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Old 11-24-15, 08:22 PM   #111
jeff5may
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Your conundrum lies in matching mass flow. With cap tube metered setups, it is very important to match the compressor to the cap tube to the heat exchangers. They all effect each other. Whenever some piece of the system changes, guess what? Cap tube has to change length. Swapping from one size of compressor to another, cap tube has to flow different.

How much different? We can't tell you. It's a balancing act. Too much flow and you will either flood your compressor or not get enough head pressure. Too little flow and you will have gobs of superheat but not much mass flow. Somewhere in the middle lies max performance. To make matters more difficult, when you change the cap tube, the optimum system charge changes with it.

Since your new rig (working, I hope) is set up for R-410A, that's what it will get max btu output using for refrigerant. My guess is that you will be able to do more than 15000 BTU for the first little while, due to the huge loop and water source. Much like the water heater, performance will drop off after initial start-up. For this size, a TXV (made for R410A) is worth it. The extra mass of gas the compressor can move (if it is allowed to) adds up in droves of extra heat transfer.

The downsides are many. Like you said, you will need to change the oil. Not only will you have to drain your oil tank, but the plumbing must be flushed as well. Double or triple flushing is better. The filters will need new ones. R410 is not cheap or as forgiving as propane, either. It runs at much higher pressure across the board, so your tubing and connections need to be stronger. POE oil absorbs moisture and won't let go of it. Me no likey R-410A. Too picky and pricey for me.

If you run propane, the system will not develop a full 15000 BTU of heat transfer. More like the 11000 that the last one was rated at, maybe a full ton. I would probably run a TXV (made for r22/R502) on it, rated around 1 ton. If the system can do more heat transfer, the TXV can open up to supply the extra gas.

The underlying question here is: How much heat do you really want to feed the greenhouse? If the unit works, will it be a good match for your heat load? If it is too large, it will short cycle and waste lots of energy starting and ramping up with each cycle.

I would definitely rig it up and play with it for a while to see how it does while it is above freezing outside. You can install it backwards and fool the thermometer to make it heat, or just vent the cold air outdoors with the whole rest of the unit indoors. Running on low fan, it won't suck too much air in from elsewhere.

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Old 11-25-15, 08:39 AM   #112
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Jeff, I almost made a quick reply last night but I knew there was more to what you were saying than I was understanding. So I just sat and continued to work things out in my head. And I think I understand more now. Thank you for your replies they are shaping my view of HVAC everytime.

I think my biggest mental block was that I didn't think that going over about 55psi suction was a good thing. My goal was it to run with as little power as possible, BUT if I'm not making heat then I'm wasting power.

SO I open up the needle valve and charged to 235psi discharge and 90psi suction, and it made a HUGE difference. In fact it dropped my suction temp to 59F. I will need to make my cap tube even shorter to go much more than that though.
I believe I still need a larger compressor, like the 10,500btu I had ().
So I'm still looking and will make a few changes to see if I can get this design to work before I hack the PTAC.
I am also worried I might over power my condenser?
Thanks again
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Old 11-25-15, 08:43 PM   #113
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When I get the larger compressor I'm thinking of going with this. Sporlan Ruud Cbive 1 61 26018 25 TXV 1 Ton 3 8"x1 4" 151619 | eBay
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Old 11-25-15, 11:21 PM   #114
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Quote:
Originally Posted by MEMPHIS91 View Post
When I get the larger compressor I'm thinking of going with this. Sporlan Ruud Cbive 1 61 26018 25 TXV 1 Ton 3 8"x1 4" 151619 | eBay
Here it is on their website for a WHOLE lot less:
Surplus City Liquidators

Parker/sporlan breakdown:
C- internal check valve (for reverse cycle bypass in cooling mode)
BI- brazed internal connection
V- R-22
E- external equalizer port
1- tons of capacity

The powerhead (aka thermostat element) says '43 VGA' plainly. That's what size the head is and what kind of gas is in the sensing bulb. No. 43 head with VGA gas charge. It is a MOP (maximum operating pressure) charge, it limits the evaporator to around 100 PSIG to prevent overloading your compressor.

This correlates to a 65 degF max evaporating temp for r290. Super awesome for a heat pump.

