DIY Air-Source Water HP

[Geo NR Gee]

[QUOTE=launboy;18957]

I need to flow at least 30 GPM through this[/QUOTE

Adam

Adam,
What makes you think you need to run that much? My 8 person hot tub barely flows the heated water. I feel the heat and water flow coming up from the recirculating pump opening. Its very low flow when the jets are on or off.
After looking in the pump area of my unit (enclosed area), it has extra piping to install an a ultra violet light. For mine it would make it easy to install an inline heat exchanger. From the looks of your setup the pump looks to be outside the hot tub. Could you just use a lower flowing pump?

[AC_Hacker]

Originally Posted by launboy

...I calculated an Average COP of 2.6 to 3.1 which I'm not exactly sure of because I'm not exactly sure if my heat loss over the time period was 4* or 6*. How does that COP sound with an avg 42* outdoor temp?

I would say that COP of 2.6 to 3.1 is very good... almost too good, considering the small air HX. Your water side is in direct connection with water, and its surface area is large, but I don't know about the water flow past the HX. It would be difficult to guess about that.

On my initial HP test, with brazed plate HXs on both sides and water pumps working merrily away, I was getting this:

I had two 50-gallon barrels. One of them, 'SOURCE' started off at about 72F, indicated by the declining black line (this one pumped into the evaporator HX) and the other barrel, 'SINK' started off at about 45F, indicated by the inclining magenta line (this side pumped into the condenser HX). The light blue line, that was bouncing all over the place, was the COP that was calculated every 5 minutes. If you can imagine a generalized line through the crazy COP points, you can see that the COP was declining at a rate that approximated the SOURCE. If you could imagine the SOURCE declining to 42F, the COP would likely be something less than 4.

I was using over-sized brazed plate HXs, with dedicated pumps assuring really good flow on each one, and I repeated this run several times, with very close results. So, I think your COP calculation is more realistic than 1.2 or whatever you came up with before, but, not to take away from the work you have done, I suspect that your results are maybe a bit too good.

Originally Posted by launboy

Where'd you find those exchangers so cheap?

My son has a friend, also something of a HVAC hacker, who is trying to raise money for a road trip.

Originally Posted by launboy

I think I might have issues with corrosion or clogging and flow rate problems using a brazed plate exchanger on pool water. I need to flow at least 30 GPM through this. I'm thinking a tube in tube with a PVC pipe outer shell for ease of connectivity to pool plumbing, and an inner copper coil, much like Acuario did, seen here :Pool heater using recycled air conditioner though I’m not sure how to properly size that, any tips? My hacking unit will be the 8000 btu window A/C I’m using now.

For long time use you're probably right about the HX, and pool chemicals. There are brazed plate HXs that are stainless and soldered with nickle, that could hold up to pool chemicals. But you are hacking, right? If you could get something that would last a year or two, might it serve the purpose?

Your 40 gpm number seems pretty large to me. Where did you get such a number?

As far as sizing an Acquario type HX, he didn't know either, he just dove in and tried it, and then made changes to the second attempt, which he was happy with.

Be brave.

Originally Posted by launboy

Since I’ll have the circuit open, I’m improving my evaporator coil too. I’m just going to use what I’ll have, so my choices are 8000 btu evap coil, 8000 btu condensing coil, 5000 btu evap coil, and a 5000 btu condensing coil. I’d Like to only use two coils because the way the pipes are set up I could use two coils easily(the evap right now has 2 circuits). I’m thinking the 5000 and 8000 btu condensing coils because they’re both thinner, allowing air to flow easier and size wise, they’re just larger.

If I were you, I would look about a bit longer to see if there is a really big-*** HX you can find for it. Maybe you could locate a Ton or Ton and a half A/C for free (do you now have an ad running on Craigs List stating that you are looking for a DEAD 1 Ton to 1.5 Ton A/C for free? You should do that.) One of the best part of this HVAC hacking madness is that there are so many freely available parts to work with!

