Heat Recovery Unit for shower

Discussion in 'Shower & Bathtub Forum & Blog' started by C Williams, Jan 13, 2012.

  1. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona
    quote; Contact time, residence time or whatever one calls it is not a relevant criteria for heat transfer

    That is a ridiculous statement. The concept for "multipass" heat exchangers is to maintain the two fluids in contact as long as possible. As for the square tube being the most efficient, a trianglar cross section would be even better, because it only has two unheated sides which are radiating the captured heat into the surrounding area.
  2. jch

    jch Member

    I have had a Drainwater Heat Recovery Unit installed in my house for the past year (since Dec 2010). I had what is probably a relative of yours (Dave W.) install it here in Victoria -- he had never seen one before.

    Full results and actual temperature measurements are in my thread here:

    In a nutshell, this $650 PowerPipe unit saves me 50% on all my shower hot water bills. Payback period for our family of 4 is less than 3 years.

    If I were to move to another house, I would immediately install another one. No brainer.
  3. Runs with bison

    Runs with bison Member

    Completely wrong. As I said before this is a common misconception. It is not a reflection on one's intelligence or educational level. Other engineers often make the same mistake because few of them have actually done their own heat exchanger design. In the design groups I've worked with we can tell when someone has never designed an exchanger when they start talking about residence or contact time with respect to the heat transfer. I've designed a lot of exchangers and redesigned or reconfigured a lot of others that were either not meeting requirements or needed to be able to handle greater heat loads and/or throughput. Multipass exchangers are used for a variety of reasons, but not for the reason you claim.

    The primary reason for using a multipass is to increase the tubeside velocity, and therefore the heat transfer coefficient. Look it up. (Increasing velocity can also prevent settling/fouling, and even corrosion in some systems...while it can cause erosion in others.) Multiple passes can be used to keep the overall length of an exchanger at a reasonable value or to balance resistance to flow from a common header (most exchangers are not throttled on the cooling water side so if one exchanger is designed with only 1 psi of drop and another with 5 psi of drop, the lower resistance exchanger will get far more than its design flow, robbing others in the network.

    Here's the kicker: For a given shell diameter and length increasing the number of tube passes REDUCES the total residence time in the heat exchanger when the flows are fixed (metered.) The reason is simple. Everytime you add another pass you have to add a partition in the exchanger head (or employ U-tubes that have an empty zone within the minimum bend radius of the tubes.) The partition obstructs part of the tube sheet and requires the elimination of some tubes. Fewer tubes = less tube side volume and lower residence time. Again, this comes from experience. Doing tube counts and examining tubesheet layouts is part of the design process.

    Another reason I've seen multipass exchangers used was because a refinery had specified that all of the exchangers in a unit have a given tube length when it was built decades ago. They varied the pass count and shell diameter to achieve the desired duties. While I was doing some cooling water distribution analysis and redesign for them I recognized that I could debottleneck their capacity limiting alkyl unit by putting a new head on a small exchanger to reduce the pass count and thereby increase the cooling water flow and duty--a super cheap, easy project. To put it in terms more familiar to you: it had way too much "residence time" for the cooling water and this was restricting the amount of cooling it could accomplish. The tubside film coefficient was high...but so was the exit water temp.

    There are various negatives to using multipass tube exchangers: reduction in the LMTD, potential for temperature crosses, increased bypass stream flow on the shell side (empty zones in the tube sheet for partitions or U-tubes), higher pressure drop on the tubeside, complexity/reliability problems of the gasketing of the head partitions, limitations on head side nozzle sizes and orientations, etc.

    An interesting proposal, but it would most likely have the opposite impact because of the geometry. Square tubes against one another will have less external surface area than triangular tubes with one face to the pipe. Essentially the two sides of the square are not exposed to the environment. The flat outside wall is. But for the same triangular tube width, neither of the two outside walls will be touching one another. They will both be losing heat to the environment. So the effective losses for the same width could be as much as twice as high for a triangular tube. However, various effects would tend to suppress the result for the triangular tube and increase that of the square tube somewhat.