You don't need an internal check valve for a one-way heat pump or an external equalizer for an evaporator that drops less than 20 PSI. Consider these "freebie upgrades". If you were considering buying 1 for 46 bucks plus shipping, just go to their website and buy one for 13 bucks, spend 20 bucks on more hardware (they have lots), and consider the savings as "freebie shipping". Or buy one of these for your water heater and see if it helps or not.
Surplus City Liquidators
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Old 11-26-15, 01:03 PM   #115
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I charged more last night, got to 105psi suction and and 245psi discharge side the entire unit was only pulling 810 watts. This makes me happy, it was heating nicely. I looked up the original design pressure for the window unit I harvested the compressor from and found 400psi/150psi. So I feel much better knowing I can go up to 150psi (though I don't think its needed).

Jeff, would the 100psi overload pressure for the TXV be enough? Thanks for the link, I will be hacking the water heater again soon.

Here is what is REALLY confusing me. In the refrigeration cycle when the gas goes through the metering device and drops to a low low pressure then makes its way through the evaporator and comes back up to the compressor. I am measuring the psi and temp right before the compressor. WHY WHY WHY do they not match the PT chart? Example 50psi suction, 68F temp!? Now 105psi suction, 64F temps!? I know it is liquid coming out of the metering device and I should have differing psi/temps in the evap but when its at the compressor it SHOULD all be a gas, unless I overloaded the evap, right?

AC, thank you for this awesome thread http://ecorenovator.org/forum/geothe...at-pump-6.html
I really think there should be an archive of helpful threads with a short description of what they are about. I have search this forum for a long time and never saw this til now.
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Old 11-26-15, 05:20 PM   #116
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Ok, this is a really good question. The pt chart describes the saturation temperatures at a certain pressure for the refrigerant being used. I say temperatureS because for a blend, the refrigerant will exhibit "glide". The temperature where the blend begins to bubble or boil is different than the dewpoint where it completely vaporizes. While the refrigerant is changing phase, it will stay at the saturation temperature as closely as physics will allow. If there is not enough heat flow to complete the phase change, a mixture of gas and liquid will come out the other side of the hx at or near the saturation(pt chart) temperature.

After all the refrigerant has changed phase, it loses its ability to gain or shed heat without changing temperature. In an evaporator, heat is absorbed only when the refrigerant is colder than its surroundings. Higher dT means more heat transfer. When the liquid all evaporates, as it finds equilibrium with its surroundings the heat transfer tapers off real quick.

This is how we measure heat exchanger loading and effectiveness. If we feed, say 75 psi into your evaporator, it may only be half saturated. Let's say you have a 25 degree dT at this condition. The refrigerant boils off quickly due to the high heat flow in the first half of its journey. Once it changes phase, it has the whole second half to gain superheat and leaves the evaporator not cold. We say the evaporator is only half loaded and the leaving refrigerant is highly superheated. Not too good with a TXV, maybe not too bad with a cap tube.

Ok, lets say something changes and the entering liquid pressure rises to 90 psi. This drops your dT to 10 degF, so the refrigerant travels 90% the way through the hx before it all boils off. Once it leaves saturation, it only has the remaining 10% to be superheated. It comes out colder than in the first example. We say the evaporator is 90% loaded and the leaving refrigerant has x amount of superheat above SST (saturated suction temperature). This amount of loading usually only happens in TXV metered systems with a massive heat source.

Whether the first or second set of conditions will transfer more heat depends on lots of specific details with the rest of the system. Since evaporators can freeze or flood, we usually monitor them and superheat.

Condensers work much the same way: higher head pressure -> higher condensation temp -> higher dT -> more heat flow and subcooling.

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Old 11-26-15, 05:46 PM   #117
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That makes more sense, thank you. What pressure in your opinion should I aim for? With 400/150 as the design, I am thinking maybe something like 330/120 at max temp? Of course staying under about 820 watts or so. Who knows maybe this 7,500BTU comp is exactly what I needed. I will not change anything til I test it on a colder night.

I now see a few reasons why the last comp died.
-Lack of oil
-To small of oil return line
-To small of power cord gauge/wiring
-Not enough suction pressure to properly cool the comp

Lessons learned!
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Old 11-26-15, 10:38 PM   #118
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Ok, with your smallish compressor of under a ton, your loop will most likely be able to digest whatever you throw at it until something breaks. For the most mass flow, a suction pressure near where it is now (90ish psig) is about as high as it should get. This correlates to around 60 degF SST, and you were getting 65 degree suction line temperature. This 5 degrees of superheat is dangerously low for a cap-tube metered rig! I don't know what your ground temperature is, but unless it's really deep, it's not going to be above 65 degrees all year long.

