Question:
In the shower the other morning, it occurred to me I was throwing water at 40C down the plughole and drawing cold water at 5C to replace it. Sounds like time for a heat exchanger to get back say, 80% of the difference and pre-warm the incoming water. Has anyone done this before? Any pitfalls? — Iain Blackie
Response:
>In the shower the other morning, it occurred to me I was throwing water >at 40C down the plughole and drawing cold water at 5C to replace it. >Sounds like time for a heat exchanger to get back say, 80% of the >difference and pre-warm the incoming water.
Good idea. I’ve been thinking of putting an 6"x8′ horizontal pressurized cold water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled greywater pipe that drains to the sewer through a large bulkhead fitting. McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. The cold water might enter the 6" pipe via a reinforced garden hose, then flow into the water heater via the drain fitting. Who sells 8" DWV pipe? Nick
Response:
– Hide quoted text — Show quoted text ->In the shower the other morning, it occurred to me I was throwing water >at 40C down the plughole and drawing cold water at 5C to replace it. >Sounds like time for a heat exchanger to get back say, 80% of the >difference and pre-warm the incoming water. >Good idea. I’ve been thinking of putting an 6"x8′ horizontal pressurized >cold water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled >greywater pipe that drains to the sewer through a large bulkhead fitting. >McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. >The cold water might enter the 6" pipe via a reinforced garden hose, then >flow into the water heater via the drain fitting. Who sells 8" DWV pipe? >Nick
I had in mind a tank, rather than a pipe-in-pipe. As the volume of outgoing water is the same as the incoming, at best you can only get 50% of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C. The more surface area, the better the efficiency. The tank would take in the warm water at the top and drain it out at the bottom. The incoming water would go through a coiled pipe starting at the bottom, up to the warmest water at the top. In the UK, I’m told the average family of 4 uses 4000 kWh of hot water, the scheme above should get 80% of 50% back – 40% overall. — Iain Blackie
Response:
>I had in mind a tank, rather than a pipe-in-pipe. As the volume of >outgoing water is the same as the incoming, at best you can only get 50% >of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C.
Not so if the system is counter current. >The more surface area, the better the efficiency.
True but the higher the capital cost. >The tank would take in the warm water at the top and drain it out at the >bottom. The incoming water would go through a coiled pipe starting at >the bottom, up to the warmest water at the top.
I think it can be simpler than this because a volume of water running down a drain "sticks" to the sides, forming a thinnish layer. So Nick’s system could achieve sufficient heat exchange area in a simpler device. If I follow his idea, hot water runs down a central drain, the walls of this drain form the heat exchange surface with an up rising cold water supply in an outer concentric pipe. AJH
Response:
>I think it can be simpler than this because a volume of water running >down a drain "sticks" to the sides, forming a thinnish layer. So >Nick’s system could achieve sufficient heat exchange area in a simpler >device. If I follow his idea, hot water runs down a central drain, the >walls of this drain form the heat exchange surface with an up rising >cold water supply in an outer concentric pipe.
Somebody makes this, with a cold water pipe coiled around a metal section of drain pipe. It needs a long-ish section of vertical pipe, which makes it unsuitable for my house, but I remember we discussed this here a while back. Check google’s archives… — ComputerSmiths Consulting, Inc. www.compusmiths.com
Response:
>>…I’ve been thinking of putting an 6"x8′ horizontal pressurized cold >water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled >greywater pipe that drains to the sewer through a large bulkhead fitting. >McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. >The cold water might enter the 6" pipe via a reinforced garden hose, then >flow into the water heater via the drain fitting. Who sells 8" DWV pipe? >I had in mind a tank, rather than a pipe-in-pipe. As the volume of >outgoing water is the same as the incoming, at best you can only get 50% >of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C.
Not true, with a counterflow heat exchanger. >The tank would take in the warm water at the top and drain it out at the >bottom. The incoming water would go through a coiled pipe starting at >the bottom, up to the warmest water at the top.
