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General: Flat plate heat exchanger performance?

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    Hi all, wondering if anyone can provide any experience of flat plate heat exchangers? We are using one with our DHW thermal store (heated by solar/gas boiler/PV and future wood burner) but I am disappointed with how hot I need to keep the tank to get a decent temperature out the tap. Its currently set at 66 degrees and the shower is hot but the wife likes it hotter. I would have expected less of a drop between incoming and outgoing water. Does that seem correct? I have the mixer on FPH set to maximum and the flow rate as low is reasonable for mains shower. Any thoughts/experiences appreciated.
  2.  
    Wow that's a big question for little detail! The exchanger will have a specification sheet with curves and stuff and will define the output temp for a given throughput. Unless it is furred up/blocked this is what it will deliver. Pipe runs, material, what the pipes are in, what insulation there is, can all effect the temp drop of the water after it leaves the exchanger. 66 deg would scald!!! 43 deg is the HSE max. Try using an an ir temp gun to measure the temp out of the exchanger. Also measure the litres/min out of the shower and check the spec sheet. This should be easy to get to the bottom of. You'll know if it suddenly starts working because of the loud scream and bright red skin glowing on your wife.....
  3.  
    Thanks for feedback, I think your answer says it all, there is clearly a problem! I will check the documentation, we did have a problem when the system was first installed with rust coming out of our large heating TS so wonder if this has blocked the heat exchanger. If it was blocked, would this behaviour fit i.e. the flow rate would remain good but the output temperature would be low? I assume it could be cleaned out by blasting water the other way through the FPH as it wasn't cheap!
    • CommentAuthorowlman
    • CommentTimeMay 23rd 2014
     
    There are firms that specialise in cleaning/descaling etc., plate heat exchangers. I think some of them may even just replace the heat exchange module for a cleaned one, so no down time.
    • CommentAuthorCWatters
    • CommentTimeMay 23rd 2014
     
    Is there a pump that circulates water from the store through the plate heat exchanger? What speed is set to?

    On my store I could measure the temperature of the pipes at the plate heat exchanger and/or blender to see if the exchanger or mixer is the fault.
    • CommentAuthortony
    • CommentTimeMay 23rd 2014
     
    More plates in the exchanger might help or a lower flow on the HW side
    • CommentAuthorSprocket
    • CommentTimeMay 24th 2014 edited
     
    You can spec a plate exchanger to operate at pretty much whatever parameters you need.
    A good supplier should have confirmed the specification with you before purchase and you would have a chance to upsize or change the flows (and see results) if necessary.

    You could add a second identical exchanger in parallel but most of the initial cost is the frame and it's not hard to add more plates.

    We bought from UK Exhangers. They were recommended to me by our heat pump manufacturer and I can second that recommendation. They were very helpful and they do some nice insulating jackets for the exchangers too.
    • CommentAuthorCWatters
    • CommentTimeMay 24th 2014
     
    Before you rush to change anything you should try and confirm it really is the heat exchanger at fault. Run the shower and put a thermometer on the input and output pipes!
    • CommentAuthorSprocket
    • CommentTimeMay 24th 2014
     
    CWatters is quite right.
    Before you mess with your exchanger setup you should try to get some better idea of flow rates and temperatures. The exchanger could be fine and/or the problem could be more easily fixed by changing a flow rate or temperature somewhere.
    • CommentAuthorEd Davies
    • CommentTimeMay 24th 2014 edited
     
    Posted By: Sprocket…you should try to get some better idea of flow rates and temperatures.
    Indeed. Temperatures are easy and the flow rate on the output side is pretty obvious. It's unfortunate that the most likely candidate for a problem, the flow rate in the primary loop, is the hardest to measure. Any suggestions?
  11.  
    Oh and if you take anything apart take the opportunity to put in a filter. I have put in a magnet type and a centrifugal one but that's just because I expect to live here for 30 more years!
    • CommentAuthorCWatters
    • CommentTimeMay 25th 2014
     
    Posted By: Ed Davies
    Posted By: Sprocket…you should try to get some better idea of flow rates and temperatures.
    Indeed. Temperatures are easy and the flow rate on the output side is pretty obvious. It's unfortunate that the most likely candidate for a problem, the flow rate in the primary loop, is the hardest to measure. Any suggestions?


