Can Reheating Steam Locally Improve Air Outlet Temperature in Heat Exchangers?

[rnmconnor]

Our tin plating line has several of these. Original design was superheated steam to 300 degrees Celcius, but the plant reduced steam pressure and now our steam inlet temperature is much lower (less than 150 degrees Celcius). Inlet air is what we want to heat up for the drying. It comes in between 15 degrees Celcius in winter to up to 40 degrees Celcius in the summer.

Since I am limited to steam inlet temperature being lower, is there a simple way to locally heat the steam again to make my outlet air temperature higher? Would an tankless boiler be effective? I have a tank of condensate at about 100 degrees Celcius in the basement below the line, but I am no better off unless I can heat this up to superheated steam again, but at least I have a source to start from. Any feedback would be great!

Thanks;
Rob

 

[russ_watters]

Why would reducing the pressure reduce the superheat? Or do you really mean saturated steam? The main problem with reducing pressure is generally a loss of flow rate and adding superheat won't fix that. Can you increase the pipe size?

 

[rnmconnor]

Hi Russ;

Thanks for your response.

I guess my numbers are off. I am new to the area and still coming up to speed with the equipment.

You are right, it is saturated steam, and the temperature follows the steam table. Reduce the pressure and temperature goes as well. From what I am told, looking at the drawings as well, the original spec on the steam to air heat exchanger was 150 psi for steam at 366 deg. F. But they applied 250 psi saturated steam into the system, which was over 400 deg. F, my guess is with a high temperature like that, it doesn't matter so much efficiency wise for the heat exchanger, you could get outlet air temperature at 250 to 300 F no problem. But now with saturated steam pressure at 100 PSI we are seeing steam inlet temperature of 300 deg F in and outlet temp of 295 F on the piping. Our air outlet temp is only 146 deg. F

The heat exchanger has been rebuilt once. We were thinking of adding a tankless boiler locally since there is no plant plantwise to increase steam pressure. Somehow we have to boost either steam inlet temperature or heat exchanger efficiency to get the maximum out of air outlet temperature. If we need to go back to the drawing board to increase the heat exchanger iefficiency, then that may be our best cost effective option.

Regards;
Rob

 

[philrainey]

Reducing the Air flow will increase the Air temp but the heat output of the heat exchanger will drop badly.Ie must less air coming off at a higher temp. less total heating value of said air as there is less of it. Make sure there is no air trapped in the heat exchanger (steam side) Don't burn yourself I know nothing about how to do that.

 

[philrainey]

I guess if the end bends pipe temp. matches the saturation temp. for the pressure of the steam there will be no air trapped.

 

[philrainey]

otherwise some one is going to have to spend some money bigger heat exchangers or another one in series in the air flow, a higher pressure boiler or perhaps one could add electric elements or heat some other way.

 

[philrainey]

Can exhaust air from the system be used to preheat the incoming air in a air to air heat exchanger to save heaps of energy?