    Another problem with a triangular shape will be the lower flow. The tube will have half as much open cross section as a square tube, actually even less than that because of wall thickness. So at a given pressure drop the flow rate would be half that of the square tube. This would defeat the purpose of having parallel flow paths. Without running through calcs to verify I suspect the change in the hydraulic radius would improve heat transfer coefficient marginally for a given velocity though.
  4. Runs with bison

    Runs with bison Member

    Thanks for the info. Always helpful to see actual installs and get some numbers. Your install is very similar to how mine would look, although I hope to squeeze in a slightly longer version. According to their specs a 54" would fit here.

    I've used digital thermometers to read shower temps and water at the drain. The loss you saw there is similar to what I observed.

    One quibble I have is that your numbers seem to indicate less than 50% reduction in hot water from showering: $188.55 reduction from a $493.14 baseline. That's a 38% reduction. The confusion is coming when comparing that to efficiency of the unit itself, which appears to be 50% based on the incoming water temps and coldside temp rise.
  5. jch

    jch Member

    For maximum efficiency, the flow of warm water down the drain pipe should match the flow of cold fresh water up the outside of the pipe. i.e. The DWHR unit should supply *both* the cold (now warm) feed to the shower *and* the hot water tank inlet.

    If you only feed one with your DWHR unit, then your heat recovery will be less.
  6. jch

    jch Member

    I have a DWHR unit that I use with an old-fashioned two-knob hot and cold handled shower.

    The temperature stabilizes within 30 seconds of turning on the shower. No need for a thermostatic valve when you have a Drainwater Heat Recovery Unit.
  7. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    North Vancouver, BC
    And if your daughter flushes the toilet while your showering???
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    The original question was what if you install it in the cold supply to the shower...then, the cold water is getting warmer and the hot water is cooling off as you empty the tank...the shower outlet temp will change. If it is installed per the instructions, to the WH inlet, things should be much more stable over time.
  9. jch

    jch Member

    You feel it!

    The most efficient way to install a DWHR unit is to have its output feed both:
    - the cold water side of your shower mixing valve, *and*
    - the cold inlet of your water heater.

    That way, the water flow rate down the drain will match the water flow rate up the outside of the DWHR unit and maximum heat transfer will occur.

    Aside from that, yes, a thermostatic valve is always a welcome step up from a regular shower valve, regardless of whether you have a DWHR unit or not.

    My point was that the temperature of the Cold/Warm output stabilizes very quickly (<30 seconds) and so you don't need to be constantly adjusting the hot/warm mix in your shower.
  10. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    North Vancouver, BC

    I thought so.

    I'm going to tackle this issue and do some solid number crunching on the money it takes to heat up one 60 gallon tank with no thought to pre-heating the supply water and another with my secondary tank working.

    If I isolate the shower's supply line and feed the showers mixing value with the drain water pre heated cold I should improve things even more.

    I had pushed these projects to the side lines but I think this summer I'll tackle them.

  11. C Williams

    C Williams New Member

    Vancouver Island
    Thank you! Nothing like real world experience.
  12. jch

    jch Member

    It's a huge stress reliever when I hear my kids' showers stretching on and on....
  13. TonyG

    TonyG New Member

    Northern Ireland

    Try this system, more efficient than powerpipe in SAP for the same size, 2.1m length. installation methods outlined on web site www.showersave.net
  14. jch

    jch Member

    Cool. Hadn't heard of them before.

    I chose Power-Pipe because:
    - it had been tested by the National Research Council (Canadian Govt Agency) for efficiency (rather than just having manufacturer-reported numbers)
    - it was on the Government approved list for rebates (ecoEnergy program) http://oee.nrcan.gc.ca/residential/personal/retrofit/13302
    - it had the highest efficiency rating (for the size) of all units tested
    - it was available at Sears and Big Orange (in case I had to return it).

    ShowerSave says their 2.1 metre tall unit has an efficiency of 66% (http://www.showersave.net/about/what-is-shower-save/), whereas the same size of Power-Pipe (R4-84) has an efficiency of 66.7% (essentially the same).

    So I'd choose whichever one was more convenient to get (or cheaper). It looks like the ShowerSave is around $1500, whereas the Power-Pipe only cost me $695 from Sears.ca

    Either way, it's worth doing.
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