    The pump is probably removable - perhaps remove the pump and reconnect it with some temporary pipes in series? However not sure if you can get a fitting that's similar to a pump flange?
    • CommentAuthorDantenz
    • CommentTimeMay 25th 2014
     
    Indeed. Temperatures are easy and the flow rate on the output side is pretty obvious. It's unfortunate that the most likely candidate for a problem, the flow rate in the primary loop, is the hardest to measure. Any suggestions?

    Observe flow rate on output side then turn the H/Ex'r around and compare.
    • CommentAuthorEd Davies
    • CommentTimeMay 25th 2014
     
    That might provide useful information if the problem is a blockage in the input side of the exchanger but if the problem's elsewhere in that circuit it won't help. Could also introduce crud from the thermal store water into the DHW and be tricky because I think these exchangers have different fittings for the input and output (could be wrong on that).

    Might be better to just take the exchanger out of the circuit and make sure there's free flow though the primary side.
    • CommentAuthorbarney
    • CommentTimeMay 25th 2014
     
    If you have the temperatures at the primary side and the secondary side and the flow on the secondary side, then you can deduce the mass flow through the primary from the manufacturers NTU's or efficiency.

    From there, you can determine what the mass flow was at design and decide if it's significantly lower than what you think you should be getting.

    I presume this is a lack of HWS temperature at all outlets - rather than something going on with the TMV side of the shower mixer ?

    You could just crack open the primary side at the lower point of the HX to see what the water quality looks like - if it's reasonably clean then you probably don't have much impediment to flow - backed up by you getting the HWS temp you want when the TS is up to temperature.

    You don't need much of a temperature drop on the primary side mean water temperature to really drop the delivery temp on the secondary side if you have reasonable volume flow expectations on the secondary

    Regards

    Barney
    • CommentAuthorEd Davies
    • CommentTimeMay 25th 2014
     
    Posted By: barneyIf you have the temperatures at the primary side and the secondary side and the flow on the secondary side, then you can deduce the mass flow through the primary from the manufacturers NTU's or efficiency.
    Once you get it to steady-state conditions you can deduce the primary mass flow directly if losses to the outside of the exchanger are small enough to be negligible (the exchanger is insulated, isn't it?) by simple conservation of energy. No need for manufacturer's data.
    • CommentAuthorbarney
    • CommentTimeMay 25th 2014
     
    Agreed, I was trying to suggest you could get an estimation of the performance for comparison with stated performance under reference criteria to see if there really is a problem physically on the primary side, rather than a problem of expectation

    Regards

    Barney
    • CommentAuthorSprocket
    • CommentTimeMay 25th 2014
     
    >the flow rate in the primary loop, is the hardest to measure. Any suggestions?

    Not 100% answer but... you could try to find pump curves for the particular pump model and power setting.
    That would at least give you a working range and if you had figures for pressure drop across the exchanger at different flows you could probably get close to real figures, if you assume that the exchanger is the main thing limiting flow.

    Not as good as measuring something but it's a start.
  19.  
    Thanks for all the feedback...we'll after two days of investigation I have found the problem, it's not the heat exchanger at all but a fault with thermostatic mixer value in the wet room upstairs. I realised that when I switched off the hot feed to upstairs, all the taps still had cold coming out of them! This was obviously responsible for cooling the hot output, fitted an isolation valve to shower room and had to lower the mixer output as it was so hot, hurray :) I hope it's simply the replaceable ceramic mixer value unit that is letting the cold through to hot side and not something more serious that is buried in the wall but that's another days challenge.
    • CommentAuthorEd Davies
    • CommentTimeMay 30th 2014 edited
     
    Heard of somebody having a TMV buried behind shower tiling leak from cold to hot like that. Workaround was to put a non-return valve in the hot feed rather than dig the thing out.

    If I remember correctly his problem was that the cold side was mains pressure but the hot side had a header tank (vented cylinder) resulting in the cold pass-through water feeding back up and causing the hot header tank to overflow. It was pumped so when the pump was on the hot low-pressure water overcame the valve, I think.

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