On the condenser side, a 230 psig discharge pressure translates to 120 degF saturation temperature. I imagine the compressor discharge temperature was higher than that. Since you are moving a lot of mass through the compressor at a high compression ratio, the compressor is working as hard as it can. You are probably at that 50% loaded condition, where the entering gas is highly superheated. It travels a while losing superheat until it reaches the saturation temperature. At that point, it begins to condense at constant (120 degrees) temperature. Half way through, it is all liquid and heat transfer tapers off as the liquid temperature drops. Liquid travels way much slower than gas, so the liquid sits around doing nothing for a long time in the end of the condenser. The more charge goes in, the more useless work the compressor does and the more liquid backs up in the condenser.

In the air-source units I have put together, the compressor was never working against this much head pressure unless it was cooling on the hottest days of the year. The high head pressure lays waste to your energy efficiency. It is much more efficient to move more air (or water)flow through your hx at a lower saturation temperature. The air (or water) doesn't "feel hot" coming out, but you are moving more of it, and more of the effective surface area of your hx will be in a saturated state with less pressure. For the same btu transfer, your compressor runs cooler and uses less power.

When you step up to compressors with larger capacities, they have enough muscle to push your system out of its comfort zone. If nothing is in place to sense and stop bad things from happening, they will. More energy causes bigger problems faster.
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Old 11-27-15, 05:12 PM   #119
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Awesome info thanks!

So a larger compressor to push my unit out of its comfort zone is a good thing, so I'll still be looking for one close to a ton.

I ordered the TXV so we shall see how that goes as well.

I believe you are saying my discharge psi is to high? Or should I push a higher psi? Little confused on that, I know you said with higher charge more liquid gets stored in the condenser but is that related to psi too?

Or could I blend some 410a into the mix to bring down the suction temp some?

I will install a high and low pressure cut off as well just to make things safer and save a compressor.
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Old 11-28-15, 01:18 PM   #120
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I'm going to relate this situation to cars.

How you set up your machine depends on how you want to use it. If you are trying to make a top fuel dragster that will do a super fast quarter mile, it will end up much different than one designed for ultimate fuel economy. Every component selected will serve different design goals.

If you want to go fast quick, you will run higher compression and a super-rich fuel mixture. It will drink something like nitromethane, which is uncommon and expensive. The drivetrain and wheels will be sized for maximum torque and power transfer to the road. If at some point during that quarter mile, you feed a little too much fuel, flames may shoot from the exhaust and the engine may explode. That may be OK as long as it happens near the finish line.

For an ultra-efficient fuel saver, you may find that running a higher compression ratio is a good way to develop enough power. That way, you can use an engine that would normally be too weak for your car. With the boost in performance, it uses less fuel than a larger, stronger engine would in the same car. A cheap, abundant fuel will do the job nicely. The drivetrain and wheels will be specified that keep the engine in its "sweet spot" for efficiency at the cost of raw output power. The machine may travel 100 miles or more on the same amount of fuel as the dragster.

Running high discharge (head) pressure and filling your condenser up with liquid is much like the drag racer. You will end up with hotter air coming out of your condenser, even if only half of it is actively exchanging heat, if you slow your air handler down enough. The added compression ratio will kill the efficiency (and maybe longevity) of the compressor, more so if you feed it high suction pressure. More total mass will flow, and lots of liquid subcooling in your condenser will transfer a few btu to the space being heated. Presto! Hyper heat mode has been achieved! Expect a higher electric bill.

On the other end of the spectrum, running lower head pressure will reduce the power draw of your compressor, due to the reduced compression ratio. Somewhat cooler air will be coming out of your condenser, due to the lower saturation temperature. However, due to the reduced refrigerant charge, less liquid in the condenser means more active surface area to change phases and less subcooling. The air handler can be sped up, and the increased airflow carries more btu's to your space being heated. Expect longer run time.

If your indoor temperature is low and the hx is effective, running less charge and head pressure will save you lots of energy in the compressor. In the same manner, running a higher suction pressure and less superheat in the evaporator flows more mass with the same amount of displacement. The newest, super high SEER split units employ a huge outdoor hx and a computer-guided electronic expnsion valve to try to keep as much liquid in the evaporator as it can handle without flooding the compressor. The variable-speed compressors are guided to keep the head pressure high enough to effectively shed heat in the condenser, but low enough not to waste energy.


Last edited by jeff5may; 11-30-15 at 05:36 PM.. Reason: last sentence din't make sense
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