Heat Recovery Off Screw Type Air Compressors
Heat Recovery Off Screw Type Air Compressors
(OP)
Hello,
I'm trying to spec out a way to recover heat off two 400hp screw type air compressors. These units have (or will shortly) their own four pass heat exchangers using an EG/water mix as cooling on the tube side. The oil from the compressor is in a closed loop.
Now the units developed by the air compressor company should be rejecting about 900,000-1MBTU/hr in the form of ~105 deg F EG/water flowing at about 70 GPH.
My scheme is to set up a secondary HX to recover this heat into a 55 deg F stream of city water which will then be mixed with the water headed to our [future] direct fire heaters.
One compressor is running pretty much 24/7 whereas the other might kick in 60% of the time when we enter peak production. We also have several 150hp compressors from the same company which might be worked into the system at a later date.
Now I'm fairly familar with shell and tube variety HXs, but I need to know if there are more efficient variaties for my application. One manufacturer suggested I consider a "Plate and Frame" type.
Suggestions?
I'm trying to spec out a way to recover heat off two 400hp screw type air compressors. These units have (or will shortly) their own four pass heat exchangers using an EG/water mix as cooling on the tube side. The oil from the compressor is in a closed loop.
Now the units developed by the air compressor company should be rejecting about 900,000-1MBTU/hr in the form of ~105 deg F EG/water flowing at about 70 GPH.
My scheme is to set up a secondary HX to recover this heat into a 55 deg F stream of city water which will then be mixed with the water headed to our [future] direct fire heaters.
One compressor is running pretty much 24/7 whereas the other might kick in 60% of the time when we enter peak production. We also have several 150hp compressors from the same company which might be worked into the system at a later date.
Now I'm fairly familar with shell and tube variety HXs, but I need to know if there are more efficient variaties for my application. One manufacturer suggested I consider a "Plate and Frame" type.
Suggestions?





RE: Heat Recovery Off Screw Type Air Compressors
Get quotes for all of them and look at the cost per BTU recovered. Oh, don't for get the installation, here again, the plate and frame are very compact so installation should bve lower.
Q = U A delta T ln mean
RE: Heat Recovery Off Screw Type Air Compressors
I think in this instace with the ability to use city water this system could recover most of the energy. It's all low grade heat afterall.
RE: Heat Recovery Off Screw Type Air Compressors
while the information you've received is good, do be aware that plate heat exchangers are best suited for clean fluids. any minerals, salts, etc. can foul/restrict the SMALL passages in the plate type exchangers.
please note this thread: thread391-190222: Settled sediments in plate exchanger
meanwhile, good luck and it is good that the waste heat is being used!
good luck!
-pmover
RE: Heat Recovery Off Screw Type Air Compressors
I assume that "EG" means Exhaust Gas. Are the compressors dirven by and internal combustion engine? Or when you say EG do you mean the compressed air stream? What is the water that is mixed in? What is on the other side of this heat exchanger?
Closed through what? Much of the cooling of a screw compressor is done through the lube oil. Are you planing to recover heat from the lube oil?
Is the compressor single or multi-stage? Where is the heat from the intercoolers and aftercoolers going?
Is there a refrigerated drier?
RE: Heat Recovery Off Screw Type Air Compressors
Each air compressor has it's own shell and tube heat exchanger that cools the oil from the compressor down from about 225 deg F to 180 deg F shell side. On the tube side these units are using what I believe to be a 50/50 EG water mix that comes in at 75 deg F and leaves at 105 deg F.
I want to introduce another heat exchanger that can capture the rejected heat from the 50/50 EG mix to preheat heater water instead of just cycling this stuff up to the rooftop cooling towers.
As for water cleanliness... well we have hardness issues here. Total water hardness ranges from 125-150 ppm seasonably. I think any exchanger used will need to be cleaned with sulfonic acid at least annually.... suggestions?
RE: Heat Recovery Off Screw Type Air Compressors
RE: Heat Recovery Off Screw Type Air Compressors
This EG water mix is prepared by a 3rd party contractor who also does the makeup water for our POS boiler system. It's possible he softens it. The city water is definitely hard, but the temperature rise may be low enough to avoid scale. (I hope!)
RE: Heat Recovery Off Screw Type Air Compressors
The city water headed to the heaters is for domestic use?
Ethylene-glycol is toxic. If you are going to run a toxic fluid opposite potable water in the same exchanger you might as well simply run the city water through what is currently the ethylene-glycol side and eliminate the second stage.
If you want to attempt to protect against contamination of the potable water then you need to change to something other than ethylene-glycol as the intermediate loop. This is probably a code requirement anyway.
Is the city water flow constant? Is the flow rate sufficient to absorb the heat?
RE: Heat Recovery Off Screw Type Air Compressors
The coolant water is strictly for sanitation. I think I'd better investigate what water codes are required.
The city water flow can be adjusted. This water will be headed towards a digitally controlled mixing valve so the flow rate will only be dictated by the allowable pressure drop across the HX. If there really is a coding issue with the EG mix intermediate fluid then a separate pumping system will need to be introduced along side the HX.
I had really hoped to retain the option of at least being able to redirect the flow of coolant from the primary HXs to the cooling towers in the event of secondary HX failure. This might still be possible as the volume of liquid cycling through the secondary system should be minor compared to the ammonia, limiting any dilution upon mixing.
How often does tube failure/leakage occur in operating heat exchangers? As I get further into this project more and more obstacles pop up, damn.
RE: Heat Recovery Off Screw Type Air Compressors
David
RE: Heat Recovery Off Screw Type Air Compressors
I would also use a PG/Water system in lieu of The EG/Water system. PG is less toxic than EG.
zdas04
You cannot depend on a pressure differential to prevent mixing of the two streams. The biggest problem is that where the connection/leak occurs it is possible to have aspiration between the two systems.
RE: Heat Recovery Off Screw Type Air Compressors
RE: Heat Recovery Off Screw Type Air Compressors
David
RE: Heat Recovery Off Screw Type Air Compressors
RE: Heat Recovery Off Screw Type Air Compressors
As for the makeup water flowing through these HXs I'll just spec out an appropriate sized pump. I was hoping to be able to avoid a pump on the secondary HXs using city water pressure to flow the city water shell side.
RE: Heat Recovery Off Screw Type Air Compressors
RE: Heat Recovery Off Screw Type Air Compressors
RE: Heat Recovery Off Screw Type Air Compressors
I also found a shell and tube made by ITT Fluid Technology that was in McMaster Carr. its 7'Dx29"L, 4 pass with 29 3/8" tubes, made of 316 stainless.
None of the plate and frames listed meet our capacity though.
RE: Heat Recovery Off Screw Type Air Compressors
Lesson Learned: Double check those conversions before optimizing/sizing!
I small plate and frame might work.. but the payback would be like 10 years.
RE: Heat Recovery Off Screw Type Air Compressors
Tom
Delta T Heat Exchangers
www.deltathx.com