Heat injection v Heat exchangers 1

[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??

 

[j2bprometheus]

remp-

Sounds odd to me but I have never designed such a system.

Doesn't the process water have anti-corrosion / anti-algae additives in it that the CHP water does not?

A heat exchanger with a temperature controlled bypass valve would allow temperature to be maintained in a tight band but would keep the system simple.

The initial cost of the pumps and the maintenence would be high compared to a heat exchanger. I suggest that you get a price for similar pumps from a pump company and get a price of a suitable heat exchanger and a temperature controlled bypass valve. My guess is that the heat exchanger will be much cheaper than the pumps.

If there was a space constraint that was critical, then the pumps might be able to be located somewhere else convenient. That could be why they did this. But I doubt it.

Was maintaining the outlet temperature in a tight band critical in this application?



j2bprometheus

 

[FredRosse]

Assuming that the water quality issues are satisfactory, it seems that a simple mixing scheme could be much less costly than a heat exchanger. If process temperature control is required, then a control valve on either the process or CHP streams is required reguardless of the selected scheme (heat exchanger or mixing valve).

As I understad it, the CHP flow is already pumped, and that leads to the question: could the existing CHP pumps handle the job of injecting hot water into the process stream? Then there may not be a need for the new injection pumps.

Can the pumping system for both the CHP water and the process water both handle the extra pressure drop of the heat exchanger, and how do the operating pressures for both loops compare? These issues (plus water quality issues)can make the heat exchanger a requirement, depending on the numbers.

Another issue is maximum required temperature, the mixing scheme can be designed to attain 40C process water output, wheras the heat exchanger scheme cannot ever really reach 40C process stream temperature with the parameters you have presented.

 

[vpl]

remp

This is the third time you've posted the same question -- in three different forums now. See your previous threads: thread794-253670 and thread403-253630. In the latter thread you voiced some very strong opionions, in fact I believe you used the word "rubbish" at one point.

The first response you received, on September 4, 2009 was from JMiles. His second reply ended with the sensible suggestion that:

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.

Rather than posting multiple times on an internet forum I also suggest that you move on to obtaining the hard facts of an actual cost-benefit analysis. If you're so certain about the benefits, then put pen to paper and demonstrate it.


Patricia Lougheed

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