Preheating of atmospheric boiler feedwater tanks?

[YungPlantEng]

Our condensate return systems aren't very complex and return to a common feedwater tank where chemicals are applied - this tank isn't kept under pressure and there is always a continuous vent of steam that leaves it in a maybe 20' long vent.

We have two air compressor waste heat ducts that are nearly adjacent to this feedwater tank and aren't being used for anything else. Is it practical to pre-heat either the make-up water that gets injected into this boiler (approximately 50 F) or is trying to use waste heat from air compressors (maybe 190 F?) generally a losing mans' game? Has anyone developed previous systems that used something like this?

 

[georgeverghese]

You mean waste heat from gas or diesel engine exhaust or is this waste heat from recip air compressor jacket cooling, or both ?
Have you worked out the economics on this? What is the payout time? If it is anymore than 5years, I wouldnt bother for a small heat recovery unit like this.

 

[YungPlantEng]

I don't have an approximate cost on the skid system, etc. required to convert the heat from the compressors.

It's heat from approximately 200 HP oil-flooded rotary screw compressors. A rule-of-thumb I saw was 50K btu/hr per 100 cfm is recoverable so we'd be seeing approximately 400K btu/hr or only around 12K/year in savings assuming 50% recovery of that heat and 100% efficiency on therms for our boiler. The bigger reason I wanted this would be as a partial justification to relocate the heat exhaust to an area of the plant where it isn't impacting our chiller. We're kind of seeing maybe 75% of that heat go into the air draw for one of our chiller circuits and while that's a little harder to quantify I'm pretty confident it's why the chiller never reaches capacity.

I guess my question is really "what do those types of heat recovery systems look like for low temperature heat recovery?" These aren't massive capacities we're dealing with but as I figure it using waste heat from air compressors to do anything beyond heating buildings is usually a losing man's game.

 

[georgeverghese]

So this is currently the hot air exhaust from the fin fan coolers for compressor hot discharge and for lube oil ?
The GPSA lists pros and cons for induced draft fan vs forced draft fan configuration on fin fan coolers (see chapter on air cooled exchangers). ID fan is much preferred to prevent short circuiting of hot air cooler exhaust to neighbouring equipment. But feed temperature of HP compressed air or hot lube oil is to be limited to < 350degF.