Maximizing Boiler Efficiency regard to steam injection method

[DRK93]

I'm currently working on a project to increase the efficiency of a wet back fire tube 3 pass boiler. We have 2 chemical tanks( with indirect steam injection) and 3 water tanks (direct steam injection)and the condensate from 2 tanks are collected and fed to feed water tank.my question is will it be profitable if we install heat exchangers to 3 water tanks and please guide me through a proper calculation procedure.
Feed Water 38 C
Average Feed Water 8732 L
Average Make up Water 7525 L
Condensate Water(per day) 1207 L
Feed Water Rate 758 L/hr.
Average run time(per day) 11.52 hr.
Condensate return temp. 86 C
Thank You!!

 

[poetix99]

You are essentially proposing to convert a (mostly) open system to a (mostly) closed system.
The potential efficiency increase is from the reduction of energy that is needed to heat the make-up water to the temperature of the feedwater, or the condensate return temperature (see my questions below). Another operating cost savings will result from the avoided cost of treating the make-up water before it enters the boiler (demineralizing, deaerating…). Heating liquid water requires little energy compared to boiling it, and the opportunities for efficiency are relatively small; ensure that you are also operating the boiler optimally for best cycle efficiency.

Some questions for you to further consider are:
• What is the feedwater temperature required by the boiler? If the water tanks are indirectly heated, and all of the condensate returns at 86C, can the feedwater pump, and the boiler handle that temperature? If you must cool all of the returning condensate in order to handle it on the feedwater & boiler side, you will have thrown away most of the potential thermodynamic gain.
• Does the volume of water in the tanks need to be increased by the amount of steam that is now directly injected (i.e. ~7525 liter)? The heat balance for indirect heating will also change slightly because the condensate leaving the heat exchanger will be hotter than the water that is being heated, and you will need slightly more steam in the HX than with direct injection.
• What parts of your system must be significantly re-sized (e.g. pipes, pumps, filters, larger return system, smaller m/u system…)? If you can’t use what you have, and must obtain new equipment (in addition to the heat exchangers themselves), this should be factored into the “payback” calculations.

 

[Jordan Hribar]

If I understand correctly, you wish to see if it is more economical to use an exchanger and re-coop the condensate to feed back to boiler versus direct injection steam to heat the water tanks which is currently installed and working now ?

Unless you have a very good application calling for the return condensate from the potential exchanger, or if direct sparing into the water tanks is causing you problems (i.e noise or turbulant flow), I don't see much gains from this.

« Rien ne se perd, rien ne se crée : tout se transforme ».
— Antoine Laurent de Lavoisier (1743-1794)

 

[GenB]

Lot to gain. Indirect heat is always better for the boiler

 

[EdStainless]

But the indirect heating will be less efficient requiring higher steam flows.
The largest cost is usually the polishing of feedwater, so first look at what he water is costing you.
Are you preheating feedwater now? Perhaps staying with direct heating but improving feedwater pre-heating would be a good compromise.

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P.E. Metallurgy, Plymouth Tube