Mixing valve instead of 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 Hheat 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??

 

[cry22]

I see two reasons for the designer's approach (from your description of things):

1. He is cost conscious, no second heat exchanger, a 3-way valve is a simple thing to control. The cost of heat exchanger and control valves, expansion tank, air separator, floor space, etc.
2. Sust'ainability. The deisgner's design is maximizing the delta "T" to get the most out of the CHP heat exchange system.

The designer is killing three (3) birds with one stone when compared to your approach: Cost reduction, sustainability, and ease of control (simplicity).

I like the designer's approach, and do not aqgree with the way you see things. You should question yourself a little more when you see a different design.

The only thing I do not like about the designer's approach is his use of flat plate heat exchangers for heating water (for condenser water OK, HW? no). They leak, and they are not reliable for HW applications.

You should recommend a shell and tube heat exchanger and you will look good in your comments. While at it, look at 1/3, 2/3 control valve arrangements with OA temperature reset (I suspect that this designer has addressed it adequately).

 

[ChrisConley]

Do you really have problems with plate and frame HX's leaking cry22? I've never seen a plate and frame leak, pumping a wide variety of fluids: hot and cold.

A huge shell and tube is a cost/space ineffecive way to exchange heat on anything but steam... and we've started to use plate and shell technology for steam.

 

[ccfowler]

Shell & tube heat exchangers can leak, too. Sometimes they leak very badly. As we all know, nothing's perfect.

I agree with cry22's assessment. The mixing valve arrangement makes more sense to me.

 

[remp]

Folks ye are worng re: the cost.

The designers current arrangement includes the installation of 2 new pumps, an MCC panel for these pumps, variable speed drives, and all the electrical wiring that comes with that..... Between the CHP and the process water side the is already a set of pumps. The designer is putting in another set in series futher upstream of the CHP to inject the hot water from the CHP to the Process hot water return. With a heat exchanger there is only the cost of the heat exchanger and a control valve. There would be no extra cost for expansion tank, air separator and lees floor space is required "Cry22" as all this equipment already exists on the CHP side and on the process side.

The plate heat exchanger is super efficient negligable loss of heat and its weel insulated too..and does not leak like you say "cry22" as it is fully welded...so the thing on sustainability is rubbish...

As for easy of control nothing is more simpler to explain to an operator that and control valve on a heat exchanger...

Consulatant sometimes miss the point on the amount of work and money that is needed on site to full fill there dreams.

And by the way "cry22" i am always quesitoning myself more when I see a different design.... why do you think i posted this question....

 

[ccfowler]

remp,

Sorry! In haste, I missed that additional pumps are needed. I was thinking that the CHP supply was already at a high enough pressure. I agree that a suitable heat exchanger (either plate or shell & tube--your choice for your needs) does indeed make more sense. I have only limited experience with plate HE's and have no reason to object to them. In general, I would expect the plate type to be lighter and more compact.

 

[Slofke]

I sailed on a dredge before( it was build in 1997) and that had a mixing pipe instead of a heat exchanger for cooling the jacket water(HT+/- 85 degrees) of the main engines. each engine had it's LT (about 45 degrees) system and it's HT system. each with a pump, with one pump each stand-by.
In the beginning we had some troubles, but after a while, when we had the correct orifices in place it was doing just fine.
One good thing about it is space: mixing just needs a pipe, and can be build in anywhere in the system. The cooler needs it's space, with extra (service) space aroud it.
I had good experience with it.

 

[TBP]

Is the hot water supply from the CHP supposed to be a closed loop? If it is, and is returned to the CHP, they may have some serious issues with direct mixing and/or not getting water back in their system.

You may wish to see just how the contract is worded.

 

[ChasBean1]

Remp, I get a little stumped halfway through your system's description - a sketch would make it so much easier (e.g, napkin sketch of what's there and one of what you think would be better)...