DIY Air-Source Water HP

[launboy]

I may be going in the totally wrong direction with this one, but I was bored and thought I'd try it. I have a theory that by pulling more air through the coil I can slow frosting because more air means warmer temps. I'm not even sure if I can pull much more air through this small coil than the stock fan does, but I tried it. The electricity the second fan uses probably isn't worth the added flow it creates. We'll see if it makes a difference tomorrow.


5000btu blower on top with cardboard ducting. Surprisingly there's hardly any leakage.

AC, I reviewed your graph further and I noticed that from about 62* on, the evap was actually warmer than the water it was supposed to be pulling heat from. I'm guessing this was just measurement inaccuracy?

Adam

[AC_Hacker]

Originally Posted by launboy

AC, I reviewed your graph further and I noticed that from about 62* on, the evap was actually warmer than the water it was supposed to be pulling heat from. I'm guessing this was just measurement inaccuracy?

The temperature sensor on the ("LS temp") was the outdoor sensor from a thrift store indoor/outdoor thermometer, so 'lab quality' can not be claimed.

And the temperature probe ("CHILL") that measured the water in the barrel was a similar kind of thing. The barrel probe was right in the middle of the barrel with water swirling around constantly... not on the HX.

So, maybe this was one of those 'digital does not guarantee accuracy' things.

But that's not all that was going on...

The ("LS temp") sensor was attached, with insulating foam tape, to the tube that was exiting the evaporator brazed plate HX. I'm sure that the temperature at the point on the HX where the evaporator was spraying was very much colder... I don't recall measuring the very coldest part of the HX, but I suspect that it was in the 'teens' as there was almost always frost around the point where the cap tube joined the HX. The result is that the refrigerant was being heated and the water was being cooled.

There was never a moment when the water in the barrel was colder than the evaporating refrigerant.

I didn't put a sensor on the frosted part of the evaporator HX.

I did that test at the very beginning of my personal investigation of HVAC hacking... to see what kind of results I could get from a simple homemade heat pump. I learned a lot about the potential that was within my reach.

I also learned a lot about the need for better measurement. I bought a simple data logger board from SparkFun and made an 8-channel data logger, which has been very useful. I also learned about 1-wire temperature sensors, and I got another board from a local guy that can run loads of 1-wire sensors

I think that were I to do the same test over now that I have real data loggers, my results would be better.

I've really learned a lot from all of this, in so many ways.

-AC_Hacker

[AC_Hacker]

Hey Adam,

I thought you might be interested in seeing what your competition looks like.

Craigs List Pool & Spa Heater

-AC_Hacker

[launboy]

So I think my experimental phase of my hottub heatpump may have come to its end. Our temps have dropped and we got almost a foot of snow over the past few days, rendering my little HP basically useless. I haven't even turned it on since the first day it started snowing. I've learned quite a bit that I will incorporate into version 2.0. including active defrost(comparing O.D. temp and evap line temp), actual tub temp control, a tube and shell condensing coil, and a much larger, at least twice the capacity evaporating coil. Oh, and version 2.0 will be propane powered.

I was reading that on heatpumps a TXV is alot more beneficial than it is on an AC due to the larger differential of temps between hot and cold side and the variations on the cold side. I think for now I'll stick with the cap tube for simplicity sake.
"Capillary tubes cannot adjust the refrigerant modulation which is fixed by the physical length of the tube. Like a broken clock that is right only twice a day, capillary tubes either over-modulate or under-modulate the refrigerant as the water temperatures vary. As an extreme example, if the water is 40° and the air is 95° the modulation will be excessive and the head pressure much too low. Expansion valves adjust the modulation according to the conditions."

That add states that HP is "standard rating: 125,000 btuh, low temp 75,000 btuh. That's huge! 10.5 ton and 6.25 ton respectively. It's interesting to me that when you compare the size of modern house HP to a pool HP, that a 5 ton house unit is similar in size to a pool hp of more than double that capacity.

People have recommended an evap coil from up to a 2 ton residential condenser. While I'm sure that would work incredibly well, I'm not sure it'd be worth it. The fan alone would draw around 500w. That added to the compressor wattage(800w, if it drew that) reduced my perfect world(producing 8000 btuh)COP to about 2. I'd like to see higher, and I'm sure most of the time I wouldn't be achieving full output in 35-40*avg temps.

Adam

[AC_Hacker]

Originally Posted by launboy

So I think my experimental phase of my hot tub heat pump may have come to its end...

It is amazing, how much you have learned from this one experience. If you look back to your earliest posts, and compare them to what you have just written, it's clear that you have come a long way.

And thanks for sharing your experiences with us... This way we all learn from your experiences, too.

BTW, I hope you saw the post I left on the 'manifesto' thread, regarding the sale on vac pumps...

Also there was a very interesting post HERE from a guy in Australia who was using a method that required no equipment... however, now he does have a righteous HVAC kit.

Originally Posted by launboy

I was reading that on heatpumps a TXV is alot more beneficial than it is on an AC due to the larger differential of temps between hot and cold side and the variations on the cold side. I think for now I'll stick with the cap tube for simplicity sake.
"Capillary tubes cannot adjust the refrigerant modulation which is fixed by the physical length of the tube. Like a broken clock that is right only twice a day, capillary tubes either over-modulate or under-modulate the refrigerant as the water temperatures vary. As an extreme example, if the water is 40° and the air is 95° the modulation will be excessive and the head pressure much too low. Expansion valves adjust the modulation according to the conditions."