A coiled pipe has to be very large for 80% efficiency at, say, 2 gpm. Many basements have sewer outlets near the ceiling, so a greywater output near the floor probably needs a pump. A heat exchanger up in the rafters uses no floorspace and contributes more space heat upstairs, vs wasting heat to the basement. A tank (vs coil) within a tank can smooth out the daily hot water flow and do heat exchange and long-term tempering even with no flow, preheating a significant volume of cold water with basement air. >In the UK, I’m told the average family of 4 uses 4000 kWh of hot water, >the scheme above should get 80% of 50% back – 40% overall.
An average 1557 Btu/h, enough to heat 28 pounds of water per hour from 55 to 110 F. C = 28 Btu/h-F and A = Pi6/12×8 = 12.6 ft^2 and U = 4 make NTU = 12.6×4/28 = 1.8 and E = NTU/(NTU+1) = 0.64, ie 64% efficiency. An 8" inner pipe ($85+2 $25 endcaps) would raise NTU to 2.4 and E to 71%. A 19′ 4" pipe inside a 20′ 6" pipe would make NTU = 2.8 and E = 0.74. How would you improve this? Nick
Response:
> In the shower the other morning, it occurred to me I was throwing water > at 40C down the plughole and drawing cold water at 5C to replace it. > Sounds like time for a heat exchanger to get back say, 80% of the > difference and pre-warm the incoming water. > Has anyone done this before? Any pitfalls?
The shower water is cooled quite a bit through the air, depending on how fine the nozzle. Compare the temperature right out of the nozzle to water just before it hits the tub. Plus the tub must warm up before it stops absorbing heat. Plus the amount of pipe it must travel from the heat exchanger to hot water tank is a lot of mass to heat up in the little time it runs. You could insulate the cold water pipe, but that might only function to add more mass to heat up. The energy reclaimed could be minuscule. OTOH, if the cold water to the shower ran through the heat exchanger, it would be more efficient, but would require the user to adjust the shower more often. But metal drainage pipes don’t last that long. imho it’s a neat idea, but i don’t think it’s practical.
Response:
- Hide quoted text — Show quoted text ->>In the shower the other morning, it occurred to me I was throwing water >>at 40C down the plughole and drawing cold water at 5C to replace it. >>Sounds like time for a heat exchanger to get back say, 80% of the >>difference and pre-warm the incoming water. >Good idea. I’ve been thinking of putting an 6"x8′ horizontal pressurized >cold water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled >greywater pipe that drains to the sewer through a large bulkhead fitting. >McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. >The cold water might enter the 6" pipe via a reinforced garden hose, then >flow into the water heater via the drain fitting. Who sells 8" DWV pipe? >Nick > I had in mind a tank, rather than a pipe-in-pipe. As the volume of > outgoing water is the same as the incoming, at best you can only get 50% > of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C. > The more surface area, the better the efficiency. > The tank would take in the warm water at the top and drain it out at the > bottom. The incoming water would go through a coiled pipe starting at > the bottom, up to the warmest water at the top. > In the UK, I’m told the average family of 4 uses 4000 kWh of hot water, > the scheme above should get 80% of 50% back – 40% overall.
A counterflow heat exchanger would be able to recover more than 50% of the waste energy. The tank is really unnecessary since you only create shower drain water when you need hot water anyway. I would think the biggest issue would be piping this up in a practical manner. In many homes getting to the shower drains and piping that water somewhere it can be run through a heat exchanger might not be all that easy. I am also fairly sure that a homebrew arrangement as suggested by Nick would be seriously frowned on by the building inspectors. AFAIK, there are requirements that generally you may not mix waste and potable water in the same container. To do this in a "legel" fashion may well requirement some kind of intermediate heat exchange fluid.