NOTE: Just to see if my advice is correct, I just now did a search in the FREE section of my local Craigs List.

• Searching for "hot tub" I found for free, four different hot tubs.
• Searching for "heat pump" I found for free, (yet another) hot tub with a non-working heat pump heating system.
• Searching for "air conditioner" I found for free, a large still-working (needs freon charge) air conditioner that has "enough power to cool a house."

...parts are not only available, they are begging to be taken...

NOTE: I just started a thread on where to get donor units and tools.

-AC_Hacker

[randen]

Launboy
I was just thinking the evaporator That may work well for your endeavor might be an outside unit air to air 2-3 T the larger the better. They have the built in fan and coupled to your 8K compressor maybe the ticket. You want to gather as much heat as possible in the cold air and these things push alot of air through. And you can locate it away from a source of moisture (the hot tub) with a small line set. Look around and ask I bet you could get one for next to nothing. A length of stainless tube 5/16 to 3/8 dia inside one of the circ. tubes approx 6 ft would be all you require for the condensor side.

Randen

Good project Adam

[launboy]

The 30 gpm came from a conservative estimate of what my 1 HP pump is flowing. In the set up I have now it's probably closer to 60 gpm. I can't believe I forgot about the circ pump when I wrote that. I actually have a pump from a small Intex pool someone threw out. It's rated at 530 gph so ~8gpm, and actually I put it in today to circulate the water instead of the 1HP pump, to reduce my electric consumption, works very well. This pumps flow rate is my new target for my HX. I'm still thinking DIY tube in shell.

Randen, 6 feet for my condenser for an 8000 btu compressor is really all I need? Wow, there's 21ft of pipe on my 5000 btu condenser coil. I guess refrigerant to water is alot more efficient then to air.

You also mention getting a 2-3 ton condenser cheap and using that. Can you over size an evaporator to a point where it causes problems? Another thing is, does this have any benefit aside from slower frosting up? Say right now with a dry coil I'm getting 6000 btu with a 35* outside temp, would I get 7000 - 8000 btu out of it?

Geo NR Gee, my pump is indeed outside of the cabinet of the tub. It's probably not ideal for heat loss, but originally I designed it not to have a pump there at all. It was piped underground to an equipment pad where it interconnected with the pool plumbing and solar heat. Unfortunately, despite the pipes being wrapped in foil bubble wrap, it lost too much heat to the ground and a 1350w element could only maintain 95* running continuously. This works much better.

AC, the COP is definitely closer to the 2.5-2.6 side of that number based on further heat loss testing of the tub with everything off. Keep in mind my HP was picking up some of the heat from the 1HP(~1400w) pump which exhausted to the coil. Also keep in mind that my water is starting around 60* in the morning and getting to about 70*, with an O.D temp averaging 35* to 45*F.

Looking at your graph I can't see how your COP bounced around so much! Isn't COP calculated by taking the btu/watt of the HP divided by 3.41(btu=1 watt)? If not than I've been doing this wrong.

AC Hacker
If I were you, I would look about a bit longer to see if there is a really big-*** HX you can find for it. Maybe you could locate a Ton or Ton and a half A/C for free (do you now have an ad running on Craigs List stating that you are looking for a DEAD 1 Ton to 1.5 Ton A/C for free? You should do that.) One of the best part of this HVAC hacking madness is that there are so many freely available parts to work with!

Well, using both condensing coils from the units I have now would give me a capacity of 13,000 btu; so that's a little more then a ton, and I've already got all the parts. I'll definitely put the add out for dead units though, that's a good idea, and who know's, maybe I'll even get a dehumidifier or something with a reversing valve.

Thanks for the help and info everyone, it's really helpful as I progress in my planning stages of version 2.0. I'm thinking I'll talk to the HVAC tech down the street next time I see him out and ask if he'd be willing to help my out with evacuating and charging the system ,and maybe he'd even be able to hook me up with some parts from old units. If he's willing to help, he may be my most valuable resource.

