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Old 06-13-16, 11:46 PM   #41
jeff5may
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I would start optimization by slowing your blower down enough at this charge level so that at least part of your evaporator tries to freeze. It should try to do this while the tank is cool. As the tank warms, the coil should melt, allowing the captured moisture to drain. This higher temperature gradient across the evap will reduce its effectiveness slightly, but the reduced blower draw should more than make up for it in energy savings.

Once a larger gradient is established across the evap, you may benefit by adding charge. A higher dischargepressure will speed the heating process. In a system this size, a few grams extra will make a difference. You are not far from your optimum charge.

I have a big question to ask. Can you explain what equipment you are using to log your temperature measurements? And how are you getting that data onto a computer and into a spreadsheet? Many members are mystified and intimidated by the process.

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Old 06-14-16, 06:10 AM   #42
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I need to respectfully disagree with the first step. Slowing the fan will cause more of the gas to stay in the evap further starving the condenser. The condenser is already so close to starved that it wouldn't surprise me if there isn't sufficient liquid at the captube. Adding a small amount of charge will increase the flow rate and reduce liquid line temp which will by effect cause a lower evap temp. I still suspect the fan to be far too fast, but it needs more gas first, we dont want to risk hurting the compressor!
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Old 06-14-16, 06:16 AM   #43
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Thank you guys both for your comments. I kinda understand it, but still need to read it over a few more times to absorb it all. My next move will be to add some more propane and retest, it's all set up and that is the easiest move. I also have a small fan and will see if I can find a way to mount it.

Here's how I take the data. I have 2 thermocouple readers that take 2 thermocouples each. I have colored the connectors red, blue, green, black so I don't mix them up when taking data. I don't have a data logger so I am just writing the numbers on paper and later entering them into a spreadsheet. A watt meter is plugged into the wall and it shows elapsed time.

As the elapsed time approaches a minute mark I write down the time and 10 sec before the minute I start taking data. I read the red & green temps and write them down, then the blue and black, then I read & record the pressures, and then the watts. The data is not all taken at the exact time, but it is always shifted by the same amount.

Later I copy it into a spreadsheet and look up the temps that correspond to the pressures and enter those. The spreadsheet does the math to get SC, SH, and temp rise rate. Now that it is in a spreadsheet I can make charts to help visualize and smooth out the data
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Old 06-14-16, 07:52 AM   #44
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Thanks for the rundown on your data logging process. What equipment are you using between the wiring and your eyeballs? What level of accuracy does this mysterious device provide? Is it some obscure, laboratory grade instrument, or a Chinese hobbyist contraption? Or something in between?

Being a captube driven rig, the flow rate is mainly determined by the difference in pressure the compressor can force. When the tank is cool, the delta T between tank temp and ambient air is small, so the compressor will be moving the most volume possible due to the low compression ratio. As tank temp rises to your target temp, discharge pressure will follow. This rise in compression ratio has a double effect: on the one hand, there is a larger delta P across the cap tube, so it will pass more liquid. OTOH, the compressor needs to do more work on every stroke.

What this says simply is that as tank temp rises, mass flow will generally rise, due to the increased delta P across the cap tube. At high tank temps, you can flood your evaporator if it cannot boil off all of the refrigerant. With what you have logged, this will not happen unless the suction saturation (pt chart) temperature rises close to ambient. This condition can occur regardless of fan speed if you add too much charge. This is why you want to set your system charge above your max tank temp during normal operation. Let's say above 130 degF, since you are going to be popping your thermostat around 110 degF.

If you slow your evap fan down, this potential for flooding will increase, due to its reduced effectiveness. The refrigerant will travel further through the evaporator before it's fully boiled off. This condition is desirable, since more working surface area generally equals more heat transfer. With the high superheat values exhibited, you are not fully loading your evaporator, due to the massive airflow in relation to the compressor capacity. In simple terms, the surface area in your evap coil is being wasted.

Matching airflow to compressor flow can be done either way. You can add charge, moving more energy faster at higher power draw, or you can slow the fan speed, slowing heat flow over more surface area at lower power draw. There is no way to know where your optimum design conditions will lie until you define what you strive to accomplish with this rig. If you want max power savings, everything will end up different than if you want maximum recovery rate. In a way, me and rb are both correct in our recommended courses of action.