Response:
In our country you can buy this equipment. Look at http://www.nemegeer.com/html/body_requperatie.html The text is in Dutch, but you can understand the pictures. Best regards, Zwerius Kriegsman
– Hide quoted text — Show quoted text -> >>In the shower the other morning, it occurred to me I was throwing water > >>at 40C down the plughole and drawing cold water at 5C to replace it. > >>Sounds like time for a heat exchanger to get back say, 80% of the > >>difference and pre-warm the incoming water. > >Good idea. I’ve been thinking of putting an 6"x8′ horizontal pressurized > >cold water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled > >greywater pipe that drains to the sewer through a large bulkhead fitting. > >McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. > >The cold water might enter the 6" pipe via a reinforced garden hose, then > >flow into the water heater via the drain fitting. Who sells 8" DWV pipe? > >Nick > I had in mind a tank, rather than a pipe-in-pipe. As the volume of > outgoing water is the same as the incoming, at best you can only get 50% > of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C. > The more surface area, the better the efficiency. > The tank would take in the warm water at the top and drain it out at the > bottom. The incoming water would go through a coiled pipe starting at > the bottom, up to the warmest water at the top. > In the UK, I’m told the average family of 4 uses 4000 kWh of hot water, > the scheme above should get 80% of 50% back – 40% overall. > A counterflow heat exchanger would be able to recover more than 50% of > the waste energy. The tank is really unnecessary since you only > create shower drain water when you need hot water anyway. > I would think the biggest issue would be piping this up in a practical > manner. In many homes getting to the shower drains and piping that > water somewhere it can be run through a heat exchanger might not be > all that easy. > I am also fairly sure that a homebrew arrangement as suggested by Nick > would be seriously frowned on by the building inspectors. AFAIK, > there are requirements that generally you may not mix waste and > potable water in the same container. To do this in a "legel" fashion > may well requirement some kind of intermediate heat exchange fluid.
Response:
- Hide quoted text — Show quoted text -> In the shower the other morning, it occurred to me I was throwing water > at 40C down the plughole and drawing cold water at 5C to replace it. > Sounds like time for a heat exchanger to get back say, 80% of the > difference and pre-warm the incoming water. > Has anyone done this before? Any pitfalls? > The shower water is cooled quite a bit through the air, depending on how > fine the nozzle. Compare the temperature right out of the nozzle to > water just before it hits the tub. > Plus the tub must warm up before it stops absorbing heat. Plus the > amount of pipe it must travel from the heat exchanger to hot water tank > is a lot of mass to heat up in the little time it runs. You could > insulate the cold water pipe, but that might only function to add more > mass to heat up. The energy reclaimed could be minuscule. > OTOH, if the cold water to the shower ran through the heat exchanger, it > would be more efficient, but would require the user to adjust the shower > more often. > But metal drainage pipes don’t last that long. > imho it’s a neat idea, but I don’t think it’s practical.
It is. Not that long ago I read about someone sells a commercial unit to do just this. You could do something similar using PVC and copper pipe. In just a few minutes I designed one that should work fine and shouldn’t cost more than $50 and take a couple of hours to make. The major problems are making it in such a way you can take it apart to clean and keeping it from leaking where the copper enters and exits.
Response:
> In the shower the other morning, it occurred to me I was throwing water > at 40C down the plughole and drawing cold water at 5C to replace it. > Sounds like time for a heat exchanger to get back say, 80% of the > difference and pre-warm the incoming water. > Has anyone done this before? Any pitfalls?
A high-school student in New Zealand has designed and built one. http://nzherald.co.nz/storydisplay.cfm?thesection=news&thesubsection=… It would be easier if your shower was upstairs, so you could just gravity-feed the waste water through. You could heat the cold water on the intake side of your hot water system. — Geoff
Response:
– Hide quoted text — Show quoted text ->>…I’ve been thinking of putting an 6"x8′ horizontal pressurized cold >>water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled >>greywater pipe that drains to the sewer through a large bulkhead fitting. >>McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. >>The cold water might enter the 6" pipe via a reinforced garden hose, then >>flow into the water heater via the drain fitting. Who sells 8" DWV pipe? >I had in mind a tank, rather than a pipe-in-pipe. As the volume of >outgoing water is the same as the incoming, at best you can only get 50% >of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C. >Not true, with a counterflow heat exchanger. >The tank would take in the warm water at the top and drain it out at the >bottom. The incoming water would go through a coiled pipe starting at >the bottom, up to the warmest water at the top. >A coiled pipe has to be very large for 80% efficiency at, say, 2 gpm. >Many basements have sewer outlets near the ceiling, so a greywater output >near the floor probably needs a pump. A heat exchanger up in the rafters >uses no floorspace and contributes more space heat upstairs, vs wasting >heat to the basement. A tank (vs coil) within a tank can smooth out the >daily hot water flow and do heat exchange and long-term tempering even with >no flow, preheating a significant volume of cold water with basement air. >In the UK, I’m told the average family of 4 uses 4000 kWh of hot water, >the scheme above should get 80% of 50% back – 40% overall. >An average 1557 Btu/h, enough to heat 28 pounds of water per hour from >55 to 110 F. C = 28 Btu/h-F and A = Pi6/12×8 = 12.6 ft^2 and U = 4 make >NTU = 12.6×4/28 = 1.8 and E = NTU/(NTU+1) = 0.64, ie 64% efficiency. An >8" inner pipe ($85+2 $25 endcaps) would raise NTU to 2.4 and E to 71%. >A 19′ 4" pipe inside a 20′ 6" pipe would make NTU = 2.8 and E = 0.74. >How would you improve this?
