Heat injection vs heat exchanger

[remp]

HI

I am reviewing a design where by the return water temperature of a process water heating system is heated up (from 40 to 80 degC) by the hot water from a Combined Heat and Power (CHP) plant.

Hot water is pumped from the CHP plant via a plate heat exchanger at 80 deg C. When it gets as far as the return water pipe of the process water heating system the designer has put in another set of pumps to inject water from the CHP line into the process heating line. He uses a mixing valve to mix CHP water with process return to give the desired temperature returning back to the process hot water system. I think this is crazy when a simple plate heat exchanger would do the job at a fraction of the cost. Am i missing something??

 

[jmiles]

Well i think the real question is would the plate exchanger really dot he job at a fraction of the cost?

typically, and i say typically cause i dont knwo all your flow rates and ahve not priced everythign out, but typically a pump is much cheaper capital cost than a plate and frame exchanger.

Both pieces of equipment will ahve maintenance issues, though the pump will burn HP all the time.

The truth is both methods will work, you just need to doa cost benefit analysis, but all my experience tells me that the exchanger is not so obviously cheaper as to make the pump and mixing valve be a crazy idea.

 

[remp]

jmiles

the pumps are quite large 10l/s each and we need to buy an MCC and variable speed drives for them, wire them up etc.... this is a lot more that an plate heat exchanger where ther is no associated electrical cost....

 

[jmiles]

Hi Remp,

im at home and i dont have any of my notes and stuff to look anything up, so this is all from memory, but it still does not sound like a run away to me.

your talking a flow of 36 m3/h say you use two pumps each doing this service, we are talking maybe 10-20k each pump, with the motor for inverter duty, so thats 40k total, add in the vfd's at 10k a piece, thats 60k, now room on the MCC's that might run you another 10k a piece? so we are at 80k and then some wiring and cables to be pulled, thats another 5k a piece, so we are talking about 90k for two pumps to get installed?

considering the heat exchanger will likely need to handle about 50% more flow than than that of the pumps your putting almost 100 m3/h though the exchanger, cause you really wont get the same efficiency through the exchanger as you will from direct injection. now you are also assuming that you dont need additional pumping power to overcome losses in the exchanger, but even so its not insane for a heat exchanger to cost you 100k

look my point is this, im just pulling number from the air and my experience, i dont know most of whats surrounding your process here, but the case ive laid out is somewhat reasonable.

what you really need to do is talk to your designer ask them why they did it this way, and then actually do your cost benefit analysis, call a few vendors ask for budget pricing, and ask for budget delivery, heat exchangers take alot longer than pumps which may be almost stock items.