Keep running trials, what you just posted for numbers is better performance than some store-bought units. How much ROI you get out of the unit depends on longevity as well as COP, so if you run close to the edge of your operation envelope it may not live a long time. Either way, this is an excellent rig to learn about the factors at play. Since you aren't dealing with thousands of watts of energy, it is much more forgiving than a larger system. It ain't gonna run away on you and blow something up in a hurry.

I hope this helps and doesn't confuse you too much.

Last edited by jeff5may; 06-14-16 at 12:02 PM.. Reason: spelling qand details
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Old 06-14-16, 05:44 PM   #45
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I have no intention to ever be more than a hobbyist and DIYer so I have cheap equipment. The thermocouple readers were around $19 each and the manifold set was around $50. I am taking lots of data points in the hopes that error will average out.

I managed to get in two tests today and each one made improvements. First I recovered the propane and vacuumed the system. Then recharged and weighed the cyclinder before and after. I put in 4.25 oz which I was expecting to be overcharged. These are the results.



Then I disconnected the 85 watt puller fan and cobbled up a 15 watt pusher as shown in photo. No change in the charge.



And here are those results.



That is a really wimpy fan but it sure works good as a dehumidifier and it's using less kWh for heating. I believe there is a small leak so I can repeat this test every day or two and see how it performs with smaller charges.
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Old 06-14-16, 07:36 PM   #46
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Must say that these results look really good! The first run is almost perfect with the exception of the motor using so much power. The second run, the airflow looks like it might be a little low, low ambient could be a concern with the SH so low, but I bet it dehumidifys really well. Without a outlet air temp I'm not able to check airflow, but given your wb/db readings, it should be about 17* Delta T across evap. The second test lower airflow did take a hit on btu produced, but it did use less power getting the final result (with a 4* water temp discrepancy). I have a little concern about the condenser efficiency now as it's on average 20ish degrees warmer than water, but that might just be stagnant water in the tank not able to quickly extract the heat, as it does take a bit of a temperature gradient to get convection after all. I'd be curious to look into some DC fans and a cheap pwm for fan speed control at this point since the ref cycle looks nearly spot on at 4.25oz. (To think my hpwh uses about 1.5lb! Lol)
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Old 06-15-16, 07:12 PM   #47
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As an afterthought.... you said you linked both coils. Their arrangement is pretty important. In evaporators , you feed from the bottom and pull from the top, and suction should be closest to the air inlet for best efficiency. I see that original condensing coil (evaporator) is a dense coil so the top/bottom rule doesn't apply the same, but air direction is important.
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Old 06-16-16, 06:25 PM   #48
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I like it with the small fan because it uses the least watts to heat water, lets more of the evaporator get cold enough to dehumidify, and it's much quieter. I am leaving it like this until I get motivated to try something else.

The fan is so weak that I didn't think I could get a good temp reading for air out of evaporator, not much air passes through it.

The thermostat is a bit inconsistent in where it shuts down and the tank temp does rise about 2 more deg after shut down. Just something to keep in mind.

This has been a much more valuable project for me than I was expecting. Of course at this scale there absolutely no economic justification. My main objective has always been to learn a few new tricks and you guys helped a lot.

When I started I thought I could make a passive evaporator. That didn't work, but the fan is so small that someday I might try that again. And I believe this approach would work well with a larger water heater, so I might try that someday. Putting the condenser coils inside the tank would be more efficient, but this approach allows the water heater to be used in its original manner as well as as a heat pump.

I am getting ready to start an AC project and will soon start a thread for that and hope you all find it interesting and are willing to offer advise.
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Old 06-16-16, 10:42 PM   #49
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Actually, you can run one of these with a passive evaporator. The thing resembles a solar thermal panel without the glass. Definitely not something mass produced commercially for dehumidifiers, but it can be done. The closest thing to what you seek is used in deep freezers and marine fridges and freezers in the capacity range you are operating. Lots of these units have no fans at all, and work reliably for decades. Number one cause of death: failed thermostat.



Don't underestimate the savings that these little projects can produce. The return on investment happens faster the more you run your machine. After that, It's money in the bank. Please make sure and let us know how much (or little) this project is costing (or saving) you during regular operation.

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Old 06-17-16, 07:31 AM   #50
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I had in mind an 8 inch heat duct about 2 ft long wrapped with a spiral of copper tube soldered to it. It's mounted over the laundry tubs so it wouldn't need a catch bucket. Anyway where I live the summers are too short to spend in my basement, I will get back to this project next winter.

I am tracking kWh compared to the resistance heater and will be able to report that next week. I am also measuring the water taken from the air and will compare that to a dehumidifier that I use.

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