Adam

[launboy]

Forgot to add, Craigslist, at least this time of year, isn't so great for HVAC equipment. Heat pumps especially, aren't popular at all in my area. We have cold winters, plenty of snow, and natural gas available everywhere so I'd say 90% of houses have gas furnaces.

Adam

[AC_Hacker]

Originally Posted by launboy

Isn't COP calculated by taking the btu/watt of the HP divided by 3.41(btu=1 watt)? If not than I've been doing this wrong.

COP = (energy out) / (energy in)

To calculate COP, the same units must be used for energy out and energy in. You can use BTUs or you can use Watts and the answer will be the same, but you can’t use Watts for one and BTUs for the other and get a correct answer.

I had two barrels that held 55 gallons of water.

Water weighs 8.34 pounds per gallon so:

Total weight of water in barrel is 55 x 8.34 = 458.7 pounds.

One BTU is the amount of energy required to raise 1 pound of water 1 degree F.

I took a temperature measurement every 5 minutes, using a thermometer that measured to tenths of a degree F.

So, if the temperature of the barrel went up 1 degree, it was evidence that 458.7 BTUs of energy had been used.

This calculation gave me energy out.

To get energy in, I plugged the compressor into a Kill-a-Watt meter and read how many watts were being used.

I converted from Watts to BTU by multiplying Watts by 3.412.

I took Watts readings every 5 minutes and converted to BTUs.

This calculation gave me energy in.

COP = (energy out) / (energy in)

When I did my graphs, I calculated the BTUs for each 5 minute period, because I wanted to see if there was a COP change during the test, and if there was, I wanted to know if it related to anything else I was measuring.

-AC_Hacker

[launboy]

Ok then I think I may have been doing that right. That little formula was a condensed bit of what I actually do. To calculate COP I've been converting everything to BTUs, with the assumption that my compressor was pulling 800w(I know that assumption could put me off but on the performance curve it should be between 700 and 900 so I split it).

My formula:
energy out= HP BTU output
Energy in= 700w*3.412=2388.4 BTU Input
Energy Out/Energy in
BTU out/BTU in
BTU Output/2388.4=COP

My Quoted formula still uses the same units...
(BTU/Watt output)/(BTU/Watt)=COP

Example:
Output = 8000 BTU
Input = 700 Watts

(8000/700)/(3.412/1)=COP
11.43/3.412=COP
3.35=COP

Check:
Energy Out/Energy In
BTU out/BTU in
Energy in= 700w*3.412=2388.4 BTU
8000/2388.4=COP
3.35=COP

My equation states how many BTU's were actually produced for every watt consumed, compared to the number of BTU's a watt actually is. My formula is the BTU in/out one with a watt conversion factor built in, so it's more plug and play.

Adam

[AC_Hacker]

Originally Posted by launboy

...with the assumption that my compressor was pulling 800w...

If you ever get your hands on an instrument that will measure power, you will be in for some real surprises... like how much the power consumed by a vapor-compression device fluctuates, and when, and what causes the fluctuations and what effect the fluctuations have on the overall system.

My little de-humidifier compressor was rated at 550 watts. There were a few moments when it actually drew that amount.

I think that the Watts number is required to be printed on devices, so that someone can then calculate what maximum load a home electric circuit might be exposed to, by adding up all of the estimated watt ratings of all the devices connected.

For COP purposes, it should really be measured with a true RMS instrument.

To my knowledge, a Kill-a-Watt is probably the cheapest true RMS power measuring instrument available.

-AC_Hacker

[launboy]

So you're saying even though your dehumidifier was rated at 550w most of the time it was actually pulling less? That'd be good news if my compressor did the same.

Adam

[AC_Hacker]

Originally Posted by launboy

So you're saying even though your dehumidifier was rated at 550w most of the time it was actually pulling less? That'd be good news if my compressor did the same.

Adam

And sometimes it was pulling more.

-AC_Hacker