HEAT RECOVERY SYSTEMS 4

[Welchita]

Hi Fellows!

I have this real case:

Some years ago I draw and have supervised the refrigerating system installation in printing industry (serving as well as climatization system for the office-building) to serve a high rotating speed machine, which has an hot camera/room in its top to dry and fix the colours to the strip paper.

This hot space consumes a lot of energy. It burns natural-gas and combustion gas reaches temperatures nearly 350 ºC or 662 ºF.

That time, I strongly suggested to the client to consider (it should be a non-option condition since the start-up!) an obviously heat recovery system applied to the 26" diam. isolated steel chimney of that rotating machine to heat the industrial dome and/or a warehouse. Now he call on in hurry, Let´s do it, "for yesterday"!

I thinking to install a heat-exchanger, using thermal-fluid system with terminal ventilated thermal units and above all, having a very simple and reliable control solution.

Yours comments, ideas, tips, hints will be recognized and very appreciated.
Thank you.
Welch


So i have a few days to decide which is the best solution. applie thermal very beguinning s oblized sin-quoi- adviserecommeded

 

[willard3]

As always with heat recovery, how you recover the heat will depend upon how and where you are going to use the recovered heat.

How much heat is there?
How much of this heat can you use?

You haven't given much information.

 

[Welchita]

Hi WILLARD

Y're right. Here is some data from that time, which now I'll confirm:

Flue-gases flow rate: 4950 Nm3/h or 173042 N(cfm) at 0ºC or 32 ºF;

Max. exhaust temp flue gases: 450 ºC or 842 ºF

Estimated mean flue gases specific heat: 1,080 kJ/kgºK
Suppose a recovery system introducing a deltaT in the flue gases side from 400 down to 180 ºC (or 698 to 356 ºF)with a transfer efficiency of 60 %, we can recover something like 250 kW or 335 Hp
Thanks again.
Welch

 

[Welchita]

Hi!

There's something else I've forgotten: I can use all the recovered heat in the cold season, that's for sure!
Welch

 

[thermcool]

you can gave a try to heat pipe heat exchanger but as willard3 commented you need to define the application where you want to use this heat. Depending on the application you will decide the recovery technique.

In one of the project we use the recovered heat from the flue gases to heat fresh air using heat pipe heat exchanger. This fresh air was use to preheat and dry the charge going to the main furnace and provided some fuel saving for the main furnace.

Lets say if you want to use this recovered heat for air/water heating purpose, heat pipe heat exchanger (HPHE) may be one of the alternative you can think about. HPHE is a bundle of copper tubes that are evacuated and sealed after charging partially with water. The latent heat of the water is used to transfer heat from one end of the pipe (evaporator or flue gas end) to another (condenser or recovery side).

 

[Welchita]

Hi THERMCOOL!

May I call your attention to the initial thread:
The application is to heat a warehouse where there are an hudge quantity of cylinders stip-paper, before going to production, that mean's quite a lot of thermal inertia and the requirement to have that material at least at 18 ºC or 64.4 ºF, before going into the machine. So I should use that heat to help to keep the ambiance temperature at 18 ºC, and only in 2nd step to call up an existing boiler.

I have done a more refined calculations and I got a recovery heat potential of 350 kW or 470 hp.
Thanks.
Welch

 

[willard3]

You could use an heat-pipe system, but, when you don't need the heat, they are expensive to control.

A runaround coil is easier to control, but, given that one side of the circuit is in combustion exhaust gasses, the material of the recovery coil will be specialized to control corrosion from water and acids in the exhaust stream.

How do you heat the warehouse now? With air handlers? Unit heaters?

What is the climate where you want to recover heat?

 

[thermcool]

Welchita

have a go through this link, it is not directly related to what you are doing but might give you some idea about drying techniques,

http://www.nrel.gov/docs/fy99osti/25885.pdf

willard3
you have a good point about the control of the heat pipe system. Most of the time, the isolation of the unit from the heat is not required as heat can be use for some secondary purpose or just exhaust to atmosphere as waste heat. About the second point, generally the heat pipes are coated with some material to minimise the corrosion effect e.g. copper heat pipe are coated with nickel layer to avoid the oxidation of the copper which add resistance to heat inflow process.

 

[1952mike]

If you are willing to spend some money, contact a heat recovery silencer manufacturer, like Maxim or Vaporphase, and give them your parameters. They can sizeup one for you that will heat up water and you can connect outlet to your boiler inlet. This will reduce the load on your boiler. This way if you don't need the heat, you can do whatever you do with the boiler outlet. Of course you will need engineered piping & valve system.

 

[itdepends]

Time for a silly question- are there levels of noxious/harmful compounds in the furnace stack gas that would prevent you from venting a portion of the gas directly to atmosphere?- i.e. the workshop. I assume the answer is yes and you've already considered this- otherwise it wouldn't be any different from a (large) gas fired space heater.

 

[willard3]

snip
Time for a silly question- are there levels of noxious/harmful compounds in the furnace stack gas that would prevent you from venting a portion of the gas directly to atmosphere?
snip


YES, there are noxious and harmful compounds in a furnace stack.

 

[itdepends]

The question needed to be asked- not all furnace off gases are necessarily be noxious/harmful- it depends on what the furnace is being used for- and I don't know enough about "colour fixing" to know if it was an issue- I assume you do............

 

[Welchita]

Hello Dear Fellows

The process has reach some evolution and i have some data, which I want to share with the Forum, (comments are very welcome):

1. We have a natural gas combustion, so a lots of CO2, mainly;
2. Suppose at the beginning of the chimney, at that position the installation of a confined space with baffles elements, a kind of a chicana to force the combustion gases go along a labyrinthine path;
3.Suppose also a tranverse tubing surface normal placed to the combustion gases. Inside of those tubes we plane to induce circulating water which is heated then to a set-point temperature.

Thanks.
Welch

 

[MedicineEng]

And an air/air heat exchanger? Could it work for you? Just remember that no matter the solution that you choose, the "clean" fluid side should have a higher pressure than the waste stream side in order that in case you have a leak in the heat exchanger you don't fill your warehouse with noxious gases.

You refer that you use the water heat during cold season? And during hot season, what will you do?
Did you considered an adsorption chiller?

 

[thermcool]

Good point, MedicineEng

 

[willard3]

I will also ask if this is a condensing heater....standard efficiency heaters have enough noxious stuff that you would harm people's respiratory systems.

CO increases
CO2 increases
Moisture increases
NOX increases
and etc.......

 

[MedicineEng]

If it is a good point, the star should be for me...

 

[thermcool]

Here you go

 

[Welchita]

Hello there! I'm back!

To MedicineEng: The heat-exchanger is suppose to be combustion gases/water. So I don't have any harm people's respiratory problems possibility, at least from this source. This water will flow to serve terminal heating air-units in the factory area.
In summer, of course the heat-recover process is off.
Thanks for keeping alive this thread! By the way you just got a 2nd star, MedicineEng!
welch