Hmmm, not sure I follow your units system?!
But I think your reasoning may be flawed – If there were an unlimited flow of the warm water, you COULD get the incoming cold up to that same temperature, with a very large exchanger. With a smaller one, the efficiency WOULD be greater than 50% The difficulty is that outgoing volume matches the incoming exactly, so I’m pretty sure you can’t do better than 50%. – see my other post in the same thread. I wanted to dump a whole showerful in the tank to extract the maximum from it. If I understand your arrangement correctly, much of your warm water will leave the end of the 20 foot (6.096m) pipe still with heat to give. Iain >Nick
– Iain Blackie
Response:
– Hide quoted text — Show quoted text -> >>In the shower the other morning, it occurred to me I was throwing water > >>at 40C down the plughole and drawing cold water at 5C to replace it. > >>Sounds like time for a heat exchanger to get back say, 80% of the > >>difference and pre-warm the incoming water. > >Good idea. I’ve been thinking of putting an 6"x8′ horizontal pressurized > >cold water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled > >greywater pipe that drains to the sewer through a large bulkhead fitting. > >McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. > >The cold water might enter the 6" pipe via a reinforced garden hose, then > >flow into the water heater via the drain fitting. Who sells 8" DWV pipe? > >Nick > I had in mind a tank, rather than a pipe-in-pipe. As the volume of > outgoing water is the same as the incoming, at best you can only get 50% > of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C. > The more surface area, the better the efficiency. > The tank would take in the warm water at the top and drain it out at the > bottom. The incoming water would go through a coiled pipe starting at > the bottom, up to the warmest water at the top. > In the UK, I’m told the average family of 4 uses 4000 kWh of hot water, > the scheme above should get 80% of 50% back – 40% overall. >A counterflow heat exchanger would be able to recover more than 50% of >the waste energy. The tank is really unnecessary since you only >create shower drain water when you need hot water anyway. >I would think the biggest issue would be piping this up in a practical >manner. In many homes getting to the shower drains and piping that >water somewhere it can be run through a heat exchanger might not be >all that easy. >I am also fairly sure that a homebrew arrangement as suggested by Nick >would be seriously frowned on by the building inspectors. AFAIK, >there are requirements that generally you may not mix waste and >potable water in the same container. To do this in a "legel" fashion >may well requirement some kind of intermediate heat exchange fluid.
You have a point, though it would be grey water only. If the incoming pipe entered and left the exchanger complete with no internal fittings, it would be OK with me. Also the pressure in the cold would be greater than the outgoing warm, so it would leak outward. — Iain Blackie
Response:
>>I had in mind a tank, rather than a pipe-in-pipe. As the volume of >outgoing water is the same as the incoming, at best you can only get 50% >of the heat back – the 40C drops to 22.5C and the 5C rises to 22.5C. >Not so if the system is counter current.
Now that I think about it, you’re right. The temperature rise would be greater with a counterflow, but I’m not sure the efficiency improves. I’ve lost my notes on heat exchangers so, from first principles on a very large exchanger with very conductive walls: Parallel flow: dT at input is 40-5=35C dT at output is 17.5-17.5=0C so average dT is 35-0/2=17.5C Counter Flow: dT at input 40-22.5 = 17.5 dT at output 40-22.5 = 17.5 average dT is 17.5, same as in the parallel case. All other things being equal, only the delta T causes heat transfer. As they are the same in both cases, the efficiency is the same. I accept the higher the temperature is more useful,If I could slow the rate of the incoming cold water, I would get a lesser quantity of warmer water, but I cannot control the flowrate of the incoming water – it must be the same as the outgoing, hence the 50% efficiency. It seems I have intuitively made a counterflow without thinking it through – see original description below. >The more surface area, the better the efficiency. >True but the higher the capital cost.
