit and flow rate controller.

Regards!

Azd.

Dear Azd,

any progress with your device?

Best regards

Bastian

@Bastian Schmandt

Going ahead, I hope to complete it soon :-)

Sincerely,

Azd.

As Sameh Nada, Benha faculty of Engineering, Benha University, said you have poor bypass factor.  

If you have tried all these already, think of having heat exchanger - gas from a burner and air heat exchanger

Regards, 

Dr.S.Ravindran

i TRIED USING ROD HEATER BUT THE MAX CAN I REACH IS 170 c

I've used long tube containing IAP( Impervious Alumina Porcelain) as a heat transfer medium. This IAP has the ability to absorb and reflect heat to obtain high temperature control under a variety of air flow conditions. Maximum temperature obtained by using this technique was 600 C at 120 l/min air flow rate.

Hint; The tube is surrounded by preheater type Tube furnaces-LTF .