Good info on TXVs!

Originally Posted by launboy

It's interesting to me that when you compare the size of modern house HP to a pool HP, that a 5 ton house unit is similar in size to a pool hp of more than double that capacity.

The space taken up by the air-source heat exchanger alone could account for this. I have seen photos of water-in-water-out heat pumps and they are quite small.

Originally Posted by launboy

People have recommended an evap coil from up to a 2 ton residential condenser. While I'm sure that would work incredibly well, I'm not sure it'd be worth it. The fan alone would draw around 500w.

I'm actually thinking along the same lines for another project I have in mind, and I think that with a larger HX, you could get away with less power in the fan.

In fact, in THIS DISCUSSION about a fine wire heat exchanger and the accompanying high-efficiency fan...

How does it work?

A slowly turning fan drives air (typically 500 liters per second) through a maze of copper fine wire gauze, woven around water carrying copper capillaries: a fine wire water-to-air heat exchanger.... The fans from this unit will have a diameter of 510 mm, maximum speed 250 rpm, electric power 60 W max., maximum heat transfer capacity 200 W/m2K

So the idea here is that they are using a slowly-turning centrifugal fan, with the HX positioned circumferentially around the fan. Very smart, very efficient.

All things being equal, centrifugal fans and their variants are more efficient than axial fans.

Interesting work being done in this area.

-AC_Hacker

[launboy]

Originally Posted by AC_Hacker

It is amazing, how much you have learned from this one experience. If you look back to your earliest posts, and compare them to what you have just written, it's clear that you have come a long way.

Thank you, I definitely have learned a lot along the way of this project. Hopefully that knowledge will lead to a final product that works very well.

Originally Posted by AC_Hacker

Also there was a very interesting post HERE from a guy in Australia who was using a method that required no equipment... however, now he does have a righteous HVAC kit.

I saw BradC's post, very interesting indeed. Got the wheels turning in my head, because a recovery machine is definitely an expensive part of the deal. I think I'd still need a vac pump though, considering the changes I need to make to the system. Hopefully I can find an HVAC guy who finds my little project interesting and is willing to help out a little.

Originally Posted by AC_Hacker

The space taken up by the air-source heat exchanger alone could account for this. I have seen photos of water-in-water-out heat pumps and they are quite small.

I wasn't surprised it was as big as it was, on the contrary I was surprised it was as small at it was. I figured it would be around twice the size to handle twice the capacity. The pool HP is putting out twice the BTU's of a home HP with the same size evap coil(or OD unit for home HP).

Originally Posted by AC_Hacker

I'm actually thinking along the same lines for another project I have in mind, and I think that with a larger HX, you could get away with less power in the fan.

All things being equal, centrifugal fans and their variants are more efficient than axial fans.

Are you saying since the coil is so oversized it will still transfer enough BTU's with less airflow? The comment about centrifugal vs axial is something I noticed in my experimenting with some fans I have here and the 5000 btu evap coil. I made a funnel on a floor type axial fan, similar in size to a box fan but not designed to be used in a window, and compared it to the stock blower from the A/C, which is obviously smaller, and with no restriction the axial moved more air, but once I put the coil over both, the axial's output dropped by a huge amount, while the centrifugal's did not.

I'm thinking about using the condensing coils from the 5000 and 8000 btu A/C's (unless I can find something bigger that's dead), each with their own blower, but powered by a single motor. Essentially, I'm going to take the evap blower from the 5000 btu unit, and stick it on the shaft of the 8000 btu unit's motor where the condensing coils axial fan was. The big idea for my project is low budget and reuse, re-purpose, recycle. I've got to pay for my own college tuition so the less I spend on other things the better. Another advantage to this is this motor is 3 speed so I can play with it and run on low or medium if that's all I need.

Do you know if it's more efficient to restrict the suction or exhaust side of a centrifugal fan if one much be picked?

Adam

[AC_Hacker]

Originally Posted by launboy

Are you saying since the coil is so oversized it will still transfer enough BTU's with less airflow?

Yes, exactly...

Originally Posted by launboy

Do you know if it's more efficient to restrict the suction or exhaust side of a centrifugal fan if one must be picked?

I was chatting with Mike about that just today, I will ask him to comment on this issue, he is in the middle of testing various fan configurations for his HRV project.

-AC_Hacker

[launboy]

Originally Posted by AC_Hacker

I was chatting with Mike about that just today, I will ask him to comment on this issue, he is in the middle of testing various fan configurations for his HRV project.

I'll be eagerly awaiting word on this.

Adam

[AC_Hacker]

Originally Posted by launboy

I'll be eagerly awaiting word on this.

Adam

Here's what Mike said:

I did some tests on mine.
I get about 20 pascals static pressure. Didn't try to measure the volume.
While the ports are at 90 degrees, the airflow is indeed tangential.
The air wants to come out back in the direction of the input port.
If I try to deflect the air at 90 degrees to the input port, the flow
goes way down.

I think the airflow will be ok if I tilt it 45 degrees, but I can't
figure out any mounting configuration that doesn't blast the incoming air right back into the outgoing fan. Still thinkin' about it...

... and he was referring to this:

LTG Tangential Fan 24Vdc Direct Drive | eBay

-AC_Hacker

[ThomSjay]

Not that I know that much, but rather than using a larger fan, couldn't one run the same fan using a fan shroud, like a car engine?