True indeed – as always its cost vs benefit. >The tank would take in the warm water at the top and drain it out at the >bottom. The incoming water would go through a coiled pipe starting at >the bottom, up to the warmest water at the top. >I think it can be simpler than this because a volume of water running >down a drain "sticks" to the sides, forming a thinnish layer. So >Nick’s system could achieve sufficient heat exchange area in a simpler >device. If I follow his idea, hot water runs down a central drain, the >walls of this drain form the heat exchange surface with an up rising >cold water supply in an outer concentric pipe. >AJH
– Iain Blackie
Response:
>>A 19′ 4" pipe inside a 20′ 6" pipe would make NTU = 2.8 and E = 0.74. >The difficulty is that outgoing volume matches the incoming exactly, so >I’m pretty sure you can’t do better than 50%.
NTU = AU/Cmin, the heat transfer area times the conductance divided by the heat capacity flow rate. With lots of surface (A) we can make NTU arbitrarily large. With the same incoming outgoing flow rates (Z = 1), the efficiency E = NTU/(NTU+1)… >If I understand your arrangement correctly, much of your warm water >will leave the end of the 20 foot (6.096m) pipe still with heat to give.
With infinite pipe, the incoming and outgoing water temps would be equal, but that’s beyond most budgets, and it’s hard to fit in a basement. Nick
Response:
– Hide quoted text — Show quoted text -> > In the shower the other morning, it occurred to me I was throwing water > > at 40C down the plughole and drawing cold water at 5C to replace it. > > Sounds like time for a heat exchanger to get back say, 80% of the > > difference and pre-warm the incoming water. > > Has anyone done this before? Any pitfalls? > The shower water is cooled quite a bit through the air, depending on how > fine the nozzle. Compare the temperature right out of the nozzle to > water just before it hits the tub. > Plus the tub must warm up before it stops absorbing heat. Plus the > amount of pipe it must travel from the heat exchanger to hot water tank > is a lot of mass to heat up in the little time it runs. You could > insulate the cold water pipe, but that might only function to add more > mass to heat up. The energy reclaimed could be minuscule. > OTOH, if the cold water to the shower ran through the heat exchanger, it > would be more efficient, but would require the user to adjust the shower > more often. > But metal drainage pipes don’t last that long. > imho it’s a neat idea, but I don’t think it’s practical. >It is. Not that long ago I read about someone sells a commercial unit to do >just this. You could do something similar using PVC and copper pipe. >In just a few minutes I designed one that should work fine and shouldn’t >cost more than $50 and take a couple of hours to make. The major problems >are making it in such a way you can take it apart to clean and keeping it >from leaking where the copper enters and exits.
Hear Hear! "Impossible" is not a Scottish word. I had in mind a plastic or Fibreglass tank, (old water drum?) warm entry at the top and warm exit from the bottom with a U-bend such that the final exit is a bit below the top of the tank – the tank is then at atmospheric pressure. In the UK water is supplied in 25mm plastic pipe <nearly one inch for our American Cousins : )>. The cold pipe would enter and leave from the top, being one complete coil inside – no fittings to leak. A hatch on the top with a gasket and self – tapping screws completes the job 50 – 100 pounds or dollars sounds about right to me. It would payback in less than a year. — Iain Blackie
Response:
– Hide quoted text — Show quoted text – > I’ve lost my notes on heat exchangers so, from first principles on a > very large exchanger with very conductive walls: > Parallel flow: > dT at input is 40-5=35C > dT at output is 17.5-17.5=0C > so average dT is 35-0/2=17.5C > Counter Flow: > dT at input 40-22.5 = 17.5 > dT at output 40-22.5 = 17.5
Nope. dT at warm inlet is 40 – <cold outlet> and <cold outlet> is 5 + Q/F. Q is function of surface area and heat transfer coefficient and delta T. So this problem sometimes gets kind of messy. > All other things being equal, only the delta T causes heat transfer. As > they are the same in both cases, the efficiency is the same.
Sorry, Iain, you miss your notes more than you realize. In a large enough heat-exchanger, the delta t across the tube from warm water to cold water at each point approaches zero. In a counter-flow heat exchanger with infinite surface area, the 5 degree cold water coming in would be 40 going out, and the 40 degree warm water going in would be 5 degree going out. If a parallel and counter HX’s both have the same surface area, the counter flow one has a larger *average* delta T as the fluid’s flow. In a parallel one, the delta T at the inlet is maximum, but it drops continously through the hx. Counter flow has less delta T at any given point than the parallel’s inlet, but it remains almost constant (if Cp for two fluids are matched) throughout the length. Of course infinite surface area heat exchangers are a bit bulky and hard to install
. But counter flow hx’s that achieve 80% or better are eminently practical to build in commercial applications. The commercially available one that I have installed in my house is from www.endlessshower.com . This is the unit that we’ve discussed before on this and other NG. I think it is the same design as the Dutch site you mentioned in another post. That web site has a link to a DOE (US Dept. Of Energy) report about the design. Mine has the whole house waste pipe going through it (it’s a straight pipe so there is no obstruction to catch/collect ‘waste’). And I pipe the cold water supply feeding both my shower and HW heater through the potable side. The various sizes do require some amount of vertical drop in the waste line, but it is important to not have the waste pipe completely flooded since that reduces effectiveness. I’ve been monitoring mine every quarter since I installed it, and the effectiveness has dropped from a high of 63% when first installed to 54% last month (guess it’s time to inject some ‘cleaner’ to remove the scum
It uses two pipes, one for waste and one for supply so it meets most building codes about avoiding mixing waste & potable. It uses the film-effect on the waste pipe to improve the film coefficient. As far as recovery, my home has PVC waste pipes and I’ve found the following. With 2.9 GPM flow and shower (in second floor bathroom) at 110 degrees F, I get 107 degrees F in the basement at the HX inlet after about 3 minutes. So there is some temperature drop, but not too bad. With 39 degree supply water temperature going in, I get 76 degree supply water leaving the unit (last month’s numbers). The drain outlet is cooled to 70 deg F. I’ve noticed that since I installed it, I don’t run out of HW even when taking a long ‘Hollywood’ shower. And, the faucet temperature control is more towards the ‘C’ than the ‘H’ than what it used to be. daestrom
Response:
– Hide quoted text — Show quoted text -> I’ve lost my notes on heat exchangers so, from first principles on a > very large exchanger with very conductive walls: > Parallel flow: > dT at input is 40-5=35C > dT at output is 17.5-17.5=0C > so average dT is 35-0/2=17.5C > Counter Flow: > dT at input 40-22.5 = 17.5 > dT at output 40-22.5 = 17.5 > Nope. dT at warm inlet is 40 – <cold outlet> and <cold outlet> is 5 + Q/F. > Q is function of surface area and heat transfer coefficient and delta T. So > this problem sometimes gets kind of messy. > All other things being equal, only the delta T causes heat transfer. As > they are the same in both cases, the efficiency is the same. > Sorry, Iain, you miss your notes more than you realize. In a large enough > heat-exchanger, the delta t across the tube from warm water to cold water at > each point approaches zero. In a counter-flow heat exchanger with infinite > surface area, the 5 degree cold water coming in would be 40 going out, and > the 40 degree warm water going in would be 5 degree going out. > If a parallel and counter HX’s both have the same surface area, the counter > flow one has a larger *average* delta T as the fluid’s flow. In a parallel > one, the delta T at the inlet is maximum, but it drops continously through > the hx. Counter flow has less delta T at any given point than the > parallel’s inlet, but it remains almost constant (if Cp for two fluids are > matched) throughout the length. > Of course infinite surface area heat exchangers are a bit bulky and hard to > install
. But counter flow hx’s that achieve 80% or better are eminently > practical to build in commercial applications. > The commercially available one that I have installed in my house is from > www.endlessshower.com . This is the unit that we’ve discussed before on > this and other NG. I think it is the same design as the Dutch site you > mentioned in another post. That web site has a link to a DOE (US Dept. Of > Energy) report about the design. > Mine has the whole house waste pipe going through it (it’s a straight pipe > so there is no obstruction to catch/collect ‘waste’). And I pipe the cold > water supply feeding both my shower and HW heater through the potable side. > The various sizes do require some amount of vertical drop in the waste line, > but it is important to not have the waste pipe completely flooded since that > reduces effectiveness. > I’ve been monitoring mine every quarter since I installed it, and the > effectiveness has dropped from a high of 63% when first installed to 54% > last month (guess it’s time to inject some ‘cleaner’ to remove the scum
Have you calculated the payback time of the capital cost of installation? In domestic homes these things are rarely worth it. Commercial system tend to perform better in economy.
Response:
>Have you calculated the payback time of the capital cost of installation? >In domestic homes these things are rarely worth it…
Would you have any evidence for this article of faith? Nick
Response:
– Hide quoted text — Show quoted text -> I’ve lost my notes on heat exchangers so, from first principles on a > very large exchanger with very conductive walls: > Parallel flow: > dT at input is 40-5=35C > dT at output is 17.5-17.5=0C > so average dT is 35-0/2=17.5C > Counter Flow: > dT at input 40-22.5 = 17.5 > dT at output 40-22.5 = 17.5 >Nope. dT at warm inlet is 40 – <cold outlet> and <cold outlet> is 5 + Q/F. >Q is function of surface area and heat transfer coefficient and delta T. So >this problem sometimes gets kind of messy. > All other things being equal, only the delta T causes heat transfer. As > they are the same in both cases, the efficiency is the same. >Sorry, Iain, you miss your notes more than you realize. In a large enough >heat-exchanger, the delta t across the tube from warm water to cold water at >each point approaches zero. In a counter-flow heat exchanger with infinite >surface area, the 5 degree cold water coming in would be 40 going out, and >the 40 degree warm water going in would be 5 degree going out.
Ah, yes – it makes sense now. Thanks. – Hide quoted text — Show quoted text ->If a parallel and counter HX’s both have the same surface area, the counter >flow one has a larger *average* delta T as the fluid’s flow. In a parallel >one, the delta T at the inlet is maximum, but it drops continously through >the hx. Counter flow has less delta T at any given point than the >parallel’s inlet, but it remains almost constant (if Cp for two fluids are >matched) throughout the length. >Of course infinite surface area heat exchangers are a bit bulky and hard to >install
. But counter flow hx’s that achieve 80% or better are eminently >practical to build in commercial applications. >The commercially available one that I have installed in my house is from >www.endlessshower.com . This is the unit that we’ve discussed before on >this and other NG. I think it is the same design as the Dutch site you >mentioned in another post. That web site has a link to a DOE (US Dept. Of >Energy) report about the design. >Mine has the whole house waste pipe going through it (it’s a straight pipe >so there is no obstruction to catch/collect ‘waste’). And I pipe the cold >water supply feeding both my shower and HW heater through the potable side. >The various sizes do require some amount of vertical drop in the waste line, >but it is important to not have the waste pipe completely flooded since that >reduces effectiveness. >I’ve been monitoring mine every quarter since I installed it, and the >effectiveness has dropped from a high of 63% when first installed to 54% >last month (guess it’s time to inject some ‘cleaner’ to remove the scum
>It uses two pipes, one for waste and one for supply so it meets most >building codes about avoiding mixing waste & potable. It uses the >film-effect on the waste pipe to improve the film coefficient. >As far as recovery, my home has PVC waste pipes and I’ve found the >following. With 2.9 GPM flow and shower (in second floor bathroom) at 110 >degrees F, I get 107 degrees F in the basement at the HX inlet after about 3 >minutes. So there is some temperature drop, but not too bad. With 39 >degree supply water temperature going in, I get 76 degree supply water >leaving the unit (last month’s numbers). The drain outlet is cooled to 70 >deg F. >I’ve noticed that since I installed it, I don’t run out of HW even when >taking a long ‘Hollywood’ shower. And, the faucet temperature control is >more towards the ‘C’ than the ‘H’ than what it used to be. >daestrom
– Iain Blackie
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Try here. This is the one that was discussed here a few months ago. http://www.endlessshower.com/welcome.htm Hope this helps, Gordon Richmond
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Actually, a single common PVC pipe inside the greywater drain pipe will only raise the temp on the fresh water a few percent at best, even if the pipe run were very long. PVC pipe isn’t a good heat transfer medium, its actually an insulator. A true heat exchanger would consist of an assembly of many thin channeled stainless steel baffle plates, one channel being pressurized greywater, the next neighboring channel if supply water, the next grey water, and so on. The number of baffle plates depends on the maximum volume of heated fluid, and the corresponding volume of fluid to heat. One of the names of this type of device is "Chill Water Press Heat Exchanger." The most common form of plate heat exchangers to be found is in the local modern dairy farm, where the fresh hot milk is pumped through a heat exchanger to cool it on the way to the raw milk storage tank. I worked as a Pasteurizer in a large dairy and packaging plant in the USA in the 1970’s. The extracted milk left the milking machines around 103 degrees F., and exited the heat exchanger at around 38 degrees F. The chilled water entering the exchange side of the plates entered at 34 degrees F, and exited back to a cooling tower around 115 degrees F. Part of the increase was due to high efficiency of the device, as well as high pump pressure and friction. There are several commercial uses of heat exchangers, and several variation on the construction, but none manufactured for spent home waste water (which is not all hot water). Capturing the hot water heater exhaust flue heat would be a more constant heat source, and more cost effective. I believe that a series of small metering vanes (that operate much like a common mechanical water meter) could be setup to generate a small amount of power ever time greywater passes out of a house or business. This would be universal, not dependant on the temperature of the water, and easily inverted into household current or battery storage. Hope this was helpful. RG >In the shower the other morning, it occurred to me I was throwing water >at 40C down the plughole and drawing cold water at 5C to replace it. >Sounds like time for a heat exchanger to get back say, 80% of the >difference and pre-warm the incoming water.
Good idea. I’ve been thinking of putting an 6"x8′ horizontal pressurized cold water thick-walled PVC pipe inside an 8"x10′ unpressurized thin-walled greywater pipe that drains to the sewer through a large bulkhead fitting. McMaster-Carr sells 10′ of 6" pipe for $56.61 and 6" caps for $9.75 each. The cold water might enter the 6" pipe via a reinforced garden hose, then flow into the water heater via the drain fitting. Who sells 8" DWV pipe? Nick
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>Actually, a single common PVC pipe inside the greywater drain pipe will only >raise the temp on the fresh water a few percent at best, even if the pipe >run were very long. PVC pipe isn’t a good heat transfer medium, its >actually an insulator.
As I recall, it’s about R1 per inch, ie 4 Btu/h-F-ft^2 for a 1/4" pipe wall, so a sufficiently long horizontal common PVC pipe inside a larger pipe full of greywater will make a dandy heat exchanger, approaching 100% efficiency as the owner’s wallet grows thinner. >A true heat exchanger would consist of an assembly of many thin channeled >stainless steel baffle plates…
Ah, a True Heat Exchanger… >The number of baffle plates depends on the maximum volume of heated fluid,
One might imagine it depends on the flow rate. >There are several commercial uses of heat exchangers, and several variation >on the construction, but none manufactured for spent home waste water (which >is not all hot water).
Can you say "GFX"? >Capturing the hot water heater exhaust flue heat would be a more >constant heat source, and more cost effective.
Would you have any evidence for this article of faith? How is "capturing" a heat source? Why would we need to heat hot water?
>I believe that a series of small metering vanes (that operate much like a >common mechanical water meter) could be setup to generate a small amount of >power ever time greywater passes out of a house or business. This would be >universal, not dependant on the temperature of the water, and easily >inverted into household current or battery storage.
This is just fine in concept, but falls apart with a few numbers, like microhydro using rainwater from a roof. >Hope this was helpful.
Not at all. Now go away. Nick
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> I believe that a series of small metering vanes (that operate much like a > common mechanical water meter) could be setup to generate a small amount of > power ever time greywater passes out of a house or business. This would be > universal, not dependant on the temperature of the water, and easily > inverted into household current or battery storage.
Just out of curiousity, how much potential energy do you think is contained in unpressurized waste water? Figure an average drop of 10 feet to the sewer. A hint – it ain’t much.
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>Have you calculated the payback time of the capital cost of installation? >In domestic homes these things are rarely worth it… > Would you have any evidence for this article of faith?
Just from people who have costed it and rejected it. No firm figures to hand, that is why I asked.
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